A 19-year-old male presents with a gunshot wound to the axilla. On examination, he has loss of elbow flexion and supination, with intact shoulder abduction and external rotation. Wrist extension and finger extension are also preserved. Which nerve is injured?

Explanation

## Musculocutaneous Nerve Injury Pattern **Nerve Anatomy and Course:** - **Origin:** Terminal branch of the lateral cord (C5–C6–C7) - **Course:** Pierces the coracobrachialis muscle, runs between biceps and brachialis in the anterior arm - **Motor innervation:** - Coracobrachialis - Biceps brachii (primary elbow flexor AND supinator) - Brachialis (elbow flexor) - **Sensory:** Lateral antebrachial cutaneous nerve (lateral forearm) **Clinical Presentation Analysis:** - **Loss of elbow flexion** → Biceps and brachialis paralyzed - **Loss of supination** → Biceps is the primary supinator; loss indicates musculocutaneous nerve injury - **Preserved shoulder abduction** → Supraspinatus intact (suprascapular nerve from upper trunk) - **Preserved external rotation** → Infraspinatus intact (suprascapular nerve) - **Preserved wrist extension** → Radial nerve intact - **Preserved finger extension** → Posterior interosseous nerve (radial nerve branch) intact **Key Point:** The combination of elbow flexion loss with supination loss is pathognomonic for musculocutaneous nerve injury. Brachialis alone (innervated by musculocutaneous) can flex the elbow, but loss of supination specifically indicates biceps involvement. **Clinical Pearl:** Musculocutaneous nerve injury = "no biceps" = no flexion + no supination.