The forces that determine joint alignment are called vectors. Force vectors move through your knee in all planes of motion. Vectors have both magnitude and direction. Vectors are a function of the position of your body in space. Muscles are the primary controllers of your body’s position, therefore muscles control vectors. Vectors determine the stress concentration in your joint.

Alignment of the knee is significantly affected by 1) the actions of powerful muscles around the hip that initiate subtle changes to control torso position and balance, and 2) actions of the foot and ankle that compensate for propulsion over ever-changing ground surfaces. Hence, the knee can be viewed as a “differential joint” driven by controlled or uncontrolled tri-plane motions at both the hip and foot. Cartilage wear patterns are a function of the resultant multi-plane motion at the knee. The force that may have the most significant effect on cartilage is shear, a translatory vector. Excessive shear on knee cartilage is controlled, in large part, by controlling the adjacent hip, ankle and foot joints. Excessive motion of the foot in pronation (foot-flat) or supination (high-arched) will alter knee alignment and weight-bearing surfaces in the sagittal, frontal and transverse planes. Weakness of hip abductors, extensors, and lateral rotators will contribute to excessive shear forces at the knee in similar planes of motion. Thus, instruction in proper knee alignment controlling excessive shear forces influenced by the linked foot and hip joints in weight-bearing postures should be a focus of rehabilitation programs for osteoarthritis of the knee.