Which nerve injury results in loss of wrist extension with preserved finger extension, and is most commonly caused by fracture of the distal radius?

Explanation

## Radial Nerve Lesions: High vs. Low Palsy ### Comparison Table | Feature | High Radial Palsy | Low Radial (PIN) Palsy | |---------|-------------------|------------------------| | **Site of injury** | Above spiral groove (axilla, proximal humerus) | Distal to branch to ECRB (forearm) | | **Wrist extension** | Lost | Preserved (ECRB spared) | | **Finger extension** | Lost | Lost | | **Thumb extension (IPJ)** | Lost | Lost | | **Thumb abduction** | Lost | Lost | | **Sensory loss** | Dorsal hand + dorsal forearm | None (PIN is motor) | | **Common cause** | Humeral fracture, axillary compression | Distal radius fracture, supinator syndrome | | **Presentation** | "Wrist drop" + sensory loss | "Finger drop" without wrist drop | ### Why Low Radial (PIN) Palsy Preserves Wrist Extension **Key Point:** The posterior interosseous nerve (PIN) branches off the radial nerve distal to the origin of the extensor carpi radialis brevis (ECRB). Therefore, ECRB remains innervated and wrist extension is preserved. **High-Yield:** The extensor carpi radialis longus (ECRL) is innervated by the main radial nerve proximal to the spiral groove, so it is affected in high radial palsy but spared in PIN lesions. ### Innervation Sequence (Proximal to Distal) 1. **Radial nerve** → ECRL (proximal, in upper arm) 2. **Radial nerve** → ECRB (distal, in proximal forearm) 3. **PIN (branch of radial)** → Extensor digitorum, extensor digiti minimi, extensor carpi ulnaris, extensor pollicis longus, extensor pollicis brevis, abductor pollicis longus ### Clinical Pearl **Posterior Interosseous Nerve Syndrome:** PIN compression at the supinator muscle ("supinator syndrome") causes finger drop without wrist drop — a pathognomonic finding that distinguishes it from high radial palsy. **Mnemonic — PIN Motor Branches:** **EDCAP** = Extensor Digitorum, Extensor Digiti minimi, Extensor Carpi Ulnaris, Abductor Pollicis longus, and the two Extensor Pollicis muscles (longus and brevis).