Scapula

The scapula is a flat triangular bone of the shoulder girdle that connects the humerus and the clavicle. It has three margins, two surfaces, and three angles.


Margins of scapula

The medial margin (also called the medial border, or vertebral border) is the longest of the three margins, extending from the superior angle to the inferior angle. Four muscles attach to the medial margin of the scapula - the serratus anterior, levator scapulae, rhomboid minor, and rhomboid major.

The lateral margin (also lateral border, or axillary border) is the thickest of the three margins. It starts at the lower margin of the glenoid cavity above, inclines obliquely downward and backward, reaching the inferior angle of the scapula. 

The lateral margin is marked by a rough impression known as the infraglenoid tuberosity. It serves as the origin site for the long head of the triceps brachii. The lateral margin also gives origin to the subscapularis muscle, and also to a few fibers of the teres major.

The superior margin (or superior border, upper border) is the shortest and thinnest of all three. It is marked by a deep, semicircular notch on its lateral part, called the scapular notch. The notch is converted into a foramen by the superior transverse scapular ligament. This foramen serves as a passage for the suprascapular nerve.


Surfaces of scapula

The anterior or costal surface of the scapula is slightly concave and projects forward. It is marked by the subscapular fossa, to which the subscapularis muscle attaches. Also ridges or lines of muscle attachment can be distinguished on the anterior surface.

The dorsal surface of the scapula is arched from above downward and is divided into two unequal parts by the spine of the scapula. The part above the spine is known as the supraspinous fossa, while below it is the infraspinous fossa. The two parts are connected by the spinoglenoid notch, which is positioned lateral to the root of the spine.

The spine of the scapula transverses the dorsal surface of the bone from its medial margin to the lateral margin. The spine terminates as the acromion, which forms the summit of the shoulder. 

The acromion is a large, somewhat triangular or oblong process, projecting at first laterally, and then curving forward and upward, so as to overhang the glenoid cavity. It is marked by the clavicular facet for articulation with the acromial end of the clavicle. The acromion gives origin to the acromial part of the deltoid muscle.


Angles

The superior angle of the scapula is formed by the junction of the superior and medial margins of the bone. It is located approximately at the level of the second thoracic vertebra (T2). The superior angle is covered by the trapezius muscle and it gives attachment to some fibers of the levator scapulae muscle. 

The inferior angle is the lowest part of the scapula and is formed by the junction of the medial and lateral margins. It is covered by the latissimus dorsi muscle. The posterior aspect of the inferior angle gives attachment to the teres major and often to a few fibers of the latissimus dorsi muscle.

The lateral angle of the scapula is also known as the glenoid angle or the head of the scapula. It is the thickest part of the scapula and it bears the glenoid cavity on its articular surface. The glenoid cavity articulates with the head of the humerus. There is a small projection called the supraglenoid tubercle at the upper border of the glenoid cavity.

Below the glenoid cavity lies the infraglenoid tubercle. The supraglenoid and infraglenoid tubercles serve as origin sites for the long heads of the triceps brachii and biceps brachii muscles. Medially to the glenoid cavity and adjacent to it, is a narrowed region called the neck of the scapula. The coracoid process lies above it.