Movements of the joints

The movements actually occurring between articular surfaces can be complex; the terms spin, roll and slide are used to help explain them. Spin, where one surface spins relative to the other, occurs about a fixed central axis(figure a). Roll is where one surface rolls across the other so that new parts of both surfaces are continually coming into contact with each other, as in a wheel rolling along the ground(figure a). Slide occurs when one surface slides over the other so that new points on one surface continually make contact with the same point on the other surface(figure a).

Normally, it is unusual for spin, roll and slide to occur separately as they complement one another in order that the complex movements available at joints are possible. Combinations of spin, roll and slide are the basic components underlying movement at all joints. This concept can best be illustrated at the knee joint, a modified hinge joint, because of the type of movement available: a true hinge joint would permit slide only as one surface slides past the other about a fixed axis. For example, if only sliding movements were possible at the knee movement would soon be restricted because of contact of popliteal surface of the femur with the posterior part of the tibial condyle(figure b - i). Similarly, if the femoral condyles only rolled over the tibial plateaux the situation would soon be reached where the femur would hypothetically roll of the tibia, because the profile of the femoral articular condyle is much longer than that of the tibia(b – ii). The actual movement at the knee joint is a combination of both rolling and sliding between the two articular surfaces, under the control of the ligaments of the joint, allowing a greater range of movement to be achieved(figure b – iii). Spin also occurs at the knee joint as it approaches full extension, as the femur spins on the tibia about its longitudinal axis so that the medial femoral condyle moves backwards(figure b – iv). The resultant effect is to place the knee into its close-packed position of maximum congruity between the joint surfaces. As with the combination of rolling and sliding, the ligaments of the joint are primarily responsible for bringing about spin at the knee.