Avascular necrosis

1-The most common site for occurrence of avascular necrosis is head of femur.

2-The second site is the dome of the talus bone.

3-Causes:

-Spontaneous.

-Trauma.

-Cortico-steroids.

-Marrow infiltrative lesions.

4-Stages of avascular necrosis of dome of talus:

-Stage 1: Bone marrow edema(Hypo intense in T1,hyper intense in T2).
-Stage 2: Formation of a groove in dome of  talus containing fluid(hypo intense in T1, hyper intense in T2).
The difference between stage 1 and 2 is in the former edema is not well delineated while in the latter, the groove is well delineated.
-Stage 3: The dead bone is separated but not moved from its site.
-Stage 4: The dead bone is moved from its site after its separation.