Coracobrachialis Anatomy

Origin

• Tip of the coracoid process of the scapula, arising with the short head of biceps brachii as the conjoined tendon.
• Within the conjoined tendon, coracobrachialis occupies the medial and posterior portion and the short head of biceps the lateral and anterior portion. Distally the two separate; the short head continues as a tendon, coracobrachialis becomes fleshy.
• A cadaveric study in a Sri Lankan population found the muscle to arise from the coracoid tip and additionally from the lateral, posterior and medial aspects of the short head of biceps tendon — the two are more intimately fused than a simple side-by-side arrangement.

Insertion

• 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 the brachialis anteriorly.
• The nutrient foramen of the humerus lies near this insertion, directed distally.

Length and Dimensions

• The muscle is roughly 17 to 19 cm in overall length in an adult, with the muscle 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.

Musculocutaneous Nerve (C5, C6, C7)

• Root values are C5, C6 and C7 for coracobrachialis specifically. 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 unique relationship: in the great majority of people the musculocutaneous nerve pierces the substance of coracobrachialis and emerges on its lateral side to continue between biceps and brachialis. In the Sri Lankan cadaveric series of 312 upper limbs the nerve 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 makes coracobrachialis the standard textbook example of a muscle routinely pierced by its own major supplying nerve.

Entry Distances from the Coracoid — The Numbers That Matter

• 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.
• Practical rule: treat 3 cm below the coracoid as the zone in which you must see the nerve, not assume it. Beyond that, still identify and protect it before applying any medial retraction force.

Variations

• Non-piercing musculocutaneous nerve (passing lateral to or superficial to coracobrachialis) occurs in roughly 10 to 20 per cent of limbs depending on the series. When the nerve does not pierce the muscle it usually runs a more superficial and more vulnerable course.
• 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.

Named Vessels

• Muscular branches of the axillary artery, principally from its second and third parts, entering the muscle in its proximal third close to the conjoined tendon.
• Anterior circumflex humeral artery contributes to the proximal and lateral part.
• Brachial artery muscular branches supply the distal part in the arm.

Pedicle Position

• The dominant supply enters proximally, within 4 to 6 cm of the coracoid, on the deep (posteromedial) surface — that is, in the same territory as the musculocutaneous nerve.
• Practical significance:
  • 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.
  • 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 Conjoined Tendon and the Clavipectoral Fascia

• The clavipectoral fascia 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.
• It is pierced by four structures: the cephalic vein, the thoracoacromial artery, the lateral pectoral nerve and lymphatics.
• In the deltopectoral approach the clavipectoral fascia is incised lateral to the conjoined tendon to enter the subdeltoid space and expose the subscapularis.

Medial to Coracobrachialis

• Axillary artery and vein and the cords of the brachial plexus proximally; the brachial artery, median nerve, ulnar nerve and basilic vein distally.
• The muscle forms a protective curtain between a laterally placed retractor and these structures — but it is a thin one.

Lateral to Coracobrachialis

• Short head of biceps distally, and the humerus with the subscapularis and anterior glenohumeral joint deep and lateral proximally.

Posterior (Deep)

• 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 are ligated in the deltopectoral approach.
• Axillary nerve passes below the inferior border of subscapularis into the quadrangular space. Deep and inferior retraction beneath the conjoined tendon endangers it.

Anterior (Superficial)

• Pectoralis major and, proximally, pectoralis minor and the clavipectoral fascia.
• Deltoid laterally.

Mechanisms

Clinical Picture

• Motor: weak elbow flexion (biceps and brachialis lost; brachioradialis intact so flexion is around grade 3), weak supination, and absent or weak coracobrachialis (undetectable clinically).
• Sensory: reduced sensation 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.
• Reflex: diminished or absent biceps jerk (C5, C6).

Prognosis and Salvage

• Traction neurapraxia after retraction recovers in the great majority of cases within weeks to a few months. In the Gartsman series of 416 Latarjet procedures, 11 of 13 nerve injuries recovered completely or near-completely.
• Persistent deficit 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 (the double fascicular transfer).

Coracoid Process Fractures

• Uncommon; roughly 3 to 13 per cent of scapular fractures. Two mechanisms: direct trauma, and avulsion by the conjoined tendon (in adults) or through the coracoid physis (in adolescents, where the coracoid apophysis is a weak link).
• Ogawa classification:
  • Type I: fracture posterior to the coracoclavicular ligament attachment (that is, 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 coracoclavicular attachment (tip or distal) — the coracoclavicular ligaments remain in continuity with the scapula, so the injury is stable and is managed non-operatively.
• Association: a coracoid base fracture with an acromioclavicular joint disruption is a classic combination requiring operative consideration.
• Clinical: pain on resisted shoulder flexion and adduction (conjoined tendon pull) and on resisted elbow flexion (short head of biceps).
• Imaging: the Stryker notch view and axillary lateral are the plain films; CT is definitive.

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. Managed with rest and load modification.

Diagnosis

• Symptom: deep anterior shoulder pain provoked by flexion, adduction and internal rotation — reaching across the body, the throwing follow-through, or a golf swing.
• Sign: pain on the passive coracoid impingement test; a palpable or audible click as the lesser tuberosity passes under the coracoid.
• Imaging: axial MRI or CT measuring the coracohumeral distance; less than 6 mm is stenotic (normal is around 8.7 to 11 mm). Look for subcoracoid oedema or fat effacement and for an associated upper subscapularis tear and biceps pulley lesion.

Causes of a Narrowed Subcoracoid Space

• Congenital coracoid over-length or lateral over-hang.
• Post-traumatic — malunited coracoid, lesser tuberosity or proximal humeral fracture.
• Iatrogenic — over-medialisation of the glenoid component in shoulder arthroplasty, or an over-tightened anterior capsulorrhaphy driving the head posteriorly and superiorly is the opposite problem; a Bristow or Latarjet graft that is too medial narrows the space further.
• 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 performed with subscapularis repair when both are present. The conjoined tendon origin must be preserved; resect from the posterolateral surface, not the tip attachment.

Conjoined Tendon Rupture

• Genuinely rare in isolation. Reported after violent resisted shoulder extension with the elbow flexed, and after coracoid fracture.
• 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.

Anatomical Variants of Surgical Consequence

• Non-piercing musculocutaneous nerve (roughly 10 to 20 per cent) — the nerve runs superficial or lateral to the muscle and is more, not less, vulnerable, because it is not protected within muscle substance.
• Absent musculocutaneous nerve with the median nerve supplying the anterior arm muscles directly — rare but described; the pattern of deficit after injury is then atypical.
• Musculocutaneous-to-median communicating branches in up to 20 to 30 per cent of limbs.
• A third head of coracobrachialis (coracobrachialis longus, reaching the medial epicondyle) is a described variant which can compress the median nerve or brachial artery — the analogue of the ligament of Struthers problem.

The Workhorse Anterior Shoulder Approach

• Skin incision: from the tip of the coracoid, running distally and slightly laterally toward the deltoid insertion, roughly 10 to 15 cm.
• Interval: deltoid (axillary nerve) and pectoralis major (medial and lateral pectoral nerves) — a true internervous plane.
• Cephalic vein: lies in the groove. Convention is to take it laterally with the deltoid, because most of its tributaries come from the deltoid side; taking it medially requires ligating more branches. Either is acceptable if done deliberately.
• Clavipectoral fascia: incised lateral to the conjoined tendon, which opens the plane onto subscapularis.
• Conjoined tendon: retracted medially. This is the critical step for the musculocutaneous nerve.
• The three sisters: the anterior circumflex humeral vessels run transversely across the inferior border of subscapularis; identify, coagulate or ligate them rather than tearing them.
• Subscapularis: managed by tenotomy, peel or lesser tuberosity osteotomy depending on the procedure.

Structures at Risk with Distances

Coracoid Transfer: Technique and Coracobrachialis

• 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.
• 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. The pectoralis minor is released from the medial coracoid; the coracoacromial ligament is divided leaving a stump on the lateral coracoid for later capsular repair; 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 the coracobrachialis on the deep, posteromedial surface within 2 to 5 cm of the coracoid. Blunt dissection along the lateral border of the conjoined tendon is safe; blind dissection on its medial or deep surface is not.
• Subscapularis split: made at the junction of the upper two-thirds and lower third. This level is chosen because it (a) allows the conjoined tendon to sit inferiorly where the sling effect is maximal, (b) avoids the nerves entering subscapularis superiorly, and (c) stays above the axillary nerve inferiorly.
• Graft position: flush with the glenoid articular surface (not lateral, which causes arthritis; not medial, which fails to restore the arc), and centred at the 3 to 5 o'clock position on a right shoulder.
• Capsular repair: the coracoacromial ligament stump is sutured to the anterior capsule and glenoid rim, keeping the graft extra-articular.

Complication Data

• In a consecutive series of 416 open Latarjet procedures, the overall immediate and early complication rate was 5.0 per cent. Thirteen neurological injuries (3.1 per cent) occurred: axillary nerve in 7, musculocutaneous nerve in 4, and suprascapular nerve in 2. Eleven of 13 recovered completely or near-completely. Infection occurred in 1.4 per cent. Increasing age was associated with a higher complication rate.
• This is the number to quote when asked how often the musculocutaneous nerve is injured: roughly 1 per cent of open Latarjet procedures, almost all transient.

Bristow Procedure

• 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; the bony effect is smaller. Higher rates of non-union of the small graft are reported. 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.
• The instrument should hug the inferior surface of the coracoid from lateral to medial under direct vision, and the medial limb should exit against bone.

Pectoralis Minor Lengthening or Release

• Released from the medial coracoid in the Latarjet and in thoracic outlet decompression. Release is performed on the bone, from lateral to medial, keeping the dissection subperiosteal to avoid the cords of the plexus medially.

Conjoined Tendon Transfer

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