INTRODUCTION. Ceramic-on-ceramic hip resurfacing offers a bone conserving treatment for more active patients without the potential metal ion risks associated with resurfacing devices. The Biolox Delta ceramic material has over 15 years of clinical history with low wear and good biocompatibility but has been limited previously in total hip replacement to 48mm diameter bearings [1]. Further increasing the diameter for resurfacing bearings and removing the metal shell to allow for direct fixation of the ceramic cup may increase the wear of this material and increase the risk of fracture. METHODS. Eighteen implants (ReCerf™, MatOrtho, UK; Figure1) were wear tested; six were ⊘40mm (small) and twelve ⊘64mm (large). All small and six large implants were tested under ISO 14242 standard conditions for 5 million cycles (mc) at 30° inclination (45° clinically). The six remaining large implants were tested under microseparation conditions in which rim contact was initiated during heel strike of the gait cycle for 5mc. Cups were orientated at 45° inclination (60° clinically) to allow for separation of the head and cup with a reduced 50N swing phase load and a spring load applied to induce a 0.5mm medial-superior translation of the cup. Wear was determined gravimetrically at 0.5mc, 1mc and every mc after. RESULTS. Wear was low in both standard and microseparation tests, less than 1mm. 3. cumulatively over 5mc (Figure 2). Standard conditions showed a run-in wear phase over the first mc followed by negligible wear in both diameters. The run-in wear significantly increased from 0.2mm. 3. /mc in the 40mm diameter bearings to 0.5mm. 3. /mc with the larger diameter implants. Under microseparation conditions, there was low wear over the first mc, increasing to 0.28mm. 3. /mc between 1–3mc. The wear rate reduced to 0.11mm. 3. /mc from 3=5mc. Stripe wear was evidenced on the microseparated components. There were no incidences of fracture or squeaking. DISCUSSION. Biolox Delta is known for its low wear rates but published results have only reported testing up to ⊘36mm [2]. Increasing the diameter to 64mm showed increased wear compared to smaller diameters but this was only significant over the first mc suggesting similar performance long term. Microseparation testing of these large sized bearings doubled the cumulative wear produced over 5mc but wear measured was still much lower than other bearing combinations. Wear of metal-on-metal resurfacing implants under these high angle, microseparation conditions has been reported up to 10.5mm. 3. /mc [3], significantly higher than any wear rate reported in the current study. Despite the 3mm wall thickness, no fracture of the cup occurred but stripe wear was observed in the ceramic components. SIGNIFICANCE. Biolox Delta ceramic is appropriate for use in larger diameters without excessive wear or damage to the bearings. The improved biocompatibility of the material may allow for hip resurfacing to be offered to more patients than currently available. For any figures or tables, please contact the authors directly

Introduction:. Microseparation has resulted in more than ten-fold increase in ceramic-on-ceramic and metal-on-metal bearing wear, and even fracture in a zirconia head [1–4]. However, despite the greater microseparation reported clinically for metal-on-polyethylene wear, less is known about its potential detrimental effects for this bearing couple. This study was therefore designed to simulate the effects of micromotion using finite element analysis and to validate computational predictions with experimental wear testing. Methods:. Experimental wear rates for low and highly crosslinked polyethylene hip liners were obtained from a previously reported conventional hip wear simulator study [5]. A finite element model of the wear simulation for this design was constructed to replicate experimental conditions and to compute the wear coefficients that matched the experimental wear rates. We have previous described out this method of validation for knee wear simulation studies [6,7]. This wear coefficient was used to predict wear in a Dual-Mobility hip component (Fig 1). Dual mobility total hip arthroplasty components, Restoration ADM (Fig 1), with highly crosslinked acetabular liners were experimentally tested: the control group was subjected to wear testing using the ISO 14242-1 waveform on a hip wear simulator. The microseparation group was subjected to a nominal 0.8 mm lateral microseparation during the swing phase by engaging lateral force springs and reducing the swing phase vertical force. Results:. The wear coefficients that matched experimental wear rates for the low and highly crosslinked polyethylene liners were 4.57×10. −10. and 5.89×10. −11. mm. 3. N. −1. mm. −1. , respectively. Introducing microseparation in the conventional hip increased the wear rate by 15.59 mm. 3. /million cycles in the low crosslinked liner and by 1.12 mm. 3. /million cycles in the highly crosslinked liner (Fig 2). Discussion:. Microseparation did increase predicted wear rates for the low crosslinked polyethylene liner and supports the hypothesis that microseparation can adversely affect the wear of hip arthroplasty. However, the predicted and experimental increase for the dual mobility highly crosslinked liners due to microseparation was low (3.3 mm. 3. and 2.9 mm. 3. /million cycles, respectively) and below the threshold for clinical relevance. The small increase in wear rate in our study supports the high wear tolerance to wear of a dual-mobility sequentially crosslinked polyethylene liner

Introduction. Translational surgical mismatch in the centres of rotation of the femoral head and acetabular cup in hip joint replacements can lead to dynamic microseparation resulting in edge loading contact [1]. Increased wear in retrieved ceramic-on-ceramic bearings has been associated with edge loading [2]. Hip joint simulators were used to replicate increased wear rate, stripe wear and bimodal wear debris size distribution, as seen clinically [3,4]. Recently developed electromechanical simulators are able to comply with the latest international standards, which include three axes of rotation conditions [5]. Previous simulators had applied two axes of rotation under microseparation conditions [6]. Therefore, the aim of this study was to compare the wear of ceramic-on-ceramic bearings obtained under edge loading due to microseparation conditions during gait using the same electromechanical hip joint simulator with two axes of rotation and three axes of rotation conditions. Materials and Methods. 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. Results. Under standard conditions, the mean wear rate of BIOLOX® delta ceramic-on-ceramic bearings was 0.03±0.01 mm3/million cycles. The mean wear rates under microseparation conditions for two axes and three axes of rotation conditions were 0.14±0.01 mm3/million cycles and 0.14±0.03 mm3/million cycles respectively. There was no statistically significant difference between the wear rates using two axes and three axes of rotation conditions under microseparation conditions (p=0.86). Stripe wear was observed and wear depth measured on the femoral heads under microseparation conditions using two axes (Figure 2a) and three axes (Figure 2b) of rotation. Conclusion. Higher wear rates were observed under microseparation compared with standard conditions, as reported in a previous study [6]. Similar wear rates were obtained under microseparation conditions with two axes and three axes of rotation conditions using the same simulator

INTRODUCTION. Retrieval and clinical studies of metal-on-metal (MoM) bearings have associated increased wear. 1. and elevated patient ion levels. 2. with steep cup inclination angles and edge loading conditions. The University of Leeds have previously developed a hip simulator method that has been validated against retrievals and shown to replicate clinically relevant wear rates and wear mechanisms. 3,4. This method involves introducing lateral microseparation to represent adverse joint laxity and offset deficiency. This study aimed to investigate the effect of microseparation representing translational malpostion, and increased cup inclination angle, representing rotational malposition, in isolation and combined on the wear of different sizes (28 and 36mm) MoM bearing in total hip replacement (THRs). MATERIALS AND METHODS. The wear of size 28mm and 36mm MoM THRs bearings was determined under different in vitro conditions using the Leeds II hip simulator. For each size bearing, two clinical cup inclination angles were considered, 45° (n=3) and 65° (n=3). The first three million cycles were run under standard gait conditions and subsequently three million cycles were run under microseparation conditions. Standard gait cycles included a twin peak load (300N–3000N), extension/flexion (−15°/+30°) and internal/external rotation (±10°). Microseparation. 4. was achieved by applying a 0.4–0.5mm medial displacement to the cup relative to the head during the swing phase of the standard gait cycle resulting in edge loading at heel strike. The lubricant was 25% (v/v) new-born calf serum. The wear volume was determined through gravimetric analysis every million cycles. One way ANOVA was performed (significance: p<0.05), and 95% confidence limits were calculated. RESULTS. Under standard gait conditions, the 28mm MoM bearing showed head-rim contact and increased wear rate with increased cup inclination angle but the 36mm bearing did not show any increase in wear. Microseparation and edge loading increased the wear rate of MoM bearings for all cup inclination angle conditions and bearing sizes (Figure 1). DISCUSSION. With the larger size bearings, head-rim contact occurred at a steeper cup inclination angle (>65°) providing an advantage over smaller bearings. Under standard gait conditions, where head-rim contact did not occur, wear was low, due to mixed lubrication and wear reduction through a protein boundary film. However, edge loading of the cup, with elevated stress, caused excess damage and wear. This effect was more dominant with microseparation conditions to that of head-rim contact due to increased cup inclination angle alone. Under microseparation conditions, there were no significant differences in the wear rates of the 28mm and the 36mm size bearings. However, the wear rates obtained in this study for 28mm and 36mm bearings were significantly lower than those obtained for size 39mm surface replacement MoM bearings (8.99 mm. 3. /million cycles) tested under the same adverse conditions. 5. . CONCLUSION. This study shows the importance of acetabular cup design and correct surgical positioning of the femoral head and acetabular cup and restoration of offset and cup centre. ACKNOWLEDGEMENT. This study was supported by the Furlong Research Charitable Foundation (FRCF) and the National Institute of Health Research (NIHR) as part of a collaboration with the Leeds Musculoskeletal Biomedical Research Unit (LMBRU). The components were custom made specifically for this project by Corin Ltd

INTRODUCTION. Ceramic-on-ceramic hip replacements have generated great interest in recent years due to substantial improvements in manufacturing techniques and material properties. 1. Microseparation conditions that could occur due to several clinical factors such as head offset deficiency, medialised cup combined with laxity of soft tissue resulting in a translation malalignment, have been shown to cause edge loading, replicate clinically relevant wear mechanisms. 2,3. and increase the wear of ceramic-on-ceramic bearings. 3,4. The aim of this study was to investigate the influence of increasing the femoral head size on the wear of ceramic-on-ceramic bearings under several clinically relevant simulator conditions. MATERIALS AND METHODS. The wear of size 28mm and 36mm ceramic-on-ceramic bearings (BIOLOX® Delta, CeramTec, Germany) was determined under different in vitro conditions using the Leeds II hip simulator. For each size bearing, two clinical cup inclination angles were considered, 55° (n=3) and 65° (n=3) for the 28mm bearing and 45° (n=3) and 65° (n=3) for the 36mm bearing. The first two (28mm study) or three (36mm study) million cycles ran under standard gait conditions and a subsequent three million cycles ran under microseparation conditions. A standard gait cycle included a twin peak load (300N–3000N), extension/flexion (−15°/+30°) and internal/external rotation (±10°). Microseparation. 3. was achieved by applying a 0.4–0.5mm medial displacement to the cup relative to the head during the swing phase of the standard gait cycle resulting in edge loading at heel strike. The lubricant was 25% (v/v) new-born calf serum which was changed approximately every 333,000 cycles. The wear volume was ascertained through gravimetric analysis every million cycles. One way ANOVA was performed (significance: p<0.05), and 95% confidence limits were calculated. RESULTS AND DISCUSSION. The mean wear rate under standard gait conditions was 0.05mm. 3. / million cycles for the 28mm bearings and significantly lower (p=0.003) for the 36mm bearings (Figure 1) which could be due to improved lubrication regime. The wear of ceramic-on-ceramic bearings was not influenced by the increase in cup inclination angle for either bearing size (Figure 1). The introduction of microseparation into the gait cycle resulted in stripe wear on the femoral head with a corresponding wear area at the rim of the acetabular cup and significantly higher wear rates of the ceramic-on-ceramic bearings (Figure 2). The wear rate of BIOLOX® Delta bearings under microseparation conditions was still low (<0.25mm. 3. /million cycles) compared to the third generation alumina ceramic-on-ceramic bearings (1.84mm. 3. /million cycles). 4. under the same adverse conditions. Under microseparation conditions, the wear rate of size 36mm bearings was significantly higher (p=0.004) than that for size 28mm bearings. This was thought to be due to the larger contact area for the larger bearings and deprived lubrication under edge loading conditions. For both bearing sizes, the combination of both steep cup inclination angles and microseparation conditions did not increase the wear rates any further compared to microseparation conditions alone (Figure 3). This study shows the importance of surgical positioning of the femoral head and acetabular cup and the importance of testing new bearing materials and designs using these adverse simulator methods. ACKNOWLEDGEMENT. This study was supported by the Furlong Research Charitable Foundation (FRCF) and the National Institute of Health Research (NIHR) as part of a collaboration with the Leeds Musculoskeletal Biomedical Research Unit (LMBRU)

Background

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

Introduction and Aims. In order to improve the longevity and design of an implant, a wide range of pre-clinical testing conditions should be considered including variations in surgical delivery, and patients' anatomy and biomechanics. The aim of this research study was to determine the effect of the acetabular cup inclination angle with different levels of joint centre mismatch on the magnitude of dynamic microseparation, occurrence and severity of edge loading and the resultant wear rates in a hip joint simulator. Methods. 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). Conclusions. The condition with the acetabular cups positioned at an inclination of 45° exhibited greater resistance to dynamic microseparation for any given medial-lateral mismatch in the centres of rotation. Higher wear rates correlated with higher levels of dynamic microseparation. These results highlight how different conditions can alter the severity of edge loading, and highlight the necessity of understanding how the surgical positioning can affect the occurrence of edge loading and wear. Future studies will look into the other factors which can influence the microseparation conditions such as joint laxity, swing phase load and version angles

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

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. 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 replacement. 1. . 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. Materials and Methods. The head-on-cup configurations of three ceramic materials (see Figure 1), were ATZ-on-ATZ, ZTA-on-ZTA, Al. 2. O. 3. -on-Al. 2. O. 3. , ATZ-on-ZTA, ZTA-on-ATZ, Al. 2. O. 3. -on-ATZ, ATZ-on-Al. 2. O. 3. and Al. 2. O. 3. -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 microseparation. 2,3,4. 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). Results. The wear of ATZ-on-ZTA, ATZ-on-Al. 2. O. 3. and Al. 2. O. 3. -on-Al. 2. O. 3. was biphasic with a bedding in wear rate between zero and one million cycles under translational malpositioning conditions and a lower steady state wear rate between one and four million cycles. The bedding in and steady state wear rates of ATZ-on-ZTA (1.16mm. 3. /million cycles bedding in and 0.18mm. 3. /million steady state) and ATZ-on-Al. 2. O. 3. (0.66mm. 3. /million cycles bedding in and 0.20mm. 3. /million steady state) were lower than that of Al. 2. O. 3. -on-Al. 2. O. 3. (1.54mm. 3. /million cycles bedding in and 0.55mm. 3. /million steady state) bearing combination (see Figure 2). However, there was statistically no significant difference (p=0.35) between the wear rates. The wear rates of the other bearing combinations under these adverse microseparation conditions, ZTA-on-ZTA and ATZ-on-ATZ, Al. 2. O. 3. -on-ATZ, Al. 2. O. 3. -on-ZTA, and ZTA-on-ATZ were very low with no clear bedding in and steady state phases (see Figure 1). The wear rates of these combinations, over the four million cycles of test under adverse microseparation conditions, were all lower than 0.14mm. 3. /million cycles. Discussion and Conclusion. The mixed material combinations (ATZ-on-ZTA, ATZ-on-Al. 2. O. 3. , Al. 2. O. 3. -on-ATZ, Al. 2. O. 3. -on-ZTA and ZTA-on-ATZ) tested in this study have shown slightly higher wear rates when compared to ATZ in like-on-like configuration, but superior wear resistance when compared to Alumina BIONIT® (Mathys) and BIOLOX® forte (CeramTec) Al. 2. O. 3. -on-Al2O. 3. bearings tested under the same adverse microseparation conditions

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)

Hip simulator studies have shown reduced hip offset can cause microseparation and increased wear in hard-on-hard hip bearings. However this has not been analysed yet in vivo. We studied the effect of reduced hip offset on serum metal ion levels in patients with metal-on-metal (MoM) hip arthroplasty. From all patients who underwent unilateral MoM bearing hip arthroplasty between 2005 and 2009, 63 patients had complete clinical evaluation, measurement of serum chromium and cobalt ion levels as well as biomechanical measurements on pre- and post operative radiographs (cup inclination, head inclination, change in hip offset and change in hip length.). Ten arthroplasties were revised due to adverse metal reaction and six patients awaiting revision. 55% of ASR hips showed higher metal ions (>7 ppb) whereas only 15% of non-ASR hips had higher ion levels. Patients with reduced postoperative hip offset by more than 5 mm had significantly higher mean metal ion levels compared to the the rest of the hips (31.8 ppb vs. 7.4 ppb, p=0.002). On subgroup analysis this effect was present in non-ASR hips (18.7 ppb vs. 4.7 ppb, p=0.025) but was not significant in ASR hips (29.6 ppb vs. 16.3 ppb, p=0.347). Our study demonstrated significantly higher serum metal ion levels in patients who lost more than 5 mm hip offset after arthroplasty. Reduced soft tissue tension leading to microseparation of the articulation and edge loading is a theoretical explanation for this effect. This may be relevant in other hard bearings such as ceramic-on-ceramic as well

Introduction. Cementless unicondylar knee implants are intended to offer surgeons the potential of a faster and less invasive surgery experience in comparison to cemented procedures. However, initial 8 week fixation with micromotion less than 150µm is crucial to their survivorship1 to avoid loosening2. Methods. Test methods by Davignon et al3 for micromotion were used to assess fixation of the MAKO UKR Tritanium (MAKO) (Stryker, NJ) and the Oxford Cementless UKR (Biomet, IN). Data was analyzed to determine the activities of daily living (ADL) that generate the highest forces and displacements4, 5. Stair ascent with 3.2BW compressive posterior tibial load was identified to be an ADL which may cause the most micromotion5. Based on previous studies6, 10,000 cycles was set as the run-time. The AP and IE profiles were scaled back to 60% for the Oxford samples to prevent the congruent insert from dislocating. A four-axis test machine (MTS, MN) was used. The largest size UKRs were prepared per manufacturer's surgical technique. Baseplates were inserted into Sawbones (Pacific Research, WA) blocks1. Femoral components were cemented to arbors. The medial compartment was tested, and the lateral implants were attached to balance the loads. Five tests were conducted for each implant with a new Sawbones and insert for each test per the test method3. The ARAMIS System (GOM, Germany) was used to measure relative motion between the baseplate and the Sawbones at three anteromedial locations (Fig. 1). Peak-Peak (P-P) micromotion was calculated in the compressive and A/P directions. FEA studies replicating the most extreme static loading positions for MAKO micromotion were conducted to compare with the physical test results using ANSYS14.5 (ANSYS, PA). Results. MAKO had a maximum axial motion of 36µm (SD=5.28) at gage 2. Oxford had an average gage 1 axial and A/P motion of 109µm (SD=31.77) and 44mm (SD=28.62) respectively (Fig. 2A). FEA correlated well with the MAKO results (Fig. 2B). Discussion. Oxford has been shown to have microseparation in lab testing conditions and the studies by Liddle et al7 under the same stair ascent activity. However, based on our results, MAKO and Oxford are both expected to allow interdigitation for long-term fixation. The Sawbones model does not allow plastic deformation in axial compression and subsequent stabilization, which could allow Oxford to achieve the fixation and clinical success shown in outcome studies. A/P prep for Oxford allows for 3mm gap between the keel and the bone which may explain the variability in the X direction. Distal flatness of the Oxford varied by 0.5mm as shown on Figure 3. The flatness of the boundary of the implant may explain the elevated micromotion observed for Oxford implant. Future studies will concentrate on FEA of manufactured Oxford components to take into account the geometric discrepancies from a perfectly flat model. Davignon et al3 and this study show that the MAKO is expected to achieve long-term fixation in the initial fixation stages similar to the clinically successful Oxford cementless UKR