Posterior Interosseous Nerve Syndrome/Palsy

Posterior Interosseous Nerve (PIN) Palsy is a syndrome characterised by a reduction in extensor movements of the fingers. It most commonly due to a compression at the radial tunnel. This article explores the causes, signs and symptoms, and management of this entrapment.

In this Article

• 5 Key Points
• Anatomy of PIN
• Causes of PIN Syndrome/Palsy
• Diagnosis of PIN Syndrome/Palsy
• Treatment of PIN Syndrome/Palsy

5 Key Points

1. Definition of PIN Syndrome

• PIN syndrome results from a compressive lesion causing motor extension weakness. This functional loss is painless, and sensory disturbances are absent.

2. Anatomy of Posterior Interosseous Nerve  

• Motor branch of the radial nerve
• Origin: the radial tunnel at the Arcade of Froshe
• Course: between superficial and deep extensors
• Innervation: All Extensors except brachioradialis, ERCB, ERCL

3. Causes of PIN Syndrome

• Compression or entrapment is the cause of a Posterior Interosseous Nerve palsy. This compression may be due to trauma, inflammation or iatrogenic.

4. Clinical Picture of PIN Syndrome

• Pain and tenderness is less common than in radial tunnel syndrome
• Loss of finger and thumb extension
• Wrist extension with radial deviation as ERCL is preserved
• No sensory disturbance

5. Clinical Picture of PIN Syndrome

1. Non-Surgical: Physiotherapy and Splinting
2. Surgical: Radial Tunnel Decompression +/- removing cause of compression

Anatomy of Posterior Interosseous Nerve

The Posterior Interosseous Nerve is the motor branch of the radial nerve. This nerve originates at the Arcade of Froshe and travels between the superficial and deep extensors. It innervates all the extensor muscles/tendons except for brachioradialis, ERCB, ERCL.

Origin

The Posterior Interosseous nerve (C7,8) is the motor branch of the radial nerve, which arises in the radial tunnel.

The Radial Tunnel is a 5 cm tunnel from the radiocapitellar joint to the distal edge of the supinator with well-defined anatomical boundaries:

• Lateral: Brachioradialis, ERCL, ECRB
• Medial: Biceps tendon, brachial
• Floor: Capsule of the radiocapitellar joint

Course

After the Arcade of Froshe, the posterior interosseous nerve travels in a fascial plane between the superficial and deep extensors.

Innervation

The Posterior Interosseous Nerve innervates all forearm extensors apart from ECRL, ECRB and brachioradialis – these muscles are innervated by the radial nerve above the elbow.

Causes of PIN Syndrome/Palsy

Posterior Interosseous Nerve syndrome is caused by compression or entrapment. This is commonly due to trauma, inflammation or iatrogenic.

Common Causes of PIN Syndrome/Palsy

The causes of posterior interosseous nerve syndrome/palsy primarily relate to compression. This can be anatomical, pathological or iatrogenic.

Radial Tunnel Compression Sites

The Posterior Interosseous Nerve is commonly compressed at the Arcade of Froshe. Whilst other compression sites do exist, these occur more proximally and are therefore more relevant to Radial Tunnel Syndrome

Diagnosis of PIN Syndrome/Palsy

Posterior Interosseous Syndrome has motor but no sensory issues. Patients with this palsy have loss of finger and thumb extension, radial deviation on wrist extension (ERCL is preserved). Pain and tenderness is less common than in radial tunnel syndrome.

Pain

• Pain is not a distinct feature of posterior interosseous nerve entrapment.
• Tenderness on the extensor compartment just distal to elbow.

Function

• Difficulty extending the fingers and thumb.
• Wrist extension is spared because ECRL has a more proximal innervation from radial nerve proper.
• Radial deviation with wrist extension may be noted due to ECRB and ECU weakness.
• There is no sensory disturbance
• There may be a Tinel’s Positive Sign

Investigations

Nerve Conduction Studies

• In contrast to radial tunnel syndrome, nerve conduction studies is useful
• Fibrillations and denervation potentials in the extensor musculature, are commonly identified.

Imaging

• A case-by-case basis
• Consideration for Ultrasound or MRI if concern for soft tissue mass.

Differential Diagnosis

• Radial Tunnel Syndrome
• Tennis Elbow – Lateral Epicondylitis will have tenderness over lateral epicondyle.
• Attritional Rupture – the presence of a normal tenodesis confirms tendon integrity (passive MPJ extension with wrist flexion)

Treatment of PIN Syndrome/Palsy

Posterior Interosseous Syndrome/Palsy can be treated non-surgically (Physiotherapy and Splinting) and surgically (Radial Tunnel Decompression +/- removing cause of compression).

Non-Surgical

• In the absence of a pathological compression, non-operative measures can be considered a period of up to 3 months before denervation and muscle atrophy.
• Physiotherapy for strength and modify provocative manoeuvres
• Intermittent extension splinting of the MP joints

Surgical Decompression

Generally speaking, surgical decompression is recommended if no recovery after 3 months. A number of different radial tunnel decompression techniques and modifications have been described. The key principles involve:

1. Correct any pathological cause of the palsy
2. Release Medial border of ERCB
3. Release Arcade of Froshe
4. Release Leash of Henry
5. Superficial head of supinator

This can be performed through different approaches:

1. Volar approach between brachioradialis and FCR in the antecubital fossa. This allows good access to proximal sites of compression
2. Dorsal approach between ECRL and ECRB or brachioradialis and ECRL. This provides good access to distal sites of compression^​8​.
3. Combined approach along the margins of the brachioradialis muscle through a single incision.