A fracture of the femoral neck in an elderly patient has the highest risk of developing avascular necrosis (AVN) due to which anatomical feature?

Explanation

## Avascular Necrosis in Femoral Neck Fractures ### Vascular Anatomy of the Femoral Head **Key Point:** The femoral head has a **retrograde blood supply** — the medial and lateral femoral circumflex arteries (branches of profunda femoris) ascend along the femoral neck and enter the femoral head proximally. The femoral head is an **end-artery territory** with minimal collateral circulation. ### Why Femoral Neck Fractures Cause AVN 1. **Retrograde supply disruption**: A fracture across the femoral neck interrupts the ascending branches of the medial and lateral femoral circumflex arteries 2. **Proximal fragment becomes ischemic**: The proximal (femoral head) fragment loses its primary blood supply 3. **End-artery territory**: Unlike other bones with rich collateral circulation, the femoral head has limited alternative vascular pathways 4. **High intracapsular pressure**: The femoral head is enclosed in a tight fibrous capsule; fracture-related bleeding increases pressure and further compromises perfusion ### Risk Factors for AVN - **Fracture displacement**: Displaced fractures have higher AVN rates (up to 30–50%) vs. non-displaced (5–10%) - **Delay in reduction**: Prolonged ischemia increases necrosis risk - **Patient age**: Elderly patients have reduced collateral circulation ### Clinical Pearls **High-Yield:** Femoral neck fractures → retrograde blood supply disruption → AVN of femoral head. This is the classic high-risk fracture for AVN. **Mnemonic: FASH** — **F**emoral neck, **A**stragalus, **S**caphoid, **H**umeral head — bones at highest risk of AVN after fracture (due to retrograde/end-artery supply). ### Comparison Table: Vascular Supply Patterns | Bone | Vascular Pattern | AVN Risk After Fracture | Key Feature | | --- | --- | --- | --- | | Femoral head | Retrograde (end-artery) | Very high (30–50% displaced) | Tight capsule, limited collaterals | | Scaphoid | Retrograde (distal entry) | High (proximal pole) | Waist fractures disrupt supply | | Astragalus | Retrograde (multiple entry points) | High | Intra-articular, tight capsule | | Humeral head | Retrograde (ascending branches) | Moderate–high | Depends on fracture pattern | **Clinical Pearl:** In femoral neck fractures, the goal is anatomic reduction within 12–24 hours to restore blood supply and minimize AVN risk.