Preoperative varus ≥10° does not compromise outcome after medial unicompartmental knee arthroplasty, whereas postoperative joint‐line convergence angle and insert thickness influence functional results: A matched‐pair analysis

Abstract Purpose To evaluate functional outcomes and implant survival following medial unicompartmental knee arthroplasty (UKA) in patients with anteromedial osteoarthritis and preoperative varus deformity ≥10° and to identify postoperative alignment thresholds that optimize outcomes. Methods Patients with preoperative varus ≥10° who underwent medial mobile‐bearing UKA were retrospectively analysed and matched 1:1 to knees with preoperative varus <10°, yielding 132 pairs (264 knees) with similar baseline characteristics. Primary outcomes were revision rates and the Oxford Knee Score (OKS). Full‐length standing radiographs were used to measure the hip–knee–ankle angle (HKA), joint‐line convergence angle (JLCA), extra‐articular deformity (EAD), mechanical medial proximal tibial angle (mMPTA), mechanical lateral distal femoral angle (mLDFA) and joint‐line obliquity (JLO) and determine functional knee phenotypes. Univariate curve‐adjusted regression assessed the influence of alignment on OKS, and receiver operating characteristic (ROC) analysis defined the alignment range within which the Patient‐Acceptable Symptomatic State (PASS) was achieved. Results At a mean follow‐up of 10.1 ± 3.3 years, OKS did not differ between groups (40.6 ± 8.0 vs. 38.8 ± 5.7, n.s.), nor did revision‐free survival (92.3% vs. 90.2% at 10 years, p = 0.618). In knees with preoperative varus ≥10°, postoperative JLCA correlated with OKS ( R 2 = 0.43, p < 0.001), whereas HKA, EAD, mMPTA, mLDFA and JLO showed no association ( p > 0.05). JLCA outside 0°–5° varus yielded inferior OKS (35.6 ± 11.0 vs. 42.4 ± 6.2, p = 0.022) and lower PASS achievement (33% vs. 70%, p = 0.011). Insert thickness ≥7 mm was associated with lower OKS (32.1 ± 12.1 vs. 41.2 ± 5.9, p = 0.004) and reduced PASS (30% vs. 81%, p = 0.046). Conclusions Medial UKA in knees with preoperative varus deformity ≥10° achieved functional outcomes and implant survival comparable to matched knees with varus <10°, provided postoperative JLCA is restored to 0°–5° varus and excessive insert thickness is avoided. Intraoperative insert upsizing should prompt reassessment for ligamentous insufficiency or compartment overstuffing. Level of Evidence Level III.