Sex-related differences in final knee balance during robotic-assisted 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 cohort study including 254 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 (< 0.5 mm and negative values), balanced (0.5-2 mm), or loose (> 2 mm). Patients were stratified by sex and preoperative varus severity, and gaps were analyzed quantitatively and categorically. The independent effects of sex, varus deformity, and operating surgeon on extension and flexion gaps has been assessed. Results: Mean preoperative alignment was 174.2 ± 2.8°, with no sex-related difference in baseline varus (p = 0.058). Flexion gaps were significantly tighter in female patients (p = 0.0045). A tight F90 gap was observed in 17.2% more female than male patients (p = 0.023), with an absolute mean difference of 0.35 mm (95% CI - 0.62 to - 0.11 mm). Multivariate linear regression and multinomial logistic regression confirmed tighter F90 gaps in female patients (β = -0.38; 95% CI - 0.63 to - 0.14; p = 0.002; OR = 2.25; 95% CI 1.30-3.88; p = 0.0036, respectively). Patient sex did not influence the extension gap (p = 0.686). Increasing preoperative varus showed a potential association with tighter extension spaces (ANOVA p = 0.0064), but not with flexion balance. The operating surgeon significantly influenced both extension and flexion gaps. Conclusion: Female patients tended to present a tighter flexion gap at 90° in robotic-assisted medial unicompartmental knee arthroplasty. Although the absolute difference was small, it was measurable, suggesting that studies focused on clinical outcomes will be needed to justify adjustments in the execution of mUKA in female patients. Surgeon-related decisions continued to exert a measurable influence on results.