Adverse wear reaction after metal-on-metal hip resurfacing has been described as a new failure mechanism. Established causes include poor implant design, small implant size and acetabular malpositioning. The purpose of this study is to establish a safe zone of acetabular inclination angle (AIA) in order to reduce the high metal ion level rate after hip resurfacing.
THOMAS P. GROSS, MD, FEI LIU, PHD MIDLANDS ORTHOPAEDICS, P.A.
COLUMBIA, SC, USA
761 cases in 613 patients with minimal two years follow-up had both metal ion levels and quality pelvis X-ray available in our database and are included in this study. The UCLA activity score, femoral shaft angle, body mass index, weight, American Society of Anesthesiologists (ASA) score, combined range of motion (CROM), diagnosis, age, implant brand, gender, AIA, bearing size, and duration of implantation were analyzed to determine the potential risk factors for elevated metal ion levels with use of uni- and multi-variable logistic regression models. A safe zone for hip resurfacing (RAIL: Relative Acetabular Inclination Limit) was calculated based on implant size and AIA on AP pelvis X-ray.
For AIA below the RAIL, there were no adverse wear failures or dislocations, and only 1% of cases with ion levels above 10ug/L. We have not found a lower limit of AIA where failures occurred. Other than high inclination angle and small bearing size, female gender was the only other factor that correlated with high ion levels in the multivariate analysis.
We have described the robust "safe zone" for acetabular component position based on metal ion levels in a large patient cohort for metal-on-metal hip resurfacing arthroplasty. Our study suggests that adverse wear failures with hip resurfacing may be highly predictable and avoidable. If the AIA is below the RAIL, rare dislocations are also prevented.