In Saturday night palsy (radial nerve compression in the spiral groove of humerus), which muscle action is PRESERVED?

• A Extension of the elbow (triceps) ✓
• B Extension of the fingers at metacarpophalangeal joints
• C Extension of the wrist (wrist drop is absent)
• D Supination of the forearm by supinator muscle

Explanation

In spiral groove compression (Saturday night palsy), the radial nerve is injured after it has already given off branches to the triceps. Therefore, elbow extension (triceps) is PRESERVED — this distinguishes spiral groove lesions from more proximal posterior cord injuries where triceps is also weak. Wrist drop, finger drop (extensor digitorum, extensor digiti minimi), thumb extension and abduction, and brachioradialis weakness/loss of forearm rotation are all affected. Supinator is innervated by the deep branch (posterior interosseous nerve) distal to the spiral groove and is also affected.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.