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 (R²= 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.