Type

Ball-and-socket synovial joint (glenohumeral joint). Most mobile, most commonly dislocated major joint.

Articular Surfaces

• Head of humerus (~4× larger) ↔ glenoid cavity of scapula (small, shallow)
• Glenoid labrum — fibrocartilaginous rim; deepens cavity; attaches long head of biceps superiorly

Capsule

• Medial: glenoid margin (beyond labrum)
• Lateral: anatomical neck; inferiorly extends to surgical neck
• Encloses supraglenoid tubercle → long head of biceps intracapsular but extrasynovial
• Weakest inferiorly — no rotator cuff reinforcement → subglenoid primary displacement in dislocation

Capsule Openings

Ligaments

Bursae

Rotator Cuff (SITS)

Inferior capsule = only aspect with no rotator cuff reinforcement.

Nerve Supply

• Axillary nerve (C5, C6) — inferior capsule; most clinically important
• Suprascapular nerve (C5, C6) — superior + posterior capsule
• Lateral pectoral nerve (C5, C6, C7) — anterior capsule

Movements and Muscles

Mechanism of Abduction

• Scapulohumeral rhythm: 2:1 (glenohumeral : scapulothoracic); GH = ~120°, scapulothoracic = ~60° for full 180°
• Scapular upward rotation: serratus anterior + upper + lower trapezius

Shoulder Girdle Movements

• Long thoracic nerve injury → winging of scapula, cannot raise arm fully

Sternoclavicular Joint

• Saddle (sellar) synovial joint — only bony joint connecting upper limb to axial skeleton
• Articular disc: prevents medial clavicle displacement; divides cavity into 2 compartments
• Costoclavicular ligament — strongest; prevents lateral + upward displacement
• Sternal epiphysis fuses ~23–25 yrs → apparent dislocation in young = physeal injury
• Posterior dislocation dangerous (mediastinal structures)

Acromioclavicular Joint

• Plane synovial joint, ~2.5 cm medial to point of shoulder
• Coracoclavicular ligament (conoid + trapezoid) = main bond between clavicle + scapula; suspends scapula + upper limb
• Both AC + coracoclavicular torn in severe shoulder separation → lateral clavicle prominent (scapula drops)

Applied Anatomy

Shoulder Dislocation

• Most common: anterior-inferior (primary displacement subglenoid → anterior)
• Mechanism: forced abduction + extension + lateral rotation; FOOSH
• Features: loss of rounded contour, prominent acromion, hollow below acromion, arm abducted + laterally rotated
• Complication: axillary nerve injury → deltoid paralysis + sensory loss over regimental badge area

Rotator Cuff Disorders / Painful Arc

• Supraspinatus most commonly affected; vulnerable to impingement under coracoacromial arch
• Painful arc: 60–120° of abduction (inflamed bursa/tendon pinched under arch)
• Dawbarn’s sign: tenderness below acromion with arm adducted; disappears on abduction (bursa slides under acromion)

Frozen Shoulder (Adhesive Capsulitis)

• Age 40–60; capsule shrinkage + fibrosis → painful restriction of all movements; normal radiograph

Referred Shoulder-Tip Pain

• Diaphragmatic irritation → C3–C5 (phrenic) shared with C3–C4 (supraclavicular nerves supplying shoulder tip)

The shoulder joint is the most mobile joint in the body. In anatomy exams, “shoulder joint” usually means the glenohumeral joint, but full shoulder movement depends on the whole shoulder complex:

• Glenohumeral joint
• Sternoclavicular joint
• Acromioclavicular joint
• Scapulothoracic movement

The essential idea is simple: the shoulder gains mobility by sacrificing bony stability, and then regains functional stability through the rotator cuff, glenoid labrum, coracoacromial arch, and surrounding muscles.

---

Type

The shoulder joint proper, or glenohumeral joint, is a ball-and-socket type of synovial joint.

It is:

• The most mobile joint in the body
• One of the least stable major joints
• The most commonly dislocated major joint

---

Articular Surfaces

The head of the humerus is large and rounded, while the glenoid cavity is shallow and much smaller. The humeral head is about four times larger than the glenoid cavity.

This mismatch explains both:

• Wide range of movement
• Tendency to dislocation

---

Glenoid Labrum

The glenoid labrum is a fibrocartilaginous rim attached to the margin of the glenoid cavity.

Functions:

• Deepens the glenoid cavity
• Increases the articular surface area
• Improves stability of the humeral head
• Provides attachment superiorly to the tendon of the long head of biceps brachii

The labrum does not make the socket deep enough to prevent dislocation by itself; it works along with the capsule, ligaments, and rotator cuff.

---

Orientation of the Joint

The glenoid cavity faces forwards, laterally, and slightly upwards. The plane of the scapula is oblique, approximately 30-45 degrees anterior to the coronal plane.

Therefore, shoulder movements are best understood in the plane of the scapula, not purely in standard anatomical planes.

---

Capsule

The capsule is thin, loose, and lax. This allows free movement but reduces stability.

Attachments

The capsule encloses the supraglenoid tubercle, so the tendon of the long head of biceps is intracapsular but extrasynovial.

Important Features

• The capsule is strengthened by rotator cuff tendons anteriorly, superiorly, and posteriorly.
• It is weak inferiorly because there is no rotator cuff reinforcement below.
• Inferior weakness explains why the primary displacement in shoulder dislocation is commonly subglenoid.

Openings in the Capsule

---

Ligaments of the Shoulder Joint

Glenohumeral Ligaments

The superior, middle, and inferior glenohumeral ligaments are thickenings of the anterior capsule. They are best seen from inside the joint.

The gap between the superior and middle glenohumeral ligaments is called the foramen of Weitbrecht, through which the subscapular bursa may communicate with the joint.

Coracohumeral Ligament

The coracohumeral ligament extends from the root of the coracoid process to the upper part of the humerus near the greater tubercle and capsule.

Functions:

• Strengthens the capsule superiorly
• Helps suspend the head of humerus
• Supports the joint in the dependent position

Transverse Humeral Ligament

The transverse humeral ligament bridges the bicipital groove between the greater and lesser tubercles.

Function:

• Converts the bicipital groove into a canal
• Holds the tendon of the long head of biceps within the groove

Coracoacromial Ligament and Arch

The coracoacromial ligament extends from the coracoid process to the acromion.

Together, the coracoid process + coracoacromial ligament + acromion form the coracoacromial arch.

Functions of the arch:

• Forms a protective roof over the humeral head
• Prevents superior displacement of the humeral head
• Acts as a secondary socket

The supraspinatus tendon passes beneath this arch, making it vulnerable to impingement.

---

Bursae Around the Shoulder Joint

Subacromial / Subdeltoid Bursa

The subacromial bursa is the largest bursa in the body.

Location:

• Below the coracoacromial arch and deltoid
• Above the supraspinatus tendon and greater tubercle of humerus

Functions:

• Reduces friction during abduction
• Allows smooth movement of supraspinatus tendon under the arch
• Permits the greater tubercle to pass under the acromion during elevation

It does not normally communicate with the shoulder joint cavity. It may communicate if the rotator cuff is torn.

Subscapular Bursa

Location:

• Between subscapularis tendon and neck of scapula

It usually communicates with the shoulder joint through an anterior capsular opening below the coracoid process.

Infraspinatus Bursa

Location:

• Between infraspinatus tendon and posterolateral capsule

It may communicate with the joint cavity.

---

Rotator Cuff

The rotator cuff, or musculotendinous cuff, is formed by the flattened tendons of four muscles that blend with the shoulder capsule.

Memory: SITS

Capsular Reinforcement

Functions:

1. Holds the head of humerus against the glenoid cavity
2. Stabilises the joint during movement
3. Prevents unwanted translation of the humeral head
4. Works with deltoid during abduction
5. Produces rotation of the humerus

---

Stability of the Shoulder Joint

The stability of the shoulder is mainly muscular, not bony.

Weakest point:

• Inferior capsule
• No rotator cuff reinforcement
• Closely related to axillary nerve

This explains the frequency of inferior/anterior-inferior dislocation.

---

Relations of the Shoulder Joint

The axillary nerve is the most important relation clinically because it may be injured in shoulder dislocation and fracture of the surgical neck of humerus.

---

Blood Supply

The shoulder joint is supplied by an arterial anastomosis around the scapula and proximal humerus.

Main arteries:

---

Nerve Supply

The shoulder joint is supplied mainly by articular branches from:

This follows Hilton’s law: nerves supplying a joint also supply muscles moving the joint and skin over those muscles.

---

Movements of the Glenohumeral Joint

The shoulder joint permits:

1. Flexion and extension
2. Abduction and adduction
3. Medial and lateral rotation
4. Circumduction

Muscles Producing Movements

---

Mechanism of Abduction

Full abduction of the arm to 180 degrees is not produced by the glenohumeral joint alone. It requires coordinated movement of the humerus, scapula, clavicle, sternoclavicular joint, and acromioclavicular joint.

Phase 1: 0-15 Degrees

Main muscle:

• Supraspinatus

Role:

• Initiates abduction
• Holds the head of humerus firmly against the glenoid cavity

Phase 2: 15-90 Degrees

Main muscle:

• Deltoid, especially middle fibres

Role:

• Powerful abduction of the arm

Supraspinatus continues to stabilise the humeral head during this phase.

Phase 3: Above 90 Degrees

Abduction above the horizontal requires:

1. Lateral rotation of humerus
2. Upward rotation of scapula

Lateral rotation is produced by:

• Infraspinatus
• Teres minor

This moves the greater tubercle posteriorly so it does not impinge on the acromion.

Upward rotation of scapula is produced by:

• Serratus anterior
• Upper and lower fibres of trapezius

These muscles turn the glenoid cavity upwards.

Scapulohumeral Rhythm

After the early phase, shoulder elevation follows an approximate 2:1 ratio:

For full 180-degree elevation:

• Glenohumeral joint contributes about 120 degrees
• Scapulothoracic rotation contributes about 60 degrees

---

Movements of the Shoulder Girdle

Shoulder girdle movements are movements of the scapula on the thoracic wall, coordinated through the sternoclavicular and acromioclavicular joints.

Serratus Anterior in Overhead Abduction

Serratus anterior rotates the inferior angle of scapula laterally and forwards. It is essential for raising the arm above 90 degrees.

Long thoracic nerve injury causes:

• Winging of scapula
• Loss of pushing/punching action
• Inability to raise arm fully above the head

---

Sternoclavicular Joint

The sternoclavicular joint is included in shoulder mechanics because it is the only true bony articulation between the upper limb and the axial skeleton.

Type

It is a saddle type of synovial joint.

Articular Surfaces

The surfaces are covered with fibrocartilage.

Articular Disc

The joint contains a fibrocartilaginous articular disc that divides the cavity into two compartments.

Functions:

• Improves fit between articular surfaces
• Acts as a shock absorber
• Prevents medial displacement of clavicle

Ligaments

Movements

• Elevation and depression of lateral end of clavicle
• Protraction and retraction
• Rotation of clavicle around its long axis during overhead movement

Blood and Nerve Supply

Clinical Notes

• Dislocation is uncommon because the joint is strong.
• Anterior dislocation is more common than posterior dislocation.
• Posterior dislocation is dangerous because of mediastinal structures behind the joint.
• The sternal epiphysis of clavicle fuses late, around 23-25 years, so apparent dislocation in young people may be physeal injury.

---

Acromioclavicular Joint

Type

The acromioclavicular joint is a plane type of synovial joint.

It lies about 2.5 cm medial to the point of the shoulder.

Articular Surfaces

The surfaces are covered with fibrocartilage. A small incomplete articular disc may be present.

Ligaments

Acromioclavicular Ligament

• Strengthens the capsule superiorly
• Connects acromion to lateral clavicle

Coracoclavicular Ligament

The coracoclavicular ligament is the main bond of union between the clavicle and scapula.

It has two parts:

Functions:

• Suspends the scapula and upper limb from the clavicle
• Transmits weight of upper limb to the clavicle
• Allows controlled rotation of scapula during overhead movement

Blood and Nerve Supply

Clinical Notes

• Injury is called shoulder separation.
• It commonly follows a fall or blow on the point of the shoulder.
• If only the acromioclavicular ligament is torn, displacement is mild.
• If the coracoclavicular ligament is also torn, the scapula and upper limb drop, and the lateral clavicle appears prominent.

---

Applied Anatomy of the Shoulder Joint

Dislocation of the Shoulder Joint

The shoulder joint is the most commonly dislocated major joint.

Reasons:

• Large humeral head
• Small shallow glenoid cavity
• Loose capsule
• Weak inferior capsule
• No inferior rotator cuff reinforcement

Direction

The primary displacement is commonly inferior / subglenoid. Clinically, the head often comes to lie anteriorly, producing an anterior-inferior dislocation.

Common Mechanism

• Fall on outstretched hand
• Forced abduction, extension, and lateral rotation

Clinical Features

• Loss of rounded contour of shoulder
• Prominent acromion
• Hollow below acromion
• Arm held slightly abducted and laterally rotated
• Pain and restricted movement

Complication

Axillary nerve injury:

• Deltoid paralysis
• Loss of abduction from about 15 to 90 degrees
• Loss of rounded shoulder contour
• Sensory loss over regimental badge area

---

Axillary Nerve Injury

Causes:

• Shoulder dislocation
• Fracture of surgical neck of humerus
• Improper intramuscular injection in deltoid region

Effects:

• Deltoid paralysis
• Weak abduction at shoulder
• Flattening of shoulder
• Sensory loss over lower half of deltoid

---

Rotator Cuff Disorders

Rotator cuff disease is one of the commonest causes of shoulder pain.

Important points:

• Supraspinatus is most commonly affected.
• It passes beneath the coracoacromial arch and is vulnerable to impingement.
• Degeneration may lead to calcification, inflammation, or tear.

Clinical features:

• Painful abduction
• Weak initiation or maintenance of abduction
• Painful arc between 60 and 120 degrees
• Difficulty with overhead activity

---

Subacromial Bursitis and Dawbarn’s Sign

Subacromial bursitis commonly occurs secondary to supraspinatus tendinitis.

Dawbarn’s sign:

• Pressure below the acromion with the arm by the side causes pain.
• When the arm is abducted, pressure at the same point becomes painless because the bursa moves under the acromion.

---

Frozen Shoulder

Frozen shoulder, or adhesive capsulitis, is characterised by pain and progressive stiffness due to thickening and contraction of the capsule.

Features:

• Usually age 40-60 years
• Painful restriction of all movements
• External rotation commonly restricted early
• Altered scapulohumeral rhythm
• Muscle wasting from disuse
• Radiographs often normal

It is often self-limiting but recovery may take many months to years and is helped by physiotherapy.

---

Painful Arc Syndrome

Painful arc occurs due to impingement of the supraspinatus tendon or subacromial bursa under the coracoacromial arch.

Classically:

• Pain occurs between 60 and 120 degrees of abduction.
• Pain may reduce after the inflamed tissue passes beyond the arch.

---

Aspiration of the Shoulder Joint

The shoulder joint may be aspirated:

• Anteriorly through the deltopectoral triangle
• Laterally just below the acromion

Needle placement should avoid the axillary nerve inferiorly.

---

Optimum Position for Immobilisation

In shoulder disease requiring immobilisation, the arm is kept in a functional position, commonly with abduction, to avoid ankylosis in a useless adducted position.

---

Referred Pain to Shoulder Tip

Irritation of diaphragmatic peritoneum can cause shoulder-tip pain.

Basis:

• Phrenic nerve carries pain from diaphragm: C3-C5
• Supraclavicular nerves supply shoulder-tip skin: C3-C4
• Shared spinal segments cause referred pain

---

Rapid Revision Table

Diagram Placeholder

Diagram content will be added later.

• Placeholder 1
• Placeholder 2
• Placeholder 3

Previous Year Questions

1. Placeholder question
2. Placeholder question
3. Placeholder question
4. Placeholder question
5. Placeholder question