Comparison of unicompartmental knee prosthesis alignment with functional outcomes

Introduction. Unicompartmental knee arthroplasty (UKA) is a treatment reserved for single-component osteoarthritis. There are two types of UKA prosthesis: mobile bearing (MB) with a moving implant between the femoral and tibial component, and fixed bearing (FB) with a stable one. It is still under debate what kind of UKA is better, with some studies suggesting no difference between MB and FB. Aim of the study. This study aimed to assess and compare the alignment of unicompartmental knee arthroplasties (UKA) and its impact on functional outcome. Material and methods. Sixty-nine adult patients referred for UKA with a set of X-ray pictures were included in this retrospective study. 28 patients were treated with FB and 41 with MB. Two independent researchers evaluated short radiographs for the alignment of both prosthesis components in anterior-posterior and lateral projections. The functional outcome of arthroplasty was assessed by Knee Osteoarthritis and Outcome Score (KOOS) and International Knee Documentation Committee Subjective Knee evaluation form (IKDC) in 41 patients. Results. The femoral component in MB was placed 4° more varus than in FB. On the lateral view, the difference between placements was 5° for MB placed in flexion and FB placed in extension. There were no significant differences in tibial component alignment. The Spearman correlation test showed a moderately strong negative correlation (R = –0.55) between KOOS Q and femur varus/valgus parameters in the MB group with p = 0.03. Conclusions. More varus femoral component placement in the MB group resulted in better functional outcomes. Other components in both FB and MB did not reveal any correlation with functional outcomes.