Medial joint opening on immediate postoperative supine radiographs predicts inferior midterm functional outcomes in women after total knee arthroplasty.

Routine postoperative radiographs are common after total knee arthroplasty (TKA), yet their value for assessing early joint balance is unclear. Immediate supine, non-weight-bearing anteroposterior radiographs may show coronal gap asymmetry, but its long-term significance remains uncertain. This study evaluated whether medial-lateral joint space differences on day-one radiographs correlate with midterm patient-reported outcomes. A retrospective cohort of 637 mechanically aligned primary TKAs (556 patients, 2012-2017) was analysed. Medial and lateral gaps were digitally measured on immediate postoperative supine AP and lateral radiographs. Gap asymmetry was expressed as an absolute difference and as a medial-to-lateral ratio. Patient-reported outcome measures (PROMs)-Oxford Knee Score (OKS), pain VAS, and EQ-5D VAS-were assessed at ≥ 3 years. Multivariable linear and logistic regression adjusted for age, sex, and BMI examined associations between radiographic imbalance and outcomes. Median follow-up was 5.4 years. The median lateral-medial gap difference improved from 4.9 mm preoperatively to 0 mm postoperatively. In women < 70 years, each 1-mm medial opening was associated with a 1-point lower OKS and 2.4-point lower EQ-5D VAS. A 100% increase in the medial-lateral ratio predicted an 8.8-point lower OKS in women < 60 years and a 4-point lower OKS in women 60-70 years. Medial opening ≥ 1.5 mm predicted poorer OKS and EQ-5D scores, whereas mild lateral opening showed outcomes comparable to symmetric knees. No significant associations were found in men. Medial joint space widening ≥ 1.5 mm or ≥ 100% ratio increase on day-one radiographs predicts poorer midterm outcomes in women < 70 years, suggesting that early coronal imbalance has lasting clinical relevance in this group.