Migration patterns of acetabular cups: a systematic review and meta-analysis of RSA studies

Background and purpose - We conducted a systematic review and meta -analysis of RSA studies to investigate the early and long-term migration patterns of acetabular cups and the influence of implant factors on cup migration over time. Methods - We performed a systematic search of PubMed, Embase, and Scopus databases to identify all RSA studies of cup migration following primary total hip replacement (THR). Proximal migration at 3 and 6 months, 1, 2, 5, and 10 years were considered for analysis. Implant factors investigated included fixation type, head size, bearing surface, uncemented coating design, and the decade of RSA introduction. Results - 47 studies reported the proximal migration of 83 cohorts (2,328 cups). Besides 1 threaded cup design, no implant factor investigated was found to significantly influence proximal migration. The mean pooled 2 -year proximal migration of cemented cups (0.14 mm, 95% confidence interval [CI] 0.08-0.20) was not significantly different from uncemented cups (0.12 mm, CI 0.04-0.19). The mean pooled proximal migration at 6 months was 0.11 mm (CI 0.06-0.16) and there was no significant increase between 6 months and 2 years (0.015 mm, CI 0.000-0.030). 27 of 75 cohorts (36%) reported mean proximal migration greater than 0.2 mm at 2 years, which has previously been identified as a predictor of implants at risk of long-term loosening. Conclusion - Our meta -analysis demonstrated that the majority of cup migration occurs within the first 6 months. With one exception, no implant factors influenced the 2 -year proximal migration of acetabular cups. 36% of studies with 2 -year migration were considered at risk of long-term loosening. Further investigation and comparison against longterm survivorship data would validate 6 -month and/or 1 -year proximal migration measurements as an earlier predictor of long-term loosening than the current 2 -year threshold.