The movement of the knee studied by magnetic resonance imaging.

The author's work using magnetic resonance imaging to study the relative movements (the kinematics) of the tibia and femur is reviewed. The description is understood best by reference to comparative anatomy and by dividing the flexion arc into three components. Knee activities take place mainly between 10 degrees and 120 degrees. Over this arc, the articulating surfaces of the femoral condyles are circular in sagittal section and rotate around their center. The medial condyle does not move anteroposteriorly (roll-back does not occur medially). The lateral condyle tends to roll back producing tibial internal rotation with flexion. From full extension to 10 degrees to 30 degrees tibial internal rotation is coupled with flexion. The articulating surfaces medially are a larger radiused anterior femoral facet, which articulates with an upward-sloping tibial facet. Laterally, the femoral condyle rolls forward onto the anterior horn. Flexion beyond 120 degrees only can be achieved passively. Medially, the femur rolls up onto the posterior horn. Laterally, the femur and the posterior horn drop over the posterior tibia.