Progression of varus deformity in osteoarthritic knees induces anterior paradoxical motion of the femur during early knee flexion

Purpose The purpose of this study was to investigate the position of the femur relative to the tibia throughout range of motion in the osteoarthritic knee to evaluate knee kinematics and assess its relationship with the degree of varus deformity. Methods In this study, 116 preoperative knees with varus deformity were evaluated using a navigation system. The internal–external, anteroposterior, and mediolateral positions of the femur relative to the tibia were measured at maximum extension, 15°, 30°, 45°, 60°, 90°, 105°, and 120°, and maximum flexion angles. From these parameters, two-dimensional translation of the surgical epicondylar axis was projected onto the tibial axial plane, and the femoral movement was evaluated relative to the tibia. In addition, the knees were retrospectively classified into three groups according to their degrees of preoperative hip–knee–ankle angle: mild (< 10°), moderate (10°–20°), and severe (> 20°). Then, the differences in each parameter between these groups were investigated. The Steel–Dwass test was performed to identify the difference among three groups. Statistical significance was set at p values < 0.05. Results There was a significant difference in the anteroposterior position of the femur relative to the tibia among the three groups, especially from extension to early flexion ( p < 0.05). The anteroposterior position at knee extension deviated posteriorly according to the progression of varus deformity. Rotational and mediolateral translation were not significantly different among the groups. Normal knee kinematics were diminished in almost all cases in each group. In addition, anterior paradoxical motion of the femur during early knee flexion was observed in 45.6% ( n = 26), 57.1% ( n = 28), and 80.0% ( n = 8) of cases in the mild, moderate, and severe groups, respectively. The anteroposterior position of the femur relative to the tibia at knee extension was significantly more posterior in patients with than in those without anterior paradoxical motion ( p < 0.0001). Conclusion The anteroposterior position of the femur relative to the tibia changed according to the progression of varus deformity in osteoarthritic knees, especially from knee extension to early flexion. Posterior deviation of the femur at knee extension induced its anteroposterior movement relative to the tibia, resulting in anterior paradoxical motion. Level of evidence III.