Workings Of The Knee

Published: 23rd October 2009
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Like the elbow, the knee joint is an example of a hinge joint with added complexity. The condyles of the femur are enlarged rounded areas at the base of the femur and they make up the knee joint with the enlarged flattened area of the upper end of the tibia. On the outer side of the shin lies the fibula, a thin, long bone which does not make up part of the knee nor bear much weight, mainly acting as an area of muscle origin for the muscles which move the foot, ankle and toes. The hinge joint of the knee splits the leg in two, allowing tidy folding in resting or active positions, the necessary shortening of the leg to allow effective walking and the large levels of of propulsive power required.

The quadriceps muscles, the main thigh group, act to bring the knee into its locking position as the joint get closer towards full straightening or extension. The apes and monkeys are not capable of extending their knees in standing so this posture takes high levels of energy, while full extension plus locking of the knee joint allows standing without much effort. The knee joint is rotated inwards by the main quadriceps muscle as it gets close to fully straight and begins to lock. Human upright posture has evolved by this process to permit prolonged standing with very low energy expenditure due to the inactivity in the knee and hip muscles.

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 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 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 140 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.

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