In order to understand the actual weight-bearing condition of lower extremity, the three dimensional (3D) mechanical axis of lower limb was compared with the loading direction of ground reaction force (GRF) in standing posture.

Three normal subjects (male, 23–39 yo) participated in the study. A bi-planar radiograph system with a rotation table was used to take frontal and oblique images of entire lower limb. Each subject’s lower limb was CT scanned to create 3D digital models of the femur and tibia. The contours of the femur and tibia in both radiographs and the projected outlines of the 3D digital femur and tibia models were matched to recover six-degree of freedom parameters of each bone. The 3D mechanical axis was a line drawn from the centre of the femoral head to the centre of the ankle. A surface proximity map was created between the distal femoral articular surface and the proximal tibial articular surface. A force plate was positioned on the rotation table to measure GRF during biplanar X-ray exposure. Each subject put one’s foot measured on the force plate and the other on the shield. Bi-planar radiographs were taken in double-limb standing, double-limb standing with toe up in the leg measured, and single-limb standing. The anterior and medical deviations of the loading direction of GRF from the 3D mechanical axis were determined at the proximal tibia and normalized by the joint width in anteroposterior direction and by the joint width in lateral direction.

For all subjects the passing points of the 3D mechanical axis at the proximal tibia were almost in the middle of the joint width in lateral direction. Compared to the 3D mechanical axis, the loading direction of GRF passed through the anterior region in double-limb standing and single-limb standing, and anteromedial region in single-limb standing. The normalized medial deviation was significantly greater in singlelimb standing than in double-limb standing (p=0.023). The separation distance tended to decrease in the medial compartment in single-limb standing, and to increase in toe up in the entire region.

Deviation of the loading direction of GRF from the 3D mechanical axis at the proximal tibia varied among standing postures, relating to the change in weightbearing condition as indicated in the separation distance map. These results provide the mechanical perspective related to the causes and progression of knee OA and may contribute to the improvement of surgical treatments such as arthroplasty and osteotomy.