Case of the Month - June

Left sided hip pain in a renal transplant patient.

Diagnosis: Avascular necrosis of the femoral head 

Investigation

Right Image: Plain radiograph demonstrating sclerosis, subchondral fracture and collapse of the femoral head, consistent with advanced vascular necrosis. It also shows joint space narrowing and marginal osteophyte formation, consistent with secondary osteoarthritis.

Middle and Right Images: MRI of the hip demonstrating the double line sign, subchondral fracture and collapse of the left femoral head. It also shows the chondral defects of secondary osteoarthritis. The middle image also shows early AVN in the right hip. The right hip appeared normal on plain radiographs (not shown). 

Discussion

Avascular necrosis of the femoral head is an increasingly recognised cause of morbidity, estimated to account for approximately 10% of hip joint arthroplasties performed in the USA.

Avascular necrosis is the sequelae of a number of insults to the femoral head which result in infarction of the cellular components of the marrow. Once infarction has occurred, the body attempts to repair the infarcted bone. Infarcted bone is resorbed and new bone deposited at the margin of the infarct. This is usually ineffective and the infarcted bone becomes weakened, resulting in microfractures that can coalesce to produce subchondral fractures ('crescent' sign) and eventually collapse of the subarticular cortex and secondary osteoarthritis.

Causes include trauma (fracture and dislocation), drugs (commonly steroids and alcohol), renal failure, haemoglobinopathies, marrow disorders, burns, pancreatitis, radiation, vascular disease, connective tissue disorders and prothrombotic states.

Plain radiographs are initially normal. The earliest changes are mixed sclerosis and lucency, usually geographic in distribution. This is followed by subchondral fractures ('crescent sign'), collapse of the subarticular cortex, and finally osteoarthritis.

MRI is the most sensitive and specific test for AVN. Changes are seen on MRI prior to any other imaging modality, including bone scintigraphy. It has a sensitivity and specificity of at least 90%. The earliest change is marrow oedema. This in itself is non specific. The earliest specific change is the double line sign, a serpigenous line of adjacent high and low signal on T2 sequences. These changes are followed by subchondral fracture, collapse and finally osteoarthritis.