The Human Hip

By Jonathan Blood Smyth

The trunk and the legs are connected mechanically at the large ball and socket joints of the hips. The acetabulum, the large rounded and deep hip socket with a cartilage rim, neatly covers the similarly shaped femoral head, the ball at the top of the thigh bone. This is a stable joint with deep structure and strong stabilisers. The surfaces of the hip joint are covered by an almost friction-free material called articular cartilage which allows movement to occur easily under stress, a vital ability in this joint. The greatest weight is taken in the roof of the socket and the uppermost part of the femoral head and this is where cartilage is thickest.

The capsule of the hip joint, a fibrous bag significantly strengthened all around by the large ligaments of the hip, surrounds the hip from the base of the neck up to the margins of the socket. It is lined by synovial membrane which is responsible for secretion of synovial fluid which feeds and lubricates the joint. For the thigh bone to fit into the pelvic socket effectively to transfer the required loads the upper femur is designed to take an inward angle of 125 degrees from the upright so the ball can enter the socket. This allows the appropriate transmission of locomotive and weight bearing forces through the joint.

Inside the hip the bony anatomy of the supporting struts or trabeculae are also mechanically aligned along the lines of most force, responding to the requirements of bodily motion and bearing weight. The densest areas reinforce the parts suffering the highest stresses. If the hip copes with these stresses by developing strengthened areas it also has areas of less strength which can become relevant in older people as they fall and suffer fractured necks of femur across these areas. As the number of older people dying after this fracture is relatively high, this matter is of concern.

Standing, the control of locomotion and coping with moving the body are the main functions of the hip joints. The muscles of the hip area are the strongest in the body and are capable of keeping the body stable or of moving it with speed and power. The main muscle groups are the gluteal muscles, the abductors and the adductors. The side to side stability of the pelvis is performed mostly by the hip abductor muscles and the most powerful muscle, the gluteus maximus, functions to move the body weight.

The mechanical forces which are transmitted across the hip joint when we are doing things such as going up stairs, jumping or running are much higher than just our body weight. The areas subject to the greatest stresses are consequently endowed with much thicker cartilage to cope with these loads. The feeding of the cartilage is provided by the synovial fluid and through the underlying bone but tissue turnover of cartilage is slow, new cells forming at the base as the upper areas suffer stresses and slough off.

The synovial membrane lines the capsule of the hip and performs the secretion of small amounts of synovial fluid. This fluid is not present in large amounts but is thought to lubricate the movements of the joint, help particles of wear be absorbed to prevent a grinding paste being developed and spread mechanical loads throughout the joint. If the joint is under great load, the synovial membrane can respond to the cyclical stresses by increasing its secretion rate to protect the joint and lubricate it during periods of high activity.

If the hip movements are compromised in some way this is likely to have knock on effects on its primary function which is walking. Normal daily locomotion is allowed by having a gait pattern which is balanced and an even length of stride. The stride pattern of any individual is fairly stereotypical and limited and the earliest movement limitation noticed is usually hip extension. The hip has about twenty degrees of extension, which is the movement of taking the leg behind the body, particularly when the other leg is moving forward. Flexion of the hip is typically 130 degrees and any loss much less noticeable. - 32188

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