Scapula (Shoulder blade) : Structure, Attachments, Side Determination, Ossification, Functions & Clinical Anatomy
Overview
- The scapula is a thin bone placed on the posterolateral aspect of the thoracic cage that connects the humerus (upper arm bone) with the clavicle (collar bone).
Structure
The scapula has two surfaces, three borders, three angles, and three processes.
Surfaces
- Costal surface or subscapular fossa is concave and is directed medially and forwards. It is marked by three longitudinal ridges. Another thick ridge adjoins the lateral border. This part of the bone is almost rod-like: It acts as a lever for the action of the serratus anterior in overhead abduction of the arm.
- Dorsal surface gives attachment to the spine of the scapula which divides the surface into a smaller supraspinous fossa and a larger infraspinous fossa. The two fossae are connected by the spinoglenoid notch, situated lateral to the root of the spine.
Borders
- Superior border is thin and shorter. Near the root of the coracoid process it presents the suprascapular notch.
- Lateral border is thick. At the upper end it presents the infraglenoid tubercle.
Angles
- Superior angle is covered by the trapezius.
- Inferior angle is covered by the latissimus dorsi. It moves forwards round the chest when the arm is abducted.
- Lateral or glenoid angle is broad and bears the glenoid cavity or fossa, which is directed forwards, laterally and slightly upwards.
Processes
- Spine or spinous process is a triangular plate of bone with three borders and two surfaces. It divides the dorsal surface of the scapula into the supraspinous and infraspinous fossae. Its posterior border is called the crest of the spine. The crest has upper and lower lips.
- Acromion process has two borders, medial and lateral; two surfaces, superior and inferior; and a facet for the clavicle.
- Coracoid process is directed forwards and slightly laterally.
Side Determination
- The lateral or glenoid angle is large and bears the glenoid cavity.
- The dorsal surface is convex and is divided by the triangular spine into the supraspinous and infraspinous fossae. The costal surface is concave to fit on the convex chest wall.
- The lateral thickest border runs from the glenoid cavity above to the inferior angle below.
Attachments
Ossification
The scapula ossifies from one primary centre and seven secondary centres.
- The primary centre appears near the glenoid cavity during the eighth week of development.
- The first secondary centre appears in the middle of the coracoid process during the first year and fuses by the 15th year.
- The subcoracoid centre appears in the root of the coracoid process during the 10th year and fuses by the 16th to 18th years (Fig. 2.15).
- The other centres, including two for the acromion, one for the lower two-thirds of the margin of the glenoid cavity, one for the medial border and one for the inferior angle, appear at puberty and fuse by the 25th year.
- The fact of practical importance is concerned with the acromion. If the two centres appearing for acromion fail to unite, it may be interpreted as a fracture on radiological examination. In such cases a radiograph of the opposite acromion will mostly reveal similar failure of union.
Clinical Anatomy
- ‘Winging’ of the scapula : Paralysis of the serratus anterior causes ‘winging’ of the scapula. The medial border of the bone becomes unduly prominent, and the arm cannot be abducted beyond 90 degrees.
- Scaphoid scapula : In a developmental anomaly called scaphoid scapula, the medial border is concave.
- Impingement syndrome
- Scapular fractures
