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Avascular necrosis of the hip

The hip is one of the most frequent sites for avascular necrosis, presumably due to a combination of precarious blood supply and high loading when standing. It can be thought of as traumatic (secondary to neck of femur fractures) or non-traumatic. In non-traumatic cases it is bilateral in 40%.

Typically it affects the superior articular surface (between 10-2 o'clock) and begins in the more anterior part of the hip.

Pathology

Aetiology
  • traumatic
  • chronic corticosteroid therapy
  • alcoholism
  • smoking
  • SLE
  • hyperlipidemias
  • HIV
  • haemoglobinopathies
  • chronic renal failure
  • diabetes

Clinical presentation

The most common presenting symptom is pain in the region of affected hip, thigh, groin and buttock. Although few patients may remain asymptomatic until late stages.

Radiographic features

Specific staging system (Ficat staging) exists for the hip which includes X-ray, MRI and bone scan appearance, and covers much of the imaging appearances, thus please refer to that article. 

Other than describing the general appearance of the affect region the following are important to include in the report as they have and effect on prognosis and treatment:

  • position 
  • estimating percentage volume of the head involved (axial) and percentage weight bearing surface involved (coronal)
  • coexisting osteoarthritis or secondary degenerative change
  • joint effusion
  • presence of a potentially unstable osteochondral fragment: rim sign
MRI

MRI is the most sensitive modality, with sensitivity of 71-100% and specificity of 94-100%1. As there is a high rate of bilateral involvement both hips should be included in the field of view of at least some sequences.

  • T1: usually the initial specific findings are areas of low intensities represent oedema, which can be bordered by a hyper-intense line which represents blood products
  • T2: it may show a second hyper-intense inner line between normal marrow and ischaemic marrow. This appearance is highly specific for AVN hip and known as "double line sign" 

Differential diagnosis

General imaging differential considerations include

See also 

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