Problems with the hip can occur at any age, and for each age group the most likely diagnosis is different.

The condition is not common, but is most easily treated if it is spotted early. There are many associations such as family history or the child being a first born female, particularly if the delivery is breech. If there is any suspicion from the history or on examination at birth, then the diagnosis should be confirmed or excluded using a dynamic ultrasound examination of both hips. If the hip is abnormal, then if it can be held in the acetabulum, using splints that hold the legs in abduction and internal rotation, the femoral head will then grow within the acetabulum, and the hip joint will develop normally. If this treatment fails (or was never instituted), then the lack of congruence in the hip joint will result in abnormal development, which will require surgical correction later in life.

When children start walking it may be noted that they are limping and that the buttock creases are asymmetrical. The most likely diagnosis is a congenital dislocation of the hip that was not spotted at birth. After the age of 6 weeks the femoral head has started to ossify, so it is possible to see that the hip is dislocated on X-ray. An ultrasound scan will also confirm the diagnosis. The key to treatment is to get the hip into the joint as soon as possible and to hold it there, so that the acetabulum and femoral head can develop congruently. Splints that hold the hip abducted may be adequate; if not surgery will be needed to clear out any soft tissue blocking reduction.

In a child too young to speak it may be difficult to work out where they are getting pain, but if the child is watched for a time it will be noticed that, however fretful they are, they do not move the painful limb. The diagnosis that must be considered is septic arthritis of the joint (see Chapter 28).

Young children are frequently referred as emergencies with a limp. It can be very difficult to make the diagnosis, as they may not be able to explain why they will no longer weight bear normally on that leg. Even when they are old enough to talk and explain the problem, they frequently complain of pain in the knee when the problem is in fact in the hip.

Any child who is limping should have a careful examination of the whole leg. If the problem is in the hip then there will be painful limitation to movement of internal rotation of the hip (see p. 39). In more severe cases the hip is held flexed, internally rotated and adducted (the position of maximum comfort). Most cases of irritable hip are of unknown cause but may be some form of transient post-viral arthritis. The condition settles spontaneously within a few days or weeks.

In children aged 4–10 years, Perthes’ disease produces a sustained irritable hip. The underlying pathology is thought to be avascular necrosis of the femoral head for no known reason (idiopathic). The femoral head will revascularise in time but if the segment of dead bone is large or the hip is subjected to a high load before it has had time to repair, then the femoral head collapses and the early onset of arthritis is inevitable. The hip should therefore be protected as much as possible during this period. Operations to protect the vulnerable segment of cartilage are of unproven value.

In children around the age of puberty, the ephiphyseal plate seems to be especially weak. Failure of the plate leads to slip of the femoral head downwards and backwards in relation to the femoral neck. The deformity is easy to miss on an anteroposterior X-ray, but is obvious on a lateral or ‘frog’s leg’ view. The child presents with a limp and often pain in the knee (referred pain), so the diagnosis is easy to miss. The foot is often turned outwards compared with the other side. If pins are run across the epiphysis, any further slip can be prevented until the epiphysis has fused at the end of growth. The condition is often bilateral.

This may develop in young people as a result of trauma, inflammatory joint disease or hip dysplasia (an abnormal-shaped hip from birth). However, it is normally a disease of the elderly, causing a limp because of pain, stiffness, weakness or a shortened leg. Hip replacement is the treatment of choice (see Chapter 17) , although in younger patients with dysplasia an osteotomy may be used to improve the shape of the hip and so prolong its life.

High alcohol intake, steroids and some storage disorders predispose patients to develop avascular necrosis of the hip. The femoral head may take some years to collapse after the initial insult, but once it has collapsed a total hip replacement is the only option for restoring function and relieving pain.