Health & Fitness

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.

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.

The flexion and extension plane is the natural plane of knee movement as this normal alignment makes knee pain problems occur less commonly. A bow-legged or knock-kneed posture allows abnormal sideways stresses to be applied to the knee, forcing pressure onto one side of the joint and increasing wear stresses which with time can cause pain symptoms or arthritic changes. Patellar misalignment can also occur, forcing one of its facets against the side of the femoral condyle groove and causing impingement pain due to increase in the friction forces.

The high mechanical forces transmitted through the knee are responsible for a variety of knee conditions, often affecting the kneecap or the knee cartilages (more properly called menisci). The knee typically has a range of motion from full extension (stated as zero) to full flexion at around one hundred and forty degrees, depending to some extent on body size and bodily mobility. There is an important degree of internal glide and slide of the femoral condyles as they move on the reciprocally moving tibial surfaces.

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.