Question

Avascular necrosis (AVN) of hip on X-ray and MRI.

Hint:

MRI is the most sensitive imaging modality for early detection of AVN, showing bone marrow edema and the double line sign. X-rays are useful for assessing late-stage findings like the crescent sign and femoral head collapse.

Answer 1

Step 1: Introduction to Avascular Necrosis (AVN) of the Hip.
Avascular necrosis (AVN), also known as osteonecrosis, is a condition characterized by the death of bone tissue due to lack of blood supply. The hip is one of the most commonly affected joints, and AVN typically involves the femoral head. It can result from various causes such as trauma, steroid use, alcoholism, sickle cell disease, or autoimmune diseases. Imaging plays a crucial role in diagnosing AVN, and both X-ray and MRI are commonly used to assess the extent and stage of the disease. Step 2: Imaging Features of AVN of the Hip on X-ray.
(1) Early Stage (Normal X-ray or Subtle Changes): - In the early stages of AVN, X-ray may be normal or show only minor changes such as mild joint space narrowing or bone sclerosis. In some cases, there may be soft tissue swelling around the joint. Early detection is challenging on X-ray. (2) Late Stage (Classic Findings): - Crescent sign: This is one of the classic features of AVN on X-ray, representing subchondral bone collapse. The crescent sign is seen as a radiolucent line or defect beneath the articular surface, indicating collapse and fracture of the subchondral bone. - Joint space narrowing: As the bone necrosis progresses, the affected hip joint may show narrowing of the joint space due to cartilage loss. - Sclerosis: Bone sclerosis may develop in the affected femoral head as the necrotic bone heals or compensates for the loss of blood supply. - Femoral head collapse: In advanced stages, the femoral head may collapse, leading to deformity of the femoral head and acetabulum, which can cause osteoarthritis. (3) Secondary Degenerative Changes: - In advanced AVN, degenerative changes such as osteoarthritis can develop, with osteophyte formation, subchondral cysts, and joint deformity being seen on X-ray.
Step 3: Imaging Features of AVN of the Hip on MRI.
(1) Early Detection (MRI as the Gold Standar: - MRI is the most sensitive imaging modality for detecting early AVN, especially before any changes are visible on X-ray. It can detect bone marrow edema (a hallmark of early AVN) as high signal intensity on T2-weighted images.
- MRI findings include: - Bone marrow edema: High signal intensity on T2-weighted images at the femoral head, indicating inflammation or early ischemia. - Low signal intensity on T1-weighted images in the affected area, indicating necrosis. (2) Characteristic MRI Findings in Established AVN: - Necrotic bone: On T1-weighted MRI, the necrotic femoral head may appear as a low signal area. The subchondral region is often involved. - Double line sign: A double line sign is another classic feature seen on T2-weighted MRI. It represents a dark inner line of necrotic bone surrounded by a bright outer line of reparative tissue (fibrous tissue and new bone formation). - Femoral head collapse: MRI can show structural collapse of the femoral head in later stages of AVN. - Joint effusion: An increase in synovial fluid or joint effusion is commonly seen as a secondary feature in later stages of AVN. Step 4: Role of MRI in Staging AVN.
- MRI plays a crucial role in staging AVN using classification systems like the Ficat and Arlet classification, which is based on MRI findings and guides treatment decisions. The stages range from early bone edema (stage I) to femoral head collapse (stage IV).