Avascular necrosis (AVN) of the femoral heads in a patient with sickle cell disease

The images in the top series (A-D) show a patient with sickle cell disease and bilateral avascular necrosis of the hips. Images E-H show a normal patient. In image A an arrow points to two arcs of alternating densities. This finding is magnified in image B, revealing a superior subchondral lucent arc (arrowhead) adjacent to a more inferior sclerotic and denser arc of sclerotic bone (arrow). The subarticular lucent arc reflects resorbed bone and is known as the crescent sign. The sclerotic arc represents the reparative process. Image C is an MRI STIR sequence and reflects edema (arrowhead) in the superior subchondral arc. It is seen bilaterally but is more prominent on the right. Image D is a T1 weighted MRI image showing abnormal heterogeneous signal in both femoral heads. The dark signal in the femoral bones reflects increased hematopoiesis relative to normal. Images E and F reveal normal femoral heads that are less dense than the abnormal hips. On the MRI STIR image (G) there is no edema and no crescents are present. Image H is a normal T1 weighted MRI sequence and shows normal signal in the femoral heads bilaterally compared to the heterogeneous appearance in image D. The bright signal in the femoral epiphyses in H reflects a greater amount of fat compared to the femoral neck which contains hematopoietic bone marrow.