Proximal tibia vara and tibial overhang are associated with inferior outcomes after medial unicompartmental knee arthroplasty

PURPOSE: Medial unicompartmental knee arthroplasty (UKA) provides reliable outcomes in appropriately selected patients with medial compartment knee osteoarthritis. However, the effect of proximal tibia vara on implant positioning, alignment correction and patient-reported outcome measures (PROMs) is not completely defined. This study aimed to evaluate the association between proximal tibia vara, tibial component positioning, postoperative coronal alignment and clinical outcomes after medial UKA. METHODS: Two hundred twelve patients who underwent medial UKA between 2014 and 2020 were included. Analysed parameters included medial proximal tibial angle (MPTA), hip-knee-ankle angle (HKA), joint line convergence angle (JLCA) and lateral distal femoral angle (LDFA). CPAK (Coronal Plane Alignment of the Knee) type and functional knee phenotypes were calculated. Patients were stratified into a normal tibia group (MPTA ≥ 85°, n = 118) and a tibia vara group (MPTA < 85°, n = 94). Clinical outcomes were assessed using the Oxford Knee Score (OKS), Knee Society Score (KSS clinical [KSS] and functional [KSS-F] subscales) and the Forgotten Joint Score-12 (FJS-12). RESULTS: Outcomes were significantly better in the normal group, with higher KSS (p = 0.036), OKS (p = 0.005) and FJS-12 (p = 0.006) scores. The majority of patients were classified as CPAK Type 1 (n = 80), while the most frequent phenotype was Var9Var3Var3 (n = 15). Alignment correction was smaller in the tibia vara group, with a lower mean ΔHKA (3.0 ± 2.8° vs. 3.7 ± 2.7°, respectively, p = 0.015). In subgroup analysis, mobile-bearing UKA demonstrated a significantly lower FJS-12 in tibia vara knees (p = 0.018), whereas fixed-bearing results were comparable between groups. The presence of component overhang was associated with worse clinical outcomes. CONCLUSION: Proximal tibial vara was associated with lower postoperative scores after medial UKA. Tibial component overhang independently impaired all clinical outcomes, underscoring the clinical relevance of tibial morphology and precise component positioning in medial UKA. LEVEL OF EVIDENCE: Level III.