Why avascular necrosis in sickle cell disease involve long bone only?

The ends of : The long bones have the most tenuous blood supply, and thus are at the greatest risk.

Not only long bones: Sickle cell avn can occur in any bone that has limited blood supply. The obstruction to blood flow caused by sickle cells can cause the bone to die and distort normal shape and function, causing pain. The head of the femur (a long bone) and the talus bone (not long bone) in the ankle are common sites of avn in my sickle cell patients.