Posterior Interosseous Nerve Syndrome/Palsy: Aetiology, Diagnosis and Management

Anatomy of Posterior Interosseous Nerve

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

This anatomical description can be visualised in the image below:

Course

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

The course of the posterior interosseous nerve is visualised in the image below:

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.

This innervation by the Posterior Interosseous nerve is visualise below.

Causes of Posterior Interosseous Syndrome

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

The potential compressions are illustrated below:

Diagnosis of Posterior Interosseous Syndrome

Function

Posterior Interosseous Nerve (PIN) Palsy presents a unique set of clinical manifestations primarily affecting the motor function of the forearm and hand, with distinct characteristics as follows:

• 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

To diagnose Posterior Interosseous Nerve (PIN) Palsy Syndrome, the following specific investigations are conducted to assess nerve integrity and identify potential contributing factors:

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 Posterior Interosseous Syndrome

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.

Conclusion