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.