hip The hip joint is an example of a ‘ball and socket’ (multiaxial) type of joint, with the top (head) of the long bone of the leg (femur) being the ‘ball’ and the socket being a depression in the bone of the pelvis known as the acetabulum. This arrangement permits movements in three planes — forwards and backwards (extension/flexion); inwards and outwards (adduction/abduction); and inward twist and outward twist (internal and external rotation). Combination of these movements also gives rise to ‘circumduction’, a circular movement of the whole leg which describes a ‘cone’ with the foot at the base and the hip at the apex. The joint is spanned by powerful muscles, which are required not only for postural control and movement but also to confer stability at the hip. The large muscles constituting the buttocks (gluteal muscles) are particularly important for maintenance of hip stability and for promoting a normal gait. The hip is one of the most strong, secure, and stable joints in the body. This stability is due to the depth of the acetabulum; the powerful muscles surrounding the joint; and a strong, fibrous capsule reinforced by ligaments. Such securing of the joint is understandable when one considers the considerable strains placed on the hip during everyday activities such as walking, running, and particularly jumping. The hip is a large, weight-bearing joint which, because of the continued stresses placed on it throughout life, can develop osteoarthritis in later years. The cause of osteoarthritis is still poorly understood, but it may be the consequence of thinning of the cartilage covering the bones (acetabulum and head of femur), followed by bone overgrowth. This condition can be debilitating and can lead to severe pain and stiffness of the hip. One of the success stories of modern medicine is the surgical treatment of this condition by a hip joint replacement (arthroplasty). The technique, perfected by the British orthopaedic surgeon Sir John Charnley, involves an operation to remove the diseased joint and insert an artificial one made of metal and tough polyethylene. In this operation both the head of the femur and the acetabulum are replaced (total hip arthroplasty). Hip joint replacement can also be performed for other types of hip joint disease or malformations, and has been used after severe injury. These are sometimes sporting injuries, and there are recorded cases of individuals being able to return to sporting activities after successful operation.

The neck of the femur is a common site of fracture in the elderly, often resulting from a fall; the fracture is often called a ‘broken hip’. This is more common in females, due to the greater thinning of bones with advancing years (osteoporosis) in women than in men. There is evidence to suggest that hormone replacement therapy (HRT) after the menopause can slow the rate of bone loss. Treatment of a fractured neck of femur is usually by total hip joint replacement, as bone healing becomes poorer with advancing age and osteoporotic bone poorly supports metallic implants. The pelvis is wider in females than males because of the need to permit childbirth, and this results in the femoral heads being further apart. Consequently the thighs tend to be more sharply inclined inwards at the knees (valgus).

William R. Ferrell