A 32-year-old man sustains a fracture of the mid-shaft of the humerus after a fall. On examination, he has wrist drop and loss of sensation over the dorsal first web space. Which clinical feature best distinguishes a radial nerve lesion at the level of the spiral groove from a lesion at the axilla?

## Discriminating Feature: Triceps Involvement **Key Point:** The radial nerve supplies the triceps muscle via branches that arise proximal to the spiral groove. A lesion at the spiral groove (mid-shaft humeral fracture) occurs DISTAL to these triceps branches, whereas an axillary lesion occurs PROXIMAL to them. ### Anatomical Course of Radial Nerve | Level of Lesion | Triceps | Wrist Extensors | Sensation (Dorsal Web Space) | Elbow Extension | |---|---|---|---|---| | **Axilla** | Paralyzed | Paralyzed | Lost | Lost | | **Spiral Groove** | Intact | Paralyzed | Lost | Preserved | | **Below Elbow** | Intact | Paralyzed | Lost | Preserved | **High-Yield:** In mid-shaft humeral fractures with radial nerve injury at the spiral groove, triceps function is PRESERVED because the nerve to triceps branches off proximal to this level. ### Clinical Findings in Spiral Groove Lesion 1. **Wrist drop** — loss of wrist extension (extensor carpi radialis longus & brevis, extensor carpi ulnaris) 2. **Loss of finger extension** — extensor digitorum communis paralyzed 3. **Sensory loss** — dorsal first web space (radial sensory branch) 4. **Elbow extension preserved** — triceps intact 5. **Thumb abduction weak** — extensor pollicis longus involved **Clinical Pearl:** The "Saturday night palsy" (radial nerve compression from sleeping with arm over chair back) typically occurs at the spiral groove level, and patients retain elbow extension ability, which helps distinguish it from higher lesions. **Warning:** Do not confuse preservation of triceps function with preservation of all motor function — wrist and finger extensors are still paralyzed in spiral groove lesions.