What causes posterior interosseous nerve (PIN) syndrome?

Posterior Interosseous Nerve Syndrome: Causes and Development

Posterior interosseous nerve syndrome (PINS) primarily develops due to compression of the deep branch of the radial nerve as it passes through the supinator muscle in the forearm, most commonly at the arcade of Frohse.

Anatomical Considerations

The posterior interosseous nerve (PIN) is the terminal motor branch of the radial nerve that provides innervation to the extensor muscles of the forearm. Understanding its course helps identify potential compression sites:

• The PIN originates from the deep branch of the radial nerve near the lateral epicondyle
• It travels through the radial tunnel and passes beneath the arcade of Frohse (fibrous arch at the proximal edge of the supinator muscle)
• After exiting the supinator, it continues distally to innervate the extensor muscles of the wrist and fingers

Common Causes of Compression

1. Anatomical Structures:

   • Arcade of Frohse: The most common site of compression (fibrous arch at the proximal edge of the supinator muscle) 1
   • Distal border of the supinator muscle 2
   • Fibrous bands within the radial tunnel 2
   • Vascular structures: Ramifications of anterior and posterior interosseous vessels 2
   • Fascial septa: Between extensor carpi ulnaris and extensor digitorum minimi 2

2. Mechanical Factors:

   • Forearm rotation: The nerve is stressed during passive supination (elongation and rotation) and pronation (compression) 2
   • Repetitive forearm rotation: Common in occupational activities
   • Recurrent radial head dislocation: Can cause PIN compression in pediatric cases 3

3. Space-Occupying Lesions:

   • Synovial cysts or ganglia
   • Lipomas
   • Bicipital bursa: Documented as a cause of PIN syndrome 4
   • Tumors: Both benign and malignant

4. Traumatic Causes:

   • Forearm fractures: Particularly radial head/neck fractures
   • Elbow dislocations
   • Penetrating injuries
   • Iatrogenic injuries during surgery

5. Inflammatory Conditions:

   • Rheumatoid arthritis: Synovial proliferation at the elbow
   • Tennis elbow: Often coexists with PIN syndrome (52% of cases) 5

Diagnostic Approach

When PIN syndrome is suspected, a thorough evaluation should include:

1. Imaging Studies:

   • MRI: Superior for evaluating nTOS and delineating nerve anatomy 6
   • Ultrasound: Can assess compression and dynamic changes 6
   • CT: Helpful for evaluating bony abnormalities 6

2. Electrodiagnostic Studies:

   • EMG/NCS to confirm the diagnosis and localize the site of compression

Clinical Implications

PIN syndrome typically presents with:

• Weakness of finger and thumb extension
• Preserved wrist extension (due to sparing of extensor carpi radialis longus and brevis)
• Absence of sensory symptoms (PIN is a pure motor nerve)

Treatment Considerations

Treatment options include:

• Conservative management with physical therapy and activity modification
• Surgical decompression for persistent symptoms or space-occupying lesions
• Indirect decompression techniques may be preferred to avoid recompression by scarring (17% complication rate with direct decompression) 5

Important Clinical Distinctions

It's crucial to differentiate PIN syndrome from:

• Radial tunnel syndrome (pain without motor weakness)
• Tennis elbow (lateral epicondylitis)
• Cervical radiculopathy
• Posterior cord brachial plexopathy

Understanding these anatomical and mechanical factors is essential for accurate diagnosis and appropriate management of PIN syndrome.