Coracobrachialis Anatomy

Origin. The tip of the coracoid process, shared with the short head of biceps brachii as the conjoined tendon. Within the tendon coracobrachialis occupies the medial and posterior portion and the short head the lateral and anterior; distally the two separate, the short head continuing as a tendon while coracobrachialis becomes fleshy. A cadaveric study in a Sri Lankan population found the muscle arising from the coracoid tip and additionally from the lateral, posterior and medial aspects of the short head tendon, so the two are more intimately fused than a simple side-by-side arrangement.

What else attaches to the coracoid. Three muscles and two ligaments:

• Pectoralis minor on the medial border, the only attachment on the medial side
• Coracobrachialis at the tip, the medial part of the conjoined tendon
• Short head of biceps at the tip, the lateral part of the conjoined tendon
• Coracoacromial ligament on the lateral border, the stump used in the Latarjet capsular repair
• Coracoclavicular ligaments on the superior surface: conoid posteromedially, trapezoid anterolaterally

Insertion. The middle third of the medial surface of the humeral shaft, on and just anterior to the medial border, roughly opposite the deltoid tuberosity on the lateral side. The insertion is a linear ridge, typically 5 to 7 cm long, lying between the origin of the medial head of triceps posteriorly and brachialis anteriorly; the nutrient foramen of the humerus lies near it, directed distally.

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Dimensions. The muscle is roughly 17 to 19 cm long in an adult, the belly occupying most of that. It does not cross the elbow, an important negative: coracobrachialis has no elbow action whatsoever, unlike the other two muscles of the anterior arm compartment.

A third head. Coracobrachialis longus, a described variant reaching the medial epicondyle, can compress the median nerve or brachial artery, the analogue of the ligament of Struthers problem.

Musculocutaneous nerve, C5, C6 and C7. The branch to coracobrachialis arises before the nerve pierces the muscle and often carries a C7 contribution, whereas the branches to biceps and brachialis are predominantly C5 and C6.

The relationship that defines the muscle. In the great majority of people the nerve pierces the substance of coracobrachialis and emerges on its lateral side to continue between biceps and brachialis. In the Sri Lankan series of 312 upper limbs it perforated the muscle in 83.3 per cent; in 50 per cent it pierced the middle third, and in none did it pierce the lower third. This is what makes coracobrachialis the standard textbook example of a muscle routinely pierced by its own major supplying nerve.

Where it enters, measured from the coracoid. The distances come from three cadaveric studies:

Twigs versus trunk. The distinction is not academic. Small proximal twigs to coracobrachialis arise before the main trunk enters and are the structures most often avulsed by a medially placed retractor. Losing a twig costs coracobrachialis function only; traction on the main trunk costs biceps, brachialis and lateral forearm sensation.

The practical rule. Treat 3 cm below the coracoid as the zone in which the nerve must be seen, not assumed. Beyond that, still identify and protect it before applying any medial retraction force.

Variations. A non-piercing nerve, passing lateral or superficial to coracobrachialis, occurs in roughly 10 to 20 per cent of limbs depending on the series; it usually runs a more superficial course and is more, not less, vulnerable, because it is not protected within muscle substance. Communicating branches between the musculocutaneous and median nerves are common, reported in up to 20 to 30 per cent of limbs, and explain occasional atypical patterns of weakness after injury. An absent musculocutaneous nerve, with the median nerve supplying the anterior arm muscles directly, is rare but described, and the pattern of deficit after injury is then atypical.

Named vessels. Muscular branches of the axillary artery, principally from its second and third parts, enter the muscle in its proximal third close to the conjoined tendon. The anterior circumflex humeral artery contributes to the proximal and lateral part, and muscular branches of the brachial artery supply the distal part in the arm.

Where the pedicle enters. The dominant supply enters proximally, within 4 to 6 cm of the coracoid, on the deep, posteromedial surface, the same territory as the musculocutaneous nerve. Two consequences follow. In the Latarjet the coracoid graft carried on the conjoined tendon retains a soft-tissue vascular supply through the tendon and muscle, which is one argument for the reliability of union of the coracoid to the glenoid. And coracobrachialis is not used as a flap: it is small, its pedicle is short and shared with the motor nerve, and there is no defect it covers that a better donor cannot.

The clavipectoral fascia. It runs from the clavicle to the axillary fascia, splitting to enclose subclavius and pectoralis minor. Between the coracoid and the upper border of pectoralis minor it thickens as the costocoracoid ligament; below pectoralis minor it continues as the suspensory ligament of the axilla. Four structures pierce it:

• Cephalic vein
• Thoracoacromial artery
• Lateral pectoral nerve
• Lymphatics

Medial. Proximally the axillary artery and vein and the cords of the brachial plexus, immediately medial to the conjoined tendon; distally the brachial artery, median nerve, ulnar nerve and basilic vein. The muscle forms a protective curtain between a laterally placed retractor and these structures, but it is a thin one.

Lateral. The short head of biceps distally, and proximally the humerus, with the subscapularis and anterior glenohumeral joint deep and lateral.

Posterior. Subscapularis and the anterior glenohumeral joint capsule. The anterior circumflex humeral vessels, the "three sisters", cross the inferior border of subscapularis deep to the conjoined tendon, and the axillary nerve passes below that border into the quadrangular space, so deep and inferior retraction beneath the conjoined tendon endangers it.

Anterior. Pectoralis major and, proximally, pectoralis minor and the clavipectoral fascia, with deltoid laterally.

How it is injured. Five mechanisms, and the setting in which each occurs:

The clinical picture. Elbow flexion is weak but not absent, because biceps and brachialis are lost while brachioradialis is intact, and supination is weak; the coracobrachialis weakness itself is undetectable clinically. Sensation is reduced over the lateral forearm in the distribution of the lateral cutaneous nerve of the forearm, the terminal continuation of the musculocutaneous nerve emerging lateral to the biceps tendon at the elbow. The biceps jerk (C5, C6) is diminished or absent.

Prognosis. Traction neurapraxia after retraction recovers in the great majority of cases within weeks to a few months. A deficit persisting at 3 to 6 months warrants electrodiagnostic testing.

Salvage. The Oberlin transfer, a redundant fascicle of the ulnar nerve (typically a flexor carpi ulnaris fascicle) transferred to the biceps motor branch, is the standard for restoring elbow flexion, often augmented by a median nerve fascicle to the brachialis branch as the double fascicular transfer.

Coracoid process fractures. Uncommon, roughly 3 to 13 per cent of scapular fractures, with two mechanisms: direct trauma, and avulsion by the conjoined tendon in adults or through the coracoid physis in adolescents, where the apophysis is a weak link. Clinically there is pain on resisted shoulder flexion and adduction, the conjoined tendon pull, and on resisted elbow flexion, the short head of biceps. The Stryker notch view and the axillary lateral are the plain films; CT is definitive.

Ogawa classification. The line is drawn at the coracoclavicular ligament attachment:

• Type I: fracture posterior to the attachment, at the base. The coracoclavicular ligaments are detached from the scapular body, so the injury behaves as a double disruption of the superior shoulder suspensory complex and is usually unstable
• Type II: fracture anterior to the attachment, at the tip or distally. The ligaments remain in continuity with the scapula, so the injury is stable and is managed non-operatively

A coracoid base fracture with an acromioclavicular joint disruption is a classic combination requiring operative consideration.

Adolescent coracoid apophysitis. Repetitive traction through the conjoined tendon in throwing and gymnastics can produce a traction apophysitis of the coracoid, the shoulder equivalent of Osgood-Schlatter disease. It is managed with rest and load modification.

The functional unit. The coracoid, the conjoined tendon and the subscapularis form a unit whose crowding produces subcoracoid impingement. Normal coracohumeral distance is roughly 8.7 to 11 mm on axial imaging; less than 6 mm is taken as stenotic.

Mechanism. In flexion, adduction and internal rotation, the throwing follow-through and the cross-body reach, the lesser tuberosity and the upper subscapularis are abraded against the coracoid tip and the undersurface of the conjoined tendon. Burkhart described this as the roller-wringer effect: the subscapularis tendon is compressed and wrung against the coracoid like cloth through a mangle, producing articular-sided partial tearing of the upper subscapularis. Subcoracoid impingement is strongly associated with upper subscapularis tears and biceps pulley lesions.

Diagnosis. Deep anterior shoulder pain provoked by reaching across the body, the throwing follow-through or a golf swing; pain on the passive coracoid impingement test, with a palpable or audible click as the lesser tuberosity passes under the coracoid. Axial MRI or CT measures the coracohumeral distance; look for subcoracoid oedema or fat effacement and for an associated upper subscapularis tear and biceps pulley lesion.

Why the space narrows. The causes are congenital, post-traumatic, iatrogenic or degenerative:

• Congenital coracoid over-length or lateral over-hang
• Post-traumatic: a malunited coracoid, lesser tuberosity or proximal humeral fracture
• Iatrogenic: over-medialisation of the glenoid component in shoulder arthroplasty, or a Bristow or Latarjet graft placed too medially, which narrows the space further; an over-tightened anterior capsulorrhaphy driving the head posteriorly and superiorly is the opposite problem
• Degenerative osteophytes

Treatment. Non-operative first: activity modification, physiotherapy focused on scapular control and posterior capsule stretching, and image-guided subcoracoid injection, which is both diagnostic and therapeutic. Coracoplasty, arthroscopic or open resection of the posterolateral coracoid tip aiming to restore a coracohumeral distance of about 7 mm, is a recognised adjunct performed with subscapularis repair when both are present. The conjoined tendon origin must be preserved: resect from the posterolateral surface, not the tip attachment.

Genuinely rare in isolation. It is reported after violent resisted shoulder extension with the elbow flexed, and after coracoid fracture. It presents as anterior shoulder pain with weakness of resisted flexion and adduction and a palpable defect below the coracoid. Because the deficit is functionally minor, most are treated non-operatively unless the patient is a high-demand athlete or the rupture accompanies a displaced coracoid fracture.

The workhorse anterior approach. The interval is between deltoid, supplied by the axillary nerve, and pectoralis major, supplied by the medial and lateral pectoral nerves: a true internervous plane. The sequence:

1. Skin incision from the tip of the coracoid, running distally and slightly laterally toward the deltoid insertion, roughly 10 to 15 cm.
2. Cephalic vein, lying superficially in the groove. Convention is to take it laterally with the deltoid, because most of its tributaries come from the deltoid side and taking it medially requires ligating more branches; either is acceptable if done deliberately.
3. Clavipectoral fascia incised lateral to the conjoined tendon, which enters the subdeltoid space and opens the plane onto subscapularis.
4. Conjoined tendon retracted medially. This is the critical step for the musculocutaneous nerve.
5. The three sisters run transversely across the inferior border of subscapularis at the level of the surgical neck and mark the inferior extent of the subscapularis; identify, coagulate or ligate them rather than tearing them.
6. Subscapularis managed by tenotomy, peel or lesser tuberosity osteotomy depending on the procedure.

The axillary nerve. It lies approximately 1 to 2 cm below the inferior border of subscapularis, and as little as 0.7 cm from the surgical neck posteriorly. Palpate it with a finger sweeping along the inferior subscapularis before any inferior capsular release.

Why it works. The Latarjet coracoid transfer is not simply a bone block, and examiners expect the three separate mechanisms of the triple-blocking effect. Coracobrachialis is central to the second.

1. Bony effect. The transferred coracoid extends the anteroinferior glenoid arc, restoring the articular surface and increasing the critical distance the humeral head must travel to dislocate. This addresses glenoid bone loss.
2. Sling effect. The conjoined tendon, coracobrachialis plus the short head of biceps, passes through the subscapularis split and acts as a dynamic anteroinferior sling. In abduction and external rotation, the position of apprehension, it tensions the inferior third of the subscapularis and pushes the humeral head posteriorly. This is widely believed to be the most important of the three, and it is entirely dependent on placing the split at the junction of the upper two-thirds and lower third.
3. Capsular effect (Bankart effect). The coracoacromial ligament stump, left attached to the lateral coracoid, is repaired to the anterior capsule and glenoid rim, restoring the capsulolabral restraint.

The sling effect is why a Latarjet works even in patients with an intact glenoid, and why it remains effective in contact athletes and in patients with capsular deficiency where a soft-tissue Bankart repair fails. It is also why a split placed too superiorly, or a graft placed too high on the glenoid, loses the sling and produces a mechanically inferior construct.

Indication. Anterior glenohumeral instability with significant glenoid bone loss (classically greater than 20 to 25 per cent, or an off-track Hill-Sachs lesion), failed soft-tissue stabilisation, or a high-risk contact athlete.

The osteotomy. The coracoid is osteotomised at its knee, the angle between the horizontal and vertical limbs, typically yielding a graft 2 to 3 cm long. Pectoralis minor is released from the medial coracoid, the coracoacromial ligament is divided leaving a stump on the lateral coracoid for the later capsular repair, and the conjoined tendon is left attached to the graft.

The nerve step. Before mobilising the graft, the surgeon must appreciate that the musculocutaneous nerve and its twigs are entering coracobrachialis on its deep, posteromedial surface, at the distances measured in the innervation tab. Blunt dissection along the lateral border of the conjoined tendon is safe; blind dissection on its medial or deep surface is not.

The subscapularis split is made at the junction of the upper two-thirds and lower third. That level lets the conjoined tendon sit inferiorly where the sling effect is maximal, avoids the nerves entering subscapularis superiorly, and stays above the axillary nerve inferiorly.

Graft position. Flush with the glenoid articular surface, not lateral, which causes rapid glenohumeral arthritis, and not medial, which fails to restore the arc, and centred at the 3 to 5 o'clock position on a right shoulder. The coracoacromial ligament stump is then sutured to the anterior capsule and glenoid rim, keeping the graft extra-articular.

The complication data. In Gartsman's consecutive series of 416 open Latarjet procedures the overall immediate and early complication rate was 5.0 per cent. There were 13 neurological injuries (3.1 per cent): axillary nerve in 7, musculocutaneous nerve in 4, suprascapular nerve in 2, and 11 of the 13 recovered completely or near-completely. Infection occurred in 1.4 per cent, and increasing age was associated with a higher complication rate. Asked how often the musculocutaneous nerve is injured, quote roughly 1 per cent of open Latarjet procedures, almost all transient.

Bristow. A smaller coracoid transfer: only the tip is transferred, fixed with a single screw, and the conjoined tendon passes through the subscapularis split. The sling effect is retained and the bony effect is smaller; higher rates of non-union of the small graft are reported, and the musculocutaneous nerve considerations are identical.

Coracoclavicular reconstruction. In acromioclavicular joint reconstruction, grafts and loops are passed around or through the base of the coracoid. The structures at risk when passing an instrument around the coracoid medially are the musculocutaneous nerve and the brachial plexus, and inferiorly the axillary artery, so the instrument should hug the inferior surface of the coracoid from lateral to medial under direct vision, with the medial limb exiting against bone.

Pectoralis minor release. Performed from the medial coracoid in the Latarjet and in thoracic outlet decompression. The release is done on the bone, from lateral to medial, keeping the dissection subperiosteal to avoid the cords of the plexus medially.

Conjoined tendon transfer. Transfer of the conjoined tendon to the lateral glenoid has been described as a soft-tissue alternative to a bone block for instability in patients without significant bone loss. The construct depends entirely on the sling effect and remains a niche option.