Posterior Interosseous Nerve Anatomy

Division of the radial nerve

• The radial nerve divides at the level of the radiocapitellar joint, roughly at the lateral epicondyle, deep in the interval between brachialis and brachioradialis.
• The superficial branch continues distally on the deep surface of brachioradialis and remains sensory.
• The deep branch turns laterally and posteriorly around the radial neck to reach the supinator.

The radial tunnel

• A fibro-osseous and muscular corridor roughly 5 cm long, running from the level of the radiocapitellar joint to the distal edge of the supinator.
• Lateral wall and roof: brachioradialis, extensor carpi radialis longus, extensor carpi radialis brevis (the mobile wad).
• Medial wall: brachialis proximally, biceps tendon distally.
• Floor: the anterior capsule of the radiocapitellar joint, then the deep head of supinator.
• Radiocapitellar synovitis, a radial head fracture haematoma, or a proximal radioulnar joint ganglion all raise pressure inside this corridor.

Through the supinator

• The nerve enters between the superficial and deep heads of supinator, passing under the arcade of Frohse - the arched proximal border of the superficial head.
• It winds around the radial neck from anterior to posterior within the muscle, crossing the shaft obliquely.
• It emerges from the distal border of supinator onto the deep extensor plane, typically 4 to 8 cm distal to the radiocapitellar joint depending on forearm rotation.

Terminal division

• Superficial (recurrent) descending branches: to extensor carpi ulnaris, extensor digitorum communis and extensor digiti minimi.
• Deep descending branch: runs on the interosseous membrane, on the surface of abductor pollicis longus, supplying abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus and extensor indicis.
• The terminal fibres run in the floor of the fourth dorsal compartment, expand into a pseudoganglion, and end as articular branches to the wrist.

The parent trunk is developed in radial nerve anatomy, and the injuries that most often threaten this segment in radial head fractures and Monteggia fractures.

Supplied before the supinator

• Supinator itself (branches enter the superficial head just proximal to and within the arcade).
• Extensor carpi radialis brevis in a minority - its innervation is genuinely variable, arising from the radial nerve proper, the superficial branch or the deep branch in different individuals. This variability is why isolated ECRB weakness is an unreliable localising sign.

Supplied after the supinator

NOT supplied by the PIN

• Triceps and anconeus - radial nerve in the arm.
• Brachioradialis and extensor carpi radialis longus - radial nerve proper, above the division. Their preservation is what makes PIN palsy look the way it does.
• Everything in the flexor compartment - median and ulnar nerves.

There is no cutaneous territory

This is the defining feature. The PIN supplies no skin. Sensation over the dorsum of the hand and the first dorsal web space belongs to the superficial branch of the radial nerve, which separates from the PIN at the radiocapitellar joint and never enters the supinator.

Articular sensory supply

• Branches to the radiocapitellar and proximal radioulnar joints arise within the radial tunnel.
• The terminal branch, after the pseudoganglion, supplies the dorsal capsule of the distal radioulnar, radiocarpal, intercarpal and carpometacarpal joints.
• Deep aching wrist pain can therefore be carried by the PIN even though the skin is normally innervated.

Clinical consequences

• Diagnostic: numbness in the first dorsal web space in a patient with finger drop means the lesion is proximal to the division - a radial nerve lesion in the spiral groove or a high radial neuropathy, not a PIN lesion.
• Therapeutic: the terminal articular branch is deliberately divided in partial or total wrist denervation for degenerative wrist pain. Dividing it does not produce anaesthetic skin and does not weaken the extensors, because the motor branches have already left proximally.
• Reconstructive: the terminal segment of the PIN in the floor of the fourth dorsal compartment is a small-calibre, expendable nerve graft donor for digital nerve gaps, harvested through the same dorsal approach.
• Diagnostic block: a local anaesthetic block placed in the fourth compartment that abolishes wrist pain predicts a good response to denervation.

The five compression points, proximal to distal

1. Fibrous bands at the radial head Bands of the radiocapitellar joint capsule and overlying fascia tether the nerve as it turns laterally. Radial head fracture, radiocapitellar synovitis or a displaced radial head all tighten this segment.

2. The leash of Henry The radial recurrent vessels fan across the nerve on their way to the mobile wad. They are the reason for bleeding during radial tunnel release and can form a genuine constricting band.

3. The tendinous medial edge of ECRB A sharp, aponeurotic border which the nerve is pressed against, particularly in resisted wrist extension. This is the anatomical link between radial tunnel syndrome and lateral epicondylitis - the two share an origin.

4. The arcade of Frohse The arched proximal margin of the superficial head of supinator. Spinner showed it is tendinous in roughly 30 per cent of adults and never tendinous in the fetus, making it an acquired structure and the commonest site of compression.

5. The distal edge of supinator The distal arcade, where the nerve emerges onto the deep extensor plane. It is the site most often left unreleased in a decompression, and a recognised reason for persistent symptoms.

Relations at the radial neck

• Anterior and lateral: the mobile wad and the radial recurrent vessels.
• Deep: the radial neck and shaft, with only the deep head of supinator between nerve and bone in the mid part of its course.
• Medial: the biceps tendon and the bicipital tuberosity. The nerve crosses the radius at approximately the level of, or just distal to, the bicipital tuberosity - a reliable intra-operative landmark.

The clinical picture

• Finger drop: loss of active metacarpophalangeal extension of all digits, often asymmetric between digits.
• Thumb: loss of extension and retropulsion.
• Wrist: extends, but in radial deviation.
• Sensation: entirely normal.
• Pain: often a prodrome of deep aching in the proximal forearm for weeks before the weakness.

Causes

• Compressive: any of the five FREAS sites, most commonly the arcade of Frohse.
• Space-occupying: lipoma, ganglion of the proximal radioulnar joint, synovial cyst, nerve sheath tumour. A mass is found in a meaningful minority of spontaneous palsies, which is why imaging is worthwhile before labelling a palsy idiopathic.
• Inflammatory: rheumatoid synovitis of the radiocapitellar joint compressing the nerve in the tunnel - the reason PIN palsy and extensor tendon rupture occur in the same patient population.
• Traumatic: radial head and neck fracture, Monteggia injury, proximal radius fracture.
• Iatrogenic: proximal radius plating, radial head arthroplasty, bicipital tuberosity exposure, elbow arthroscopy portals.
• Neuralgic amyotrophy: the hourglass fascicular constriction described in neuralgic amyotrophy causes a painful, spontaneous, often partial palsy in an otherwise healthy person. Severe pain preceding the weakness is the clue.

Imaging

• Ultrasound: first line. Shows nerve calibre change at the arcade, a mass lesion, or an hourglass constriction, and allows dynamic assessment during pronation and supination.
• MRI: denervation oedema in supinator and the extensor group in the acute phase; fatty atrophy when chronic. Isolated supinator oedema without extensor changes is a recognised incidental finding and should not by itself be reported as PIN syndrome.
• Radiographs: always look at the radial head and the radiocapitellar relationship. A missed Monteggia is a diagnosis made on the plain film.

A pain syndrome, not a palsy

• Presentation: aching lateral forearm pain, worse with repetitive pronation and supination, often with night pain, and no measurable motor deficit.
• Site: maximal tenderness 3 to 5 cm distal to the lateral epicondyle.
• Overlap: it coexists with lateral epicondylitis in a substantial proportion of patients. In one large surgical series more than half of the patients operated for posterior interosseous nerve compression also had tennis elbow, and the authors argued the two are anatomically inseparable, sharing the common extensor and supinator origin.
• Treatment: activity modification, splinting in supination, physiotherapy and corticosteroid injection first. Surgery only after prolonged failure of non-operative care and a convincing clinical picture.
• Outcome: the least reliable of the decompressions. Kalb's 110-patient series is the honest long-term account, and it is worth quoting exactly: only 60 per cent excellent or good by Roles and Maudsley, a mean DASH of 32, and recompression of the nerve by scar in 17 per cent. Results are poorer again with coexisting lateral epicondylitis and with compensation claims.
• The finding that is usually left out: those authors stopped doing the operation. Because of the scar recompression rate, Kalb's group abandoned direct decompression of the nerve and moved to Wilhelm's denervation procedure, which relieves the arcade of Frohse indirectly by dividing the superficial origin of supinator. Their early results with it were reported as promising, and no more than that. A 60 per cent success rate is not the whole story when the surgeons who generated it changed operation as a result.

Three causes of one finding

Loss of active metacarpophalangeal extension in the rheumatoid hand has three explanations, and the operation is completely different for each.

The Vaughan-Jackson pattern

• Attritional rupture over a dorsally prominent, eroded ulnar head (the caput ulnae syndrome).
• Ruptures progress ulnar to radial: extensor digiti minimi first, then extensor digitorum to the little finger, then ring, then middle.
• Loss of independent little finger extension with an intact common extensor is the earliest sign - test it specifically.
• Treatment addresses the ulnar head (Darrach or Sauve-Kapandji) as well as the tendons, or the reconstruction ruptures again.

Why the two coexist

Radiocapitellar synovitis raises pressure in the radial tunnel; distal radioulnar joint synovitis erodes the tendons. Both are driven by the same disease, and a rheumatoid patient with finger drop can have both simultaneously. Perform the tenodesis test in every case.

Radial head and neck fractures

• PIN palsy accompanies a minority of radial head and neck fractures, most often from the fracture haematoma and the displaced head stretching the nerve over the tunnel floor.
• Most are traction neuropraxias that recover. Document the palsy before any intervention.

Monteggia fracture dislocations

• Nerve injury complicates a significant minority of Monteggia lesions. The posterior interosseous nerve is the nerve most often involved, because the radial head displaces anteriorly (Bado type I) or laterally (Bado type III) and drags the nerve across the arcade of Frohse.
• In children and adults, the palsy usually resolves once the radial head is reduced and the ulna is anatomically restored - the nerve is stretched, not divided.
• Explore the nerve if: the palsy appears or worsens after manipulation or fixation; the injury is open; the ulnar fracture requires open reduction and the nerve lies in the field; or there is no recovery by roughly 8 to 12 weeks with electrodiagnostic evidence of denervation.

Tardy PIN palsy

• A chronically dislocated radial head - typically a missed paediatric Monteggia presenting years later - can produce a late, slowly progressive PIN palsy by chronic compression at the arcade of Frohse.
• The same tardy mechanism has been reported after childhood osteomyelitis of the proximal radial metaphysis, where scarring and traction against inflamed periosteum compressed the nerve decades later.
• Management addresses the underlying deformity and decompresses the nerve; neurolysis alone, without correcting the radial head, is inadequate.

Proximal radius fractures

• Fractures at the level of the bicipital tuberosity and proximal shaft sit exactly where the nerve crosses. Both the fracture and the plate can injure it.
• In an open injury or a high-energy fracture with a palsy, assume the nerve needs to be seen.

Posterolateral (Thompson) approach to the proximal radius

• Superficial interval: extensor carpi radialis brevis and extensor digitorum communis proximally; abductor pollicis longus and extensor carpi radialis brevis distally.
• Deep: the supinator is encountered with the PIN inside it. The nerve must be identified if the exposure extends beyond the safe zone.
• Technique: pronate fully, identify the distal border of supinator, trace the nerve proximally under direct vision, and elevate the supinator subperiosteally from the radius with the nerve protected within the muscle.

Extensor digitorum communis splitting approach

• A direct lateral approach that gives better proximal radius exposure than the standard posterolateral interval.
• Schimizzi and colleagues measured the nerve in 15 cadaveric arms: mean radiocapitellar-joint-to-nerve distance 48.2 plus or minus 7.9 mm in pronation, 44.5 mm in neutral, 40.8 mm in supination. Mean lateral-epicondyle-to-nerve distance was 64.7 mm in pronation.
• Crucially, the shortest distances observed were 29 mm from the radiocapitellar joint and 42 mm from the lateral epicondyle in pronation. Those minima, not the means, define the safe zone.
• The nerve lay a mean 10.2 plus or minus 2.4 mm deep to the most superficial aspect of extensor digitorum communis - shallow enough that an aggressive muscle split reaches it.
• Put the two studies side by side and the trade-off appears. Diliberti's posterolateral approach was safe for at least 38 mm in every one of 32 specimens in pronation. Schimizzi's EDC-splitting approach came within 29 mm of the joint in its closest specimen. So the approach that gives the better view of the proximal radius is also the one whose worst case sits 9 mm nearer the nerve, and its nerve lies only about a centimetre below the muscle surface. Exposure and safety move in opposite directions here; choose the split when you need the view, and identify the nerve rather than trusting a number.

Kocher approach (anconeus and extensor carpi ulnaris)

• Used for radial head fracture and arthroplasty. It is the posterior-most of the lateral intervals and therefore the furthest from the nerve at the level of the radial head.
• The trade-off is the lateral ulnar collateral ligament: keep the capsular incision anterior to the midaxial line of the radial head to avoid creating posterolateral rotatory instability.

Anterior (Henry) approach

• The PIN is protected by full supination, which brings the supinator insertion onto the anterolateral radius into view and rotates the nerve posteriorly, away from the incision.
• Elevate the supinator strictly subperiosteally from its radial insertion, staying on bone, with the nerve carried away inside the muscle. Never dissect into the substance of the supinator from the front.
• Note the deliberate opposite of the posterolateral rule: the anterior approach uses supination, the posterolateral approach uses pronation. The principle is identical - rotate the nerve away from the side you are working on.

Proximal radius plating

• Plates that extend proximal to the bicipital tuberosity cross the nerve. It must be seen and protected before the plate is slid proximally.
• Practical rule: with the forearm pronated, exposure of the proximal 3 cm of the lateral radius distal to the radiocapitellar joint is safe without seeing the nerve. Beyond that, identify it.
• Retractors injure nerves more often than knives. Place blunt retractors directly on bone, subperiosteally, and never lever a Hohmann around the anterior radial neck.

Radial head arthroplasty and excision

• Keep the forearm pronated throughout the exposure and while broaching.
• The commonest mechanism of palsy is a retractor placed around the radial neck to expose the proximal shaft for broaching.
• When resecting the radial head, cut the neck perpendicular to the shaft at the head-neck junction and resist the temptation to expose further distally.

Bicipital tuberosity and distal biceps repair

• A single-incision anterior repair with a retractor placed on the lateral side of the tuberosity risks the nerve. Keep the forearm supinated so the tuberosity presents anteriorly and the nerve is carried laterally and posteriorly.
• A two-incision (Boyd-Anderson) technique risks the nerve at the posterior incision if the dissection strays anterior off the ulna, and additionally risks radioulnar synostosis.

Elbow arthroscopy

• The radial nerve and its deep branch are the structures closest to the anterolateral portal. The distal anterolateral portal lies nearest the nerve; the proximal anterolateral portal places the cannula further from it and is preferred for that reason.
• Elbow flexion to 90 degrees and joint distension both increase the distance from portal to nerve. Use a blunt trocar, never a sharp one, and never place a portal into a decompressed joint.

Radial tunnel and PIN decompression

• Approach options: anterior (the brachioradialis and brachialis interval, or Henry), or a brachioradialis-splitting / posterolateral approach. Both are acceptable; the anterior approach gives better access to the proximal sites.
• The release must include all five FREAS sites. The most frequently missed is the distal supinator edge, and incomplete release is a well-documented cause of failure.
• Ligate or coagulate the radial recurrent vessels deliberately rather than tearing them - bleeding obscures the field exactly where the nerve is.
• Release the tendinous medial edge of extensor carpi radialis brevis, which is easy to overlook.
• Divide the superficial head of supinator along the line of the nerve; the deep head is left intact.

Recompression by scar

• Direct decompression is followed by recompression from perineural scarring in a meaningful minority. One series reported recompression by scarring in 17 per cent of patients after direct decompression, which led those authors to abandon direct release in favour of an indirect technique that divides the superficial supinator origin and relaxes the arcade.
• Early active mobilisation after release matters for that reason.

The nerve as a graft and as a target

• Donor graft: the terminal PIN in the floor of the fourth dorsal compartment provides a short segment of small-calibre nerve for digital nerve gaps, harvested through a dorsal wrist incision with no motor and no cutaneous sensory cost.
• Recipient for transfer: in high radial nerve palsy the PIN is a standard recipient. Median nerve donors - the branch to flexor digitorum superficialis transferred to the PIN, and the branch to flexor carpi radialis transferred to extensor carpi radialis brevis - restore finger and wrist extension.
• Denervation: dividing the terminal articular branch, usually together with the corresponding anterior interosseous nerve branch, is the basis of partial wrist denervation for degenerative wrist pain. Confirm with a diagnostic block first.