INTRODUCTION. The introduction of hard-on-hard bearings and the consequences of increased wear due to edge-loading have renewed interest in the importance of acetabular component orientation for implant survival and functional outcome following hip arthroplasty. Some studies have shown increased dislocation risk when the cup is mal-oriented which has led to the identification of a safe-zone1. The aims of this prospective, multi-centered study of primary total hip arthroplasty (THA) were to: 1. Identify factors that influence cup orientation and 2. Describe the effect of cup orientation on clinical outcome. METHODS. In a prospective study involving seven UK centers, patients undergoing primary THA between January 1999 and January 2002 were recruited. All patients underwent detailed assessment pre-operatively as well as post-op. Assessment included data on patient demographics, clinical outcome, complications and further surgery/revision. 681 primary THAs had adequate radiographs for inclusion. 590 hips received cemented cups. The primary functional outcome measure of the study was the change between pre-operative and at latest follow up OHS (OHS). Secondary outcome measures included dislocation rate and revision surgery. EBRA was used to determine acetabular inclination and version. The influence of patient's gender, BMI, surgeon's grade and approach on cup orientation was examined. Four different zones tested as possibly ± (Lewinnek Zone, Callanan's described zone and zones ± 5 and ±10 about the study's mean inclination and anteversion) for a reduced dislocation risk and an optimal functional outcome. RESULTS. There were 21 dislocations (3.1%) and 8 (1.2%) patients required revision at a mean follow up of 7 years. Experienced surgeons (2=0.047) and those operating with the patient in the lateral decubitus position (p=0.04) were more likely to achieve a cup orientation within any of the tested zones. Surgical approach (2=0.14) and patient's BMI (2=0.93) had no influence on whether a cup was within or outside any zone. There was no difference in dislocation rate between the posterior and anterio-lateral approaches (2=0.88). None of the zones tested had a significantly reduced dislocation risk (2=0.13), nor revision risk (2=0.55). OHS was not different for patients with cups within or outside any of the zones tested (p=0.523). DISCUSSION. There was a wide variation in cup orientation. Despite the wide scatter in cup orientation, no safe zone could be identified that would reduce dislocation and revision rate, nor improve patient reported outcome (OHS). Hence, these data suggest that acetabular component orientation should not be considered predictive of patients' early/mid-term complication/revision rate and outcome following THA

Introduction. The same cup orientation is classically applied to all cases of hip replacement (45° abduction, 20° anteversion). We hypothesize that this orientation must be adapted to the patient's hip range of motion. We tested this hypothesis by means of an experimental study with respect to hip range of motion, comparing the classical orientation (45° and 20°), and the orientation obtained with computer-assisted navigation. Material and Methods. The experimental model included a hemipelvis equipped with a femur whose mobility was controlled for three configurations: stiff (60°/0°, 15°/10°, 10°/10°), average (80°/10°, 35°/30°,35°/25°), mobile (130°/30°, 50°/50°, 45°/35°). The hemipelvis and the cup holder were equipped with an electromagnetic system (Fastrack ™) to measure cup orientation. The Pleos™ navigation system (equipping the hemipelvis, the femur, and the cup holder) guided the cup orientation by detecting the positions risking impingement through a kinematic study of the hip. Nine operators each performed 18 navigation-guided implantations (162 hip abduction, anteversion, and range of movement measurements) in two series scheduled 2 months apart. Results. The model used herein showed intra and interobserver reliability. Compared to the navigation-assisted surgery, the arbitrary orientation gave a mean anteversion error of only 1° ± 6° (−12 to +19°) but 5° ± 8° (−26° to +13°) for abduction. However, 16% of the errors were more than 10° in anteversion (1/2 in the mobile configuration) and 11% of the errors were more than 15° in abduction (for the most part in the mobile configuration). With arbitrary orientation, the errors consisted in excess anteversion and insufficient abduction. Discussion and Conclusion. The experimental model developed was reliable and can be used to evaluate different prosthetic configurations. This study emphasizes that the ideal arbitrary cup orientation cannot be applied to all hips. All the surgeons are very reproducible but the only way to integrate the range of motion in there ‘own way to do’ in vitro, is to use a navigation system witch can guide the surgeon so as to reduce the risk of impingement and instability

INTRODUCTION. Although simulation studies have shown superior wear properties of metal-on-metal articulations, increased concern exists regarding the excess in-vivo wear of a small number of Metal-on-Metal-Hip-Resurfacing (MoMHRA) implants. Serum ion levels of Chromium (Cr) and Cobalt (Co) are surrogate markers of wear. Risk factors associated with increased wear include female gender, small components, dysplasia, cup orientation outside safe zone and femoral head downsize during surgery with an associated decrease in Head-Neck-Ratio (HNR). However, these factors are interlinked. This study aims to identify the factors that are most important for subsequent wear of MoMHRA, by performing a multivariate analysis. METHODS. 206 patients (124M: 82F) with unilateral MoMHRA were included in this study. The average follow up was 3.3 years. All patients had Cr/Co levels measured at follow up. Inclination and anteversion of each cup were measured using EBRA. Cups were analysed as being within or outside the previously defined optimum-zone. HNR measurements were made from pre-operative (HNRpre) and post-operative (HNRpost) radiographs. The immediate changes in HNR (downsize/upsize of femoral head) as a result of the operation were expressed as:. HNRprepost=HNRpost–HNRpre. Multivariate linear regression modelling was used to explore the association between measures of ions with the following predictor variables (gender, age, diagnosis, femoral component size, orientation of the acetabular component, head/neck ratio and position of femoral stem). Analyses were carried out separately for each outcome (Cr and Co). Classification and Regression Tree (CART) models were fitted as a complimentary approach to regression modelling. RESULTS. Articular surface downsize followed by cup orientation within/outside optimum zone, followed by cup anteversion followed by gender were the strongest predictors of ion levels. A percentage decrease in HNRartpost, predicted an increase in Cr ion level by 5% (and 6% for Co). If the cup was within the optimum zone this was associated with decreased levels of Cr and Co ions. As acetabular component anteversion increased, levels of ions increased. Gender had a strong effect on ion levels. Adjusting for other variables, the effect of gender was attenuated due to a confounding effect of component size and the amount of femoral head downsize. Predictors identified as important in regression analyses were similar to those produced in the CART model, where the highest levels of ions were seen in patients with a percentage decrease in pre-operative HNR −11.3. DISCUSSION. This analysis shows that (surgical) factors, such as amount of femoral head downsize at operation and whether the cup was orientated within the optimum zone, explained the majority of the variability in ion levels in MoMHRA

INTRODUCTION. Whilst there is a great deal of research on hip implants, few studies have looked at implant orientation and the subsequent effect upon the wear performance of a hip resurfacing. This study aimed to measure implantation angles through radiographic analysis and linear wear for retrieved acetabular cups in order to investigate possible causal links between wear and implant orientation. MATERIALS & METHODS. Seventy Birmingham Hip Resurfacing (Smith & Nephew, UK) cups with known time in vivo were analysed. Linear wear of retrieved cups were assessed using a Talyrond 290 roundness machine. Deviations from the characteristic manufactured profile, was identified as a region of wear. Polar measurements across the wear region were taken to determine wear. The linear wear rate (LWR) of a component was defined as the linear wear (μm) divided by the duration of the implant life in vivo (years). Cups which showed the wear crossing over the edge of the cup were classified as edge loaded (EL). For all non-edge loaded (NEL) cups, the wear area was within the bearing surface. Cup orientation angles were conducted for 31 cups. This was determined by superimposing BHR models of appropriate size, generated by CAD ProEngineer Wildfire 4, onto anterior-posterior x-rays. Anatomical landmarks and specific features of the BHR were used as points of reference to determine cup version and inclination angles. RESULTS. Forty two cups were classed as EL, showing regions of wear extending beyond the edge of the cup. Twenty eight were classed as NEL. The EL group had an average LWR of 25.4(±8.05 95% CL) μm/yr, whilst the NEL group generated an average LWR of 1.45 (±0.34 95% CL) μm/yr, a statistically significant difference (p<0.05).a Following radiographic analysis, 23 cups were classed as EL, showing regions of wear extending beyond the edge of the cup. Eight were classed as NEL. Cups in the EL group showed average inclination and version angles of 54.35° (±5.37° 95% CL) and 22.43° (±5.23° 95% CL). Average inclination and version angles of cups in the NEL group were 45° (±7.20° 95% CL), and 14.88° (±3.38° 95% CL) respectively. Inclination and version angles between the two groups were statistically significant (p<0.05). DISCUSSION. Through linear wear and radiographic analysis, the current study has shown that mal-positioned resurfacing devices classed as EL had higher linear wear than the NEL cups. Edge loaded cups examined in this study showed significantly higher inclination and anteversion (p<0.05) than the non-edge loaded devices. This indicates that component wear is closely associated with in vivo orientation. The success of any implant is dependent upon implant orientation both in version and inclination angles. The correct implant orientation will help to ensure that wear occurs within the bearing surfaces, maintaining an optimal lubrication regime and low wear