The Human Knee Joint

By Jonathan Blood Smyth

The knee joint is a hinge joint as is the elbow but has a complex hinging action. At the end of the thigh bone is the expanded area known as the femoral condyles, with a similar area on the upper part of the shin bone (the condyles of the tibia), making up the knee. The fibula is a smaller bone on the outside of the shin but not part of the knee joint and does not bear significant weight, functioning as an area of attachment for the ankle, foot and toe muscles. The knee divides the leg by its hinge, functioning to allow the delivery of high levels of muscular propulsive force, to enable shortening of the limb during gait and to bend the knee in sitting and other resting postures.

As our knees come towards straight in order to weight bear the quadriceps comes into action to straighten the leg towards full extension, engaging the locking position of the knee. One of the characteristics of the human knee is that it ensures efficient and safe weight bearing when standing along with very low energy requirements, unlike the knees of apes. As the knee approaches fully straight the inside part of the quadriceps muscle helps the joint swivel inwards into the locked position. When we stand with our knees straight, we can remain in that position with no activity in the quadriceps and so with very low energy output.

With the joint itself are the menisci, cartilaginous structures in a crescent-like structure rather like a banked track, fitting the rounded femoral condyles. It is not entirely clear what their function is but they may help make the knee more stable by keeping the condyles centred during movement, restraining small unhelpful movements as the joint moves and guiding the knee towards the effective position for locking. The patella or knee cap is the third part of of the knee joint and this small bone has its underneath lined with joint cartilage and is held in front of the knee.

The kneecap or patella is placed within the tendon the main thigh muscle or quadriceps, the muscle which enables us to move our body weight up and down stairs and up from a chair. The patella is shaped on its inner surface with two facets, fitting into the groove formed between the condyles of the femur, sliding along the groove as the knee moves. The kneecap is present to allow the muscular forces developed by the quadriceps to be amplified across the knee and so enable application of high levels of power.

The knee is designed to move effectively in a flexion and extension direction and this alignment prevents mechanical problems developing. If a sideways pressure is applied to the knee continually by abnormal posture such as bow leg and knock knee then a painful functional condition can develop. Abnormal forces cause joint compression as the forces are thrown onto one side of the joint and increase the friction and wear. The kneecap can also suffer from misalignment as it glides on one side, causing pain by the increased friction against the side of the groove.

During motion of the knee the menisci (the knee cartilages) and the kneecap are subject to high mechanical forces and are the cause of a number of common knee problems. The movement range of the knee is usually from straight (zero degrees) to full bend at about 140 degrees, varying with the body size and joint mobility. As the knee goes through the movements of extension and flexion the large rounded condyles glide and slide on the top of the tibia, with the tibial surface moving underneath them also.

The forward and backward gliding of the condyles of the femur ensure that they will not slide off the back of the shin bone during motion. In motion one bone does not move in isolation, rather one bone moves in a complicated manner on a complementary bone which is also moving to complete the overall function. This permits a much larger range of motion than would be achievable without such a technique. The femur exhibits an amount of rotation also at the knee which is most discernible as the knee approaches fully straight and the thigh rotates inwards to lock the knee safely. - 32188

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