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Mechanics and Pathomechanics of the Shoulder

Detailed Notes for Physiotherapy Students

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PART 1: ANATOMY OVERVIEW - THE SHOULDER COMPLEX

Key Point: All 4 joints must move in a coordinated manner for full, pain-free shoulder function. Dysfunction in any one joint disrupts the entire complex.

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PART 2: GLENOHUMERAL JOINT - MECHANICS

2.1 Bony Architecture

• Head of humerus: large, spherical (articular surface ~3x larger than glenoid)
• Glenoid cavity: small, shallow, pear-shaped fossa on scapula
• The glenoid faces anteriorly, laterally, and superiorly (~5° superior tilt)
• This arrangement allows mobility but sacrifices bony stability

2.2 Glenoid Labrum

• Fibrocartilaginous ring attached to the glenoid margin
• Deepens the glenoid by ~50% (without it, the glenoid is too shallow)
• Increases contact area, acts as a suction cup, serves as attachment point for ligaments
• Superiorly continuous with the long head of biceps (LHB) tendon (attached to supraglenoid tubercle)

2.3 Joint Capsule

• Attaches medially to the glenoid margin, laterally to the anatomical neck of humerus
• Capsule is twice the area of the humeral head - allows full ROM
• Redundant axillary recess inferiorly - accommodates abduction; if scarred = adhesive capsulitis (frozen shoulder)
• Reinforced anteriorly by the glenohumeral ligaments (SGHL, MGHL, IGHL)

2.4 Glenohumeral Ligaments

2.5 Subacromial Space

• Space between the inferior surface of acromion/coracoacromial arch and the superior surface of the rotator cuff
• Normally: ~9-10 mm
• Contains: supraspinatus tendon, subacromial-subdeltoid bursa, LHB tendon (partial)
• Critical zone: Any reduction in this space (swelling, bony spur, muscle hypertrophy) = impingement

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PART 3: THE ROTATOR CUFF - MECHANICS

3.1 The Four Muscles (SITS)

3.2 Force Couple Mechanism - The Core of GHJ Mechanics

This is the single most important concept in shoulder mechanics.

• Deltoid pulls the humeral head superiorly (toward acromion)
• Rotator cuff (especially infraspinatus, teres minor, subscapularis) pulls the humeral head inferiorly and medially (into the glenoid)
• Together, they create a net upward rotation while keeping the humeral head centered in the glenoid

• Subscapularis (anterior) and infraspinatus + teres minor (posterior) create opposing forces
• This force couple stabilizes the humeral head against anterior or posterior translation

If the rotator cuff is weak or torn: Deltoid action goes unopposed → humeral head migrates superiorly → supraspinatus tendon gets crushed under acromion → impingement and pain.

3.3 Compression Function (Concavity-Compression)

• Rotator cuff muscles co-contract during shoulder movement
• This compresses the humeral head into the glenoid concavity
• Creates a concavity-compression mechanism - dynamic stability without restricting motion
• The glenoid concavity provides a larger reactive force when more compression force is applied

3.4 Critical Zone of Supraspinatus

• The distal portion (~1 cm from insertion) has a poor blood supply - called the "critical zone"
• This zone is a watershed area between vessels entering from the musculotendinous junction and those from the bony insertion
• Repeated microtrauma + poor healing = degeneration, calcification, tears

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PART 4: SCAPULAR MECHANICS AND SCAPULOHUMERAL RHYTHM

4.1 Scapular Movements (6 Movements)

Note: Scapular movements are coupled. Protraction = internal rotation + anterior tilt + abduction (lateral translation). Retraction = external rotation + posterior tilt + adduction.

4.2 Scapulohumeral Rhythm (SHR) - EXAM FAVOURITE

• For every 3° of shoulder abduction: 2° occur at GHJ + 1° at STJ (scapular upward rotation)
• To achieve 180° total elevation:
  • 120° at GHJ
  • 60° at STJ

• Scapular upward rotation elevates the acromion away from the greater tuberosity during abduction
• This preserves the subacromial space and prevents impingement
• The clavicle acts as a strut - its elevation and rotation at the SCJ and ACJ enables scapular upward rotation

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PART 5: STERNOCLAVICULAR AND ACROMIOCLAVICULAR JOINT MECHANICS

5.1 Sternoclavicular Joint (SCJ)

• Only bony connection between the upper limb and axial skeleton
• Type: saddle joint with an intraarticular disc (makes it functionally bicondylar)
• Stabilized primarily by posterior SCL (A-P stability) and costoclavicular ligament
• Movements: elevation/depression, protraction/retraction, axial rotation

• Clavicle elevates ~35-40° at SCJ
• Clavicle retracts ~15°
• Clavicle axially rotates posteriorly ~40-50°

5.2 Acromioclavicular Joint (ACJ)

• Plane synovial joint between lateral clavicle and acromion
• Key ligaments: ACL (horizontal stability) + coracoclavicular ligaments - conoid + trapezoid (vertical stability)
• Allows ~20° of rotation
• Acts as a "hinge" allowing scapular upward rotation relative to clavicle

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PART 6: BURSAE - MECHANICS

Key Bursae Around the Shoulder

The subacromial bursa is the most clinically important. It does NOT normally communicate with the GHJ. If it does (on MRI) = full-thickness rotator cuff tear.

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PART 7: PATHOMECHANICS OF THE SHOULDER

7.1 Subacromial Impingement Syndrome

Weak rotator cuff / SHR disturbance
        ↓
Deltoid unopposed → humeral head migrates SUPERIORLY
        ↓
Subacromial space narrows (< 7mm = impingement threshold)
        ↓
Supraspinatus tendon + bursa compressed against acromion
        ↓
Pain (painful arc 60-120°), inflammation, tendon degeneration
        ↓
Possible partial or full-thickness rotator cuff tear

1. Intrinsic (tendon side): Rotator cuff weakness, tendon swelling, calcium deposits
2. Extrinsic (structural): Hooked (Type III) acromion, AC joint osteophytes, thickened coracoacromial ligament
3. Functional: Scapular dyskinesis (failure of scapular upward rotation), loss of SHR, poor posture (protracted, anteriorly tilted scapula)

• Stage I: Edema + hemorrhage (reversible, age < 25)
• Stage II: Fibrosis + tendinosis (25-40 years)
• Stage III: Tendon tears, bone changes (>40 years)

7.2 Rotator Cuff Tears

1. Repeated impingement or eccentric overload → microtrauma in critical zone
2. Poor blood supply → incomplete healing
3. Progressive degeneration (tendinosis) → partial thickness tear
4. Continued loading → full thickness tear

• Supraspinatus tear → loss of superior compression → humeral head migrates up
• Subscapularis tear → anterior instability, loss of internal rotation strength
• Infraspinatus/teres minor tear → posterior instability, ER weakness

7.3 Scapular Dyskinesis

• Failure of scapular upward rotation → reduced subacromial space → functional impingement
• Anterior tilt + protraction (SICK scapula) → coracoid impingement of subscapularis
• Loss of stable scapular base → GHJ force couples disrupted → rotator cuff overload

• Scapular malposition
• Inferior medial border prominence (winging)
• Coracoid pain and tenderness
• Kinesis (movement) abnormality

• Serratus anterior weakness (long thoracic nerve palsy → classic winging)
• Upper trapezius dominance with lower trapezius inhibition
• Tight pectoralis minor (pulls scapula anteriorly)
• Poor thoracic spine mobility (kyphosis)

7.4 Glenohumeral Instability

1. Forceful abduction + external rotation (e.g., throwing)
2. IGHL (anterior band) fails to restrain humeral head
3. Bankart lesion (anterior labrum avulsion) - detachment of labrum + IGHL from glenoid
4. Bony Bankart (glenoid rim fracture) or Hill-Sachs lesion (posterolateral humeral head compression fracture)
5. Loss of labral depth → decreased concavity-compression → cycle of recurrent dislocation

• Primary static: IGHL anterior band, anteroinferior labrum
• Primary dynamic: Subscapularis, long head of biceps

7.5 Adhesive Capsulitis (Frozen Shoulder)

1. Synovitis → capsular inflammation
2. Fibroblast proliferation → dense, fibrotic capsule
3. Axillary recess obliteration → loss of capsule redundancy → restricted abduction
4. Coracohumeral ligament thickening → restricted ER
5. Capsular volume reduced from normal ~28 mL to as little as ~5-10 mL

• Stage 1 (Freezing/Painful): Synovitis, pain with all movements - 3-9 months
• Stage 2 (Frozen/Stiffening): Fibrosis, stiffness > pain - 9-18 months
• Stage 3 (Thawing): Gradual return of motion - 18-24+ months

7.6 Shoulder Instability from Poor Force Coupling

• Subscapularis (anterior) vs. Infraspinatus + Teres Minor (posterior)

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PART 8: MUSCLE FORCE COUPLES FOR PHYSIOTHERAPY PRACTICE

8.1 Deltoid-Rotator Cuff Force Couple (Coronal Plane)

Normal: Deltoid (superior force) + RC (inferior force) = Pure rotation
Pathological: RC fails → Superior translation → Impingement
PT aim: Strengthen RC (especially IR + ER muscles) to restore inferior glide

8.2 Trapezius-Serratus Anterior Force Couple (Scapular Upward Rotation)

Upper Trapezius (rotates scapula up + retracts)
Lower Trapezius (rotates scapula up + depresses)
Serratus Anterior (rotates scapula up + protracts)
→ Together = Pure upward rotation of scapula
Pathological: Lower trap + serratus weak = scapular dyskinesis
PT aim: Specifically strengthen lower trapezius and serratus anterior

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PART 9: QUICK REVISION SUMMARY TABLE

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PART 10: CLINICAL CORRELATIONS FOR PHYSIOTHERAPY

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