Why damage to posterior interosseus nerve (PIN) does not cause wrist drop?

1. PIN arises from radial nerve. Radial nerve pulak arises from posterior cord of the brachial plexus
2. It passes posterior to the axillary artery between long and media heads of triceps muscle, to lie in the spiral groove between medial & lateral heads of triceps muscle.
3. Here it is accompanied by the profunda brachii artery before it pierces it lateral intermuscular septum of the lower third humerus to run between brachialis & brachioradialis
4. At the lateral of epicondyle humerus, it gives rises to PIN & superficial radial nerve
5. Radial nerve supply all the extensor muscle of forearm & arm
6. However it also supply brachioradialis, which is flexor of elbow when forearm pronated
7. Damage to the nerve in the spiral groove causes wrist drop but no loss of elbow extension, as fibres of triceps remain intact proximal to this site
8. Only damage in the axilla will causes loss of elbow extension & wrist drop
9. Damage to posterior interosseus nerve (PIN) does not cause wrist drop because extensor carpi radialis longus receives its innervation from the main radial nerve
10. Pin only cause unable to extend metacarpophalangeal joint à finger drop.