Attachments
The posterior shoulder muscles run from the neck and back to the scapula, and from the scapula to the humerus.
| Muscle | Origin | Insertion | Nerve | Main action |
|---|---|---|---|---|
| Trapezius | Superior nuchal line, external occipital protuberance, ligamentum nuchae, spines of C7–T12 | Lateral third of clavicle, acromion, spine of scapula | Accessory nerve; C3–C4 sensory | Elevates, retracts and upwardly rotates the scapula |
| Levator scapulae | Transverse processes of C1–C4 | Medial border from superior angle to root of spine | C3–C4; dorsal scapular (C5) | Elevates the scapula, rotates the glenoid down |
| Deltoid | Lateral third of clavicle, acromion, spine of scapula | Deltoid tuberosity | Axillary (C5–C6) | Abducts the arm; anterior fibres flex, posterior fibres extend |
| Supraspinatus | Supraspinous fossa | Superior facet of greater tubercle | Suprascapular (C5–C6) | Starts abduction; stabilises the head |
| Infraspinatus | Infraspinous fossa | Middle facet of greater tubercle | Suprascapular (C5–C6) | External rotation |
| Teres minor | Upper two-thirds of lateral border, dorsal surface | Inferior facet of greater tubercle | Axillary (C5–C6) | External rotation |
Trapezius inserts along the same continuous line from which deltoid arises: the lateral clavicle, acromion and spine of the scapula. Supraspinatus, infraspinatus and teres minor are three of the four rotator cuff muscles; subscapularis, on the front of the scapula, is the fourth.
Relations
Under the superficial layer, the posterior shoulder contains three gaps and two notches that carry its nerves and vessels.
- Suprascapular notch: on the upper border of the scapula, bridged by the superior transverse scapular ligament. The suprascapular nerve passes under the ligament and the suprascapular artery over it.
- Spinoglenoid notch: between the lateral edge of the spine and the glenoid, where the nerve and artery pass from supraspinatus down to infraspinatus.
- Quadrangular space: between teres minor above, teres major below, the long head of triceps medially and the humerus laterally; it carries the axillary nerve and posterior circumflex humeral artery.
- Triangular space: medial to the long head of triceps, between teres minor and teres major; it carries the circumflex scapular artery.
- Triangular interval: below teres major, between the long head of triceps and the humerus; it carries the radial nerve and profunda brachii artery.
Above the scapula, the accessory nerve crosses the posterior triangle of the neck on the surface of levator scapulae to reach the deep surface of trapezius.
Blood supply and innervation
The posterior shoulder muscles are fed by the scapular anastomosis, a network round the scapula that links the subclavian and axillary arteries.
The suprascapular artery and the dorsal scapular artery (or deep branch of the transverse cervical) from the subclavian system join the circumflex scapular branch of the subscapular artery from the third part of the axillary artery. This collateral route keeps the arm perfused when the axillary artery is blocked above the subscapular origin. Trapezius also receives the superficial branch of the transverse cervical artery, and deltoid the posterior circumflex humeral artery.
The nerve supply spans four nerves: the accessory nerve (cranial nerve XI) for trapezius, the suprascapular nerve from the upper trunk for supraspinatus and infraspinatus, the axillary nerve from the posterior cord for deltoid and teres minor, and cervical rami with the dorsal scapular nerve for levator scapulae.
Clinical relevance
Each nerve of the posterior shoulder has a typical injury and a matching pattern of weakness.
- Accessory nerve palsy: most often after lymph node biopsy in the posterior triangle. Trapezius wastes, the shoulder droops, shrugging and overhead abduction weaken, and the scapula wings mildly on abduction.
- Suprascapular nerve entrapment: compression at the suprascapular notch weakens and wastes both supraspinatus and infraspinatus; compression at the spinoglenoid notch, often by a ganglion from a labral tear, affects infraspinatus alone.
- Axillary nerve injury: after anterior dislocation or surgical neck fracture; deltoid wastes and abduction beyond the first few degrees is weak.
- Rotator cuff tear: supraspinatus is the tendon most often torn, giving a painful arc and weakness on the empty-can test. Infraspinatus and teres minor are tested by resisted external rotation with the elbow at the side.
- Injections: intramuscular injections go into the upper part of deltoid, clear of the axillary nerve below.
On the specimen
The posterior shoulder station shows the scapula from behind with trapezius and deltoid reflected or cut, exposing the cuff muscles.
- Trapezius: the broad flat sheet from the midline to the spine of the scapula; its upper fibres run down to the clavicle.
- Levator scapulae: the strap running up from the superior angle towards the neck, deep to trapezius.
- Deltoid: the thick muscle capping the shoulder from the spine and acromion down to the lateral humerus.
- Supraspinatus: in the fossa above the spine, disappearing under the acromion.
- Infraspinatus: filling the fossa below the spine.
- Teres minor: the narrow muscle along the upper lateral border, immediately below infraspinatus and often fused with it. Teres major is the thicker muscle below it from the inferior angle; the long head of triceps passes between the two.