Brachialis Anatomy

Origin

• 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.
• Fibres also arise from the medial and lateral intermuscular septa. The lateral septal origin brings the muscle into contact with the radial nerve as the nerve pierces the septum.

Two Heads (Leonello, JBJS Am 2007)

• Superficial head — the larger. Origin more proximal on the anterolateral humerus; longitudinal fibres converging into a thick round tendon inserting on the ulnar tuberosity. This head has the mechanical advantage of a long proximal origin and a distal insertion, and generates the bulk of flexion strength.
• Deep head — smaller, fan-shaped, oblique fibres converging into an aponeurosis onto the coronoid process, more anteriorly. Better placed to initiate flexion from full extension.
• The two heads are separable by blunt dissection — practically relevant because the superficial head tendon is a described local donor for anular ligament or medial collateral ligament reconstruction, and for restoring supination by transfer to the radial tuberosity when the distal biceps is unreconstructable.

Insertion

• Ulnar tuberosity (superficial head tendon) and coronoid process (deep head aponeurosis).
• The insertion is entirely on the ulna. There is no radial attachment, so the muscle contributes nothing to supination or pronation.
• The footprint sits 1 to 2 cm distal to the coronoid tip, immediately anterior to the anterior band of the medial collateral ligament complex.

Primary: Musculocutaneous Nerve (C5, C6)

• The musculocutaneous nerve pierces coracobrachialis and then runs distally between biceps and brachialis, giving branches to both.
• Motor branches to brachialis enter the anteromedial surface of the muscle, typically as two or three twigs in the middle third of the arm.
• It emerges lateral to the biceps tendon at the elbow as the lateral cutaneous nerve of the forearm — the sensory territory tested (lateral forearm) to confirm musculocutaneous integrity.

Secondary: Radial Nerve (C7)

• In every specimen in the Leonello cadaveric series a branch of the radial nerve supplied the inferolateral fibres of the deep head.
• The branch arises as the radial nerve passes anterior to the lateral epicondyle in the interval between brachialis and brachioradialis.
• Functional significance: the radial-innervated portion is small; in isolated musculocutaneous palsy the brachialis is effectively paralysed for practical purposes despite this dual supply.
• Surgical significance: the boundary between the musculocutaneous-supplied medial two-thirds and the radial-supplied lateral third is an internervous plane within a single muscle. This is what makes the brachialis-splitting anterolateral approach to the distal humerus safe.

Root Values Summary

Named Vessels

• Brachial artery — muscular branches directly into the anteromedial surface throughout the arm. This is the dominant supply.
• Radial recurrent artery — supplies the distal lateral portion, entering close to the radial nerve in the brachialis-brachioradialis interval.
• Inferior ulnar collateral artery and the profunda brachii contribute to the distal and lateral portions respectively.

Pedicle Position

• The vascular supply is segmental and multi-pedicled, not a single dominant pedicle. Mathes-Nahai type IV pattern.
• Practical consequence: brachialis is a poor free or pedicled muscle flap donor. When cover is needed around the elbow the surgeon reaches for brachioradialis (radial recurrent pedicle) or anconeus (recurrent posterior interosseous pedicle), both of which have a single reliable proximal pedicle.
• In contracture release the segmental supply means the muscle tolerates extensive elevation from the anterior capsule without devascularisation, provided the medial neurovascular bundle is not stripped.

Superficial (Anterior)

• Biceps brachii throughout, and the bicipital aponeurosis (lacertus fibrosus) at the elbow.
• Musculocutaneous nerve in the plane between biceps and brachialis.

Deep (Posterior)

• Anterior surface of the humerus proximally.
• Anterior capsule of the elbow joint distally — the muscle and capsule are adherent, and this is the whole point of the muscle for elbow surgeons.

Medial

• 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 crosses the brachial artery from lateral to medial at about the mid-arm.
• Ulnar nerve is further posteromedial, behind the medial intermuscular septum — not in the brachialis plane, but reached from it.

Lateral

• Brachioradialis and extensor carpi radialis longus — the mobile wad.
• 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 radial nerve is safe as long as you stay medial to the lateral third of the brachialis, and it is found by developing the brachialis-brachioradialis interval.

Myositis Ossificans of Brachialis

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

Elbow Stiffness and the Brachialis

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 it matters operatively: an anterior capsular release that ignores a scarred brachialis will not deliver the intra-operative gain to the ward. In an open anterior release the brachialis is elevated off the capsule as a sheet, protecting the median nerve and brachial artery medially, and only then is the capsule excised. In arthroscopic release the brachialis is the muscular curtain between the arthroscope and the median nerve — the reason anterior capsulectomy is done with the elbow flexed and with the capsule lifted away from the nerve.

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

Brachialis Strain

• Common in climbers and in eccentric loading during resisted flexion, presenting as deep anterior arm pain, worse on resisted pronated flexion.
• MRI shows oedema in the muscle belly, typically distally and laterally.
• Managed non-operatively: relative rest, eccentric loading programme, 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.
• 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

• 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.
• Radiographs are the correct first investigation for a firm antecubital mass after trauma — heterotopic bone is a radiographic diagnosis.

Musculocutaneous Nerve

• Injured in anterior shoulder dislocation, in coracoid transfer (Latarjet) by retractor traction on the conjoined tendon, and in vigorous medial retraction in the deltopectoral approach.
• Findings: weak elbow flexion (grade 3, brachioradialis-powered), weak supination, and numbness over the lateral forearm.
• Recovery is usually spontaneous for traction neurapraxia; 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 radial 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 is in the brachialis-brachioradialis interval, tethered where it pierces the lateral intermuscular septum.
• Holstein-Lewis distal-third spiral humeral fractures classically tension the nerve at the septum.

Anterior (Henry) Approach to the Humerus

• Interval: proximally between the deltoid and pectoralis major (deltopectoral), then between biceps (medial) and brachialis in the middle of the arm.
• Distally, the brachialis is split longitudinally — this is the key manoeuvre. The split is made in the line of the fibres, keeping 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; it pierces the lateral intermuscular septum a mean of 12 cm (range 7.4 to 16.6 cm) proximal to the olecranon fossa.
  • 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.
• Exposure obtained: 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 — use a posterior approach for that.

Anterolateral Approach

• Interval: brachialis (medial) and brachioradialis (lateral) — a true internervous plane (musculocutaneous versus radial).
• The radial nerve must be identified and protected first; it lies in this interval.
• Preferred for the distal third of the humeral shaft when the surgeon wants direct control of the radial nerve.

Open Anterior Capsular Release

• Approach options: lateral column (Mansat-Morrey), medial over-the-top, or a combined approach 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. Keeping the muscle intact and elevated turns it into a living retractor protecting the median nerve and brachial artery.
• The capsule is then excised, not merely incised — a capsulotomy alone frequently recurs.
• Distances that matter: the median nerve and brachial artery 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.

Arthroscopic Release

• The brachialis is the protective curtain between the anterior capsule and the median nerve.
• Safety rules: work with the elbow at approximately 90 degrees of flexion (which relaxes and displaces the anterior neurovascular structures away from the capsule), insufflate the joint before portal placement, use retractors rather than suction, and keep the shaver blade facing the capsule.
• Anteromedial portal endangers the median nerve; anterolateral portal endangers the radial nerve, which lies closest to the joint of any structure in that quadrant.
• A contracted capsule pulls the neurovascular bundle closer to the joint — the stiffer the elbow, the smaller the margin.

Brachialis Tendon as a Local Graft

Leonello and colleagues specifically proposed the tendon of the superficial head as a local donor:

• 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

• Poor choice. Segmental (type IV) blood supply, no reliable dominant pedicle, and its loss costs elbow flexion power. Use brachioradialis for anterior elbow cover (radial recurrent pedicle, arc covering the distal half of the arm and proximal two-thirds of the forearm) or anconeus for posterior cover.

Brachialis as a Landmark in Distal Biceps Repair

• In the single-incision anterior approach, the retracted biceps tendon is retrieved from a tunnel between the brachialis and the mobile wad. Brachialis defines the floor of the antecubital fossa dissection.
• The radial tuberosity is reached by supinating the forearm, which rotates the tuberosity anteriorly and moves the posterior interosseous nerve laterally and posteriorly, away from the field. Blunt dissection is carried out directly on the tuberosity; retractors placed around the radial neck endanger the posterior interosseous nerve.
• 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.
• Intact brachialis is why patients with a chronic unrepaired distal biceps rupture still flex the elbow well and complain principally about supination endurance.