PIN (Posterior Interosseous Nerve) Injury
What is a Posterior Interosseous Nerve (PIN) Injury?
A Posterior Interosseous Nerve (PIN) injury affects the deep motor branch of the radial nerve. The PIN innervates several extensor muscles of the forearm, which control wrist and finger extension. Injury to this nerve results in weakness or paralysis of these muscles, leading to functional difficulties like wrist drop or inability to extend the fingers. Unlike radial nerve injuries higher up the arm, PIN injuries do not cause sensory loss because the PIN is a purely motor nerve.
Causes of Posterior Interosseous Nerve Injury
PIN injuries can result from a variety of mechanical and inflammatory sources. Common causes include:
- Compression at the radial tunnel, particularly between the two heads of the supinator muscle (Arcade of Frohse)
- Repetitive motion or overuse, especially in athletes or manual laborers
- Trauma, such as fractures or dislocations of the elbow or forearm
- Mass lesions, like ganglion cysts or lipomas compressing the nerve
- Inflammatory neuritis, such as in Parsonage-Turner syndrome
What are the Symptoms of Posterior Interosseous Nerve Injury?
PIN injury typically presents with specific motor deficits and no sensory changes. Common symptoms include:
- Wrist drop, due to loss of extensor carpi ulnaris and radialis brevis function
- Inability to extend fingers or thumb, while wrist extension may be spared
- Weak grip strength, because of poor wrist stabilization
- Forearm pain or aching, particularly if the nerve is compressed
- No numbness or tingling, distinguishing it from more proximal radial nerve injuries
These signs are key to differentiating PIN injury from other radial neuropathies.
How is Posterior Interosseous Nerve Injury Diagnosed?
Diagnosis involves both clinical evaluation and imaging:
- Physical examination, including testing finger and thumb extension
- Provocative maneuvers, such as resisted supination, to reproduce symptoms
- Electromyography (EMG) and nerve conduction studies, to assess motor function and confirm diagnosis
- MRI or ultrasound, to detect compressive lesions like cysts or tumors
- X-rays, if trauma or bony abnormalities are suspected
Accurate diagnosis is essential to distinguish PIN syndrome from other causes of weakness.
What are the Treatment Options for Posterior Interosseous Nerve Injury?
Treatment depends on the severity and duration of symptoms:
- Activity modification and rest, especially for overuse-related compression
- NSAIDs, for inflammation and pain relief
- Splinting, to support wrist and hand function during recovery
- Physical therapy, to maintain mobility and strengthen unaffected muscles
- Surgical decompression, if symptoms persist or a mass is identified
- Nerve repair or grafting, in cases of traumatic laceration