The scapula, also known as the shoulder blade, is a flat triangular bone located at the back of the trunk and resides over the posterior surface of ribs two to seven. The scapula, along with the clavicle and the manubrium of the sternum, make up the pectoral (shoulder) girdle which connects the upper limb of the appendicular skeleton to the axial skeleton. The scapula is an important bone as each scapula provides a point of attachment for a number of muscles that make up the arm and shoulder. It also articulates with the humerus and clavicle, forming the glenohumeral (shoulder) joint and acromioclavicular joint respectively.
However, because the medial aspect of the scapula is not attached to the axial skeleton but are rather held in place to the thorax and vertebral column by muscles, the scapula can move freely across the posterior thoracic wall (scapulothoracic joint). This allows the arm to move with the scapula, providing a wide range of movement and mobility for the upper limb compared to the lower limb.
Borders and Angles
Like any triangle, the scapula consists of three borders: superior, lateral and medial. The superior border is the shortest and thinnest border of the three. The medial border is a thin border and runs parallel to the vertebral column and is therefore often called the vertebral border. The lateral border is often called the axillary border as it runs superolaterally towards the apex of the axilla and it is the thickest and strongest of the three borders for muscle attachment. It also has the glenoid cavity or socket along this border, a shallow fossa which articulates with the head of the humerus, forming the glenohumeral joint. There are also three angles to the scapula. The superior border meets the lateral border at the lateral angle and with the medial border at the superior angle. The third angle is the inferior angle where the medial and lateral borders meet.
The scapula has two surfaces; on the anterior aspect is the smooth costal surface which is concave in shape and is majorly taken up by the subscapular fossa. At the back of the scapula is the convex and uneven posterior surface which has a protruding ridge of bone (spine of the scapula) that unevenly separates it into two divisions: the superior supraspinous fossa and the much bigger, inferior infraspinous fossa.
Along with the spine, there are two more processes: the coracoid and acromion process. The coracoid process is a beak-like bent anterolateral projection from the superior border. Inferior to the coracoid process is the glenoid cavity, superiorly lies the lateral part of the clavicle and medial to the coracoid process is the suprascapular notch (for nerve passage) which connects the base of the coracoid process to the superior border. The coracoid process allows the attachment of various muscles and ligaments. Ligaments of the coracoid process are:
- Coracohumeral ligament - to the greater tubercle of the humerus
- Coracoclavicular ligament - to the clavicle
- Coracoacromial ligament - to the acromion process
The acromion process is a palpable lateral and enlarged extension of the posterior spine of the scapula which projects anterolaterally to the spine. It arches over the glenohumeral joint and articulates with the lateral acromial end of the clavicle to make up the synovial acromioclavicular joint. This joint is supported by the acromioclavicular ligament which attaches to the acromion process at one end and the clavicle at the other.
Blood supply to the posterior scapular region
Several arteries form an anastomosis to supply blood to this area:
- Suprascapular artery: a branch of the thyrocervical trunk, which in turn arises from the subclavian artery. It runs along the suprascapular nerve and mostly supplies the supraspinatus and infraspinatus muscles
- Posterior circumflex humeral artery: a branch of the axillary artery supplying the glenohumeral joint
- Circumflex scapular artery: originating from the subscapular artery, which in turn is a branch of the axillary artery
- Transverse cervical artery: a branch of the thyrocervical trunk running along the medial border of the scapula
Due to the large surface area of the scapula there are a large number of muscles attached (17 in total) which fix the scapula to the thoracic wall and allow it to move. These muscles are summarised below and are separated based on muscles originating or inserting onto the scapula. Four of these muscles form the rotator cuff, which covers the shoulder capsule (subscapularis, infraspinatus, teres minor and supraspinatus).
Muscles originating from the scapula, where they originate on the scapula, their action and innervation:
- Deltoid muscle – inferiorly along the scapula spine to the acromion (and lateral third of the clavicle). Flexion and medial rotation (anterior fibres), abduction (middle fibres), extension and lateral rotation (posterior fibres) at the shoulder joint. Axillary nerve.
- Supraspinatus muscle – supraspinous fossa. Abduction at the shoulder joint. Suprascapular nerve.
- Infraspinatus muscle – infraspinous fossa. Lateral rotation at the shoulder joint. Suprascapular nerve.
- Triceps brachii muscle (long head) – infraglenoid tubercle found on the lateral border inferior to the glenoid cavity. Elbow extension. Radial nerve.
- Teres minor muscle – lateral border of the posterior surface. Lateral rotation at the shoulder joint. Axillary nerve.
- Teres major muscle – posterior surface of the inferior angle and the lower part of the medial border. Adduction and medial rotation at the shoulder joint. Subscapular nerve.
- Latissimus dorsi muscle – inferior angle (inconstant). Adduction, extension and medial rotation at the shoulder joint. Thoracodorsal nerve.
- Coracobrachialis muscle – coracoid process. Adduction and flexion at the shoulder joint. Musculocutaneous nerve.
- Biceps brachii muscle (long and short head) – long head: supraglenoid tubercle; short head: coracoid process. Elbow flexion. Musculocutaneous nerve.
- Subscapularis muscle – subscapular fossa. Adduction and medial rotation at the shoulder joint. Subscapular nerve.
- Omohyoid muscle – superior border (adjacent to the suprascapular notch). Depression of hyoid bone. Ansa cervicalis (from cervical plexus).
Muscles inserting on the scapula, their insertion sites on the scapula, their action and innervation:
- Trapezius muscle – superiorly along the spine, acromion process (and clavicle). Elevation of the scapula, rotation of scapula during abduction of humerus beyond 90 degrees. Accessory nerve.
- Levator scapulae muscle – superior angle and medial border (superior to the spine). Elevation of scapula. Innervation by branches of C3-C5.
- Rhomboid major muscle – medial border (inferior to the spine). Elevation and retraction of scapula. Dorsal scapular nerve.
- Rhomboid minor muscle – above the scapular spine. Elevation and retraction of scapula. Dorsal scapular nerve.
- Serratus anterior muscle – along the medial border from the superior angle to inferior angle. Protraction and rotation of the scapula. Long thoracic nerve.
- Pectoralis minor muscle – coracoid process. Protraction and depression of the scapula. Medial pectoral nerve.