WHEN IS 45 DEGREES NOT 45 DEGREES? ANALYSIS OF THE TRUE INCLINCATION ANGLE OF RESURFACING SOCKETS

Abstract

Introduction: Acetabular cup inclination is associated with higher wear in both UHMWPE and ceramic-on-ceramic hips. It has also emerged recently as a critical factor in the success of hip resurfacing. A direct correlation between socket inclination and serum metal ion levels has been demonstrated and concerns remain regarding edge loading and hypersensitivity. Resurfacing socket designs vary and manufacturers do not recommend specific inclination angles for their products. The aim of this study was to investigate the effect of cup geometry on the effective inclination for a variety of different designs of resurfacing socket.

Methods: The internal geometry of every available size of Birmingham Hip Resurfacing(BHR-Smith & Nephew), ASR(DePuy) and MITCH TRH(Stryker) socket was measured using X-ray templates. This data was used to calculate the difference between the apparent inclination and the true inclination for all sockets.

Results: BHR and MITCH TRH cups have lateralised internal geometries in that the centre of the internal surface is lateralised by 3.5mm in the case of the BHR and varies from 2 to 3.8mm in the MITCH TRH system. The ASR is concentric but the bearing surface is less than a hemisphere and there is also an internal groove which further reduces the bearing surface. The difference between apparent and true inclination was greatest in the ASR socket (13o–18 o) followed by the BHR (7o–10o) and then the MITCH TRH (6o–8o). This difference increased with decreasing socket size in both the BHR and ASR but was more or less constant in the MITCH TRH.

Conclusion: These results show that a socket measured at 45o of inclination may have a true inclination of up to 63o. This has important implications for surgeons who should have full knowledge of the design geometry of the socket being implanted and adjust the inclination angle accordingly.