Stage III Avascular Necrosis of Hip / Stage III Osteonecrosis of Hip
[Description: Geographical areas of altered signal intensity are seen involving the femoral head with flattening and fragmentation and subchondral crescentric fracture along the anterosuperior aspect. Marrow edema with joint effusion is also seen. Significant cartilage loss is however not seen]
Avascular Necrosis of Hip, as the name suggests, is postulated to occur due to cell death as a consequence of decreased/interrupted blood supply. It may occur in any bone, though the femoral head is most commonly involved. The epiphysis as well as the metadiaphysis may be involved. It is often labeled as bone infarcts when they involve the metadiaphysis, though the underlying pathophysiology remains the same.
MRI is today the mainstay for diagnosing and staging avascular necrosis and is the most sensitive and specific techique for the same.
The characteristic and most specific appearance of MRI is that of a geographical area of altered signal best seen on the T1 and T2W coronal and sagittal images.
The Ficat Arlet, described below, is the most widely used classification system for osteonecrosis of the hip.
Stage I : Normal Xray , Marrow edema on MRI
Stage II: Geographical areas of altered signal intensity
Stage III: Flattening /Subchondral Crescent
Stage IV: Secondary degenerative changes
This, was however developed in the early days of MRI, and with the advent of the high resolution MRI, a few changes are needed. Specially, the Stage I and II are clubbed together, as detection of geographical areas of altered marrow signal on MRI is must for diagnosis of AVN. Only marrow edema on MRI without geographical areas of altered signal is not sufficient for diagnosis of AVN and other possibilities especially insufficiency fractures [earlier known as transient osteoporosis of hip] needs strong consideration.
The findings that require mention, and decide management in cases of Avascular necrosis of Hip are as follows:
Presence of collapse of the femoral head [Flattening /fragmentation/subchondral crescent]
Presence of cartilage loss
Percentage of involvement of head volume
Presence of marrow edema/Joint effusion
Associated complications like stress fracture of femoral neck
The presence of marrow edema and joint effusion suggests active disease and such patients are more often symptomatic. Stage I and II avascular necrosis, without marrow edema are often labeled as quiescent osteonecrosis.
Management algorithm for avascular necrosis based on radiological findings:
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