Several electrical fields are known to be present in bone tissue as originally described by Fukada and Yasuda in the year 1957. Intrinsic voltages can derive from bone deformation and reversely lead to mechanical modifications, called the piezoelectric effect. This effect is used in the clinic for the treatment of bone defects by applying electric and magnetic stimulation directly to the bone supplied with an implant such as the electroinductive screw system. Through this system a sinusoidal alternating voltage with a maximum of 700 mV can be applied which leads to an electric field of 5–70 V/m in the surrounding bone. This approach is established for bone healing therapies. Despite the established clinical application of electrical stimulation in bone, the fundamental processes acting during this stimulation are still poorly understood. A better understanding of the influence of electric fields on cells involved in bone formation is important to improve therapy and clinical success.

To study the impact of electrical fields on bone cells in vitro, Ti6Al4V electrodes were designed according to the pattern of the ASNIS III s screw for a 6-well system. Osteoblasts were seeded on collagen coated coverslip and placed centred on the bottom of each well. During four weeks the cells were stimulated 3×45 min/d and metabolic and alkaline phosphatase (ALP) activity as well as gene expression of cells were analysed. Furthermore, supernatants were collected and proteins typical for bone remodelling were examined.

The electrical stimulation did not exert a significant influence on the metabolic activity and the ALP production in cells over time using these settings. Gene expression of BSP and ALP was upregulated after the first 3 days whereas OPG was increased in the second half after 14 days of electrical stimulation. Moreover, the concentration of the released proteins OPG, IL-6, DKK-1 and OPN increased when cells were cultivated under electrical stimulation. However, no changes could be seen for essential markers, like RANKL, Leptin, BMP-2, IL-1beta and TNF-alpha.

Therefore, further studies will be done with osteoblasts and osteoclasts to study bone remodelling processes under the influence of electrical fields more in detail. This study was supported by the German Research Foundation (DFG) JO 1483/1-1.

Introduction

Persistent patellofemoral (PF) pain is a common postoperative complication after total knee arthroplasty (TKA). In the USA, patella resurfacing is conducted in more than 80% of primary TKAs [1], and is, therefore, an important factor during surgery. Studies have revealed that the position of the patellar component is still controversially discussed [2–4]. However, only a limited number of studies address the biomechanical impact of patellar component malalignment on PF dynamics [2]. Hence, the purpose of our present study was to analyze the effect of patellar component positioning on PF dynamics by means of musculoskeletal multibody simulation in which a detailed knee joint model resembled the loading of an unconstrained cruciate-retaining (CR) total knee replacement (TKR) with dome patella button.

Introduction

It is well-known that wear debris generated by metal-on-metal hip replacements leads to aseptic loosening. This process starts in the local tissue where an inflammatory reaction is induced, followed by an periprosthetic osteolysis. MOM bearings generate particles as well as ions. The influence of both in human bodies is still the subject of debate. For instance hypersensitivity and high blood metal ion levels are under discussion for systemic reactions or pseudotumors around the hip replacement as a local reaction. The exact biopathologic mechanism is still unknown. The aim of this study was to investigate the impact of local injected metal ions and metal particles.

Introduction

Total knee replacement (TKR) is an established and effective surgical procedure in case of advanced osteoarthritis. However, the rate of satisfied patients amounts only to about 75 %. One common cause for unsatisfied patients is the anterior knee pain, which is partially caused by an increase in patellofemoral contact force and abnormal patellar kinematics. Since the malpositioning of the tibial and the femoral component affects the interplay in the patellofemoral joint and therefore contributes to anterior knee pain, we conducted a computational study on a cruciate-retaining (CR) TKR and analysed the effect of isolated femoral and tibial component malalignments on patellofemoral dynamics during a squat motion.

The biological reaction in metallosis and pseudotumor generation after metal on metal total hip arthroplasty or corroding metal implants remains unsettled. Clinically, still lethal cases appear with massive bone loss and metal ions are suspected to be responsible for this inflammatory reaction, solid metal wear particles instead are usually not observed in the common literature. The aim of this study was to compare the biological reactions of metal ions and metal wear particles in a murine in vivo model. Metal ions (CoCr), metal particles (CoCr), polyethylene particles (UHMWPE) and phosphate buffered saline (PBS) were injected into the left knee joint of female BALB/c mice. 7 days after injection, the microcirculation was observed using intravital fluorescence microscopy, followed by euthanasia of the animals. After the assessment of the knee diameter, the knees underwent histological evaluations of the synovial layer. Throughout all recorded data, CoCr particles caused higher inflammatory reactions compared to metal ions and UHMWPE particles. The mice treated with the solid particles showed enlarged knee diameters, more intensive leukocyte–endothelial cell interactions and an elevated functional capillary density. Pseudotumor-like tissue formations in the synovial layer of the mice were only seen after the exposition to solid CoCr particles. Even if the focus of several national guidelines concerning metallosis and pseudotumor generation is on metal ions, the present data reveal that solid CoCr particles have the strongest inflammatory activity compared with metal ions and UHMWPE particles in vivo.

Introduction

Migration of bone cells and precursor cells to the site of a bone defect can accelerate bone regeneration. Therefore, guidance of these cells by direct current (DC) is an interesting approach to improve implant ingrowth or fracture healing. To allow a better understanding of DC-induced directed migration, a specific stimulation chamber was established and the influence of DC on calcium channel expression in osteoblasts was investigated.

Introduction

In total hip arthroplasty, press-fit anchorage is one of the most common fixation methods for acetabular cups and mostly ensures sufficient primary stability. Nevertheless, implants may fail due to aseptic loosening over time, especially when the surrounding bone is affected by stress-shielding. The use of acetabular cups made of isoelastic materials might help to avoid stress-shielding and osteolysis.

The aim of the present numerical study was to determine whether a modular acetabular cup with a shell made of polyetheretherketone (PEEK) may be an alternative to conventional titanium shells (Ti6Al4V). For this purpose, a 3D finite element analysis was performed, in which the implantation of modular acetabular cups into an artificial bone stock using shells made of either PEEK or Ti6Al4V, was simulated with respect to stresses and deformations within the implants.

Introduction

Modern acetabular cups require a convenient bone stock for sufficient cup fixation. Thereby, fixation stability is influenced by the chosen interference fit of the acetabular cup, the cup surface structure, circularity of the reamed acetabulum and by the acetabular bone quality. The ideal implantation situation of the cup is commonly compromised by joint dysplasia and acetabular bone defects. The aim of the present experimental study was to characterise implant fixation of primary acetabular cups in case of definite acetabular cavity defects.

Introduction

Despite decades of clinical research in artificial joints and underlying failure mechanisms, systematical and reproducible identification of reasons for complications in total knee replacements (TKR) remains difficult. Due to the complex dynamic interaction of implant system and biological situs, malfunction eventually leading to failure is multifactorial and remains not fully understood. The aim of present study was to evaluate different TKR designs and positions with regard to joint kinematics and stability under dynamic conditions by using a robot-based hardware-in-the-loop (HiL) setup.

The biomechanical evaluation of tendon repair with collagen-based scaffolds in rat model is a common method to determine the functional outcome of the tested material. We introduced a magnetic resonance imaging (MRI) approach to verify the biomechanical test data. In present study different collagen scaffolds for tendon repair were examined.

Two collagen test materials: based on bovine stabilized collagen, chemically cross-linked with oriented collagenous fibres (material 1) and based on porcine dermal extracellular matrix, with no cross-linking (material 2) were compared. The animal study was approved by the local review board. Surgery was performed on male Sprague-Dawley rats with a body weight of 400 ± 19 g. Each rat underwent a 5 mm transection of the right Achilles tendon. The M. plantaris tendon was removed. The remaining tendon ends were re-joined with a 5 mm scaffold of either the material 1 or 2. Each scaffold material was sutured into place with two single stiches (Vicryl 4–0, Ethicon) each end. A total of 16 rats (n= 8 each group) were observed for 28 days follow up. The animals were sacrificed and hind limbs were transected proximal to the knee joint. MRI was performed using a 7 Tesla scanner (BioSpec 70/30, Bruker). T2-weighted TurboRARE sequences with an in-plane resolution of 0.12 mm and a slice thickness of 0.7 mm were analysed. All soft and hard tissues were removed from the Achilles tendon-calcaneus-foot complex before biomechanical testing. Subsequently, the specimens were fixed in a materials testing machine (Z1.0, Zwick, Ulm, Germany) for tensile testing. All tendons were preloaded with 1 N and subsequently stretched at a rate of 1 mm/s until complete failure was observed. Non-operated tendons were used as a control (n=4).

After 28 postoperative days, MRI demonstrated that four scaffolds (material 1: n=2, material 2: n=2) were slightly dislocated in the proximal part of hind limb. In total five failures of reconstruction could be detected in the tendon repairs (material 1: n=3, material 2: n=2). Tendons augmented with the bovine material 1 showed a maximum tensile load of 57.9 ± 17.9 N and tendons with porcine scaffold material 2 of 63.1 ± 19.5 N. The native tendons demonstrated only slightly higher loads of 76.6 ± 11.6 N. Maximum failure load of the tendon-scaffold construct in both groups did not differ significantly (p < 0.05). Stiffness of the tendons treated with the bovine scaffold (9.9 ± 3.6 N/mm) and with the porcine scaffold (10.7 ± 2.7 N/mm) showed no differences. Stiffness of the native healthy tendon of the contralateral site was significantly higher (20.2 ± 6.6 N/mm, p < 0.05). No differences in the mechanical properties between samples of both scaffold groups could be detected, regardless of whether the repaired tendon defect has failed or the scaffold has been dislocated.

The results show that MRI is important as an auxiliary tool to verify the biomechanical outcome of tendon repair in animal models.

Nowadays, biomaterials can be used to maintain or replace several functions of the human body being constricted or lost due to tumors, fractures, injuries as well as chronic diseases, infections or simply aging. Titanium and its alloys, i.e. Ti6Al4V are the most common materials (70 to 80%) used for structural orthopedic implants due to their unique combination of good mechanical properties, corrosion resistance and biocompatibility. Addition of β-stabilizers, e. g. niobium (Nb), can improve the mechanical properties of such titanium alloys further, simultaneously offering excellent biocompatibility. Previous studies concerning biocompatibility analyses with niobium and especially Ti-42Nb specimens are rarely described; none for niobium and Ti-42Nb powders examining human cell viability, collagen and interleukin synthesis. In this in vitro study, human osteoblasts were cultured on different roughened niobium specimens (Nb Amperit, Nb Ampertec), Nb sheets and spherical Ti-42Nb (sintered and 3D-printed by selective laser melting, SLM) and compared with forged Ti6Al4V specimens. Furthermore, human osteoblasts were incubated with particulates of the Nb and Ti-42Nb specimens in three particle concentrations over four and seven days to imitate influence of wear debris against the background of osteolysis and aseptic implant loosening. Thereby, the specimens with the roughest surfaces, i.e. Ti-42Nb and Nb Ampertec, revealed excellent and similar results concerning cell viability (WST-1 test, live-dead staining) and collagen-I synthesis superior to forged Ti6Al4V. Examinations with particulate debris disclosed a significant dose-dependent influence of all powders with Nb Ampertec showing the highest decrease of cell viability and collagen-I synthesis. Furthermore, interleukin expression was only slightly increased for all powders. In summary, from a cell-biological point of view Nb Ampertec (sintered Nb) and Ti-42Nb materials seem to be superior alternatives for medical applications compared to common materials like forged Ti6Al4V.

Objectives

Enhanced micromotions between the implant and surrounding bone can impair osseointegration, resulting in fibrous encapsulation and aseptic loosening of the implant. Since the effect of micromotions on human bone cells is sparsely investigated, an in vitro system, which allows application of micromotions on bone cells and subsequent investigation of bone cell activity, was developed.

Introduction

Metal on metal bearings are used especially in hip resurfacing. On the one hand, small bone preserving implants can be used. On the other hand recent studies found a variety of local and systemic side effects, for instance the appearance of pseudotumors, that are explained by pathologic biological reaction of the metal wear debris. The detailed mechanisms are still not understood until now. Thus it was the aim of this study to investigate the local reaction of metal wear particles and metal ions in a murine model. The hypothesis was that mainly metal ions provoke adverse histopathological reactions in vivo.

Introduction

The purpose of this study was to experimentally evaluate impingement and dislocation of total hip replacements while performing dynamic movements under physiological-like conditions. Therefore, a hardware-in-the-loop setup has been developed, in which a physical hip prosthesis actuated by an industrial robot interacts with an in situ-like environment mimicked by a musculoskeletal multibody simulation-model of the lower extremity.

Introduction

The influence of the bone mineral density (BMD) on the mechanical behavior of bones can be examined using computer tomography (CT) data and finite element (FE) simulations, because the BMD correlates with the Hounsfield scale (HU) of the CT data. Therefor the material mapping strategy, which is required to assign the HU values to the FE mesh, is of crucial importance. In this study a nodal mapping strategy was analyzed concerning its sensitivity towards FE mesh parameters and an averaging of HU values from the area around the respective nodes.

Current implant designs and materials provide a high grade of quality and safety, but aseptic implant loosening is still the main reason for total hip revision. Highly cross-linked polyethylene (HX-PE) is used successfully in total hip replacements (THR) since several years. The good wear properties lead to a reduction of wear debris and may contribute to a longer survival time of the THRs. Furthermore, thin HX-PE liner allows the use of larger femoral heads associated with a decreased risk of dislocation and an improved range of motion. However, the cross-linking process is associated with a loss of mechanical properties of the polyethylene material which compromise the use of thin HX-PE liner in terms of high stress situations.

The aim of the present study was the experimental wear analysis of HX-PE liner under steep acetabular cup position. Furthermore, a finite element analysis (FEA) was performed in order to calculate the stress within the HX-PE material in case of steep cup position under physiological loading.

Experimental wear testing was performed for 5 Mio load cycles, using highly cross-linked polyethylene (HX-PE) acetabular liner combined with 44 mm ceramic femoral heads at a standard position of the acetabular cup (30° inclination) according to ISO 14242 as well as at 60° cup inclination. The wall thickness of the HX-PE liner was 3.8 mm. A hip wear simulator, according to ISO 14242 (EndoLab GmbH, Rosenheim, Germany), was used and wear was determined gravimetrically. Moreover, finite element models of the THR system at standard and steep cup position was created by Abaqus/CAE (Dessault Systemes Providence, USA). Using the finite element software Abaqus (Dessault Systemes Providence, USA) the total hip implants were physiologically loaded with maximum force of the gait cycle (3.0 kN). Thereby, the stresses within the HX-PE material were analysed.

The average gravimetrical wear rates of the HX-PE liners at standard implant position (30°) and 60° cup inclination showed small wear amounts of 3.15 ± 0.32 mg and 1.92 ± 1.00 mg per million cycles, respectively. The FEA revealed a clear increase of stresses at the HX-PE liner with respect to steep cup position (von Mises stress of 8.78 MPa) compared to ISO standard implant position (von Mises stress of 5.70 MPa).

The wear simulator tests could not demonstrate significant differences of gravimetrical wear amount of HX-PE liners under steep hip cup position compared to standard implant position. The small contact surface between the femoral head and the SX-PE liner during the wear testing may lead to the low wear rate of the misaligned acetabluar cup. Moreover, the FEA showed that the effect of a misaligned acetabular cup on the stresses within the polyethylene liner can be critical. Although an increase of wear could not be detected a steeper acetabular cup position using thin HX-PE liners should be avoided due to higher stresses preventing implant failure in clinical application.

Introduction:

The higher resisting torque against dislocation and the large range of motion due to the enlarged effective head diameter substantiate the use of eccentric dual-mobility cups in case of total hip joint instability [1,2]. As a result of force-dependent self-centering mechanism, an increased movement of the intermediate-component can be expected whose effect on wear propagation is unknown so far. Currently available hip joint simulators are only able to vary the load by the absolute value and not by the direction of resulting force. Therefore, the uniaxial force transmission may lead to a unique and stable alignment of the intermediate-component during testing. The purpose of this numerical study was to evaluate relative movements of the intermediate-component during daily life activities with respect to wear propagation.

Clinically applied methods of assessing implant fixation and implant loosening are of sub-optimal precision, leading to the risk of unsecure indication of revision surgery and late recognition of bone defects. Loosening diagnosis involving measuring the eigenfrequencies of implants has its roots in the field of dentistry. The changing of the eigenfrequencies of the implant-bone-system due to the loosening state can be measured as vibrations or structure-borne sound. In research, vibrometry was studied using an external shaker to excite the femur-stem-system of total hip replacements and to measure the resulting frequencies by integrated accelerometers or by ultrasound. Since proper excitation of implant components seems a major challenge in vibrometry, we developed a non-invasive method of internal excitation creating an acoustic source directly inside the implant.

In the concept proposed for clinical use, an oscillator is integrated in the implant, e.g. the femoral stem of a total hip replacement. The oscillator consists of a magnetic or magnetisable spherical body which is fixed on a flat steel spring and is excited electromagnetically by a coil placed outside the patient. The oscillator impinges inside the implant and excites this to vibrate in its eigenfrequency. The excitation within the bending modes of the implant leads to a sound emission to the surrounding bone and soft tissue. The sound waves are detected by an acoustic sensor which is applied on the patient's skin. Differences in the signal generated result from varying level of implant fixation.

The sensor principle was tested in porcine foreleg specimens with a custom-made implant. Influence of the measurement location at the porcine skin and different levels of fixation were investigated (press-fit, slight loosening, advanced loosening) and compared to the pull-out strength of the implant. Evaluation of different parameters, especially the frequency spectrum resulted in differences of up to 12% for the comparison between press-fit and slight loosening, and 30% between press-fit and advanced loosening. A significant correlation between the measured frequency and the pull-out strength for different levels of fixation was found.

Based on these findings, an animal study with sensor-equipped bone implants was initiated using a rabbit model. The implants comprised an octagonal cross-section and were implanted into a circular drill hole at the distal femur. Thereby, definite gaps were realized between bone and implant initially. After implantation, the bone growth around the implant started and the gaps were successively closed over postoperative period. Consequently, since the tests had been started with a loose implant followed by its bony integration, a reverse loosening situation was simulated. In weekly measurements of the eigenfrequencies using the excitation and sensor system, the acoustic signals were followed up. Finally, after periods of 4 and 12 weeks after implantation, the animals were sacrificed and pull-out tests of the implants were performed to measure the implant fixation. The measured implant fixation strengths at the endpoint of each animal trial were correlated with the acoustic signals recorded.

Introduction

Dislocation of total hip replacements (THRs) remains a severe complication after total hip arthroplasty. However, the contribution of influencing factors, such as implant positioning and soft tissue tension, is still not well understood due to the multi-factorial nature of the dislocation process. In order to systematically evaluate influencing factors on THR stability, our novel approach is to extract the anatomical environment of the implant into a musculoskeletal model. Within a hardware-in-the-loop (HiL) simulation the model provides hip joint angles and forces for a physical setup consisting of a compliant support and a robot which accordingly moves and loads the real implant components [2]. The purpose of this work was to validate the HiL test system against experimental data derived from one patient.

At present, wear investigations of total hip replacement (THR) are performed in accordance with the ISO standard 14242, which is based on empirically determined relative motion data and exclusively describes the gait cycle. However, besides continuous walking, a number of additional activities characterize the movement sequences in everyday life and influence the wear rates as well as the size and shape of wear debris. Disagreements of in vitro and in vivo wear mechanisms seemed to be a result of differences between in vitro and in vivo kinematics and dynamics. This requires an optimization of the current test procedures and parameters. Hence, the aim of the present study was to evaluate most frequent activities of daily living, based on available in vivo data, in order to generate parameter sets according to loading and rotational movements close to the physiological situation.

For the generation of angular patterns, time-dependent three-dimensional trajectories of reference points were used from the HIP98 database of Bergmann. The data set was evaluated and interpolated using analytical techniques to simulate consecutive smooth motion cycles in hip wear simulators or further test devices. The calculated relative joint movement was expressed by an ordered set of three elementary rotations and was complemented with three force components of the joint contact force to generate kinematically and dynamically consistent parameter sets. The obtained sets included the activities walking, knee bending, stair climbing and a combined load case of sitting down and standing up for an averaged patient.

Generated slide tracks, created by the use of the angular patterns, demonstrated differences according to the kinematics between selected daily life activities and those established for the ISO standard 14242. In particular, for the relative flexion-extension rotational movement, routine activities showed significant higher ranges of motion. Additionally, the depicted force pattern underlined that the prevailing force component varied considerably between different activities.

These deviations in range of motion and joint forces could be attributed to disagreements between in vitro and in vivo results of THR wear testing. The Integration of frequent activities of daily living in the in vivo test protocol could be realized by means of the sequential arrangement of the four investigated activities.

The prevalent cause of implant failure after total joint replacement is aseptic loosening caused by wear debris. Improvement of the wear behaviour of the articulating bearing between the cup and femoral head is essential for increased survival rate of artificial hip joints. Cross-linking of the polyethylene (PE) material is one attempt to reduce wear particle release at the articulating surface. Various cross-linked polyethylenes (X-PE) are used in orthopaedics since several years.

In total hip arthroplasty (THA) the use of larger femoral head sizes has specific reasons. Larger heads lead to a decreased risk of total hip dislocation and impingement as well as an improved range of motion in comparison to smaller head sizes like 28mm or less. However, the increasing diameter of femoral head can be associated with lower thickness of the PE liner and increased wear rate. Cross-linking of PE can improve the wear rate of the liner and hence supports the use of larger femoral heads. The aim of this experimental study was to evaluate the wear of standard vs. sequential X-PE (X3-PE) liner in combination with different ceramic femoral head sizes.

Wear testing was performed for 5 million load cycles using standard UHMW-PE liners (N2Vac) and X3-PE liners (each Stryker GmbH & Co. KG, Duisburg, Germany) combined with 28mm ceramic ball heads and the Trident PSL acetabular cup (Stryker). Furthermore, X3-PE liners with an internal diameter of 36mm and 44mm and decreased wall thickness (5.9mm and 3.8mm) were combined with corresponding ceramic heads. An eight station hip wear simulator according to ISO 14242 (EndoLab GmbH, Rosenheim, Germany) was used to carry out the standard wear tests. The tests were realised in temperature-controlled chambers at 37°C containing calf serum (protein content 20g/l).

The average gravimetrical wear rates of the standard UHMW-PE (N2Vac) liners combined with 28mm ceramic heads amounted to 12.6 ± 0.8mg/million cycles. Wear of X3-PE liners in combination with 28 mm ceramic heads was not detectable. The average gravimetrical wear rates of the X3-PE liners in combination with 36mm and 44mm ceramic heads amounted to 2.0 ± 0.5mg and 3.1 ± 0.3mg/million cycles, respectively.

The purpose of this study was to evaluate the effect of femoral head size at THA on standard and sequential X-PE liner. The wear simulator tests showed that the wear rate of PE liners with small heads (28mm) decreased by cross-linking of the PE significantly. The amount of wear at X-PE increased slightly with larger head size (36mm and 44mm). However, by sequential cross-linking, the wear rate using thinner liners and larger femoral heads is reduced to a fractional amount of wear at conventional UHMW-PE. Hence, the above-mentioned advantages of larger femoral head diameters can be realised by improved wear behaviour of sequential X-PE.

Due to increased life expectancy of human population, the amount of total knee replacements (TKR) is expected to increase. TKR reached a high grade of quality and safety, but most often it fail because of aseptic implant loosening caused by polyethylene (PE) wear debris. Wear is generated at the articulating surfaces, e.g. caused by three body particles, like bone fragments or bone cement particles. The aim of this experimental study was to compare the wear of tibial PE inserts combined with metallic and ceramic femoral components at three body wear situation induced by polymethylmethacrylate (PMMA) and zirconia (ZrO2) particles from the bone cement.

Wear testing was performed for 5 Mio load cycles, using tibial standard PE inserts combined with the same CR femoral component, in two different materials, Cobalt Chromium (CoCrMo) and Biolox delta ® ceramic (Multigen Plus Knee System, Lima Corporate, Italy). A knee wear simulator, according to ISO 14243 (EndoLab GmbH, Rosenheim, Germany), was used to carry out the tests. The tests were performed in temperature-controlled test chambers at 37 °C, containing calf serum with a protein content of 30 g/l. Polymethylmethacrylate (PMMA) and zirconia (ZrO2) bone cement particles (Palacos R ®) were manufactured to a size of 30 μm. The three body particles were added at all stations onto the articulating surface of the tibial PE insert (7mg per condyle) at every 500,000 cycles. Wear was determined gravimetrically and the surfaces of tibial inserts were analysed by scanning electron microscope (SEM) after finishing the 5 million cycles. Furthermore, roughness of the PE insert surfaces and the articulating surfaces of the different femoral components were detected and the PE wear particles were analysed by SEM.

The average gravimetrical wear rates of the tibial PE inserts in combination with CoCr and Biolox delta ® ceramic femoral components amounted to 6.4 ± 0.9 mg and 2.6 ± 0.4 mg per million cycles, respectively. Beside bone cement particles on the articulating surface of the PE inserts, polished surfaces and scratches were detected by SEM. In comparison to the untreated surfaces of the PE inserts at both material pairings the surface roughness at the articulating areas showed deep scratches and polished regions. Analyses of the metallic femoral components showed scratches at the articulating surfaces, none on ceramics.

The present study pointed out the effect of femoral component material in an abrasive three body wear situation on the wear properties of TKR. The wear simulator tests showed that wear of PE inserts under three body wear conditions, in combination with ceramic femoral components, was significantly lower than with metallic femoral components. With regard to anti-allergic properties, ceramic femoral components are promising products for TKR.

Introduction

Due to the commercial launch of newly developed ceramic-on-metal (COM) bearings, we compared the deformation and stresses in the liner with ceramic-on-ceramic (COC), metal-on-metal (MOM) as well as ceramic-on-polyethylene (COP) bearings using a finite-element (FE)-model, analyzing a variety of head size and implant position. Liner deformation in terms of change in inner diameter as well as peak stresses were evaluated.

The objective of this prospective duo-center study was to evaluate the clinical and radiological outcome of the unconstrained Multigen Plus total knee system (Lima Lto, San Daniele, Italy) with the new BIOLOXÒ Delta ceramic femoral component.

40 patients underwent cemented total knee arthroplasty in two university hospitals. Clinical evaluations were undertaken preoperatively and at 3 as well as 12 months postoperatively using the HSS-Score, WOMAC-Score and SF-36-Score. The radiological investigations included ant-post. radiographs (whole leg in two leg stance and lateral view of the knee) and patella tangential radiographs (Merchant view).

During 12 months follow-up three patients underwent revision surgery. One patient had to be revised due to infection after postoperative opening of the knee joint due to direct trauma. One patient sustained an osteosynthetic procedure due to periprosthetic fracture after trauma. In one patient a retropatellar replacement was inserted one year postoperatively. Implant related complications were not found. The mean preoperative HSS-Score amounted to 57.8±11.7 points. At 3 and 12 month follow-up the mean HSS-Score was 76.0±12.3 and 83.3±11.9 points respectively.

Therefore HSS, as well as WOMAC and SF-36 Score improved significantly from preoperativly to both postoperative evaluations (Wilcoxon-Test p< 0.002). Radio-lucent lines around the femoral ceramic component were found in six cases.

However, subsequent long-term studies must be carried out in order to prove the good early clinical results and to clarify if progression of radiolucent lines may influence the clinical outcome of the presented newly ceramic total knee system.

Sufficient primary stability of the acetabular cup is essential for stable osseous integration of the implant after total hip arthroplasty. By means of under-reaming the cavities press-fit cups gain their primary stability in the acetabular bone stock. These metal-backed cups are inserted intra-operatively using an impact hammer.

The aim of this experimental study was to obtain the forces exerted by the hammer both in-vivo and in-vitro as well as to determine the resulting primary stability of the cups in-vitro.

Two different artificial bone models were applied to simulate osteoporotic and sclerotic bone. Polymeth-acrylamid (PMI, ROHACELL 110 IG, Gaugler & Lutz, Germany) was used as an osteoporotic bone substitute, whereas a composite model made of a PMI-Block and a 4 mm thick (cortical) Polyvinyl chloride (PVC) layer (AIREX C70.200, Gaugler & Lutz, Germany) was deployed to simulate sclerotic bone. In all artificial bone blocks cavities were reamed for a press-fit cup (Trident PSL, Size 56mm, Stryker, USA) using the original surgical instrument. The impactor of the cup was equipped with a piezoelectric ring sensor (PCB Piezotronics, Germany). Using the standard surgical hammer (1.2kg) the acetabular cups were implanted into the bone substitute material by a male (95kg) and a female (75kg) surgeon. Subsequently, primary stability of the implant (n=5) was determined in a pull-out test setup using a universal testing machine (Z050, Ziwck/Roell, Germany).

For validation the impaction forces were recorded intra-operatively using the identical press-fit cup design.

An average impaction force of 4.5±0.6kN and 6.3±0.4kN using the PMI and the composite bone models respectively were achieved by the female surgeon in vitro.

7.4±1.5kN and 7.7±0.8kN respectively were obtained by the male surgeon who reached an average in-vivo impaction force of 7.5±1.6kN.

Using the PMI-model a pull-out force of 298±72N and 201±112N were determined for the female and male surgeons respectively. However, using the composite bone model approximately half the pull-out force was measured for the female surgeon (402±39N) compared to the male surgeon (869±208N).

Our results show that impact forces measured in-vitro correspond to the data recorded in-vivo. Using the osteoporotic bone model the pull-out test revealed that too high impaction forces affect the pull-out force negatively and hence the primary implant stability is reduced, whereas higher impact forces improve primary stability considerably in the sclerotic bone model. In conclusion, the amount of impaction force contributes to the quality of the obtained primary cup stability substantially and should be adjusted intra-operatively according to the bone quality of each individual patient.

Introduction: Total knee replacement has become a common procedure with good clinical results. Today many different designs of the femoral component of bicondylar endoprostheses are offered by industry. The femoral components show similar designs however different angles and length of the cross sections are specific. Because of these design differences the preoperative planning and sparing bone resection are difficult at the revision surgery. The aim of this experimental study was to compare the design of femoral components at their cross section contours to find congruence and differences of common bicondylar endoprostheses to prove the possibility of design exchange during revision surgery.

Material and method: Ten femoral components (e.motion^®, Genesis II, Genia^®, Innex^®, LCS^®, Multigen Plus, NexGen^®, P.F.C.^®, Scorpio^®, Vanguard^®) of similar implant size were analysed with regard to their cross section design. Therefore the constructional properties of the inner surface (direction and length of cross sections) of the components were determined. The components were scanned with a three-dimensional laser scanner and were transferred to two dimensional CAD models to the lateral and frontal view in order to compare the inner contours. The contours of the cross sections were overlaid with congruence of the posterior and anterior cross section of all components at lateral view.

Results: Four of the ten analysed femoral components showed good congruence of the cross sections. Here, only a few additional bone resections or extra bone cement have to be done at the diagonal cross sections to change the femoral design among each other. Four other components show wide differences between the inner contours in comparison to the first four components especially at their posterior and diagonal cross sections. Two components can not be compared with the others due to their diagonal distal cross section.

Discussion: The numerical results shows good congruence of cross section contours of some analysed femoral components. Furthermore there were clear design differences which complicate the exchange of the femoral component at revision surgery. The use of an elementary inner contour of femoral components of bicondylar endoprostheses could be an advantage for revision arthroplasty in regard to bone sparing surgical treatment.

Introduction: To obtain secondary implant stability of acetabular press-fit cups, sufficient primary stability is essential. The aim of this study was to investigate the influence of cup insertion force and bone quality on the primary implant stability.

Materials and Methods: The experiments were carried out using two commercially available press-fit acetabular cups (Trident PSL, Stryker und EP-FIT PLUS, PLUS Ortho-peadics), comparable in design and with identical diameters, which were inserted axially into artificial bone by a female and a male surgeon. Two bone substitute material models were used. To imitate osteoporotic bone, a PMI-model (ROHACELL 110 IG, Gaugler & Lutz oHG) was employed. To simulate sclerotic bone, a composite-model made of a PMI-bloc with a 4 mm thick PVC-layer (AIREX C70.200, Gaugler & Lutz oHG) was used. The cups were inserted using an insertion device, equipped with a force sensor, and an 1100 g surgical hammer. Additionally, all experiments were carried out using a dynamic testing machine (25 kN, Instron) utilising insertion forces of 4.0 kN and 8.0 kN respectively. Primary implant stability was determined via lever-out tests using a static universal testing machine (Z050, Zwick/Roell).

Results: On average an insertion force of 4.8 kN (female) and 7.0 kN (male) using the PMI-model and 6.2 kN (female) and 7.5 kN (male) for the composite-model was assessed for the two different surgeons. The machined forces averaged 3.8 kN and 7.9 kN.

Lever-out-moments of 17 Nm were determined for both the PMI- and composite-model for the female surgeon using the PSL cup, whereas 27 Nm and 70 Nm, respectively, were reached for the EP-FIT shell.

For the male surgeon using the PSL cup, lever-out moments of 15 Nm and 30 Nm for the PMI- and composite-model respectively were determined. Insertion of the EP-FIT cup resulted in lever-out moments of 10 Nm using the PMI-model and 82 Nm using the composite-model.

The low machined insertion force led to average lever-out moments of 34 Nm for the PSL and 71 Nm for the EP-FIT cups using the composite-model. For the high machined force, the highest lever-out moments of 44 Nm and 99 Nm for the PSL and EP-FIT shells respectively were determined.

Conclusion: Using the composite-model (sclerotic bone), higher insertion forces lead to higher lever-out moments and hence higher primary implant stability for both tested cups. However, a high, non axial applied force can result in loss of stability using the PMI-model (osteoprotic bone). Compared to the manually inserted acetabular cups, the machined insertion resulted in higher primary stability for both implants and artificial bone types.

For orthopaedic implants the adhesive strength of bone cells on implant surfaces is of high interest. In some cases the adherence of cells is desirable, e.g. on endoprosthetic implants, in others, mainly temporarily used implants, e.g. intramedullary nails, it is not favourable for the cells to attach to the implant. Therefore, besides cell spreading and proliferation on surfaces the adhesion strength with which cells bond to the substrate is of high interest. There are different approaches to determine bone cell adhesion, but no easy to operate quantitative methods are available. For this purpose, based on the spinning disc principle, we have developed a new adhesion device in conjunction with an inverse confocal laser scanning microscope (LSM).

Polished disc-shaped test samples made of Ti6Al4V were seeded with bone cells (MG-63), stained with a fluorescent dye, at defined radial positions and were incubated for 18 h with cell medium. After incubation the test samples were placed into the adhesion chamber filled with 250 ml cell medium (DMEM). The test samples were rotated at various velocities until a minimum detachment of 50% was achieved. Using the LSM the detachment of the bone cells at the defined radial positions was determined and the cell count was recorded before and after rotation by means of imaging software.

An average shear stress of 50 N/m2 was determined for polished Ti6Al4V surfaces. To calculate the adhesion force, the cross-sectional cell area has to be measured by the xz-scan of the LSM.

Our results are reproducible and comparable to the data found in literature. The advantage of our new approach is that the same cells can be observed before and after rotation as well as different rotational speeds can be applied to the same cell population. Further investigations e.g. using different surfaces are carried out.

Introduction: The failure of total hip endoprosthesis is usually caused by aseptic implant loosening which can be a result of inflammatory reactions of the periprosthetic tissue on released metallic and bone cement wear particles. The objective of the study was to analyse the abrasive interfacial wear behaviour of cemented stems depending on the composition of the bone cement. Material and methods: With a test device cemented anatomical hip stems with different surface topography and material composition were investigated. Following bone cements were used: high viscosity PMMA cement with ZrO2 (Palacos R), high viscosity PMMA cement with BaSO4 (CMW 2000) as radiopaque material, low viscosity PMMA cement with ZrO2 (Sulcem 3) and an experimental high viscosity cement without ZrO2.

Results and Discussion: The abrasive wear behaviour in the interface between the implant and the bone cement is clearly affected by the surface topography of the stem. Moreover, the composition of the bone cement had a substantial impact on the abrasive wear behaviour in the interface. The tests revealed that the commercial bone cements with ZrO2 particles caused a higher polishing effect on rough stems and increased release of metallic particles. The Ti6Al7Nb and Co28Cr6Mo stems, which were tested against the bone cement without ZrO2 and the bone cement with BaSO4, showed no surface damage in the macroscopic analysis, whereas in the SEM analysis abrasive wear on the stem surface could be detected. However, in case of the Palocos R cement the added ZrO2 particles led to an increased wear resistance of the cement mantle and therefore to a reduced release of cement wear particles compared to the other cements tested. Whether this result is based on a ball bearing effect of the hard ZrO2 particles, which may reduce the friction in the interface, or a reinforcement of the bone cement matrix, is still unclear. If the use of high viscosity bone cements with ZrO2 particles (Palacos R) leading to a reduced release of cement particles may compensate the increased accumulation of abraded metallic particles, has to be examined by subsequent cellbiological studies, whereas in clinical studies the Palacos R cement showed superior survial rate of cemented hip stems. The low-viscosity cement tested seems to be less wear resistant than the high-viscosity cements. However, the quantity of metallic particles released has still to be analysed with atomic absorption spectrometry.

Introduction: An insufficient range of motion (ROM) can lead to prosthetic impingement causing dislocation of a total hip replacement. The objective of this study was to analyze the influence of the wear coupling on ROM and dislocation stability.

Material and Methods: By means of an experimental test device, a total hip system (Alloclassic) with four different insert materials, standard ultra-high-molecular-weight-polyethylene (UHMW-PE), highly cross-linked-polyethylene (XL-PE), aluminium-oxide-ceramic and cobalt-chromium, was investigated concerning ROM and stability against dislocation. The tests were carried out under dry conditions as well as after lubrication of the articulating surfaces with fetal calf serum. In a supplementary test procedure, the force vector-induced dislocation, i.e. dislocation without previous prosthetic impingement, was analyzed.

Results: No significant differences in the ROM until impingement(ROMImp)weredeterminedbetweenthe UHMW-PE and XL-PE inserts. The overall ROMImp of ceramic and metal inserts was approximately 5° less than with PE because no plastic deformation of the rim surface occurred. There was no significant difference in the maximum resisting moment prior to dislocation between the metal-on-polyethylene couples, whereas ceramic-on-ceramic showed the lowest moments and metal-on-metal the highest. Generally, slightly decreased moments for subluxation were determined after lubrication of the sliding surfaces for all couples. In a proper cup position (45° inclination and 15° ante-version) varying the wear coupling had a minor impact on the ROM until dislocation (ROMLux). However, in a poor implant position, ceramic-on-ceramic revealed a clear decrease in the ROMLux of approximately 40° after lubrication of the articulating surfaces. In general, metal-on-metal provided the highest ROMLux. The force vector-induced dislocation provided similar results for the different wear couples.

Conclusion: The study underlines the importance of optimized implant orientation and the impact of the wear couple used on ROM and dislocation stability. Recurrent impingement with subsequent release of wear particles has to be considered for all wear couples. However, ceramic-on-ceramic couples should be used in optimal implant position to avoid impingement and dislocation.

Aims:Malpositioning of implant components plays a signiþcant role in instability of THR. Our aim was the determination of the inßuence of anteversion of the acetabular cup. Methods: The biomechanical study was performed on a model which enables different deþned implant positions. Rotation of the femoral stem are carried out in different anteversion positions of the acetabular cup with the hip joint in neutral position and in 90¡ ßexion, as well as inclination of the cup. The range of motion (ROM) is determined until impingement or dislocation is evident, as well as the recording of the resisting moment. Results: While the resisting moment shows minor deviation in several anteversion (AV) positions with the joint in neutral position, being almost independent from the inclination, a major difference is determined with minimal resisting moment with minor retroversion (RV) and with the hip joint in 90¡ ßexion (0,51 Nm/15¡ RV vs. 3,69 Nm/30¡ AV). Dislocation occurs very early due to low ROM at retroversion. Variation of inclination of 30¡ can only increase ROM until dislocation by 2,8¡. With same inclinations angles ROM is increased by 38,1¡ in the 30¡ AV. Conclusions: Correct anteversion of the acetabular component is a signiþcant factor in prevention of dislocation. Minor differences in anteversion are more important than inclination variation to improve stability of THR.

Limitations of the range of motion (ROM) of total hip prostheses lead to impingement causing dislocation and material failure. Due to wear, the femoral head penetrates polyethylene (PE)-sockets by about 0,1 mm/year (ceramic on PE) and 0,2–0,6 mm/year (metal on PE). Wear rate increases with steep acetabular cup position. In contrast to polyethylene, wear of alumina-ceramic cups appears to be independent from inclination angle and is only about 0,001 mm/year. Wear and design features may restrict the artificial joint mobility. The purpose of this study is to determine the effects of head penetration on ROM in relation to different cup positions.

Computer simulation was carried out with a three-dimensional CAD-program. 3-D models of modular cup, spherical head, and stem with cylindrical neck and 12/14 taper were generated. The femoral head was shifted 0, 1, 2, and 5 mm towards the pole of the cup. According to mean direction of penetration measured in retrieved PE-sockets, femoral head was also moved 0, 1, 2, and 5 mm in vertical direction. The joint motions were measured at different cup positions.

The study demonstrates that ROM is clearly reduced by increasing head penetration. After 2 mm penetration, e.g. maximum flexion is reduced by approx. 15° at 45° cup inclination. Restriction of flexion is more pronounced in the vertical penetration path. If the socket is placed in more horizontal position, less ROM of flexion, extension and abduction is observed. With steeper cup positions ROM of flexion increases but, as well as risk of dislocation, wear and penetration rate of PE sockets increase.

Modern hip prostheses should provide sufficient joint movements, precise implant positioning and low wear bearing couples avoiding penetration of femoral head. Additionally, design aspects like liner geometry, head-neck ratio have to be considered preventing impingement, dislocation or early failure by aseptic loosening.