Introduction

This is a self-limiting hip disorder caused by varying degrees of ischaemia and subsequent necrosis of the femoral head.

Characteristic features of this disorder are:

• The essential lesion is loss of blood supply (avascular necrosis) of the nucleus of the proximal femoral epiphysis.
• Abnormal growth of the epiphysis results.
• Eventual remodelling of regenerated bone.

Epidemiology

• Usually seen in a 4 to 8 year-old boy with delayed skeletal maturity.
• Approximately 1 in 1,200 children aged under 15 years.
• Male to female ratio: 4-5:1.^[1]
• Rare in non-Caucasians.
• Increased incidence with a positive family history, low birthweight and abnormal pregnancy or delivery.

Pathogenesis

The primary event is avascular necrosis of the femoral epiphysis, which results in delayed ossific nucleus:

• The articular cartilage is nourished by synovial fluid and continues to grow.
• The cartilage columns become distorted with some loss of their cellular components.
• They do not undergo normal ossification, which results in excess of calcified cartilage in the primary trabecular bone.
• Revascularisation proceeds from peripheral to central.
• Symptoms occur with subchondral collapse and fracture.

Presentation

• It presents with pain in the hip or knee and causes a limp.
• There is pain (often in the knee) and an effusion (from synovitis).
• Up to 12% of cases are bilateral, but will be at different stages and are asymmetrical.
• On examination all movements at the hip are limited:
  • The early phase has limited abduction of the hip and limited internal rotation in both flexion and extension.
  • There is an antalgic gait (due to pain).
  • Trendelenburg gait is seen in the late phase.
• No history of trauma.
• Roll test: with the patient lying in the supine position, the examiner rolls the hip of the affected extremity into external and internal rotation. This test should invoke guarding or spasm, especially with internal rotation.

Investigations

• FBC and ESR.
• Early X-rays may show widening of the joint space (the best view is frog lateral), or may be normal.
• Technetium bone scan or MRI scanning can be used to identify pathology (seen as an area of reduced perfusion).^[2]
• Later, there is a decrease in size of the nuclear femoral head with patchy density on X-ray.
• Later still, there may be collapse and deformity of the femoral head with new bone formation. Severe deformity of the femoral head risks early arthritis.
• An arthrogram and/or MRI scan are often needed to assess congruency throughout full range of movement. A flat-topped incongruent head has the worst prognosis. It can rule out hinge abduction where the enlarged femoral head impinges on the acetabular rim.
• Hip aspiration if a septic joint is suspected.

Differential diagnosis

Bilateral Perthes' disease

This requires a skeletal survey as part of the work-up.

• Hypothyroidism.
• Multiple epiphyseal dysplasia (MED).
• Spondyloepiphyseal dysplasia tarda.
• Sickle cell disease.

Unilateral Perthes' disease

• Septic arthritis.
• Sickle cell disease.
• Spondyloepiphyseal dysplasia tarda.
• Gaucher's disease.
• Eosinophilic granuloma.
• Transient synovitis (this was thought to lead to Perthes' disease, however it is now believed there is no causal relationship).

Management

Children who have a chronological age of 8.0 years or less, or a skeletal age of 6.0 years or less at the onset of the disease, do well without treatment.^[3]

Physiotherapy

Physiotherapy using muscle strengthening and stretching exercises, produces significant improvement in articular range of motion, muscular strength and articular dysfunction, but these improvements are not seen on X-ray.^[4]

Surgical treatment

Surgery is indicated for children greater than age 6 years as they do better with intervention - see 'Prognosis', below.

Proximal varus osteotomy has been recommended.^[1] Salvage procedures are reserved for patients with severe impairment.
Procedures include:

• Excision of extruded portion of head for hinge abduction.
• Acetabular osteotomy.
• Chiari osteotomy to cover femoral head.
• Lateral shelf osteotomy (the best results are seen in patients in the earliest stages of the disease).^[5]
• Valgus osteotomy to increase abduction and bring more normal medial femoral head into weight-bearing area.
• Arthrodesis at skeletal maturity (in unilateral involvement) for patients with severe functional impairment.
• Patients aged older than 9 years may benefit from combined innominate and femoral osteotomies if done in the early stages.

Prognosis

At least 50% of involved hips do well with no treatment.^[3]. For patients who are less than 6.0 years of age, outcome is good, regardless of treatment.^[6]

• Those who have more serious disease have significantly better outcomes with surgical treatment than those with non-operative treatment in children over the age of 8.0 years at the onset of the disease.
• Patients who were 8.0 years old or less (at the onset of the disease), with mild-to-moderate disease, did equally well with non-operative and operative treatment.
• Hips with the most severe disease had the least favorable outcomes, with no differences between the operative and non-operative groups. Female patients did significantly worse than male patients if they were over the age of 8.0 years at the onset of the disease.
• Many others will do well up until the 5th decade.

Other factors

• A flat-topped femoral head which is incongruent with the acetabulum has the worst prognosis.^[7]
• Ability to maintain hip motion.
  • The shape of the femoral head after healing.
  • Development of subluxation of the joint.
  • Decreased hip range of motion (decreased abduction).
  • Collapse of lateral pillar more than 50%.
• Degree of epiphyseal involvement; the greater the involvement, the worse the prognosis.

Complications

• Coxa magna - broadening of the head and neck of the femur.
• Coxa plana - osteochondritis of the femoral head.
• Coxa breva - structural shortening of the neck of the femur (which results in lever arm dysfunction).
• Hinged abduction (this occurs when an enlarged femoral head is pushed laterally and it impinges on the acetabular rim when the hip is abducted).