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Scapula

The scapula (Latin: scapula) is a flat triangular bone that connects the humerus and the clavicle. The scapula is also known as the shoulder bone, shoulder blade, wing bone, and blade bone. The name “scapula” derives from Latin, meaning “trowel” or “small shovel,” which it seemed to resemble.

Together with the clavicle, the scapula forms the shoulder girdle. It is positioned on the posterolateral aspect of the thoracic cage.

The scapula has three margins, two surfaces, and three angles.

Margins of scapula

The three margins (or borders) of the scapula are:

  • medial (vertebral) margin,
  • lateral (axillary) margin, and
  • superior margin, which is marked by the scapular notch.

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. This margin is marked by a rough impression known as the infraglenoid tuberosity, which serves as the origin site for the long head of the triceps brachii. The lateral margin also gives origin to the subscapularis and teres minor, and even 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, which 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 scapula is a flat bone that has two surfaces:

  • anterior (costal) surface, and
  • dorsal surface.

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 seen on the anterior surface.

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

  • The supraspinous fossa is the smallest of the two fossae, and its medial two-thirds give origin to the supraspinatus muscle. 
  • The infraspinous fossa is much larger than the supraspinous fossa. Its medial two-thirds give origin to the infraspinatus muscle, and this muscle covers the lateral one-third of this fossa.

The spine of the scapula transverses the dorsal surface of the bone from the 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, 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 of scapula

The scapula has a triangular shape, so it has three angles:

  • superior (upper) angle,
  • inferior (lower) angle, and
  • lateral (genoid) angle.

The superior angle of the scapula is formed by the fusion of the superior and medial margins. It is located approximately at the level of the second thoracic vertebra. The trapezius muscle covers the superior angle. The angle also gives attachment to a few 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. The latissimus dorsi muscle covers this angle. 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 cavity lies the infraglenoid tubercle. These tubercles serve as origin sites for the long heads of the muscles m. triceps brachii and m. biceps brachii. 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.