Posterior Interosseous Syndrome

Definition. Posterior interosseous nerve (PIN) syndrome is a compressive neuropathy of the deep motor branch of the radial nerve. It typically occurs in the proximal forearm within the radial tunnel, most commonly at the arcade of Frohse.

The clinical picture. It presents as weakness of finger and thumb extension, the finger drop. Wrist extension is preserved because ECRL is innervated proximal to the PIN, but the wrist deviates radially as it extends because ECU is paralysed.

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The course. The radial nerve divides into its superficial (sensory) and deep (motor) branches at the level of the radiocapitellar joint; the deep branch is the PIN. It enters the radial tunnel under the edge of ECRB, then pierces the supinator between its two heads, passing beneath the proximal edge, the arcade of Frohse. It runs 4-5 cm within the supinator and exits distally to supply the deep extensor compartment.

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What the nerve supplies. The sequence of innervation helps localise the lesion. The PIN supplies the deep (distal) forearm extensors after innervating the supinator; the superficial wrist extensor, ECRL, is supplied by the radial nerve proximally and is spared.

• Radial nerve, proximal to the PIN: triceps, anconeus, brachioradialis, ECRL
• PIN, proximal to or within the supinator: ECRB (variable), supinator
• PIN, distal to the supinator: EDC, EDM, ECU, APL, EPB, EPL, EIP

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Where it is compressed. The "radial tunnel" is a misnomer; it is a muscular cleft. Five sites can compress the nerve, remembered as FREAS:

1. Fibrous bands anterior to the radiocapitellar joint
2. Recurrent vessels, the leash of Henry (branches of the radial recurrent artery)
3. ECRB, its sharp medial tendinous edge
4. Arcade of Frohse, the proximal tendinous arch of the supinator and the most common site; normal in 30%, fibrous in 70%
5. Supinator, its distal edge

Clinical types. PIN compression presents in one of three clinical types.

• Type I (complete PIN): all PIN muscles paralysed, giving finger drop, thumb drop and ECU paralysis
• Type II (partial PIN): only some muscles, such as the thumb or index alone
• Type III (radial tunnel): pain only, from dynamic compression

Type II mimics tendon rupture and often confuses the diagnosis.

Aetiology. Always consider inflammatory arthritis (RA) as a cause.

• Compressive: the arcade of Frohse (fibrous band)
• Space-occupying: lipoma (the commonest soft-tissue tumour here), ganglion, rheumatoid synovitis
• Traumatic: Monteggia fracture (radial head dislocation)
• Iatrogenic: radial head fixation or replacement

Examination. Test the PIN muscles: EDC (MCP extension), EIP (index extension) and EPL (thumb extension). Check the tenodesis effect to rule out tendon rupture. The pain is a vague ache in the proximal forearm, unlike lateral epicondylitis, which is at the epicondyle; pain with weakness is PIN syndrome.

Special tests. Injection of local anaesthetic can also be diagnostic (and therapeutic).

• Middle finger test: resisted middle finger extension with the elbow extended. Pain in the proximal forearm is radial tunnel syndrome; weakness is PIN syndrome.
• Supinator compression: palpation 4 cm distal to the lateral epicondyle reproduces the symptoms.

A "dropped finger" or refractory lateral elbow pain has a wide differential. The decisive discriminators are motor weakness, the sensory status and the tenodesis effect. The sites of the four radial nerve syndromes: PIN and radial tunnel syndromes arise at the arcade of Frohse, Wartenberg syndrome at the forearm fascia, and a high radial palsy at the spiral groove.

Non-operative. Suitable for neuritis (Parsonage-Turner), transient compression, and a palsy with no mass. A dynamic cock-up splint with an outrigger holds the fingers in extension and prevents overstretching, and the patient is observed for 3-6 months. Activity modification, avoiding pronation and supination, is key; steroids have a limited role unless the cause is inflammatory.

Surgical decompression. Decompress for a mass lesion such as a lipoma, for failure to recover after 3-6 months, and after trauma if the nerve is caught. The procedure releases the ECRB edge, the arcade of Frohse and the distal supinator, and complete release of all five sites is mandatory. The approach is anterior (Henry) or posterior (Thompson), described under Surgical Technique.

Tendon transfer. For a permanent palsy, greater than 1 year, tendon transfer is the salvage, and its outcome is generally better than nerve repair for gaps. The classic set is the Jones transfer:

1. PT to ECRB - restores central wrist extension
2. FCU to EDC - restores finger extension
3. PL to EPL - restores thumb extension

Anterior approach (Henry). Stay on the nerve visually at all times.

1. Volar curvilinear incision
2. Develop the interval between PT and brachioradialis
3. Identify the radial nerve and trace it distally
4. Ligate the leash of Henry, retract ECRB and identify the arcade of Frohse
5. Divide the arcade and the superficial head of the supinator

Posterior approach (Thompson). A dorsal forearm incision through the interval between ECRB and EDC exposes the supinator directly. It suits tumours within the supinator and distal supinator pathology and gives a direct view of the nerve within the muscle, at the cost of less proximal control of the radial nerve at the elbow.

Choosing the approach. The choice depends on the level of pathology and surgeon preference. The anterior approach gives better visualisation of the arcade and better proximal control for arcade-level decompression; the posterior approach is favoured for supinator masses.

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On the way in. Identify and protect the cutaneous nerve before deepening the interval; unrecognised traction or division creates an avoidable sensory neuroma.

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The release. Maintain direct visual control rather than dividing the supinator blindly. Open the fibrous edge of the arcade under direct vision with the nerve protected below; then trace the PIN distally through the supinator to exclude a second constriction. A final inspection confirms continuity, freedom from residual bands and the absence of a retractor-related injury.

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Mass lesions. The nerve should be identified proximally before circumferential tumour mobilisation. Excision of a ganglion should remove the cyst and its stalk while preserving the nerve and releasing any additional radial-tunnel constrictions. Once the mass is out, inspect the PIN along the former contact zone for hourglass narrowing, loss of continuity or residual compression before closure.

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Where the nerve ends. The PIN does not simply end in muscle. After supplying the deep extensors, its slender terminal continuation runs distally on the dorsal aspect of the interosseous membrane to the dorsal wrist capsule, where it becomes a purely articular (afferent) branch carrying pain and proprioceptive fibres from the dorsal radiocarpal and intercarpal joints. It is found in the floor of the fourth extensor compartment.

Partial wrist denervation. Selective neurectomy of the terminal PIN, classically combined with the terminal articular branch of the anterior interosseous nerve on the volar side, ablates the dorsal wrist joint pain afferents. It is a motion-preserving option for chronic dorsal wrist pain, particularly when a patient wishes to avoid or defer fusion or arthroplasty. Examples include degenerative SLAC or SNAC wrist, post-traumatic arthritis and a recalcitrant occult dorsal ganglion.

Predicting the response. A diagnostic local anaesthetic block of the terminal PIN over the dorsal distal forearm that relieves the pain predicts a good response before committing to neurectomy.

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Why it is at risk. Because it lies directly against the proximal radius within the supinator, the PIN is among the most frequently iatrogenically injured nerves of the forearm. It is at risk during any exposure of the proximal radius: radial head or neck fracture fixation or arthroplasty, Monteggia surgery, plating of the proximal radial shaft, and dissection towards the bicipital tuberosity.

Pronate the forearm. Pronation is the single most useful protective manoeuvre, and the one to use when working on the proximal radius or radial neck. It rotates the radius so that the PIN is carried medially and distally, away from the lateral operative field and the bicipital tuberosity, increasing the distance between the nerve and the radiocapitellar joint. Supination does the reverse, drawing the nerve anterolaterally, closer to the radiocapitellar joint and bicipital tuberosity and into the danger zone, and is avoided during lateral exposure.

The safe window. For lateral work it is the few centimetres immediately distal to the radiocapitellar joint; with the forearm pronated the nerve sits roughly two fingerbreadths beyond the joint. Keep subperiosteal dissection proximal to the bicipital tuberosity, avoid circumferential retractors around the radial neck, and positively identify the nerve whenever exposure must extend further distally. In the dorsal (Thompson) approach the PIN must be actively sought and protected within the supinator rather than relying on forearm position alone.

Intraoperative. Meticulous technique with loupe magnification is essential.

• Direct injury to the PIN during release, especially at the arcade of Frohse
• Damage to the leash of Henry (radial recurrent vessels), causing significant bleeding
• Incomplete release of the five compression sites, most commonly missing the distal supinator edge
• Unexpected ECRB weakness if its variable motor branch is damaged
• Injury to the superficial radial nerve during the anterior approach, causing sensory loss on the dorsal hand

Postoperative. Early mobilisation reduces scar formation around the nerve.

• Wound infection: low risk (1-2%) in clean surgery with appropriate prophylaxis
• Haematoma: may require evacuation if significant; compress during early recovery
• Recurrence: scarring around the nerve can cause recurrent symptoms (5-10%)
• Persistent weakness: may reflect irreversible nerve damage before decompression
• Traction neuropathy: over-aggressive retraction during exposure can worsen function
• Heterotopic ossification: rare, particularly after traumatic cases

The pace of recovery. Recovery progresses at 1 mm per day from the site of compression, and full recovery may take 6-12 months depending on severity. Sensory re-education is not required, because the PIN has no cutaneous territory. Recovery is serial, so document the return of individual muscles rather than relying on a single composite hand position.

The order of recovery. Recovery follows the order of innervation. In a PIN lesion, ECU recovers first, EDC next, and EPL and EIP last. In a high radial nerve lesion, brachioradialis and ECRL recover first, and the sequence runs brachioradialis, ECRL, ECRB, supinator, EDC, EPL.

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Outcome by cause. Expected outcomes differ by aetiology.

• Compression at the arcade: excellent recovery if decompressed within 6 months, with greater than 85% returning to full function
• Mass lesion (lipoma): excellent with excision and neurolysis; function returns 3-6 months after surgery
• Traumatic (Monteggia): variable, depending on mechanism and timing; neurapraxia recovers well, neurotmesis requires grafting
• Inflammatory (RA): good if synovectomy is performed early; ongoing disease may cause recurrence
• Radial tunnel syndrome: unpredictable, with 60-70% success for pain relief even with surgery

Prognostic factors. A shorter duration of symptoms (under 3 months) is associated with better recovery, partial palsies recover better than complete ones, and younger patients have better nerve regeneration capacity. Mass lesions with clear compression do better than inflammatory conditions. Decompression within 6 months maximises recovery potential, and early intervention correlates strongly with better outcomes.

Serial EMG. Reinnervation potentials at 3 months are a positive prognostic sign, and serial EMG monitoring helps guide the surgical decision.

PIN syndrome and radial tunnel syndrome are uncommon and have no dedicated society guideline or arthroplasty-style registry. The "evidence" is therefore best summarised as global epidemiology, where the major society positions converge or differ, and how practice changes with available resources.

Global Epidemiology:

• Compressive radial neuropathy at the elbow is rare relative to carpal and cubital tunnel syndrome; isolated spontaneous PIN palsy is described mainly in case series and reviews rather than population studies (McGraw, J Hand Surg Eur Vol 2019).
• No sex predominance is firmly established; reported cases span young adults (often neuralgic amyotrophy / Parsonage-Turner) to older adults (compressive and mass lesions).
• Recognised causes worldwide: the arcade of Frohse and supinator fibrous bands, space-occupying lesions (lipoma is the classic solid mass, also ganglion, synovial cyst, nerve sheath tumour), rheumatoid elbow synovitis, Monteggia and radial head fracture–dislocation, and iatrogenic injury during radial head or proximal radius surgery.

Side-by-Side Society Positions:

Anterior versus posterior approach. No randomised data show superiority of the Henry over the Thompson exposure; the choice is driven by the level of pathology and surgeon familiarity, as set out under Surgical Technique.

Timing of surgery in compressive palsy. Common practice is to decompress a confirmed compressive lesion promptly and to observe an imaging-negative palsy for roughly 3 months, but the exact threshold is not standardised. Outcomes worsen with age over 50 and with delay (McGraw, 2019).

Nerve transfer versus tendon transfer. For recent lesions (under ~12 months), nerve transfer can give superior wrist motion and grip compared with tendon transfer (Bertelli, 2020), but tendon transfer remains the established, microsurgery-free salvage for late or irrecoverable palsy. The optimal algorithm is still debated.

Hourglass fascicular constriction. Increasingly recognised in spontaneous palsy, within the neuralgic amyotrophy spectrum. Whether to observe, perform neurolysis, resect and graft, or transfer is unresolved, and intraoperative findings often dictate the decision.

The role of imaging. MRI is favoured to exclude a mass, but high-resolution ultrasound increasingly detects fascicular constriction and dynamic compression; the comparative diagnostic accuracy for the specific question of operability is not well defined.