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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 socket, head and neck of the hip joint are completely enveloped by the hip capsule, a ligamentous bag which is well strengthened by the large and powerful ligaments of the hip. Lined by the synovial membrane which secretes nutritional and lubricating synovial fluid, the capsule runs from the margins of the socket, down over the head and neck to its base. The thigh bone travels upwards towards the body but the hip socket is at the side of pelvis and faces outwards and down, requiring the femur to make a 125 degree turn inwards at the top to allow the hip joint to form. This facilitates the appropriate mechanical structure.

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.

When performing activities such as climbing stairs, jumping or running, the joint surfaces of the hip are exposed to much higher mechanical stresses than just the body weight. To allow for this the most exposed areas to the weight are covered by the thickest cartilage. The underlying bone and the synovial membrane fluid are responsible for nutrition to the cartilage but this is not a very strong process. Cartilage turnover is slow with some forming at the base as the upper layers get damaged and are lost.

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 is the Superintendent of Physiotherapists at an NHS hospital in the South-West of the UK. He writes articles about back pain, neck pain, and injury management. If you are looking for physiotherapists in Edinburgh visit his website.

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