8.1 The Pectoral Girdle

Bones and Features of the Pectoral Girdle

The pectoral girdle (also called the shoulder girdle) consists of just two bones: the clavicle and the scapula. Together, they connect the upper limb to the axial skeleton and provide the structural base for shoulder movement. What makes this girdle unique is how little bony attachment it has to the trunk, which sacrifices some stability but gives you a huge range of motion in the arm.

Clavicle

The clavicle is an S-shaped bone that acts as a strut between the upper limb and the axial skeleton.

• The sternal (medial) end articulates with the manubrium of the sternum at the sternoclavicular (SC) joint. This is the only bony attachment of the entire pectoral girdle to the axial skeleton.
• The acromial (lateral) end articulates with the acromion process of the scapula at the acromioclavicular (AC) joint.
• On the inferior surface of the lateral clavicle, the conoid tubercle and trapezoid line serve as attachment points for the coracoclavicular ligament, which is the primary stabilizer of the AC joint.

Scapula

The scapula is a triangular, flat bone that lies on the posterior thoracic wall, roughly over ribs 2 through 7.

• Acromion process: Projects laterally and anteriorly from the spine of the scapula. It articulates with the clavicle at the AC joint and forms the bony "roof" over the shoulder.
• Coracoid process: A hook-shaped projection from the superior border of the scapula. It serves as an attachment point for muscles (pectoralis minor, coracobrachialis, short head of biceps brachii) and ligaments (coracoclavicular, coracoacromial).
• Glenoid cavity: A shallow socket on the lateral angle of the scapula. It articulates with the head of the humerus to form the glenohumeral joint (the shoulder joint). Its shallowness allows great mobility but provides relatively little bony stability.
• Spine of the scapula: A prominent bony ridge running horizontally across the posterior surface. It serves as an attachment point for the trapezius (superior border) and deltoid (inferior border).
• Other landmarks to know: the superior border, medial (vertebral) border, lateral (axillary) border, superior angle, inferior angle, supraspinous fossa, infraspinous fossa, and subscapular fossa (anterior surface).

Pectoral Girdle Attachment and Function

Attachment to the Axial Skeleton

The sternoclavicular joint is the sole bony connection between the pectoral girdle and the axial skeleton. This joint allows limited movement in multiple planes (elevation/depression, protraction/retraction, and some rotation), giving the clavicle enough freedom to follow the scapula's movements while still anchoring the girdle to the trunk.

Because the scapula itself has no direct bony articulation with the axial skeleton, it's held in place entirely by muscles. This is a key concept: the scapula essentially "floats" on the posterior thoracic wall.

Support for Upper Limb Movement

The clavicle and scapula together provide a stable platform for upper limb muscles like the deltoid and the rotator cuff group.

• The glenohumeral joint (glenoid cavity + humeral head) allows flexion, extension, abduction, adduction, and internal/external rotation of the arm. It's the most mobile joint in the body.
• The scapulothoracic "joint" is not a true synovial joint. It's a functional articulation between the anterior scapula and the thoracic wall. It allows the scapula to glide during arm movements (elevation, depression, protraction, retraction, and upward/downward rotation).
• Muscles attached to the clavicle and scapula (trapezius, serratus anterior, pectoralis minor, rhomboids) work to stabilize and reposition the scapula so the glenoid cavity stays optimally oriented as you move your arm.

Pectoral Girdle Movement and Muscles

Scapular Movements

The scapula can move in several directions along the thoracic wall:

• Elevation (shrugging up) and depression (pulling down)
• Protraction (sliding laterally, as when reaching forward) and retraction (pulling medially, as when squeezing shoulder blades together)
• Upward rotation (glenoid cavity tilts upward, essential for raising the arm overhead) and downward rotation (glenoid returns to resting position)

Clavicular Movements

The clavicle moves at the SC joint in coordination with scapular movement. It can elevate, depress, protract, retract, and rotate slightly. These movements aren't independent; they occur as the clavicle follows the scapula.

Key Muscles

• Trapezius: Elevates, retracts, depresses, and upwardly rotates the scapula (different fiber regions perform different actions)
• Rhomboids (major and minor): Retract and downwardly rotate the scapula
• Levator scapulae: Elevates the scapula and assists with downward rotation
• Serratus anterior: Protracts and upwardly rotates the scapula; holds the medial border of the scapula against the thoracic wall
• Pectoralis minor: Depresses and protracts the scapula; tilts it anteriorly

These muscles work as a coordinated group to position the scapula during any upper limb movement.

Injuries to the Clavicle and AC Joint

Clavicle Fractures

The clavicle is one of the most commonly fractured bones in the body.

• Mechanism: Usually a direct blow to the shoulder or a fall onto an outstretched hand (FOOSH injury).
• Location: Most fractures occur in the middle third of the clavicle, which is the thinnest part and lacks reinforcing ligament attachments.
• Signs and symptoms: Visible deformity (the lateral fragment often drops due to the weight of the arm), pain, swelling, and limited shoulder movement.
• Treatment: Mild, non-displaced fractures are typically managed with a sling for immobilization. Displaced or comminuted fractures may require surgical fixation (open reduction and internal fixation with plates and screws).

Acromioclavicular (AC) Joint Injuries

AC joint separations are common in contact sports and falls.

• Mechanism: Direct blow to the top of the shoulder or a fall onto an outstretched hand.
• Classification (based on degree of ligament damage and separation):
  1. Type I: Sprain of the AC ligament; no joint separation. Coracoclavicular ligaments intact.
  2. Type II: Partial tear of the AC ligament and sprain of the coracoclavicular ligaments; slight separation visible.
  3. Type III: Complete tear of both the AC and coracoclavicular ligaments; significant separation with the clavicle visibly elevated.
• Signs and symptoms: Pain at the top of the shoulder, swelling, and a visible bump or "step-off" deformity at the AC joint (especially in Types II and III).
• Treatment: Type I and II injuries are usually managed conservatively with rest, ice, and a sling. Type III injuries are treated on a case-by-case basis; some respond to conservative management, while others require surgical reconstruction (e.g., coracoclavicular ligament repair or graft reconstruction). Higher-grade separations (Types IV-VI exist but are less commonly tested at this level) almost always need surgery.