Introduction

PEEK-OPTIMA™ has been considered as an alternative to cobalt chrome in the femoral component of total knee replacements. Whole joint wear simulation studies of both the tibiofemoral and patellofemoral joints carried out to date have shown an equivalent wear rate of UHMWPE tibial and patella components against PEEK and cobalt chrome (CoCr) femoral components. In this study, the influence of third body wear on UHMWPE-on-PEEK was investigated, tests on UHMWPE-on-CoCr were carried out in parallel to compare PEEK to a conventional femoral component material.

INTRODUCTION

There is great potential for the use of computational tools within the design and test cycle for joint replacement devices.

The increasing need for stratified treatments that are more relevant to specific patients, and implant testing under more realistic, less idealised, conditions, will progressively increase the pre-clinical experimental testing work load. If the outcomes of experimental tests can be predicted using low cost computational tools, then these tools can be embedded early in the design cycle, e.g. benchmarking various design concepts, optimising component geometrical features and virtually predicting factors affecting the implant performance. Rapid, predictive tools could also allow population-stratified scenario testing at an early design stage, resulting in devices which are better suited to a patient-specific approach to treatment.

The aim of the current study was to demonstrate the ability of a rapid computational analysis tool to predict the behaviour of a total hip replacement (THR) device, specifically the risk of edge loading due to separation under experimental conditions.

Component alignment and soft tissue constraints are key factors affecting function and implant survival after total knee replacement (TKR). Knee kinematics contribute to knee function whilst soft tissue constraints and component alignment impact polyethylene wear. This study experimentally investigated the effect of soft tissue constraints and component alignment on the kinematics and wear of a TKR.

A six station electromechanical ProSim knee simulator was used with the ISO 14243-1:2009 standard force control inputs; axial force, flexion-extension (FE), tibial rotation (TR) torque and anterior-posterior (AP) force. This allowed the kinematics to vary with the test conditions. The soft tissue constraints were simulated using virtual springs.

DePuy Sigma XLK fixed bearing TKRs were tested in 25% bovine serum (in 0.04% sodium azide) lubricant. The average output kinematics across 6 stations were found for each test and the peak values compared. The wear rates were calculated over 2 million cycles (MC), the serum was changed every 350,000 cycles and the tibial inserts weighed after every MC. A one way ANOVA and post hoc Tukey's test was used to compare the kinematics and wear with significance taken at p<0.05.

The kinematics and wear rates for three soft tissue conditions were established under ideal alignment (Table 1). The ISO standard springs for a cruciate substituting (CS) and a cruciate retaining (CR) prosthesis were used to represent a knee with a resected ACL and PCL and a knee with a resected ACL respectively. The third spring condition was based on clinical data to represent a “stiff” knee.

Three other alignment conditions were then assessed using “stiff” knee springs; 4° varus, 14° rotational mismatch and 10° posterior tibial slope. These alignments were chosen to represent the range found in clinical data.

Under ideal alignment the “stiff” knee springs had significantly lower peak AP and TR displacements (0.9mm, 2mm, 2mm and 3.6°, 7.1°, 7.8° for the “stiff”, CR and CS springs respectively) than the other springs (p<0.01). The “stiff” knee spring had a significantly lower wear rate than the CR spring; 1.58 ±1.20mm³/MC compared to 4.71±1.29 mm³/MC (p<0.01).

The varus and rotated components had significantly larger peak AP displacements of 2.56mm and 2.42mm respectively, than the ideal and tibial slope fixtures (1.97mm and 0.92mm respectively) (p<0.01). The rotated components had significantly higher internal rotation of 12.2° compared to 4.4°, 3.7° and 3.5° for the tibial slope, varus and ideal components respectively (p<0.01).

The ideal and varus components had significantly lower wear than the tibial slope and rotated components (1.58±1.20mm³/MC and 0.15±0.83mm³/MC compared to 8.24±7.72mm³/MC and 5.19±1.12mm³/MC respectively) (p<0.01). This may be due to increased AP and TR displacements with the rotated components and the increased anterior AP displacement with the tibial slope components, resulting in wear on the posterior edge of the tibial insert.

Soft tissue constraints and component alignment had a significant effect on the kinematics and wear. Experimental simulation should test a variety of soft tissue and alignment conditions to reflect the range observed clinically and determine causes for early failure.

For any figures or tables, please contact the authors directly.

Introduction

PEEK-OPTIMA™ has been considered as an alternative to cobalt chrome in the femoral component of total knee replacements. Wear simulation studies of both the tibiofemoral and patellofemoral joints carried out to date have shown an equivalent wear rate of UHMWPE tibial and patella components against PEEK and cobalt chrome (CoCr) femoral components implanted under optimal alignment conditions. In this study, fundamental pin-on-plate studies have been carried out to investigate the wear of UHMWPE-on-PEEK under a wider range of contact pressure and cross-shear conditions.

Introduction

The number of young and more active patients requiring total knee replacement (TKR) is increasing. Preclinical evaluation and understanding the long-term failure of TKR is therefore important. Preclinical wear simulation of TKR is usually performed according to the International Standards Organization (ISO) recommendations. Two international standards for preclinical wear simulation of TKRs have been developed so that the anterior-posterior (AP) translation and internal-external (IE) rotation can be driven in either force or displacement control. However, the effects of using different control regimes on the kinematics and wear of the same TKR have not been investigated. The current study investigated the kinematics, contact mechanics and wear performance of a TKR when running under ISO force and displacement control standards using an experimentally validated computational model.

Decellularised extracellular matrix scaffolds show great promise for the regeneration of damaged musculoskeletal tissues (cartilage, ligament, meniscus), however, adequate fixation into the joint remains a challenge. Here, we assess the osseo-integration of decellularised porcine bone in a sheep model. This proof-of-concept study supports the overall objective to create composite decellularised tissue scaffolds with bony attachment sites to enable superior fixation and regeneration.

Porcine trabecular bone plugs (6mm diameter, 10mm long) were decellularised using a novel bioprocess incorporating low-concentration sodium dodecyl sulphate with protease inhibitors. Decellularised bone scaffolds (n=6) and ovine allograft controls (n=6) were implanted into the condyle of skeletally mature sheep for 4 and 12 weeks. New bone growth was visualised by oxytetracycline fluorescence and standard resin semi-quantitative histopathology.

Scaffolds were found to be fully decellularised and maintained the native microarchitecture. Following 4-week implantation in sheep, both scaffold and allograft appeared well integrated. The trabecular spaces of the scaffold were filled with a fibro-mesenchymal infiltrate, but some areas showed a marked focal lymphocytic response, associated with reduced bone deposition. A lesser lymphocytic response was observed in the allograft control. After 12-weeks the lymphocytic reaction was minimised in the scaffold and absent in allografts. The scaffold showed a higher density of new mineralized bone deposition compared to allograft. New marrow had formed in both the scaffold and allografts.

Following the demonstration of osteointegration this bioprocess can now be transferred to develop decellularised composite musculoskeletal tissue scaffolds and decellularised bone scaffolds for clinical regeneration of musculoskeletal tissues.

Experimental simulation is the gold standard wear testing method for total knee replacements (TKR), with reliable replication of physiological kinematic conditions. When combined with a computational model, such a framework is able to offer deeper insight into the biomechanical and wear mechanisms. The current study developed and validated a comprehensive combined experimental and computational framework for pre-clinical biomechanics and wear simulation of TKR.

A six-station electro-mechanical knee simulator (SimSol, UK), capable of replicating highly demanding conditions with improved input kinematic following, was used to determine the wear of Sigma fixed bearing curved TKRs (DePuy, UK) under three different activities; standard-walking, deep-squat, and stairs-ascending. The computational model was used to predict the wear under these 3 conditions. The wear calculation was based on a modification of Archard's law which accounted for the effects of contact stress, contact area, sliding distance, and cross-shear on wear. The output wear predictions from the computational model were independently validated against the experimental wear rates.

The volumetric wear rates determined experimentally under standard-walking, deep-squat, and stairs-ascending conditions were 5.8±1.4, 3.5±0.8 and 7.1±2.0 [mm3/mc] respectively (mean ± 95% CI, n=6). The corresponding predicted wear rates were 4.5, 3.7, and 5.6 [mm3/mc]. The coefficient of determination for the wear prediction of the framework was 0.94.

The wear predictions from the computational model showed good agreement with the experimental wear rates. The model did not fully predict the changes found experimentally, indicating other factors in the experimental simulation not yet incorporated in the framework, such as plastic deformation, may play an additional role experimentally in high demand activities. This also emphasises the importance of the independent experimental validation of computational models.

The combined experimental and computational framework offered deeper insight into the contact mechanics and wear from three different standard and highly demanding daily activities. Future work will adopt the developed framework to predict the effects of patients and surgical factors on the mechanics and wear of TKR.

The concept of decellularised xenografts as a basis for anterior cruciate ligament (ACL) reconstruction was introduced to overcome limitations in alternative graft sources such as substantial remodelling delaying recovery and donor site morbidity. This study aimed to measure the biomechanical properties of decellularised porcine super flexor tendon (pSFT) processed to create ACL grafts of varying diameters, with a view to facilitating production of stratified ‘off the shelf’ products with specified functional properties for use in ACL reconstructive surgery.

Decellularisation was carried out using a previously established procedure, including antibiotic washes, low concentration detergent (0.1% sodium dodecyl sulphate) washes and nuclease treatments. Decellularised pSFTs were prepared to create double-bundle grafts of 7, 8 and 9mm diameter (n=6 in each group). Femoral and tibial fixations were simulated utilising Arthrex suspension devices (Tightrope®) and interference screws in bovine bone respectively.

Dynamic stiffness and creep were measured under cyclic loading between 50–250N for 1000 cycles at 1Hz. This was followed by ramp to failure at 200mm/min from which linear stiffness and load at failure were measured. Data were analysed using either 1- or 2-way ANOVA as appropriate with Tukey post-hoc analysis (p<0.05).

Significant differences were found between all groups for dynamic stiffness and between 7 & 9mm and 8 & 9mm groups for dynamic creep. Significant differences were also found between 7, 8 & 9mm groups for linear stiffness (167.8±4.9, 186.9±16.6 & 216.3±12.4N/mm respectively), but no significant differences were found between groups for load at failure (531.5±58.9, 604.1±183.3 & 627.9±72.4N respectively).

This study demonstrated that decellularised pSFTs possess comparable biomechanical properties to other ACL graft options (autografts and allografts). Furthermore, grafts can be stratified by their diameter to provide varying biomechanical profiles depending on the anatomy and individual needs of the recipient.

We have developed a decellularised porcine superflexor tendon (pSFT), which has shown promising regenerative capacity in an ovine model of anterior cruciate ligament (ACL) repair. This study investigated the strain rate dependent and dynamic mechanical properties of native and decellularised pSFTs.

Decellularisation was carried out using a previously established procedure, including antibiotic washes, low concentration detergent (0.1% sodium dodecyl sulphate) washes and nuclease treatments.

Three different strain rates were employed: 1, 10 & 100%s-1 (n=6 for all groups). Toe-region modulus (E0), linear-region modulus (E1), transition coordinates (εT, σT), tensile strength (UTS) and failure strain were calculated. For DMA, specimens were loaded between 1 & 5MPa with increasing frequency up to 2Hz. Dynamic (E*), storage (E') and loss (E'') moduli, and tan delta were calculated for native and decellularised groups (n=6). Data was analysed by 2-way ANOVA and Tukey post-hoc test (p<0.05).

For decellularised tendons, altering the strain rate did not affect any of the static tensile properties. For native pSFTs, the UTS, failure strain and E1 were not affected by changing the strain rate. Increasing the strain rate significantly increased E0 (1% vs 10% and 1% vs 100%) and σT (1% vs 100%) and decreased εT (1% vs 10% and 1% vs 100%) for native pSFT. E*, E' and E'' were all significantly reduced in decellularised specimens compared to native controls across all frequencies investigated. No significant differences were found for tan delta.

Evidence of strain rate dependency was witnessed in the native pSFTs by increase of the toe region modulus and displacements of the transition point coordinates. This response was not seen in the tissue following decellularisation. DMA demonstrated a reduction in dynamic, storage and loss moduli. Tan delta (E''/E') remained unchanged, indicating reductions in solid and fluid components are interlinked.

A pre-clinical experimental simulation model has been previously successfully developed, and was shown to have the potential for investigation of the biomechanical and tribological performance of early stage knee therapies. In order to investigate interventions that may necessitate sacrifice of the natural ligaments, it is necessary to replicate their function. This study investigated the most effective spring constraint conditions for the porcine knee model with the aim of replicating the natural ligament function.

The replication of natural ligament function was achieved through the use of physical springs in the anterior-posterior (AP) axis. Spring-9 (9 N/mm) and spring-20 (20 N/mm) were set at different free lengths in a natural knee simulator. The A/P displacement and shear force outputs from porcine knee samples (N=6) were measured and the most appropriate spring setting was determined by comparing the outputs at different spring settings with intact knee.

The A/P displacement of both spring-9 and spring-20 showed similar shapes to the all ligament control. Spring-9 with a free length of 4 mm and spring-20 with a free length of 5 mm showed minimal differences in A/P displacement output compared to the all ligament controls. There was no statistical difference between the two minimal differences either in A/P displacement or in shear force (paired t-test, p>0.05), which indicated that both conditions were appropriate spring constraint settings for the natural porcine knee model.

A porcine knee simulation model with refined spring constraint conditions was successfully developed in this study. Human knee model is currently under investigation using the methodology developed in porcine knee model, which will be more appropriate to investigate the effect of early stage knee therapies on the tribological function of the natural knee.

Introduction

PEEK-OPTIMA™ has been considered as an alternative bearing material to cobalt chrome in the femoral component of total knee replacements. To better understand the tribology of UHMWPE-on-PEEK-OPTIMA™ and to find the most appropriate environmental conditions under which to test this novel bearing material combination, a series of tests under different protein lubricant concentrations at rig (∼24°C) and elevated temperature (∼35°C) were carried out in simple geometry wear and friction rigs. Under all conditions, the wear of UHMWPE-on-PEEK-OPTIMA™ was compared to UHMWPE-on-cobalt chrome (CoCr).

Variation in soft tissue constraints influence the kinematics and wear of total knee replacements (TKRs). The aim of this study was to experimentally investigate the effect of variation in the soft tissue constraints on the output kinematics of a fixed bearing TKR with different insert geometries. The kinematics have been shown to affect the wear rate of TKRs; increased output displacements may result in an increased wear rate. The soft tissue constraints were simulated experimentally using virtual springs.

A new generation six station electromechanical ProSim knee simulator was used with the ISO 14243–1:2009 standard force control inputs; axial force, flexion-extension (FE), tibial rotation (TR) torque and anterior-posterior (AP) force. This allowed the kinematics to vary due to the test conditions. The ISO standard spring tensions of 44N/mm and 0.36Nm/° and gaps of 2.5mm and 6° were used for the AP and TR springs respectively.

Different combinations of the input profiles were run in order to test the effect of their absence. The spring gaps were varied between 0mm–3mm and 0°–6° and the tensions between 0N/mm–250N/mm and 0Nm/°–1Nm/° for the AP and TR respectively. Three tibial insert designs were tested; high conformity curved (CVD), partially lipped (PLI) and flat.

DePuy PFC Sigma fixed bearing components were tested in 25% bovine serum (in 0.04% sodium azide) lubricant. For each test 100 cycles were recorded on each station and then averaged. The CVD insert was used for all tests, the PLI insert was also used to test the effect of spring tension.

The TR and AP output displacement profiles were affected by the FE position along with the TR torque and AP force respectively. The absence of these inputs changed the shape of the output profiles significantly. The spring gaps affected the peak AP and TR displacements (6.4mm to 3.7mm and 8° to 5.8° for maximum and zero spring gaps respectively). The spring tensions had a higher effect on the peak AP than TR position due to the design of the CVD insert restricting the TR movement (8.3mm to 3.7mm and 8.8° to 7.4° for no springs and maximum tension respectively). The spring gaps and tensions affected the amplitudes of the output profiles not their shape.

The lower conformity inserts had a higher peak TR position (23° for the flat and 8.1° for the CVD insert) which occurred earlier in the cycle. The flat insert resulted in more anterior displacement, potentially due to the high conformity on the anterior side of the CVD and PLI inserts. The spring tension test had an increased effect on the PLI than the CVD insert. The PLI insert resulted in a higher change in displacements due to the spring tensions (10.4mm to 3.5mm and 13.6° to 8.8°).

Soft tissue constraints and insert design had a significant effect on the kinematic outputs. Spring tensions and gaps for experimental testing should be chosen to reflect those of a specific patient group.

Introduction

Experimental wear simulation of an all-polymer knee implant has shown an equivalent rate of wear of UHMWPE tibial components against PEEK-OPTIMA™ and cobalt chrome femoral components of a similar initial geometry and surface topography. However, when the patella is resurfaced with an UHMWPE patella button, it is important to also ascertain the wear of the patella. Wear debris from the patella contributes to the total volume of wear debris produced by the implant which should be minimised to reduce the potential for osteolysis and subsequent implant loosening. The aim of this study was to investigate the wear of the patellofemoral joint in an all-polymer knee implant. The wear of UHMWPE patellae articulating against PEEK-OPTIMA™ femoral components was compared to UHMWPE articulating against cobalt chrome femoral components.

Introduction and Aims

There are many surgical, implant design and patient factors that should be considered in preclinical testing of hip replacement which are not being considered in current standards. The aim of this study was to develop a preclinical testing method that consider surgical positioning, implant design and patient factors and predict the occurrence and severity of edge loading under the combination of such conditions. Then, assess the safety and reliability of the implant by predicting the wear, deformation and damage of the implant bearings under worst case conditions.

INTRODUCTION

Avascular necrosis (AVN) of the femoral head (FH) initiates from biological disruptions in the bone and may progress to mechanical failure of the hip. Mechanical and structural properties of AVN bone have not been widely reported, however such understanding is important when designing therapies for AVN. Brown et al.[1] assessed mechanical properties of different regions of AVN FH bone and reported 52% reduction in yield strength and 72% reduction in elastic modulus of necrotic regions when compared to non-necrotic bone. This study aimed to characterise structural and mechanical properties of FH bone with AVN and understand the relationship between lesion volume and associated mechanical properties.

Purpose and Background

Strengthening the lumbar extensor musculature is a common recommendation for CLBP. Although reported as effective, variability in response in CLBP populations is not well investigated. This study investigated variability in responsiveness to isolated lumbar extension (ILEX) resistance training in CLBP participants by retrospective analysis of 3 RCTS.

The ability to pre-clinically evaluate new cartilage substitution therapies in viable physiological biotribological models, such as the femoral-tibial joint would be advantageous. Methods for osteochondral (OC) plug culture have been developed and the aim of this study was to extend these methods to organ culture of whole femoral condylar and tibial osteochondral tissues.

Porcine femoral condyles and tibial plateau were aseptically dissected. The majority of cancellous bone was removed leaving intact cartilage and a layer of cortical bone. OC plugs were from porcine knee condyles. “Whole joint” tissues and OC plugs were cultured in defined medium and the viability of the cartilage at day 0, 8 or 14 days of culture assessed by XTT assay and LIVE/DEAD staining. Histological analysis (H&E; alcian blue staining) was used to determine cell number and visualise glycosominoglycans (GAGs). GAG levels were quantified in the cartilage using the dimethylene blue assay.

XTT conversion by OC plug cartilage reduced significantly between day 0 and day 8 with no further change between day 8 and 14. GAG levels did not change. “Whole joint” tissue behaved similarly with reduced XTT conversion between days 0 and 8 (femoral only) and days 0 and 14 (femoral and tibial). LIVE/DEAD staining showed the majority of cells remained alive in the mid and deep cartilage zones. There was a band of mainly dead cells in the surface zone, from day 0. There was no change in the GAG levels over the 14 day culture period.

In conclusion, large cuts of femoral and tibial osteochondral tissues were maintained in organ culture for extended periods. Surface zone chondrocytes rapidly lost membrane integrity ex-vivo whereas mid- and deep zone chondrocytes remained viable. It is hypothesised that physiological loading in a novel physically interactive bioreactor will improve the viability and will be the focus of future studies.

Background

Many factors contribute to the occurrence of edge-loading conditions in hip replacement; soft tissue tension, surgical position, patient biomechanical variations and type of activities, hip design, etc. The aim of this study was to determine the effect of different levels of rotational and translational surgical positioning of hip replacement bearings on the occurrence and severity of edge-loading and the resultant wear rates.

Introduction and Aims

Clinically many factors such as variations in surgical positioning, and patients' anatomy and biomechanics can affect the occurrence and severity of edge loading which may have detrimental effect on the wear and durability of the implant. Assessing wear of hundreds of combinations of conditions would be impractical, so a preclinical testing approach was followed where the occurrence and severity of edge loading can be determined using short biomechanical tests. Then, selected conditions can be chosen under which the wear can be determined. If a wear correlation with the magnitude of dynamic separation or the severity of edge loading can be shown, then an informed decision can be made based upon the biomechanical results to only select important variables under which the tribological performance of the implant can be assessed. The aim of this study was to determine the relationship between the wear of ceramic-on-ceramic bearings and the (1) magnitude of dynamic separation, (2) the maximum force reached during edge loading and (3) the severity of edge loading resulting from component translational mismatch between the head and cup centres.

Introduction and Aims

There are many variables that can affect the occurrence and severity of edge loading in hip replacement. A translational mismatch between the centres of rotation of the head and cup may lead to dynamic separation, causing edge loading and increased wear. Combining a steep inclination angle with such translational mismatch in the medial-lateral axis caused a larger magnitude of separation and increased severity of edge loading. Previous studies have shown variation in the hip Swing Phase Load (SPL) during gait between different patients. The aim of this study was to apply a translational mismatch and determine the effect of varying the SPL on the occurrence and severity of edge loading under different cup inclination angles in a hip joint simulator.

Introduction

Total ankle replacement (TAR) is surgically complex; malalignment can arise due to surgical technique or failure to correct natural varus/valgus malalignment. Across joint replacement, malalignment has been associated with pain, component edge loading, increased wear and higher failure rates. Good component alignment is considered instrumental for long term TAR success. The conforming surface geometry of mobile bearing TARs leaves no freedom for coronal plane malalignment. The aim of this study was to investigate the biomechanical effect of coronal alignment on a mobile bearing TAR.

INTRODUCTION

Mal-positioning of the acetabular component in total hip replacement (THR) could lead to edge loading, accelerated component wear, impingement and dislocation [1,2]. In order to achieve a successful position for the acetabular component, the assessment of the acetabular orientation with reference to different coordinate systems is important [3]. The aims of the present study were to establish a pelvic coordinate system and a global body coordinate system, and to assess the acetabular orientations of natural hips with reference to the two coordinate systems.

Introduction

Variations in component position can lead to dynamic separation and edge loading conditions. In vitro methods have been developed to simulate edge loading conditions and replicate stripe wear, increased wear rate, and bimodal wear debris size distribution, as observed clinically [1, 2]. The aim of this study was to determine the effects of translational and rotational positioning on the occurrence of dynamic separation and severity of edge loading, and then investigate the wear rates under the most severe separation and edge loading conditions on an electromechanical hip joint simulator.

Introduction

The input mechanical properties of knee replacement bearing materials, such as elastic modulus and Poisson's ratio, significantly contribute to the accuracy of computational models. They should therefore be determined from independent experimental studies, under similar test conditions to the clinical and experimental conditions, to provide reliability to the models. In most cases, the reported values in the literature for the elastic modulus and Poisson's ratio of the bearing materials have been measured under tensile test conditions, in contrast to the compressive operating conditions of the total knee replacements (TKR). This study experimentally determined the elastic modulus and Poisson's ratio of conventional and moderately cross-linked ultra-high molecular weight polyethylene (UHMWPE) under compressive test conditions. These material parameters will be inputs to future computational models of TKR.

Introduction

Geometric variations of the hip joint can give rise to abnormal joint loading causing increased stress on the articular cartilage, which may ultimately lead to degenerative joint disease. In-vitro simulations of total hip replacements (THRs) have been widely reported in the literature, however, investigations exploring the tribology of two contacting cartilage surfaces, and cartilage against metal surfaces using complete hip joint models are less well reported.

The aim of this study was to develop an in-vitro simulation system for investigating and comparing the tribology of complete natural hip joints and hemiarthroplasties with THR tribology. The simulation system was used to assess natural porcine hip joints and porcine hemiarthroplasty hip joints. Mean friction factor was used as the primary outcome measure to make between-group comparisons, and comparisons with previously published tribological studies.

Introduction

The performance of total hip replacement (THR) devices can be affected by the quality of the tissues surrounding the joint or the mismatch of the component centres during hip replacement surgery. Experimental studies have shown that these factors can cause the separation of the two components during walking cycle (dynamic separation) and the contact of the femoral head with the rim of the acetabular liner (edge loading), which can lead to increased wear and shortened implant lifespan¹. There is a need for flexible pre-clinical testing tools which allow THR devices to be assessed under these adverse conditions. In this work, a novel dynamic finite element model was developed that is able to generate dynamic separation as it occurs during the gait cycle. In addition, the ability to interrogate contact mechanics and material strain under separation conditions provides a unique means of assessing the severity of edge loading. This study demonstrates these model capabilities for a range of simulated surgical translational mismatch values, for ceramic-on-polyethylene implants.

Introduction

Surface wear of polyethylene is still considered a long-term risk factor for clinical success, particularly as life expectancy and activity levels increase. Computational models have been used extensively for preclinical wear prediction and optimization of total knee replacements (TKR). In most cases, the input wear parameters (wear factors and coefficients) to the computational models have been experimentally measured under average contact stresses to simulate standard activities. These wear studies are not therefore applicable for more adverse conditions that could lead to edge loading and high stress conditions, including higher levels of activities and severe loading conditions. The current study investigated the multidirectional pin-on-plate wear performance of moderately cross-linked ultra-high molecular weight polyethylene (UHMWPE) under high applied nominal contact stress, to be used as inputs to a computational model investigating adverse high stress conditions.

Introduction

Contact between the femoral head and rim of the acetabular liner in total hip replacements has been linked to adverse tribological performance that may potentially shorten the lifespan of the prosthesis. Predicting the size and location of the contact area can be done computationally, however, experimental validation of these models is challenging due to the conforming nature of the bearing surfaces.

This study aimed to develop a method of accurately determining the in-vitro contact area between the femoral head and acetabular cup in metal-on-polyethylene and ceramic-on-polyethylene bearings under different component orientations.

Introduction

Previous studies have shown that third body damage to the femoral head in metal-on-polyethylene hip replacement bearings can lead to accelerated wear of the polyethylene liners. The resulting damage patterns observed on retrieved metal heads are typically scratches and scrapes. The damage created in vitro must represent the third body damage that occurs clinically. A computational model was developed to predict the acceleration of wear of polyethylene articulating against in vitro damaged femoral heads. This involved using a damage registry from retrieval femoral heads to develop standardized templates of femoral head scratches statistically representative of retrieval damage

The aim of this study was to determine the wear rates of polyethylene liners articulating against retrievals and artificially damaged metal heads for the purpose of validating a computational wear prediction model; and to develop and validate an in vitro standardised femoral head damage protocol for pre-clinical testing of hip replacements.

Introduction

Edge loading of hip replacements may result in plastic deformation, creep and wear at the rim of the cup and potentially fatigue failure. Variations in component positioning can lead to dynamic separation and edge loading [1]. The aim of this study was firstly to investigate the effects of translational and rotational positioning on the dynamic separation and severity of edge loading, and secondly to determine the wear rates of metal-on-polyethylene bearings under the more severe separation and edge loading conditions.

Edge loading due to dynamic separation can occur due to variations in component positioning such as a steep cup inclination angle (rotational) or mismatch between the centres of rotation of the head and the cup (translational). The aim of this study was to determine the effect of variations in rotational and translational positioning of the cup on the magnitude of dynamic separation, wear and deformation of metal-on-polyethylene bearings.

Eighteen 36mm diameter metal-on-polyethylene hip replacements were tested on an electromechanical hip simulator. Standard gait with concentric head and cup centres were applied for cups inclined at 45° (n=3) and 65° (n=3) for two million cycles. A further two tests with translational mismatch of 4mm applied between the head and cup bearing centres for cups inclined at 45° (n=6) and 65° (n=6) were run for three million cycles. Wear was determined using a microbalance and deformation by geometric analysis. Confidence intervals of 95% were calculated for mean values, and t-tests and ANOVA were used for statistical analysis (p<0.05).

Under 4mm mismatch conditions, a steeper cup inclination angle of 65° resulted in larger dynamic separation (2.1±0.5mm) compared with cups inclined at 45° (0.9±0.2mm). This resulted in larger penetration at the rim under 65° (0.28±0.04mm) compared to 45° (0.10±0.09mm) cup inclination conditions (p<0.01). Wear rates under standard concentric conditions were 12.8±3.8 mm³/million cycles and 15.4±5.0 mm³/million cycles for cups inclined at 45° and 65° respectively. Higher wear rates were observed under 4mm of translational mismatch compared with standard concentric conditions at 45° (21.5±5.5 mm³/million cycles, p<0.01) and 65° (23.0±5.7 mm³/million cycles, p<0.01) cup inclination.

Edge loading under dynamic separation conditions due to translational mismatch resulted in increased wear and deformation of the polyethylene liner. Minimising the occurrence and severity of edge loading through optimal component positioning may reduce the clinical failure rates of polyethylene.

Acellular porcine super flexor tendon (pSFT) offers a promising solution to replacement of damaged anterior cruciate ligament [1]. It is desirable to package and terminally sterilise the acellular grafts to eliminate any possible harmful pathogens. However, irradiation techniques can damage the collagen ultra-structure and consequently reduce the mechanical properties [2]. The aims of this study were to investigate the effects of irradiation sterilisation of varying dosages on the biomechanical properties of the acellular pSFT.

Tendons were decellularised using a previously established protocol [1] and subjected to irradiation sterilisation using either 30 kGy gamma, 55 kGy gamma, 34 kGy E-beam, 15 kGy gamma, 15 kGy E-beam and (15+15) kGy E-beam (fractionated dose). Specimens then underwent stress relaxation and strength testing at 0 and 12 months post sterilisation to determine whether any effect on these properties was progressive. For stress relaxation testing, specimens were analysed using a Maxwell-Wiechert model. For strength testing, the ultimate tensile strength, Young's modulus and failure strain were assessed.

Significant differences were found which demonstrated that all irradiation treatments had an effect on the time-independent and time-dependent viscoelastic properties of irradiated tendons compared to per-acetic acid only treated controls. Interestingly, no significant differences were found between the irradiated groups. Similar trends were found for the strength testing properties. No significant differences were found between groups at 0 and 12 months.

Tendons retained sufficient biomechanical properties following sterilisation, however it was notable that there were no significant differences between the irradiated groups, as it was believed higher dosages would lead to a greater reduction in the mechanical properties. The changes observed were not altered further after 12 months storage, indicating the acellular pSFT graft has a stable shelf-life.

Edge loading due to dynamic separation can occur due to variations in component positioning such as a steep cup inclination angle (rotational) or mismatch between the centres of rotation of the head and the cup (translational). The aim of this study was to determine the effect of variations in rotational and translational positioning of the cup on the magnitude of dynamic separation, wear and deformation of metal-on-polyethylene bearings.

Eighteen 36mm diameter metal-on-polyethylene hip replacements were tested on an electromechanical hip simulator. Standard gait with concentric head and cup centres were applied for cups inclined at 45° (n=3) and 65° (n=3) for two million cycles. A further two tests with translational mismatch of 4mm applied between the head and cup bearing centres for cups inclined at 45° (n=6) and 65° (n=6) were run for three million cycles. Wear was determined using a microbalance and deformation by geometric analysis. Confidence intervals of 95% were calculated for mean values, and t-tests and ANOVA were used for statistical analysis (p<0.05).

Under 4mm mismatch conditions, a steeper cup inclination angle of 65° resulted in larger dynamic separation (2.1±0.5mm) compared with cups inclined at 45° (0.9±0.2mm). This resulted in larger penetration at the rim under 65° (0.28±0.04mm) compared to 45° (0.10±0.05mm) cup inclination conditions (p<0.01). Wear rates under standard concentric conditions were 12.8±3.8 mm³/million cycles and 15.4±5.0 mm³/million cycles for cups inclined at 45° and 65° respectively. Higher wear rates were observed under 4mm of translational mismatch compared with standard concentric conditions at 45° (21.5±5.5 mm³/million cycles, p<0.01) and 65° (23.0±5.7 mm³/million cycles, p<0.01) cup inclination.

Edge loading under dynamic separation conditions due to translational mismatch resulted in increased wear and deformation of the polyethylene liner. Minimising the occurrence and severity of edge loading through optimal component positioning may reduce the clinical failure rates of polyethylene.

Wear particles produced by alumina ceramic-on-ceramic (CoC) bearings cause a minimal immunological response with low cytotoxicity and inflammatory potential^{1, 2}. However, more comprehensive immunological studies are yet to be completed for the composite CoC (zirconia-toughened, platelet reinforced alumina) hip replacements due to difficulties in isolating the very low volume of clinically relevant wear debris generated by such materials in vitro. The aim of this study was to compare the cytotoxic effects of clinically relevant cobalt chromium (CoCr) nano-particles with commercial composite ceramic particles.

Composite ceramic particles (commercial BIOLOX® delta powder) were obtained from CeramTec, Germany and clinically relevant CoCr wear particles were generated using a six station pin-on-plate wear simulator. L929 fibroblast cells were cultured with 50µm³ of CoCr wear debris or composite ceramic particles at low to high volumes ranging from 500µm³–0.5µm³ per cell and the cyctotoxic effects of the particles were assessed over a period of 6 days using the ATP-Lite™ cell viability assay.

The composite ceramic particles were bimodal in size (0.1–2µm & 30–100nm) and showed mild cytotoxic effects when compared with equivalent particle volumes (50µm³) of clinically relevant CoCr nano-particles (10–120nm). The CoCr nano-particles had significant cytotoxic effects from day 1, whereas the composite ceramic particles only showed cytotoxic effects at particle concentrations of 50 and 500µm³ after 6 days. The increased cytotoxicity of the clinically relevant CoCr nano-particles may have been attributed to the release of Co and Cr ions.

This study demonstrated the potential cytotoxic effects of model ceramic particles at very high volume concentrations, but it is unlikely that such high particle volumes will be experienced routinely in vivo in such low wearing bearing materials. Future work will investigate the longer-term effects on genotoxicity and oxidative stress of low volumes of clinically-relevant generated BIOLOX® delta ceramic wear particles.

Introduction

There is a demand for longer lasting arthroplasty implants driving the investigation of novel material combinations. PEEK has shown promise as an arthroplasty bearing material, with potentially relatively bio inert wear debris [1]. When coupled with an all-polyethylene tibial component this combination shows potential as a metal-free knee. In this study, the suitability of PEEK Optima® as an alternative to cobalt chrome for the femoral component of total knee replacements was assessed using experimental knee wear simulation under two kinematic conditions.

Introduction

To meet the demands of younger more active patients more robust pre-clinical wear testing methods are required, in order to simulate a wider range of activities. A new electromechanical simulator (Simulation Solutions, UK) with a greater range of motion, a driven abduction/adduction axis and improved input kinematic following has been developed to meet these requirements, as well as requirements of the relevant international standards. This study investigated the wear of a fixed bearing total knee replacement using this new electromechanical knee simulator, comparing with previous data from a pneumatic simulator.

Introduction

Translational surgical mismatch in the centres of rotation of the femoral head and acetabular cup in hip joint replacements can lead to dynamic microseparation resulting in edge loading contact [1]. Increased wear in retrieved ceramic-on-ceramic bearings has been associated with edge loading [2]. Hip joint simulators were used to replicate increased wear rate, stripe wear and bimodal wear debris size distribution, as seen clinically [3,4]. Recently developed electromechanical simulators are able to comply with the latest international standards, which include three axes of rotation conditions [5]. Previous simulators had applied two axes of rotation under microseparation conditions [6]. Therefore, the aim of this study was to compare the wear of ceramic-on-ceramic bearings obtained under edge loading due to microseparation conditions during gait using the same electromechanical hip joint simulator with two axes of rotation and three axes of rotation conditions.

Introduction

Total ankle replacements (TAR) are a much debated alternative to ankle fusion for treatment of end stage arthritis. Compared with hip and knee replacements these are implanted in small numbers with less than 500 per year recorded by the joint registry for England and Wales. The small numbers are a likely result of typically low mid-term survival rates, as well as extensive contra-indications for surgery. There have been multiple generations of TARs consisting of both constrained and unconstrained designs but due to device classification pre-clinical testing has been minimal.

Introduction and Aims

In order to improve the longevity and design of an implant, a wide range of pre-clinical testing conditions should be considered including variations in surgical delivery, and patients' anatomy and biomechanics. The aim of this research study was to determine the effect of the acetabular cup inclination angle with different levels of joint centre mismatch on the magnitude of dynamic microseparation, occurrence and severity of edge loading and the resultant wear rates in a hip joint simulator.

Introduction

Total ankle replacement (TAR) has been used as a surgical intervention for arthritis since the 1970s. However, unlike clinically successful hip and knee replacements, TARs are renowned for extensive contraindications to surgery and high failure rates with an average of 83% survival at 5 years. The majority cite aseptic loosening as the reason for failure. The aim of this study wais to analyse retrieved TARs visually and through interferometry to identify potential the failure mechanisms associated with these devices.

Introduction

Ceramic composites have been developed to further improve the mechanical properties, reduce risk of fracture, and increase the survivorship of ceramic-on-ceramic bearings in total hip replacement¹.

The aim of this study was to evaluate the wear of two novel ceramic composite materials under edge loading conditions due to translational mal-positioning when used in both like-on-like and mixed pairing configurations; and to compare their performance to earlier generation ceramic-on-ceramic bearings.

Introduction

When third body particles originating from bone cement or bone void fillers become trapped between articulating surfaces of joint replacements, contact surfaces may be damaged leading to accelerated wear and premature failure of the implant. In this study, the damage to cobalt chrome counterfaces by third body particles from PMMA bone cement (GMV, DePuy) and various bone void fillers was investigated; then wear tests of UHMWPE were carried out against these surfaces.

Introduction

UHMWPE articulating against PEEK-OPTIMA® has the potential for use as a novel bearing couple in joint arthroplasty due to its potentially low wear rates and the bioinertness of its wear debris. The aim of this study was to investigate the role of protein in the lubricant on the wear of UHMWPE articulating against PEEK at both room and physiological temperature.

Introduction

Hip replacements are falling short of matching the life expectancy of coxarthritis patients, due to implanting THR in younger patients and due to increasingly active patients. The most frequently implanted hip prostheses use cross linked (XL) polyethylene (PE) on metal bearings in the USA and most of the Western world. Concerns remain in the long term around the potential of wear debris-induced aseptic loosening. Thus exploring lower-wearing alternative bearings remains a major research goal.

PEEK (poly-ether-ether-ketone) is a thermoplastic polymer with enhanced mechanical properties. This study compared the wear of PEEK to the wear of cross linked polyethylene, when sliding against cobalt chrome (CoCr) metallic counterfaces, and compared the wear of carbon-fibre reinforced (CFR)-PEEK to cross linked polyethylene when sliding against metallic and ceramic counterfaces under different contact stresses within the hip joint.

Introduction

Increased wear rates [1, 2] and acetabular rim fracture [3] of hip replacement bearings reported clinically have been associated with edge loading, which could occur due to rotational and/or translational mal-positioning [4]. Surgical mal-positioning can lead to dynamic microseparation mechanisms resulting in edge loading conditions. In vitro microseparation conditions have replicated stripe wear and the bi-modal wear debris distribution observed clinically [5, 6]. The aim of this study was to investigate the effect of steep cup inclination, representing rotational mal-positioning, on the magnitude of dynamic microseparation, severity of edge loading, and the resulting wear rate of a ceramic-on-ceramic bearing, under surgical translational mal-positioning conditions.

Introduction

Wear debris induced osteolysis and loosening continue to cause clinical failure in total knee replacement (TKR). To improve longevity and reduce wear alternative materials have been examined. Carbon-fibre-reinforced poly–ether-ether-ketone (CFR-PEEK) has shown promising results in wear studies [1–2].

The aim of this study was to explore the use of CFR-PEEK and PEEK as alternative bearing materials for polyethylene in TKR through experimental knee joint wear simulation.

Summary Statement

Wear of total knee replacement (TKR) is a clinical concern. This study demonstrated low-conformity moderately cross-linked-polyethylene fixed bearing TKRs showed lower volumetric wear than conventional-polyethylene curved fixed bearing TKRs highlighting potential improvement in TKR performance through design and material selection.

Summary Statement

The frictional torque of ceramic-on-ceramic bearings tended to increase with increasing the bearings size (32, 48, 56mm). However, the frictional torque was significantly lower than that measured on metal-on-metal bearings under well positioned and well lubricated conditions.

Introduction:

Backside wear has been previously reported through in-vitro and in-vivo to have a significant contribution to the total wear in rotating bearing TKRs.

The present study investigated the contribution of backside wear to the total wear in the PFC Sigma rotating platform mobile bearing TKR. In addition, the wear results were compared to the computed wear rates of the PFC Sigma fixed bearing TKR, with two different bearing materials.

Introduction

Stripe wear, observed on retrieved ceramic hip replacements, has only been replicated in vitro under translational mal-positioning conditions where the centres of rotation of the head and the cup are mismatched^1,2; an in vitro condition termed “microseparation”.

The aim of this study was to compare the edge loading mechanisms observed under microseparation conditions due to translational mal-positioning conditions simulated on two different hip joint simulators.

Introduction

Wear debris induced osteolysis and loosening continue to be causes of clinical failure in total knee replacement (TKR). Laboratory simulation aims to predict the wear of TKR bearings under specific loading and motion conditions. However, the conditions applied may have significant influence on the study outcomes (1)

The aim of this study was to examine the influence of femoral setup and kinematic inputs on the wear of a conventional polyethylene fixed bearing TKR through experimental and computational models.

Introduction

Patella femoral joint bearings in total knee replacements have shown low wear (3.1 mm³/MC) under standard gait simulator conditions¹. However, the wear in retrieval studies have shown large variations between 1.3 to 45.2 mm³/year². Previous in vitro studies on the tibial femoral joint have shown wear is dependent on design, materials and kinematics³.

The aim of this study was to investigate the influence of the design (geometry) and shape on the wear rate of patella femoral joints in total knee replacements.

Introduction

Wear of polyethylene continues to be a significant factor in the longevity of total knee replacement (TKR). Moderately cross-linked polyethylene has been employed to reduce the wear of knee prostheses, and more recently anti-oxidants have been introduced to improve the long-term stability of the polyethylene material. This is the initial study of the wear of a new anti-oxidant polyethylene and a new TKR design, which has modified femoral condylar geometry.

INTRODUCTION

Ceramic-on-metal hip replacements (COM, where the head is a Biolox Delta ceramic and liner is Co Cr alloy), have demonstrated reduced wear under standard conditions in vitro compared to metal-on-metal (MOM) [1]. Early clinical results are also encouraging [2]. Recently concerns have been raised regarding the poor clinical performance of MOM hip resurfacings [3], particularly when cups are steeply inclined. Laboratory hip simulator testing has been used to replicate edge loading, also demonstrating elevated wear [4]. Therefore, a range of conditions to replicate sub-optimal use clinically to better predict in vivo performance should be used. The aim of this study was to compare the wear rates of MOM and COM under adverse edge loading conditions in an in vitro hip simulator test.

Introduction

Concerns regarding UHMWPE wear particle induced osteolysis in total hip replacement (THR, [1]) have led to alternative materials to be sought. Carbon-fibre reinforced poly-ether-ether-ketone (CFR-PEEK) has shown reduced wear in hip and knee configurations compared with conventional polyethylene [2-4]. The aim of this study was to investigate the wear performance of a ceramic-on-CFR PEEK THR through a simulator study.

INTRODUCTION

Ceramic-on-ceramic hip replacements have generated great interest in recent years due to substantial improvements in manufacturing techniques and material properties¹. Microseparation conditions that could occur due to several clinical factors such as head offset deficiency, medialised cup combined with laxity of soft tissue resulting in a translation malalignment, have been shown to cause edge loading, replicate clinically relevant wear mechanisms^2,3 and increase the wear of ceramic-on-ceramic bearings^3,4. The aim of this study was to investigate the influence of increasing the femoral head size on the wear of ceramic-on-ceramic bearings under several clinically relevant simulator conditions.

Introduction

Hip wear simulation is a widely used technique for the pre-clinical evaluation of new bearing designs. However, wear rates seen in vitro can often be significantly different to those seen clinically. This can be attributed to the difference between the optimal conditions in a simulator and wide ranging conditions in real patients.

This study aimed to develop more clinically relevant simulator tests, looking specifically at the effects of cup inclination angle (in vivo) and stop-dwell-start (SDS) protocols on a clinically available product.

INTRODUCTION

Retrieval and clinical studies of metal-on-metal (MoM) bearings have associated increased wear¹ and elevated patient ion levels² with steep cup inclination angles and edge loading conditions. The University of Leeds have previously developed a hip simulator method that has been validated against retrievals and shown to replicate clinically relevant wear rates and wear mechanisms^3,4. This method involves introducing lateral microseparation to represent adverse joint laxity and offset deficiency. This study aimed to investigate the effect of microseparation representing translational malpostion, and increased cup inclination angle, representing rotational malposition, in isolation and combined on the wear of different sizes (28 and 36mm) MoM bearing in total hip replacement (THRs).

In vitro the introduction of microseparation and edge loading to hip simulator gait cycle has replicated clinically relevant wear rates and wear mechanisms in ceramic-on-ceramic bearings^[1], and elevated the wear rates of MoM surface replacements (SR) to levels similar to those observed in retrievals^[2]. The aim was to assess the wear of two different sized MoM total hip replacement bearings under steep cup inclination angles and adverse microseparation and edge loading conditions.

Two tests were performed on the Leeds II hip joint simulator using two different size bearings (28mm and 36mm). Cups were mounted to provide inclination angles of 45 degrees (n=3) and 65 degrees (n=3). The first three million cycles were under standard gait conditions. Microseparation and edge loading conditions as described by Nevelos et al^[1] were introduced to the gait cycle for the subsequent three million cycles. The lubricant was 25% new born calf serum. The mean wear rates and 95% confidence limits were determined and statistical analysis was performed using One Way ANOVA.

Under standard gait conditions, when the cup inclination angle increased from 45 degrees to 65 degrees, the wear of size 28mm bearing significantly (p=0.004) increased by 2.7-fold, however, the larger bearings did not show any increase in wear (p=0.9). The introduction of microseparation conditions resulted in a significant (p=0.0001) increase in wear rates for both bearing sizes under both cup inclination angle conditions. Under microseparation conditions, the increase in cup inclination angle had no influence on the wear rate for both bearing sizes (Figure 1).

With larger bearings, head-rim contact occurs at a steeper cup inclination angle providing an advantage over smaller bearings. The introduction of edge loading and microseparation conditions resulted in a significant increase in wear rates for both bearing sizes. The wear rates obtained in this study under combined increased cup inclination angle and microseparation were half of those obtained when SR MoM bearings were tested under similar adverse conditions^[2]. This study shows the importance of prosthesis design and accurate surgical positioning of the head and acetabular cup in MoM THRs.

Tribology and wear of articular cartilage is associated with the mechanical properties, which are governed by the extracellular matrix (ECM). The ECM adapts to resist the loads and motions applied to the tissue. Most investigations take cartilage samples from quadrupeds, where the loading and motions are different to human. However, very few studies have investigated the differences between human and animal femoral head geometry and the mechanical properties of cartilage.

This study assessed the differences between human, porcine, ovine and bovine cartilage from the femoral head; in terms of anatomical geometry, thickness, equilibrium elastic modulus and permeability.

Diameter of porcine (3-6 months old), bovine (18-24 months old), ovine (4 years old) and human femoral heads were measured (n=6). Plugs taken out of the superior region of each femoral head and creep indentation was performed. The human femoral heads were obtained from surgery due to femoral neck fracture. Cartilage thickness was measured by monitoring the resistive force change as a needle traversed the cartilage and bone at a constant feed rate using a mechanical testing machine. The percentage deformation over time was determined by dividing deformation by thickness. A biphasic finite element model was used to obtain the intrinsic material properties of each plug. Data is presented as the mean ± 95% confidence limits. One-way ANOVA was used to test for significant differences (p < or = 0.05).

Significant differences in average femoral head diameter were observed between all animals, where bovine showed the largest femoral head. Human cartilage was found to be significantly thicker than cartilage from all quadrupedal hips. Human cartilage had a significantly larger equilibrium elastic modulus compared to porcine and bovine cartilage. Porcine articular cartilage was measured to be the most permeable which was significantly larger than all the other species. No significant difference in permeability was observed between human and the other two animals: bovine and ovine (Table 1).

The current study has shown that articular cartilage mechanical properties, thickness and geometry of the femoral heads differ significantly between different species. Therefore, it is necessary to consider these variations when choosing animal tissue to represent human.

Spinal total disc replacement (TDR) designs rely heavily on total hip replacement (THR) technology and it is therefore prudent to check that typical TDR devices have acceptable friction and torque behaviour. For spherical devices friction factor (f) is used in place of friction coefficient (mju). The range of loading for the lumbar spinal discs is estimated at perhaps 3 times body weight (BW) for normal activity rising to up to 6 times BW for strenuous activity^[1]. For walking this equates to around 2000 N, which is the maximum load required by the ISO standard for TDR wear testing^[2].

Three Prodisc-L TDR devices (Synthes Spine) were tested in a single station friction simulator. Bovine serum diluted to 25% was used as a lubricating medium. Flexion-extension was ±5 deg for all experiments with constant axial loading of 500, 2000 and 3000 N. The cycle run length was limited to 100 and the f and torque (T) values recorded around the maximum velocity of the cycle point and averaged over multiple cycles.

Preliminary results shows that the 500 N loading produced the largest f of 0.05 ± 0.004. The 2000 N load, which approximates daily activity, gave f = 0.036 ± 0.05 and the 3000 N load gave f = 0.013 ± 0.003. The trend was for lower f with increasing loads.

A lumbar TDR friction factor of 0.036 for a 2000N load and the reduction in f for increasing loads is comparable to the lower end of the range of values reported for THR in similar simulator studies using metal-on-polyethylene bearing materials^[3]. The 3000 N result showing that increasing the load above that expected in daily activity does not raise the f could be important when considering rotational stability and anchorage in a TDR device because frictional torque at the bearing surfaces is proportional to the product of load, device radius and f.

Background

High cup abduction angles generate increased contact stresses, higher wear rates and increased revision rates. However, there is no reported study about the influence of cup abduction on stresses under head lateralisation conditions for ceramic-on-Ceramic THA.

INTRODUCTION

Squeaking after total hip replacement has been reported in up to 10% of patients. Some authors proposed that sound emissions from squeaking hips result from resonance of one or other or both of the metal parts and not the bearing surfaces. There is no reported in vitro study about the squeaking frequencies under lubricated regime. The goal of the study was to reproduce the squeaking in vitro under lubricated conditions, and to compare the in vitro frequencies to in vivo frequencies determined in a group of squeaking patients. The frequencies may help determining the responsible part of the noise.

Introduction

Articular hyaline cartilage has a unique structural composition that allows it to endure high load, distribute load to bone and enables low friction movement in joints. A novel acellular xenogenic graft is proposed as a biological cartilage replacement, for repair of osteochondral defects. Acellular porcine cartilage has been produced using repeated freeze thaw cycles and washing using hypotonic buffers and sodium dodecyl sulphate solution (SDS; Keir, 2008). DNA content of the acellular matrix was reduced by 93.3% compared to native cartilage as measured by nanodrop spectrophotometry of extracted DNA, with a corresponding reduction in glycosaminoglycan (GAG) content.

Introduction

It is believed that wear of replacement joints vivo in is strongly dependent on input motions (kinematics) and loading. There is difficulty in accurately measuring total disc replacement (TDR) kinematics in vivo. It is therefore desirable to ascertain the sensitivity of implant wear in vitro to perturbations of the standard testing parameters. An anterior-posterior (AP) shear force input is not currently included in the present ISO and ASTM testing standards for lumbar TDRs but is known to exist in in vivo. Other joint-replacement wear tests have shown that the phasing of input motions influences the ‘cross-shear’ process of polyethylene wear. Polyethylene bearing materials do not behave linearly to axial loading changes and so the effect on wear rate is difficult to predict. The study aim was to assess the effects on wear of a ProDisc-L TDR under the following conditions: ISO 18191-1 standard inputs; an additional input AP shear; input kinematics phasing changes; axial loading changes.

Introduction

The biological response to UHMWPE particles generated by total joint replacements is one of the key causes of osteolysis, which leads to late failure of implants. Particles ranging from 0.1-1.0μm have been shown to be the most biologically active, in terms of osteolytic cytokine release from macrophages [1]. Current designs of lumbar total disc replacements (TDR) contain UHMWPE as a bearing surface and the first reports of osteolysis around TDR in vivo have appeared recently in the literature [2]. The current wear testing standard (ISO18192-1) for TDR specifies only four degrees of freedom (4DOF), i.e. axial load, flexion-extension, lateral bend and axial rotation. However, Callaghan et al. [3] described a fifth DOF, anterior-posterior (AP) shear. The aim of this study was to investigate the effect that this additional AP shear load component had on the size and morphology of the wear particles generated by ProDisc-L TDR devices over five million cycles in a spine simulator.

The search for the ideal bearing surface in Total Hip Replacements continues. The current ‘best’ materials are felt to be combinations of metal, ceramics and cross-linked polyethylene. Laboratory studies suggest that ceramic-on-metal articulations may provide distinct advantages. This study aims to identify the best bearing surface combination with the lowest adverse side effect profile.

Between February 2004 and September 2007, 164 hips were replaced in 142 patients. 39% were male and 69% were female. The average age at surgery was 53 years (17-72 years). Follow-up assessment included radiographs, the Harris Hip Score and whole blood samples for metal ion levels. Complications to date included 3 hips which needed femoral revision because of surgery related factors, and 3 cases of sepsis of which 1 settled and 2 needed revision. One hip needed revision of head and liner to a larger bearing size for recurrent dislocations, and is no longer being followed up for blood metal ions.

Post-operative whole blood metal ion levels were compared to pre-operative levels to determine the increase or decrease in metal ion levels. There were no changes in those patients with ceramic-on-ceramic and ceramic-on-polyethylene articulations. Moderately raised whole blood metal ion levels were noted at 3 months in the ceramic-on-metal group, while the metal-on-metal group show the greatest increase.

This study agrees with laboratory bearing surface wear studies demonstrating lower wear rates in the ceramic-on-metal group compared to the metal-on-metal group. With concerns related to high blood metal ion levels in metal-on-metal articulations, ceramic-on-metal bearing surfaces may well become a bearing surface of choice in the future, but progress needs to be monitored in the longer term.

Purpose of the study: Implantation of the acetabular socket with high inclination generates increased contract stress, wear and revision rate for total hip arthroplasty (THA). Study of ceramic-on-ceramic THA explants has revealed a high wear rate in bands, suggesting a microseparation effect generating edge loading. There have not been any studies examining the influence of the cup inclination on the contact pressures in ceramic-on-ceramic THA exposed to microseparation between the head and the cup.

Material and methods: A finite elements model of a ceramic-on-ceramic hip prosthesis was developed with ABAQUS in order to predict the surface contact and the distribution of the contract pressures, first during ideal centred function then under conditions of microseparation. A 32mm head and a radial clearance head (30μm) were used. The cup was positioned in zero anteversion and 45, 65, 70, and 90° anteversion. Progressive microseparation (0 to 500 μm) was imposed. A 2500N loading force was applied to the centre of the head.

Results: For 45° inclination, edge loading appeared for mediolateral separation greater than 30 μm and became complete for 60 μm separation. When edge loading appeared, the contact surface was elliptic. The length of the lesser axis converged towards 0.96mm; the greater axis towards 8.15mm, respectively in the anteroposterior and mediolateral directions. For inclinations of 45°, the contact pressure was 66 Mpa for the centred force. As the mediolateral separation increased, the maximal contact pressure increased, converging towards an asymptotic value of 205 MPa. Increasing the inclination angle of the cup generated an increase in the maximal contact pressure. However, this increase in contact pressure generated by the increasing inclination angle was negligible if the microseparation increased.

Discussion: Cup inclination and mediolateral laxity increase stress forces of ceramic-on-ceramic THA and should be avoided. However, the influence of the cup inclination becomes negligible beyond a separation value of 240 μm, the stress forces already having reached their asymptotic value.

Purpose of the study: Ceramic-on-ceramic THA explants exhibit a higher wear rate than that predicted by classical simulators. This appears to be related to edge loading, which could perhaps be reproducible in vitro by creating a microseparation between the two components. The purpose of this study was to evaluate this coefficient of friction for ceramic-on-ceramic THA with edge loading. This should enable prediction of wear in the event of microseparation.

Material and methods: Three 32mm alumina inserts (Biolox Forte Ceramtec^®) were tested on a friction simulatior (Prosim^®). The cup was positioned vertically (75° inclination) to reproduce edge loading. The metal-back and the acetabular insert were sectioned to avoid impingement between the neck and cup. Contact was imposed along the border of the cup, then perpendicularly to it. The tests were performed under lubrication conditions (25% bovine serum). In order to simulate severe contact pressures, the tests were also conducted with a third body inserted between the head and the edge of the cup. To obtain reference values of the centred regimen, tests were first run with identical components positioned horizontally.

Results: Edge loading was achieved for cups inclined at 75°. The coefficient of friction was 0.02±0.001 under centred conditions. For edge loading conditions, the coefficient of friction was significantly increased, to a mean 0.09±0.00 for movement along the acetabular border and 0.034±0.001 for movement perpendicular to the border. Squeaking occurred for 15 s when the third body was introduced, corresponding to a coefficient of friction 15-fold higher (0.32±0.003) than under ideal conditions.

Discussion: For the first time, the coefficient of friction of edge loading was determined under conditions of lubrication. The friction coefficient of ceramic-on-ceramic THA was greater for a very vertical cup, but remained (0.1) equivalent to the metal-on-metal coefficient under optimal conditions. When a third body was introduced, transient squeaking occurred with a very high coefficient of friction.

Conclusion: Implantation of cups with a high abduction angle induces edge loading and an increased coefficient of friction, and should be avoided.

Dislocation remains one of the most common complications after total hip arthroplasty.

Precise cup position appears to be a main factor as significant variations occur for frontal and sagittal acetabular tilt and anteversion according to sitting or standing positions.

An innovative dual mobility ceramic-on-ceramic joint has been developed to solve these problems.

The dual mobility ceramic-on-ceramic joint allows to move the rotation center much deeper inside the insert in order to increase the joint stability without negative impact on the ROM. This device revealed higher torques against subluxation in comparison to the classical Al-Al systems, even with 36mm head diameters, or 41 mm metal on metal bearings.

The additional outer-bearing surface motion creates a second “adjustable acetabulum” due to the eccentration between the rotation center of the ball head and the rotation center of the bipolar head. This offset creates a resultant force that rotates the bipolar component.

Using two bearing ceramic surfaces, the intermediate component acts as a “self adjusting cup”, dealing with the variations of pelvic orientation and acetabulum anteversion.

The use of the dual mobility ceramic-on-ceramic joint seems an interesting alternative when facing difficult or unexpected situations for cup adjustment and cases with hip instability In a hip simulator in micro separation condition, the wear of the dual mobility ceramic-on-ceramic was less than 0.01 mm3/million cycles, the detection limit for wear measurement. There was no change in the surface roughness of the inserts.

The design of the joint with the mobile ceramic head prevented edge loading of the head on the edge of the cup. No stripe wear was observed.

Since 2006 more than 2000 dual mobility ceramic-on-ceramic systems have been implanted in Europe and clinical studies are conducted. The aim is to demonstrate the resistance to dislocation in primary total hip arthroplasty. Previous results over 125 patients in a prospective multicentric study show a Harris and Womac score equivalent to a standard hip prosthesis. No dislocations have been reported. No ceramic breakage or “squeaking” phenomenon appears.

Dislocation and microseparation are major causes of failure for ceramic-ceramic hip prosthesis. When no ideal solution has been found for acetabular implantation, the dual mobility ceramic-on-ceramic device is a real alternative. The exclusive design of the bipolar head give the high resistance to wear and stripe wear to the dual mobility ceramic-on-ceramic joint. Reducing the risk of dislocation and reducing wear drastically are two advantages that can place the dual mobility ceramic-on-ceramic joint as the best choice in primary Total Hip Arthroplasty. Obviously this choice applies to recurrent dislocation also.

Bioengineering reasons for increased wear and failure of metal-on-metal (MoM) bearings in hip prostheses have been described. Low wear occurs in MoM hips when the centre of the femoral head is concentric with the centre of the acetabular component and the implants are correctly positioned. Translational or rotational malpositioning of the components can lead to the contact-patch of the femoral component being displaced to the rim of the acetabular component, resulting in a ten- to 100-fold increase in wear and metal ion levels. This may cause adverse tissue reactions, loosening of components and failure of the prosthesis.

Ceramic-on-metal (ceramic head and metal liner, COM) hip replacements have shown reduced wear in comparison to metal-on-metal (MOM) bearings. This has been attributed to reduced corrosive and adhesive wear, and differential hardness. The study assessed the performance of ceramic and metal bearings in different configurations under adverse conditions, ceramic heads on metal liners (COM) were compared to metal heads on ceramic inserts (MOC), with head on cup rim loading under micro-separation hip joint simulation.

Components used were made of zirconia-platelet toughened alumina (Biolox Delta) and CoCrMo alloy. Hip simulator testing applied a twin-peak loading cycle and walking motions with the prosthesis in the anatomical position. Testing was conducted in calf-serum for 2-million cycles. A standard simulator cycle was adapted, the head sub-luxed in the swing-phase forcing the head onto the cup rim at heel strike.

The overall mean wear rate for the MOC bearings (0.71±0.30mm3/Mc) was significantly higher than the wear rate for the COM bearings (0.09±0.025mm3/Mc). The contact of the head against the rim of the cup caused deep stripe wear on the metallic heads of the MOC bearings. This region on the head is exposed to high stress conditions and susceptible to damage in edge contact, the effect of this is increased when the cup is a harder material than the head. The wear of a metal-on-metal (MOM) couple under similar conditions was almost two-fold greater than the MOC couple (1.58mm3/Mc, Williams et al., 2006) providing further evidence of the reduced wear with COM in comparison to MOM.

The COM concept allows thin metal shells to be used with larger ceramic heads and protects against ceramic liner chipping. COM bearings are undergoing clinical trials, early data suggests reduced metal ion release in patients compared to metal-on-metal.

Introduction: UHMWPE wear particles induce osteolysis and loosening of total joint replacements. Much effort has been directed at reducing the wear volume of UHMWPE, such as crosslinking treatments [1]. Recently, interest in UHMWPE with vitamin E (VE) has increased due to its improved wear resistance in knee prostheses [2], as well as improved mechanical properties. The aim of this study was to culture human peripheral blood mononuclear cells (PBMNCs) with known volumes of clinically relevant wear debris from UHMWPE with and without VE in order to quantify and compare their respective biological activities.

Methods: For UHMWPE with VE, GUR1050 UHMWPE powder was mixed with VE at 0.3% (w/w) and 3% (w/w) using a screw cone mixer. The wear rates were evaluated using a six-station multidirectional pin on plate wear simulator against a smooth CoCr plate (Ra 0.01–0.03 micrometres), in 25% bovine serum, under a load of 160N and a frequency of 1 Hz. Endotoxin-free clinically relevant wear debris was generated aseptically for cell culture studies, using a single-station multidirectional pin on plate wear rig housed in a class II safety cabinet. PBMNCs were isolated from blood collected from three healthy donors then cultured with debris at particle volume (μm3) to cell number ratios of 100:1 using the agarose gel technique [3]. Cells without particles were used as the negative control, and LPS at 200 ng/ml was the positive control. Cell viability was assessed by ATP-Lite assay, and TNF-alpha, interleukin (IL)-1beta, IL-6 and IL-8 were measured by ELISA at 12 and 24 h.

Results: The 3% VE UHMWPE was found to have a higher wear rate than both the Virgin and the 0.3% VE UHMWPE, although there were no significant differences. Particle size and volume distributions were similar for all materials, with the mode of the frequency distributions being in the 0.1–1 micron size range. Cell viability was not adversely affected by any of the treatments. Cells cultured with virgin UHMWPE debris secreted significantly higher quantities (P< 0.05) of TNF-alpha compared to debris from both the 0.3% and the 3% VE UHMWPE, which released comparable levels of TNF-alpha to the cell only control group. The results for the other cytokines, IL-1beta, IL-6 and IL-8, and for the two additional donors showed similar trends as the results for TNF-alpha.

Discussion: The biological response to wear particles is strongly influenced by particle size and volume [3]. Cells cultured with wear debris of UHMWPE containing VE released very low levels of cytokines in comparison with virgin UHMWPE, even there were no significant differences in particle size. Differences in the chemical composition of the particles or different rates of protein adsorption may explain these differences. VE has anti-inflammatory properties, which may act by free radical scavenging. VE has been shown to reduce production of reactive oxygen species and pro-inflammatory cytokines such as TNF-alpha and IL-1beta from monocytes [4]. The anti-inflammatory effects of UHMWPE particles containing VE are currently being investigated.

Ceramic-on-ceramic total hip replacements (THRs) have shown low wear volumes in standard gait hip simulator studies1. However clinical reports have indicated a variation in wear rates and formation of stripe wear on the ceramic femoral heads2. The aim of this study was to investigate the influence of different clinical conditions such as cup inclination angle and microseparation (head offset deficiency) on the wear of ceramic-on-ceramic THRs. The six station Leeds II hip joint simulator was used to investigate the wear of size 28mm ceramic-on-ceramic bearing couples. The alumina matrix composite ceramic material (AMC, Biolox Delta, CeramTec AG, Germany) was used in this study. The lubricant used was 25% bovine serum. The study was carried out for a total of five million cycles; the first two million cycles under standard gait conditions and a further three million cycles under microseparation conditions. During microseparation, a lateral movement of 0.5mm was applied to the cup relative to the head during the swing phase of the gait cycle3. Three of the cups were mounted to provide a clinical angle of 55°, which is referred to as the ‘standard’ condition; and the other three cups were mounted to provide a clinical angle of 65°, which is referred to as the ‘steep angle’ condition. These combinations provided four different testing conditions: standard, steep cup angle, microseparation, and combination of steep cup angle and microseparation conditions. Volumetric wear was determined gravimetrically and statistical analysis was performed using One Way ANOVA (significance at p< 0.05). Increasing the cup inclination angle from 55° to 65° had no significant effect on the wear rate in Biolox Delta ceramic-on-ceramic THRs under both standard (p> 0.42) and microseparation (p> 0.55) conditions. Under standard gait conditions, the mean wear rate for both cup inclination angles was very low at 0.05 mm3/million cycles. The introduction of microseparation to the standard gait cycle significantly increased the mean wear rates (p< 0.01) to 0.13 mm3/ million cycles for the ‘standard’ cup inclination angle of 55° and 0.11 mm3/million cycles for the ‘steep’ cup inclination angle of 65°. A stripe of wear on the head also formed, with corresponding superior rim wear on the cup. For comparison, the steady state wear rate of HIPed third generation alumina ceramic (Biolox Forte) under microseparation conditions was 1.3 mm3/million cycles [4]. In conclusion, increasing the cup inclination angle by 10° had no influence on the wear rate of Biolox Delta ceramic-on-ceramic bearings. The introduction of microseparation conditions significantly increased the wear rate and resulted in stripe-like wear on the femoral head, which has previously been observed on retrieved ceramic prosthesis. However, these wear rates were still low, and were ten times lower than those previously reported for Biolox Forte.

Introduction: The aim of this study was to develop a technique to decellularise a porcine cartilagebone construct with a view to using this as a biological scaffold for transplantation into human osteochondral defect as a cartilage substitute.

Methods: Decellularisation was based on a modification of the technique of Booth et al (2002). Cartilage bone matrix (n=9) were decellularised by exposing the tissue to 2 cycles of dry freeze-thaw followed 2 more cycles with the addition of hypotonic (10mM tris-HCl, pH8.0) buffer. Samples were then cycled through hypotonic buffer, followed by ionic detergent (0.1% [w/v] sodium dodecyl sulphate [SDS]) in the presence of protease inhibitors (aprotinin 10 KIU/ml) and 0.1% (w/v) ethylene diamine tetraacetic acid (EDTA). This was followed by washes in PBS with aprotinin and incubation in nuclease solution containing DNase (50U/ml) and RNase (1U/ml). Decontamination using 0.1% (v/v) peracetic acid in PBS was then incorporated to achieve disinfection of the tissue samples. Finally, samples were washed in PBS. Three decellularisation protocols were used depending on the number of hypotonic/SDS cycles: this was either done once, three or six times referred to as DC1, DC3 and DC6 respectively. Fresh & decellularised cartilage were compared histologically using haematoxylin and eosin staining, to visualize cellular content, sirius red, to visualise collagen fibres & alcian blue, to visualise glycosaminoglycans (GAG). Immunohistochemistry staining for galactose-α-1,3-galactose (α-gal), collagen I, II & VI was performed for fresh and decellularised samples. DNA assay: Genomic DNA was extracted using a DNA isolation kit for tissues (Roche Applied Sciences). Collagen and DMB sulphated sugar assay, as described by Stapleton et al. (2008), were performed to measure collagen and GAG content. The biphasic property of fresh and decellularised cartilage was determined using a pin on plate indentation test.

Results: H& E staining revealed the absence of visible whole cells. Sirius red stain gave evidence of the retention of collagen following decellularisation. In contrast, the acellular matrix showed evidence of loss of GAGs. There was no evidence of the expression of α-gal in the acellular scaffold. DNA analysis revealed the absence of genomic DNA in comparison to fresh tissues (ANOVA, p< 0.05). The decellularisation process had minimal effect on the collagen content of the cartilage. Nevertheless there was a significant difference in the sulphated sugar content of the fresh tissue when compared to the decellularised tissue (ANOVA, p< 0.05), indicating loss of 92% GAG. Biomechanical testing of decellularised tissues showed a significant change (ANOVA, p< 0.05) in comparison to the fresh cartilage.

Discussion: In conclusion this study has generated data on the production of an acellular cartilage bone matrix scaffold for use in osteochondral defect repair. To our knowledge, this is the first study that has successfully removed whole cells and α-gal from xenogeneic cartilage and bone tissue. Future studies are required to investigate methods to recellularise the acellular matrix using an appropriate cell type and mechanical conditioning and to investigate replenishing GAG loss following decellularisation.

Introduction: Nanometre sized UHMWPE particles have recently been isolated from periprosthetic tissues and hip simulator lubricants [1,2]. The biological response to UHMWPE particles of 0.1 μm and above has been well characterised, with particles in the 0.1–1.0 μm size range having the highest biological activity [3]. The purpose of the study was to determine the biological activity of nanometre-sized particles in terms of osteolytic cytokine release from primary human monocytes.

Methods: Monocytes were isolated from peripheral blood from 5 healthy donors by density gradient centrifugation over Lymphoprep. Cells were cultured using the agarose gel technique [3] at particle volume (μm3):cell number ratios of 10:1 and 100:1. The particles used were:

1 Polystyrene FITC-conjugated FluoSpheres (FS; Invitrogen) in 20 nm, 40 nm, 0.2 μm and 1.0 μm sizes.

All particles were tested for the presence of endotoxin prior to culture with cells. Cells without particles were used as a negative control and 200 ng/ml LPS was used as a positive control. Cell viability was assessed using the ATP Lite assay (Perkin Elmer) and ELISA was used to determine TNF-alpha, IL-1beta, IL-6 and IL-8 release at 3, 6, 12 and 24 h.

Results: FluoSpheres and CD had no effect on cell viability at 10 or 100:1. Clinically relevant UHMWPE particles had no effect on cell viability at 10:1, however, at 100:1 significant differences (P< 0.05) were seen at 3, 12 and 24 h for Donors 1 and 3. The 40 nm, 0.2μm and 1.0 μm FS caused significant elevation of TNF-α release at the 12 and 24 h time points at 100:1. There was no significant increase in TNF-α release for the 20 nm FS (3/5 donors). Particle volume and particle size showed correlation with cellular response, with the 20 nm FS showing the lowest biological activity. Clinically relevant UHMWPE particles and nanometre sized CD produced significantly higher quantities of TNF-alpha at 100:1. Release of interleukins IL-1beta, IL-6 and IL-8 followed a similar trend to TNF-alpha release.

Discussion: This study found that all nanometre-sized particles had the potential to provoke inflammatory cytokine release from macrophages. Particle volume and particle size played critical roles in initiating cellular responses. There was a lower particle size limit, with the 20 nm FS showing the lowest activity. Nanometre-sized polyethylene particles (CD) caused elevated TNF-α release, and since it has been shown that nanometre-sized UHMWPE particles are produced in large numbers in vivo [2], the relative contribution of these particles to osteolysis should be considered. The biological response to nanometre-sized clinically relevant UHMWPE particles is currently under investigation.

Ceramic-on-metal (COM) bearings have shown reduced wear and friction compared with metal-on-metal (MOM) bearings in-vitro. Lower wear has been attributed to a reduction in corrosive wear, smoother surfaces, improved lubrication and differential hardness reducing adhesive wear. Clinical studies have also shown reduced metal ion levels in-vivo compared with MOM bearings. The aim of this study was to examine two explanted COM bearings (one head and cup, one head only), and to assess the effect of in-vivo changes on the wear performance of the COM bearings by comparing the wear of the explanted bearings with three new COM implants in a hip wear simulator.

Two 28mm diameter COM bearings were provided for analysis. These were visually examined and surface profilometry was performed using a 2-D contacting profilometer (Form Talysurf, Taylor Hobson, UK). Scanning electron microscopy was used to image the regions of transfer on the ceramic heads, and EDX to assess the transfer composition (Philips XL30 ESEM).

Hip simulator testing was conducted for 2 million cycles (Mc) comparing the explanted bearings with three new 28mm COM bearings. Tests were performed in a Prosim simulator (SimSol, UK), which applied a twin peak loading cycle, with a peak load of 3kN. Flexion-extension of − 15 to 30 degrees was applied to the head and internal-external rotation of +/− 10 degrees was applied to the cup, components were mounted in the anatomical position. The lubricant was 25% (v/v) calf serum supplemented with 0.03% (w/v) sodium azide and was changed approximately every 0.33Mc. Wear was measured gravimetrically at 0.5, 1 and 2 Mc.

Regions of material transfer, identified on both ceramic explant heads, were shown to be CoCr material by EDX analysis, suggesting metallic transfer from the metal cup. Profilometry traces across metallic transfer showed comparable surface roughness measurements compared to unworn material.

The overall mean wear rate for the new COM bearings at 2Mc was 0.047 ± 0.06mm3/Mc. The mean wear rate for the explanted head articulated with a new cup was slightly lower at 0.034mm3/Mc. The mean wear rate for the explanted head and cup was highest at 0.15mm3/Mc. It was noted that the explanted head/cup had higher bedding in wear compared with the other bearings, but still significantly less than a new MOM bearing (mean bedding-in wear rate 2.03 ± 2.59 mm3/Mc). The steady-state wear was comparable with the new bearings. As the orientation of these implants in-vivo was unknown, it is proposed that the elevated wear during bedding-in of the explanted head/cup bearing may be due to the alignment of the components. The wear rates of the explanted ceramic head against a new cup were comparable with the new bearings, suggesting that the presence of metallic transfer on the ceramic head does not adversely affect the wear behaviour of COM bearings.

The combination of a ceramic head articulating against a metal acetabular liner (CoM) has shown reduced metal ion levels compared with a metal-on-metal bearing (MoM) in hip simulator studies. A randomized prospective clinical trial was undertaken using CoM and MoM bearings in an otherwise identical total hip procedure. The initial clinical results were encouraging. This report comprises a further review of metal ion data.

Patients received identical components with the exception of the bearing surface material but all were 28mm diameter. All components were supplied by DePuy International Ltd. Patients were assessed pre-operatively, 3m, 12m and > 24m (median 32m). Whole blood samples were collected at regular follow-ups, frozen and analysed in batches using high resolution Inductively Coupled Plasma – Mass Spectrometry (ICP-MS). All recruited patients are included irrespective of outcome. However some patients failed to attend specific follow-ups and some contaminated samples had to be discarded. Statistical significance was analyzed using a non-parametric comparison (Mann-Whitney test). After 3m and 12m implantation there were between 21 and 24 patients available for analysis in both the CoM and MoM cohort and after > 24m point 10 and 9 respectively.

There were four outliers (either Cr or Co > 10ug/l) in both the CoM and MoM groups. In common with previous studies (with the exception of two marginal outliers), these were related to component position. They were implanted with either a cup abduction angle of > 55°, an anteversion angle of > 30° or both. Other studies with the same design of component have reported no significant outliers.

The median Cr and the Co levels are lower with the CoM bearing compared with the MoM at all measurements points following implantation. The median background (pre-operative) levels for the combined CoM and MoM group were Cr: 0.22ug/l and Co: 0.49ug/l. These were significantly different (p=0.006).

In the CoM group, the median 12m Cr and Co values were 0.43ug/l and 0.72ug/l respectively. The comparable values for MoM are 0.68ug/l and 0.83ug/l. Increases in metal ion levels from pre-operative levels are used as the primary ion level outcome in this study because the background level will comprise of the order of 30–50% of the overall value. The increase in Cr for CoM and MoM from pre-op levels to 12m significantly different for Cr (p=0.015). It has a lower significance for combined metal ion levels (p=0.029). This difference in not significant for Co (p=0.195).

In agreement with predictions from hip simulator studies, CoM bearings in this study produced lower levels of metal ions than comparable MoM bearings at all time points. However the difference is less than that predicted in the laboratory and is much more pronounced with Cr than with Co.

Introduction: Squeaking after total hip replacement has been reported in up to 10% of patients. Some authors proposed that sound emissions from squeaking hips result from resonance of one or other or both of the metal parts and not the bearing surfaces. There is no reported in vitro study about the squeaking frequencies under lubricated regime. The goal of the study was to reproduce the squeaking in vitro under lubricated conditions, and to compare the in vitro frequencies to in vivo frequencies determined in a group of squeaking patients. The frequencies may help determining the responsible part of the noise.

Methods: Four patients, who underwent THR with a Ceramic-on-Ceramic THR (Trident^®, Stryker^®) presented a squeaking noise. The noise was recorded and analysed with acoustic software (FMaster^®). In-vitro 3 alumina ceramic (Biolox Forte Ceramtec^®) 32 mm diameter (Ceramconcept^®) components were tested using a PROSIM^® hip friction simulator. The cup was positioned with a 75° abduction angle in order to achieve edge loading conditions. The backing and the cup liner were cut with a diamond saw, in order to avoid neck-head impingement and dislocation in case of high cup abduction angles. The head was articulated ± 10° at 1 Hz with a load of 2.5kN for a duration of 300 cycles. The motion was along the edge. Tests were conducted under lubricated conditions with 25% bovine serum without and with the addition of a 3rd body alumina ceramic particle (200 μm thickness and 2 mm length).

Results: Edge loading was obtained incompletely. In-vitro, no squeaking occurred under edge loading conditions. However, with the addition of an alumina ceramic 3rd body particle in the contact region, squeaking was obtained at the beginning of the tests and stopped after ~20 seconds (dominant frequency 2.6 kHz). In-vivo, recordings had a dominant frequency ranging between 2.2 and 2.4 kHz.

Discussion: For the first time, squeaking was reproduced in vitro under lubricated conditions. In-vitro noises followed edge loading and 3rd body particles and despite, the severe conditions, squeaking was intermittent and difficult to reproduce. However, squeaking is probably more difficult to reproduce because the cup was cut and the head was fixed in the simulator, preventing vibration to occur

Squeaking noises of a similar frequency were recorded in-vitro and in-vivo. The lower frequency of squeaking recorded in-vivo, demonstrates a potential damping effect of the soft tissues. Therefore, the squeaking in the patients was probably related to the bearing surfaces and modified lubrication conditions that may be due to edge loading. The determined values of frequencies may help to analyze the squeaking patients in order to determine the mechanism generating the sound.

Introduction: There is increasing interest in the use of ceramic on ceramic bearings for hip replacement, due to recognition of their extremely low wear and biocompatibility of the wear debris [1].

The aim of this study was to investigate the influence of cup inclination angle and head position on the wear of ceramic-on-ceramic total hip replacements.

Methods: The wear of Biolox Delta alumina matrix composite ceramic (CeramTec AG, Germany) was investigated using the six station Leeds II Physiological Anatomical hip joint simulator, using 25% bovine serum as a lubricant. Three ceramic-on-ceramic bearings were mounted with the cup providing a clinical angle of 55o (representing the standard condition) and three were mounted to provide a clinical angle of 65o (representing the steep cup angle condition). Simulator studies were carried out under standard gait conditions for 2 million cycles, and under micro-separation conditions for a further 3 million cycles. Micro-separation and dynamic lateralisation of the position of the head replicate head/cup rim contact at heel strike and simulate stripe wear on a ceramic femoral head as found on ceramic-on-ceramic retrievals [2]. Volumetric wear was determined gravimetrically and statistical analysis was performed using One Way ANOVA.

Results: There was no difference in the wear rates under standard gait conditions for the standard and steep cup angles, with a wear rate of 0.05 mm3/million cycles. Under micro-separation conditions the wear rates increased significantly to 0.13 and 0.11 mm3/million cycles for the standard and steep cup angles respectively. However, there was no significant difference between the standard and steep cup angle groups.

Discussion: Micro-separation and dynamic lateralisation of the position of the head during gait simulation significantly increased wear. However, the inclination of the cup in ceramic-on-ceramic THRs did not have a significant effect on the wear under either standard gait or micro-separation conditions.

Ceramic-on-metal (ceramic head and metal liner, COM) hip replacements have shown reduced wear in comparison to metal-on-metal (MOM) bearings (Firkins et al., 1999). This has been attributed to a reduction in corrosive wear, differential hardness and a reduction in adhesive wear. In a clinical report on the use of a metal-on-ceramic hip replacement (Valenti et al., 2007) which consisted of a stainless steel head and alumina ceramic insert at revision 6-months post-op massive metallosis and macroscopic wear was observed.

The aim of this study was to assess the performance of ceramic and metal bearings in different configurations under adverse conditions, ceramic heads on metal liners (COM) were compared to metal heads on ceramic inserts (MOC), with head on cup rim loading under micro-separation hip joint simulation.

Components used were made of zirconia-platelet toughened alumina (Biolox Delta) and high carbon (0.2wt%) CoCrMo alloy (DePuy International Ltd, UK). Hip simulator testing applied a twin-peak loading cycle and walking motions with the prosthesis in the anatomical position. The lubricant (25% calf-serum) was changed every 0.33Mc, wear was measured gravimetrically. Testing was conducted for 2-million cycles, a standard simulator cycle was adapted so the head subluxed in the swing phase forcing the head onto the cup rim at heel strike (Williams et al., 2006).

The total overall mean wear rate for the MOC bearings (0.71±0.30mm3/Mc) was significantly higher than the wear rate for the COM bearings (0.09±0.025mm3/Mc). The contact of the head against the rim of the cup at heel strike caused deep stripe wear on the metallic heads of the MOC bearings. This region on the head is exposed to high stress conditions and susceptible to damage in edge contact, the effect of this is increased when the cup is a harder material than the head. The wear of a metal-on-metal (MOM) couple under similar conditions was almost two-fold greater than the MOC couple (1.58mm3/Mc, Williams et al., 2006) providing further evidence of the reduced wear with COM in comparison to MOM.

The explant described Valenti et al. included a stainless steel head, this is a softer material compared to CoCr, and wears at a higher level. It can be postulated that the wear under adverse conditions would be further increased.

The COM concept can provide increased design flexibility; thin metal shells can be used with larger ceramic heads. Additionally the design protects against ceramic liner chipping. COM bearings are currently undergoing clinical trials, early data suggests reduced metal ion release in patients with COM bearings compared to metal-on-metal (Williams et al., 2007).

Total meniscectomy has been shown to induce osteoarthritic changes in the underlying articular cartilage(AC) and bone in the natural knee (Fairbank 1948; McDermott 2006). This indicates the meniscus plays an important protective role, providing joint congruity and distributing contact forces, hence reducing contact stress. However, no friction and wear studies have been performed on meniscectomy. The aim of this study was to study the tribological response of the medial compartmental natural knee with and without the intact meniscus, under physiological dynamic loading and motion. The effect of normal and reduced loading was investigated.

Eighteen month old bovine medial compartmental knees were used. A pendulum friction simulator (Simulation Solutions, UK) was used to apply a dynamic axial loads with peak loads of 1000N (normal) and 260N (reduced). Flexion-extension of amplitude 23degrees was applied and the experiments ran for 3600 cycles at 1Hz. Lubricant was 25% bovine serum in saline. A 9.4 Tesla MRI (Bruker) scanner and Analyze software (Mayo Clinic, US) were used to calculate wear volumes. A surface profilometer (Talysurf, Taylor-Hobson, UK) was used to measure the surface roughness of the specimen before and after the test.

Coefficient of friction was found to increase with increased loading, with and without meniscus. With meniscus intact, no wear was found on AC and contact stresses were 4.9MPa and 2.8MPa, for normal and reduced loading respectively. On removal of meniscus, friction was higher at both loading conditions and surface fibrillation found on some of the AC surfaces. Contact stresses rose to 17.2MPa and 8.6MPa for normal and reduced loading.

This study has shown for the first time, the direct elevation of the coefficient of friction, immediate surface fibrillation and biomechanical wear of AC upon removal of the meniscus. On removal of meniscus, peak stresses rose and surface damage occurred on AC surfaces. The removal of the meniscus means forces act across smaller areas and contact stresses are increased. Wear is increased due to the subsequent increase in direct solid-solid contact and loss of fluid support due to the unique biphasic nature of AC. This further supports retaining meniscus whenever possible in knee joint surgery.

Metal-on-metal hip resurfacing has been introduced recently, due to its potential advantages of biomechanics and biotribology. However, a number of problems have been identified from clinical retrievals, including significant elevation of wear when the implant is mal-positioned. Our hypothesis is that implant mal-position and micro-lateralisation can result in edge contact, leading to increases in wear. The aim of this study was to investigate the combined effect of cup position and micro-lateralisation on the contact mechanics of metal-on-metal hip resurfacing prosthesis, in particularly to identify conditions which resulted in edge contact.

Finite element (FE) method was used. A generic metal-on-metal hip resurfacing prosthesis was modelled. The bearing diameters of the femoral head and acetabular cup components were 54.49mm and 54.6mm respectively, with a diametral clearance between the head and the cup of 0.11mm. The resurfacing components were implanted into a hemi-pelvic hip joint bone model and all the materials in the FE model were assumed to be homogenous, isotropic and linear elastic (Udofia et al 2007). The FE models consisted of approximately 80,000 elements, which were meshed in I-DEAS (Version 11, EDS, USA) and solved using ABAQUS (Version 6.7-1, Dassault Systèmes). For this study, the femoral component was fixed with an inclination angle of 45° and an anteversion angle of 10°. The orientation of the acetabular cup was varied, using inclination angles of 35° and 65°, and anteversion angles between −10° to 30°. Contact at the bearing surface between the cup and femoral head was modelled using frictionless surface-based elements, simulating a fully lubricated situation, as coefficients of friction less than 0.1 would not have appreciable effects on the predicted contact mechanics. The femoral component was fixed into the femur (except the guide pin) using PMMA cement with an average thickness of approximately 1mm. The other contact interfaces in the FE model (cup/acetabulum, cement/bone and cement/femoral component) were all assumed to be rigidly bonded. The hip joint model was loaded through a fixed resultant hip joint contact force of 3200N, and was applied through medial, anterior muscle forces and subtrochanteric forces to simulate the mid-to-terminal stance phase (approximately 30% – 50%) of the gait cycle (Bergmann et al., 1993). Micro-lateralisation was modelled through displacing the femoral head laterally, up to 0.5mm, relative the centre of the cup.

Edge contact was detected once the inclination angle became greater than 65°. The effect of ante-version was to further shift the contact area towards the edge of the cup, nevertheless no edge contact was found for ante-version angles up to 25° and inclination angles below 55°. However, when the micro-lateralisation was introduced, edge contact was detected at a much smaller inclination angle. For example, even with a micro-lateralisation of 0.5 mm, edge contact occurred at an inclination angle of 45°. This study highlights the importance of surgical techniques on the contact mechanics and tribology of metal-on-metal hip resurfacing and potential outcome of these devices.

Introduction: The use of hard-on-hard hip prostheses has highlighted specific problems like the “stripe-wear” and the squeaking. Many authors have related these phenomena to a micro-separation between the cup and the head. The goal of the study was to model the hip kinematics under micro-separation regime in order to develop a computational simulator for total hip prosthesis including a joint laxity, and to use it to perform a sound analysis.

Method: A three-dimensional model of the Leeds II hip simulator was developed on ADAMS^® software. A spring was used to introduce a controlled micro-separation (less than 500 microns) during the swing phase of the walking cycle. The increase of the load during the stance phase induced a relocation of the head in the cup. Values of the medial-lateral separation predicted from the model were compared to experimental data measured using a LVDT of less than 5 microns precision. Theoretical wear path predicted from the model was compared to the literature data. The frequencies of the vibratory phenomena were determined, using the Fourier transformation.

Results: There was an excellent correlation between the theoretical prediction and the experimental measurement of the medial-lateral separation during the walking cycle (0.92). Edge-loading contact occurred during 57% of the cycle according to the model and 47% according to the experimental data. Velocity and acceleration were increased during the relocation phase in a chaotic manner, leading to vibration. The contact force according to the model had also a chaotic variation during the micro-separation phase, suggesting a chattering movement. Fourier transformation showed many frequencies in the audible area.

Discussion: A three-dimensional computational model of the kinematics of the hip after total replacement was developed and validated with an excellent precision under microseparation. It highlighted possible explanations for the squeaking that may occur during either relocation phase or edge loading.

Young and active patients require bearing materials that can last up to 200 million walking steps, ten fold greater than conventional polyethylene bearings.

Cross linked polyethylene provides reduced wear rate compared to conventional polyethylene, and further advantage is gained from using ceramic femoral heads. However in polyethylene bearings wear increases with the head diameter, and there is currently little opportunity to use head sizes greater then 36mm diameter. There is evidence of polyethylene fracture with steeply positioned cups.

Ceramic on ceramic bearings provide substantially lower wear rates than polyethylene bearings. Steep cups, lateralised heads or neck impingement can lead to head contact on superior rim of the cup and stripe wear, but this still results in very low wear rates. Recently developed ceramic matrix composites Biolox Delta provide greater resistance to stripe wear. In a few patients stripe wear may lead to squeaking.

Metal on metal bearings also provide substantially lower wear than polyethylene bearings. However there remains concern about elevated metal ion levels in a few patients and resultant risk of hypersensitivity reactions. In metal on metal bearings larger head sizes and reduced diametrical clearance can lead to reduced wear. Increased wear is associated with steep cups and lateralised heads resulting in rim wear.

Ceramic on metal bearings have been introduced recently as the first differential hard on hard bearings. These bearings show substantial reduction in wear, corrosive wear mechanisms, metal ion levels in laboratory simulators and initial clinical studies have shown a reduction in metal ion levels in vivo compared to metal on metal bearings.

Total disc replacement is an alternative to spinal fusion in treating degenerative disc disease, whilst preserving motion and reducing the risk of subsequent DDD at adjacent levels. Current designs have evolved from technology used in total hip replacements with metal-metal or metal-PE bearing surfaces. These articulating systems may be prone to wear and it is essential the medical engineering community assess their performance using appropriate simulators

Utilising previous Leeds simulation design experience, current knowledge on spinal kinetics and prevailing Standards for spinal testing, a comprehensive set of requirements was generated from which a simulator design was produced. The Leeds Spine wear simulator, developed in conjunction with Simulation Solutions Ltd, incorporates five active degrees of freedom: axial compression, axial rotation, flexion-extension, lateral bending and anterior-posterior displacement. The fifth DOF, unique to the Leeds simulator, is anticipated to be particularly important for the study of mobile bearing devices such as the Charité. Loads and motions are applied by electro-mechanical actuators, providing accurate and precise control without the low band width suffered from pneumatics or contamination from hydraulic systems. This validation study determines the accuracy and precision of the simulator with regards to the degrees of freedom required by the newly published standard ISO 18192-1. Here, loads and motions have to be within ±5% of the maximum value and ±0.5degrees, respectively. The simulator’s response to demand input signals was determined for load and motion using independent measuring devices; a digital inclinometer for motions and load cell for force.

The load calibration was found to be within ±1% of the maximum load within the specified load range of 600–2000N. Flexion-extension, lateral bending and axial rotation were found to be within ±0.5, ±0.3 and ±0.5 degrees respectively, within and beyond the operating ranges specified by ISO.

The Leeds spine wear simulator is the first orthopaedic wear simulator to include electro-mechanical actuators for all active DOF, and the first spinal wear simulator to include a minimum of 5 active DOF. This novel simulator meets the demanding tolerances required by ISO for testing of total disc replacements. Validation of the simulator is currently being undertaken to determine its suitability against explanted devices and debris located within tissues.

Introduction: The goal of the study was to compare the squeaking frequencies of Ceramic-on-Ceramic THR in-vitro and in-vivo among patients who underwent THR.

Method: Four patients, who underwent THR with a Ceramic-on-Ceramic THR (Trident^®, Stryker^®) presented a squeaking noise. The noise was recorded and analysed with acoustic software (FMaster^®). In-vitro 2 alumina ceramic (Biolox Forte Ceramtec^®) 32 mm diameter (Ceramconcept^®) components were tested using a PROSIM^® hip friction simulator. The cup was positioned with a 70° abduction angle in order to achieve edge loading conditions and the head was articulated ± 10° at 1 Hz with a load of 2.5kN for a duration of 300 cycles. Tests were conducted under lubricated conditions with 25% bovine serum and with the addition of a 3rd body alumina ceramic particle (200 μm thickness and 2 mm length).

Results: In-vivo, recordings had a dominant frequency ranging between 2.2 and 2.4 kHz. In-vitro no squeaking occurred under edge loading conditions. However, with the addition of an alumina ceramic 3rd body particle in the contact region squeaking was obtained at the beginning of the tests and stopped after ~20 seconds (dominant frequency 2.6 kHz).

Discussion and Conclusion: Squeaking noises of a similar frequency were recorded in-vitro and in-vivo. In-vitro noises followed edge loading and 3rd body particles and despite, the severe conditions, squeaking was intermittent and difficult to reproduce. The lower frequency of squeaking recorded in-vivo, demonstrates a potential damping effect of the soft tissues. No damage was observed on the components, however, the test duration was very short. Squeaking may be related to third body particles that could be generated by wear or impingement between the femoral neck and the metal back. Cup design seems to be of particular importance in noisy hip, leading to a high variability of squeaking rate according to the implants.

Ceramic head and metal liner hip replacements (COM) have shown reduced wear in comparison to metal-on-metal (MOM) bearings. The aim of this study was to further assess the performance by a wear simulator study under standard and adverse conditions, including the wear of a metal head against a ceramic liner.

Components were Biolox Delta and CoCrMo alloy. Hip simulator testing applied a simplified walking cycle to anatomically mounted prostheses. The lubricant was 25% calf-serum and wear was measured gravimetrically. In hip simulator testing with edge loading a standard cycle was adapted so the head sub-luxed in the swing phase forcing the head onto the edge of the cup at heel strike, this was applied to ceramic on metal and metal on ceramic material combinations.

Under standard conditions the total overall mean wear rate of the MOM THR (1.01±0.38mm3/Mc) was significantly higher in comparison to the COM and COC (< 0.015mm3/Mc). The overall mean wear rate for the MOC bearings (0.71±0.30mm3/Mc) was significantly higher than the wear rate for the COM bearings (0.09±0.025mm3/Mc). The contact of the head against the rim of the cup caused deep stripe wear on the metallic heads of the MOC bearings. This region on the head is exposed to high stress conditions and susceptible to damage in edge contact, the effect of this is increased when the cup is a harder material than the head. The wear of a metal-on-metal (MOM) couple under similar conditions was almost two-fold greater than the MOC couple (1.58mm3/Mc, Williams et al., 2006) providing further evidence of the reduced wear with COM in comparison to MOM.

Reduced wear from COM bearings will address some concerns associated with MOM THRs regarding reports of elevated ion levels clinically. These studies have provided valuable data demonstrating reduced wear with COM bearings. COM bearings are undergoing clinical trials, early data suggests reduced metal ion release in patients compared to metal-on-metal

A randomised prospective study of four bearing surfaces in hip replacements is being conducted. The primary objective is to identify the best long term bearing surf ace clinically and radiographically, and metal ion levels have been measured in all cases.

Patients have been randomised to the four bearing surfaces viz. Ceramic-on-XLinked Polyethelene, Ceramic-on-Ceramic, Metal-on-Metal and Ceramic-on-Metal. Pre-operative blood samples and follow-up blood samples for metal ion analysis using ICP-MS method have been taken in all patients. As at February 2008 187 patients have been recruited, and metal ion levels at one year are available in 52 patients.

Metal ion levels are not increased with Ceramic-on-XLPE or Ceramic-on-Ceramic bearings. At one year follow-up the metal ion levels in Ceramic-on–Metal bearings is half that of Metal-on-Metal bearings using mean levels, and one third using median levels. Of note is that chromium levels in Ceramic-on-Metal bearings is the least elevated.

Due to the laboratory evidence that ceramic-on-metal bearings have the best surf ace wear characteristics with no head stripe wear on a ceramic head, and the laboratory and clinic al evidence of lower metal ion levels, Ceramic-on-Metal hip replacements could be one of the bearing surfaces of the future.

A randomised prospective study of 4 bearing surfaces in hip replacements is being conducted. The primary objective is to identify the best long term bearing surface clinically and radiographically, and metal ion levels have been measured in all cases.

Patieents have been randomised to the 4 bearing surfaces viz. Ceramic on XLinked Poly, Ceramic on Ceramic, Metal on Metal and Ceramic on Metal. Pre operative blood samples and follow up blood samples for metal ion analysis using the ICPMS method have been taken in all patients. As at February 2008 187 patients have been recruited, and metal ion levels at 1 year are available in 52 patients.

Metal ion levels are not increased with Ceramic on XLinked Poly or Ceramic on Ceramic bearings. At 1 year follow up the metal ion levels in Ceramic on Metal bearings is half that of Metal on Metal bearings using the mean levels, and one third using the madian levels. Of note is that the chromium levels in Ceramic on Metal bearings is the least elevated.

Due to laboratory evidence that Ceramic on Metal bearings have the best surface wear characteristics with no head stripe wear, and laboratory and clinical evidence of lower metal ion blood levels, Ceramic on Metal hip replacements could be a bearing surface of the future.

The use of hard-on-hard hip prostheses has highlighted specific problems like the “stripe-wear” and the squeaking. Many authors have related these phenomena to a micro-separation between the cup and the head. The goal of the study was to model the hip kinematics under micro-separation regime in order to develop a computational simulator for total hip prosthesis including a joint laxity, and to use it to perform a sound analysis.

A three-dimensional model of the Leeds II hip simulator was developed on ADAMS® software. A spring was used to introduce a controlled micro-separation (less than 500 microns) during the swing phase of the walking cycle. The increase of the load during the stance phase induced a relocation of the head in the cup. Values of the medial-lateral separation predicted from the model were compared to experimental data measured using a LVDT of less than 5 microns precision. Theoretical wear path predicted from the model was compared to the literature data. The frequencies of the vibratory phenomena were determined, using the Fourier transformation.

There was an excellent correlation between the theoretical prediction and the experimental measurement of the medial-lateral separation during the walking cycle (0.92). Edge-loading contact occurred during 57% of the cycle according to the model and 47% according to the experimental data. Velocity and acceleration were increased during the relocation phase in a chaotic manner, leading to vibration. The contact force according to the model had also a chaotic variation during the micro-separation phase, suggesting a chattering movement. Fourier transformation showed many frequencies in the audible area.

A three-dimensional computational model of the kinematics of the hip after total replacement was developed and validated with an excellent precision under micro separation. It highlighted possible explanations for the squeaking that may occur during either relocation phase or edge loading.

This study reports on ceramic-on-metal (CoM) bearings in total hip replacement. Whole blood metal ion levels were measured. The median increase in chromium and cobalt at 12 months was 0.08 μg/1 and 0.22 μg/1, respectively, in CoM bearings. Comparable values for metal-on-metal (MoM) were 0.48 μg/1 and 0.32 μg/1. The chromium levels were significantly lower in CoM than in MoM bearings (p = 0.02). The cobalt levels were lower, but the difference was not significant. Examination of two explanted ceramic heads revealed areas of thin metal transfer. CoM bearings (one explanted head and acetabular component, one explanted head and new acetabular component, and three new heads and acetabular components) were tested in a hip joint simulator. The explanted head and acetabular component had higher bedding-in. However, after one million cycles all the wear rates were the same and an order of magnitude less than that reported for MoM bearings. There were four outliers in each clinical group, primarily related to component malposition.

Ceramic on metal bearings for hip replacement have shown reduced friction, wear, and metal ion levels in comparison to metal on metal bearings. Lower wear has been attributed to a reduction in corrosive wear, smoother surfaces and improved lubrication and differential hardness and reduction in adhesive wear. The aim of this study was to further assess the performance of novel differential hardness COM THRs in two different bearing configurations. The effect of bearing configuration was examined by comparing COM (ceramic head on metal liner) with metal-on-ceramic (metal head on ceramic liner) (MOC) bearings in micro-separation hip simulator testing.

Components used were zirconia-platelet toughened alumina (Biolox Delta) heads and high carbon (0.2wt%) CoCrMo alloy cups, tested in COM and MOC configurations, (, (DePuy International Ltd, UK). Micro-separation hip simulator testing was conducted for 2 million cycles (Mc) using a Prosim hip simulator (SimSol, UK), which applied a twin peak loading cycle and walking motions with the prosthesis positioned in the anatomical position. The lubricant (25% calf serum) was changed approximately every 0.33Mc and wear was measured gravimetrically. A negative force was applied to the head during the swing phase, to produce a joint laxity and head – rim contact, the head relocated in the stance phase.

The total overall mean wear rate for the MOC bearings (0.71±0.30mm3/Mc) was significantly higher than the wear rate for the COM bearings (0.09±0.025mm3/ Mc). The contact of the head against the rim of the cup at heel strike caused deep stripe wear on the metallic heads of the MOC bearings. Stripes of metallic transfer were visible on all the ceramic heads (COM bearings). The COM bearings had much lower wear rates than the MOC bearings under harsh micro-separation conditions. This suggests that the head in a differential hardness bearing should be the harder material. The COM concept also provides increased design flexibility; thin metal shells can be used with larger ceramic heads. Additionally the design protects against ceramic liner chipping. COM bearings are currently undergoing clinical trials.

Large diameter metal-on-metal (MOM) bearings are becoming increasingly popular for young, active patients. Clearance is a particularly important consideration for designing MOM implants, considering historical experience of equatorial contact and high frictional torque. Lubrication theory predicts increasing the clearance will result in diminished lubrication, resulting in increased friction and wear. Clinical cases of transient squeaking in patients with resurfacing bearings have been noted in recent years, with some reporting an incidence of up to 10% between 6 months and 2 years post-implantation. This study aimed to investigate the impact of increasing clearance on the lubrication, friction and squeaking of a large diameter metal-on-metal resurfacing bearing through frictional studies.

Clinical-grade MOM implants of 55mm diameter and 100μm diametric clearance, and custom-made, 55mm bearings with diametric clearances of approximately 50μm and 200μm (DePuy International Ltd) were tested in a friction simulator. Components were inverted with a flexion-extension of ±25o applied to the head and lubricated with 25% and 100% newborn bovine serum. A peak load of 2kN, with swing-phase loads of 25N, 100N and 300N were applied.

Sound data was recorded during each friction test using a MP3 recorder and pre-amplifier. A microphone was set up at a distance of 50mm from the implant, and data recorded over a minimum of 10 seconds where sound was generated. Sound data was assessed through narrow band analysis on Frequency Master software (Cirrus Research, UK).

Lubrication was assessed by directly measuring the separation between the head and cup during the test cycle by ultrasonic methods (Tribosonics, UK). An ultrasound sensor was bonded to the back of the cup and reflection measurements were taken during the friction tests with a sampling rate of 100Hz. Using equations which related reflection coefficient to lubricant properties and thickness, values for the film thickness were calculated.

The surface replacement with the largest clearance yielded the highest friction factor for each test condition. The difference between the large clearance bearing and the smaller clearance samples was statistically significant in 25% bovine serum, the more clinically relevant lubricant (ANOVA, p< 0.05). The 50μm clearance group yielded similar results to the 100μm clearance bearing, although a slight increase in friction was observed.

Squeaking occurred during every test in the large clearance group. There was a reduced incidence of squeaking in the smaller clearances, with the lowest incidence observed in the 100μm clearance group.

The smallest separation of the head and cup was observed within the large clearance bearings. The best lubrication condition measured ultrasonically was observed within the 100μm clearance bearing. There appeared to be good correlation between friction, lubrication and the incidence of squeaking. This study suggests a large diametric clearance results in reduced lubrication, increased friction and an increased incidence of squeaking. However, there is a minimum diametric clearance that can be tolerated, as clearance must accommodate the manufacturing tolerance.

Clinical reports of surgical intervention options, such as spacers or hemi-arthroplasties, particularly for treatment of young arthritic patients, have been poor [1]. Knowledge of the tribology of the cartilage-prosthesis interaction of these devices would potentially provide an insight to the reasons for the premature failure of these devices and the development of more appropriate intervention treatment solutions for arthritic patients. Frictional studies of articular cartilage have been reported, using simple pin-on-plate geometric configurations [2], which do not accurately represent the geometric and stress conditions in the natural joint. A more representative model, based on the medial compartment of the knee joint has been developed in the Part 1 of this study [3] for the pre-clinical tribological testing the natural joint and their related arthroplasty devices. Bearing geometry is an important consideration in limiting wear, as shown in congruous meniscal knee replacement, which exhibited lower wear rates than incongruous designs [4,5]. The aim of this study was to use a unicompartmental hemi-arthroplasty model to examine the effect of tibial conformity and stress on the friction and wear of articular cartilage.

Experiments were conducted in an anatomic pendulum friction simulator (SimSol, UK) using the medial femoral condyle of a bovine knee joint articulating against two conforming stainless steel (316L) tibial plates (R=50mm and 100mm). A simplified physiologic knee loading profile was applied represent both low loading and much higher physiological loading conditions, with peak load between 259N – 1.5kN). Tests were conducted in 25% bovine serum and run for 3600 and 300 cycles under the low and high loading conditions respectively. The motion was cycled at 1Hz with amplitude between −10°–13.1°. Cartilage wear was assessed qualitatively from surface roughness measurements using a surface profile using a surface profilometer (Taylor Hobson, UK). The friction and wear of cartilage articulating against the conforming tibial plates were compared to a positive control flat tibial plate model [3]. The conforming plate models were found to produce significantly lower cartilage friction and surface damage (μ=0.022–0.035, Ra=0.136–0.145μm) than the flat plate model (μ=0.078, Ra=2.70μm). No damage on the cartilage surface was observed under low loads, however, under higher, more physiological loading cartilage friction increased (μ=0.08) in the conforming plate model, with a significant surface damage. An anatomic unicompartmental knee joint model has been developed to successfully examine the effect of counterface conformity on cartilage friction and wear for pre-clinical testing of a hemi-arthroplasty device. Counterface conformity was shown to significantly reduce cartilage friction and wear. This was attributed to the increased surface area and reduced stresses experienced in comparison to an incongruent bearing articulation.

Introduction: There is increasing interest in the coupling of highly cross-linked polyethylene with large diameter heads in the hip. The aim of this study was to determine the wear of large (size 36 mm) highly cross-linked polyethylene inserts against ceramic and cobalt chrome femoral heads using a physiological hip simulator.

Methods: Size 36 mm Biolox® Forte alumina and cobalt chrome femoral heads were coupled with highly cross-linked polyethylene inserts in the ten station Leeds ProSim Physiological Anatomical Hip Joint Simulator. The simulator was run for 10 million cycles and the change in volume of the polyethylene inserts was determined geometrically.

Results: The volume change of the ceramic/cross-linked polyethylene bearing combinations during the first two million cycles of the hip simulator test was twice that of the cobalt chrome/cross-linked polyethylene bearing combinations due to increased creep. After 2 million cycles a steady state wear rate was reached. In contrast the cobalt chrome/cross-linked polyethylene bearing combinations reached their steady state at 1 million cycles.

The steady state wear rate for the ceramic/cross-linked polyethylene bearing combinations was 4.7 mm³/million cycles. This was a significant 40% reduction compared to the wear rate of the cobalt chrome/cross-linked polyethylene bearing combinations at 8.1 mm³/million cycles (p< 0.01).

Discussion: The clinical implications of this study relate to the measurement of in vivo wear, which is routinely assessed using penetration measured from radiographs. However, penetration is a measure of both wear and creep. This means that although the penetration of polyethylene inserts coupled with metal and ceramic femoral heads may be similar, the actual wear is likely to be lower with the ceramic heads due to their elevated creep

Nanometre-sized particles of ultra-high molecular weight polyethylene have been identified in the lubricants retrieved from hip simulators. Tissue samples were taken from seven failed Charnley total hip replacements, digested using strong alkali and analysed using high-resolution field emission gun-scanning electron microscopy to determine whether nanometre-sized particles of polyethylene debris were generated in vivo. A randomised method of analysis was used to quantify and characterise all the polyethylene particles isolated.

We isolated nanometre-sized particles from the retrieved tissue samples. The smallest identified was 30 nm and the majority were in the 0.1 μm to 0.99 μm size range. Particles in the 1.0 μm to 9.99 μm size range represented the highest proportion of the wear volume of the tissue samples, with 35% to 98% of the total wear volume comprised of particles of this size. The number of nanometre-sized particles isolated from the tissues accounted for only a small proportion of the total wear volume. Further work is required to assess the biological response to nanometre-sized polyethylene particles.

Considerable differences in kinematics between different designs of knee prostheses and compared to the natural knee have been seen in vivo. Most noticeably, lift off of the femoral condyles from the tibial insert has been observed in many patients. The aim of this study was to simulate lateral femoral condylar lift off in vitro and to compare the wear of fixed bearing knee prostheses with and without lift off.

Twelve PFC Sigma cruciate retaining fixed bearing knees (DePuy, Leeds, UK) were tested using six station simulators (Prosim, Manchester, UK). The kinematic input conditions were femoral axis loading (maximum 2.6 kN), flexion-extension (0–58°), internal/external rotation (±5°) and anterior/posterior displacement (0–5 mm). Six knees were tested under these standard conditions for 4 million cycles. Six knees were tested under these conditions with the addition of lateral femoral condylar lift off, for 5 million cycles. The lubricant used was 25% newborn calf serum. Wear of the inserts was determined gravimetrically.

Under the standard kinematic conditions the mean wear rate with 95% confidence limits was 8.8 ± 4.8 mm 3/million cycles. When femoral condylar lift off was simulated the mean wear rate increased to 16.4 ± 2.9mm 3/million cycles, which was statistically significantly higher (p < 0.01, Students t-test). The wear patterns on the femoral articulating surface of all the inserts showed more burnishing wear on the medial condyle than the lateral. However, in the simulation of lift off the medial condyle was more aggressively worn with evidence of adhesion and surface defects.

The presence of lateral femoral condylar lift off accelerated the wear of PFC Sigma cruciate retaining fixed bearing knees. The lateral lift off produced uneven loading of the bearing, resulting in elevated contact stresses and hence more wear damage to the medial side of the insert. The implications of condylar lift off include increased wear of the polyethylene and possible osteolysis.

Different wear rates have been reported for ceramic-on-ceramic (COC) and metal-on-metal (MOM) hip replacements tested in simulators with different loading conditions and lubricants. We postulate that differences in wear rates may be associated with changes in lubrication and friction in the joint. This study aimed to compare the friction of COC and MOM bearings under different lubrication regimes, simulated by varying swing-phase loads and lubricants.

Alumina COC and CoCr MOM 28mm-diameter bearings were studied in a pendulum friction simulator. Flexion-extension of +/−25 degrees was applied to the head, a peak load of 2kN and swing-phase loads of 25N,100N, 300N used. Lubricants used included water, 25% and 100%-bovine serum.

COC and MOM bearings showed increased friction as the swing-phase load increased. COC bearings produced higher friction in 100%-serum compared to 25%-serum. In contrast, friction was lower when MOM bearings were tested in 100%-serum compared to 25%-serum. When COC bearings were tested in water, the friction decreased in comparison to testing in serum, however, MOM friction was higher in water.

Increasing the swing-phase load reduced the thickness of the fluid-film in the stance-phase and this increased friction. The increase in friction when COC bearings were tested in 100%-serum (compared to 25%) may be due to the increased forces required to shear the increased concentration of proteins, similarly friction is reduced in water. MOM bearing friction was reduced in 100%-serum, in this instance increased proteins may be acting as solid-phase lubricants, and similarly MOM friction increased in water.

The product recall of Desmarquest Zirconia heads in 2001 was associated with specified batches of material. Despite of this fact, concerns raised over the stability of Zirconia led to a vast decrease in the use of Zirconia for hip prostheses. While there is evidence in the literature suggesting that Zirconia may become unstable, there remain many prostheses with Zirconia heads in use today. The purpose of this study was to report the condition observed in retrieved Zirconia heads not included in the product recall.

The bearing surfaces of seven retrieved 22mm diameter Zirconia on UHMWPE hip prostheses were investigated to determine whether any degradation of the Zirconia occurred in-vivo. All seven of the Zirconia heads were manufactured by Saint-Gobain Cerammiques Avancees Desmarquest and implantation time varied from 1 to 10 years. Components were analysed by Talysurf, Interferometer, SEM and XRD and compared to new components.

Talysurf of the components revealed an average surface roughness ranging from 0.004 to 0.007 micrometers Ra. This was only slightly rougher than new ceramic components which generally have an Ra of 0.003 micrometers. SEM of the surfaces did not reveal any difference between the retrieved components and new components. Further surface XRD of 4 of the 7 heads, as shown in Figure 1, showed very small percentages of monoclinic phase (28 degrees 2 theta) with predominantly tetragonal phase (30 degrees 2theta), similar to what is observed in new components. Figure 1: XRD of typical retrieved ZR head surface.

All seven retrieved heads demonstrated no evidence suggesting that degradation of the Zirconia had occurred in-vivo.