Risk Factors and Thresholds for Minimal Clinically Important Difference in Worsening after Unicompartmental Knee Arthroplasty.

The rising demand for unicompartmental knee arthroplasty (UKA) in the United States has led to high 1-year patient satisfaction rates. However, some patients experience substantial declines in patient-reported outcome measures (PROMs) postoperatively, which we refer to as "minimal clinically important difference for worsening" (MCID-W). We sought to define MCID-W values for specific PROMs and identify risk factors associated with PROMIS Physical Function Short Form 10a (PROMIS PF-10a) declines after UKA. We conducted a retrospective study of 760 patients undergoing UKA at our institution between 2016 and 2023. Preoperative and postoperative PROMIS PF-10a, PROMIS global physical, and knee injury and osteoarthritis outcome score-physical function short-form (KOOS-PS) scores were collected. Patients were stratified upon reaching minimal clinically importance difference for improvement (MCID-I), MCID-W, or "no significant change" (score between MCID-W and MCID-I). MCID-W and MCID-I values were determined using a distribution-based method. Logistic regression was performed to identify risk factors for scoring below MCID-W. We established the following MCID-I and MCID-W thresholds: PROMIS PF-10a (+3.00 and -1.64), KOOS-PS (+6.25 and -3.42), and PROMIS Global-Physical (+2.72 and -1.55). Bivariate analysis revealed differences in terms of revision ( = 0.02), reoperation ( = 0.03), postoperative complications ( = 0.002), deep venous thrombosis (DVT;  < 0.001), and pneumonia ( = 0.01) between cohorts. Body mass index >35 (odds ratio [OR] = 2.49), postoperative complications (OR = 5.09), pneumonia (OR = 22.39), DVT (OR = 9.27), and preoperative PROMIS PF-10a scores (OR = 1.07) were risk factors for scoring below the MCID-W threshold, whereas age > 80 (OR = 2.89) and preoperative PROMIS PF-10a scores (OR = 1.05) were risk factors for failing to achieve MCID-I. Our study established MCID-W values for pivotal PROMs after primary UKAs. We found that 8.8% of patients scored below MCID-W, highlighting the need to improve patient selection and perioperative care in UKA.