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

Abstract

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 long-term 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.

Figure 1

PRISMA flowchart for inclusion and exclusion of the literature.

Figure 2

Early migration in percentiles of 83 study cohorts, 2,328 cups. The migration of 2 known acetabular cup failures is also plotted as they were outliers: the Link V cup and the ReCap hydroxyapatite coated cup.

Figure 3

Long-term migration of 8 study cohorts. A significant number of patients are often lost at long-term follow-up (10 years); therefore the mean migration may be significantly influenced by an outlier. For example, a patient in the cemented Opticup cohort (retention of sub-subchondral bone) had been radiologically diagnosed with acetabular cup loosening, which may have significantly influenced the mean migration of the cohort. PE = standard polyethylene liner, XLPE = crosslinked-polyethylene liner, CSF = cancellous screw fit.

Figure 4

Reported mean proximal migration from RSA studies that reported proximal migration at 6 months, 1 year, and 2 years. The green dashed line represents the 0.2 mm threshold.

Figure 5

The mean pooled migration overtime (CI) for cemented and cementless acetabular cups.

Figure 6

Mean pooled migration overtime (CI) according to the cementless cup surface type. The threaded cup is only 1 study cohort comprising the Link V cup. TCP = tricalcium phosphate.

Figure 7

Mean pooled migration overtime (CI) by decade of RSA introduction (1990s, 2000s, and 2010s).

Figure 8

Mean pooled migration overtime (CI) according to femoral head size.

Figure 9

Mean pooled migration overtime (CI) according to PE type. XLPE = crosslinked polyethylene, AO = antioxidant.

Figure 10

Mean pooled migration overtime (CI) according to study design. RCT = randomized controlled trial