A patient sustains a fracture of the mid-shaft humerus and develops wrist drop with inability to extend the fingers at the metacarpophalangeal joints. Which branch of the radial nerve is responsible for innervating the muscles that produce these movements?

Explanation

## Radial Nerve Branches and Motor Innervation ### Understanding Wrist Drop and Finger Extension Loss **Key Point:** Wrist drop (loss of wrist extension) and inability to extend fingers at the metacarpophalangeal (MCP) joints result from paralysis of the extensor muscles of the forearm. These muscles are innervated by the **Posterior Interosseous Nerve (PIN)**, the deep motor branch of the radial nerve. ### PIN Motor Innervation Pattern The posterior interosseous nerve innervates all the extensor muscles of the forearm: | Muscle | Action | PIN Branch | |--------|--------|------------| | Extensor carpi radialis brevis | Wrist extension | PIN | | Extensor carpi ulnaris | Wrist extension + ulnar deviation | PIN | | Extensor digitorum | MCP extension of fingers 2–5 | PIN | | Extensor digiti minimi | MCP extension of little finger | PIN | | Extensor pollicis longus | IP extension of thumb | PIN | | Extensor pollicis brevis | MCP extension of thumb | PIN | | Abductor pollicis longus | Thumb abduction | PIN | | Extensor indicis | MCP extension of index finger | PIN | **High-Yield:** The **Posterior Interosseous Nerve (PIN)** is the deep motor continuation of the radial nerve after it divides in the cubital fossa. It passes through the supinator muscle and supplies all extensor muscles of the forearm. ### Clinical Presentation of PIN Palsy 1. **Wrist drop** — loss of wrist extension (ECRB, ECU paralyzed) 2. **Loss of finger extension** — inability to extend fingers at MCP joints (Extensor digitorum paralyzed) 3. **Loss of thumb extension** — inability to extend thumb at IP joint (EPL paralyzed) 4. **Preserved wrist flexion** — flexor muscles (FCR, FCU) are innervated by median and ulnar nerves 5. **Preserved grip strength** — intrinsic hand muscles intact **Clinical Pearl:** In mid-shaft humeral fractures, the radial nerve is injured within the spiral groove, but the PIN (its deep motor branch) is affected downstream, resulting in the classic wrist drop syndrome. ### Mnemonic for PIN Innervation **"EABCDE"** — **E**xtensor carpi radialis brevis, **A**bductor pollicis longus, **B**oth extensor pollicis (longus and brevis), **C**arpi ulnaris, **D**igitorum, **E**xtensor indicis ## Why Each Option is Correct or Wrong **Correct answer: Posterior Interosseous Nerve (PIN)** - PIN is the deep motor branch of the radial nerve - Innervates all extensor muscles of the forearm - Injury causes wrist drop and loss of finger MCP extension **Why the other options are wrong:** - **Superficial radial nerve:** Sensory only; no motor function in the forearm - **Radial nerve to brachioradialis:** This branch comes off proximal to the PIN and supplies only brachioradialis (elbow flexion); loss does not cause wrist drop - **Radial nerve to ECRB:** The ECRB is innervated by PIN, not a separate proximal radial nerve branch ### Anatomical Basis ```mermaid flowchart TD A["Radial Nerve<br/>(C5-T1)"]:::outcome --> B["Spiral Groove<br/>in Humerus"]:::outcome B --> C{"Exit at Middle-Distal<br/>Third Junction"}:::decision C -->|"Anterior Compartment"| D["Superficial Branch<br/>(Sensory)"]:::outcome C -->|"Posterior Compartment"| E["Deep Branch<br/>= PIN"]:::action E --> F["Supinator Muscle<br/>Penetration"]:::outcome F --> G["Motor Innervation<br/>All Forearm Extensors"]:::action G --> H["Extensor Digitorum<br/>Extensor Carpi Radialis<br/>Extensor Carpi Ulnaris<br/>Extensor Pollicis L & B"]:::outcome ``` **High-Yield:** PIN palsy is a classic presentation in mid-shaft humeral fractures, and the examiner will expect you to identify the specific nerve branch responsible for the motor deficit.