The Knee Joint
The knee, like the elbow to some extent, is a hinge joint but not a simple one. The lower part of the thigh bone expands into the femoral condyles and the upper shin bone likewise expands into the tibial condyles, the junction of these two enlarged areas forming the knee joint. The small bone on the outside of the knee, known as the fibula, is not involved in weight bearing nor is actually part of the knee joint, serving only as an attachment area for the muscles controlling the ankle, foot and toes. The knee functions as a hinge dividing the leg, allowing the exertion of high levels of muscular propulsive force, the folding of the leg in bending and the shortening of the leg in stepping to allow efficient gait.
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.
Inside the joint are two crescent-shaped structures made of cartilage, looking a little like banked tracks, accommodating the large rounded femoral condyles. Their exact function is not clear but they may contribute to guiding the knee towards locking, stabilise the knee by centring the condyles during bending and straightening and evening out any potential unwanted small movements during joint motion. The kneecap is the other part of the knee joint and is a small bone with an inner lining of articular cartilage which is suspended in front of the knee joint.
The patella or kneecap is situated in the tendon of the quadriceps muscle which is the major muscle at the front of thigh and responsible for pushing us up from sitting and up and down stairs. The joint surface of the patella fits into the large groove between the front of the femoral condyles, sliding up and down the groove as the knee bends and straightens. The function of the patella appears to be to magnify the power of the quadriceps muscle and so improve its ability to exert the very large forces needed to move the body weight.
When a knee continues to bend and straighten in a forward and backwards direction the alignment is good and problems are less likely to arise. When a sideways misalignment is added however, the knee can develop painful conditions. A sideways alignment (knock knee or bow leg type conditions) throws the stresses onto one side of the knee by compressing that side of the joint, exposing it to increased wear. A misalignment also changes the angles of function of the patella and causes it to track off to one side, increasing joint friction and causing pain.
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 gliding of the tibia backwards and forwards makes certain that the femoral condyles will not slide off the tibia during movement. During knee motion one of the bones moves in relation to the other one, in the sense that the movement of one is occurring over the other which is also moving in a complementary pattern. This allows a much greater range of movement than would otherwise be possible. There is a degree of rotation of the femur which occurs at the knee joint and this is obvious as the knee approaches full extension and the femur turns in slightly to achieve the knee lock.
Jonathan Blood Smyth, editor of the Physiotherapy Site, writes articles about Physiotherapists, physiotherapy, physiotherapists in Birmingham, back pain
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