The mechanical axis of the lower extremity extends from the center of rotation of the hip to the center of the ankle joint. This axis normally crosses the knee joint in the lateral third of the medial tibial plateau.
The normal anatomic alignment of the knee is in 7 degrees of valgus. When the knee is loaded, the medial compartment experiences 60% and the lateral compartment 40% of the weight-bearing stress.
This difference in the applied load in the normal knee is the reason the medial tibial plateau and medial femoral condyle are larger than the lateral side.
Patients with a significant angular deformity in the knee have altered weight-bearing, which results in increased stress in the medial (with varus or bowlegged deformity) or lateral (with valgus or knock-knee deformity) compartment.
The increased stress frequently results in early arthritis in the overused section of the knee.