A 32-year-old man presents with wrist drop following a fracture of the mid-shaft humerus. On examination, he has weakness of wrist extension but preserved finger extension. Which clinical feature best distinguishes a lesion at the spiral groove from a lesion distal to the posterior interosseous nerve (PIN) origin?

Explanation

## Radial Nerve Anatomy and Lesion Localization ### Course of the Radial Nerve The radial nerve follows a critical anatomical path through the upper limb: 1. Exits the axilla posterior to the axillary artery 2. Travels in the **spiral groove** of the humerus (vulnerable to mid-shaft fractures) 3. Pierces the lateral intermuscular septum 4. Divides into the **superficial radial nerve** (purely sensory) and the **posterior interosseous nerve (PIN)** (purely motor) ### Key Anatomical Distinction | Feature | Spiral Groove Lesion | Lesion Distal to PIN Origin | |---------|---------------------|----------------------------| | Wrist extension | Lost | Lost (ECRL spared if very distal) | | Finger extension | Lost | Lost | | Thumb extension | Lost | Lost | | Sensation — dorsal 1st web space | **Lost** | **Preserved** | | Sensation — dorsal hand | **Lost** | **Preserved** | ### Why Sensation Is the Discriminator **High-Yield:** The superficial radial nerve (sensory branch) diverges from the main radial nerve **proximal to** the PIN origin, in the lateral forearm. A lesion at the **spiral groove** damages the radial nerve before this bifurcation, injuring BOTH the superficial sensory branch AND the PIN → sensory loss over the dorsal first web space AND finger/wrist drop. A lesion **distal to the PIN origin** (e.g., posterior interosseous nerve syndrome at the radial tunnel) affects only the motor PIN; the superficial radial nerve is already separate and unaffected → **no sensory loss** over the dorsal first web space. **Clinical Pearl:** The best clinical feature distinguishing a **spiral groove lesion** from a **lesion distal to the PIN origin** is the **status of sensation over the dorsal first web space** (Option A): - **Spiral groove lesion** → Sensory loss over dorsal first web space (superficial radial nerve involved) - **Distal PIN lesion** → Sensation over dorsal first web space **preserved** (superficial radial nerve spared) ### Why Option C Is Incorrect as a Discriminator Option C ("weakness of finger extension with preserved wrist extension") describes the classic finding of a **proximal PIN lesion** (e.g., radial tunnel syndrome), not a feature that distinguishes spiral groove from distal-to-PIN lesions. Furthermore, the stem itself already states preserved finger extension, making Option C circular and non-discriminating. Both spiral groove and distal PIN lesions can produce finger extension loss; the sensory status is the reliable localizing feature. ### Mnemonic **"Spiral = Sensory + Motor lost; PIN-only = Motor lost, Sensation Spared"** *Reference: Gray's Anatomy for Students, 4th ed.; Aids to the Examination of the Peripheral Nervous System (Medical Research Council); Netter's Clinical Anatomy.*