This is a large, flat, triangular plate of bone on the posterolateral aspect of the thorax, overlying the second to the seventh ribs. Suspended in muscles, the scapula is held in position by the strut-like clavicle, but retains great mobility relative to the thorax. Being a triangular bone it has three angles, three borders and two surfaces which support three bony processes.
Figure. The right scapula: (A) posterior view, (B) anterior view.
The costal surface which faces the ribs is slightly hollowed and ridged with a smooth, narrow strip along its entire medial border. It is also known as the subscapular fossa.
The dorsal surface faces posterolaterally and is divided by the spine of the scapula into a smaller supraspinous fossa above and a larger infraspinous fossa below. The supraspinous and infraspinous fossae communicate via the spinogle-noid notch between the lateral end of the spine and the neck of the scapula. The spine of the scapula has upper and lower free borders which diverge laterally enclosing the acromion.
The thin medial border lies between the inferior and superior angles, being slightly angled at the medial end of the spine. The lateral border is thicker, being deeply invested in muscles, and runs down from the infraglenoid tubercle below the glenoid fossa to meet the medial border at the inferior angle. The superior border, which is thin and sharp, is the shortest, and has the suprascapular notch at the junction with the root of the coracoid process.
Figure. (A) Left scapula, lateral view, (B) right clavicle, superior view, (C) right clavicle, inferior view.
Inferiorly the thick inferior angle lies over the seventh rib and is easily palpated. The superior angle lies at the junction of the medial and superior borders, whilst the lateral angle is truncated and broadened to support the head and glenoid fossa of the scapula.
The head of the scapula is an expanded part of the bone joined to a flat blade by a short inconspicuous neck. The glenoid fossa (or cavity) is found on the head as a shallow, pear-shaped concavity, facing anterolaterally. The glenoid fossa is broader below and articulates with the head of the humerus forming the shoulder (glenohumeral) joint. Immediately above the glenoid fossa is the supraglenoid tubercle.
The acromion, which is the expanded lateral end of the spine, is large and quadrilateral, projecting forwards at right angles to the spine. The lower border of the crest of the spine continues as the lateral border of the acromion; the junction of these two borders forms the palpable acromial angle. The upper border of the crest becomes continuous with the medial border of the acromion and presents an oval facet for articulation with the clavicle at the acromioclavicular joint. The superior surface of the acromion is flattened and subcutaneous.
The coracoid process is a hook-like projection with a broad base directed upwards and forwards from the upper part of the head, and a narrow more horizontal part which passes anterolaterally from the upper edge of the base. The tip lies below the junction of the middle and lateral thirds of the clavicle.
Starting at the lowest point, the inferior angle can readily be gripped between thumb and index finger and, if the subject relaxes sufficiently, can be lifted away from the thorax. The medial border can be followed along its whole length from inferior to superior angles. The spine of the scapula can be palpated as a small triangular area medially increasing in size as the fingers move laterally along it. The flat crest with its upper and lower borders can be identified. Continuing along the lower border of the crest to its most lateral point, the sharp 9 0° acromial angle can be felt; this continues as the palpable lateral border of the acromion. Running onto this lateral border, the flat upper surface of the acromion can be felt above the shoulder joint. The coracoid process can be palpated as an anterior projection below the junction of the middle and lateral thirds of the clavicle, and therefore is a useful reference point for surface marking the shoulder joint as it lies just medial to the joint line.
The scapula ossifies from a number of centres. The primary ossification centre appears in the region of the neck by the eighth week in utero, so that at birth the coracoid process, acromion, glenoid cavity, medial border and inferior angle are still cartilaginous. Secondary centres appear in each of these regions, except the coracoid, between the ages of 12 and 14 years, fusing with the body between 20 and 25 years. The secondary centre for the coracoid process, however, appears during the first year and fuses with the body between 12 and 14 years.