Abstract
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.
