BACKGROUND: The rising demand for unicompartmental knee arthroplasty (UKA) in the United States has led to high one-year patient satisfaction rates. However, some patients experience substantial declines in patient-reported outcome measures (PROMs) post-operatively, which we refer to as "Minimal Clinically Importance 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. METHODS: 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. RESULTS: 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 revision (P=0.02), reoperation (P=0.03), postoperative complications (P=0.002), deep venous thrombosis (DVT) (P<
.001), and pneumonia (P=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 pre-operative PROMIS PF-10a scores (OR=1.07) were risk factors for scoring below the MCID-W threshold, whereas age >
80 (OR=2.89) and pre-operative PROMIS PF-10a scores (OR=1.05) were risk factors for failing to achieve MCID-I. CONCLUSION: 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.