A 65-year-old man with a history of chronic alcohol abuse and poor nutritional status presents with a displaced intracapsular fracture of the femoral neck (Garden grade III). He is medically fit for surgery. At 6 weeks post-operatively following internal fixation with three cannulated screws, he develops severe groin pain and progressive loss of hip abduction strength. Radiographs show lucency around the screw heads with evidence of femoral head collapse. Which of the following is the most likely explanation for this complication, and what is the primary mechanism underlying its development?

Explanation

## Clinical Context & Pathophysiology This patient presents with classic features of **avascular necrosis (AVN)** of the femoral head following intracapsular femoral neck fracture repair: - **Timing**: 6 weeks post-op (AVN typically becomes radiographically evident 6–12 weeks post-injury) - **Clinical signs**: Groin pain, loss of hip abduction (gluteus medius weakness from pain inhibition), femoral head collapse on imaging - **Risk factors**: Displaced intracapsular fracture (Garden III), chronic alcohol use (impairs bone healing and increases AVN risk), malnutrition ### Mechanism of AVN in Intracapsular Femoral Neck Fractures The femoral head has a **precarious blood supply** that is highly vulnerable to disruption in intracapsular fractures: ```mermaid flowchart TD A[Intracapsular femoral neck fracture]:::outcome --> B{Fracture displacement}:::decision B -->|Displaces > 2mm| C[Disrupts lateral femoral circumflex artery<br/>and its ascending cervical branches]:::urgent C --> D[Loss of retrograde blood flow<br/>to femoral head]:::urgent D --> E[Ischemic necrosis of femoral head]:::outcome B -->|Minimal displacement| F[Blood supply may be preserved<br/>via ligamentum teres artery]:::action F --> G[Lower AVN risk]:::outcome E --> H[6-12 weeks: Radiographic changes<br/>lucency, collapse, flattening]:::outcome ``` **Key Point:** In displaced intracapsular fractures, the ascending cervical branches of the lateral femoral circumflex artery (which supply the femoral head) are torn at the fracture site. The medial femoral circumflex artery (main supply) is also compromised because it runs along the posterior capsule, which is disrupted in displaced fractures. The ligamentum teres artery (from the obturator artery) provides only minimal collateral supply and is insufficient in displaced fractures. **High-Yield:** The risk of AVN is directly proportional to the degree of fracture displacement: - **Non-displaced or minimally displaced (Garden I–II)**: 10–15% AVN risk - **Displaced (Garden III–IV)**: 30–50% AVN risk ### Why Chronic Alcohol Use Increases AVN Risk **Clinical Pearl:** Alcohol impairs: 1. Osteoblast function and bone healing 2. Microvascular perfusion (direct endothelial toxicity) 3. Immune response, increasing infection risk 4. Nutritional status (thiamine, folate deficiency) These factors compound the ischemic insult from the fracture itself. ### Radiographic Evolution of AVN | Stage | Timeline | Radiographic Finding | |-------|----------|----------------------| | 1 (Asymptomatic) | 0–3 months | Normal or subtle sclerosis | | 2 (Lucency) | 3–6 months | Lucent crescent sign, sclerotic rim | | 3 (Collapse) | 6–12 months | Femoral head flattening, subchondral fracture | | 4 (Osteoarthritis) | > 12 months | Secondary OA with joint space narrowing | This patient is at **Stage 2–3**, consistent with the 6-week timeline and imaging findings. ## Why the Correct Answer is Right **Avascular necrosis** is the most likely diagnosis because: 1. **Timing**: Radiographic evidence at 6 weeks is typical for AVN in displaced intracapsular fractures 2. **Mechanism**: Displacement of the fracture disrupts the lateral femoral circumflex artery branches, which are the primary blood supply to the femoral head 3. **Clinical presentation**: Groin pain and femoral head collapse are pathognomonic for AVN 4. **Risk factors**: Displaced Garden III fracture + chronic alcohol use = very high AVN risk 5. **Lucency pattern**: Peri-screw lucency with femoral head collapse is characteristic of early AVN, not simple loosening or infection ## Management Implications **Key Point:** Once AVN is established (Stage 2+), treatment options are limited: - **Conservative**: Activity restriction, NSAIDs (controversial), weight-bearing precautions - **Surgical**: Core decompression (Stage 1–2), vascularized fibular graft, or total hip arthroplasty (THA) for advanced disease This patient will likely require **THA** given the advanced collapse at 6 weeks.