Summary Statement. A new 28mm-diameter ceramic-on-ceramic (COC) acetabular bearing couple (Biomet Orthopedics) showed extremely low wear, even under adverse microseparation conditions∗. The wear results are similar or more favorable than those reported for clinical retrievals and wear testing of similar ceramic bearings. Introduction. A new acetabular shell and ceramic insert design (Biomet) incorporates features to help prevent malalignment during implantation, while still providing secure fixation within the acetabular shell. The incorporation of Biolox. ®. Delta (zirconia toughened alumina, CeramTec) material should provide improved wear resistance over pure alumina ceramics. The goal of this study is to evaluate the wear durability of this system for standard and microseparation testing. Materials & Methods. The 28 mm diameter ceramic heads and inserts (CeramTec) were seated on taper spigots and within acetabular shells (Biomet), respectively. Six sets of parts were tested for 5M cycles of standard hip wear testing (ISO 14242) and an additional six sets of parts for 2M cycles of microseparation testing. The microseparation testing protocol included a steep cup angle (60° in-vivo), side load, and reduced axial load to induce head-liner separation. The lateral displacement was increased from 0.5mm, to 1mm, and then to 2mm in order to replicate wear features observed in extreme situations of clinical retrievals. [1]. The parts were weighed (gravimetric wear rates) and photographed throughout the test. SEM, transformation, and wear debris analyses were completed. Results. The steady-state wear rate throughout standard testing was 0.0094 +/− 0.0029 mm. 3. /10. 6. cycles (+/-95% CI). The initial 0.5mm microseparation distance (0–1M cycles) showed no signs of wear. Most heads showed wear stripes after increasing to 1.0mm (1–1.5M cycles), and then all test parts showed stripes after increasing to 2mm. The increased visibility in wear stripes correlated with an increased level of measured wear. For the 2mm separation-distance testing interval, the wear rate was 0.178 +/− 0.052mm. 3. /10. 6. cycles. Discussion/Conclusion. The lack of wear stripes during 0.5mm of microseparation is an indication of the strength of the implants. A distance of 1–2mm is an extreme level of microseparation and the 60° in-vivo cup inclination created an even worse-case situation for wear; however, the implants showed excellent mechanical strength and low wear rates. SEM and transformation analyses showed minimal wear and evidence of stress-induced ceramic toughening. Microseparation testing at another lab . [2]. has shown a similarly low wear rate (0.5 mm. 3. /10. 6. cycles) for Biolox. ®. Delta ceramic, with Biolox. ®. Forte (alumina ceramic, without zirconia) showing a considerably higher wear rate (6.3mm. 3. /10. 6. cycles). The standard testing wear rate (0.0094+/-0.0029 mm. 3. /10. 6. cycles) was much lower than the average wear rate (0.69+/-0.63 mm. 3. /10. 6. cycles) of several COC implant retrievals by Walter . [1]. The 28mm steady-state wear rate of this test is better than or equal to the wear rate (0.0101 mm. 3. /10. 6. cycles) observed in other 28mm COC systems.∗∗. ∗Ceramic-on-Ceramic articulation is not cleared for use in the United States. ∗∗Laboratory results are not necessarily indicative of clinical performance

In vitro the introduction of microseparation and edge loading to hip simulator gait cycle has replicated clinically relevant wear rates and wear mechanisms in ceramic-on-ceramic bearings. [1]. , and elevated the wear rates of MoM surface replacements (SR) to levels similar to those observed in retrievals. [2]. The aim was to assess the wear of two different sized MoM total hip replacement bearings under steep cup inclination angles and adverse microseparation and edge loading conditions. Two tests were performed on the Leeds II hip joint simulator using two different size bearings (28mm and 36mm). Cups were mounted to provide inclination angles of 45 degrees (n=3) and 65 degrees (n=3). The first three million cycles were under standard gait conditions. Microseparation and edge loading conditions as described by Nevelos et al. [1]. were introduced to the gait cycle for the subsequent three million cycles. The lubricant was 25% new born calf serum. The mean wear rates and 95% confidence limits were determined and statistical analysis was performed using One Way ANOVA. Under standard gait conditions, when the cup inclination angle increased from 45 degrees to 65 degrees, the wear of size 28mm bearing significantly (p=0.004) increased by 2.7-fold, however, the larger bearings did not show any increase in wear (p=0.9). The introduction of microseparation conditions resulted in a significant (p=0.0001) increase in wear rates for both bearing sizes under both cup inclination angle conditions. Under microseparation conditions, the increase in cup inclination angle had no influence on the wear rate for both bearing sizes (Figure 1). With larger bearings, head-rim contact occurs at a steeper cup inclination angle providing an advantage over smaller bearings. The introduction of edge loading and microseparation conditions resulted in a significant increase in wear rates for both bearing sizes. The wear rates obtained in this study under combined increased cup inclination angle and microseparation were half of those obtained when SR MoM bearings were tested under similar adverse conditions. [2]. This study shows the importance of prosthesis design and accurate surgical positioning of the head and acetabular cup in MoM THRs

Background. Many factors contribute to the occurrence of edge-loading conditions in hip replacement; soft tissue tension, surgical position, patient biomechanical variations and type of activities, hip design, etc. The aim of this study was to determine the effect of different levels of rotational and translational surgical positioning of hip replacement bearings on the occurrence and severity of edge-loading and the resultant wear rates. Method. The Leeds II Hip-Joint Simulator and 36mm diameter alumina matrix composite ceramic bearings (BIOLOX delta, DePuy Synthes, UK) were used in this study. Different levels of mismatch between the reconstructed rotational centres of the head and the cup were considered (2, 3 and 4mm) in the medial-lateral axis. Two cup inclination angles were investigated; an equivalent to 45 and 65 degrees in-vivo, thus six conditions (n=6 for each condition) were studied in total with three million cycles completed for each condition. The wear of the ceramic-on-ceramic bearings were determined using a microbalance (Mettler Toledo, XP205, UK) and the dynamic microseparation displacement was measured using a Liner Variable Differential Transformer. Results. When a translational joint centre mismatch was coupled with a higher cup inclination angle, the severity of edge-loading increased when compared with the effect of those variables applied individually. Increasing the medial-lateral joint centre mismatch from 2 to 3 to 4mm resulted in increased wear rates under both cup inclination angles, with the 65 degree cup inclination angle having significantly higher wear rate than the cup inclination angle of 45 degree (p=0.02, p=0.02, and p<0.01 respectively). Conclusion. The cups with a 45 degree inclination angle showed greater resistance to dynamic microseparation as a result of joint centre mismatch. This study demonstrated that optimal position should not only consider the rotational position of the acetabular cup but also the relative centres of rotation of the head and the cup. Disclosure. John Fisher is a paid consultant to DePuy Synthes. Jonathan Thompson and Graham H. Issac are employees at DePuy Synthes

Abstract. Objectives. Hip joint laxity after total hip arthroplasty (THA) has been considered to cause microseparation and lead to complications, including wear and dislocation. In the native hip, the hip capsular ligaments may tighten at the limits of range of hip motion and provide a passive stabilising force preventing edge loading and reduce the risk of dislocation. Previous attempts to characterise mechanical properties of hip capsular ligaments have been largely variable and there are no cadaveric studies quantifying the force contributions of each ligament in different hip positions. In this study we quantify the passive force contribution of the hip capsular ligaments throughout a complete range of motion (ROM). Methods. Nine human cadaveric hip specimens (6 males and 3 females) with mean age of (76.4 ± 9.0 years) were skeletonised, preserving the capsular ligaments. Prepared specimens were tested in a 6 degree of freedom system to assess ROM with 5 Nm torque applied in external and internal rotation throughout hip flexion and extension. Capsular ligaments were resected in a stepwise fashion to assess internal force contributions of the iliofemoral (superior and inferior), pubofemoral, and ischiofemoral ligaments during ROM. Results. In external rotation, the superior and inferior iliofemoral ligament minimum force contributions were (136.52 ± 27.15 N) in flexion and (82.40 ± 27.85 N) in extension, respectively. In internal rotation, the ischiofemoral ligament force contributions were dominant in adducted-flexion positions and abducted-extension positions. Conclusions. These findings provide insights into the primary capsular structures that stabilise the hip joint in different manoeuvres. This data allows for an improved understanding of which capsular ligaments contribute the most to hip stability and has important implications for choosing surgical approaches and repair strategies to minimise complications related to joint instability. Declaration of Interest. (b) declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported:I declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project

Hip simulators have been used for ten years to determine the tribological performance of large-head metal-on-metal devices using traditional test conditions. However, the hip simulator protocols were originally developed to test metal-on-polyethylene devices. We have used patient activity data to develop a more physiologically relevant test protocol for metal-on-metal devices. This includes stop/start motion, a more appropriate walking frequency, and alternating kinetic and kinematic profiles.

There has been considerable discussion about the effect of heat treatments on the wear of metal-on-metal cobalt chromium molybdenum (CoCrMo) devices. Clinical studies have shown a higher rate of wear, levels of metal ions and rates of failure for the heat-treated metal compared to the as-cast metal CoCrMo devices. However, hip simulator studies in vitro under traditional testing conditions have thus far not been able to demonstrate a difference between the wear performance of these implants.

Using a physiologically relevant test protocol, we have shown that heat treatment of metal-on-metal CoCrMo devices adversely affects their wear performance and generates significantly higher wear rates and levels of metal ions than in as-cast metal implants.