What are the plain film features of avascular necrosis?
In general there is initial minor osteopaenia, followed by variable density. Gradually microfractures accumulate in the dead bone, which is unable to repair leading to collapse of the articular surface and the crescent sign of AVN. Eventually the cortex collapses and fragments, with superimposed secondary degenerative change.
What is the best diagnostic tool of AVN, CT or MRI?
MRI is the most sensitive ( 90 - 100%) modality and demonstrates changes well before plain film changes are visible.
There is now more extensive signal abnormality within the left femoral head with a serpiginous contour best seen on the sagittal sequence. The femoral head is flattened but there is no clear collapse. There is progression of the joint space loss and osteophytes in keeping with osteoarthritis. Persisting hip joint effusion.
No evidence of avascular necrosis on the right side.