The objective of this study was to compute the in vivo dynamic tibiofemoral contact forces for normal alignment, and then evaluate the change in contact forces and pressures with increasing varus-valgus and internal-external rotational malalignment of the femoral component. A three-dimensional computational model of the lower limb during deep knee bend was created using Kane’s method of dynamics. The change in forces from normal with malalignment of up to 10° valgus, 10° varus, 10° internal axial femoral rotation, and 10° internal axial femoral rotation were determined. In this study, varus-valgus malalignment had the greatest effect on medial-lateral pattelofemoral contact forces, with a maximum increase of 2.25 times body weight for 10° valgus malalignment. Axial malalignment had the greatest influence on tibiofemoral contact forces.

Numerous dynamic studies have evaluated the tibiofemoral contact pressures that follow total knee arthroplasty (TKA), and several static studies utilizing finite elements and pressure sensitive film have evaluated malalignment. The objective of this study was to compute the in vivo dynamic tibiofemoral contact forces for normal alignment and evaluate the change in contact pressure with increasing malalignment of the femoral component.

A three-dimensional computational model of the lower limb during deep flexion was created using Kane’s method of dynamics. A hybrid approach was used to determine the boundary conditions of the model. The motions of a total knee arthroplasty patient were measured using fluoroscopy. The motions of the patient were varied from the normal motions to simulate malalignment of the femoral component. The change in forces with malalignments of up to 10° valgus, 10° varus, 10° internal rotation, and 10° internal rotation were determined.

An increase in the axial tibiofemoral contact force from 2.44 times body weight (BW) to 2.62 BW and a decrease in the quadriceps force from 6.8 to 5.65 BW were observed with varus malalignment. The medial-lateral patellofemoral contact force decreased from 0.95 BW to 0.1 BW with 10° varus positioning of the femur and increased to 2.2 BW with 10° valgus positioning of the femur and a decrease in the patellar ligament forces from 1.70 to 1.63 BW was observed.

Changes in the tibiofemoral and patellofemoral forces of 1–2 BW were observed as the femur was malaligned with respect to the tibia. The most significant of these changes was the medial-lateral patellofemoral contact force. The implications of these findings are that malalignment could result in increased patellar subluxation or increased wear of the polyethylene component. Concerns were raised that this initial subject evaluated may not have had optimum alignment, thus leading to more optimal bearing surface stress conditions with varus malalignment. Future studies will be evaluated for subjects having the joint line restored to conditions for non-implanted knees.