The Figure-of-4 Maneuver: Quantitative Characterization of Lateral Compartment Behavior During Robotic Total Knee Arthroplasty

Introduction Conventional intraoperative gap assessment is performed in standardized reference positions that do not replicate many functional postures patients assume during daily activities. Physiologic lateral compartment opening becomes most apparent in postures combining flexion, hip external rotation, and relative varus alignment, such as sitting cross-legged, and shoe or sock application. The Figure-of-4 position reproduces this combination intraoperatively. The purpose of this study was to describe the Figure-of-4 maneuver as an intraoperative positional assessment during robotic total knee arthroplasty (rTKA) and quantitatively characterize compartment behavior during its execution using standardized robotic measurements. Materials and Methods Eighty-three consecutive primary rTKAs performed by a single surgeon were retrospectively analyzed. After bone preparation and balancing, with trial components in situ, each knee underwent conventional and Figure-of-4 assessment. The VELYS standardized robotic measurement framework was used for both conditions; only limb posture differed. Compartment behavior was expressed as the medial-to-lateral gap difference (ΔML; medial minus lateral) in flexion and extension. Conventional and Figure-of-4 ΔML were compared within each knee using paired t-tests and Wilcoxon signed-rank tests. Preoperative hip-knee-ankle (HKA) angle was correlated with the change in ΔML using Pearson and Spearman coefficients. Results Figure-of-4 assessment produced consistently more negative flexion ΔML than conventional assessment (mean difference −2.181 mm; 95% CI −2.414 to −1.947; p < 0.001; Cohen's d = 2.039). Extension ΔML did not differ significantly (mean difference 0.080 mm; 95% CI −0.019 to 0.178; p = 0.113). Preoperative HKA angle did not correlate with the change in flexion ΔML (r = 0.091, p = 0.416) or extension ΔML (r = −0.111, p = 0.318). Conclusion Figure-of-4 assessment consistently demonstrated greater lateral compartment opening in flexion than conventional assessment, while extension measurements remained essentially unchanged. The standardized robotic measurement framework enabled objective quantification of compartment behavior in a positional condition not evaluated during routine assessment. The maneuver characterizes behavior of the prepared knee in a posture associated with common activities of daily living and provides information complementary to conventional assessment.