Sex and Preoperative Varus Deformity Have Distinct Impacts on Final Gap Balance Following Robotic-Assisted Image-Based Medial Unicompartmental Knee Arthroplasty

Introduction In robotic medial unicompartmental knee arthroplasty (mUKA), a preoperative alignment strategy can be defined and subsequently adjusted through stress testing to achieve optimal soft-tissue balance. However, the application of uniform planning principles does not necessarily yield equivalent balance in all patients. The aim of this study was to examine differences in intraoperative balance according to patient sex and the degree of preoperative varus deformity. Materials and Methods A single-center retrospective study of MAKO® robotic-assisted medial UKAs (2018–2025) was performed using a uniform intraoperative planning protocol. Gap widths were measured in millimeters at full extension and 90° of flexion (F90) and classified as tight (< 0.5mm), balanced (0.5–2mm), or loose (> 2mm). Patients were stratified by sex and preoperative varus severity, and gaps were analyzed quantitatively and categorically. Multivariate linear and multinomial regression assessed the independent effects of sex, varus deformity, and surgeon on extension and flexion gaps. Results Mean preoperative alignment was 174.2° ± 2.8°, with no sex-related difference in baseline varus (p = 0.058). The mean extension and F90 were 0.79 ± 0.87 mm and 0.14 ± 1.0 mm, respectively. Flexion gaps were significantly tighter in women (p = 0.0045), whereas extension gaps did not differ by sex (p = 0.686). Categorically, 68 knees were tight in extension and 142 in flexion, with significantly tighter flexion balancing in females (p = 0.023). Increasing preoperative varus showed a significant association with tighter extension spaces (ANOVA p = 0.0064; multinomial p = 0.03), but not with flexion. In multivariate analysis, preoperative varus and surgeon predicted extension gaps, whereas female sex and surgeon independently predicted tight flexion gaps (p = 0.0036). Conclusion Using a uniform planning strategy intraoperative balance remained heterogeneous: increasing preoperative varus independently predicted tight extension gaps, whereas female sex independently predicted tight flexion gaps. Surgeon-related decisions continued to exert a measurable influence on results.