Shoulder Region
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Direct answer
The glenohumeral joint is the most mobile and the most frequently dislocated joint in the body, anteriorly in the great majority of cases — the humeral head comes to lie below the coracoid process, threatening the axillary nerve. Stability depends less on the shallow glenoid cavity than on the rotator cuff (supraspinatus, infraspinatus, teres minor, subscapularis) and the glenoid labrum, which is why FMGE stems pair shoulder trauma with deltoid paralysis and regimental badge numbness. Abduction beyond 90 degrees is a scapular movement powered by trapezius and serratus anterior.
What you must remember
- Abduction arc: supraspinatus initiates the first 15 degrees, deltoid (axillary nerve, C5–C6) carries the arm from 15 to 90 degrees, and beyond 90 degrees the scapula rotates through trapezius (CN XI) and serratus anterior (long thoracic nerve).
- Rotator cuff order back to front spells SITS — supraspinatus, infraspinatus, teres minor, subscapularis; the supraspinatus tendon passes beneath the coracoacromial arch and degenerates first, the commonest cuff tear.
- Anterior dislocation damages the anteroinferior labrum (Bankart lesion) and impacts the posterolateral humeral head (Hill–Sachs lesion), the anatomy of recurrent dislocation.
- The axillary nerve runs through the quadrangular space — subscapularis and teres minor above, teres major below, long head of triceps medially, surgical neck of humerus laterally — with the posterior circumflex humeral artery.
- The scapular anastomosis links the subscapular artery (third part of axillary) with the suprascapular and transverse cervical branches of the subclavian, the collateral route that preserves the limb after axillary ligation.
- The subacromial bursa, the largest bursa of the body, does not normally communicate with the joint; a painful arc between 60 and 120 degrees points to it or to the supraspinatus tendon.
- The sternoclavicular joint is the only articulation between upper limb and axial skeleton; the long head of biceps arises from the supraglenoid tubercle and traverses the joint cavity.
A typical exam case
A 22-year-old cricketer lands on the outstretched hand; the shoulder loses its square profile and the arm is held slightly abducted, with no power to abduct further and numbness over the lower deltoid. The reasoning the options demand runs like this. Loss of the rounded contour with a palpable emptiness below the acromion is anterior dislocation — the capsule is weakest inferiorly and the mechanism is abduction plus external rotation. Deltoid weakness here is almost always neurapraxia of the axillary nerve stretched over the dislocated head, and the numb strip over the lower deltoid (the regimental badge patch) confirms it; document power and that skin before reduction and retest after, since most deficits recover. If the stem instead described an elderly person after a fall on the point of the shoulder who cannot abduct but has intact sensation, the answer shifts from nerve to a rotator cuff tear or a surgical neck fracture. The lesson of every such question: name the structure at risk for the mechanism described.
High-yield viva angles
Viva examiners love the paradox of abducting the arm with a paralysed deltoid: supraspinatus and scapular rotation can raise the arm nearly to the horizontal, so absent abduction alone never proves nerve injury — test sensation first. The second angle is the loss of the final 30 degrees of overhead abduction when serratus anterior is paralysed, because deltoid cannot act against an unrotated scapula. The third is the quadrangular space itself: asked to name its walls and contents, candidates routinely swap the borders — a quick frame is muscles above and below, muscle medially and bone laterally.
Frequently asked questions
Which nerve is injured in anterior dislocation of the shoulder?
The axillary nerve, producing deltoid weakness and anaesthesia over the regimental badge area — usually a recoverable neurapraxia.
Which muscle initiates abduction of the shoulder?
Supraspinatus initiates the first 15 degrees before deltoid takes over up to 90 degrees.
What is the arterial collateral pathway after ligation of the axillary artery?
The scapular anastomosis connecting subscapular branches with the suprascapular and transverse cervical arteries from the subclavian.
What are the boundaries of the quadrangular space?
Subscapularis and teres minor above, teres major below, long head of triceps medially, and the surgical neck of the humerus laterally.
Which lesions mark recurrent anterior dislocation?
A Bankart lesion of the anteroinferior labrum and a Hill–Sachs defect of the posterolateral humeral head.
Why does full overhead abduction fail with a long thoracic nerve palsy?
Serratus anterior paralysis stops scapular rotation beyond the horizontal, so the last 30 degrees are lost despite a normal deltoid.