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 directly attached to the axial skeleton, but is rather held in place and connected 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 & 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. 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 that projects anterolaterally 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.
The 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.
Several arteries form an anastomosis to supply blood to the posterior scapular region:
- 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).
Originating from the Scapula
- Deltoid muscle – it originates inferiorly along the scapula spine to the acromion (and lateral third of the clavicle). Its actions include flexion and medial rotation (anterior fibres), abduction (middle fibres), extension and lateral rotation (posterior fibres) at the shoulder joint. It is innervated by the axillary nerve.
- Supraspinatus muscle – it originates from supraspinous fossa. It is responsible for abduction at the shoulder joint and it is innervated by the suprascapular nerve.
- Infraspinatus muscle – it originates from the infraspinous fossa. Its action involves lateral rotation at the shoulder joint. The muscle is also innervated by the suprascapular nerve.
- Triceps brachii muscle (long head) – its origin is the infraglenoid tubercle found on the lateral border, inferior to the glenoid cavity. It is responsible for elbow extension and it is innervated by the radial nerve.
- Teres minor muscle – it originates from the lateral border of the posterior surface. Its action consists of lateral rotation at the shoulder joint. This muscle is innervated by the axillary nerve.
- Teres major muscle – its origins are the posterior surface of the inferior angle and the lower part of the medial border. Its role is to perform adduction and medial rotation at the shoulder joint. It is innervated by the subscapular nerve.
- Latissimus dorsi muscle – it originates from the inferior angle (inconstant). It performs a variety of actions, such as adduction, extension and medial rotation at the shoulder joint. It receives its innervation via the thoracodorsal nerve.
- Coracobrachialis muscle – its origin is the coracoid process. Its actions include adduction and flexion at the shoulder joint. It is innervated by the musculocutaneous nerve.
- Biceps brachii muscle (long and short head) – the long head originates from the supraglenoid tubercle, while the short head from the coracoid process. This muscle is responbile for elbow flexion. It is innervated by the musculocutaneous nerve.
- Subscapularis muscle – it originates from the subscapular fossa. It performs adduction and medial rotation at the shoulder joint. The subscapular nerve innervates it.
- Omohyoid muscle – its origin is the superior border (adjacent to the suprascapular notch) and causes depression of hyoid bone. It is innervated via the Ansa cervicalis (from cervical plexus).
Inserting on the Scapula
- Trapezius muscle – it inserts superiorly along the spine, acromion process, and clavicle. Its actions include elevation of the scapula and rotation of scapula during abduction of humerus beyond 90 degrees. It is innervated by the accessory nerve.
- Levator scapulae muscle – they insert into the superior angle and medial border (superior to the spine). Their roles are to elevation the scapula. They are innervation by branches of C3-C5.
- Rhomboid major muscle – its insertion is the medial border (inferior to the spine). This muscle performs elevation and retraction of scapula and it is innervated by the dorsal scapular nerve.
- Rhomboid minor muscle – it inserts above the scapular spine. It performs actions like elevation and retraction of the scapula. It is supplied via the dorsal scapular nerve.
- Serratus anterior muscle – its insertion is along the medial border, from the superior angle to the inferior angle. This muscle protracts and rotates the scapula. It is innervated by the long thoracic nerve.
- Pectoralis minor muscle – it inserts into the coracoid process. Its actions consist of protraction and depression of the scapula. The muscle is innervated by the medial pectoral nerve.
The most common form is winging of the scapula. Surgery to the axilla, e.g. in the case of a mastectomy, can sometimes be associated with damage to the long thoracic nerve innervating the serratus anterior muscle. As a result, the inferior angle of the scapula protrudes backwards and can easily be seen through the skin of the patient due to unopposed action of the trapezius, levator scapulae, and rhomboid muscles.
Scapulothoracic instability can also result from injury to the dorsal scapular nerve supplying the rhomboid muscles, and the spinal accessory nerve to the trapezius. Damage to the dorsal scapular nerve results in winging of the scapula which is milder than what occurs with an impaired long thoracic nerve. Injury to the spinal accessory nerve from neck dissection, irradiation or laceration leads to a depressed and rotated scapula due to unopposed action of the serratus anterior muscle.
Another cause of winging of the scapula is fascioscapulohumeral dystrophy, an autosomal dominant condition affecting several muscles related to the scapula: serratus anterior, rhomboids, trapezius, teres major and minor, pectoralis minor and major, biceps, and triceps muscle. As a result, only the deltoid can move the shoulder and winging of the scapula occurs.
Scapular dysplasia describes an abnormal morphology of the scapula which can either be primary or acquired, secondary to obstetric brachial plexus palsy. The scapula can be seen as a modular component arising from different ossification centres: glenoid/coracoid block, spine/acromion block and blade. Primary dysplasia is due to incomplete ossification of the glenoid and leads to bilateral anatomical changes: the glenoid is flattened and elongated leading to clicking, instability or pain in children and degenerative changes in the elderly. Morphological changes of the scapula can also be seen in infants featuring a brachial plexus injury at the time of delivery due to an abnormal development of the cartilage of the posterior glenoid.
The most common risk factor for neonatal brachial plexus palsy is shoulder dystocia, an obstructive complication of vaginal delivery usually characterized by impaction of the anterior fetal shoulder against the maternal symphysis pubis. Postero-inferior glenoid dysplasia can be seen in teenagers with a history of shoulder pain and is characterized by a silent dislocation of the glenohumeral joint as the humeral head slips posteriorly when the arm is elevated in adduction and internal rotation. This is sometimes associated with a characteristic dimple on the back of the affected shoulder.
Snapping Scapula Syndrome
For the scapula to smoothly glide over the chest wall (termed the scapulothoracic joint) there are a number a muscles that lie between the ribs and scapular to facilitate this. Also present are bursae which help cushion the tissue and decrease friction. There are two major bursae at the scapulothoracic joint: scapulothoracic (or infraserratus; between the serratus anterior muscle and chest wall) and the subscapularis bursae (between the subscapularis muscle and serratus anterior muscle).
Snapping scapula syndrome is when there is abnormality at the scapulothoracic joint which leads to non-smooth articulation. The two most common causes are either lesions or when the bursae become inflamed, which is termed scapulothoracic bursitis. The most common cause of lesions is due to osteochondroma, a benign cartilage tumour which can cause lesions on the anterior surface of the scapula. Scapulothoracic bursitis is often due to repeated movements of the joint usually due to an over-the-head arm motion.
Like any bone, the scapula is subjective to fractures. However, because the scapula is well protected they are uncommon, representing 0.5 to 1% of all fractures. Anteriorly, the scapula is protected by the rib cage and thoracic cavity and posteriorly. It is also covered with a lot of soft tissue (i.e. muscle). Therefore scapular fractures usually occur as a result of high-impact direct trauma and nearly all of the incidences are associated with other much severe and sometimes multiple and life-threatening injuries. Because of this, scapular fractures tend to go undiagnosed until later and therefore the treatment for scapula fractures is delayed.