Spine of scapula

The spine of scapula is a prominent, transverse bony ridge situated on the posterior surface of the scapula. It extends from the medial (vertebral) border laterally to the acromion, which then arches superiorly to overlie the glenohumeral joint and articulates with the clavicle at the acromioclavicular (AC) joint. This spine effectively divides the posterior scapular surface into two fossae: the supraspinous fossa superiorly and the infraspinous fossa inferiorly.

This flattened, ridge-like spine possesses both a superior and an inferior surface. The superior surface forms part of the supraspinous fossa, while the inferior surface contributes to the infraspinous fossa. The supraspinatus muscle, a rotator cuff muscle initiating arm abduction, originates from the supraspinous fossa and the adjacent superior surface of the spine. Conversely, the infraspinatus muscle, another rotator cuff muscle responsible for external (lateral) rotation of the arm, originates from the infraspinous fossa and the adjacent inferior surface of the scapular spine.

Anteriorly, the base of the spine is firmly attached to the posterior surface of the scapula. The posterior border of the spine, also known as the crest of the spine, is broad and features a superior lip, an inferior lip, and a rough interval between them. The trapezius muscle attaches to the superior lip of the crest and the adjoining part of the acromion. The deltoid muscle attaches along the inferior lip of the crest and the adjoining part of the acromion. The interval between the superior and inferior lips of the crest is subcutaneous and is partially covered by tendinous fibers of the trapezius and deltoid muscles.