A 28-year-old man presents with acute-onset severe pain in the lateral aspect of the elbow after a fall on an outstretched hand (FOOSH injury). On examination, he has weakness of finger extension and wrist extension, but normal thumb opposition and sensation in the palm. Sensation is intact over the dorsal first web space. Imaging reveals a fracture-dislocation of the radial head. Which branch of the radial nerve is most likely injured?

Explanation

## Clinical Presentation Analysis **Key Point:** Radial head fractures and dislocations can entrap or stretch the **posterior interosseous nerve (PIN)** as it passes through the supinator muscle, causing a pure motor syndrome without sensory loss. ### Anatomical Vulnerability of PIN at Radial Head Level The radial nerve divides into its terminal branches **distal to the elbow**: - **Superficial radial nerve** → sensory (dorsal hand/thumb) - **Posterior interosseous nerve (PIN)** → motor (finger & wrist extension) The PIN is particularly vulnerable at the **supinator muscle level** (just distal to radial head) due to: 1. Passage through the supinator arcade (fibrous tunnel) 2. Proximity to radial head fractures and callus 3. Traction injury from radial head displacement ### Clinical Features of PIN Injury | Feature | PIN Injury | Radial Nerve Proper Injury | |---------|-----------|---------------------------| | **Wrist extension** | Weak (ECRB, ECRL innervated by PIN) | Weak | | **Finger extension** | Weak/absent (EDC, EIP innervated by PIN) | Weak | | **Thumb IP extension** | Weak (EPL innervated by PIN) | Weak | | **Thumb opposition** | Normal (median nerve) | Normal | | **Dorsal web space sensation** | Normal (superficial radial nerve intact) | Absent | | **Palm sensation** | Normal (median/ulnar) | Normal | **High-Yield:** PIN injury produces a **pure motor syndrome** — no sensory loss because the superficial sensory branch is spared. This is the key discriminator from proximal radial nerve injury. ### Anatomical Course of PIN ```mermaid flowchart TD A[Radial Nerve Proper]:::outcome --> B[Divides distal to elbow]:::action B --> C[Superficial Radial Nerve]:::action B --> D[Posterior Interosseous Nerve]:::action D --> E[Passes through Supinator Muscle]:::action E --> F[Vulnerable to radial head fractures]:::urgent F --> G[Motor loss: wrist + finger extension]:::outcome C --> H[Sensory: dorsal hand - SPARED in PIN injury]:::outcome ``` **Clinical Pearl:** The **supinator muscle acts as a tourniquet** around PIN; radial head fractures, swelling, or callus can compress the nerve within this fibrous tunnel, causing delayed or acute PIN palsy ("supinator syndrome"). ## Why Correct The patient has **motor loss** (weak finger and wrist extension) but **intact sensation** (normal dorsal web space sensation and palm sensation). This dissociation—motor loss without sensory loss—is pathognomonic for **PIN injury**, which is a pure motor nerve. The radial head fracture-dislocation is the anatomical mechanism: PIN passes through the supinator muscle just distal to the radial head and is vulnerable to compression or traction. [cite:Clinically Oriented Anatomy Moore 8e Ch 6]