Rotator Cuff Muscles & Shoulder Joint

The shoulder (glenohumeral) joint is the most mobile joint in the body and consequently the most unstable. It is sacrificed stability for mobility, and the rotator cuff is the dynamic stabiliser that compensates for this trade-off. This is one of the most reliably tested regions in NEET PG anatomy, integrating muscle attachments, nerve supply, dislocation mechanics, and resultant nerve injuries.

The Glenohumeral Joint — Quick Anatomy

• Type: Synovial, ball-and-socket (multiaxial).
• Articulation: Head of humerus with the glenoid cavity of the scapula, deepened by the fibrocartilaginous glenoid labrum.
• Mismatch: Only about one-third to one-fourth of the humeral head contacts the shallow glenoid at any time — hence the inherent instability.
• Capsule: Loose and lax inferiorly (allows abduction); attaches medially to the glenoid margin and laterally to the anatomical neck of the humerus.
• Stability factors: Glenoid labrum, glenohumeral ligaments (superior, middle, inferior — the inferior is the most important anterior stabiliser), coracohumeral ligament, the long head of biceps tendon (passes intracapsularly), and most importantly the rotator cuff muscles.

High-yield: The capsule is weakest inferiorly (anteroinferiorly), which is why dislocations occur in that direction. The rotator cuff reinforces the joint on all sides except inferiorly — the unprotected inferior aspect is the anatomical basis of dislocation.

The Rotator Cuff — SITS

The rotator cuff (musculotendinous cuff) is formed by four muscles whose tendons fuse with the joint capsule and hold the humeral head in the glenoid. Remember them by the mnemonic SITS:

• S — Supraspinatus
• I — Infraspinatus
• T — Teres minor
• S — Subscapularis

A useful clinical mnemonic for nerve supply: "SITS muscles get the suprascapular and axillary mostly, but the subscapular nerves run the front."

Insertion pattern — tubercles

• Supraspinatus, Infraspinatus, Teres minor → insert on the GREATER tubercle (superior, middle, and inferior facets respectively).
• Subscapularis → inserts on the LESSER tubercle.

High-yield: Three muscles go to the greater tubercle (Supraspinatus = superior facet, Infraspinatus = middle facet, Teres minor = inferior facet) and only one (Subscapularis) goes to the lesser tubercle. Mnemonic: "S-I-T from above down on the GREATER, Subscapularis alone on the LESSER."

Rotator Cuff Muscles — Comparison Table

Muscle	Origin (on scapula)	Insertion	Nerve supply	Root value	Main action
Supraspinatus	Supraspinous fossa	Superior facet, greater tubercle	Suprascapular nerve	C5, C6	Initiates abduction (0–15°)
Infraspinatus	Infraspinous fossa	Middle facet, greater tubercle	Suprascapular nerve	C5, C6	Lateral (external) rotation
Teres minor	Upper/middle lateral border	Inferior facet, greater tubercle	Axillary nerve	C5, C6	Lateral (external) rotation
Subscapularis	Subscapular fossa (costal surface)	Lesser tubercle	Upper & lower subscapular nerves	C5, C6, C7	Medial (internal) rotation

High-yield: Teres minor is the odd one out — it is the only rotator cuff muscle supplied by the axillary nerve (the other three are suprascapular ×2 and subscapular ×2). This is a classic single-best-answer trap.

High-yield: Suprascapular nerve supplies TWO rotator cuff muscles — supraspinatus and infraspinatus — and gives sensory branches to the shoulder joint. It does NOT supply any skin.

Detailed Muscle Actions

Supraspinatus

• Initiates the first 15° of abduction — this is the single most tested fact. After ~15°, the deltoid takes over as the main abductor, being maximally effective from 15° to 90°.
• Also acts as a "compressor" holding the humeral head against the glenoid (stabilising fulcrum for deltoid action).
• Beyond 90°, abduction continues by scapular rotation (trapezius + serratus anterior).

Flow of abduction: Supraspinatus (0–15°) → Deltoid (15°–90°) → Trapezius + Serratus anterior rotate scapula (90°–180°)

High-yield: A patient who cannot initiate abduction but can complete it (by leaning/swinging the arm to ~15° and then using deltoid) has a supraspinatus tear / suprascapular nerve lesion. A patient who can initiate but not continue abduction has a deltoid (axillary nerve) problem.

Infraspinatus & Teres minor

• Both are lateral (external) rotators of the humerus.
• Together with deltoid (posterior fibres) they resist posterior subluxation.

Subscapularis

• The only anterior rotator cuff muscle.
• Powerful medial (internal) rotator; reinforces the joint anteriorly.
• Its laxity/tear contributes to anterior instability.

Supraspinatus Tendon — The Vulnerable One

The supraspinatus tendon is the most commonly torn part of the rotator cuff and the site of most shoulder pathology.

• The tendon passes through a narrow space beneath the coraco-acromial arch (acromion + coracoacromial ligament + coracoid process).
• A "watershed" / critical zone of relative avascularity lies about 1 cm proximal to its insertion — predisposing to degeneration and tears.
• The subacromial (subdeltoid) bursa lies between the tendon and the acromion/deltoid; it reduces friction and is the site of subacromial bursitis.

Painful arc & impingement

• Painful arc syndrome: Pain felt between 60° and 120° of abduction, when the supraspinatus tendon/subacromial bursa is compressed against the coraco-acromial arch.
• Subacromial impingement results from repetitive overhead activity, a hooked (type III) acromion, or bursal/tendon swelling.

High-yield: Painful arc = 60°–120°. Pain in the last 10–20° (170–180°) instead suggests acromioclavicular joint pathology.

Clinical Tests Table

Test	Muscle/structure assessed	Method
Empty can (Jobe's) test	Supraspinatus	Arm abducted 90°, thumb pointing down; resist downward pressure
External rotation against resistance	Infraspinatus / teres minor	Elbow flexed 90° at side, resist outward rotation
Lift-off (Gerber's) test	Subscapularis	Dorsum of hand on lower back, push away from back
Drop-arm test	Large/complete cuff tear	Passively abduct arm, ask patient to lower slowly; sudden drop is positive
Hawkins–Kennedy / Neer test	Subacromial impingement	Forced internal rotation in flexion / forward flexion

Diagnosis & Investigation of Choice

• First-line imaging: Plain radiograph to exclude bony lesions, calcific tendinitis, acromial morphology, and to assess the acromiohumeral interval (reduced in chronic tears).
• Investigation of choice for rotator cuff tear: MRI — best for soft-tissue detail, tear size, retraction, and muscle atrophy/fatty degeneration.
• Ultrasound: Operator-dependent but excellent, dynamic, and cheap for cuff tears; good for partial tears.
• MR arthrography: Most sensitive for partial-thickness tears and labral (Bankart/SLAP) lesions.
• Arthroscopy: Gold standard (also therapeutic).

High-yield: MRI is the investigation of choice for rotator cuff and labral pathology. For suspected anterior instability with a Bankart lesion, MR arthrography is preferred.

Management / Treatment Principles

• Conservative (first-line for most partial tears, tendinopathy, impingement): Rest, NSAIDs/paracetamol for analgesia, physiotherapy (strengthening, scapular stabilisation), and subacromial corticosteroid injection.
• Surgical: Indicated for full-thickness tears in active/young patients, failed conservative treatment, or large/acute traumatic tears. Options — arthroscopic rotator cuff repair, subacromial decompression/acromioplasty.
• Calcific tendinitis: NSAIDs, needle barbotage, or ESWT.

Shoulder Dislocation — Direction & Mechanics

High-yield: The shoulder is the most commonly dislocated major joint, and >95% of dislocations are ANTERIOR (specifically antero-inferior, "subcoracoid").

• Mechanism: Force on an abducted, externally rotated, extended arm pushes the head antero-inferiorly through the weak inferior capsule.
• The head comes to lie in the subcoracoid position most commonly.
• Posterior dislocations are rare (~2–4%) and classically follow seizures, electrocution, and electric shock (sustained internal rotators overpower external rotators) — often missed on AP X-ray ("light bulb sign").
• Inferior dislocation = luxatio erecta (arm fixed overhead).

Associated lesions

• Bankart lesion: Avulsion of the anteroinferior glenoid labrum — the essential lesion of recurrent anterior dislocation.
• Hill–Sachs lesion: Compression fracture of the posterolateral humeral head caused by impaction against the glenoid rim.

Axillary Nerve Injury — The Critical Integration

This is the single most frequently asked integrated MCQ around the shoulder.

• The axillary nerve (C5, C6) is a terminal branch of the posterior cord of the brachial plexus.
• It winds around the surgical neck of the humerus, passing through the quadrangular space with the posterior circumflex humeral artery.
• Supplies: Deltoid and teres minor (motor); upper lateral cutaneous nerve of arm carries sensation over the lower deltoid — the "regimental badge" area.

Causes of axillary nerve injury: Anterior shoulder dislocation → Surgical neck fracture of humerus → Improper use of crutches → Intramuscular injection (incorrect site) into deltoid

Effects of axillary nerve palsy

• Loss/weakness of abduction (deltoid paralysed; only the initial 15° via supraspinatus may remain, but power is lost beyond).
• Deltoid atrophy → loss of rounded shoulder contour → "square shoulder/flat shoulder" deformity.
• Sensory loss over the regimental badge area (upper lateral arm).
• Weak lateral rotation (teres minor).

High-yield: Anterior shoulder dislocation injures the AXILLARY nerve, causing deltoid paralysis, square-shoulder deformity, and sensory loss over the regimental badge area. Always test deltoid sensation/power before and after reduction.

Quadrangular Space

Boundary	Structure
Superior	Teres minor (and capsule)
Inferior	Teres major
Medial	Long head of triceps brachii
Lateral	Surgical neck of humerus
Contents	Axillary nerve + posterior circumflex humeral artery

High-yield: Quadrangular space syndrome = compression of the axillary nerve in this space, causing posterior shoulder pain and patchy deltoid/teres minor weakness, classically in throwing athletes.

Complications

• Recurrent dislocation (especially in young patients with a Bankart lesion).
• Axillary nerve injury (deltoid weakness, square shoulder).
• Rotator cuff tear (more common with dislocation in the elderly).
• Adhesive capsulitis (frozen shoulder) — global restriction of active and passive movement, classically external rotation lost first; associated with diabetes mellitus.
• Avascular necrosis of the humeral head and axillary artery injury (rare).

Key Differentials of a Painful Shoulder

Condition	Distinguishing feature
Rotator cuff tear	Weakness on resisted testing; painful arc; positive drop-arm
Subacromial impingement	Painful arc 60–120°; positive Neer/Hawkins
Adhesive capsulitis	Both active and passive movements restricted; external rotation lost first
Acromioclavicular arthritis	Pain in last 10–20° of abduction; tenderness over ACJ
Calcific tendinitis	Acute severe pain; calcific deposit on X-ray
Referred pain (cardiac/diaphragm/cervical)	No local tenderness; movements full and painless

High-yield: Frozen shoulder restricts BOTH active and passive movement, whereas a rotator cuff tear typically restricts active movement with relatively preserved passive range. This single distinction answers many MCQs.

Recently asked / exam angle

• "Which muscle initiates abduction of the arm?" → Supraspinatus (0–15°).
• "Most common rotator cuff muscle to be torn?" → Supraspinatus.
• "Nerve injured in anterior shoulder dislocation?" → Axillary nerve (causing square-shoulder, regimental badge sensory loss).
• "Teres minor is supplied by which nerve?" → Axillary nerve (the odd SITS muscle out).
• "Which rotator cuff muscle inserts on the lesser tubercle?" → Subscapularis.
• "Contents of the quadrangular space?" → Axillary nerve + posterior circumflex humeral artery.
• "Painful arc syndrome range?" → 60°–120°.
• Image-based: identifying the Hill–Sachs (humeral head) vs Bankart (glenoid labrum) lesion.
• "Which part of the shoulder capsule is weakest?" → Inferior (antero-inferior).
• "Most commonly dislocated joint / direction of dislocation?" → Shoulder; antero-inferior (subcoracoid).

Rapid revision

1. SITS = Supraspinatus, Infraspinatus, Teres minor, Subscapularis — the rotator cuff.
2. Three insert on the greater tubercle (S, I, T); subscapularis on the lesser tubercle.
3. Supraspinatus initiates abduction (0–15°); deltoid does 15–90°; scapular rotators beyond 90°.
4. Suprascapular nerve supplies supraspinatus + infraspinatus; axillary nerve supplies teres minor (and deltoid).
5. Subscapularis = only medial rotator and only anterior cuff muscle; supplied by upper & lower subscapular nerves.
6. Glenohumeral capsule weakest inferiorly → dislocations are antero-inferior (>95%).
7. Anterior dislocation → axillary nerve injury → square shoulder + regimental badge sensory loss.
8. Posterior dislocation is classically caused by seizures and electric shock.
9. Bankart = anteroinferior labrum avulsion; Hill–Sachs = posterolateral humeral head dent.
10. MRI is the investigation of choice for cuff tears; MR arthrography for labral/partial lesions.
11. Painful arc 60°–120° = supraspinatus/subacromial impingement; last 10–20° = ACJ pathology.
12. Frozen shoulder restricts both active and passive movement (external rotation lost first) — distinguishes it from a cuff tear.