Long-term migration of monoblock vs modular design in uncemented total knee arthroplasty: a secondary report of a randomized trial using radiostereometric analysis

Background and purpose — Backside wear of the polyethylene insert in total knee arthroplasty (TKA) has been described to produce clinically significant levels of polyethylene debris, which can lead to aseptic loosening and osteolysis. Monoblock design eliminates backside wear of the polyethylene and therefore could improve long-term fixation. This randomized clinical trial (RCT) using radiostereometric analysis (RSA) compares micromotion of monoblock and modular polyethylene inserts with 7 years’ follow-up. Methods — 65 patients (mean age 61 years) were randomized to receive either monoblock (n = 32) or modular (n = 33) uncemented trabecular metal tibial components. 35 patients (monoblock = 18, and modular = 17) completed 7 years’ follow-up. The primary endpoint of the study was maximum total point motion (MTPM). Implant translation and rotation are reported as secondary endpoints. Results — After 84 months, the modular group had a statistically significant higher mean MTPM of 1.17 (95% confidence interval [CI] 0.90–1.41) mm compared with the monoblock group of 0.78 (CI 0.55–0.88) mm (P = 0.02). However, there was no difference in continuous migration (from 12–84 months), which was 0.13 mm in the monoblock group and 0.16 mm in the modular group. Conclusion — There was significantly lower early migration in the monoblock group compared with the modular group but no difference in continuous migration after 12 months, which confirms the finding of previous publications.