Attachments
The scapula is a mobile plate with muscles attached to every border, both surfaces and the coracoid process.
| Muscle | Attachments | Nerve | Main action |
|---|---|---|---|
| Subscapularis | Subscapular fossa to lesser tubercle | Upper and lower subscapular (C5–C6) | Internal rotation; anterior cuff |
| Serratus anterior | Outer surfaces of the upper eight ribs to the costal surface of the medial border, mostly the inferior angle | Long thoracic (C5–C7) | Protraction and upward rotation; holds the scapula to the chest |
| Pectoralis minor | Ribs 3–5 to the coracoid process | Medial pectoral (C8–T1) | Draws the scapula forwards and down |
| Short head of biceps | Tip of the coracoid to the radial tuberosity | Musculocutaneous | Flexes elbow, supinates, flexes shoulder |
| Coracobrachialis | Tip of the coracoid to the middle of the medial humerus | Musculocutaneous | Flexes and adducts the arm |
| Omohyoid (inferior belly) | Upper border of the scapula near the suprascapular notch | Ansa cervicalis | Depresses the hyoid |
| Rhomboid major | Spines of T2–T5 to the medial border below the root of the spine | Dorsal scapular (C5) | Retracts and elevates; rotates the glenoid down |
| Teres major | Dorsal surface of the inferior angle to the medial lip of the intertubercular sulcus | Lower subscapular | Adducts, extends and internally rotates the arm |
| Latissimus dorsi | Thoracolumbar fascia, lower thoracic spines, iliac crest and lower ribs, crossing the inferior angle, to the floor of the sulcus | Thoracodorsal (C6–C8) | Adducts, extends and internally rotates the arm |
Supraspinatus, infraspinatus and teres minor complete the attachments on the back of the scapula; with subscapularis they form the rotator cuff.
Structure: the coracoacromial arch
The coracoacromial arch is the roof over the shoulder joint, formed by the acromion behind, the coracoid process in front and the coracoacromial ligament between them.
The coracoacromial ligament is a strong triangular band from the lateral border of the coracoid to the tip of the acromion. Beneath the arch lie the subacromial bursa and the supraspinatus tendon, which glide under it during abduction. The arch prevents upward dislocation of the humeral head.
The coracoid process is the anterior anchor of the shoulder: pectoralis minor inserts on its medial border, the conjoint tendon of the short head of biceps and coracobrachialis arises from its tip, the coracoacromial ligament attaches to its lateral border, and the coracoclavicular ligament rises from its upper surface to the clavicle.
Relations
The scapular muscles form most of the walls of the axilla and separate the shoulder from the chest wall.
- Anterior wall of the axilla: pectoralis minor, under pectoralis major. It divides the axillary artery into three parts and the axillary lymph nodes into three levels.
- Posterior wall: subscapularis, teres major and latissimus dorsi.
- Medial wall: serratus anterior, with the long thoracic nerve running down its outer surface.
- Scapulothoracic plane: between serratus anterior and the chest wall, a gliding space rather than a true joint.
During abduction the scapula rotates upwards as the humerus moves, roughly one degree of scapular rotation for every two of glenohumeral movement. Serratus anterior and the upper and lower fibres of trapezius produce the rotation.
Clinical relevance
Weakness of the scapular muscles shows as winging, and problems under the arch show as impingement.
- Medial winging: long thoracic nerve palsy paralyses serratus anterior, so the medial border lifts off the chest when the patient pushes against a wall. The nerve is at risk in axillary clearance and chest drain insertion.
- Lateral winging: accessory nerve palsy weakens trapezius, and the inferior angle swings laterally.
- Subacromial impingement: the supraspinatus tendon and bursa are compressed under the arch, giving a painful arc of abduction. Decompression removes the under-surface of the acromion and may release the coracoacromial ligament.
- Subscapularis tear: the lift-off and belly-press tests show weak internal rotation; it may accompany anterior dislocation.
- Latarjet procedure: the coracoid tip with the conjoint tendon is transferred to the front of the glenoid for recurrent anterior instability, putting the musculocutaneous nerve at risk.
- Axillary node levels: level I lies lateral to pectoralis minor, level II behind it, and level III medial to it.
On the specimen
The shoulder stations show the scapula from the front and from behind, with the attached muscles cut near their origins.
- Costal surface: subscapularis fills the fossa; serratus anterior is the fleshy sheet along the medial border, its digitations running forwards to the ribs.
- Coracoid: pectoralis minor inserts on its medial border and runs down and medially to the ribs; the conjoint tendon of the short head of biceps and coracobrachialis hangs from its tip; the coracoacromial ligament runs laterally from it to the acromion.
- Upper border: the thin inferior belly of omohyoid arises near the suprascapular notch.
- Dorsal surface: supraspinatus above the spine, infraspinatus below it, teres minor along the upper lateral border, teres major from the inferior angle.
- Medial border: rhomboid major inserts below the root of the spine; latissimus dorsi sweeps across the inferior angle.