A 6-year-old boy is brought to the emergency department with a supracondylar fracture of the humerus. Which clinical feature most reliably distinguishes an extension-type supracondylar fracture from a flexion-type fracture?

Explanation

## Extension-Type vs Flexion-Type Supracondylar Fractures Supracondylar fractures are classified by the direction of displacement of the distal fragment, which reflects the mechanism of injury and has important implications for reduction and complications. ### Mechanism and Displacement Pattern | Feature | Extension-Type | Flexion-Type | |---------|----------------|---------------| | **Frequency** | 95–98% of all supracondylar fractures | 2–5% of all supracondylar fractures | | **Mechanism** | Fall on outstretched hand (FOOSH) with elbow in extension | Direct blow to posterior elbow or fall on flexed elbow | | **Distal fragment position** | **Posterior and proximal** (displaced backward) | **Anterior and proximal** (displaced forward) | | **Radiographic finding** | Loss of anterior humeral line; posterior angulation | Anterior angulation; disruption of posterior cortex | | **Reduction method** | Hyperextension then flexion | Gentle traction and extension | | **Neurovascular injury** | Anterior interosseous nerve (AIN), brachial artery (more common) | Posterior interosseous nerve (PIN), posterior structures | **Key Point:** The **direction of distal fragment displacement** is the single most reliable and objective radiographic discriminator. Extension-type shows posterior displacement; flexion-type shows anterior displacement. **High-Yield:** Extension-type fractures are vastly more common (>95%) in the pediatric population. Flexion-type fractures are rare and often missed on initial radiographs because they are unexpected. **Clinical Pearl:** On lateral X-ray, trace the anterior humeral line (from the anterior cortex of the proximal humerus through the center of the capitellum). In extension-type, this line passes anterior to the capitellum (loss of alignment). In flexion-type, the distal fragment is displaced anteriorly, creating a different angulation pattern. **Mnemonic:** **"Extension = Posterior; Flexion = Anterior"** — the distal fragment goes in the direction opposite to the elbow position at injury.