How the knee moves

Abstract

The arc of flexion used in almost all the activities of everyday life extends from about 20°±10° to 110°/120°. During this arc, the human knee corresponds to the quadrupedal mammalian knee. Both the femoral surfaces are circular with a similar radius and rotate around their geometrical centres as the knee flexes. The medial femoral condyle does not move antero-posteriorly with flexion, i.e. stability depends on the medial side of the knee. In contrast, the lateral femoral condyle is antero-posteriorly mobile and as it moves it carries the meniscus with it. This AP movement results in longitudinal tibial rotation which is facultative rather than obligatory: if posterior motion occurs, the femur rotates externally around a medial axis with flexion whereas if no AP motion occurs, the knee can flex as would a uniaxial hinge. Rotation first appears in arboreal quadrupeds (apes) and may be becoming vestigial in Man. The axis of longitudinal rotation during flexion, parallel to the tibia and perpendicular to the flexion axis, approximately intersects the latter in the centre of the medial femoral condylar sphere. Varus/valgus rotation, around an AP axis which also passes through the centre of the femoral sphere, permits the lateral femoral condyle to lift away from the tibia because the lateral collateral ligament (LCL) is slack at 90° in mid external/internal rotation. Thus, in the arc ‘20’–120° the medial femoral condyle resembles the femoral head: it is spherical, it does not translate during flexion and all three axes of rotation intersect at its centre. At 90°, forced longitudinal rotation does result in AP movement of the medial condyle and on the lateral side in a reciprocal translation which is almost sufficient to abolish the translation accompanying flexion. This movement occurs around a vertical axis which is slightly lateral to that representing longitudinal rotation with flexion.The arc from 10° to full extension is accompanied by the so-called ‘locking’ and ‘screw-home’. It appears to be a feature of bipedal terrestrial gait with an erect stance, i.e. human gait. Although the arc exists, it is rarely used fully in everyday life. The motion is complex and involves asymmetrical articular surfaces other than those used from 20° to 120°. On the medial side, the femur ‘rocks’ forward onto the upward-sloping anterior surface of the tibia and then rotates into extension around an anterior, larger radiused circular surface. On the lateral side, the femur rolls down onto the anterior horn. The result is ‘lift-off’ of the posterior facets used in the arc ‘20’–120° and progressive tightening of the structures attached posteriorly to the femur, in particular the ACL. This ligament, as it tightens, may move the lateral femoral condyle anteriorly so that extension is accompanied by about 5° of obligatory femoral internal rotation. Flexion and longitudinal rotation occur by rotation around, and translation along, a 20° oblique screw axis penetrating medially the epicondyle and, laterally, the region of the tibio-femoral contact surface.From 120° to full flexion, the motion is passive rather than active. Both femoral condyles move backwards and both lose contact with the tibia. Thus, the tibio-femoral joint is strictly speaking subluxed. Medially, the femoral condyle rolls up onto the posterior horn. Laterally, the femoral condyle rolls backwards and downwards, finally to lie posterior to the tibia, resting on the posterior horn.Although the motion of the knee is complex, it can be (and has been) imaged by MRI in the unloaded cadaveric knee, the unloaded living knee and the loaded living knee. The keys to its understanding are to divide flexion into three arcs and to appreciate that in the functional active arc (‘20’–120°) the medial femoral condyle, like the femoral head, is spherical, that it does not translate and that it rotates around three axes which intersect at its centre. By contrast, the lateral femoral condyle rolls and slides antero-posteriorly on the tibia to result in longitudinal rotation (a possibly vestigial movement in Man) around a medial axis.