A 52-year-old woman from Mumbai presents with acute right hip pain following a fall on an icy pavement. She is unable to bear weight and the right lower limb is held in flexion, adduction, and internal rotation. On examination, the right leg appears shortened compared to the left. Plain radiograph confirms a displaced intracapsular fracture of the femoral neck. During surgical planning, the orthopedic surgeon is concerned about potential avascular necrosis (AVN). Which anatomical feature of the femoral neck blood supply makes intracapsular fractures particularly prone to AVN?

Explanation

## Femoral Neck Blood Supply and Intracapsular Fracture Complications ### Vascular Anatomy of the Femoral Head and Neck **Key Point:** The femoral head has a precarious blood supply that is entirely dependent on small vessels running along the femoral neck. Intracapsular fractures disrupt this supply, leading to a high risk of avascular necrosis (AVN). ### Blood Supply Sources | Source | Vessel | Course | Relevance to Fracture | |--------|--------|--------|----------------------| | **Primary (90%)** | Medial femoral circumflex artery (MFCA) | Extracapsular, runs posteriorly along femoral neck | **TORN by fracture fragments** | | **Primary (90%)** | Lateral femoral circumflex artery (LFCA) | Extracapsular, runs anteriorly along femoral neck | **TORN by fracture fragments** | | **Secondary (10%)** | Ligamentum teres artery (from obturator) | Intracapsular, via ligamentum teres | Insufficient in adults; mainly fetal supply | | **Tertiary** | Nutrient artery | Enters femoral shaft distally | Does NOT supply femoral head | ### Why Intracapsular Fractures Cause AVN **High-Yield:** The **medial and lateral femoral circumflex arteries** are the dominant blood supply to the femoral head. These vessels: 1. Run **extracapsularly** along the posterolateral and anterolateral aspects of the femoral neck 2. Are **stretched, kinked, and often torn** by displaced fracture fragments 3. Supply the femoral head via **retinacular branches** that penetrate the capsule near the femoral head **Clinical Pearl:** In intracapsular fractures: - Displaced fractures (Garden III–IV) have a 30–50% risk of AVN - Non-displaced fractures have a 5–10% risk - The risk increases with: - Degree of displacement - Delay to reduction - Age > 60 years (less collateral circulation) - Disruption of the retinacular vessels ### Mechanism of Vascular Compromise ```mermaid flowchart TD A[Intracapsular femoral neck fracture]:::outcome --> B{Fracture displaced?}:::decision B -->|Yes, displaced| C[Fracture fragments compress/tear<br/>MFCA and LFCA]:::urgent B -->|No, non-displaced| D[Vessels stretched but intact]:::action C --> E[Retinacular vessels occluded]:::urgent D --> E E --> F[Femoral head ischemia]:::urgent F --> G[Avascular necrosis develops<br/>over months to years]:::outcome H[Ligamentum teres artery<br/>insufficient in adults]:::action --> I[Cannot compensate for MFCA/LFCA loss]:::urgent ``` **Mnemonic:** **MFCA = Most Frequent Culprit in Avascular necrosis** — The medial femoral circumflex artery is the primary blood supply and the most commonly disrupted vessel in femoral neck fractures. [cite:Moore's Clinically Oriented Anatomy 8e Ch 5; Harrison 21e Ch 333]