Four articulations functioning in coordinated sequence:

1. Glenohumeral joint
2. Acromioclavicular joint
3. Sternoclavicular joint
4. Scapulothoracic articulation (functional, not a true joint)

1. Glenohumeral Joint

Type: Ball-and-socket synovial joint — most mobile, least stable.

Articular surfaces: Head of humerus ↔ glenoid cavity (deepened by fibrocartilaginous glenoid labrum)

Capsule:

• Medial: glenoid margin (beyond labrum); lateral: anatomical neck; inferiorly extends to surgical neck
• Weakest inferiorly — no rotator cuff reinforcement

Ligaments

Bursae

Rotator Cuff (SITS)

Movements

Mechanism of Abduction

Scapulohumeral rhythm: 2:1 (GH : scapulothoracic); simultaneous from the start.

2. Acromioclavicular Joint

• Plane synovial joint; ~2.5 cm medial to point of shoulder
• Articular surfaces covered with fibrocartilage; incomplete wedge-shaped disc
• Coracoclavicular ligament = strongest ligament of upper limb; two parts:
  • Conoid — apex to coracoid root; base to conoid tubercle of clavicle
  • Trapezoid — horizontal; coracoid upper surface → trapezoid line of clavicle

3. Sternoclavicular Joint

• Saddle (sellar) synovial joint — only joint connecting upper limb to axial skeleton
• Articular disc — prevents medial clavicle displacement; divides cavity into 2 compartments
• Costoclavicular ligament — strongest; anchors clavicle to 1st rib; prevents lateral + upward displacement
• Permits elevation, depression, protraction, retraction, rotation of pectoral girdle

4. Scapulothoracic Articulation

• Functional linkage; facilitated by loose areolar tissue between serratus anterior + subscapularis
• Serratus anterior provides linkage + enables upward scapular rotation during arm elevation

Applied Anatomy

Shoulder Dislocation

• Most commonly dislocated joint; anterior (~95%)
• Cause: excessive extension + lateral rotation; primary displacement inferior → anterior
• Features: hollowing of shoulder, prominent acromion, arm abducted + laterally rotated
• Complication: axillary nerve injury → deltoid paralysis + sensory loss (regimental badge area)

Rotator Cuff / Painful Arc

• Calcific degeneration of supraspinatus → subacromial bursitis
• Painful arc: 60–120° (bursa pinched under coracoacromial arch)
• Dawbarn’s sign: tenderness below acromion arm adducted; disappears on abduction

Frozen Shoulder (Adhesive Capsulitis)

• Age 40–60; capsule shrinkage + fibrosis; all movements uniformly restricted; normal X-ray

AC Joint Dislocation (Shoulder Separation)

• Blow to superolateral shoulder; severe: both AC + coracoclavicular ligaments torn → scapula + upper limb drop

The shoulder joint complex consists of four articulations that must function in coordinated sequence to allow full mobility of the upper limb:

1. Glenohumeral joint
2. Acromioclavicular joint
3. Sternoclavicular joint
4. Scapulothoracic articulation (functional linkage)

Impairment of any one joint produces a functional deficit of the entire complex.

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1. Glenohumeral Joint (Shoulder Joint)

The primary articulation of the shoulder complex — the most mobile and least stable joint in the body.

Type

Ball-and-socket synovial joint.

Articular Surfaces

• Head of humerus (large, spherical) articulates with the glenoid cavity of the scapula (small, shallow)
• Glenoid cavity is deepened by the fibrocartilaginous glenoid labrum

Capsular Ligament

• Attached medially to the glenoid margin (beyond the labrum) and laterally to the anatomical neck of the humerus
• Inferiorly extends onto the surgical neck (1.5 cm or more) — the weakest and most dependent part
• Contains three apertures: (a) bicipital groove for tendon of long head of biceps; (b) anterior opening communicating with subscapular bursa; (c) posterior opening communicating with infraspinatus bursa

Ligaments

Bursae

Relations

Rotator Cuff (SITS Muscles)

The rotator cuff is formed by the tendons of four muscles that blend with and reinforce the joint capsule, holding the large humeral head against the shallow glenoid cavity:

Movements

Mechanism of Abduction (Three Phases)

Scapulohumeral rhythm: For every 15° of abduction — 10° at glenohumeral joint and 5° at scapulothoracic linkage (2:1 ratio). Movement is simultaneous from the start, not sequential.

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2. Acromioclavicular Joint

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3. Sternoclavicular Joint

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4. Scapulothoracic Articulation

Not a true joint — a functional linkage between the ventral aspect of the scapula and the lateral thoracic wall. Movement is facilitated by loose areolar tissue between serratus anterior and subscapularis. Serratus anterior provides the linkage and enables upward rotation of the scapula during arm elevation.

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Applied Anatomy

Dislocation of the Shoulder Joint

Most common joint to dislocate. Occurs inferiorly first (weakest capsule), then descends anterior to infraglenoid tubercle — described clinically as anterior dislocation (most common; ~95%).

Cause: Excessive extension and lateral rotation of the humerus.

Features:

• Flattening / hollowing of the rounded shoulder contour (deltoid rides over the acromion)
• Prominence of the acromion
• Arm held in slight abduction and lateral rotation
• Axillary nerve injury — close relation to inferior capsule → deltoid paralysis + loss of sensation over regimental badge area

Rotator Cuff Disorders and Painful Arc Syndrome

Calcific degeneration of supraspinatus tendon → irritation of subacromial bursa → subacromial bursitis. On abduction, the inflamed bursa is caught between the supraspinatus tendon and the coracoacromial arch.

Painful arc: Pain during 60°–120° of abduction only (when the inflamed bursa is pinched under the arch). Pain-free below 60° and above 120°.

Dawbarn’s sign: Tenderness over the deltoid just below the acromion with the arm adducted; tenderness disappears when the arm is abducted (bursa slides under the acromion).

Frozen Shoulder (Adhesive Capsulitis)

Uniform painful limitation of all shoulder movements with no radiological changes. Caused by shrinkage and fibrosis of the joint capsule. Typically affects individuals aged 40–60 years.

Dislocation of the Acromioclavicular Joint (Shoulder Separation)

Caused by severe blow to the superolateral shoulder. In severe form, both acromioclavicular and coracoclavicular ligaments are torn — scapula and upper limb drop due to gravity, separated from the clavicle.

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