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 the knee approaches full extension or straightening the main thigh muscles (the quadriceps) engage to achieve full active extension and guide the knee into its locking position. Apes are unable to straighten their knee in standing, whilst humans can extend their knees fully in standing which allows very low energy usage. As the knee gets closer to full extension the thigh muscle rotates the knee inwards to allow it to reach the locking point. Humans can stand with their knees straight for long periods with little activity in the quadriceps and hip muscles, combining stability and low effort.
The cartilages, structures known as menisci, are shaped like a banked track in a crescent configuration and fill in to some extend between the rounded condyles of the femur and the flat tibial surface. Their function has not exactly been demonstrated, however they may contribute to easing the knee into lock, minimising inappropriate movement of the condyles and bringing the condyles towards the joint centre in motion. The third component of the knee joint is the patella or kneecap, this small bone being lined with cartilage and placed anterior to the knee.
The tendon of the quadriceps muscle holds the patella or kneecap within it, the muscle's main function being knee extension to raise the body from a chair or negotiate steps. The inner surface of the patella, divided into two facets, is lined with joint cartilage and articulates with the major groove between the condyles of the femur, sliding back and forth. The power of the quadriceps muscle to extend the knee against the body weight is significantly increased by the presence of the kneecap.
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.
It is in order to stop the femoral condyles slipping off the back of the tibial plateau that they perform the internal glide in a back and front direction. One bone does not move on its own as the joint changes its position, but rather both move in a complicated fashion on each other to allow the functionally desired movement to be achieved. A much more extensive range of motion is achievable with this design. Rotation also occurs at the knee joint but is less obvious, becoming more apparent as the knee extends more closely to the locking position and the internal rotation of the femur occurs.
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