Robot-Assisted TKA for Varus Knees: Post Hoc Exploratory Analysis of Alignment Strategy and Deformity Severity

Background: Robot-assisted total knee arthroplasty (raTKA) improves the precision of component positioning and coronal alignment restoration, but it remains uncertain whether that technical accuracy modifies the clinical effect of alignment strategy in different varus phenotypes. The present report evaluates alignment strategy and correction magnitude, explicitly as a post hoc exploratory deformity-subgroup analysis within a randomized raTKA cohort. Methods: This single-center, open-label, randomized study enrolled 296 patients with varus knee osteoarthritis who underwent raTKA between 2023 and 2025 using either mechanical alignment (MA; n = 149) or limited/restricted kinematic alignment (lim.-KA; n = 147). The parent randomized comparison was conducted at the whole-cohort level; the deformity-based subgroups reported here were defined after the whole-cohort analysis and are therefore post hoc and exploratory. Patients were stratified according to preoperative varus severity into a mild-deformity subgroup (≤10°; lim.-KA-I n = 99, MA-I n = 102) and a moderate-deformity subgroup (11-20°; lim.-KA-II n = 48, MA-II n = 47). Outcomes included hip-knee-ankle angle (HKA), correction angle, range of motion (ROM), visual analog scale (VAS; 0-10 points), Knee Society Score (KSS; knee and function), Oxford Knee Score (OKS), and Forgotten Joint Score-12 (FJS-12) over 12 months. Estimates are presented with 95% confidence intervals where applicable. Because multiple post hoc subgroup comparisons were performed without formal multiplicity adjustment, p-values are interpreted descriptively and in conjunction with effect sizes and 95% confidence intervals. Results: The primary whole-cohort randomized comparison did not demonstrate an overall between-group advantage of either alignment strategy. The post hoc moderate-varus subgroup showed favorable unadjusted 12-month differences for lim.-KA versus MA in KSS-knee (+6.8 points; 95% CI 5.3 to 8.3; nominal p < 0.001), KSS-function (+4.0 points; 95% CI 2.7 to 5.2; nominal p < 0.001), OKS (+6.4 points; 95% CI 4.5 to 8.3; nominal p < 0.001), and FJS-12 (+11.3 points; 95% CI 9.4 to 13.1; nominal p < 0.001). In contrast, ROM favored MA rather than lim.-KA in the moderate-varus subgroup (-11.8°; 95% CI -16.6 to -7.0; nominal p < 0.001), indicating greater 12-month ROM after MA, and VAS pain, reported on a 0-10 scale, did not support a lim.-KA pain advantage (+0.26 points; 95% CI 0.05 to 0.48; higher scores indicate worse pain; nominal p = 0.018). Exploratory, unadjusted, post hoc 12-month alignment-by-deformity interaction terms were significant for ROM, KSS-knee, KSS-function, OKS, and FJS-12, but not for VAS. Because multiple post hoc comparisons were performed without formal multiplicity adjustment, the results are interpreted descriptively, along with effect sizes and confidence intervals. Conclusions: The primary randomized comparison did not demonstrate a clinical advantage of lim.-KA over MA in the whole cohort. In post hoc exploratory analyses, mild varus deformity was associated with outcomes broadly similar to those after both alignment strategies. In the moderate-varus subgroup, patient-level analyses suggested a possible phenotype-dependent signal for KSS-knee, KSS-function, OKS, and FJS-12 after lim.-KA, whereas ROM favored MA, and VAS pain did not support a lim.-KA pain advantage. These subgroup findings should be interpreted separately from the primary randomized result, considered hypothesis-generating only, and not used in isolation to change clinical practice without prospective confirmation.