A 28-year-old male presents to the emergency department 6 hours after a stab wound to the posterior aspect of the upper arm at the junction of the middle and distal thirds. On examination, he has intact wrist extension but complete loss of finger extension at the metacarpophalangeal (MCP) joints, inability to extend the thumb at the interphalangeal joint, and loss of sensation over the dorsal first web space. Motor power in the forearm is otherwise intact. What is the most likely nerve injured and at what anatomical level?

Explanation

## Anatomical Basis of PIN Injury — Distal to Supinator ### Radial Nerve Branching Pattern The radial nerve descends through the spiral groove of the humerus and enters the anterior compartment of the arm. Just proximal to the elbow, it divides into two terminal branches: 1. **Superficial radial nerve (SRN)**: Pure sensory branch supplying the dorsal radial hand and first web space. 2. **Posterior interosseous nerve (PIN)**: Deep motor branch that enters and traverses the supinator muscle (through the Arcade of Frohse), then emerges posteriorly to supply finger and thumb extensors. ### PIN Segmental Innervation **Before entering the supinator (proximal PIN):** - Extensor carpi radialis brevis (ECRB) - Supinator **After exiting the supinator (distal PIN):** - Extensor digitorum communis (EDC) - Extensor digiti minimi (EDM) - Extensor indicis proprius (EIP) - Extensor pollicis longus (EPL) - Extensor pollicis brevis (EPB) - Abductor pollicis longus (APL) **Note:** Extensor carpi radialis longus (ECRL) is innervated by the **radial nerve proper**, proximal to the PIN bifurcation — it is never affected by a PIN lesion. ### Clinical Differentiation: Proximal vs. Distal PIN Injury | Feature | Proximal PIN Injury (before supinator) | Distal PIN Injury (after supinator) | |---------|----------------------------------------|--------------------------------------| | **Wrist extension** | Partially preserved (ECRL intact via radial nerve proper; ECRB lost → radial deviation on extension) | Fully preserved (both ECRL and ECRB intact) | | **Finger MCP extension** | Lost | Lost | | **Thumb IP extension** | Lost | Lost | | **Dorsal first web sensory loss** | Present (SRN also affected if injury is at radial nerve level) | Present (SRN runs separately and may be co-injured) | ### Analysis of This Case **High-Yield:** The key clinical findings are: - **Intact wrist extension** — both ECRL (radial nerve proper) and ECRB (proximal PIN) are functioning → the PIN is injured **distal to the supinator**, after it has already supplied ECRB. - **Loss of finger MCP extension and thumb IP extension** — EDC, EIP, EPL affected → distal PIN territory. - **Sensory loss over dorsal first web space** — superficial radial nerve co-injured at this level. **Clinical Pearl:** A proximal PIN injury (before the supinator) would spare ECRL but lose ECRB, causing wrist extension with radial deviation. A **distal PIN injury** (after the supinator) spares both ECRL and ECRB, producing **full wrist extension with complete loss of finger and thumb extension** — exactly as described in this vignette. **Anatomical correlation:** The stab wound is at the junction of the middle and distal thirds of the posterior upper arm. At this level, the PIN has already exited the supinator (which is located at the proximal forearm/elbow level). The nerve is now in the posterior compartment of the forearm, vulnerable to penetrating trauma. This is entirely consistent with a **distal PIN injury**. **Mnemonic: Distal PIN = Finger drop WITHOUT wrist drop (wrist extension fully preserved).** [cite: Moore & Dalley, Clinically Oriented Anatomy, 8th ed.; Gray's Anatomy, Standring, 42nd ed.; Netter's Atlas of Human Anatomy]