Avascular necrosis of the hip

The hip is one of the most frequent sites for avascular necrosis, presumably due to 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.

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 % volume of the head involved (axial) and % wt bearing surface involved (coronal)
• coexisting osteoarthritis / 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.

Differential diagnosis

• haematopoetic marrow (see bone marrow)
• Pitt's pit
• fovea centralis
• idiopathic transient osteoporosis of the hip (ITOH)
• chondroblastoma
• metastases

See also 

• Legg-Calve-Perthes disease