A 28-year-old woman sustained a stab wound to the proximal forearm just below the elbow. Examination reveals loss of thumb interphalangeal (IP) joint extension and loss of index finger distal interphalangeal (DIP) joint extension, but wrist extension and finger metacarpophalangeal (MCP) joint extension are intact. Which finding best distinguishes this as a posterior interosseous nerve (PIN) injury rather than a more proximal radial nerve lesion at the spiral groove?

Explanation

## Distinguishing PIN Injury from Proximal Radial Nerve Lesion ### Anatomical Basis The radial nerve divides into two branches in the proximal forearm: 1. **Superficial radial nerve** — sensory to dorsal hand 2. **Posterior interosseous nerve (PIN)** — motor to finger and thumb extensors The **wrist extensor muscles** (extensor carpi radialis longus and brevis) receive motor innervation from the **main radial nerve BEFORE it branches into PIN**. This is the critical anatomical distinction. ### Motor Innervation of Extensors | Muscle | Innervation | Spiral Groove Lesion | PIN Lesion | |--------|-------------|---------------------|------------| | **ECRL / ECRB (wrist extensors)** | Radial nerve (proximal) | **Paralyzed** | **Preserved** | | **EDC (finger MCP extensors)** | PIN | Paralyzed | Paralyzed | | **EIP (index DIP extensor)** | PIN | Paralyzed | Paralyzed | | **EPL (thumb IP extensor)** | PIN | Paralyzed | Paralyzed | | **EPB (thumb MCP extensor)** | PIN | Paralyzed | Paralyzed | **Key Point:** The **wrist extensor muscles branch off the radial nerve proximal to the PIN origin**. Therefore: - **Spiral groove lesion** → Loss of wrist extension + loss of finger/thumb extension - **PIN lesion** → Preserved wrist extension + loss of finger/thumb extension ### Clinical Presentation in This Case The patient has: - ✓ **Preserved wrist extension** ← This is the discriminator - ✗ Loss of thumb IP extension (EPL — PIN-innervated) - ✗ Loss of index DIP extension (EIP — PIN-innervated) - ✓ Intact finger MCP extension (EDC — PIN-innervated, but this is a motor loss, not preserved) **Clinical Pearl:** The **preservation of wrist extension in the presence of finger/thumb extension loss** is pathognomonic for a PIN lesion distal to the branching of the wrist extensor nerves. A proximal radial nerve lesion (spiral groove) would cause **both** wrist and finger extension loss. ### Mnemonic **"WRIST FIRST"** — Wrist extensors branch from radial nerve FIRST (proximal); PIN branches SECOND (distal). PIN injury spares wrist; proximal injury loses both. **High-Yield:** This is a classic NEET PG discriminator question. The single best feature distinguishing PIN from proximal radial nerve injury is **preserved wrist extension**.