Varus thrust is a multiplanar dynamic phenomenon: Evidence from three-dimensional gait analysis in a population-based cohort

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BACKGROUND: This study aimed to elucidate detailed knee kinematics associated with varus thrust in both healthy individuals and those with varus knee osteoarthritis (OA), using three-dimensional (3D) gait analysis data from the Matsudai Knee OA Epidemiological Survey. METHODS: Of 459 participants who underwent 3D gait analysis in 2019, 383 (218 females, 165 males; mean age 68 ± 13 years) with complete data were included; the right limb was analyzed in every participant. Kinematics were expressed in two frames: the Grood coordinate system (shank relative to thigh) and the world coordinate system (absolute segment orientation). Three independent examiners classified participants as thrust (-), thrust (±), or thrust (+). Six-degree-of-freedom kinematics during stance were compared using repeated-measures ANOVA with Tukey's post hoc test. Analysis of covariance (ANCOVA) was additionally performed for the loading response phase, adjusting for age, sex, BMI, femorotibial angle, Kellgren-Lawrence grade, and gait velocity. RESULTS: During the loading response phase, the thrust (+) group showed greater shank adduction and internal rotation relative to thigh in the Grood frame (p < 0.001); the same motion in the world frame corresponded to greater shank lateral inclination, thigh medial inclination, and thigh external rotation (all p < 0.001) versus thrust (-). After ANCOVA adjustment, shank adduction (Grood, p < 0.001), thigh medial inclination (world, p < 0.001), shank lateral inclination (world, p = 0.003), thigh external rotation (world, p = 0.004), and Grood internal rotation maximum (p = 0.013) remained significant. CONCLUSIONS: To our knowledge, this is the first epidemiological study to characterize knee kinematics associated with varus thrust using 3D gait analysis. Visually assessed varus thrust was associated with a coherent, segment-specific kinematic pattern in both reference frames that persisted after adjustment for major confounders. Its relevance for rehabilitation and OA progression remains to be tested in longitudinal and interventional studies.

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