Introduction. Edge loading is a common wear mechanism in Metal-on-Metal (MOM) hip resurfacing and is associated with higher wear rates and the incidence of pseudotumour. The purpose of this study was to develop a method to investigate the contributions of patient, surgical and implant design variables on the risk of edge loading. Method. We created a mathematical model to calculate the distance from the head-cup contact patch to the rim of the cup and used this to investigate the effect of component position, specific design features and patient activity on the risk of edge loading. We then used this method to calculate the contact patch to rim distance (CPRD) for 160 patients having undergone revision of their MOM hip resurfacing in order to identify any possible associations. Results. We identified several variables that reduce the CPRD and increase the risk of edge loading, including; increased cup anteversion and inclination, activities involving increased hip flexion, reduced clearance, reduced hip diameter and reduced cup arc angle. We also determined the threshold cup orientation for edge loading for five resurfacing designs (Figure 1). In patients with failed MOM hip resurfacings, there was a significant correlation between CPRD and both component wear rates (Figure 2) and blood metal ion levels (all p < 0.005). The ASR was associated with increased wear, reduced CPRD, and increased prevalence of edge loading (all p < 0.05). Conclusions. Edge loading is common and difficult to avoid in MOM hip resurfacing. Whilst some designs, such as the ASR, are more susceptible to edge loading, all are unforgiving of suboptimal cup position. Furthermore, common activities involving flexion of the hip result in edge loading even in patients with a well-positioned cup

Introduction and Aims. Clinically many factors such as variations in surgical positioning, and patients' anatomy and biomechanics can affect the occurrence and severity of edge loading which may have detrimental effect on the wear and durability of the implant. Assessing wear of hundreds of combinations of conditions would be impractical, so a preclinical testing approach was followed where the occurrence and severity of edge loading can be determined using short biomechanical tests. Then, selected conditions can be chosen under which the wear can be determined. If a wear correlation with the magnitude of dynamic separation or the severity of edge loading can be shown, then an informed decision can be made based upon the biomechanical results to only select important variables under which the tribological performance of the implant can be assessed. The aim of this study was to determine the relationship between the wear of ceramic-on-ceramic bearings and the (1) magnitude of dynamic separation, (2) the maximum force reached during edge loading and (3) the severity of edge loading resulting from component translational mismatch between the head and cup centres. Methods. 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 = ∫. t. t0. F(x) dx + ∫. t. t0. 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). Results. The wear rates of ceramic-on-ceramic bearings increased as the magnitude of dynamic separation (Figure 1), the maximum load at the rim during edge loading (Figure 2), and the severity of edge loading (Figure 3) increased. The magnitude of dynamic separation was found to have the highest correlation to the wear rate under the conditions tested in this study (R. 2. =0.94). Conclusions. A preclinical testing approach has been developed to understand the occurrence and severity of edge loading associated with variation of component positioning. A good correlation was found between the wear rates obtained for ceramic-on-ceramic bearings and the magnitude of parameters obtained under edge loading during a short-term biomechanical study. For figures/tables, please contact authors directly.

Introduction. Variations in component position can lead to dynamic separation and edge loading conditions. In vitro methods have been developed to simulate edge loading conditions and replicate stripe wear, increased wear rate, and bimodal wear debris size distribution, as observed clinically [1, 2]. The aim of this study was to determine the effects of translational and rotational positioning on the occurrence of dynamic separation and severity of edge loading, and then investigate the wear rates under the most severe separation and edge loading conditions on an electromechanical hip joint simulator. Materials and Methods. 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. Results. Dynamic separation increased significantly with 3mm (p<0.01) and 4mm (p<0.01) translational mismatch at a 45° cup inclination angle (Figure 1). At 65° the separation increased significantly as the translational mismatch increased from 1mm to 4mm (p<0.01). The most severe edge loading conditions occurred at a 65° cup inclination angle with 4mm of translational mismatch (p<0.01, Figure 2). Mean wear rates were greater at a 65° cup inclination angle compared with a 45° cup inclination angle (p<0.01, Figure 3). Conclusion. Different levels of rotational and translational mismatch affected the separation between the head and cup during gait. Higher levels of translational mismatch and a steeper cup inclination angle may lead to more severe edge loading conditions and increased wear of ceramic-on-ceramic bearings in vivo. A new preclinical testing approach was developed to study the effects of edge loading due to variations in rotational and translational surgical positioning under ISO loading and angular displacement conditions. The first stage comprised of biomechanical tests to determine the occurrence and severity of edge loading in a range of component positions. The second stage investigated the tribological performance of the bearing surface under the worst case edge loading conditions

Introduction. Edge loading of hip replacements may result in plastic deformation, creep and wear at the rim of the cup and potentially fatigue failure. Variations in component positioning can lead to dynamic separation and edge loading [1]. The aim of this study was firstly to investigate the effects of translational and rotational positioning on the dynamic separation and severity of edge loading, and secondly to determine the wear rates of metal-on-polyethylene bearings under the more severe separation and edge loading conditions. Materials and Methods. 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. Results. The largest dynamic separation measured was at a 65° cup inclination angle with 4mm translational mismatch (Figure 1). At 1mm and 2mm of translational mismatch there was insignificant or no edge loading due to dynamic separation. The most severe edge loading conditions occurred at 4mm of translational mismatch at 45° and 65° inclination angles (p<0.01, Figure 2). The wear rates under standard concentric conditions were 12.9±3.8 and 15.4±5.0 mm. 3. /million cycles for cups inclined at 45° and 65° respectively. Higher wear rates were observed under 4mm of translational mismatch at 45° (21.5±5.5mm. 3. /million cycles, p<0.01) and 65° (23.0±5.7mm. 3. /million cycles, p<0.01) cup inclination angles. The mean maximum penetration depth at the edge of the cup at three million cycles was 0.10±0.05mm and 0.28±0.04mm at 45° and 65° cup inclination angles respectively (p<0.01), indicating substantial plastic deformation due to edge loading (Figure 3). Conclusion. Surgical positioning is important for long term clinical success of hip joint implants. A method has been developed to study the effects of rotational and translational positioning of metal-on-polyethylene bearings. Severe edge loading increased the wear and deformation of polyethylene liners at the rim. Minimising the occurrence and severity of edge loading and reducing the dynamic separation in vivo may reduce revision rates associated with polyethylene bearings

Introduction and Aims. There are many variables that can affect the occurrence and severity of edge loading in hip replacement. A translational mismatch between the centres of rotation of the head and cup may lead to dynamic separation, causing edge loading and increased wear. Combining a steep inclination angle with such translational mismatch in the medial-lateral axis caused a larger magnitude of separation and increased severity of edge loading. Previous studies have shown variation in the hip Swing Phase Load (SPL) during gait between different patients. The aim of this study was to apply a translational mismatch and determine the effect of varying the SPL on the occurrence and severity of edge loading under different cup inclination angles in a hip joint simulator. Methods. 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 matrix. Results. For any given translational mismatch or cup inclination angle, increasing the SPL from 50N to 450N resulted in a decrease in the magnitude of dynamic separation (Figures 1 and 2). In some scenarios when the mismatch between the centres of rotation was low and the SPL was high, no separation was observed. Under 150N SPL, the severity of edge loading was similar to that determined for the 50N SPL conditions although the magnitude of dynamic separation was lower. Higher wear rates were found for the 70N and 150N compared to 300N SPL (Figure 3). No significant difference was found between wear rate under the SPLs of 70N and 150N (p=0.05), but significant differences were found between the wear rates under 150N and 300N SPL and between 70N and 300N SPL (p<0.01 and p<0.01 respectively). Conclusion. The SPL contributed to the resistance of separation between the head and the cup, hence a lower dynamic separation was measured under higher SPL. The wear study demonstrated that edge loading was present even under a higher SPL. For figures/tables, please contact authors directly.

Introduction and Aims. There are many surgical, implant design and patient factors that should be considered in preclinical testing of hip replacement which are not being considered in current standards. The aim of this study was to develop a preclinical testing method that consider surgical positioning, implant design and patient factors and predict the occurrence and severity of edge loading under the combination of such conditions. Then, assess the safety and reliability of the implant by predicting the wear, deformation and damage of the implant bearings under worst case conditions. Methods. 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. Results. As the level of mismatch and the cup inclination angle increased, the magnitude of dynamic separation and the force at the rim increased. The level of dynamic separation and the force on the rim correlated with the wear of CoC bearings (R= 0.96). For polyethylene, steeper inclination angle did not significantly increase the wear (p>0.05) however, edge loading under 4mm translational mismatch and steep cup inclination angle did (p<0.01). The combined wear and deformation of the polyethylene liners at the rim increased under larger levels of dynamic separation. Conclusions. The magnitude of dynamic separation and force at the rim were predictive of the severity of edge loading. These parameters can be measured using short term testing (500 cycles). This will determine the effect of variations in surgical positioning, implant design and patient factors on the occurrence and severity of edge loading. Then, the wear, deformation and/or damage on hip replacement bearings can be determined using longer term simulator testing under selected conditions. The short term tests do not only help identify worst case scenarios but may identify the boundary of surgical position under which the implants performance may be considered acceptable. A new approach for preclinical testing of hip replacement was developed:. Stage 1: Short biomechanical tests. : assess the occurrence and severity of edge loading conditions where the outputs are:. Magnitude of medial-lateral dynamic separation. Maximum force under edge loading. Stage 2: Wear assessment. : assess the tribological performance of hip replacement under selected conditions where the outputs are:. Wear rates. Deformation and/or damage on the bearing surface

Edge loading commonly occurs in all bearings in hip arthroplasty. The aim of this study compares metal bearings with edge loading to alumina bearings with edge loading and to metal bearings without edge loading. Seventeen failed large diameter metal-on-metal hip bearings (8 total hips, 9 resurfacings) were compared to 55 failed alumina-on-alumina bearings collected from 1998 to 2010. The surface topography of the femoral heads was measured using a chromatically encoded confocal measurement machine (Artificial Hip Profiler, RedLux Ltd.). The median time to revision for the metal hip bearings and the alumina hip bearings was 2.7 years. Forty-six out of 55 (84%) alumina bearings and 9 out 17 (53%) metal bearings had edge loading wear (p<0.01). The average volumetric wear rate for metal femoral heads was 7.87 mm3/yr (median 0.25 mm3/yr) and for alumina heads was 0.78 mm3/yr (median 0.18 mm3/yr) (p=0.02). The average volumetric wear rate for metal heads with edge loading was 16.51 mm3/yr (median 1.77 mm3/yr) and for metal heads without edge loading was 0.19 mm3/yr (median 0 mm3/yr) (p=0.1). There was a significant difference in gender, with a higher ratio of females in the alumina group than the metal group (p=0.02). Large diameter metal femoral heads with edge loading have a higher wear rate than smaller alumina heads with edge loading. Metal-on-metal bearings have low wear when edge loading does not occur

Introduction. Edge loading commonly occurs in all bearings in hip arthroplasty. Edge loading wear can occur in these bearings when the biomechanical loading axis reaches the edge and the femoral head loads the edge of the cup producing wear damage on both the head and cup edge. When the biomechanical loading axis passes through the polished articulating surface of the acetabular component and does not reach the edge, the center of the head and the center of the cup are concentric. The resulting wear known as concentric wear is low in metal-on-metal (MOM) bearings, and is negligible in ceramic-on-ceramic (COC) bearings. Edge loading is well defined in COC hip bearings. However, edge loading is difficult to identify in MOM bearings, since the metal bearing surfaces do not show wear patterns macroscopically. The aims of this study are to compare edge loading wear rates in COC and MOM bearings, and to relate edge loading to clinical complications. Materials and Methods. Twenty-nine failed large diameter metal-on-metal hip bearings (17 total hips, 12 resurfacings) were compared to 54 failed alumina-on-alumina bearings collected from 1998 to 2011. Most COC bearings were revised for aseptic loosening or periprosthetic bone fracture, while most MOM bearings were revised for pain, soft tissue reactions or impingement. The median time to revision was 3.2 years for the metal hip bearings and 3.5 years for alumina hip bearings. The surface topography of the femoral heads was measured using a RedLux AHP (Artificial Hip Profiler, RedLux Ltd, Southampton, UK). Results. Forty-five out of fifty-four bearings (83%) alumina bearings and 15 out 29 (52%) metal bearings had edge loading wear (p<0.01). There was no difference in the median volumetric wear rates, which were 0.25 mm. 3. /yr for metal femoral heads and 0.18 mm. 3. /yr for alumina heads (means 7.87 mm. 3. /yr and 0.78 mm. 3. /yr respectively). The median volumetric wear rate was 1.77 mm. 3. /yr (mean 16.51 mm. 3. /yr) for metal heads with edge loading and 0.01 mm. 3. /yr (mean 0.19 mm. 3. /yr) for metal heads without edge loading (p=0.1). Conclusions. The median wear rates for COC and MOM bearings were the same, however MOM bearings have the potential for much higher wear rates when edge loading occurs. Most of the reasons for revision of MOM bearings were related to a biological response to the wear debris. Therefore, it may be the reactivity of the wear debris, and not the wear rate that is an important determinant for the survivorship of MOM bearings

Introduction. Increased wear rates [1, 2] and acetabular rim fracture [3] of hip replacement bearings reported clinically have been associated with edge loading, which could occur due to rotational and/or translational mal-positioning [4]. Surgical mal-positioning can lead to dynamic microseparation mechanisms resulting in edge loading conditions. In vitro microseparation conditions have replicated stripe wear and the bi-modal wear debris distribution observed clinically [5, 6]. The aim of this study was to investigate the effect of steep cup inclination, representing rotational mal-positioning, on the magnitude of dynamic microseparation, severity of edge loading, and the resulting wear rate of a ceramic-on-ceramic bearing, under surgical translational mal-positioning conditions. Materials and Methods. 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. Results. The magnitude of dynamic microseparation was significantly (p<0.01) higher when the inclination angle of the cup was steeper (Figure 1) under the same level of translational mal-positioning of 4mm. This has resulted in significantly (p<0.01) higher wear rates of 1.01mm. 3. /million cycles for the steep cup inclination group of 65° compared to 0.32mm. 3. /million cycles for the 45° inclined cups group (Figure 2). Furthermore, the penetration on the femoral heads was significantly (p<0.01) higher for the steep cup inclination angle group with a mean (±95% confidence limit) penetration of 33±6µm under the 65° cup inclination angle condition and 15±3µm under the 45° cup inclination angle condition (Figure 3). Discussion and Conclusion. This study showed that cup inclination angle affects the magnitude of dynamic microseparation for a given surgical translational mal-position, thus leading to severe edge loading and increased wear rates with increased cup inclination angles. The occurrence and severity of the resulting edge loading causing increased wear in hip bearings will depend on the combinations of surgical variations, such as steep inclination angle, excessive version angle, medialised cups, head offset deficiencies, stem subsidence, and joint laxity. Future work will include studying the effect of these variables on the level of dynamic microseparation, severity of edge loading, the offset frictional torque and level of resulting wear

Introduction. Total ankle replacement (TAR) is less successful than other joint replacements with a 77% survivorship at 10 years. Predominant indications for revision include: Insert dislocation, soft tissue impingement and pain/stiffness. Insert edge-loading may be both a product and cause of these indications and was reported to affect 22% of patients with the, now withdrawn from market, Ankle Evolutive System (AES) TAR (Transysteme, Nimes, France). Compressive forces up to seven times body weight over a relatively small contact area (∼6.0 to 9.2 cm. 2. ), in combination with multi-directional motion potentially causes significant polyethylene wear and deformation in mobile-bearing TAR designs. Direct methods of measuring component volume (e.g. pycnometer) use Archimedes' principle but cannot identify spatial changes in volume or form indicative of wear/deformation. Quantitative methods for surface analysis bridge this limitation and may advance methods for analysing the edge loading phenomena in TAR. Aim. Determine the frequency of edge loading in a cohort of explanted total ankle replacements and compare the quantitative surface characteristics using a novel explant analysis method. Methods. Thirty-two AES TAR devices were implanted and retrieved by the same surgeon (UK Health Research Authority approval: 09/H1307/60). Mean implantation time was 7.8 years (1.5 to 12.1 range). Pain and/or loosening were the primary indications for revision. An Alicona Infinite microscope measured the entire superior surface of each insert (10× mag; 1.76µm lateral resolution). Abbott-Firestone curves were produced per insert to quantify the deviation of the insert surface from flat. Peak material volume (Vmp), core material volume (Vmc), core void volume (Vvc) and dale void volume (Vvv) were measured. Edge loading was identified visually by a depressed area in the insert surface indicative of articulation with the edge of the tibial component. Inserts were identified as either edge-loaded or not edge-loaded and the above analyses compared. Results. Seventeen inserts (53%) showed edge loading. Peak material volume (Vmp) was significantly increased for the edge loaded inserts 5.64 ± 5.42µm compared to the normal inserts 1.29 ± 0.954µm (Independent T-Test, P=0.005). No difference was found for the other volume parameters (Figure 2). A progressive change in insert form, beginning at the edges of the superior insert surface, was evident (Figure 1). Machining marks identified at the centre of several components supported this observation. Discussion. Insert edge loading affected 53% of TAR explants. The volume parameters showed a statistically significant inflection of material at the inserts' edge for the affected ankles. Spatial changes to insert form progressed over time in-vivo. Machining marks at the centre of several inserts remained which indicated the deformation/wear process commenced at the periphery of the insert. Normal ranges of volume change/redistribution are not established for TAR devices and the implications of insert form change are not yet understood. However, edge-loaded components composed over half of this cohort, which reflects the conflict between design simplicity and kinematic complexity. For any figures or tables, please contact authors directly (see Info & Metrics tab above).

Introduction:. A disturbing prevalence of painful inflammatory reactions has been reported in metal-on-metal (MoM) hip resurfacing arthroplasty. A contributing factor is localized loading of the acetabular shell leading to “edge wear” which is often seen after precise measurement of the bearing surfaces of retrieved components. Factors contributing to edge wear include adverse cup orientation leading to proximity (<10 mm) of the hip reaction force to the edge of the acetabular component. As this phenomenon is a function of implant positioning and patient posture, this study was performed to investigate the occurrence of edge loading during different functional activities as a function of cup inclination and version. Methods:. We developed a computer model of the hip joint through reconstruction of CT scans of a proto-typical pelvis and femur and virtually implanting a hip resurfacing prosthesis in an ideal position. Using this model, we examined the relationship between the resultant hip force vector and the edge of the acetabular shell during walking, stair ascent and descent, and getting in and out of a chair. Load data was derived from 5 THR patients implanted with instrumented hip prostheses (Bergmann et al). We calculated the distance from the edge of the shell to the point of intersection of the load vector and the bearing surface for cup orientations ranging from 40 to 70 degrees of inclination, and 0 to 40 degrees of anteversion. Results:. Previous studies have shown that wear of MOM bearings becomes significantly elevated once the load vector comes within 10 mm of the edge of the acetabular cup. Our simulations demonstrated that normal gait, stair climbing and stair descent do not generate edge loading unless the shell was oriented in 70° of inclination and 20° of anteversion. Conversely, edge loading was predicted during sit-to-stand and stand-to-sit activities for every orientation of the implanted components, including values within the “safe zone” (Figure 1). Cup anteversion was not a consistent predictor of edge loading during gait, stair climbing or stair descent, but did affect the distance to the edge of the cup in sit-to-stand and stand-to-sit activities. Conclusions:. We demonstrated that normal gait, stair-climbing and stair descent do not appear to explain the edge wear seen in many of the retrieved resurfacing components. Edge loading does occur during sit to stand and stand to sit activities in virtually any cup orientation and is postulated as the missing factor explaining component wear. In our work we have effectively demonstrated that, in the absence of other confounding factors, edge loading and pseudotumor formation can happen in even the “safe” acetabular orientations. We propose this as a new way to understand the forces upon the components following HRA

Introduction. A disturbing prevalence of short-term failures of metal-on-metal (MoM) hip resurfacings has been reported by joint registries. These cases have been primarily due to painful inflammatory reactions and, in extreme cases, formation of pseudotumors within periarticular soft-tissues. The likely cause is localized loading of the acetabular shell leading to “edge wear” which is often seen after precise measurement of the bearing surfaces of retrieved components. Factors contributing to edge wear of metal-on-metal arthroplasties are thought to include adverse cup orientation, patient posture, and the direction of hip loading. The purpose of this study was to investigate the role of different functional activities in edge loading of hip resurfacing prostheses as a function of cup inclination and version. Methods. We developed a computer model of the hip joint through reconstruction of CT scans of a proto-typical pelvis and femur and virtually implanting a hip resurfacing prosthesis in an ideal position. Using this model, we examined the relationship between the resultant hip force vector and the edge of the acetabular shell during walking, stair ascent and descent, and getting in and out of a chair. Load data was derived from 5 THR patients implanted with instrumented hip prostheses (Bergmann et al). We calculated the distance from the edge of the shell to the point of intersection of the load vector and the bearing surface for cup orientations ranging from 40 to 70 degrees of inclination, and 0 to 40 degrees of anteversion. Results. The low flexion activities of normal gait, stair climbing and stair descent did not demonstrate values consistent with edge loading unless the shell was oriented in 70° inclination and 20° version. Conversely, the occurrence of edge loading was predicted during sit to stand and stand to sit activities for every orientation of the implanted components (Figure 1). Cup anteversion was not a consistent predictor of edge loading during gait, stair climbing and stair descent; but did affect the distance to the edge of the cup in sit-to-stand and stand-to-sit activities. Conclusions. We demonstrated that normal gait, stair-climbing and stair descent do not appear to explain the edge wear seen in many of the retrieved resurfacing components. Edge loading does occur during sit to stand and stand to sit activities in virtually any cup orientation and is postulated as the missing factor explaining component wear. In our work we have effectively demonstrated that, in the absence of other confounding factors, edge loading and pseudotumor formation can happen in even the “safe” acetabular orientations. We propose this as a new way to understand the forces upon the components following HRA

Introduction. Robotically-assisted unicondylar knee arthroplasty (UKA) is intended to improve the precision with which the components are implanted, but the impact of alignment using this technique on subsequent polyethylene surface damage has not been determined. Therefore, we examined retrieved ultra-high-molecular-weight polyethylene UKA tibial inserts from patients who had either robotic-assisted UKA or UKA performed using conventional manual techniques and compared differences in polyethylene damage with differences in implant component alignment between the two groups. We aimed to answer the following questions: (1) Does robotic guidance improve UKA component position compared to manually implanted UKA? (2) Is polyethylene damage or edge loading less severe in patients who had robotically aligned UKA components? (3) Is polyethylene damage or edge loading less severe in patients with properly aligned UKA components?. Methods. We collected 13 medial compartment, non-conforming, fixed bearing, polyethylene tibial inserts that had been implanted using a passive robotic-arm system and 21 similarly designed medial inserts that had been manually implanted using a conventional surgical technique. Pre-revision radiographs were used to determine the coronal and sagittal alignment of the tibial components. Retrieval analysis of the tibial articular surfaces included damage mapping and 3D laser scanning to determine the extent of polyethylene damage and whether damage was consistent with edge loading of the surface by the opposing femoral component. Results. Though the individual planar alignments did not differ between the two groups, overall 69% of the 13 robotically aligned components were well-positioned in both the coronal and sagittal planes, as opposed to only 18% of the manually aligned tibial components (Fig.1). Robotically aligned inserts had significantly less pitting, burnishing, and deformation than manually aligned inserts, and the maximum surface deviations (wear and deformation) were significantly smaller, though these differences could be explained by a longer length of implantation for the manually aligned inserts. Interestingly, no difference was found in the incidence of edge loading between the robotically aligned and manually aligned groups. When comparing polyethylene damage on the basis of alignment rather than surgical technique, neither the polyethylene damage nor surface deviation was different, aside from more burnishing and deformation in mal-positioned components and greater deviation in components mal-positioned in the sagittal plane. Conclusions. Static radiographic alignment measurements were not useful in predicting the wear patterns that the tibial inserts experienced while implanted, suggesting that other factors, such as the patient's functional kinematics, influence the mechanical burden placed on the polyethylene articular surfaces (Fig. 2)

Ceramic-on-ceramic bearings provide a solution to the osteolysis seen with traditional metal-on-polyethylene bearings. Sporadic reports of ceramic breakage and squeaking concern some surgeons and this bearing combination can show in vivo signs of edge loading wear which was not predicted from in vitro studies. Taper damage or debris in the taper between the ceramic and metal may lead to breakage of either a ceramic head or insert. Fastidious surgical technique may help to minimise the risk of ceramic breakage. Squeaking is usually a benign complication, most frequently occurring when the hip is fully flexed. Rarely, it can occur with each step of walking when it can be sufficiently troublesome to require revision surgery. The etiology of squeaking is multifactorial origin. Taller, heavier and younger patients with higher activity levels are more prone to hips that squeak. Cup version and inclination are also relevant factors. Fifty-five ceramic bearings revised at our center were collected over 12 years. Median time to revision was 2.7 years. Forty-six (84%) cases had edge loading wear. The median femoral head wear volume overall was 0.2mm. 3. /yr, for anterosuperior edge loading was 2.0mm. 3. /yr, and the median volumetric wear rate for posterior edge loading was 0.15mm. 3. /yr (p=0.005). Osteolysis following metal-on-polyethylene total hip arthroplasty (THA) is well reported. Earlier generation ceramic-on-ceramic bearings did produce some osteolysis, but in flawed implants. As 3rd and now 4th generation ceramic THAs come into mid- and long-term service, the orthopaedic community has begun to see reports of high survival rates and very low incidence of osteolysis in these bearings. The technique used by radiologists for identifying the nature of lesions on Computed Tomography (CT) scan is the Hounsfield score which will identify the density of the tissue within the lucent area. Commonly the radiologist will have no access to previous imaging, especially pre-operative imaging if a long time has elapsed. With such a low incidence of osteolysis in this patient group, what, then, should a surgeon do on receiving a CT report on a ceramic-on-ceramic THA, which states there is osteolysis? This retrospective review aims to determine the accuracy of CT in identifying true osteolysis in a cohort of long-term 3rd generation ceramic-on-ceramic uncemented hip arthroplasties in our department. Methods. Pelvic CT scans were performed on the first 27 patients from a cohort of 301 patients undergoing 15-year review with 3rd generation alumina-alumina cementless THAs. The average follow-up was 15 years (15–17). The CT scans were reviewed against pre-operative and post-operative radiographs and reviewed by a second musculoskeletal specialist radiologist. Results. Eleven of the CT scans were reported to show acetabular osteolysis, two reported osteolysis or possible pre-existing cyst and one reported a definitive pre-existing cyst. After review of previous imaging including pre-operative radiographs, eleven of the thirteen patients initially reported to have osteolysis were found to have pre-existing cysts or geodes in the same size and position as the reported osteolysis, and a further patient had spot-welds with stress-shielding. One patient with evidence of true osteolysis awaits aspiration or biopsy to determine if he has evidence of ceramic wear or metallosis. Conclusions. Reports of osteolysis on CT should be interpreted with care in modern ceramic-on-ceramic THA to prevent unnecessary revision. Further imaging and investigations may be necessary to exclude other conditions such as geodes, or stress shielding which are frequently confused with osteolysis on CT scans

INTRODUCTION. Squeaking after total hip replacement has been reported in up to 10% of patients. Some authors proposed that sound emissions from squeaking hips result from resonance of one or other or both of the metal parts and not the bearing surfaces. There is no reported in vitro study about the squeaking frequencies under lubricated regime. The goal of the study was to reproduce the squeaking in vitro under lubricated conditions, and to compare the in vitro frequencies to in vivo frequencies determined in a group of squeaking patients. The frequencies may help determining the responsible part of the noise. METHODS. Four patients, who underwent THR with a Ceramic-on-Ceramic THR (Trident(r), Stryker(r)) presented a squeaking noise. The noise was recorded and analysed with acoustic software (FMaster(r)). In-vitro 3 alumina ceramic (Biolox Forte Ceramtec(r)) 32 mm diameter (Ceramconcept(r)) components were tested using a PROSIM(r) hip friction simulator. The cup was positioned with a 75° abduction angle in order to achieve edge loading conditions. The backing and the cup liner were cut with a diamond saw, in order to avoid neck-head impingement and dislocation in case of high cup abduction angles (Figure1). The head was articulated ± 10° at 1 Hz with a load of 2.5kN for a duration of 300 cycles. The motion was along the edge. Tests were conducted under lubricated conditions with 25% bovine serum without and with the addition of a 3. rd. body alumina ceramic particle (200 μm thickness and 2 mm length). Before hand, engineering blue was used in order to analyze the contact area and to determine whether edge loading was achieved. RESULTS. Edge loading was obtained. In-vitro, no squeaking occurred under edge loading conditions. However, with the addition of an alumina ceramic 3. rd. body particle in the contact region squeaking was obtained at the beginning of the tests and stopped after ∼20 seconds (dominant frequency 2.6 kHz). In-vivo, recordings had a dominant frequency ranging between 2.2 and 2.4 kHz. DISCUSSION. For the first time, squeaking was reproduced in vitro under lubricated conditions. In-vitro noises followed edge loading and 3. rd. body particles and despite, the severe conditions, squeaking was intermittent and difficult to reproduce. However, squeaking is probably more difficult to reproduce because the cup was cut and the head was fixed in the simulator, preventing vibration to occur. Squeaking noises of a similar frequency were recorded in-vitro and in-vivo. The lower frequency of squeaking recorded in-vivo, demonstrates a potential damping effect of the soft tissues. Therefore, the squeaking in the patients was probably related to the bearing surfaces and modified lubrication conditions that may be due to edge loading. Varnum et al reported recently (3) that all the revised squeaking patients had a neck-cup impingement with metal 3. rd. body particles. These metallic wear particles may generate squeaking as shown in vitro. However, a larger cohort of squeaking patients is needed to confirm these results

The functional pelvic tilt when standing and sitting forward of 7402 cases on the OPS, Optimized Ortho, Australia Data Base were reviewed. All patients had undergone lateral radiographs when standing simulating extension of the hip, and sitting forward when the hip is near full flexion. Pelvic tilt was measured as the angle of the Anterior Pelvic Plane to the vertical Sagittal Plane, rotation anteriorly being given a positive value. Pelvises that had rotated more than 13 degrees anteriorly (+ve) when sitting forward or posteriorly (-ve) when standing were considered to place the hip at increased risk of dislocation or edge loading when flexed or extending respectively. This degree of rotation has the effect of changing the acetabular version by approximately10. 0. Most safe zones that have been described have given a range of anteversion of 20. 0. as safe. A change of 10. 0. would potentially place the acetabular orientation outside this range. Further, clinical studies have supported this concept. All lateral radiographs were reviewed to confirm that 281 had undergone instrumented spinal fusion at some level between T12 and S1. There was a large variability in the number and the levels arthrodesed. The range of pelvic mobility in the non-arthrodesed group in extension was −37. 0. to 31. 0. (mean −0.9. 0. , Standard deviation 7.49) and in flexed position was −70. 0. to 49. 0. (mean −1.9. 0. , Standard deviation 14.01). For the group with any fusion the range of pelvic tilt in extension was −31. 0. to 22. 0. (mean −4. 0. , Standard deviation 8.21) and flexed −32. 0. to 46. 0. (mean 4.4. 0. , Standard deviation 13.79). Of the 7121 cases without instrumented fusion, 15.5% were considered to be at risk when in flexion and 6.1% when extended. The risk for those with any fusion was approximately doubled in both flexion and extension. Further, those with extensive arthrodesis from T12 to S1 had a range of pelvic tilts similar to the non-fused group, although they had a significantly higher percentage of cases in the ‘at risk’ zones. The proportion of the cases in the ‘at risk’ zones decreased progressively as the arthrodesed levels moved from L5/S1 to the upper lumbar spine, and with decreasing number of levels fused. Conclusion. Spinal fusion is not just one group as there are many combinations of different levels fused. Patients with instrumented spinal fusions do have a proportionately high risk of failure of their THR than the majority of cases with no instrumentation, though the risk varies significantly with the number of levels and actual levels arthrodesed. Further approximately 21% of cases with no spinal fusion have functional pelvic movements that would potentially place them ‘at risk’ of edge loading or dislocation. For any figures or tables, please contact authors directly

Introduction. Two types of ceramic materials currently used in total hip replacements are third generation hot isostatic pressed (HIPed) alumina ceramic (commercially known as BIOLOX®forte, CeramTec) and fourth generation alumina matrix composite ceramic consisting of 75% alumina, 24% zirconia, and 1% mixed oxides (BIOLOX®delta, CeramTec). Delta ceramic hip components are being used worldwide, but very few studies have analyzed retrieved delta bearings. The aim of this study is to compare edge loading ‘stripe’ wear on retrieved femoral heads from delta-on-delta, delta-on-forte and forte-on-forte ceramic bearings revised within 2 years in vivo. Material and Methods. Ceramic bearings revised at one center from 1998 to 2010 were collected (61 bearings). Eleven delta heads revised between 1–33 months were compared to 24 forte femoral heads with less than 24 months in vivo (Figure 1). The surface topography of the femoral heads was measured using a RedLux AHP (Artificial Hip Profiler, RedLux Ltd, Southampton, UK). Three representative samples were examined with a FEI Quanta 200 Scanning Electron Microscope (SEM). Results. The median time to revision for delta femoral heads was 12 months, compared to 13 months for forte femoral heads. Sixteen out of 20 forte femoral heads and 6 out of 11 delta femoral heads had edge loading wear (Figure 2). The average volumetric wear rate for forte was 0.96 mm. 3. /yr (median 0.13 mm. 3. /yr), and 0.06 mm. 3. /yr (median 0.01 mm. 3. /yr) for delta (p=0.03). There was no significant difference (p>0.05) in age, gender, time to revision or femoral head diameter between the two groups. Conclusions. Edge loading wear occurs in BIOLOX®delta ceramic bearings. Edge loading wear volumes and wear rates are less in these bearings compared to BIOLOX®forte bearings. These findings are consistent with hip simulator studies. Early analysis of retrieved implants is important to check the in vivo performance of biomaterials

Malpositioning still occurs in total hip arthroplasty (THA). As a result of mal-orientation, THA bearing can be subjected to edge loading. The main objective of the study was to assess if the wear rate of ceramic-on-ceramic and metal-on-polyethylene increases under edge loading conditions and to determine which of the most commonly used hip bearings is the most forgiving to implant mal-orientation. Materials and methods. Two different polyethylenes (UHMWPE and vitamin E blended HXLPE) and ceramics (pure aluminum PAL and alumina-toughened zirconia ATZ) were tested with a hip simulator and compared to metal-on-metal results. The inclination angle was selected at 45°, 65° and 80°. In addition, the ceramic-on-ceramic barings were tested at conditions that produced microseparation. Results. Contrary to metal-on-metal that is highly susceptible to edge loading, the wear rate of ceramic-on-ceramic and metal-on-polyethylene articulations does not increase with increasing cup inclination. In fact, the polyethylenes showed a contra-intuitive behaviour as its wear rate decreased slightly but significantly with increasing inclination angle. This behaviour can be explained when looking closely at the contact stresses and areas. (Figure 1 shows the wear area of the vitamin E blended HXLPE at 45° and figure 2 at 80° cup inclination). The newest biomaterials, vitamin E blended HXLPE and ATZ, showed markedly lower wear rates compared to their conventional counterparts, UHMWPE and PAL. The ATZ ceramic-on-ceramic articulation showed the lowest wear rate (even when microseparation is included) of all tested pairings, but the new vitamin-doped HXLPE seems to be the most forgiving materials when it comes to implant mal-orientation. It shows low wear rate even at an extremely high cup inclination angle. Therefore, a surgeon that discovers a mal-positioned polyethylene cup at the first post-op X-ray will not need to worry unduly about increased wear (but “only” about a potential dislocation)

Introduction. Stripe wear, observed on retrieved ceramic hip replacements, has only been replicated in vitro under translational mal-positioning conditions where the centres of rotation of the head and the cup are mismatched. 1,2. ; an in vitro condition termed “microseparation”. The aim of this study was to compare the edge loading mechanisms observed under microseparation conditions due to translational mal-positioning conditions simulated on two different hip joint simulators. Materials and Methods. 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 al. 2. and Stewart et al. 3. (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). Results. The wear rates of the 36 mm ceramic-on-ceramic bearings obtained under microseparation conditions on the ProSim, where the medial-lateral displacement was controlled (condition 2), and under microseparation conditions on the Leeds II simulator were 0.22 mm. 3. and 0.13 mm. 3. /million cycles respectively, however, the difference was not statistically significant (p = 0.092). The wear rates obtained under microseparation conditions on the Leeds II and the ProSim (condition 2) were both significantly higher (p = 0.006 and p = 0.009 respectively) than the wear rate obtained under microseparation conditions on the ProSim where full distraction of the head was provided (condition 1, 0.05 mm. 3. /million cycles). The wear stripe areas formed on the femoral head under the three simulator set-ups are shown in Figure 2. Discussion and Conclusion. Edge loading due to translational mal-positioning with 0.5 mm dynamic medial-lateral displacement on the Leeds II simulator has been validated against retrievals and shown to replicate stripe wear mechanisms, wear rates and bimodal wear debris distribution as that observed in vivo. 1,2,4,5. The edge loading due to translational mal-positioning simulated on different machines can result in different wear rates and wear mechanisms. One microseparation technique (condition 2) achieved on the ProSim simulator has demonstrated similar results to the validated Leeds II simulator

Malorientation of the acetabular cup in Total Hip replacement (THR) may contribute to premature failure of the joint through instability (impingement, subluxation or dislocation), runaway wear in metal-metal bearings when the edge of the contact patch encroaches on the edge of the bearing surface, squeaking of ceramic-ceramic bearings and excess wear of polyethylene bearing surfaces leading to osteolysis. However as component malorientation often only occurs in functional positions it has been difficult to demonstrate and often is unremarkable on standard (usually supine) pelvic radiographs. The effects of spinal pathology as well as hip pathology can cause large rotations of the pelvis in the sagittal plane, again usually not recognized on standard pelvic views. While Posterior pelvic rotation with sitting increases the functional arc of the hip and is protective of a THR in regards to both edge loading and risk of dislocation, conversely Anterior rotation with sitting is potentially hazardous. We developed a protocol using three functional positions – standing, supine and flexed seated (posture at “seat-off” from a standard chair). Lateral radiographs were used to define the pelvic tilt in the standing and flexed seated positions. Pelvic tilt was defined as the angle between a vertical reference line and the anterior pelvic plane. Supine pelvic tilt was measured from computed tomography. Proprietary software (Optimized Ortho, Sydney) based on Rigid Body Dynamics then modelled the patients’ dynamics through their functional range producing a patient-specific simulation which also calculates the magnitude and direction of the dynamic force at the hip and traces the contact area between prosthetic head/liner onto a polar plot of the articulating surface. Given prosthesis specific information edge-loading can then be predicted based on the measured distance of the edge of the contact patch to the edge of the acetabular bearing. Results and conclusions. The position of the pelvis in the sagittal plane changes significantly between functional activities. The extent of change is specific to each patient. Spinal pathology can be an insidious “driver” of pelvic rotation, in some cases causing sagittal plane spinal imbalance or changes in orientation of previously well oriented acetabular components. Squeaking of ceramic on ceramic bearings appears to be multi factorial, usually involving some damage to the bearing but also usually occurring in the presence of anterior or posterior edge loading. Often these components will appear well oriented on standard views [Fig 1]. Runaway wear in hip resurfacing or large head metal-metal THR may be caused by poor component design or manufacture or component malorientation. Again we have seen multiple cases where no such malorientation can be seen on standard pelvic radiographs but functional studies demonstrate edge loading which is likely to be the cause of failure [Fig 2]. Clinical examples of all of these will be shown