The aim of this study was to develop an in vitro GAG-depleted patella model and assess the biomechanical effects following treatment with a SAP:CS self-assembling hydrogel. Porcine patellae (4–6 month old) were harvested and subject to 0.1% (w/v) sodium dodecyl sulfate (SDS) washes to remove GAGs from the cartilage. Patellae were GAG depleted and then treated by injection with SAP (∼ 6 mM) and CS (10 mg) in Ringer's solution through a 30G needle. Native, GAG depleted and SAP:CS treated patellae were tested through static indentation testing, using 15g load, 5mm indenter over 1hr period. The degree of deformation of each group was assessed and compared (Mann-Whitney, p<0.05). Native, GAG depleted, sham (saline only) and SAP:CS treated paired patellae and femurs were additionally characterized tribologically through sequential wear testing when undergoing a walking gait profile (n=6 per group). The cartilage surfaces were assessed and compared (Mann-Whitney, p<0.05) using the ICRS scoring system, surface damage was illustrated through the application of Indian ink.Abstract
Objectives
Methods
The aim of this work was to develop a novel, accessible and low-cost method, which is sufficient to measure changes in meniscal position in a whole-knee joint model performing dynamic motion in a knee simulator. An optical tracking method using motion markers, MATLAB (MATLAB, The MathWorks Inc.) and a miniature camera system (Raspberry Pi, UK) was developed. Method feasibility was assessed on porcine whole joint knee samples (n = 4) dissected and cemented to be used in the simulator (1). Markers were placed on three regions (medial, posterior, anterior) of the medial meniscus with corresponding reference markers on the tibial plateau, so the relative meniscal position could be calculated. The Leeds high kinematics gait profile scaled to the parameters of a pig (1, 2) was driven in displacement control at 0.5 Hz. Videos were recorded at cycle-3 and cycle-50. Conditions tested were the capsule retained (intact), capsule removed and a medial posterior root tear. Mean relative displacement values were taken at time-points relating to the peaks of the axial force and flexion-extension gait inputs, as well as the range between the maximum and minimum values. A one-way ANOVA followed by Tukey post hoc analysis were used to assess differences (p = 0.05). The method was able to measure relative meniscal displacement for all three meniscal regions. The medial region showed the greatest difference between the conditions. A significant increase (p < 0.05) for the root tear condition was found at 0.28s and 0.90s (axial load peaks) during cycle-3. Mean relative displacement for the root tear condition decreased by 0.29 mm between cycle-3 and cycle-50 at the 0.28s time-point. No statistically significant differences were found when ranges were compared at cycle-3 and cycle-50. The method was sensitive to measure a substantial difference in medial-lateral relative displacement between an intact and a torn state. Meniscus extrusion was detected for the root tear condition throughout test duration. Further work will progress onto human specimens and apply an intervention condition.
The clinical success of osteochondral autografts is heavily reliant on their mechanical stability, as grafts which protrude above or subside below the native cartilage can have a negative effect on the tribological properties of the joint [1]. Furthermore, high insertion forces have previously been shown to reduce chondrocyte viability [2]. Commercial grafting kits may include a dilation tool to increase the diameter of the recipient site prior to insertion. The aim of this study was to evaluate the influence of dilation on the primary stability of autografts. Six human cadaveric femurs were studied. For each femur, four 8.5 × 8mm autografts were harvested from the trochlear groove and implanted into the femoral condyles using a Smith & Nephew Osteochondral grafting kit. Two grafts were implanted into dilated recipient sites (n=12) and two were implanted with no dilation (n=12). Insertion force was measured by partially inserting the graft and applying a load at a rate of 1 mm/min, until the graft was flush with the surrounding cartilage. Push-in force was measured by applying the same load, until the graft had subsided 4mm below congruency. Significance was taken as (p<0.05). Average maximum insertion force of dilated grafts was significantly lower (p<0.001) than their non-dilated equivalent [28.2N & 176.7N respectively]. There was no significant difference between average maximum push-in force between the dilated and non-dilated groups [1062.8N & 1204.2N respectively]. This study demonstrated that significantly less force is required to insert dilated autografts, potentially minimising loss of chondrocyte viability. However, once inserted, the force required to displace the grafts below congruency remained similar, indicating a similar degree of graft stability between both groups.
PEEK-OPTIMA™ has been considered as an alternative bearing material to cobalt chrome in the femoral component of total knee replacements. To better understand the tribology of UHMWPE-on-PEEK-OPTIMA™ and to find the most appropriate environmental conditions under which to test this novel bearing material combination, a series of tests under different protein lubricant concentrations at rig (∼24°C) and elevated temperature (∼35°C) were carried out in simple geometry wear and friction rigs. Under all conditions, the wear of UHMWPE-on-PEEK-OPTIMA™ was compared to UHMWPE-on-cobalt chrome (CoCr). The pins used were GUR1020 UHMWPE (conventional, non-sterile) and the plate material was either polished CoCr (Ra<0.01µm) or PEEK-OPTIMA (Ra∼0.03µm) provided by Invibio Ltd, UK. The wear simulation was carried out in a six station reciprocating rig. The kinematic conditions were consistent for all tests and reflected the average cross shear and contact pressure (3.2MPa) in a total knee replacement. Tests were carried out at either rig running temperature (∼24°C) or at elevated temperature (∼35°C) and in varying protein lubricant concentrations (0, 2, 5, 25 and 90%). Wear of the UHMWPE pins was determined by gravimetric analysis. The pin-on-plate friction rig study was carried out at rig temperature in 0, 2, 5, 25 and 90% serum and reflected the contact pressure used in the wear tests. Measurements were taken using a piezoelectric sensor and the steady state friction derived. At least 3 repeats were taken for each study, statistical analysis carried out using ANOVA with significance taken at p<0.05.Introduction
Methods
Variation in soft tissue constraints influence the kinematics and wear of total knee replacements (TKRs). The aim of this study was to experimentally investigate the effect of variation in the soft tissue constraints on the output kinematics of a fixed bearing TKR with different insert geometries. The kinematics have been shown to affect the wear rate of TKRs; increased output displacements may result in an increased wear rate. The soft tissue constraints were simulated experimentally using virtual springs. A new generation six station electromechanical ProSim knee simulator was used with the ISO 14243–1:2009 standard force control inputs; axial force, flexion-extension (FE), tibial rotation (TR) torque and anterior-posterior (AP) force. This allowed the kinematics to vary due to the test conditions. The ISO standard spring tensions of 44N/mm and 0.36Nm/° and gaps of 2.5mm and 6° were used for the AP and TR springs respectively. Different combinations of the input profiles were run in order to test the effect of their absence. The spring gaps were varied between 0mm–3mm and 0°–6° and the tensions between 0N/mm–250N/mm and 0Nm/°–1Nm/° for the AP and TR respectively. Three tibial insert designs were tested; high conformity curved (CVD), partially lipped (PLI) and flat. DePuy PFC Sigma fixed bearing components were tested in 25% bovine serum (in 0.04% sodium azide) lubricant. For each test 100 cycles were recorded on each station and then averaged. The CVD insert was used for all tests, the PLI insert was also used to test the effect of spring tension. The TR and AP output displacement profiles were affected by the FE position along with the TR torque and AP force respectively. The absence of these inputs changed the shape of the output profiles significantly. The spring gaps affected the peak AP and TR displacements (6.4mm to 3.7mm and 8° to 5.8° for maximum and zero spring gaps respectively). The spring tensions had a higher effect on the peak AP than TR position due to the design of the CVD insert restricting the TR movement (8.3mm to 3.7mm and 8.8° to 7.4° for no springs and maximum tension respectively). The spring gaps and tensions affected the amplitudes of the output profiles not their shape. The lower conformity inserts had a higher peak TR position (23° for the flat and 8.1° for the CVD insert) which occurred earlier in the cycle. The flat insert resulted in more anterior displacement, potentially due to the high conformity on the anterior side of the CVD and PLI inserts. The spring tension test had an increased effect on the PLI than the CVD insert. The PLI insert resulted in a higher change in displacements due to the spring tensions (10.4mm to 3.5mm and 13.6° to 8.8°). Soft tissue constraints and insert design had a significant effect on the kinematic outputs. Spring tensions and gaps for experimental testing should be chosen to reflect those of a specific patient group.
Experimental wear simulation of an all-polymer knee implant has shown an equivalent rate of wear of UHMWPE tibial components against PEEK-OPTIMA™ and cobalt chrome femoral components of a similar initial geometry and surface topography. However, when the patella is resurfaced with an UHMWPE patella button, it is important to also ascertain the wear of the patella. Wear debris from the patella contributes to the total volume of wear debris produced by the implant which should be minimised to reduce the potential for osteolysis and subsequent implant loosening. The aim of this study was to investigate the wear of the patellofemoral joint in an all-polymer knee implant. The wear of UHMWPE patellae articulating against PEEK-OPTIMA™ femoral components was compared to UHMWPE articulating against cobalt chrome femoral components. Six mid-size (size C) PEEK-OPTIMA™ femoral components (Invibio Knee Ltd., UK) and six cobalt chrome femoral components of similar initial surface topography and geometry were coupled with 28mm all-polyethylene GUR1020 patellae (conventional, EO sterile) (Maxx Orthopaedics, USA). The implants were set up in a ProSim 6 station electromechanical knee simulator (Simulation Solutions, UK) which was modified for testing the patellofemoral joint. 3 million cycles (MC) of wear simulation was carried out under kinematics aiming to replicate a gait cycle adapted for an electromechanical simulator from previous work by Maiti et al. The simulator used has six degrees of freedom of which four were controlled; axial force up to 1200N, flexion/extension 22°, superior-inferior (SI) displacement (22mm) and Abduction-adduction (AA) (4°). The SI and AA were displacement controlled and driven through the patella. The medial-lateral displacement and tilt (internal/external rotation) of the patella were passive so the patella button was free to track the trochlear groove. The lubricant used was 25% bovine serum supplemented with 0.03% sodium azide to retard bacterial growth. The wear of patellae was determined gravimetrically with unloaded soak controls used to compensate for the uptake of moisture by the UHMWPE. The mean wear rate ± 95% confidence limits were calculated and statistical analysis was carried out using ANOVA with significance taken at p<0.05.Introduction
Materials and Methods
There are many surgical, implant design and patient factors that should be considered in preclinical testing of hip replacement which are not being considered in current standards. The aim of this study was to develop a preclinical testing method that consider surgical positioning, implant design and patient factors and predict the occurrence and severity of edge loading under the combination of such conditions. Then, assess the safety and reliability of the implant by predicting the wear, deformation and damage of the implant bearings under worst case conditions. Ceramic-on-ceramic (CoC, 36mm, BIOLOX® delta, Pinnacle®, DePuy Synthes, UK) and metal-on polyethylene (MoP, 36mm, Marathon®, Pinnacle®, DePuy Synthes, UK) bearings were used for this study on multi-station multi-axis hip joint simulators. Two factors were varied, cup inclination angles (45° and 65°) and translational mismatch between the femoral head and acetabular cup (0, 2, 3 and 4 (mm)). Under each condition for both CoC and MoP bearings, three million cycles of gait cycle testing were completed with wear, deformation and/or damage measurements completed at one million cycle intervals. Other outputs of the study were the level of dynamic separation between the femoral head and acetabular cup during gait, the maximum force at the rim during edge loading when the head was sliding back to the cup confinement. Means and 95% confidence limits were determined and statistical analysis were done using one way ANOVA with significance taken at p<0.05.Introduction and Aims
Methods
Clinically many factors such as variations in surgical positioning, and patients' anatomy and biomechanics can affect the occurrence and severity of edge loading which may have detrimental effect on the wear and durability of the implant. Assessing wear of hundreds of combinations of conditions would be impractical, so a preclinical testing approach was followed where the occurrence and severity of edge loading can be determined using short biomechanical tests. Then, selected conditions can be chosen under which the wear can be determined. If a wear correlation with the magnitude of dynamic separation or the severity of edge loading can be shown, then an informed decision can be made based upon the biomechanical results to only select important variables under which the tribological performance of the implant can be assessed. The aim of this study was to determine the relationship between the wear of ceramic-on-ceramic bearings and the (1) magnitude of dynamic separation, (2) the maximum force reached during edge loading and (3) the severity of edge loading resulting from component translational mismatch between the head and cup centres. The Leeds II hip joint simulator with a standard walking cycle and 36mm diameter ceramic-on-ceramic bearings (BIOLOX® delta, DePuy Synthes Joint Reconstruction, Leeds, UK.) were used. The study was in two parts. Part one: a biomechanical study where the dynamic separation, the maximum load during edge loading, and the duration of edge loading alongside the magnitude of forces under edge loading (severity of edge loading) were assessed. Part two; a wear study where the wear rates of the bearing surfaces were assessed under a series of input conditions. These input testing conditions included inclining the acetabular cups at 45° and 65° cup inclination angle (in-vivo equivalent), with 2, 3, and 4mm medial-lateral component mismatch between the centres of the head and the cup. This equated to six conditions being assessed, each with three repeats for the biomechanical test, and six repeats completed for the wear study. The severity of edge loading was assessed as described in Equation 1. Severity of Edge Loading = ∫tt0 F(x) dx + ∫tt0 F(y) dy … Equation 1, where F(x) is the axial load, F(y) is the medial-lateral load and t-t0 is the duration of edge loading. The wear of the ceramic bearings were determined using gravimetric analysis (XP205, Mettler Toledo, UK).Introduction and Aims
Methods
There are many variables that can affect the occurrence and severity of edge loading in hip replacement. A translational mismatch between the centres of rotation of the head and cup may lead to dynamic separation, causing edge loading and increased wear. Combining a steep inclination angle with such translational mismatch in the medial-lateral axis caused a larger magnitude of separation and increased severity of edge loading. Previous studies have shown variation in the hip Swing Phase Load (SPL) during gait between different patients. The aim of this study was to apply a translational mismatch and determine the effect of varying the SPL on the occurrence and severity of edge loading under different cup inclination angles in a hip joint simulator. The Leeds II hip joint simulator with a standard gait cycle and 36mm diameter ceramic-on-ceramic bearings (BIOLOX® delta) were used in this study. The study was in two stages; [1] a biomechanical study where the magnitude of dynamic separation, the duration of edge loading and the magnitude of force under edge loading (severity) were assessed under variations in component positioning and SPLs. [2] A wear study to assess edge loading with selected input conditions. For the biomechanical study, a combination of four mismatches, three cup inclination angles, and eight SPLs (Table 1) were investigated. For the wear study, three SPL conditions were selected with one cup angle and one mismatch (Table 1). Three million cycles were completed under each condition. Mean wear rates and 95% confidence limits were determined and statistical analysis (one way ANOVA) completed (significance taken at p<0.05). Table 1: Study matrixIntroduction and Aims
Methods
Variations in component position can lead to dynamic separation and edge loading conditions. In vitro methods have been developed to simulate edge loading conditions and replicate stripe wear, increased wear rate, and bimodal wear debris size distribution, as observed clinically [1, 2]. The aim of this study was to determine the effects of translational and rotational positioning on the occurrence of dynamic separation and severity of edge loading, and then investigate the wear rates under the most severe separation and edge loading conditions on an electromechanical hip joint simulator. A hip joint simulator (ProSim EM13, Simulation Solutions, UK) was set up with 36mm diameter ceramic-on-ceramic (BIOLOX®delta, PINNACLE®, DePuy Synthes, UK) hip replacements. Three axes of rotation conditions (ISO 14242-1 [3]) was applied to the femoral head. This study was in two parts. I) A biomechanical test was carried out at 45° (n=3) and 65° (n=3) cup inclination angles with 1, 2, 3 and 4 (mm) medial-lateral translational mismatch between the centres of the head and cup. The amount of dynamic separation displacement between the head and cup was measured using a position sensor. The severity of edge loading was determined from the area under the axial force and medial-lateral force outputs during the time of separation [4]. II) A wear test was carried out at 45° (n=6) and 65° (n=6) cup inclination angles for three million cycles with translational mismatch of 4mm between the head and cup. The lubricant used was diluted new-born calf serum (25% v/v). Volumetric wear measurements were undertaken at one million cycle intervals and mean wear rates were calculated with 95% confidence limits. Statistical analysis was carried out using ANOVA and a t-test with significance levels taken at p<0.05.Introduction
Materials and Methods
Calcium sulfate bone void fillers (CS-BVF) are increasingly being used for dead space management in infected arthroplasty revision surgery. The use of loose beads of CS-BVF close to the articulating surfaces of an implant means there is potential for them to migrate between the articulating surfaces acting as a third body particle. The aim of this study was to investigate the influence of CS-BVF on the third body wear of total knee replacements. The influence of CS-BVF on wear was investigated using the commercially available CS-BVF ‘Stimulan’ (Biocomposites Ltd., UK) and posterior stabilised U2 total knee replacement system implants (United Orthopaedic Corp., Taiwan). The experimental wear simulation was performed using a six station ProSim electropneumatic knee simulator (Simulation Solutions, UK) running the Leeds intermediate kinematics input profile [1]. To investigate the damage that could be caused by the third body particles, 5cc of CS-BVF beads (excess) were placed on the tibial component of the implant, the simulator was run dry for 60 cycles before adding lubricant (25% bovine serum supplemented with 0.03% sodium azide) and running for an additional 115,000 cycles representative of the 6–8 weeks the CS-BVF are present in the body prior to their resorption. The surface topography of the cobalt chrome femorals was analysed using contacting profilometry to ascertain whether the third body particles of CS-BVF had damaged the surfaces. To investigate the influence of CS-BVF on the third body wear of the UHMWPE tibials, 3 million cycles (MC) of wear simulation was subsequently carried out. The wear of the UHMWPE tibials was assessed gravimetrically and the wear of implants tested with CS-BVF was compared to the wear against negative controls (initial Ra∼0.02µm) and positive controls (initial Ra ∼0.4µm) damaged with a diamond stylus. N=6 was completed for each condition, statistical analysis was carried out using ANOVA with significance taken at p<0.05.Introduction
Methods
The input mechanical properties of knee replacement bearing materials, such as elastic modulus and Poisson's ratio, significantly contribute to the accuracy of computational models. They should therefore be determined from independent experimental studies, under similar test conditions to the clinical and experimental conditions, to provide reliability to the models. In most cases, the reported values in the literature for the elastic modulus and Poisson's ratio of the bearing materials have been measured under tensile test conditions, in contrast to the compressive operating conditions of the total knee replacements (TKR). This study experimentally determined the elastic modulus and Poisson's ratio of conventional and moderately cross-linked ultra-high molecular weight polyethylene (UHMWPE) under compressive test conditions. These material parameters will be inputs to future computational models of TKR. To determine the Poisson's ratio of the conventional and moderately cross-linked UHMWPE, contact areas of 12mm diameter cylindrical specimens of 10.2mm length were measured experimentally under a compressive displacement of 1mm, at a strain rate of 12mm/min that was held for 10minutes. A computational model was developed in Abaqus, 6.14–1, to simulate this experimental test assuming different values for the Poisson's ratio of the UHMWPE cylindrical specimens. The curve fitted relationship between the computationally predicted contact area and Poisson's ratio was used to calculate the Poisson's ratio of the UHMWPE specimens, using the experimentally measured contact areas. Using a similar approach, the equivalent elastic modulus of the UHMWPE was calculated using the computationally calculated curve fitted contact area-elastic modulus relationship, from the computational simulation of a ball-on-flat compression test, and the experimentally measured contact area from a ball-on-flat dynamic compression test. This experiment used 10mm thick UHMWPE flat specimens against a 63.5mm rigid ball, under a compressive dynamic sinusoidal loading of 250N average load, and 6000 cycles. The applied test conditions maintained the stress level within the reported range for the TKR.Introduction
Materials/Methods
Surface wear of polyethylene is still considered a long-term risk factor for clinical success, particularly as life expectancy and activity levels increase. Computational models have been used extensively for preclinical wear prediction and optimization of total knee replacements (TKR). In most cases, the input wear parameters (wear factors and coefficients) to the computational models have been experimentally measured under average contact stresses to simulate standard activities. These wear studies are not therefore applicable for more adverse conditions that could lead to edge loading and high stress conditions, including higher levels of activities and severe loading conditions. The current study investigated the multidirectional pin-on-plate wear performance of moderately cross-linked ultra-high molecular weight polyethylene (UHMWPE) under high applied nominal contact stress, to be used as inputs to a computational model investigating adverse high stress conditions. Moderately cross-linked UHMWPE (GUR_1020,5Mrad gamma irradiation) pins were tested against cobalt–chrome alloy (CoCr) plates in a multidirectional pin-on-plate wear simulator. The CoCr metallic plates were polished to an average surface roughness of 0.01μm. The pin rotation and the plate reciprocation of ±30º and 28mm were in phase, having a common frequency of 1Hz, and resulted in a multidirectional motion at the pin-plate contact surface in a flat-on-flat configuration. Six different pin diameter and applied load combinations were tested, resulting in applied nominal contact stresses from 4 to 80[MPa](Fig.1). Each set was run for 1million cycles in 25% bovine serum as a lubricant. The volumetric wear was calculated from the weight loss measurements using a density 0.93mg/mm3 for the UHMWPE material. The wear factor and wear coefficient were calculated as (volumetric wear/(load x sliding distance)) and (volumetric wear/(contact area x sliding distance)) respectively[1]. Statistical analysis of the data was performed in ANOVA and significance was taken at p<0.05.Introduction
Materials/Methods
Previous studies have shown that third body damage to the femoral head in metal-on-polyethylene hip replacement bearings can lead to accelerated wear of the polyethylene liners. The resulting damage patterns observed on retrieved metal heads are typically scratches and scrapes. The damage created The aim of this study was to determine the wear rates of polyethylene liners articulating against retrievals and artificially damaged metal heads for the purpose of validating a computational wear prediction model; and to develop and validate an Twenty nine, 32mm diameter, metal-on-moderately cross-linked polyethylene bearings (MarathonTM) inserted into Ti-6Al-4V shells (Pinnacle®) were tested in this study. All products were manufactured by DePuy Synthes, Warsaw, Indiana, USA. Following a retrieval study seven different damage patterns were defined, and these were applied to the femoral heads using a four-degree-of-freedom CNC milling machine (Figure 1). The ProSim 10-station pneumatic hip joint simulator (Simulation Solutions, UK) was used for experimental wear simulation using standard gait cycles and testing each experimental group for 3 million cycles. The acetabular cups were inclined at 35° on the simulator (equivalent to 45° Introduction
Materials and Methods
Edge loading of hip replacements may result in plastic deformation, creep and wear at the rim of the cup and potentially fatigue failure. Variations in component positioning can lead to dynamic separation and edge loading [1]. The aim of this study was firstly to investigate the effects of translational and rotational positioning on the dynamic separation and severity of edge loading, and secondly to determine the wear rates of metal-on-polyethylene bearings under the more severe separation and edge loading conditions. A hip joint simulator (ProSim EM13, Simulation Solutions, UK) was set up with 36mm diameter metal-on-polyethylene hip replacements (Marathon™, DePuy Synthes Joint Reconstruction, Leeds, UK). This study was in two parts. I) A biomechanical test was carried out at 45° (n=3) and 65° (n=3) cup inclination angles with 1, 2, 3 and 4 (mm) medial-lateral translational mismatch between the head and cup centres. The severity of edge loading was calculated from the area under the axial force and medial-lateral force outputs during the time of separation when the load was acting on the edge of the cup [2]. II) For two conditions (two million cycles), the head and cup were concentric for cups inclined equivalent clinically to 45° (n=3) and 65° (n=3). For two further conditions (three million cycles), 4mm medial-lateral translational mismatch between centres was applied for cups inclined equivalent clinically to 45° (n=6) and 65° (n=6). Volumetric wear measurements were undertaken at one million cycle intervals. The lubricant was diluted new-born calf serum (25% v/v). Plastic deformation and wear were determined using a coordinate measurement machine. Mean values were calculated with 95% confidence limits. Statistical analysis was carried out using ANOVA and a t-test with significance levels taken at p<0.05.Introduction
Materials and Methods
Edge loading due to dynamic separation can occur due to variations in component positioning such as a steep cup inclination angle (rotational) or mismatch between the centres of rotation of the head and the cup (translational). The aim of this study was to determine the effect of variations in rotational and translational positioning of the cup on the magnitude of dynamic separation, wear and deformation of metal-on-polyethylene bearings. Eighteen 36mm diameter metal-on-polyethylene hip replacements were tested on an electromechanical hip simulator. Standard gait with concentric head and cup centres were applied for cups inclined at 45° (n=3) and 65° (n=3) for two million cycles. A further two tests with translational mismatch of 4mm applied between the head and cup bearing centres for cups inclined at 45° (n=6) and 65° (n=6) were run for three million cycles. Wear was determined using a microbalance and deformation by geometric analysis. Confidence intervals of 95% were calculated for mean values, and t-tests and ANOVA were used for statistical analysis (p<0.05). Under 4mm mismatch conditions, a steeper cup inclination angle of 65° resulted in larger dynamic separation (2.1±0.5mm) compared with cups inclined at 45° (0.9±0.2mm). This resulted in larger penetration at the rim under 65° (0.28±0.04mm) compared to 45° (0.10±0.09mm) cup inclination conditions (p<0.01). Wear rates under standard concentric conditions were 12.8±3.8 mm3/million cycles and 15.4±5.0 mm3/million cycles for cups inclined at 45° and 65° respectively. Higher wear rates were observed under 4mm of translational mismatch compared with standard concentric conditions at 45° (21.5±5.5 mm3/million cycles, p<0.01) and 65° (23.0±5.7 mm3/million cycles, p<0.01) cup inclination. Edge loading under dynamic separation conditions due to translational mismatch resulted in increased wear and deformation of the polyethylene liner. Minimising the occurrence and severity of edge loading through optimal component positioning may reduce the clinical failure rates of polyethylene.
Edge loading due to dynamic separation can occur due to variations in component positioning such as a steep cup inclination angle (rotational) or mismatch between the centres of rotation of the head and the cup (translational). The aim of this study was to determine the effect of variations in rotational and translational positioning of the cup on the magnitude of dynamic separation, wear and deformation of metal-on-polyethylene bearings. Eighteen 36mm diameter metal-on-polyethylene hip replacements were tested on an electromechanical hip simulator. Standard gait with concentric head and cup centres were applied for cups inclined at 45° (n=3) and 65° (n=3) for two million cycles. A further two tests with translational mismatch of 4mm applied between the head and cup bearing centres for cups inclined at 45° (n=6) and 65° (n=6) were run for three million cycles. Wear was determined using a microbalance and deformation by geometric analysis. Confidence intervals of 95% were calculated for mean values, and t-tests and ANOVA were used for statistical analysis (p<0.05). Under 4mm mismatch conditions, a steeper cup inclination angle of 65° resulted in larger dynamic separation (2.1±0.5mm) compared with cups inclined at 45° (0.9±0.2mm). This resulted in larger penetration at the rim under 65° (0.28±0.04mm) compared to 45° (0.10±0.05mm) cup inclination conditions (p<0.01). Wear rates under standard concentric conditions were 12.8±3.8 mm3/million cycles and 15.4±5.0 mm3/million cycles for cups inclined at 45° and 65° respectively. Higher wear rates were observed under 4mm of translational mismatch compared with standard concentric conditions at 45° (21.5±5.5 mm3/million cycles, p<0.01) and 65° (23.0±5.7 mm3/million cycles, p<0.01) cup inclination. Edge loading under dynamic separation conditions due to translational mismatch resulted in increased wear and deformation of the polyethylene liner. Minimising the occurrence and severity of edge loading through optimal component positioning may reduce the clinical failure rates of polyethylene.
There is a demand for longer lasting arthroplasty implants driving the investigation of novel material combinations. PEEK has shown promise as an arthroplasty bearing material, with potentially relatively bio inert wear debris [1]. When coupled with an all-polyethylene tibial component this combination shows potential as a metal-free knee. In this study, the suitability of PEEK Optima® as an alternative to cobalt chrome for the femoral component of total knee replacements was assessed using experimental knee wear simulation under two kinematic conditions. Three cobalt chrome and three injection moulded PEEK Optima® (Invibio Biomaterial Solutions, UK) femoral components of similar geometry and surface roughness (mean surface roughness (Ra) ∼0.02µm) were coupled with all-polyethylene GUR1020 (conventional, unsterilised) tibial components in a 6 station ProSim knee simulator (Simulation Solutions, UK). 3 million cycles (MC) of wear simulation were carried out under intermediate kinematics (maximum anterior-posterior (AP) displacement 5mm) followed by 3MC under high kinematics (AP 10mm) [2] with 25% serum as the lubricant. The wear of the tibial component was assessed gravimetrically. At each measurement point, the surface roughness of the femoral components was determined using contacting profilometry and throughout testing, the bulk lubricant temperature was monitored close to the articulating surfaces. Statistical analysis was carried out using ANOVA, with significance at p<0.05.Introduction
Methods
To meet the demands of younger more active patients more robust pre-clinical wear testing methods are required, in order to simulate a wider range of activities. A new electromechanical simulator (Simulation Solutions, UK) with a greater range of motion, a driven abduction/adduction axis and improved input kinematic following has been developed to meet these requirements, as well as requirements of the relevant international standards. This study investigated the wear of a fixed bearing total knee replacement using this new electromechanical knee simulator, comparing with previous data from a pneumatic simulator. The wear of six Sigma CR fixed bearing TKRs (DePuy, UK) with curved moderately cross-linked polyethylene inserts (XLK) was determined in pneumatic and electromechanical Prosim knee simulators (Simulation Solutions, UK). Standard gait displacement controlled kinematics were used, with a maximum anterior-posterior displacement of either 10mm (high) or 5mm (intermediate) [1]. The output profiles from the simulators were obtained and compared to the demand input profiles. The lubricant used was 25% new-born calf serum and wear determined gravimetrically. Statistical analysis was performed using the one-way ANOVA with 95% confidence interval and significance was taken at p<0.05.Introduction
Materials/Methods
Translational surgical mismatch in the centres of rotation of the femoral head and acetabular cup in hip joint replacements can lead to dynamic microseparation resulting in edge loading contact [1]. Increased wear in retrieved ceramic-on-ceramic bearings has been associated with edge loading [2]. Hip joint simulators were used to replicate increased wear rate, stripe wear and bimodal wear debris size distribution, as seen clinically [3,4]. Recently developed electromechanical simulators are able to comply with the latest international standards, which include three axes of rotation conditions [5]. Previous simulators had applied two axes of rotation under microseparation conditions [6]. Therefore, the aim of this study was to compare the wear of ceramic-on-ceramic bearings obtained under edge loading due to microseparation conditions during gait using the same electromechanical hip joint simulator with two axes of rotation and three axes of rotation conditions. A six-station electromechanical hip joint simulator (ProSim EM13, Simulation Solutions, UK) was set up with 36mm diameter ceramic-on-ceramic (BIOLOX® delta, PINNACLE®, DePuy Synthes, UK) hip replacements. The wear was determined for two million cycles under standard conditions with two axes of rotation conditions (n=6), two million cycles under microseparation conditions with two axes of rotation conditions (n=6) (Figure 1a), and two million cycles under microseparation conditions with three axes of rotation conditions (n=6) (Figure 1b). The loading profiles [5,7] comprised of 3kN twin peak loads and 300N swing phase load under standard conditions. The swing phase load was reduced to approximately 70N under microseparation conditions. Approximately 0.5mm of dynamic microseparation between the head and the cup was applied in the medial/lateral direction. The components were lubricated with 25% new-born calf serum supplemented with 0.03% sodium azide to minimise bacterial growth. The gravimetric wear rates were compared over two million cycles for each test (XP205, Mettler Toledo, UK). The mean wear rates of the head and cup were calculated with 95% confidence limits and statistical analysis was carried out (t-test) with significance levels taken at p<0.05. A coordinate-measurement machine (Legex 322, Mitutoyo, UK) was used to construct a three-dimensional map of the femoral head surface wear.Introduction
Materials and Methods
In order to improve the longevity and design of an implant, a wide range of pre-clinical testing conditions should be considered including variations in surgical delivery, and patients' anatomy and biomechanics. The aim of this research study was to determine the effect of the acetabular cup inclination angle with different levels of joint centre mismatch on the magnitude of dynamic microseparation, occurrence and severity of edge loading and the resultant wear rates in a hip joint simulator. The six-station Leeds Mark II Anatomical Physiological Hip Joint Simulator and 36mm diameter ceramic-on-ceramic bearings (BIOLOX® delta) were used in this study. A standard gait cycle, with a twin-peak loading (2.5kN peak load and approximately 70N swing phase load), extension/flexion 15°/+30° and internal/external ±10° rotations, was applied. Translational mismatch in the medial-lateral axis between the centres of rotation of the head and the cup were considered. In this study, mismatches of 2, 3 and 4 (mm) were applied. Two acetabular cup inclination angles were investigated; equivalent to 45° and 65° in-vivo. These resulted in a total of six conditions [Figure 1] with n=6 for each condition. Three million cycles were completed under each condition. The lubricant used was 25% (v/v) new-born calf serum supplemented with 0.03% (w/v) sodium azide to retard bacterial growth. The wear of the ceramic bearings were determined using a microbalance (XP205, Mettler Toledo, UK) and a coordinate measuring machine (Legex 322, Mitutoyo, UK). The stripe wear was analysed using RedLux software. The dynamic microseparation displacement was measured using a linear variable differential transformer. Mean wear rates and 95% confidence limits were determined and statistical analysis (one way ANOVA) completed with significance taken at p<0.05. Results Increasing the medial-lateral joint centre mismatch from 2 to 3 to 4mm resulted in an increased dynamic microseparation [Figure 2]. A similar trend was observed for the wear. A higher level of medial-lateral mismatch increased the wear rate under both 45° and 65° cup inclination angle conditions [Figure 3]. The mean wear rates obtained under 65° were significantly higher compared to those obtained under the 45° cup inclination angle conditions for a given medial-lateral mismatch in the joint centre (p=0.02 for 2mm mismatch, p=0.02 for 3 mm mismatch, and p<0.01 for 4mm mismatch).Introduction and Aims
Methods
Ceramic composites have been developed to further improve the mechanical properties, reduce risk of fracture, and increase the survivorship of ceramic-on-ceramic bearings in total hip replacement1. The aim of this study was to evaluate the wear of two novel ceramic composite materials under edge loading conditions due to translational mal-positioning when used in both like-on-like and mixed pairing configurations; and to compare their performance to earlier generation ceramic-on-ceramic bearings. The head-on-cup configurations of three ceramic materials (see Figure 1), were ATZ-on-ATZ, ZTA-on-ZTA, Al2O3-on-Al2O3, ATZ-on-ZTA, ZTA-on-ATZ, Al2O3-on-ATZ, ATZ-on-Al2O3and Al2O3-on-ZTA. All combinations were size 28mm and were supplied by Mathys Orthopädie GmbH (Morsdorf, Germany). They were tested for four million cycles on the Leeds II hip simulator under microseparation2,3,4 conditions representing translational mal-positioning. The gait cycle comprised extension/flexion (−15º/+30º), internal external rotation (+/−10º) and a twin peak load with a maximum of 3kN. Microseparation was achieved by applying a 0.5mm dynamic medial/lateral displacement using a spring load resulting in edge loading at heel strike. New-born calf serum (25%) was used as a lubricant. Wear was assessed gravimetrically every million cycles. Statistical analysis was performed using one-way ANOVA (significance taken at p<0.05).Introduction
Materials and Methods
When third body particles originating from bone cement or bone void fillers become trapped between articulating surfaces of joint replacements, contact surfaces may be damaged leading to accelerated wear and premature failure of the implant. In this study, the damage to cobalt chrome counterfaces by third body particles from PMMA bone cement (GMV, DePuy) and various bone void fillers was investigated; then wear tests of UHMWPE were carried out against these surfaces. Third body particles of polymerised GMV bone cement and the bone void fillers; OsteoSet (with tobramycin), Stimulan and Stimulan+ (with vancomycin and tobramycin) (provided by Biocomposites Ltd.) were trapped between an UHMWPE pin and a highly polished cobalt chrome plate. A load of 120N was applied to the pin and using an Instron materials testing machine, the plate was pulled beneath the pin to recreate third body damage [1]. The resulting surface topography of the plate was analysed using white light interferometry (Bruker NPFLEX). Pin on plate wear tests of GUR 1020 UHMWPE pins were carried out against the plates perpendicular to the direction of damage for 500,000 cycles in 25% bovine serum using a 6-station multi-axial reciprocating rig under conditions to replicate the kinematics in total knee replacement. Wear of the pins was determined by gravimetric analysis and results were compared to negative (highly polished) control plates and positive controls scratched with a diamond stylus (lip height 2µm). Statistical analysis was carried out using one-way ANOVA with significance taken at p<0.05.Introduction
Methods
UHMWPE articulating against PEEK-OPTIMA® has the potential for use as a novel bearing couple in joint arthroplasty due to its potentially low wear rates and the bioinertness of its wear debris. The aim of this study was to investigate the role of protein in the lubricant on the wear of UHMWPE articulating against PEEK at both room and physiological temperature. The wear of GUR1020 UHMWPE pins articulating against PEEK plates (Ra ∼0.06µm) was compared to highly polished cobalt chrome plates (Ra <0.01µm) in a 6-station multi-axial pin-on-plate rig using kinematics to replicate those in total knee arthroplasty. Tests were carried out at either ∼20°C or ∼36°C and wear was investigated under varying concentrations of bovine serum (0, 25 or 90%). Studies were carried out for 1 Million cycles with wear of the UHMWPE pins assessed gravimetrically using unloaded soak controls to compensate for moisture uptake. Statistical analysis was carried out using ANOVA with significance taken at p<0.05.Introduction
Methods
Increased wear rates [1, 2] and acetabular rim fracture [3] of hip replacement bearings reported clinically have been associated with edge loading, which could occur due to rotational and/or translational mal-positioning [4]. Surgical mal-positioning can lead to dynamic microseparation mechanisms resulting in edge loading conditions. In vitro microseparation conditions have replicated stripe wear and the bi-modal wear debris distribution observed clinically [5, 6]. The aim of this study was to investigate the effect of steep cup inclination, representing rotational mal-positioning, on the magnitude of dynamic microseparation, severity of edge loading, and the resulting wear rate of a ceramic-on-ceramic bearing, under surgical translational mal-positioning conditions. Ceramic-on-ceramic bearings where the ceramic liner was inserted into a titanium alloy cup (BIOLOX® delta and Pinnacle® respectively, DePuy Synthes, UK) were tested on the six-station Leeds II hip simulator. The first test was run with the cups inclined at an angle equivalent, clinically, to 45° (n=6) and the second test was run with the cups inclined at an angle equivalent, clinically, to 65° (n=6). A standard gait cycle was run. A fixed surgical translational mal-positioning of 4mm between the centres of rotations of the head and the cup in the medial/lateral axis was applied on all stations. Both tests ran for three million cycles each. The lubricant used was 25% new-born calf serum. Wear was assessed gravimetrically using a microbalance (XP205, Mettler Toledo, UK) and geometrically using a coordinate measuring machine (CMM, Legex 322, Mitutoyo, UK). Statistical analysis was done using one way ANOVA with significance taken at p<0.05.Introduction
Materials and Methods
Wear debris induced osteolysis and loosening continue to cause clinical failure in total knee replacement (TKR). To improve longevity and reduce wear alternative materials have been examined. Carbon-fibre-reinforced poly–ether-ether-ketone (CFR-PEEK) has shown promising results in wear studies [1–2]. The aim of this study was to explore the use of CFR-PEEK and PEEK as alternative bearing materials for polyethylene in TKR through experimental knee joint wear simulation. Two novel materials were studied as an alternative to polyethylene as the tibial bearing surface in a TKR configuration using a Cobalt chrome femoral bearing in current clinical use. Six right Sigma CR fixed bearing TKRs (DePuy Synthes, UK) were paired with either PEEK or CFR-PEEK custom-made flat inserts (Invibio, UK) in a Prosim knee simulator (Simulator Solutions, UK). The tibial inserts were 14mm thick, to give an equivalent thickness to existing insert designs. A flat geometry was selected as this has previously been shown to yield low wear in polyethylene bearings [3]. The tests were conducted under High Kinematics, with anterior-posterior and internal-external displacement control [4]. Tests were conducted for three million cycles, lubricated with 25% bovine serum, with wear assessed gravimetrically at 1 and 3Mc. Images of the wear scars were recorded at completion of the study. Visual inspection of the tibial inserts was used to identify regions of wear damage for SEM analysis (EVO MA15-Smart SEM, Zeiss, Germany)Introduction
Methods
Wear of total knee replacement (TKR) is a clinical concern. This study demonstrated low-conformity moderately cross-linked-polyethylene fixed bearing TKRs showed lower volumetric wear than conventional-polyethylene curved fixed bearing TKRs highlighting potential improvement in TKR performance through design and material selection. Wear of total knee replacement (TKR) continues to be a significant factor in the clinical performance of the implants. Historically, failure due to delamination and fatigue directed implant design towards more conforming implants to reduce contact stress. However, the new generations of more oxidatively-stable polyethylene have improved the long-term mechanical properties of the material, and therefore allowed more flexibility in the bearing design. The purpose of this study was to investigate the effect of insert conformity and material on the wear performance of a fixed bearing total knee replacement through experimental simulation.Summary Statement
Introduction
Backside wear has been previously reported through The present study investigated the contribution of backside wear to the total wear in the PFC Sigma rotating platform mobile bearing TKR. In addition, the wear results were compared to the computed wear rates of the PFC Sigma fixed bearing TKR, with two different bearing materials. The commercially available PFC Sigma rotating platform mobile bearing and PFC Sigma fixed bearing total knee replacements, size 3 (DePuy, UK) were tested, with either conventional or moderately cross-linked (5 MRad) GUR1020 UHMWPE bearing materials. The computational wear model for the knee implants was based on the contact area and an independent experimentally determined non-dimensional wear coefficient [1,2,3]. The experimental wear test for the mobile bearing was force controlled using the ISO anterior-posterior force (ISO14243-1-2009). However, due to time limitation of the explicit simulation required to run the force controlled model, the simulation was run using the AP displacements taken from the experimental knee simulator which was run under the ISO AP force. The Sigma fixed bearing TKR was run under high level of anterior-posterior displacements (maximum of 10 mm).Introduction:
Materials and Methods:
Stripe wear, observed on retrieved ceramic hip replacements, has only been replicated The aim of this study was to compare the edge loading mechanisms observed under microseparation conditions due to translational mal-positioning conditions simulated on two different hip joint simulators. The components used in this study were zirconia-toughened-alumina ceramic-on-ceramic bearings (36 mm) inserted into titanium alloy acetabular cups (BIOLOX® delta and Pinnacle® respectively, DePuy Synthes Joint Reconstruction, Leeds, UK). Six couples were tested for two million cycles under 0.5 mm dynamic microseparation conditions on the Leeds II hip joint simulator as described by Nevelos et al2 and Stewart et al3 (Figure 1). Ten bearing couples were tested for two million cycles under microseparation conditions achieved in two different ways on the ProSim pneumatic hip joint simulator (SimSol, Stockport, UK). Two conditions were tested; condition (1)- the femoral head was left to completely separate (the vertical motion was controlled at 1 mm) causing it to contact the inferior rim of the acetabular cup before edge loading on the superior rim at heel strike (n = 5) and condition (2)- springs were placed below the plate holding the femoral head to control the tilt of the head laterally towards the rim of the acetabular cup as the negative pressure was applied (n = 5; Figure 1). Wear was assessed gravimetrically every million cycles using a microbalance (Mettler AT201, UK). Three-dimensional reconstructions of the wear area on the heads were obtained using a coordinate measuring machine (Legex 322, Mitutoyo, UK) and SR3D software (Tribosol, UK).Introduction
Materials and Methods
Wear debris induced osteolysis and loosening continue to be causes of clinical failure in total knee replacement (TKR). Laboratory simulation aims to predict the wear of TKR bearings under specific loading and motion conditions. However, the conditions applied may have significant influence on the study outcomes (1) The aim of this study was to examine the influence of femoral setup and kinematic inputs on the wear of a conventional polyethylene fixed bearing TKR through experimental and computational models. Six right Sigma CR fixed bearing TKRs (DePuy Synthes, Leeds, UK) with curved polyethylene inserts (GVF, GUR1020 UHMWPE) were tested in Prosim knee simulator (Simulator Solutions, UK). The femoral bearing was set up with the centre of rotation (CoR) on either on the distal radius of the implant (Distal CoR), as indicated by the device design, or according to the ISO specification (ISO CoR; ISO14243-3). The tests were conducted under ‘High Kinematics’ (2). It was necessary to reverse the direction of the anterior-posterior displacement for the tests conducted with the ISO centre of rotation to maintain the contact region within the insert surface (Reverse High Kinematics). Tests were conducted for three million cycles, lubricated with 25% bovine serum, with wear assessed gravimetrically. The computational wear model for the TKR was based on the contact area and an independent experimentally determined non-dimensional wear coefficient, previously validated against the experimental data (3).Introduction
Methods
Patella femoral joint bearings in total knee replacements have shown low wear (3.1 mm3/MC) under standard gait simulator conditions1. However, the wear in retrieval studies have shown large variations between 1.3 to 45.2 mm3/year2. Previous in vitro studies on the tibial femoral joint have shown wear is dependent on design, materials and kinematics3. The aim of this study was to investigate the influence of the design (geometry) and shape on the wear rate of patella femoral joints in total knee replacements. The Leeds/Prosim knee simulator was used to investigate the wear of two types of commercially available patellae. The PFC Sigma cobalt chrome femoral component was coupled with 2 types of patellae buttons: round and oval dome. The UHMWPE was the same for the both types – GUR1020 GVF (gamma irradiated in vacuum and foiled packed). 25% bovine serum was used as the lubricant. The test were carried out at three conditions – high medial lateral (ML) rotations (<4°) and uncontrolled ML displacement (<4 mm), low ML rotation (<1°) and uncontrolled ML displacement (<4 mm); the physiological gait cycle; and low ML rotation and controlled ML displacement (<1.5 mm). In this abstract the two designs were tested in physiological gait condition (Figure 1). Patella ML displacement and tilt were passively controlled and measured after every 300,000 cycles. A ligament resisting force equivalent to 10 N4 was applied on the lateral side of the patella to avoid patella slip. Five samples of each design were tested for 3 million cycles at a cycle rate of 1 Hz. The wear volume was obtained gravimetrically every million cycles and presented with 95% confidence limits. Statistical significance was taken at p<0.05.Introduction
Materials and Methods
Wear of polyethylene continues to be a significant factor in the longevity of total knee replacement (TKR). Moderately cross-linked polyethylene has been employed to reduce the wear of knee prostheses, and more recently anti-oxidants have been introduced to improve the long-term stability of the polyethylene material. This is the initial study of the wear of a new anti-oxidant polyethylene and a new TKR design, which has modified femoral condylar geometry. The wear of a new TKR the Attune knee was investigated using a physiological six station Prosim knee wear simulator (Simulator Solutions, UK). Six mid-size Attune fixed bearing cruciate retaining TKRs (DePuy Inc, Warsaw, USA) were tested for a period of 6 million cycles. The inserts were manufactured from AOX™, a compression moulded GUR1020 polyethylene incorporating Covernox™ solid anti-oxidant. The AOX polymer was irradiated to 8M Rad, to give a moderately cross-linked material. High and intermediate kinematics, under anterior-posterior displacement control were used for this study (McEwen Introduction
Materials and Methods
Ceramic-on-ceramic hip replacements have generated great interest in recent years due to substantial improvements in manufacturing techniques and material properties1. Microseparation conditions that could occur due to several clinical factors such as head offset deficiency, medialised cup combined with laxity of soft tissue resulting in a translation malalignment, have been shown to cause edge loading, replicate clinically relevant wear mechanisms2,3 and increase the wear of ceramic-on-ceramic bearings3,4. The aim of this study was to investigate the influence of increasing the femoral head size on the wear of ceramic-on-ceramic bearings under several clinically relevant simulator conditions. The wear of size 28mm and 36mm ceramic-on-ceramic bearings (BIOLOX® INTRODUCTION
MATERIALS AND METHODS
Retrieval and clinical studies of metal-on-metal (MoM) bearings have associated increased wear1 and elevated patient ion levels2 with steep cup inclination angles and edge loading conditions. The University of Leeds have previously developed a hip simulator method that has been validated against retrievals and shown to replicate clinically relevant wear rates and wear mechanisms3,4. This method involves introducing lateral microseparation to represent adverse joint laxity and offset deficiency. This study aimed to investigate the effect of microseparation representing translational malpostion, and increased cup inclination angle, representing rotational malposition, in isolation and combined on the wear of different sizes (28 and 36mm) MoM bearing in total hip replacement (THRs). The wear of size 28mm and 36mm MoM THRs bearings was determined under different INTRODUCTION
MATERIALS AND METHODS
In vitro the introduction of microseparation and edge loading to hip simulator gait cycle has replicated clinically relevant wear rates and wear mechanisms in ceramic-on-ceramic bearings[1], and elevated the wear rates of MoM surface replacements (SR) to levels similar to those observed in retrievals[2]. The aim was to assess the wear of two different sized MoM total hip replacement bearings under steep cup inclination angles and adverse microseparation and edge loading conditions. Two tests were performed on the Leeds II hip joint simulator using two different size bearings (28mm and 36mm). Cups were mounted to provide inclination angles of 45 degrees (n=3) and 65 degrees (n=3). The first three million cycles were under standard gait conditions. Microseparation and edge loading conditions as described by Nevelos et al[1] were introduced to the gait cycle for the subsequent three million cycles. The lubricant was 25% new born calf serum. The mean wear rates and 95% confidence limits were determined and statistical analysis was performed using One Way ANOVA. Under standard gait conditions, when the cup inclination angle increased from 45 degrees to 65 degrees, the wear of size 28mm bearing significantly (p=0.004) increased by 2.7-fold, however, the larger bearings did not show any increase in wear (p=0.9). The introduction of microseparation conditions resulted in a significant (p=0.0001) increase in wear rates for both bearing sizes under both cup inclination angle conditions. Under microseparation conditions, the increase in cup inclination angle had no influence on the wear rate for both bearing sizes (Figure 1). With larger bearings, head-rim contact occurs at a steeper cup inclination angle providing an advantage over smaller bearings. The introduction of edge loading and microseparation conditions resulted in a significant increase in wear rates for both bearing sizes. The wear rates obtained in this study under combined increased cup inclination angle and microseparation were half of those obtained when SR MoM bearings were tested under similar adverse conditions[2]. This study shows the importance of prosthesis design and accurate surgical positioning of the head and acetabular cup in MoM THRs.
