MCQ – 93755

Injury to the posterior interosseous nerve leads to which of the following deficits?

A.	Wrist drop
B.	Loss of finger extension
C.	Inability to extend the wrist
D.	Inability to adduct the fingers/thumb

Show Answer

Correct Answer � B Explanation

The posterior interosseous nerve (PIN) is a deep branch of the radial nerve.
It supplies the muscles in the posterior compartment of the forearm responsible for extending the fingers and thumb, but it does not supply the muscles for wrist extension directly or those for intrinsic hand movements like adduction of fingers.
The PIN innervates muscles like the extensor digitorum, extensor digiti minimi, and extensor indicis, responsible for finger extension.
Damage to the PIN specifically results in the inability to extend the fingers at the MCP (metacarpophalangeal) joints while sparing wrist extension.

High-Yield Points on Posterior Interosseous Nerve

Origin: Deep branch of the radial nerve, after passing through the supinator muscle.

Innervation: Supplies the deep extensor muscles of the forearm, including:

• Extensor digitorum
• Extensor digiti minimi
• Extensor pollicis longus
• Extensor pollicis brevis
• Abductor pollicis longus
• Extensor indicis

Common Causes of Injury:

• Fractures of the radial head or shaft.
• Compression in the arcade of Frohse (a fibrous arch in the supinator).

Presentation:

• Weakness in finger and thumb extension.
• Normal wrist extension.

Pearls for Exams

• Wrist drop signifies a more proximal radial nerve injury, such as in the mid-humerus (e.g., in “Saturday night palsy”).
• Posterior interosseous nerve syndrome presents with “finger drop” but preserved wrist extension.
• Differentially diagnose from ulnar nerve lesions, which affect finger abduction/adduction, and median nerve lesions, which impair thumb opposition.