Shoulder and Scapular Region

Origin: Anterior fibres, lateral third of clavicle; middle fibres, acromion; posterior fibres, spine of scapula.

• Insertion: Deltoid tuberosity of humerus.
• Innervation: Axillary nerve (C5, C6).
• Blood supply: Posterior circumflex humeral artery, deltoid branch of thoracoacromial artery.
• Action: Anterior fibres, flexion and medial rotation; middle fibres, the main abductor of the shoulder beyond the first 15°; posterior fibres, extension and lateral rotation.
• Relations: Overlies the shoulder joint, subacromial bursa, and the quadrangular space contents (axillary nerve, posterior circumflex humeral vessels).

• Origin: Supraspinous fossa of scapula.
• Insertion: Superior facet of greater tubercle of humerus.
• Innervation: Suprascapular nerve (C5, C6).
• Blood supply: Suprascapular artery.
• Action: Initiates abduction (first 15°); works with deltoid throughout the rest of the range; stabilises the humeral head against the glenoid during deltoid contraction (prevents superior migration).
• Relations: Tendon passes beneath the coracoacromial arch, the most common site of rotator cuff impingement and tear.

• Origin: Infraspinous fossa of scapula.
• Insertion: Middle facet of greater tubercle of humerus.
• Innervation: Suprascapular nerve (C5, C6).
• Blood supply: Suprascapular artery, circumflex scapular artery.
• Action: Lateral (external) rotation of the shoulder; posterior stabiliser of the humeral head.

• Origin: Upper two-thirds of the lateral border of scapula (dorsal surface).
• Insertion: Inferior facet of greater tubercle of humerus.
• Innervation: Axillary nerve (C5, C6).
• Blood supply: Circumflex scapular artery, posterior circumflex humeral artery.
• Action: Lateral rotation; posterior stabiliser of humeral head.

• Origin: Lower third of the lateral border and inferior angle of scapula (dorsal surface).
• Insertion: Medial lip of intertubercular sulcus of humerus.
• Innervation: Lower subscapular nerve (C5, C6).
• Blood supply: Subscapular artery, circumflex scapular artery.
• Action: Adduction and medial rotation of the shoulder; not part of the rotator cuff.
• Relations: Forms the lower border of the triangular and quadrangular spaces; marks the transition of axillary artery to brachial artery (at its lower border).

• Origin: Subscapular fossa (costal surface of scapula).
• Insertion: Lesser tubercle of humerus.
• Innervation: Upper and lower subscapular nerves (C5, C6).
• Blood supply: Subscapular artery, circumflex scapular artery.
• Action: Medial (internal) rotation; the strongest rotator cuff muscle; anterior stabiliser of the humeral head.

Components: Supraspinatus, Infraspinatus, Teres minor, Subscapularis (mnemonic SITS).

Insertions: Form a musculotendinous cuff that blends with the glenohumeral joint capsule before inserting on the tubercles, supraspinatus (superior facet), infraspinatus (middle facet), teres minor (inferior facet) on the greater tubercle; subscapularis on the lesser tubercle.

Functions:

• Individually rotate the humerus (subscapularis medially; infraspinatus and teres minor laterally); supraspinatus assists abduction.
• Collectively, the prime dynamic stabilisers of the glenohumeral joint.

Contribution to stability:

• The cuff tendons compress the humeral head into the shallow glenoid cavity throughout the range of motion, counteracting the tendency of deltoid contraction to translate the head superiorly.
• This dynamic "concavity-compression" mechanism is more important for day-to-day joint stability than the static ligaments, especially in the mid-range of motion.

Quadrangular Space

• Boundaries:
• Superior: teres minor
• Inferior: teres major
• Medial: long head of triceps brachii
• Lateral: surgical neck of humerus
• Contents: Axillary nerve, posterior circumflex humeral artery and vein.

Triangular Space

• Boundaries:
• Superior: teres minor
• Inferior: teres major
• Lateral: long head of triceps brachii
• Contents: Circumflex scapular artery (and vein).

Triangular Interval

• Boundaries:
• Superior: lower border of teres major
• Medial: long head of triceps brachii
• Lateral: shaft of humerus (below the surgical neck; above the site where the radial nerve exits)
• Contents: Radial nerve, profunda brachii artery (deep artery of the arm).

These three spaces share the triceps and teres muscles as boundaries and are frequently tested together, quadrangular space = axillary nerve; triangular space = circumflex scapular artery; triangular interval = radial nerve.

• Type: Synovial ball-and-socket joint; the most mobile joint in the body, at the expense of inherent bony stability.

• Articulating surfaces: Head of humerus (large, roughly one-third of a sphere) and glenoid cavity of scapula (shallow, covers only about a third of the humeral head surface).

• Glenoid labrum: Fibrocartilaginous rim around the glenoid cavity; deepens the socket, increases congruency and stability; continuous with the long head of biceps tendon superiorly (site of SLAP lesions: superior labrum anterior to posterior tears).

• Capsule: Loose and lax, permitting the wide range of motion; attaches to the anatomical neck of the humerus (except inferiorly and medially, where it extends onto the surgical neck) and to the margins of the glenoid cavity/labrum. The axillary recess (inferior capsular fold) allows for abduction but is the site where the capsule is most vulnerable to tearing in dislocation.

• Ligaments:
• Glenohumeral ligaments (superior, middle, inferior): thickenings of the anterior capsule, main static anterior stabilisers.
• Coracohumeral ligament: coracoid process to greater tubercle, reinforces the superior capsule.
• Transverse humeral ligament: bridges the tubercles, holds the long head of biceps tendon in the intertubercular sulcus.

• Coracoacromial arch: Formed by the coracoid process, coracoacromial ligament, and acromion; a protective "roof" over the joint that prevents superior dislocation of the humeral head, but is also the site of subacromial impingement on the supraspinatus tendon and subacromial bursa.

• Bursae: Subacromial (subdeltoid) bursa: between deltoid/acromion and the rotator cuff, reduces friction; often communicates with the joint in rotator cuff tears. Subscapularis bursa: between subscapularis tendon and the scapular neck, usually communicates with the joint cavity.

• Blood supply: Anterior and posterior circumflex humeral arteries; suprascapular and circumflex scapular arteries.

• Innervation: Axillary and suprascapular nerves (Hilton's law, nerves crossing a joint also supply the joint and the muscles acting on it).

• Movements: Flexion/extension, abduction/adduction, medial/lateral rotation, circumduction, the widest range of any joint.

• Muscles responsible:

• Stability: Relies more on dynamic stabilisation by the rotator cuff than on bony congruency or static ligaments, given the shallow glenoid, trading stability for mobility. Static stabilisers (labrum, capsule, ligaments) become more important at the extremes of motion.

• Scapulohumeral rhythm: Coordinated movement between the glenohumeral and scapulothoracic joints during arm elevation; overall a roughly 2:1 ratio of glenohumeral to scapulothoracic motion (varies through range), allowing full overhead elevation beyond what the glenohumeral joint alone could achieve. Serratus anterior and trapezius drive the scapulothoracic component.

Anterior shoulder dislocation:

• Most common direction of shoulder dislocation (~95% of cases), due to the relatively weak anteroinferior capsule and the shallow glenoid.
• Typically occurs with the arm abducted and laterally rotated (e.g., forced abduction/external rotation, or falling on an outstretched hand).
• Often tears the anteroinferior labrum (Bankart lesion) and/or produces a posterolateral humeral head impaction fracture (Hill-Sachs lesion).

Axillary nerve injury:

• At risk in anterior shoulder dislocation and surgical neck fractures, due to its close relationship as it winds around the surgical neck through the quadrangular space.
• Motor loss: deltoid, teres minor, weak abduction beyond the first 15°.
• Sensory loss: regimental badge area (lower lateral shoulder).

Rotator cuff injury:

• Supraspinatus is most commonly torn, related to the relatively poor blood supply of its tendon near the insertion ("critical zone") and repetitive impingement beneath the coracoacromial arch.
• Presents with a painful arc of abduction (roughly 60–120°) and weakness initiating/maintaining abduction.
• Chronic tears can lead to superior migration of the humeral head against the acromion (loss of the cuff's depressor function).

Supraspinatus impingement:

• Compression of the supraspinatus tendon and subacromial bursa between the greater tubercle and the coracoacromial arch during abduction, especially with a hooked or degenerative acromion.
• Repetitive impingement is a common precursor to rotator cuff tendinopathy and full-thickness tears.