Avascular Necrosis (AVN) is a condition characterized by the death of bone components due to an interruption of blood supply. While it can occur in various bones (such as the scaphoid or talus), the femoral head is the most clinically significant and frequently tested site. For the board-certified physician, mastering AVN requires identifying high-risk patient profiles and understanding why MRI is the gold standard for early detection when radiographs remain misleadingly normal.

1. Etiology: The “High-Yield” Risk Factors

AVN is rarely idiopathic; it is almost always associated with specific systemic stressors. Boards often use these in clinical vignettes:

• Trauma: Femoral neck fractures or hip dislocations that disrupt the circumflex femoral arteries.
• Corticosteroids: One of the most common non-traumatic causes. The risk is dose-dependent and typically involves prolonged high-dose therapy.
• Alcohol Abuse: Chronic ethanol intake is thought to cause fatty emboli and increased intraosseous pressure.
• Sickle Cell Disease: Vaso-occlusive crises lead to micro-infarctions within the bone marrow.
• Other Factors: Systemic Lupus Erythematosus (SLE), Caisson disease (the “bends”), and Gaucher disease.

2. Pathophysiology: The “Compartment Syndrome” of Bone

The femoral head is an “end-artery” organ with limited collateral circulation.

• Vascular Compromise: Ischemia leads to death of osteocytes and bone marrow cells.
• Repair Failure: The body attempts to repair the dead bone, but the remodeling process results in the removal of dead trabeculae before new bone is strong enough to support weight.
• Structural Failure: This leads to the “Crescent Sign”—a subchondral fracture—and eventual collapse of the femoral head, resulting in secondary osteoarthritis.

3. Clinical Presentation

Patients typically present with deep, aching groin pain that is exacerbated by weight-bearing.

• Physical Exam: Internal rotation and abduction are the most painful and limited planes of motion.
• Sudden Change: A sudden increase in pain intensity often correlates with the transition from bone ischemia to structural collapse (Stage III).

4. Diagnostic Imaging and the Ficat Classification

The Ficat and Arlet Classification is the most common staging system used for boards:

Stage	Radiographic Findings	MRI Findings	Clinical Status
I	Normal X-ray	Edema/Ischemic signals	Painful
II	Sclerosis/Cystic changes	Geographic lesions	Painful
III	Crescent Sign (Flattening)	Subchondral collapse	Severe pain
IV	Secondary Osteoarthritis	Joint space narrowing	Chronic disability

High-Yield Imaging Pearls

• MRI: The gold standard. It can detect AVN within days of the ischemic event (Stage I), showing a “double line sign” on T2-weighted images.
• The Crescent Sign: A linear subchondral lucency seen on lateral X-rays. It signifies a subchondral fracture and is the hallmark of Stage III (irreversible collapse).

5. Interventional and Surgical Management

The treatment goal is “joint preservation” if diagnosed before Stage III.

• Core Decompression: A surgical procedure where a hole is drilled into the femoral head to reduce intraosseous pressure and promote revascularization. This is most effective in Stages I and II.
• Vascularized Fibular Graft: Moving a piece of the fibula (with its blood supply) to the femoral head.
• Total Hip Arthroplasty (THA): The treatment of choice once the joint has progressed to Stage III or IV.

6. The Weight-Bearing Balance

The management of early-stage AVN focuses on protecting the joint while maintaining muscle tone.

• Non-Weight-Bearing (NWB): Crutches or a walker are essential in the early stages to prevent mechanical collapse while the bone is in the “repair” phase.
• Strengthening: Isometric hip exercises are used to prevent gluteal atrophy without adding compressive loads to the femoral head.
• Outcome Optimization: For patients who proceed to THA, pre-operative “pre-hab” focusing on hip abductor strength is the best predictor of a successful functional return.

---

High-Yield Board “Fast Facts”

• MRI: Required for diagnosis in Stage I (X-ray is normal).
• Crescent Sign: Indicates subchondral fracture/collapse (Stage III).
• Groin Pain: The most common location of pain.
• Steroids/Alcohol: The most common non-traumatic etiologies.
• Internal Rotation: The most sensitive physical exam finding for hip joint pathology.