THE AXES OF ROTATION OF THE KNEE

Knee motion is believed to occur about a variable flexion-extension (F E) axis perpendicular to the sagittal plane and a longitudinal rotatio n (LR) axis. The authors used a mechanical device to locate the FE and the LR axes of six fresh anatomic specimen knees. The motion of point s on the LR axis produced circular, planar paths about the fixed FE ax is. Magnetic resonance (MR) images in planes perpendicular to the FE a xis showed a circular profile for the femoral condyles. The FE axis is constant and directed from anterosuperior on the medial side to poste roinferior on the lateral side, passing through the origins of the med ial and lateral collateral ligaments and superior to the crossing poin t of the cruciates. The LR axis is anterior and not perpendicular to t he FE axis, the anatomic planes. This offset produces the valgus and e xternal rotation observed with extension. The implications of two fixe d offset axes for knee motion on prosthetic design, braces, gait analy sis. and reconstructive surgery are profound.