The scapula (shoulder blade) is a large triangular flat bone which lies over the posterolateral aspect of the thorax. It connects the clavicle and humerus and hence the arm and thorax. It has no direct attachment to the thorax itself, lying free to glide across the posterior surface of the ribs. It usually covers an area from the second to the seventh rib.
Costal (above) and dorsal (below) surfaces of the scapula
|Search for scapula in Gray's.|
The majority of the scapula is palpable subcutaneously. The medial border of the scapular is obvious on inspection and palpation. The acromion can be felt as the most lateral portion of the scapular spine, lying immediately above the swelling of the deltoid muscle. The coracoid process lies immediately inferior to the clavicle at the junction of the middle and outer thirds, its shape obscured by the anterior fibres of the deltoid.
The scapula has two surfaces, costal and dorsal. The costal suface, anteromedially, is smooth and largely featureless, being dominated by the subscapular fossa. It has a rounded ridge near the lateral border, which arches anteriorly at its superior extreme to form a border of the glenoid fossa. The posterior dorsal surface is divided into two fossae by the triangular scapular spine. Above the spine is the supraspinous fossa, whilst below is the infraspinous fossa; these two communicate via the spinoglenoid notch between the lateral border of the spine and the neck of the scapula.
The scapula has three borders - lateral, medial and superior. The lateral border runs from the inferior angle to the glenoid cavity, and has an adjacent rough area on the dorsum for muscle attachments. It widens superiorly to form the infraglenoid tubercle. The medial border runs from the inferior to the superior angle, and the short superior border which is separated laterally from the coracoid by the suprascapular notch.
The head of the scapula is also the lateral angle, which bears the glenoid cavity. The neck is the area medial to this, is the constriction that adjoins the glenoid, being most distinct dorsally and inferiorly. The neck extends between the infraglenoid and supraglenoid tubercles, laterally to the root of the coracoid process.
Two processes project superiorly to the lateral angle of the scapula:
A number of muscles originate from or insert into the scapula.
- All four muscles of the rotator cuff:
- Teres major originates from the lower third ('oval area') of the lateral side of the inferior angle, below teres minor
- Coracobrachialis arises from the coracoid process with the long head of biceps brachii
- Biceps brachii has two heads, which both arise from the scapula:
- The long head arises from the supraglenoid tubercle
- The short head arises from the coracoid process with coracobrachialis.
- Triceps has three heads, only the long head arising from the scapula; this originates from the infraglenoid tubercle
- Latissimus dorsi originates from a large area, the most superior point being the inferior angle of the scapula
- The inferior belly of omohyoid originates from a small area of the superior border
- The deltoid muscle originates from the acromion and spine of the scapula to the deltoid tubercle
- Levator scapulae inserts into the upper part of the medial border
- Rhomboid major inserts into the lower half of the dorsal medial border, from the inferior angle to the base of the scapular spine
- Rhomboid minor inserts into a small area of the dorsal medial border at the level of the spine, below levator scapulae
- Serratus anterior has eight digitations that insert into different parts of the scapula:
- The first inserts into the costal and dorsal surface of the superior angle
- The next two or three converge to attach to almost the entire costal surface of the medial border
- The last four or five converge and attach to the inferior angle, mostly on the costal surface but also a small area dorsally.
- The medial fibres of trapezius insert into the acromion and lateral spine; the lower fibres insert into the medial spine as far as the deltoid tubercle
- Pectoralis minor inserts into the medial and upper surface of the coracoid process.
Two ligaments run from origin to insertion across the scapula:
The anterior aspect of the neck is covered by the subscapular bursa. A subacromial bursa lies inferiorly to the acromion. The joint capsule of the glenohumeral joint attaches to the edge of the glenoid fossa.
In addition, the scapula glides over the posterior surface of the thoracic cage, allowing movements of elevation, depression, protraction, retraction, lateral rotation and medial rotation.
Most of the scapula is surrounded by muscular tissue and so does not directly adjoin any other structure. The costal surface lies over the second to ninth ribs in the anatomical position.
The circumflex scapular vessels pass into the infraspinous fossa through a groove in the lateral border.
The scapula is ossified from eight or more centres. These appear with two in the coracoid and acromion, and one on the medial border, inferior angle, body and glenoid rim. Ossification usually begins in the first year from the coracoid. Most centres appear at puberty; all epiphyses fuse by the twentieth year.
The slope and length of the acromion are variable giving rise to degenerative change if the height of the coracoacromial arch is relatively small.
A spinoglenoid ligament is variably present.
- May be fractured.