Supinator Anatomy

Two Heads

The supinator has a superficial (humeral) head and a deep (ulnar) head, and the posterior interosseous nerve passes between them.

Insertion

• The two heads converge and wrap around the radius to insert on the lateral, posterior and anterior surfaces of the proximal third of the radius, from just distal to the radial tuberosity down to the insertion of pronator teres.
• The insertion is circumferential over the proximal third — which is exactly why the muscle can be elevated subperiosteally as a sleeve, carrying the nerve safely within it.

Bony Landmarks to Quote

• Supinator crest of the ulna — also the insertion of the lateral ulnar collateral ligament, so a fracture or dissection here has ligamentous as well as muscular consequences.
• Radial tuberosity — the proximal limit of the supinator insertion, and the landmark for distal biceps repair.
• The radiocapitellar joint — the fixed proximal reference from which all posterior interosseous nerve safe-zone distances are measured.

Posterior Interosseous Nerve (C5, C6)

• The radial nerve divides into the superficial radial nerve (sensory) and the posterior interosseous nerve (motor) at or just distal to the level of the radiocapitellar joint, deep to the mobile wad.
• The branches to supinator arise proximal to or at the arcade of Frohse, before the nerve enters the muscle. This is why supinator function is preserved in a posterior interosseous nerve palsy at the arcade — the nerve is compressed distal to the branches that supply it.
• Root values for supinator are C5 and C6, distinguishing it from the more distal posterior interosseous nerve muscles (extensor carpi ulnaris, extensor digitorum communis, extensor pollicis longus, extensor indicis, abductor pollicis longus) which are predominantly C7 and C8.
• In a cadaveric study of 20 forearms, two motor branches arose from the posterior interosseous nerve in the vicinity of the arcade of Frohse — one laterally to the superficial head and one medially to the deep head. Together they carried 70 per cent of the myelinated fibres of the posterior interosseous nerve, and when divided could be coapted directly to it.

The Muscles Supplied by the Posterior Interosseous Nerve

What Is NOT Affected

• Brachioradialis and extensor carpi radialis longus — supplied by the radial nerve proper, proximal to the division. Preserved brachioradialis with a finger drop is the localising sign.
• Sensation — the superficial radial nerve branches off before the supinator, so sensation over the dorsal first web is NORMAL in a posterior interosseous nerve palsy. Numbness there indicates a more proximal radial nerve lesion.
• The posterior interosseous nerve does carry afferent fibres from the wrist capsule and interosseous membrane, which is the anatomical basis of both the deep aching pain of radial tunnel syndrome and of posterior interosseous nerve neurectomy for wrist denervation in painful wrist arthritis.

Named Vessels

• Radial recurrent artery — arises from the radial artery just distal to its origin, ascends anterior to the supinator toward the lateral epicondyle, and supplies the muscle's anterior and proximal portion. As it crosses the posterior interosseous nerve it forms the leash of Henry, the second of the five radial tunnel compression sites.
• Posterior interosseous artery — arises from the common interosseous artery, passes over the proximal edge of the interosseous membrane and emerges distal to the supinator into the posterior compartment, accompanying the posterior interosseous nerve. It supplies the deep and distal portion of the muscle.
• Recurrent interosseous artery — a proximal branch of the posterior interosseous artery, running proximally deep to the anconeus. It is the pedicle of the anconeus flap.

Pedicle Position

• Supply is segmental, entering proximally from the radial recurrent and distally from the posterior interosseous artery.
• Supinator is not used as a flap. It is thin, its supply is segmental, and — decisively — the posterior interosseous nerve runs through it, so it cannot be mobilised without endangering the nerve.
• Practical relevance: the leash of Henry must be identified and ligated in both a radial tunnel decompression and a proximal volar Henry approach. Avulsion rather than deliberate ligation produces troublesome bleeding right beside the nerve.

The Radial Tunnel

A 5 cm fibro-osseous corridor extending from the radiocapitellar joint to the distal edge of the supinator, through which the posterior interosseous nerve passes.

Boundaries:

• Anterolateral (roof): the mobile wad — brachioradialis, extensor carpi radialis longus and extensor carpi radialis brevis.
• Medial: the brachialis proximally and the biceps tendon.
• Floor: the radiocapitellar joint capsule proximally and the deep head of supinator distally.

Superficial to Supinator

• Mobile wad, then the extensor digitorum communis and extensor carpi ulnaris more posteriorly.
• The superficial radial nerve runs on the deep surface of brachioradialis, superficial to the supinator, having separated from the posterior interosseous nerve proximal to the muscle.

Deep to Supinator

• Proximal radius and the interosseous membrane.
• The posterior interosseous nerve emerges from the distal edge of the supinator into the posterior compartment, where it divides into a superficial branch (to extensor digitorum communis, extensor digiti minimi and extensor carpi ulnaris) and a deep branch (to the outcropping thumb muscles and extensor indicis).

Anterior

• Biceps tendon inserting on the radial tuberosity, and the bicipital bursa.
• Brachialis proximally.

Definition

Compression of the posterior interosseous nerve within the radial tunnel producing pain without motor deficit. It is a contentious diagnosis precisely because it has no objective findings.

The Five Compression Sites (Proximal to Distal)

1. Fibrous bands anterior to the radiocapitellar joint.
2. Leash of Henry — the radial recurrent vessels crossing the nerve.
3. Tendinous medial edge of extensor carpi radialis brevis.
4. Arcade of Frohse — the tendinous proximal margin of the superficial head of supinator. The commonest site.
5. Distal edge of the supinator.

Presentation

• Deep, aching pain in the proximal extensor forearm, worse with repetitive pronation and supination.
• Night pain is characteristic and helps distinguish it from lateral epicondylitis.
• Maximal tenderness 4 to 5 cm distal to the lateral epicondyle.
• No weakness, no sensory loss, normal electrodiagnostics.

The Coexistence Problem

Radial tunnel syndrome and lateral epicondylitis coexist in a meaningful minority of patients. A lateral epicondylitis that fails to respond to appropriate treatment should prompt reassessment for a proximal nerve component — and equally, a supposed radial tunnel that improves with an epicondylar injection was probably epicondylitis.

Management

• Non-operative first, and for a prolonged period: activity modification, avoidance of repetitive forearm rotation, splinting the wrist in extension with the elbow flexed and the forearm supinated (which relaxes the supinator), and physiotherapy including nerve gliding.
• Surgical decompression is reserved for persistent symptoms after an adequate non-operative trial, and outcomes are less predictable than for carpal or cubital tunnel decompression — patients must be counselled accordingly.
• The technical imperative is completeness. Hartz and colleagues, in their pronator teres syndrome series, identified inadequate decompression or misdiagnosis as the causes of failure, and the same holds here. All five sites must be released.
• Approach: either the Thompson (dorsal, extensor digitorum communis / extensor carpi radialis brevis) interval, or the brachioradialis-splitting or anterolateral (Henry-type) approach. The anterolateral approach gives better access to the proximal sites (fibrous bands and the leash of Henry); the Thompson approach gives better access to the arcade and the distal supinator edge.

Presentation

• Finger drop — inability to extend the metacarpophalangeal joints. Interphalangeal extension is preserved because the intrinsics (ulnar and median innervated) do it.
• Thumb drop — loss of extensor pollicis longus and brevis and abductor pollicis longus.
• Wrist extension preserved but in RADIAL DEVIATION — extensor carpi radialis longus works (radial nerve proper), extensor carpi ulnaris does not.
• Sensation entirely normal.
• Supinator function preserved, because its branches arise proximal to the compression.

Causes

Management

• Traumatic or post-fracture: observe with a splint holding the wrist and fingers extended; most neurapraxias recover. Electrodiagnostics at 3 to 4 weeks and again at 3 months.
• Spontaneous: image to exclude a mass. A spontaneous posterior interosseous nerve palsy with no history of trauma or repetitive activity is a space-occupying lesion until proven otherwise.
• Iatrogenic after a procedure in which the nerve was at risk: low threshold for early exploration.
• No recovery by 3 to 6 months: exploration and decompression, with nerve grafting if a discontinuity is found.
• Late salvage: tendon transfers — pronator teres to extensor carpi radialis brevis is not needed here since wrist extension is preserved, so the reconstruction is flexor carpi radialis or flexor carpi ulnaris to extensor digitorum communis and palmaris longus to a rerouted extensor pollicis longus.

Proximal Radioulnar Synostosis

• Post-traumatic synostosis across the interosseous space at the level of the supinator is a devastating complication, abolishing forearm rotation.
• Risk factors: both-bone forearm fractures at the same level, fractures involving the proximal radius and ulna, single-incision approaches to both bones, extensive periosteal stripping (the Boyd approach is the classic culprit), head injury, delayed surgery, and bone graft or debris left in the interosseous space.
• Prevention: separate incisions for both-bone forearm fractures, minimal periosteal stripping, thorough irrigation to remove bone debris, and avoidance of the Boyd approach except where simultaneous access to both bones is genuinely required.
• Treatment: excision once mature, with interposition (fat, fascia or a vascularised flap) and early motion. Recurrence is common.
• Functional threshold: if the forearm is fused, position matters. A fusion in 10 to 20 degrees of pronation is most functional for a unilateral case, permitting hand-to-mouth and hand-to-table activities.

Congenital Radioulnar Synostosis

• Usually proximal, frequently bilateral, and typically presents with the forearm fixed in pronation.
• Many children compensate remarkably well through shoulder and wrist motion. Surgery (derotational osteotomy rather than attempted mobilisation, which fails) is reserved for severe fixed hyperpronation, generally beyond about 60 degrees.

The Supinator and Distal Biceps Repair

• The radial tuberosity, the insertion of the distal biceps, lies just proximal to the supinator insertion on the radius. The posterior interosseous nerve is therefore close to any dissection at the tuberosity.
• Single-incision anterior technique: the forearm is supinated, which rotates the tuberosity anteriorly and carries the posterior interosseous nerve laterally and posteriorly, away from the field. Dissection is blunt and carried directly onto the tuberosity; retractors placed around the radial neck are the mechanism of nerve injury. The most commonly injured structure overall is the lateral antebrachial cutaneous nerve, not the posterior interosseous nerve.
• Two-incision (Boyd-Anderson) technique: a second dorsal incision is used to retrieve the tendon and drill the tuberosity. It reduces the risk of posterior interosseous nerve injury but historically carried a higher risk of proximal radioulnar synostosis and heterotopic ossification, which the muscle-splitting modification (avoiding subperiosteal exposure of the ulna) substantially reduced.
• Consequence of loss: an unrepaired distal biceps costs roughly 40 to 50 per cent of supination power and 30 per cent of flexion power, but slow unresisted supination is preserved by the supinator — which is why patients under-report the deficit and why supination endurance rather than flexion strength drives the decision to repair.

The Four Ways In and What Each Does to the Supinator

The Volar Henry Manoeuvre in Detail

This is the technique most often asked for verbatim:

1. Develop the interval between brachioradialis (laterally, taking the superficial radial nerve with it) and flexor carpi radialis (medially, taking the radial artery with it).
2. Ligate the leash of Henry — the radial recurrent vessels crossing the field at the level of the radial neck.
3. FULLY SUPINATE the forearm. This brings the supinator insertion anteriorly into view and rotates the posterior interosseous nerve, held within the muscle, posteriorly and laterally away from you.
4. Elevate the supinator subperiosteally off the radius, starting at its insertion and working proximally. The nerve travels with the muscle, protected within its substance. At no point is the muscle transected.
5. This exposes the proximal radius safely.

Why this works: you are never dissecting toward the nerve. You are lifting the whole muscular sleeve, nerve included, off the bone in one layer.

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The Thompson Approach

• Interval: extensor digitorum communis (posterior interosseous nerve) and extensor carpi radialis brevis (variable innervation) — not a strictly reliable internervous plane, which is one of its criticisms.
• PRONATE the forearm to move the nerve away.
• The nerve is encountered within the supinator; many surgeons formally identify it, particularly if plating is planned or the exposure needs to extend proximally.
• Safe zone: at least 3.8 cm distal to the radiocapitellar joint in pronation, and often more, but this is the figure to work to.

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The Problem

Plating the proximal radius means placing an implant exactly where the posterior interosseous nerve crosses. The safe zone data are therefore not academic.

Rules

• Pronate the forearm for a dorsal or posterolateral application: this buys a mean safe zone of 52 mm from the capitellum (minimum 38 mm), versus 33 mm (minimum 22 mm) in supination.
• Do not plate blind beyond 3.8 cm distal to the radiocapitellar joint without identifying the nerve.
• Formally identify the nerve if the plate must extend into the proximal third, and place it under direct vision.
• Retractors, not just the plate, injure the nerve. A Hohmann placed around the proximal radial neck in supination is one of the classic mechanisms.
• Radial head arthroplasty and radial head fixation: the safe zone for hardware on the radial head is the non-articular arc, identified by the "safe zone" between the radial styloid and Lister's tubercle projections — with the forearm in neutral, the arc between the radial styloid and Lister's tubercle. Plates placed outside this arc impinge on the proximal radioulnar joint.

Radial Head Fixation Safe Zone

• The radial head articulates with the ulna through about 240 degrees of its circumference; the remaining 110 to 120 degree arc is non-articular and is where hardware may safely be placed.
• Surface landmark method: with the forearm in neutral rotation, the safe zone is the arc between lines drawn through the radial styloid and Lister's tubercle.
• Implants placed within the articular arc block forearm rotation and cause proximal radioulnar impingement.

Supinator Motor Branches as a Donor

The supinator receives its motor supply from the posterior interosseous nerve before the nerve enters the muscle: a cadaveric study of 20 forearms found two branches near the arcade of Frohse, one lateral to the superficial head and one medial to the deep head, together carrying 70 per cent of the myelinated fibres of the posterior interosseous nerve. Because the muscle is a secondary supinator whose function is duplicated by the biceps — which is preserved in the relevant injuries — these branches are expendable donors.

The Transfer in Practice

Two settings, with the stronger evidence in brachial plexus palsy rather than tetraplegia.

C7-T1 brachial plexus palsy (4-patient series). In a C7-T1 lesion, shoulder, elbow and wrist motion are preserved but finger motion is absent, and supinator function is preserved because its innervation stems from the C6 root. Four adult patients underwent surgery 5 to 7 months after injury, with finger flexion reconstructed by brachialis transfer and finger and thumb extension by transferring the supinator motor branches to the posterior interosseous nerve, through an incision over the proximal third of the radius developing the interval between extensor carpi radialis brevis and extensor digitorum communis. At 12 months all four patients could open the hand and fully extend the metacarpophalangeal joints, and thumb abduction improved from 0 to 5 cm from the lateral aspect of the index finger.

Tetraplegia (case report). The same transfer was performed bilaterally in one patient 7 months after spinal cord injury, with almost full thumb and finger extension at 6 months. The indication is paralysis of thumb and finger extension with STRONG WRIST EXTENSORS — the radial nerve proper and the supinator branches working while the more distal C7-C8 posterior interosseous territory is not.

Why it works:

• The supinator branches carry C5 and C6 fibres, preserved in these injuries, while the target extensors are C7 and C8.
• They contain 70 per cent of the myelinated fibres of the posterior interosseous nerve — a well-matched donor.
• They can be coapted directly to the posterior interosseous nerve with no graft, and the coaptation lies close to the target motor end plates.
• Donor morbidity is minimal: sacrificing the supinator branches does not abolish supination, because biceps function is preserved in these lower-type injuries.

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Posterior Interosseous Nerve Neurectomy for Wrist Pain

• The posterior interosseous nerve carries afferent fibres from the dorsal wrist capsule. Its terminal portion, running on the floor of the fourth extensor compartment on the interosseous membrane, can be resected to denervate the dorsal wrist.
• Indication: painful wrist arthritis (scapholunate advanced collapse, scaphoid non-union advanced collapse, Kienbock disease, post-traumatic arthritis) in a patient who wishes to preserve motion, or as an adjunct to another procedure.
• Technique: a short dorsal incision at the distal radius, the fourth compartment floor opened, and a 1 to 2 cm segment of the nerve resected. Frequently combined with anterior interosseous nerve neurectomy for a more complete denervation.
• Diagnostic test first: a selective local anaesthetic block of the posterior interosseous nerve should abolish the pain before a neurectomy is offered. If it does not, the neurectomy will not work.
• Trade-off: pain relief without loss of motion or strength, but with loss of proprioceptive feedback from the wrist — a theoretical concern about accelerated joint destruction that has not been consistently borne out.
• This is entirely different from the supinator's motor function, which is unaffected: the neurectomy is performed on the terminal sensory continuation of the nerve, far distal to the motor branches.