During the clinical examination of a 6-year-old girl with a supracondylar fracture of the humerus, the orthopedic surgeon assesses for neurovascular complications. Which of the following is the most common nerve injury associated with supracondylar fractures?

Explanation

## Neurovascular Complications in Supracondylar Fractures **Key Point:** The anterior interosseous nerve (AIN), a branch of the median nerve, is the most commonly injured nerve in supracondylar fractures, occurring in 10–20% of cases. ### Anatomical Basis The AIN arises from the median nerve just distal to the elbow and innervates: - **Flexor pollicis longus (FPL)** — thumb IP joint flexion - **Flexor digitorum profundus (FDP)** to digits 2–3 — DIP joint flexion of index and middle fingers - **Pronator quadratus** — pronation of forearm ### Mechanism of AIN Injury 1. **Traction injury** from posterior displacement of the fracture fragments 2. **Stretching** of the nerve as the distal fragment displaces posteriorly 3. **Compression** by swelling or hematoma in the anterior compartment ### Frequency of Nerve Injuries in Supracondylar Fractures | Nerve | Incidence | Clinical Test | |-------|-----------|---------------| | AIN (median nerve branch) | 10–20% | Loss of thumb IP flexion (FPL) and index DIP flexion (FDP) | | Radial nerve | 5–10% | Wrist drop, loss of thumb abduction (EPL) | | Median nerve (main trunk) | 5–8% | Loss of pronation, wrist flexion | | Ulnar nerve | 2–5% | Rare; claw hand deformity | | Brachial artery | 5–20% | Absent radial pulse, pale hand | **High-Yield:** AIN injury presents with the **"OK sign" loss** — inability to make an "OK" sign (thumb IP + index DIP flexion cannot be performed). ### Clinical Pearl AIN injury is often missed on initial examination because: - It is a pure motor nerve (no sensory component) - The loss of thumb IP flexion may be subtle in a crying, injured child - Swelling and pain limit cooperation during testing **Mnemonic:** **AIN = Anterior Interosseous Nerve** - **A** — Anterior compartment of forearm - **I** — Innervates FPL, FDP (index), Pronator quadratus - **N** — Most common nerve injury in supracondylar fractures ### Prognosis Most nerve injuries (>90%) in supracondylar fractures are **neurapraxias** and recover spontaneously within 3–6 months. Axonotmesis or neurotmesis requiring surgical exploration is rare. **Warning:** Do not confuse AIN injury with radial nerve injury. Radial nerve injury causes wrist drop (loss of wrist extension), whereas AIN injury causes loss of thumb IP flexion and index DIP flexion.