Avascular necrosis of the hip

Epidemiology

  Avascular necrosis is a rare but important problem in all age groups (Table 23.6). In the infant it may follow as a complication of septic arthritis, the pressure of the effusion denying the head of the femur its blood supply. In children, spontaneous avascular necrosis is called Perthes’ disease. In adults it may be caused by steroids or heavy alcohol intake, and can occur in people who work in an environment which is at high pressure (divers or people who work in tunnels). It is also found occurring spontaneously in patients with blood abnormalities such as sickle cell anaemia and thalassaemia. It is a very common problem in the elderly following a displaced subcapital fractured neck of femur. These are managed by replacing the dead femoral head with a hemiarthroplasty before the sequelae of avascular necrosis can develop.

Pathophysiology

The head of the femur is one of the areas of the body most susceptible to avascular necrosis. The underlying pathophysiology of the condition is not known but it is believed that the blood supply to the femoral head is lost either through thrombosis of the vessels entering the head or because of interosseous hypertension. The marrow of the femoral head becomes replaced with fat and the bone dies. Subsequently, there will be a zone of revascularisation which will be incomplete if the avascular area is large. Finally, there is a zone of reossification. During this period the joint is at risk of collapse. Once collapse has occurred full recovery is not possible and secondary arthritis sets in.

History

The patient may complain of sudden onset of pain in the hip or gradually increasing joint pain.

A careful family history will be needed to exclude sickle cell and thalassaemia. A social history should carefully cover alcohol intake, and a history of having worked underground or under water using high pressure. This exposure might have taken place some years before the onset of symptoms in the joint.

Medical history should specifically cover treatment with steroids.

Examination

On examination there may be nothing to find, except for a painful limitation of range of movement.

Investigation

Ultrasound may reveal a small effusion around the hip but X-rays are normal unless the avascular necrosis has been caused by a fracture. A radioisotope scan will show a dead area of the femoral head later surrounded by a hyperaemic area of revascularisation. Magnetic resonance imaging (MRI) may also show one or more avascular areas. If any of these areas are in the weight-bearing part of the femoral head then there is a risk that the femoral head will collapse before revascularisation occurs. If this does occur, secondary osteoarthritis of the hip starts and a total hip replacement may be needed. If the diagnosis can be made early before the femoral head has collapsed, there is a chance of protecting the femoral head until it revascularises. This may prevent collapse.  

Treatment

If the femoral head has not collapsed a ‘forage’ operation (where a core of bone is removed from the femoral neck and femoral head through a window in the lateral femoral cortex) can be used to decompress the femoral head. There is no clear evidence that this operation makes any difference to the natural history of avascular necrosis. If the avascular area is in the main weight-bearing area of the femoral head a further option is to perform an osteotomy. The head is rotated to bring a healthier part of the head into the main weight-bearing area of the hip joint to prevent collapse.

Problems

Once the femoral head has collapsed osteoarthritis is inevitable, with pronounced shortening as the hip collapses followed by stiffness as the joint becomes incongruent and osteoarthritis sets in. The options then are an arthrodesis or a hip replacement. Osteotomy is unlikely to help at this stage.

Avascular necrosis

Idiopathic in children = Perthes’ disease

Complication of alcohol and steroids in adults

  Complication of sickle cell and thalassaemia in all ages

  Avoided by hemiarthroplasty following subcapital fracture in the elderly

Pertrochanteric fractured neck of femur

Epidemiology

Fractures through the trochanters of the femur are as com­mon as subcapital fractured neck of femur. They can occur in patients with osteoarthritis of the hip and are caused by a trip or a fall on to the hip.

  Pathophysiology

Unlike subcapital fractures these can occur in association with arthritis of the hip. In the younger adult they can occur with high-velocity trauma.

History

The patient complains of sudden severe onset of pain in the hip and is unable to weight-bear.

Examination

On examination the leg is often short and internally rotated.

Treatment

The fracture is highly unstable (Fig. 23.20) and the patient cannot mobilise until it has been stabilised. This fracture can be managed on traction but it is very difficult to reduce the fragments and the patient would need to in bed for a minimum of 12—18 weeks.

The patient is usually very elderly and this length of time in bed would destroy their sense of independence and put them at risk for developing hypostatic pneumonia, urinary tract infection and bed sores. The blood supply of the femoral head is not affected by this fracture and therefore avascular necrosis is not a problem. However, the fracture requires a very strong fixation because of the enormous forces which go through the hip. The dynamic hip screw (DHS) is made up of two parts which can slide in relation to each other but do not allow binding (Fig. 23.20). The first part is a heavy-duty plate which is fixed to the lateral cortex of the femur with cortical screws. The second part is a rod which passes up through a slot in the plate into the femoral neck. Its threaded end crosses the fracture line to engage and hold the femoral head. As the patient weight-bears on the healing fracture the broken ends of the bone collapse into each other and compress the fracture. The sliding-rod mech­anism of the DHS allows this to happen without allowing the hip to fall into varus. This prevents the plate breaking at the fracture, or indeed the rod penetrating through into the femoral head and acetabulum.

  Problems

Some fractures are difficult to reduce and can result in poor placement of the DHS. If this happens the fixation will fail, and the fracture collapses into a painful nonunion which is difficult to reconstruct.

Pertrochanteric fractured neck of femur

     Common in the elderly

     Does not cause avascular necrosis

     Nonoperative treatment is slow and difficult