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Posterior cord

The posterior cord is a major division of the brachial plexus, a network of nerves originating from the ventral rami of spinal nerves C5 through T1 that provides motor and sensory innervation to the upper limb. Formed by the union of the posterior divisions from the superior, middle, and inferior trunks of the brachial plexus, it lies posterior to the axillary artery in the axilla and gives rise to several key branches that primarily supply the extensor and abductor muscles of the shoulder, arm, and forearm, as well as sensory fibers to the posterior and lateral aspects of the upper limb.^[1]^[2]^[3] The posterior cord's branches include the upper subscapular nerve (C5-C6), which innervates the superior portion of the subscapularis muscle; the thoracodorsal nerve (C6-C8), which supplies the latissimus dorsi for shoulder adduction and medial rotation; and the lower subscapular nerve (C5-C6), which innervates the inferior subscapularis and teres major muscles. Its two terminal branches are the axillary nerve (C5-C6), responsible for motor innervation to the deltoid and teres minor muscles (enabling shoulder abduction and external rotation) along with sensory supply to the lateral shoulder skin via the superior lateral cutaneous nerve of the arm, and the radial nerve (C5-T1), the largest branch, which provides motor innervation to the triceps brachii, brachioradialis, and posterior forearm extensors while carrying sensory fibers to the posterior arm, forearm, and dorsolateral hand.^[1]^[2]^[3] Clinically, injury to the posterior cord or its branches can result in significant deficits, such as deltoid weakness and sensory loss over the shoulder from axillary nerve damage, or "wrist drop" and inability to extend the elbow, wrist, or fingers due to radial nerve involvement, often seen in trauma, compression, or surgical complications affecting the axilla or humerus.^[1]^[2]^[3]

Anatomy

Formation

The posterior cord is one of the three cords of the brachial plexus, along with the lateral and medial cords, and serves as a key structural component in the innervation of the upper limb. It forms through the union of the posterior divisions arising from the upper, middle, and lower trunks of the brachial plexus.^[4]^[5] This configuration positions the posterior cord to primarily supply extensor and abductor muscles of the shoulder and arm, as well as certain sensory regions.^[6] The brachial plexus derives its neural contributions from the ventral rami of spinal nerves C5 through T1, with the posterior cord receiving input via the posterior divisions of all three trunks.^[7] The upper trunk (C5-C6), middle trunk (C7), and lower trunk (C8-T1) each contribute equally in terms of divisional components, though the upper trunk's posterior division often carries a substantial fiber load due to the prominence of C5 and C6 roots in posterior cord derivatives like the radial nerve.^[8] Formation begins proximally as the trunks traverse the scalene muscles and extend toward the clavicle. At the lateral border of the first rib, each of the three trunks divides into an anterior and a posterior division, marking the transition from the supraclavicular to the infraclavicular portion of the plexus.^[9] The three posterior divisions then converge and unite within the axilla, posterior to the axillary artery, to constitute the posterior cord.^[10] This assembly occurs in a consistent manner in the majority of individuals, reflecting the standardized developmental patterning of the brachial plexus. Anatomical variations in posterior cord formation, while infrequent, can include incomplete fusion of the posterior divisions or atypical direct contributions from individual spinal roots, such as extensions from C4 or T2.^[11] Cadaveric studies indicate that the classic pattern of cord formation from the divisions occurs in approximately 96% of cases, with variations noted in about 4%.^[11] Such anomalies may arise during embryological development between days 32 and 40, when spinal nerve extensions merge to form the plexus, and can influence surgical approaches in the axilla.^[12]

Location and relations

The posterior cord of the brachial plexus is located within the axilla, where it is formed by the union of the posterior divisions of the upper, middle, and lower trunks.^[4] It lies posterior to the axillary artery, a defining positional relationship that contributes to its nomenclature, and is situated among the surrounding structures of the axillary region.^[7] As it descends, the cord courses downward and laterally, beginning from its point of formation near the clavicle and passing posterior to the pectoralis minor muscle.^[4] In terms of its relations to adjacent tissues, the posterior cord is positioned anterior to the subscapularis muscle and runs alongside the subscapular artery, which arises from the third part of the axillary artery.^[7] It lies posterior to the lateral pectoral nerve, which emerges from the lateral cord, and is located lateral to the medial cord within the axillary space.^[4] The cord also maintains a medial relation to the coracobrachialis muscle as it transitions toward the arm.^[7] While descending in the axilla, it gives off its branches before terminating into the axillary and radial nerves.^[4] The blood supply to the posterior cord is derived primarily from branches of the axillary artery, including the subscapular artery, as well as contributions from the subclavian artery.^[7] Venous drainage occurs via tributaries of the axillary vein, accompanying the arterial supply in the axillary region.^[4]

Branches

Subscapular nerves

The subscapular nerves are the earliest branches arising from the posterior cord of the brachial plexus, consisting of the upper and lower subscapular nerves, both of which provide motor innervation exclusively to muscles of the rotator cuff and adjacent structures. These nerves emerge in the axilla, proximal to the other major branches of the posterior cord, and course posteriorly toward the subscapularis muscle. They derive primarily from the C5 and C6 spinal roots, reflecting the posterior cord's contribution from the upper trunk of the brachial plexus.^[4] The upper subscapular nerve originates from the upper portion of the posterior cord in approximately 97% of cases, with rare variations arising directly from the axillary nerve (3%). It travels inferiorly for a mean length of 5 cm before piercing the upper part of the subscapularis muscle to innervate its superior fibers. The nerve typically presents as a single trunk (90.3%), though it may divide into two (8%) or three (1.6%) branches. With a mean diameter of 2.3 mm, it lacks any sensory fibers, functioning solely to supply motor innervation.^[13]^[14] The lower subscapular nerve, also known as the inferior subscapular nerve, emerges from a more distal aspect of the posterior cord in 79% of cases, while in 21% it arises from the proximal axillary nerve. It extends for a mean of 3.5 cm to its branch for the inferior subscapularis muscle, then continues an additional 2.5 cm to innervate the teres major muscle. Like its upper counterpart, it is purely motor, with no sensory component, and usually appears as a single trunk (93.6%), occasionally bifurcating (6.4%). A notable anatomical variation occurs in about 20% of cases, where the lower subscapular nerve shares a common trunk with the thoracodorsal nerve before diverging. Its mean diameter measures 1.9 mm. These short nerves (combined lengths approximating 3-5 cm prior to terminal branching) are critical for shoulder stability but are susceptible to iatrogenic injury due to their proximal location.^[13]^[14]^[15]

Thoracodorsal nerve

The thoracodorsal nerve, also known as the middle or long subscapular nerve, arises from the posterior aspect of the posterior cord of the brachial plexus at the mid-axillary level, receiving contributions primarily from the C6, C7, and C8 spinal nerve roots.^[16]^[17] This purely motor nerve, lacking any sensory fibers, has a diameter of approximately 2–3 mm as it traverses the axilla.^[18]^[19] From its origin near the apex of the axilla, the nerve descends posteriorly along the posterior axillary wall for a distance of 10–15 cm, passing deep to the axillary vein and initially posterior to the subscapular artery before crossing anterior to the thoracodorsal artery, a branch of the subscapular artery.^[17]^[20]^[19] It courses parallel to the thoracodorsal vessels, lying between the upper and lower subscapular nerves, and crosses the inferior border of the teres major muscle.^[16]^[17] Upon reaching the latissimus dorsi muscle, the thoracodorsal nerve enters its medial surface approximately 2 cm from the medial border, distributing small terminal branches throughout the muscle for innervation.^[18]^[16] Anatomical variations occur in up to 21% of cases, including origins from the axillary nerve (reported in 8.9–17% of specimens) or from a common trunk shared with the upper and lower subscapular nerves.^[21]^[11]^[20]

Axillary nerve

The axillary nerve originates from the posterior cord of the brachial plexus, derived from the ventral rami of the C5 and C6 spinal nerves, near the termination of the cord in the axilla.^[22] It arises posterior to the axillary artery and anterior to the subscapularis muscle before passing posteriorly to exit the axilla.^[22] The nerve travels through the quadrangular space, bounded superiorly by the teres minor muscle, inferiorly by the teres major muscle, laterally by the surgical neck of the humerus, and medially by the long head of the triceps brachii muscle.^[22] Accompanied by the posterior circumflex humeral artery and vein, it courses along the surgical neck of the humerus beneath the deltoid muscle.^[22] Upon reaching the posterior aspect of the surgical neck of the humerus, the axillary nerve divides into anterior and posterior branches.^[22] The anterior branch supplies the deltoid muscle, while the posterior branch innervates the teres minor muscle and provides a small articular branch to the shoulder joint.^[22] The sensory component of the axillary nerve forms the upper lateral cutaneous nerve of the arm, which arises from the posterior branch and pierces the deep fascia of the deltoid muscle to supply sensation to the skin over the lower portion of the deltoid region, often referred to as the "regimental badge" area.^[22] The axillary nerve measures approximately 4-5 cm in length from its origin at the posterior cord to its bifurcation into anterior and posterior branches.^[23] Anatomical variations include the site of division, with bifurcation occurring within the quadrangular space in about 88% of cases and within the deltoid muscle in 12%; additionally, rare communications with the radial nerve, forming a loop, have been observed in less than 5% of specimens.^[24]^[25]

Radial nerve

The radial nerve arises as the terminal continuation of the posterior cord of the brachial plexus, immediately distal to the origin of the axillary nerve, receiving contributions from the ventral rami of spinal nerves C5 through T1.^[26] As the largest branch of the posterior cord, it provides extensive motor and sensory innervation to the posterior aspects of the arm, forearm, and hand.^[27] In the axilla, the radial nerve descends along the posterior wall, positioned posterior to the axillary artery and anterior to the subscapularis, latissimus dorsi, and teres major muscles.^[26] It then passes through the triangular interval—bounded superiorly by the teres major, laterally by the humerus, and medially by the long head of the triceps brachii—into the posterior compartment of the arm.^[27] There, it courses obliquely in the radial groove on the posterior surface of the humerus, accompanied by the profunda brachii artery, lying between the lateral and medial heads of the triceps brachii.^[26] Prior to reaching the elbow, the radial nerve emits several sensory branches in the axilla and arm. These include the posterior cutaneous nerve of the arm, which arises in the axilla and supplies the skin over the posterior arm; the lower lateral cutaneous nerve of the arm, originating in the upper arm and innervating the skin of the lower lateral arm; and the posterior cutaneous nerve of the forearm, given off in the radial groove and providing sensation to the posterior forearm skin.^[27] Additionally, it issues muscular branches to the heads of the triceps brachii, with the branch to the long head emerging in the axilla and those to the lateral and medial heads arising proximally in the arm.^[26] Anatomical variations of the radial nerve include high branching to the triceps muscles, where additional motor twigs may originate more proximally than typical, and occasional piercing of the lateral head of the triceps brachii, reported in up to 10% of cases. These variations can influence surgical approaches in the axilla and arm.

Function

Motor innervation

The posterior cord of the brachial plexus, derived from the posterior divisions of C5-T1 roots, supplies motor innervation to key muscles of the shoulder girdle, arm, and posterior forearm, enabling a range of extension, rotation, and stabilization movements essential for upper limb function.^[4] Its branches collectively target the posterior compartments, promoting antagonistic actions to the flexors and adductors supplied by anterior divisions.^[28] In the shoulder girdle, the upper and lower subscapular nerves (C5-C6) innervate the subscapularis muscle, facilitating internal rotation of the humerus at the glenohumeral joint.^[4] The lower subscapular nerve additionally supplies the teres major, which contributes to adduction and internal rotation of the arm.^[28] The thoracodorsal nerve (C6-C8) provides exclusive motor input to the latissimus dorsi, enabling powerful adduction, extension, and internal rotation of the humerus, particularly during pulling motions.^[29] The axillary nerve (C5-C6) divides into anterior and posterior branches; the anterior branch innervates the deltoid muscle to support shoulder abduction and flexion (via its anterior fibers), while the posterior branch supplies the teres minor for external rotation and glenohumeral stabilization.^[22] For arm extension, the radial nerve (C5-T1) issues branches in the axilla to the long and medial heads of the triceps brachii, with additional branches in the radial groove targeting the lateral head, collectively enabling elbow extension against gravity.^[26] This nerve also innervates the anconeus muscle, which assists in elbow extension and stabilizes the ulnohumeral joint during forearm movements.^[26] In the forearm and hand, the radial nerve's superficial branch supplies the brachioradialis for elbow flexion in the mid-prone position, while its deep branch—the posterior interosseous nerve—innervates the extensor muscles of the posterior compartment, including the extensor carpi radialis longus and brevis (wrist extension and radial deviation), extensor digitorum (extension of digits 2-5), extensor digiti minimi (extension of the little finger), extensor carpi ulnaris (wrist extension and ulnar deviation), abductor pollicis longus (thumb abduction), extensor pollicis longus and brevis (thumb extension), and extensor indicis (index finger extension).^[26] These innervations allow for precise wrist and finger extension, thumb opposition, and overall hand stabilization during gripping tasks.^[28] Overall, the posterior cord plays a critical role in innervating the posterior compartment muscles of the upper limb, facilitating extension at the elbow, wrist, and digits, as well as rotational and stabilizing actions at the shoulder to counterbalance anterior compartment functions.^[4]

Sensory contributions

The posterior cord of the brachial plexus provides sensory innervation to the upper limb primarily through its terminal branches, the axillary and radial nerves, while the subscapular and thoracodorsal nerves are purely motor and contribute no sensory fibers.^[4] This sensory supply covers the posterior and lateral aspects of the arm, posterior forearm, and dorsolateral hand, facilitating cutaneous sensation such as touch and pain in these regions.^[4] The axillary nerve (C5-C6) gives rise to the upper lateral cutaneous nerve of the arm, which innervates the skin over the deltoid muscle and the upper lateral aspect of the humerus, often referred to as the "regimental badge area."^[30] This branch emerges after the nerve passes through the quadrangular space, providing sensation to the lateral shoulder skin without overlap from adjacent nerves in its primary territory.^[30] The radial nerve (C5-T1) contributes the majority of the posterior cord's sensory distribution through several cutaneous branches. The posterior cutaneous nerve of the arm supplies the skin of the mid-posterior humerus, while the lower lateral cutaneous nerve of the arm innervates the lower lateral arm extending to the proximal forearm.^[26] Further distally, the posterior cutaneous nerve of the forearm provides sensation to the posterior aspect of the forearm and wrist, and the superficial branch of the radial nerve innervates the dorsal surface of the thumb, index finger, middle finger, and the radial side of the hand up to the proximal interphalangeal joints.^[26] Collectively, these nerves from the posterior cord cover the posterior arm, posterior forearm, and dorsolateral hand, with dermatomal contributions primarily from C5 to C8 spinal segments and minimal involvement of T1.^[4] Clinical assessment of this sensory territory typically involves testing light touch or pinprick in the affected areas to evaluate integrity.^[4]

Clinical significance

Injuries

Injuries to the posterior cord of the brachial plexus most commonly result from high-energy trauma, such as motor vehicle accidents, falls, or penetrating wounds, which can cause stretching, compression, or laceration of the cord.^[31] Anterior shoulder dislocations and humeral shaft fractures are frequent mechanisms, often leading to associated injuries of the cord's branches like the axillary or radial nerves.^[32] Compression etiologies include axillary region tumors or prolonged pressure from crutches, while iatrogenic damage may occur during procedures such as axillary lymph node dissection for breast cancer, potentially affecting the thoracodorsal or axillary nerves.^[33] Posterior cord injuries are relatively rare in obstetric settings compared to upper trunk lesions, though they can occur in extended brachial plexus palsies during difficult deliveries.^[34] A complete posterior cord lesion typically presents with profound motor deficits, including weakness or paralysis of the deltoid and teres minor muscles (impairing shoulder abduction and external rotation), loss of elbow extension due to triceps involvement, and wrist drop from radial nerve palsy, preventing wrist and finger extension.^[32] Adduction and internal rotation deficits arise from involvement of the latissimus dorsi (via thoracodorsal nerve) and subscapularis/teres major (via upper and lower subscapular nerves).^[33] Sensory loss affects the posterior arm, forearm, and dorsal hand in the radial distribution, often accompanied by pain, numbness, or paresthesia; reflexes such as triceps and brachioradialis may be diminished or absent.^[34] Isolated branch injuries are more common than full cord lesions and often stem from specific traumas. The axillary nerve is the most commonly affected, with reported incidences ranging from 5% to 55%, leading to isolated deltoid weakness and sensory loss over the lateral shoulder "regimental badge" area without broader posterior cord signs.^[35]^[36] Radial nerve injuries, which arise from the posterior cord, frequently occur with mid-humeral shaft fractures (incidence 7-17%) or compression in "Saturday night palsy," where prolonged arm draping over a chair back causes wrist drop and finger extension loss while sparing proximal posterior cord functions like deltoid action.^[37] Diagnosis relies on clinical examination to identify the pattern of deficits, followed by electrodiagnostic studies. Electromyography (EMG) and nerve conduction studies demonstrate denervation and slowed conduction in muscles innervated by C5-T1 roots via the posterior cord, such as the deltoid, triceps, and extensor digitorum, helping localize the lesion proximal to peripheral branches.^[32] Magnetic resonance imaging (MRI) of the brachial plexus identifies structural causes like hematomas, tumors, or avulsions, while distinguishing posterior cord involvement from root or trunk injuries through targeted views of the axillary region.^[31]

Surgical considerations

Surgical exposure of the posterior cord typically occurs through the deltopectoral approach, which involves an incision along the deltopectoral groove to retract the deltoid laterally and the pectoralis major medially, providing access to the infraclavicular brachial plexus including the posterior cord and its branches.^[38] Alternatively, a posterior axillary approach can be employed for direct visualization of the posterior cord's terminal branches, such as the axillary and radial nerves, by incising along the posterior axillary line and mobilizing the teres major and latissimus dorsi muscles.^[39] During these exposures, the posterior cord is identified posterior to the axillary artery and slightly lateral to it, approximately 2-3 cm distal to the lower border of the pectoralis minor muscle, where the cords form around the second and third parts of the axillary artery.^[32]^[40] In brachial plexus exploration for trauma, the posterior cord is accessed to assess and repair injuries, often via combined supraclavicular and infraclavicular approaches to evaluate cord integrity and perform neurolysis or reconstruction.^[41] During axillary lymph node dissection for breast cancer, the posterior cord's branches—particularly the thoracodorsal and axillary nerves—are at risk of iatrogenic injury due to their proximity to level II and III nodes.^[42] Nerve transfers involving the posterior cord, such as using the thoracodorsal nerve to reinnervate the axillary nerve for deltoid reanimation in isolated axillary palsy or partial brachial plexus injuries, leverage the cord's accessibility in the axilla to restore shoulder abduction, achieving meaningful functional recovery in over 70% of cases when performed within 6-12 months of injury.^[43] Key anatomical landmarks facilitate safe dissection, with the subscapular artery serving as a reliable guide to the posterior cord, as it arises from the third part of the axillary artery and parallels the lower subscapular and thoracodorsal nerves along the posterior axillary wall.^[44] Anatomical variations, such as a high origin of the radial nerve directly from the posterior cord proximal to the usual takeoff, can increase dissection complexity by altering the cord's branching pattern and requiring extended proximal exposure to avoid incomplete repairs.^[45] Postoperative monitoring emphasizes radial nerve function, as its assessment through wrist extension strength and sensory testing in the dorsal hand can detect early palsy from compression or edema, guiding timely intervention to prevent permanent deficits.^[46] For posterior cord-level repairs, nerve grafting using sural or cable grafts yields success rates of approximately 70-80% for achieving antigravity strength (MRC grade ≥3) in motor recovery, particularly when graft lengths are limited to under 15 cm and surgery occurs within 6 months of injury.^[47]^[48]

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