4.4 Pectoral Girdle, Axilla & Brachial Plexus

Pectoral Girdle

The pectoral (shoulder) girdle connects the upper limb to the trunk and consists of the clavicle and scapula. Unlike the pelvic girdle, it is mobile and lightly built, sacrificing stability for range of motion.

The clavicle is an S-shaped bone and the first bone to ossify (intramembranous ossification at weeks 5–6 of gestation) and one of the last to fuse (medial epiphysis around age 25). It is the only bony link between the upper limb and the axial skeleton, articulating medially at the sternoclavicular joint (with the manubrium and 1st costal cartilage) and laterally at the acromioclavicular joint (with the acromion of the scapula). Its middle third is the most common fracture site — the typical mechanism is a fall on the outstretched hand, and the fracture occurs between the attachments of the sternocleidomastoid (the medial fragment is pulled superiorly) and the deltoid and pectoralis major (the lateral fragment is pulled inferiorly and medially).

The scapula is a flat triangular bone with the spine separating the supraspinous and infraspinous fossae posteriorly and ending laterally in the acromion (articulates with the clavicle). The coracoid process projects anteriorly and gives attachment to the short head of biceps, coracobrachialis, and pectoralis minor. The glenoid cavity (shallow, receives the humeral head) is deepened by the glenoid labrum. The subscapular fossa is on the anterior (costal) surface.

Muscles of the Pectoral Region and Shoulder

• Pectoralis major — large fan, two heads (clavicular and sternocostal); adducts and medially rotates the arm, and flexes from the anatomical position; innervated by the lateral and medial pectoral nerves (C5–T1).
• Pectoralis minor — deep to major, from ribs 3–5 to the coracoid process; stabilizes and depresses the scapula and elevates the ribs during forced inspiration.
• Serratus anterior — "boxer's muscle," originates on ribs 1–9, inserts on the medial border of the scapula; protracts the scapula and holds it against the thorax; innervated by the long thoracic nerve (C5–C7). Injury produces a winged scapula.
• Deltoid — abducts the arm (after the first 15°, which the supraspinatus initiates); axillary nerve (C5–C6).
• Rotator cuff (SITS) — Supraspinatus (abduction initiation, suprascapular nerve), Infraspinatus (lateral rotation, suprascapular), Teres minor (lateral rotation, axillary), Subscapularis (medial rotation, upper and lower subscapular nerves); their tendons fuse with the joint capsule to stabilize the glenohumeral joint.

The Axilla

The axilla is a pyramidal space between the upper arm and the lateral thoracic wall, traversed by the axillary vessels, brachial plexus, and lymphatics. Its boundaries are:

Contents: axillary artery (divided by the pectoralis minor into three parts), axillary vein (medial to the artery), cords of the brachial plexus (lateral, medial, posterior — named by their relationship to the artery), and axillary lymph nodes (five groups: anterior [pectoral], posterior [subscapular], lateral [humeral], central, and apical — draining the upper limb and breast; the apical nodes receive all efferents).

Brachial Plexus

The brachial plexus arises from the ventral rami of C5–T1 and is organized into roots → trunks → divisions → cords → branches:

• Roots (C5–T1) — between scalenus anterior and medius; give the dorsal scapular nerve (C5, rhomboids) and long thoracic nerve (C5–C7, serratus anterior).
• Trunks — upper (C5–6), middle (C7), lower (C8–T1); the suprascapular nerve arises from the upper trunk (supraspinatus and infraspinatus).
• Divisions — each trunk splits into anterior and posterior divisions; anterior divisions supply flexors, posterior divisions supply extensors.
• Cords — named by their relation to the axillary artery: lateral (anterior divisions of upper + middle trunks → musculocutaneous, lateral root of median, lateral pectoral), medial (anterior division of lower trunk → ulnar, medial root of median, medial pectoral, medial brachial and antebrachial cutaneous), posterior (all three posterior divisions → axillary [deltoid, teres minor], radial [all extensors], thoracodorsal, upper and lower subscapular).
• Terminal branches — musculocutaneous (C5–C7, lateral cord) pierces coracobrachialis, supplies the anterior arm, and continues as the lateral cutaneous nerve of the forearm; median (C5–T1, lateral + medial cords) supplies most anterior forearm muscles and the thenar muscles; ulnar (C8–T1, medial cord) supplies most intrinsic hand muscles; radial (C5–T1, posterior cord) supplies all posterior/extensor compartments; axillary (C5–C6, posterior cord) supplies the deltoid and teres minor.

Clinical: Erb-Duchenne and Klumpke Palsies

An upper trunk injury (C5–C6), classically from a forceful downward pull on the shoulder (birth trauma or a fall onto the shoulder), is Erb-Duchenne palsy: the limb hangs adducted, medially rotated, elbow extended, forearm pronated, and hand laterally deviated — the "waiter's tip" posture — because the abductors (supraspinatus and deltoid via the suprascapular and axillary nerves), lateral rotators, biceps, and brachialis are denervated. A lower trunk injury (C8–T1, Klumpke palsy), often from pulling the arm upward, produces a claw hand (loss of intrinsic muscles) and may include Horner syndrome (ptosis, miosis, anhidrosis) if the T1 preganglionic sympathetic fibers to the head are damaged.