The Human Knee Joint

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.

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 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.

The patella and the internal cartilages (also known as menisci) take a lot of force during knee movements and many problem conditions are related to these structures. The knee joint’s range of movement is typically from zero degrees (straight) to around–0 degrees, although this does vary with general joint mobility and with body weight. During flexion and extension of the knee the femoral condyles slide and glide on the tibial condyles, which themselves slide backwards and forwards.

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 Physiotherapy, back pain, orthopaedic conditions, neck pain, injury management and physiotherapists in Edinburgh. Jonathan is a superintendant physiotherapist at an NHS hospital in the South-West of the UK.