Patients Who Have a Preoperative Flexion Contracture Demonstrate Inferior Outcomes Following Unicompartmental Knee Arthroplasty Compared to Total Knee Arthroplasty

BACKGROUND: The traditional criteria for unicompartmental knee arthroplasty (UKA) excluded patients who have a flexion contracture greater than 5°. However, the outcomes of patients undergoing UKA who have a flexion contracture remain poorly studied. METHODS: Patients undergoing primary UKA and total knee arthroplasty (TKA) between 2010 and 2022 who had a minimum of 2-years follow-up were retrospectively identified. Patients undergoing UKA who had a flexion contracture ≥ 10° were matched in a 1:1 manner based on age, sex, body mass index, smoking status, Charlson Comorbidity Index, and diabetes prevalence to two cohorts: TKA patients who had a flexion contracture ≥ 10° (TKA controls) and UKA patients, who did not have a flexion contracture (UKA controls). Changes in the range of motion as well as Knee Society Score (KSS) were compared. A total of 138 patients were included in the final analysis (n = 46 per cohort). RESULTS: There were comparable changes in the range of mation and KSS values for UKA patients who had a flexion contracture and UKA controls. Patients undergoing UKA who had a flexion contracture had a smaller change in their contracture postoperatively relative to TKA controls (8.4 versus 12.0°; P < 0.001). Despite similar preoperative KSS scores, patients undergoing UKA who had a flexion contracture had lower KSS objective scores at final follow-up relative to TKA controls (88.5 versus 94.9; P = 0.004). The UKA patients who had a flexion contracture were more likely to undergo revision relative to both TKA and UKA controls (10.9% versus 0 versus 0, respectively; P = 0.02). Revisions in the UKA flexion contracture cohort included two for unexplained pain, two for aseptic loosening, and one for infection. CONCLUSIONS: Patients undergoing UKA who had a flexion contracture experienced significantly less improvement in their flexion contracture, inferior patient-reported outcomes, and higher revision rates relative to matched TKA controls. These findings support the historical contraindication for performing UKA procedures in this patient population.