Kinematics

Four articulations

• GHJ
• Scapulothoracic
• ACJ
• SCJ

ROM

Can use either Thorax or Scapula as plane of reference

Abduction

• Elevation in coronal plane
• Range is 170°
• ER required to prevent GT impingement
• Sometimes described in plane of scapula 30° anterior to coronal plane

Flexion

• Elevation in sagittal plane
• Range is 170°

Rotation

• Varies with elevation of arm
• IR & ER range ~ 90° each

Extension

• Range is 60°

Adduction

• Range is 75°

Range ↓ by ~ 3°/decade in Normal population

Glenohumeral Joint

Humeral head

• Nearly hemispherical
• 1/3 sphere
• 135° to humeral shaft
• Retroverted 30° to axis of elbow flexion

Glenoid Fossa

• SA 1/3 of humeral head
• Longitudinal ø 75% of head
• Tranverse ø 60% of head
• Retroverted 7° to scapula
• Anteverted 30° to thorax
• Faces 5° superiorly
• Glenoid highly congruent to head
  • ~ 1mm diff
• 2° 1/2 bone & 1/2 cartilage
• Allows significant movement
• Makes joint susceptible to instability

Stability

• 1. Dynamic/ Static
• 2. ST/ Muscle/ Bone

Includes

• Glenoid labrum
• Capsule & GHL & CHL
• Coracoacromial Arch
• Rotator Cuff
• Negative Intra-articular Pressure
• H²O Cohesion

Acromioclavicular Joint

Anatomy

• Stability 2° Coracoclavicular ligaments
• Conoid resists 70%
• Trapezoid resists 18%
• Meniscus divides joint into two segments ~ Incomplete
• True diathrodial joint covered in fibrocartilage

Movement

Scapula moves on clavicle in 3 axes

• Vertical axis around conoid ligament
• Transverse axis in coronal plane (hinging on trapezoid ligament)
• Transverse axis in sagittal plane (through ACJ)

Clavicle axially rotates 40° on forward elevation

• 35° at SCJ
• 5° ACJ as scapula rotates as well

Sternoclavicular Joint

Anatomy

• 1° stabiliser is Costoclavicular ligament
• Meniscus divides joint into 2

Movement

• Costoclavicular ligament acts as fulcrum for gliding motion at joint
• AP gliding occurs between sternum & meniscus
• Superior/ Inferior gliding occurs between clavicle & meniscus
• Clavicle elevates at SCJ during shoulder elevation
• 4° Rotation/ 10° shoulder elevation from 0° to 90°
• Clavicle also rotates during shoulder elevation
• 40° during shoulder range

Scapulothoracic Articulation

Anatomy

• Scapula attached to thorax only by SCJ via Clavicle via ACJ
• Scapulothoracic articulation is bone-muscle-bone articulation
• Subscapularis & Serratus anterior lie between scapula & thorax

Movement

• Ratio of GH:ST movement
• Average 2:1
• Varies from 1:1 – 4.3:1
• Task dependent
• Ratio changes during range
• Relatively less ST movement during initial elevation (esp. first 30°)
• ER of scapula occurs during elevation
• Associated with ER of humerus
• 60° of scapular motion possible because
  • Elevation of clavicle of 20° at ACJ & 40° at SCJ
  • Rotation of clavicle of 40°

Kinetics

Complex effects of 20 muscles acting across shoulder

Abduction

• Deltoid & supraspinatus act synergistically
• Equally produce torque
• Either alone can produce full abduction in nerve block tests
• Supraspinatus
  • Initiator of abduction & maintains constant 75° line of pull to glenoid
• Deltoid
  • line of pull improves with abduction ↑ advantage with abduction
• Hence with cuff tear can maintain 90° abduction, but not obtain it
• Rest of cuff depress head to resist shear forces of above 2 muscles
• Scapular Rotation Force Couple
  • During abduction maintains Deltoid & SS at constant length
• Upper Trapezius, Levator Scapulae & Upper Serratus Anterior act with Lower Trapezius & Lower Serratus Anterior to rotate

ER

• 1° = Infraspinatus
• 2° = Teres Minor, Posterior Deltoid

IR

• 1° = Pectoralis Major sternal head, Latissimus Dorsi, Subscapularis, Teres Major

Loads

• At 90° of Abduction
• Deltoid force is 0.7 W
• Joint reaction force is 0.9 W

Anatomical Characteristics of the Shoulder

Glenoid diameter
AP	18-30 mm
Superior-inferior	30-48 mm
Inclination
Glenoid	Average 4.2 degrees (-7 to 20 degrees)
Humeral head	30-55 degrees
Version
Glenoid	1.5 degrees retroversion
Humeral head	0-55 degrees retroversion
Surface area
Glenoid	4-6 mm
Humeral head	11-19 mm
Cartilage thickness
Glenoid	2.16 mm
Humeral head	1.44 mm
Radius of curvature
Glenoid	22-28 mm
Humeral head	23-28 mm
Humeral offset
Medial (coronal)	4-14 mm
Head-shaft angle	30-55 degrees