SHOULDER BIOMECHANICS
Glenohumeral Stability | Rotator Cuff Force Couples | Scapulothoracic Rhythm
Shoulder Motion Components
Critical Must-Knows
- Scapulothoracic rhythm: 2:1 ratio (for every 2° GH abduction, 1° ST upward rotation)
- Rotator cuff force couple: Subscapularis/infraspinatus-teres minor balance humeral head centering
- Deltoid-rotator cuff force couple: Deltoid elevates, RC depresses and centers humeral head
- Glenoid covers only 25% of humeral head (inherent instability requires dynamic stabilizers)
- Concavity-compression: RC compresses head into glenoid concavity for stability
Examiner's Pearls
- "Critical arc of abduction 60-120°: supraspinatus most active, impingement risk highest
- "Capsular volume 2x humeral head volume allows large ROM but sacrifices stability
- "Labrum deepens glenoid socket 50% (increases stability)
- "Inferior glenohumeral ligament (IGHL) is primary anterior stabilizer in abduction-external rotation
Critical Shoulder Biomechanics Exam Points
Scapulothoracic Rhythm
2:1 ratio throughout abduction arc. 180° total arm elevation = 120° GH + 60° ST. Disruption causes scapular dyskinesis and impingement.
Rotator Cuff Force Couple
Coronal plane: Deltoid (superior) vs subscapularis/infraspinatus (inferior). Transverse plane: Subscapularis (anterior) vs infraspinatus-teres minor (posterior). Centers humeral head.
Concavity-Compression
RC compresses humeral head into glenoid concavity. Creates suction seal and depth. Accounts for 50% of GH stability. Loss with RC tear causes superior migration.
Labral Function
Deepens glenoid 50% (9mm to 5mm depth). Increases contact area and stability. SLAP tears compromise biceps anchor and superior stability.
SITSRotator Cuff Force Couples
Memory Hook:SITS muscles work as force couples: Supraspinatus-deltoid (coronal), Subscapularis vs Infraspinatus-Teres (transverse)!
CLASPSGlenohumeral Stability Mechanisms
Memory Hook:CLASPS hold the shoulder stable: 50% dynamic (RC), 50% static (capsule/labrum)!
Overview and Introduction
Shoulder biomechanics encompasses the complex interplay of multiple joints, muscles, and ligaments that produce the greatest range of motion of any joint in the body. Understanding these principles is critical for managing instability, rotator cuff pathology, arthroplasty, and rehabilitation.
The shoulder complex includes the glenohumeral (GH), scapulothoracic (ST), acromioclavicular (AC), and sternoclavicular (SC) joints working in concert to achieve 180 degrees of abduction.
Concepts and Principles
Key Biomechanical Principles:
- Scapulothoracic Rhythm (2:1): For every 2° of GH abduction, there is 1° of ST upward rotation
- Force Couples: Coronal (deltoid vs inferior cuff) and transverse (subscapularis vs infraspinatus-teres minor)
- Concavity-Compression: RC compresses humeral head into glenoid for dynamic stability
- Labral Contribution: Deepens glenoid by 50%, critical for stability
Glenohumeral Joint Geometry and Stability
Inherent Instability vs Mobility Trade-Off
The glenohumeral joint is the most mobile but least stable joint in the body. This is due to minimal bony constraint.
Bony Geometry
- Glenoid coverage: Only 25% of humeral head surface area
- Radius mismatch: Humeral head radius greater than glenoid concavity
- Retroversion: Glenoid retroversion averages 5° (posterior tilt)
- Result: Minimal intrinsic bony stability
Capsular Volume
- Capsule volume: 2x humeral head volume
- Function: Allows large ROM (180° abduction, 90° ER/IR)
- Trade-off: Loose capsule = less static restraint
- Folds: Capsular redundancy allows motion without tension
The labrum is critical for increasing stability by deepening the glenoid socket approximately 50% (from 2.5mm to 5mm depth). This increases contact area and creates a suction seal.
Static vs Dynamic Stabilizers
Shoulder stability is 50% static, 50% dynamic.
Static vs Dynamic Stabilizers
| Category | Structures | Contribution | Failure Mechanism |
|---|---|---|---|
| Static (50%) | Capsule, ligaments (IGHL, MGHL, SGHL), labrum, glenoid concavity | Passive restraint at end ROM | Capsular laxity, labral tear, Bankart lesion |
| Dynamic (50%) | Rotator cuff (SITS), deltoid, scapular stabilizers (serratus, trapezius) | Active concavity-compression, force couples | RC tear, scapular dyskinesis, nerve injury |
Concavity-compression is the primary dynamic stabilizing mechanism. The rotator cuff compresses the humeral head into the glenoid concavity, creating stability through increased friction and depth. This accounts for approximately 50% of GH stability.
Scapulothoracic Rhythm
The 2:1 Ratio
Normal shoulder elevation requires coordinated motion between the glenohumeral (GH) and scapulothoracic (ST) articulations. The classic ratio is 2:1.
During Abduction
- 0-30° abduction: Primarily GH motion (setting phase)
- 30-180° abduction: 2:1 ratio (2° GH for every 1° ST)
- Total 180°: 120° GH + 60° ST upward rotation
- Result: Scapula rotates upward, glenoid tracks under humeral head
Scapular Motion
- Upward rotation: 60° total (inferior angle moves laterally)
- Posterior tilt: 20-30° (acromion tilts posteriorly)
- External rotation: 15-25° (medial border moves away from spine)
- Muscles: Serratus anterior (upward rotation), trapezius (elevation)
Scapular Dyskinesis Consequences
Loss of normal ST rhythm causes impingement and instability. Serratus anterior palsy (long thoracic nerve injury) causes medial scapular winging and loss of upward rotation. This reduces subacromial space and increases impingement risk. Rehabilitation must restore scapular stability before addressing GH pathology.
Acromioclavicular Joint Contribution
The AC joint allows scapular protraction/retraction and rotation. Clavicular rotation at the sternoclavicular joint contributes 40-50° to overhead motion.
Disruption of AC joint (separation) or SC joint affects scapular position and can cause secondary impingement.
Rotator Cuff Force Couples
The rotator cuff functions as two force couples to center the humeral head during motion.
Coronal Plane Force Couple
Deltoid (superior) vs Subscapularis + Infraspinatus-Teres Minor (inferior)
Deltoid Action
- Primary abductor: Generates upward force
- Vector: Superior translation of humeral head
- Problem: Alone would cause superior migration and impingement
- Balanced by: Inferior force from rotator cuff
Rotator Cuff Action
- Depressor force: Subscapularis + infraspinatus-teres minor pull inferiorly
- Vector: Counteracts deltoid superior pull
- Result: Humeral head stays centered in glenoid
- Clinical: RC tear allows superior migration (loss of depressor)
Supraspinatus role is debated:
- Traditional view: Primary abduction initiator
- Modern view: Primarily a humeral head depressor and compressor
- Clinical: Supraspinatus tear does not abolish abduction (deltoid compensates)
Transverse Plane Force Couple
Subscapularis (anterior) vs Infraspinatus-Teres Minor (posterior)
Transverse Plane Balance
| Muscle | Action | Force Vector | Tear Effect |
|---|---|---|---|
| Subscapularis | Internal rotation, anterior compression | Prevents posterior subluxation | Anterior instability, loss of IR strength |
| Infraspinatus + Teres Minor | External rotation, posterior compression | Prevents anterior subluxation | Posterior superior migration, loss of ER |
Balance is critical: Subscapularis tear leads to anterosuperior escape of humeral head. Infraspinatus-teres minor tears lead to posterosuperior instability.
Capsular Ligaments and Stability
Inferior Glenohumeral Ligament Complex (IGHLC)
The IGHLC is the primary anterior stabilizer in the abducted, externally rotated position (late cocking phase of throwing).
Anatomy
- Anterior band: Resists anterior translation in ABD-ER
- Posterior band: Resists posterior translation in ABD-IR
- Axillary pouch: Inferior capsule, resists inferior subluxation
- Insertion: Labrum (Bankart lesion disrupts IGHL anchor)
Function by Position
- 90° ABD-ER: Anterior band maximally loaded
- 90° ABD-IR: Posterior band maximally loaded
- Adduction: Redundant, minimal restraint
- Clinical: Bankart repair restores IGHL to labrum
Middle and Superior Glenohumeral Ligaments
MGHL is primary restraint to anterior translation at 45-90° abduction. SGHL and coracohumeral ligament restrain inferior subluxation in adduction.
Clinical Applications
Rotator Cuff Tear Biomechanical Consequences
Supraspinatus Tear
- Loss of compression: Reduced concavity-compression stability
- Superior migration: Deltoid unopposed, head migrates superiorly
- Impingement: Acromiohumeral interval narrows (normal 7-10mm)
- Compensation: Deltoid and remaining cuff can maintain elevation
Subscapularis Tear
- Anterior instability: Loss of anterior force couple
- Anterosuperior escape: Head subluxes anterosuperiorly
- Loss of IR strength: Cannot internally rotate against resistance
- Clinical: Lift-off test positive, belly-press weak
Massive RC tear (greater than 2 tendons or greater than 5cm) causes:
- Loss of both force couples (coronal and transverse)
- Anterosuperior or posterosuperior escape
- Rotator cuff arthropathy (acetabularization of acromion)
Shoulder Arthroplasty Design
Understanding biomechanics guides arthroplasty:
Arthroplasty Design Considerations
| Component | Design Goal | Biomechanical Basis |
|---|---|---|
| Glenoid component | Restore native concavity and version | Concavity-compression requires proper depth and orientation |
| Humeral head size | Match native radius and offset | Maintains normal center of rotation and deltoid moment arm |
| Reverse shoulder | Medialize center of rotation, distalize humerus | Deltoid becomes primary elevator when RC absent (biomechanical compensation) |
Reverse shoulder arthroplasty (RSA) reverses normal biomechanics:
- Glenosphere becomes ball (medializes center of rotation)
- Humeral socket becomes cup (distalizes humerus)
- Deltoid becomes primary elevator (RC not needed)
- Increases deltoid moment arm and reduces joint reactive force
Evidence Base
Scapulothoracic Rhythm in Healthy Shoulders
- Classic 2:1 ratio (GH:ST) throughout abduction arc 30-180°
- 0-30° abduction is primarily GH motion (setting phase)
- Total 180° elevation = 120° GH + 60° ST upward rotation
- Disruption of rhythm occurs with RC tear or scapular dyskinesis
Concavity-Compression Stability Mechanism
- Concavity-compression accounts for 50% of GH stability
- RC compresses head into glenoid, creating friction and depth
- Labrum deepens glenoid 50% (increases stability)
- Loss of concavity (glenoid wear) or compression (RC tear) causes instability
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
Scenario 1: Scapulothoracic Rhythm
"Examiner asks: Describe the scapulothoracic rhythm during shoulder abduction. What happens when this rhythm is disrupted?"
Scenario 2: Rotator Cuff Force Couples
"Examiner shows MRI of massive rotator cuff tear and asks: Explain the biomechanical consequences of this tear and why the patient has superior migration of the humeral head."
MCQ Practice Points
Scapulothoracic Ratio Question
Q: What is the normal scapulothoracic rhythm ratio during shoulder abduction from 30-180 degrees? A: 2:1 (glenohumeral:scapulothoracic) - For every 2 degrees of GH abduction, there is 1 degree of ST upward rotation. Total 180° = 120° GH + 60° ST.
Stability Mechanism Question
Q: What percentage of glenohumeral stability is provided by concavity-compression from the rotator cuff? A: 50% - Concavity-compression (RC compressing head into glenoid) provides 50% of stability. The other 50% comes from static restraints (capsule, ligaments, labrum).
Labral Function Question
Q: By what percentage does the labrum deepen the glenoid socket? A: 50% - The labrum increases glenoid depth from approximately 2.5mm to 5mm, a 50% increase. This enhances stability by increasing contact area and creating a suction seal.
SHOULDER BIOMECHANICS
High-Yield Exam Summary
Stability Mechanisms
- •50% dynamic (RC concavity-compression) + 50% static (capsule/labrum)
- •Glenoid covers only 25% of humeral head (inherent instability)
- •Labrum deepens socket 50% (2.5mm to 5mm)
- •Capsular volume = 2x head volume (allows ROM, sacrifices stability)
Scapulothoracic Rhythm
- •2:1 ratio (GH:ST) from 30-180° abduction
- •0-30° = setting phase (primarily GH)
- •Total 180° = 120° GH + 60° ST upward rotation
- •Scapular muscles: serratus anterior (upward rotation), trapezius (elevation)
Rotator Cuff Force Couples
- •Coronal: Deltoid (superior) vs RC (inferior depressor)
- •Transverse: Subscapularis (anterior) vs infraspinatus-teres (posterior)
- •SITS muscles: Supraspinatus, Infraspinatus, Teres minor, Subscapularis
- •RC tear causes superior migration (deltoid unopposed)
Capsular Ligaments
- •IGHL = primary anterior stabilizer in ABD-ER (90°+)
- •MGHL = primary restraint at 45-90° abduction
- •SGHL + coracohumeral = inferior restraint in adduction
- •Bankart lesion = IGHL detachment from labrum
Clinical Applications
- •Acromiohumeral interval normal = 7-10mm (reduces with RC tear)
- •Reverse shoulder: medializes center, distalizes humerus, deltoid primary elevator
- •Scapular dyskinesis causes impingement (loss of upward rotation)
- •Massive RC tear (greater than 2 tendons or greater than 5cm) may need RSA
Key Numbers
- •Glenoid retroversion: 5° average
- •Clavicular rotation (SC joint): 40-50°
- •AC joint rotation: 20°
- •Supraspinatus critical arc: 60-120° abduction (highest stress)