Internal Torsion of the Knee: An Embodiment of Lower-Extremity Malrotation in Patients with Patellar Instability.

Abstract

Torsion of the lower extremities has been suggested to be a predisposing factor for patellar instability. However, no clear torsional factors have been clarified. This study aimed to elucidate the rotational geometry of the lower limb through segmental analysis in patients with patellar instability. Eighty-three patients with patellar instability were included. Computed tomography of the lower limb was acquired. Femoral and tibial torsion were measured in different segments. Femoral and tibial torsion in each segment was compared between the high- and normal-torsion groups to investigate which segment contributes the most. All segments of femoral and tibial torsion except proximal femoral torsion showed significant differences between the high- and normal-torsion groups. The average proximal femoral torsion was 1.9° ± 7.1°, indicating that there was only slight torsion in the segment of the femoral neck. The angle between the femoral neck and the foot orientation in the normal-torsion, high-torsion tibial, and combined high-torsion femoral and tibial groups was 89.1° ± 12.0°, 81.3° ± 9.4°, and 98.2° ± 11.7°, respectively (p < 0.001), which suggested that the femoral neck remained nearly perpendicular to the foot orientation in the normal-torsion group. Shaft and distal femoral torsion contributed the most to total femoral torsion. For tibial torsion, from distal to proximal, internal torsion of both the proximal and distal segments contributed to the high torsion. There was no significant difference between distal femoral torsion and proximal tibial torsion, which suggested that, rather than the distal femur rotating internally on its own, the distal femur and the proximal tibia simultaneously rotated internally. In patients with patellar instability, torsional deformity occurs along the length of the tibia and in the shaft and distal segments of the femur. The comprehensive embodiment of lower-extremity malrotation is an internal rotation deformity of the knee. This study elucidated the rotational geometry of the lower limb through a detailed segmental analysis in patients with patellar instability. It could serve as a theoretical basis for choosing a derotational osteotomy site and may be a reference for additional clinical research.