Scapula

Where is the scapula?

The scapula lies on the back of the upper thorax, over the ribs. Muscles cover both its front and its back, so it has no joint with the ribs. (StatPearls, Scapula; OpenStax A&P 2e, 8.1)

What are the parts of the scapula?

Its three borders are the superior, medial and lateral. The superior and inferior angles sit at the two ends of the medial border, and the third corner, between the superior and lateral borders, holds the glenoid cavity. (OpenStax A&P 2e, 8.1; StatPearls, Scapula; Gray’s Anatomy (1918), Scapula, p. 202)

On the back, the spine of the scapula crosses the upper part of the bone and extends sideways as the acromion. The hook-like coracoid process projects forward from the superior border and sits below the lateral end of the clavicle. (OpenStax A&P 2e, 8.1; StatPearls, Scapula; Gray’s Anatomy (1918), Scapula, p. 202 and Clavicle, p. 200)

Three fossae give muscles room: the narrow supraspinous fossa above the spine, the broad infraspinous fossa below it and the subscapular fossa on the front, against the ribs. (OpenStax A&P 2e, 8.1; StatPearls, Scapula)

What does the scapula do?

As part of the pectoral girdle, the scapula anchors the upper limb to the body. Its fossae give broad surfaces to the muscles that cross the shoulder joint to act on the humerus. (OpenStax A&P 2e, 8.1; StatPearls, Shoulder and Scapula)

Seventeen muscles attach to it. Between them they move the scapula six ways: elevation, depression, upward and downward rotation, retraction and protraction. (StatPearls, Scapula)

Each rotator cuff muscle runs from the scapula to the humerus: the supraspinatus to the top of the greater tubercle, the infraspinatus below it, the teres minor to the lowest part of the greater tubercle, and the subscapularis to the lesser tubercle. Together they keep the head of the humerus seated in the glenoid cavity. (OpenStax A&P 2e, 9.6; StatPearls, Scapula, Shoulder Muscles and Glenohumeral Joint)

Which joints does the scapula form?

The glenoid cavity meets the head of the humerus at the glenohumeral (shoulder) joint. The cavity is shallow and much smaller than the head: OpenStax puts it at about a third of the head’s size, and StatPearls gives the head four times its surface area. That mismatch and a loose capsule give the joint its wide range of motion and leave it prone to dislocation. (OpenStax A&P 2e, 8.1 and 9.6; StatPearls, Glenohumeral Joint; Gray’s Anatomy (1918), Shoulder joint, p. 317)

The acromion meets the lateral end of the clavicle at the acromioclavicular joint, and the coracoclavicular ligament ties the coracoid process to the underside of the clavicle. (OpenStax A&P 2e, 8.1; StatPearls, Acromioclavicular Joint; Gray’s Anatomy (1918), Acromioclavicular joint, p. 315)

Small tubercles just above and below the glenoid cavity anchor arm muscles; the long head of the triceps brachii arises from the lower one, the infraglenoid tubercle. (OpenStax A&P 2e, 8.1; StatPearls, Scapula)

The suprascapular nerve passes through the notch at the base of the coracoid process, under the superior transverse scapular ligament, while the suprascapular artery runs just above the ligament. (StatPearls, Scapula)

What variants of the scapula are there?

In os acromiale, a secondary ossification center in the acromion never fuses. Sprengel deformity, a scapula raised from birth, is rare, yet it is the most common variant of the scapula. (StatPearls, Scapula)