The posterior interosseous nerve is a branch of the radial nerve (answer c).
The posterior interosseous nerve (PIN) is the continuation of the deep branch of the radial nerve after it passes through the supinator muscle in the forearm 1, 2. This is fundamental neuroanatomy that is well-established across multiple anatomical studies.
Anatomical Course
The radial nerve bifurcates into two terminal branches approximately 3.6 cm proximal to the supinator muscle:
- Superficial radial nerve (sensory)
- Deep branch of the radial nerve (motor), which becomes the PIN after passing through the supinator
The PIN specifically innervates the posterior compartment muscles of the forearm 1, including most extensor muscles, and contains deep sensory fibers to the wrist ligaments and joints 2.
Key Anatomical Landmarks
The radial nerve divides into its posterior interosseous and superficial radial branches approximately 1.3 cm above the elbow joint line 3. The PIN then:
- Passes through the radial tunnel anterior to the radiohumeral joint
- Courses beneath the arcade of Frohse (proximal edge of the supinator muscle)
- Exits the supinator muscle approximately 3.8 cm distal to its proximal margin 4
Clinical Relevance
Understanding this relationship is critical because:
- PIN compression (radial tunnel syndrome) presents with dorsal forearm pain without motor paralysis in many cases 5
- The nerve is vulnerable at multiple compression sites along its course through the supinator muscle
- Surgical decompression requires precise knowledge of the nerve's relationship to surrounding structures [6-6 discuss positioning to avoid radial nerve injury in the spiral groove of the humerus]
The other options are incorrect: The median nerve innervates the anterior forearm compartment, the ulnar nerve supplies medial forearm and hand muscles, and the axillary nerve is confined to the shoulder region.