Similar tibial and femoral component fixation of mobile‐bearing and fixed‐bearing lateral unicompartmental knee arthroplasty: A radiostereometry study with 5‐year follow‐up

Abstract Purpose To compare tibial and femoral component fixation of lateral unicompartmental knee arthroplasty (UKA) with mobile‐bearing (MB) and fixed‐bearing (FB) designs up to 5 years follow‐up. The primary outcome was mean tibial component maximal total point motion (MTPM) at 1‐year follow‐up. Methods The Oxford lateral domed UKA ( n = 14) and the Oxford Fixed Lateral UKA ( n = 13) were inserted in 27 patients with posterolateral unicompartmental knee osteoarthritis. Tibial implants were cemented, while femoral implants were either cemented ( n = 17) or cementless ( n = 10). At baseline, 1‐, 2‐, and 5‐year follow‐up, radiostereometry (RSA) imaging, Oxford Knee Score (OKS) and EQ‐5D‐5L quality of life score were collected. A newly updated 1‐year migration threshold was used to determine 15‐year revision‐risk with migration above 0.4 mm termed ‘at‐risk’. Results At 1‐year follow‐up, tibial implant MTPM was 0.47 mm (confidence interval [CI]: 0.32; 0.62) for MB UKA and 0.52 mm (CI: 0.36; 0.68) for FB UKA. Femoral implant MTPM was 0.56 mm (CI: 0.34; 0.79) for MB UKA and 0.54 mm (CI: 0.30; 0.77) for FB UKA. Migration patterns were similar between cemented and cementless femoral components. No differences in tibial implant MTPM were found between groups at follow‐up, with a mean difference of −0.06 (CI: −0.28; 0.15) at 2‐year and −0.06 (CI: −0.31; 0.18) at 5‐year follow‐up. Further, 16/25 (MB: 7/13, FB: 9/12) implants were in the “at‐risk” group for 15‐year revision. Both groups demonstrated clinically relevant improvements in OKS and European quality of life score (EQ‐5D‐5L), no differences between FB and MB were found. Conclusion The MB and FB tibial components had similar good fixation and clinical improvement until 5 years of follow‐up. MTPM migration for both tibial groups was in the ‘at risk’ group for revision based on the newly established 1‐year migration threshold, but showed no measurable migration after the 1‐year follow‐up. Level of Evidence Level III.