Brachioradialis Anatomy

Where it starts. Brachioradialis arises from the proximal two-thirds of the lateral supracondylar ridge of the humerus, from roughly 10 cm to 3 cm proximal to the lateral epicondyle, and from the anterior surface of the lateral intermuscular septum. That is the highest origin of any forearm muscle, and it leaves a substantial part of the muscle belly lying in the arm rather than the forearm — which is what makes the muscle available for elbow cover as a proximally based flap.

Belly and tendon. The belly is fleshy for the proximal half of the forearm and then becomes a long, flat tendon for the distal half. That tendon is long, typically 8 to 12 cm, and a long tendon on a short belly is a liability in transfer: usable excursion has to be won by mobilising the muscle-tendon unit extensively from proximal (see the tendon transfer tab of Surgical Relevance).

Where it ends. The tendon inserts on the lateral surface of the distal radius, at the base of and just proximal to the radial styloid process, about 1 to 2 cm proximal to the styloid tip. It frequently blends with the fascia over the first dorsal compartment.

The branch. The nerve to brachioradialis leaves the radial nerve proper in the interval between brachialis and brachioradialis, proximal to the elbow joint and typically 2 to 5 cm above the lateral epicondyle. It is the first radial nerve branch distal to those to triceps and anconeus; branches to extensor carpi radialis longus follow, and only then does the nerve divide into the superficial radial nerve and the posterior interosseous nerve.

Motor entry point. The branch enters the deep (posteromedial) surface of the muscle in its proximal third, close to the vascular pedicle. In a transfer the surgeon therefore works distal to this point.

Root values. C5 and C6, which is why brachioradialis is the muscle behind the brachioradialis reflex (supinator jerk), tested at the distal radius.

Reading the table. An intact brachioradialis with a finger drop localises the lesion to the posterior interosseous nerve, distal to the branches to brachioradialis and extensor carpi radialis longus. Radial deviation on attempted wrist extension is the corroborating sign, because extensor carpi radialis longus is working and extensor carpi ulnaris is not.

The dominant pedicle is the radial recurrent artery, a branch of the radial artery arising just distal to the radial artery origin, entering the deep surface of the muscle in its proximal third alongside the motor branch. Rohrich and Ingram, dissecting 53 upper extremities, found a consistent major pedicle near the elbow, arising in descending order of frequency from the radial recurrent artery, then the radial artery, then the brachial artery.

Minor pedicles in variable number arise segmentally from the radial artery down the length of the muscle. That combination — one dominant proximal pedicle plus minor distal ones — makes brachioradialis a Mathes-Nahai type II muscle.

What the pedicles allow. Pedicled proximally on the radial recurrent artery, the distal muscle reached the elbow anteriorly and posteriorly in 100 per cent of Rohrich and Ingram's specimens; pedicled distally on radial artery perforators, the proximal muscle covered the anterior elbow in 100 per cent and the posterior elbow in 91 per cent. The arc of rotation takes in the distal half of the arm and the proximal two-thirds of the forearm, and defects up to about 3 cm in the elbow region are reliably covered. No major vessel is sacrificed and there is no functional cost, which is the reason it is the go-to local muscle flap for elbow cover.

The cubital fossa. Brachioradialis forms its lateral boundary; pronator teres is the medial boundary, an imaginary line between the epicondyles the base, and brachialis with supinator the floor.

Deep to the muscle proximally lie the radial nerve and its division into the superficial radial nerve and the posterior interosseous nerve, a bifurcation that typically occurs at or just distal to the level of the radiocapitellar joint, deep to the mobile wad. The supinator and the arcade of Frohse are there too, with the radial recurrent artery and the leash of Henry — the radial recurrent vessels crossing the posterior interosseous nerve.

Deep to the tendon distally the superficial radial nerve runs on the deep surface of the brachioradialis tendon for the middle third of the forearm, then emerges between the brachioradialis and extensor carpi radialis longus tendons to become subcutaneous. The radial artery lies immediately medial to the brachioradialis tendon in the distal forearm: the pulse point at the wrist, and the structure retracted medially in the Henry approach.

On either side. Flexor carpi radialis and pronator teres lie medially, and the brachioradialis-flexor carpi radialis interval is the Henry approach; extensor carpi radialis longus and brevis, the rest of the mobile wad, lie lateral and posterior.

Why this muscle localises the lesion. Brachioradialis is the single most useful muscle for localising a radial nerve lesion, because its branch arises above the elbow and above the division into the posterior interosseous and superficial radial nerves. A weak brachioradialis with wrist drop, finger drop and numbness in the dorsal first web places the lesion at or proximal to the spiral groove or lateral intermuscular septum — classically after a humeral shaft fracture, the Holstein-Lewis distal-third spiral pattern tethering the nerve at the septum.

Prognosis. Radial nerve palsy after a closed humeral shaft fracture recovers spontaneously in the large majority of cases, and observation with splinting and serial examination is standard. Brachioradialis, being the most proximal muscle supplied below the injury, is usually the first to recover, and its return is the earliest clinical evidence of reinnervation.

What to document at each review. A Tinel sign advancing distally, and the return of the brachioradialis cord on resisted mid-prone flexion.

Three compartments, not two. The forearm has three, and the mobile wad is one of them:

1. Volar (flexor) — superficial and deep flexors, further divided by some authors into superficial and deep.
2. Dorsal (extensor) — the extensor group and supinator.
3. Mobile wad (lateral) — brachioradialis, extensor carpi radialis longus and extensor carpi radialis brevis.

What holds the wad together. All three arise from the lateral supracondylar ridge and lateral epicondyle and all three are radial-innervated, and all three retract as one mass. Brachioradialis is the most anterior and superficial and runs to the radial styloid; extensor carpi radialis longus inserts on the base of the second metacarpal and is supplied by the radial nerve proper; extensor carpi radialis brevis inserts on the base of the third metacarpal and is usually supplied by the posterior interosseous nerve or at the radial nerve bifurcation.

Why it matters in fasciotomy. In forearm compartment syndrome a volar (Henry-type or curvilinear) release decompresses the volar compartment, but the mobile wad must be released separately — by opening its fascia laterally through the same incision, or through a separate dorsal incision. Missing the mobile wad is a documented cause of persistent compartment pressure and ongoing ischaemia after an apparently adequate fasciotomy.

Volkmann contracture. The classical ischaemic contracture principally affects the deep volar compartment, flexor digitorum profundus and flexor pollicis longus, because the deep muscles lie furthest from collateral flow. Brachioradialis, being superficial and separately compartmentalised, is frequently spared — which is exactly why it survives to be used as a donor in reconstruction of the ischaemic forearm.

The tunnel. It runs from the radiocapitellar joint to the distal edge of the supinator, roughly 5 cm in length, and brachioradialis with the rest of the mobile wad forms its anterolateral roof.

The sites of compression, from proximal to distal:

1. Fibrous bands anterior to the radiocapitellar joint.
2. The leash of Henry, the radial recurrent vessels crossing the posterior interosseous nerve.
3. The tendinous medial edge of extensor carpi radialis brevis.
4. The arcade of Frohse, the tendinous proximal edge of the superficial head of supinator — the commonest site.
5. The distal edge of supinator.

Radial tunnel syndrome versus posterior interosseous nerve palsy. Radial tunnel syndrome is a pain syndrome without motor loss; posterior interosseous nerve palsy is a motor syndrome — finger drop — usually without pain. Both involve the same nerve at the same anatomical sites, and the distinction is examined constantly.

Presentation. Brachioradialis tendinopathy and strain are uncommon in isolation, and present as pain over the lateral distal arm and proximal forearm on resisted mid-prone flexion. They occur in rowers, climbers and racquet players with repetitive rapid loaded flexion.

What else it could be. Lateral epicondylitis gives pain on resisted wrist extension and passive wrist flexion, with tenderness at the extensor carpi radialis brevis origin. Radial tunnel syndrome is tender 4 to 5 cm distal to the lateral epicondyle over the supinator, with pain on resisted supination with the elbow extended and on resisted long-finger extension.

The overlap trap. Radial tunnel syndrome and lateral epicondylitis coexist in a meaningful minority of patients, so a lateral epicondylitis that fails to respond should prompt reassessment for a proximal nerve component.

The interval. Brachioradialis laterally, supplied by the radial nerve, against flexor carpi radialis medially, supplied by the median — a true internervous plane. The skin incision runs from just lateral to the biceps tendon at the elbow crease to the radial styloid, following the medial border of the brachioradialis.

Sorting the structures. Once the interval is open, the radial artery lies deep and immediately medial to brachioradialis and is retracted medially with the flexor carpi radialis, while the superficial radial nerve lies on the deep surface of brachioradialis and is retracted laterally with the muscle. Getting these two on the correct sides is the whole trick of the approach.

Each third of the radius is exposed by moving a different muscle:

• Proximal third — fully supinate, which brings the supinator insertion anteriorly and moves the posterior interosseous nerve laterally and posteriorly away from the field, then elevate the supinator subperiosteally off the radius from its insertion, protecting the nerve within its substance.
• Middle third — release and reflect the pronator teres insertion from the lateral radius, with the forearm pronated to bring it anteriorly.
• Distal third — elevate the pronator quadratus from its radial origin, with the flexor pollicis longus retracted ulnarly.

The leash. The radial recurrent vessels (leash of Henry) cross the field proximally and are ligated to allow mobilisation. That sacrifices the dominant brachioradialis pedicle, so a brachioradialis flap has to be planned before a Henry approach, not after it.

Brachioradialis to flexor pollicis longus is the single most important transfer involving this muscle, and it is used in two very different settings.

C6 tetraplegia: the indication. A patient with an International Classification for Surgery of the Hand in Tetraplegia group 1 or 2 hand, a brachioradialis of grade 4 or better, and elbow flexion preserved by biceps and brachialis. The purpose is active thumb interphalangeal flexion and therefore key (lateral) pinch, the single most functionally valuable grip for a tetraplegic patient.

Where it sits in a reconstruction. Fridén and colleagues incorporate brachioradialis to flexor pollicis longus as one of seven components of their single-stage grip reconstruction, alongside extensor carpi radialis longus to flexor digitorum profundus, split flexor pollicis longus to extensor pollicis longus interphalangeal tenodesis, thumb carpometacarpal arthrodesis and passive intrinsic reconstruction. That split flexor pollicis longus to extensor pollicis longus tenodesis is usually added to prevent uncontrolled interphalangeal hyperflexion, which would otherwise convert pinch into a useless flexed posture.

High median nerve palsy: the deficit. Flexor pollicis longus, flexor digitorum profundus to the index and middle fingers, flexor digitorum superficialis, flexor carpi radialis, pronator teres and the thenar intrinsics are all lost. The standard reconstruction set is three transfers:

• Brachioradialis to flexor pollicis longus — restores thumb interphalangeal flexion.
• Extensor carpi radialis longus to flexor digitorum profundus of the index and middle fingers, or side-to-side tenodesis of the index and middle profundus tendons to the intact ring and little finger profundus (ulnar-innervated).
• Opponensplasty — extensor indicis proprius, or the Camitz palmaris longus transfer — for thumb opposition.

Why brachioradialis for flexor pollicis longus. Matched long excursion, adequate power for a single-digit flexor, expendable, and radial-innervated, so a median lesion leaves it untouched.

Mobilise extensively. The native muscle-tendon unit has a long tendon and a short excursion at rest. The muscle must be dissected free from the surrounding fascia and intermuscular septa for most of its length, up to the level of the motor branch entry in the proximal third, to convert its passive excursion into usable amplitude. A brachioradialis that has not been fully released will not deliver its potential travel.

Preserve the pedicle. The motor branch and the radial recurrent artery enter the deep proximal third; dissection stops short of them.

Tension and route. Tension is set with the wrist in neutral and the thumb interphalangeal joint in slight flexion, using the tenodesis effect to confirm. The tendon is routed subcutaneously and superficial to the radial artery and superficial radial nerve, avoiding a sharp angle of pull.

Expendability. The donor must be genuinely dispensable. Brachioradialis passes because biceps and brachialis power elbow flexion — but in a C5 tetraplegic with no biceps, brachioradialis may be the only elbow flexor, and is then not expendable.

Excursion match. Flexor pollicis longus requires roughly 5 cm of excursion and brachioradialis provides about 6 cm after full mobilisation. Wrist extensors require only about 3 cm, while finger flexors require about 7 cm — which is why brachioradialis to multi-digit flexor digitorum profundus is marginal.

Power. A transferred muscle loses approximately one MRC grade, so a grade 4 brachioradialis becomes a grade 3 flexor pollicis longus: adequate for pinch, not for heavy grip. Beyond that, the usual rules apply — a straight line of pull, and a synergistic donor where possible.

Brachioradialis as a deforming force. The muscle inserts on the radial styloid and metaphysis, and in a dorsally displaced, shortened distal radius fracture it is the principal deforming force: it pulls the styloid fragment proximally and into radial deviation, resisting restoration of length. Release of the brachioradialis insertion is a well-described adjunct in comminuted or delayed-presentation fractures — it makes reduction dramatically easier and reduces the risk of loss of radial length, and the functional cost is negligible.