Coronal Tibio-Femoral Subluxation Leads to Inferior 15-Year Survival After Mobile-Bearing Unicompartmental Knee Arthroplasty.

Abstract

The impact of coronal tibiofemoral subluxation (CTFS) on 15-year outcomes after unicompartmental knee arthroplasty (UKA) remains unclear. This study evaluated whether preoperative CTFS influences implant survivorship. A total of 175 patients (219 knees) underwent medial UKA between 2007 and 2017 and were prospectively followed; the mean age was 64 years (range, 20 to 92), and CTFS was 4.1 ± 2.8 mm. Patients were stratified by preoperative CTFS: ≥ five mm (subluxation group, n = 87) and less than five mm (non-subluxation group, n = 132). Outcomes were assessed with Kaplan-Meier survival, competing risk analyses, and multivariate Cox regressions. The 10- and 15-year survivorships for the entire cohort were 94.7 and 92.2%, respectively, after a minimum of eight years of follow-up (mean, 11.8 ± 3.8). Kaplan-Meier analyses demonstrated lower survivorship in the subluxation group, with 95.4, 90.5, and 88.3%, respectively, at five, 10, and 15 years compared with 99.2, 97.6, and 95.0% in the nonsubluxation group (P = 0.020). Competing risk analyses confirmed this difference (P = 0.020). Preoperative CTFS ≥ five mm was independently associated with an increased conversion risk (hazard ratio 3.9, P = 0.029), whereas the hip-knee-ankle angle was not. Neither preoperative correctability on postero-anterior flexion radiographs nor residual postoperative subluxation significantly altered the risk of conversion. The most common indications for conversion were progression of osteoarthritis (n = seven) and aseptic loosening (n = six). All cases of aseptic loosening occurred exclusively in the subluxation group (P = 0.002). Both groups exhibited survivorship higher than 90% at 15 years; however, preoperative CTFS ≥ five mm was associated with a higher risk of conversion to TKA due to aseptic loosening after mobile-bearing UKA. CTFS should be incorporated into preoperative evaluation and counseling before UKA.