The Hip

The hips are ball and socket joints between the legs and the pelvic part of the trunk. The joint consists of a rounded ball (the femoral head) which fits into the reciprocally rounded socket (the acetabulum) which is quite deep and envelops most of the head. It is a large joint, the surfaces mirror each other well and it is stable. The articular cartilage of the hip is a shiny, friction-free material which aids the easy movement of the joint even under load, and is vital to normal functioning of the hip. The cartilage is thickest where weight bearing occurs most commonly, at the uppermost point of the head and the socket.

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.

The bone’s internal structure in the upper femur reflects the need of the hip to bear weight and to move the weight of the body. Denser areas of bone struts or trabeculae have developed which reinforce the bone at the points of greatest mechanical need. These strengthened areas function well in ensuring we manage vigorous activities but this has left much weaker areas which show themselves, particularly in elderly people, by the frequency of hip fractures. This is a vital matter as many fractures in elderly people are not survived well by this group.

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 fluid is secreted in small amounts by the synovial membrane lining the capsule of all synovial joints. There are only small amounts of fluid involved but it has several possible functions: nutrition and lubrication of the joint, removal of small particles of cartilage wear to prevent a rough debris collecting and an evening out of stressful loads through the joint. The synovial membrane will respond if the joint is suddenly worked hard by increasing fluid secretion to enhance the protective qualities it brings to the joint under load.

Gait is the most likely function to be compromised if there is a reduction in the movements available at the hip, as this is our most important activity. A balanced pattern of gait with an even length of stride is required. Individuals typically have a fairly restricted range of gait motions but if hip extension is reduced this becomes rapidly noticeable. Taking the leg behind the body as the other leg steps forward is hip extension, which typically measures around twenty degrees. Hip flexion however is much greater at around one hundred and thirty degrees and any loss is much less easily noticed.

Jonathan Blood Smyth, editor of the Physiotherapy Site, writes articles about Physiotherapy, back pain, orthopaedic conditions, neck pain, injury management and physiotherapists in Harpenden. Jonathan is a superintendant physiotherapist at an NHS hospital in the South-West of the UK.