Knee joint Biomechanics

Six degree of freedom movement in knee joint

When the subject is standing upright, if the knee is flexed, the vertical line of action of the body weight passes posterior to the centre of rotation of the knee, tending to cause the body to tilt posteriorly. To counterbalance this, continuous quadriceps femoris contraction is required.

When the leg is swinging past the supporting leg, the knee must be flexed in order to avoid dragging the toes on the ground; this requires approximately 67° knee flexion.

Tibial medial–lateral rotation also occurs during gait: the tibia rotates laterally during terminal extension, a phenomenon known as ‘screw-home’. It is surmised that this rotation helps to lock the geometry and tighten the soft tissues, thereby maintaining the knee in a stable position prior to the impact load of weight-bearing. The knee acts as one link in a chain of limb

segments, and this screw-home relates to rotation of both the foot and hip. When the foot is swung forwards for heel strike, the pelvis rotates so that the hip is moved forwards, a movement that entails lateral rotation of the hip. During stance, the femur is medially rotated against the locked knee. Tibial lateral rotation also causes inversion of the foot at the subtalar joint, raising the arch and locking the structure of the foot.

The femur slides anteriorly at the same time as it rolls posteriorly, and thus remains in correct articulation.

With knee flexion, the femur lifts off the anterior horns of the menisci, leading to contact between the smaller radii of the posterior parts of the femoral condyles and the tibial plateau plus the posterior horns of the menisci. This means that the centre of contact moves posteriorly in early knee flexion. After this, the femoral condyles have approximately circular sagittal sections. The femur is now prevented from rolling back any further by tension in the anterior cruciate ligament.

Fully conforming articular surface geometry would allow the greatest area of contact, and thus would minimize contact pressure; however, this is not present in the knee. The medial compartment of the knee is semi-conforming with a convex femoral condyle articulating over a concave medial tibial plateau. The lateral compartment has less conformity; the lateral tibial plateau is flat or slightly convex in sagittal sections. These different shapes reflect the differential movement of the medial and lateral compartments. In normal movement, the screw-home rotation of the tibia occurs about a medial axis, which means that most of the rotation of the tibia is due to an anterior–posterior translation of the lateral compartment.

In order to maintain some degree of conformity, and also to minimize contact pressure between the femur and tibia, the menisci are wedge-shaped in cross-section, thereby increasing the area over which the compressive force on the knee is distributed. In th absence of the menisci, the load is carried by a much smaller area of cartilage, resulting in higher contact stresses on the articular cartilage. This helps to explain the prevalence of osteoarthrosis following meniscectomy.

The load-carrying mode of the menisci is shown in Figure When the femoral condyle presses down on the meniscus, it tends to squeeze the meniscus out of the joint because of its tapered crosssection. This causes the diameter of the circular shape of the meniscus (and therefore the meniscus circumference) to increase. This is resisted by ‘hoop tension’ in strong fibres that pass around the periphery of the meniscus, and transmit the tension to the tibial plateau via strong insertional ligaments. As the tissue is adapted to resist hoop stresses, it has much greater hoop strength (approximately 100 MPa) than radial strength (approximately 3 MPa), which explains why bucket handle tears occur. It also explains why a circumferential tear does not have such a serious effect on meniscal function because the circumferential fibres can still transmit the loads, whereas a radial tear breaks the loadcarryingfibres.

At the time of heel strike, a large impact load acts to compress the surfaces together. At a microscopic level, the surfaces do not come into contact because of the squeeze-film effect.

the synovial fluid cannot all be squeezed out of the joint space because of its viscosity and the narrowness of the space

The anterior cruciate ligament is well aligned to resist the applied anterior force. With an absent anterior cruciate ligament, the tibial collateral ligament can resist the applied force; however, it does so by being loaded to a much higher level than the original loading on the anterior cruciate ligament.