Brachialis Anatomy

Origin. The distal half of the anterior surface of the humerus, from about the level of the deltoid insertion to within 2 cm of the joint line. The origin has two limbs that embrace the deltoid tuberosity in a V shape, a reliable landmark when working proximally in an anterior approach, and fibres arise from the medial and lateral intermuscular septa as well. The lateral septal origin is what brings the muscle into contact with the radial nerve where the nerve pierces that septum.

Two heads. Leonello and colleagues found two heads in every specimen they dissected, and the difference between them is mechanical rather than decorative.

• Superficial head — the larger. A more proximal origin on the anterolateral humerus, longitudinal fibres, and a thick round tendon onto the ulnar tuberosity. The long origin and distal insertion give it the bulk of flexion strength
• Deep head — smaller and fan-shaped, with oblique fibres converging into an aponeurosis onto the coronoid process, more anteriorly. Better placed to initiate flexion from full extension
• The two are separable by blunt dissection, which is what makes the superficial head tendon available as a local graft

Insertion. The tendon of the superficial head takes the ulnar tuberosity and the aponeurosis of the deep head the coronoid process, with the footprint 1 to 2 cm distal to the coronoid tip, immediately anterior to the anterior band of the medial collateral ligament complex. The insertion is entirely on the ulna: there is no radial attachment, so the muscle contributes nothing to supination or pronation.

The musculocutaneous supply. The nerve pierces coracobrachialis and then runs distally between biceps and brachialis, giving branches to both. Its motor twigs, typically two or three, enter the anteromedial surface in the middle third of the arm, and the nerve emerges lateral to the biceps tendon at the elbow as the lateral cutaneous nerve of the forearm, so sensation over the lateral forearm is the territory tested to confirm musculocutaneous integrity.

The radial supply. A branch of the radial nerve supplied the inferolateral fibres of the deep head in every specimen of the Leonello cadaveric series, arising as the nerve passes anterior to the lateral epicondyle in the interval between brachialis and brachioradialis. Eleven of eleven is strong evidence that the branch is usual; it cannot establish that it is invariant, so identify the plane rather than assume it.

What the second nerve does, and does not, buy. The radial-innervated portion is small, and in an isolated musculocutaneous palsy the brachialis is effectively paralysed for practical purposes despite the dual supply. What the dual supply gives the surgeon is a boundary: the musculocutaneous-supplied medial two-thirds against the radial-supplied lateral third is an internervous plane within a single muscle, and that is what makes the brachialis-splitting anterolateral approach to the distal humerus safe.

Named vessels. Muscular branches of the brachial artery enter the anteromedial surface throughout the arm and are the dominant supply. The radial recurrent artery feeds the distal lateral portion, entering close to the radial nerve in the brachialis-brachioradialis interval, and the inferior ulnar collateral artery and profunda brachii contribute to the distal and lateral portions respectively.

A segmental pattern rather than a pedicle. The supply is segmental and multi-pedicled, a Mathes-Nahai type IV pattern with no single dominant vessel. That rules brachialis out as a free or pedicled muscle flap, and it means the muscle tolerates extensive elevation off the anterior capsule in a contracture release without devascularising, provided the medial neurovascular bundle is not stripped.

Superficial. The biceps brachii throughout, and the bicipital aponeurosis (lacertus fibrosus) at the elbow.

Deep. The anterior surface of the humerus proximally, and the anterior capsule of the elbow joint distally. Muscle and capsule are adherent, and for an elbow surgeon that adherence is the whole point of the muscle.

Medial. The brachial artery, median nerve and basilic vein lie on or just medial to the medial border of brachialis in the distal arm, the median nerve crossing the artery from lateral to medial at about mid-arm. The ulnar nerve is further posteromedial, behind the medial intermuscular septum — not in the brachialis plane, but reached from it.

Lateral. The brachioradialis and extensor carpi radialis longus make up the mobile wad, and the radial nerve lies in the brachialis-brachioradialis interval in the distal third of the arm, having pierced the lateral intermuscular septum. This is the single most important relation to remember: the nerve is safe as long as you stay medial to the lateral third of the brachialis, and it is found by developing that interval.

The mechanism. Myositis ossificans of brachialis is the classic post-traumatic sequel of a simple elbow dislocation, and the route to it is direct: the dislocating coronoid tears through the muscle, producing an intramuscular haematoma against a periosteally stripped humerus. That haematoma ossifies.

Risk factors.

• Elbow fracture-dislocation rather than simple dislocation — heterotopic ossification was seen in 30 per cent of surgically treated elbow fracture-dislocations in a recent randomised trial, most commonly around the radial head
• Associated head injury or burns
• Repeated forceful passive stretching and vigorous early manipulation
• Delayed surgery through an already inflamed field

Course. Pain and a firm mass in the antecubital fossa at 2 to 6 weeks, radiographic flocculent calcification from about 3 to 4 weeks, and maturation with a peripheral cortical rim by 6 to 12 months.

Telling it from a sarcoma. Heterotopic ossification matures from the periphery inward — the zonal phenomenon, a mature cortical rim outside an immature centre — whereas a parosteal or extraskeletal osteosarcoma ossifies centrally first. Never biopsy an immature-looking calcifying mass in the antecubital fossa in the weeks after trauma: the histology looks alarmingly cellular, and misdiagnosis as sarcoma is a documented disaster.

Management. Active and active-assisted motion only, with no forced passive stretching, and static progressive or turnbuckle splinting for a fixed deformity. Excision waits until the bone is mature and motion has plateaued, traditionally 6 to 12 months, though contemporary practice increasingly excises earlier when the arc is functionally limiting and the bone is radiographically defined. Prophylaxis with indometacin or single-dose radiotherapy is used in high-risk cases, principally after excision.

Three components. Elbow flexion contracture after trauma has three components, and the brachialis contributes to two of them.

1. Capsular — the anterior capsule thickens and shortens.
2. Myostatic — the brachialis itself shortens and scars to the capsule and humerus.
3. Bony or articular — coronoid and olecranon osteophytes, fossa filling, incongruity.

Why the muscle decides the result. An anterior capsular release that ignores a scarred brachialis will not deliver the intra-operative gain to the ward. The muscle has to be dealt with alongside the capsule, and the release technique is built around doing so without dividing it.

When flexion is what is lost. Loss of terminal flexion rather than extension points to posterior impingement or triceps contracture, not to brachialis.

Strain. Common in climbers and in eccentric loading during resisted flexion, presenting as deep anterior arm pain that is worse on resisted pronated flexion. MRI shows oedema in the muscle belly, typically distally and laterally. Management is non-operative: relative rest, an eccentric loading programme, and avoidance of forced passive stretch, which risks ossification if there is a significant haematoma.

Rupture. Isolated brachialis rupture is genuinely rare. It occurs with violent hyperextension and is usually accompanied by dislocation, and because the muscle is broad and multipennate with a segmental blood supply it heals reliably without operative repair. The concern is ossification of the resulting haematoma rather than loss of continuity.

Imaging.

• Radiographs are the correct first investigation for a firm antecubital mass after trauma — heterotopic bone is a radiographic diagnosis
• Ultrasound is a good first-line test for muscle discontinuity and haematoma, and is dynamic
• MRI best characterises the extent of oedema and, critically, the relation of any mass to the capsule and to the neurovascular bundle

Musculocutaneous nerve. It is injured in anterior shoulder dislocation, in coracoid transfer (Latarjet) by retractor traction on the conjoined tendon, and by vigorous medial retraction in the deltopectoral approach. Recovery is usually spontaneous for a traction neurapraxia; a persistent deficit at 3 to 6 months warrants electrodiagnostic study and consideration of nerve transfer, an ulnar nerve fascicle to the biceps branch, the Oberlin transfer.

Radial nerve. The branch to the deep brachialis head is expendable, but the parent nerve alongside it is not. In a distal humeral fracture or an anterolateral approach the nerve lies in the brachialis-brachioradialis interval, tethered where it pierces the lateral intermuscular septum, and Holstein-Lewis distal-third spiral humeral fractures classically tension it there.

The interval. Proximally between the deltoid and pectoralis major, then between biceps medially and brachialis in the middle of the arm.

The split. Distally the brachialis is divided longitudinally in the line of its fibres, and this is the manoeuvre the approach turns on. Keep the lateral third with the radial nerve and the medial two-thirds with the musculocutaneous nerve: because these are separate nerve territories, splitting the muscle denervates nothing.

Structures at risk.

• Radial nerve laterally, in the brachialis-brachioradialis interval
• Musculocutaneous nerve in the biceps-brachialis plane, and its continuation as the lateral cutaneous nerve of the forearm emerging lateral to the biceps tendon at the elbow — the commonest sensory casualty of an over-enthusiastic distal extension
• Brachial artery and median nerve medially; retract the biceps and the bundle together, medially, as one unit

What the exposure gives. The entire anterior humeral shaft down to within 2 cm of the joint. It does not give good access to the posterior column of the distal humerus, for which a posterior approach is required.

The anterolateral variant. The interval here is brachialis medially against brachioradialis laterally, a true internervous plane between musculocutaneous and radial territory, and it is preferred for the distal third of the humeral shaft when the surgeon wants direct control of the radial nerve. That nerve lies in the interval and is identified and protected first.

Approach options. A lateral column exposure (Mansat-Morrey), a medial over-the-top exposure, or both combined for a global contracture.

The brachialis manoeuvre. The muscle is elevated from lateral to medial off the anterior capsule as a sheet, using a blunt periosteal elevator kept on the capsule. Kept intact and elevated it becomes a living retractor protecting the median nerve and brachial artery, which lie within 1 cm medial to the medial border of brachialis at the level of the joint line; never place a sharp retractor blind on the medial side of the anterior capsule. Only then is the capsule dealt with, and it is excised rather than merely incised, because a capsulotomy alone frequently recurs.

Arthroscopic release. Here the brachialis is the muscular curtain between the arthroscope and the median nerve, and a contracted capsule pulls the neurovascular bundle closer to the joint — the stiffer the elbow, the smaller the margin. The safety rules follow from that.

• Insufflate the joint before portal placement
• Work with the elbow at approximately 90 degrees of flexion, which relaxes and displaces the anterior neurovascular structures away from the capsule
• Use retractors rather than suction, lifting the capsule away from the nerve, and keep the shaver blade facing the capsule
• Know what each portal threatens: the anteromedial portal endangers the median nerve, the anterolateral portal the radial nerve, which lies closest to the joint of any structure in that quadrant

The superficial head tendon as a local graft. Leonello and colleagues specifically proposed the tendon of the superficial head as a local donor, for three uses.

• Transfer to the radial tuberosity to restore supination when the distal biceps is unreconstructable
• Anular ligament reconstruction in chronic radial head dislocation, as an alternative to a triceps or fascial strip
• Medial collateral ligament reconstruction of the elbow

These remain uncommon, but the anatomical rationale — a discrete round tendon of adequate length already in the field, whose loss leaves the deep head intact and functioning — is sound and is fair game in a viva.

As a flap, a poor choice. The segmental type IV supply offers no reliable dominant pedicle, and losing the muscle costs elbow flexion power. Anterior elbow cover comes from brachioradialis on its radial recurrent pedicle, whose arc covers the distal half of the arm and the proximal two-thirds of the forearm; posterior cover comes from anconeus on its recurrent posterior interosseous pedicle.

Brachialis as the floor. In the single-incision anterior approach the retracted biceps tendon is retrieved from a tunnel between the brachialis and the mobile wad, and brachialis defines the floor of the antecubital fossa dissection.

Reaching the tuberosity. Supinating the forearm rotates the radial tuberosity anteriorly and moves the posterior interosseous nerve laterally and posteriorly, away from the field. Blunt dissection is carried out directly on the tuberosity; it is retractors placed around the radial neck that endanger that nerve.

The nerve that actually gets hurt. The lateral antebrachial cutaneous nerve — the continuation of the musculocutaneous nerve, lying just lateral to the biceps tendon at the crease — is the most frequently injured structure in this approach.