Shoulder: Clinical Anatomy & Biomechanics

Shoulder: Clinical Anatomy & Biomechanics

• The shoulder complex consists of the glenohumeral, acromioclavicular, sternoclavicular joints, and the scapulothoracic articulation and subacromial space.

Shoulder Complex

• Glenohumeral Joint: The primary articulation connecting the humerus and the scapula.
• Acromioclavicular Joint: Connects the acromion process of the scapula and the clavicle.
• Sternoclavicular Joint: Connects the sternum and the clavicle.
• Scapulothoracic Articulation: The area where the scapula moves against the chest wall, although not a true joint, it allows for scapular movement.
• Subacromial Space: The area between the acromion, the humeral head and tendons, plays a major role in shoulder movement/function.

Anatomical Orientation of the Shoulder Complex

• The scapular plane is oriented at a 35-degree angle from the sagittal plane.
• The humerus has a 30-degree retroversion.
• The clavicle sits at a 20-degree angle with the scapula.

Stability Functions of Shoulder Girdle

• Provides a stable base for shoulder muscles to generate force.
• Shoulder girdle muscles act as stabilizers.
• Correct force-length relationships are maintained.
• Shoulder joint congruence is maximized.

Mobility Functions of the Shoulder Girdle

• Permits the largest range of motion (ROM) of any complex in the body.
• The scapulohumeral rhythm allows for increased ROM with less compromise of stability (4 joints vs 1 joint).
• Facilitates movements of the upper extremity by positioning the glenohumeral joint favorably.

Shoulder Girdle Motion

• Elevation/Depression: Upward/Downward movement
• Protraction/Retraction (Abduction/Adduction): Forward/Backward movement
• Upward/Downward rotation: Rotation of the scapula

Shoulder Joint Movements

• Flexion/Extension: Bending and straightening the arm
• Abduction/Adduction: Moving the arm away from and towards the body
• Horizontal Abduction/Adduction: Moving the arm horizontally away from and towards the body
• Diagonal Abduction/Adduction: Moving the arm diagonally away from and towards the body
• Outward/Inward Rotation: Rotating the arm externally and internally

Muscular Considerations

• The muscles of the scapula and humerus are critical for the range of motion (ROM) of the shoulder.

Shoulder Girdle Muscles

• Pectoralis Minor: Downward rotation, depression, and protraction of the scapula.
• Serratus Anterior: Protraction and upward rotation of the scapula.
• Subclavius: Stabilizes the sternoclavicular joint.
• Levator Scapulae: Elevation and downward rotation of the scapula.
• Rhomboids: Downward rotation, retraction, and elevation of the scapula.
• Trapezius (Upper, Middle, Lower):
  • Upper: Elevation, upward rotation of scapula.
  • Middle: Retraction of scapula.
  • Lower: Depression, retraction of scapula

Shoulder Joint Muscles

• Pectoralis Major: Flexion, horizontal adduction, and internal rotation of the humerus.
• Deltoid: Nearly all arm movements from the shoulder joint (internal rotation, abduction, hyperextension, flexion, horizontal adduction).
• Coracobrachialis: Forward movements of the humerus.
• Subscapularis: Internal rotation of the humerus, stabilizing the glenohumeral joint.
• Biceps Brachii: Flexion and adduction, and horizontal adduction.
• Supraspinatus: Abduction of the humerus.
• Infraspinatus: External rotation, horizontal abduction, and stabilizes the humerus during flexion.
• Teres Minor: External rotation, horizontal abduction, and stabilizes the humerus during flexion.
• Latissimus Dorsi: Extension and adduction of the arm.
• Teres Major: Shoulder extension, adduction, and medial rotation.
• Triceps Brachii: Adduction, extension, and hyperextension of the humerus.

Scapulothoracic Muscles

• Elevators (Upper): Trapezius, Levator scapulae, Rhomboids
• Depressors (Lower): Trapezius, Pectoralis minor
• Protractors: Serratus anterior, Pectoralis minor
• Retractors: Mid and Lower Trapezius, Rhomboids, Levator scapulae
• Upward Rotators: Serratus anterior, Upper Trapezius
• Downward Rotators: Rhomboids, Pectoralis minor, Levator scapulae

Glenohumeral Joint Muscles

• Abductors: Deltoid, Supraspinatus
• Adductors: Pectoralis major, Latissimus dorsi, Teres major
• Internal Rotators: Pectoralis major, Subscapularis, Anterior deltoid, Latissimus dorsi, Teres major
• External Rotators: Infraspinatus, Teres minor, Posterior deltoid
• Flexors: Biceps brachii, Coracobrachialis, Anterior deltoid
• Extensors: Triceps brachii, Posterior deltoid, Latissimus dorsi, Teres major

Specific Joints Structure and Function

• Sternoclavicular Joint: Clavicle, Sternum, 1st Rib, Sternoclavicular ligaments, Costoclavicular ligament, Articular disc. Muscular attachments.
• Sternoclavicular Joint Motions: Elevation/Depression, Protraction/Retraction, Axial Rotation.

Acromioclavicular Joint

• Gliding/Planar Joint: Articulating surfaces glide over one another, rather than hinge-like or ball-and-socket joint.
• Acromioclavicular Ligament: Connects the acromion and clavicle
• Coracoclavicular Ligament (Conoid/Trapezoid): Connects the coracoid process and clavicle:
• Articular Disc: Provides cushion between bones.
• Acromioclavicular Joint Motions: Upward/Downward rotation and Horizontal & sagittal plane adjustments.

Coracoclavicular Ligaments

• Conoid Ligament (medial): Connects acromion to the clavicle,
• Trapezoid Ligament (lateral): Connects acromion to the clavicle,
• Maintains the acromioclavicular joint, supports the scapula.

Scapulothoracic Articulation

• Not a true synovial joint
• Provides movement base for the humerus
• AC joint + SC joint motion = ST joint motion

Primary Movements of Scapulothoracic Articulation

• Elevation and Depression
• Retraction and Protraction
• Downward and Upward rotation

Scapulothoracic Elevation

• SC joint elevation + AC joint downward rotation = ST Elevation

Scapulothoracic Upward Rotation

• SC joint elevation + AC joint upward rotation = ST Upward rotation

Scapular Rotation

• Enhances glenohumeral stability
• Elevates the acromion to avoid impingement
• Maintains effective length tension relationship for scapulohumeral muscles

Glenohumeral Joint

• Humeral Head: Ball-shaped end of the humerus
• Glenoid Fossa: Socket of the scapula
• Glenoid Labrum: Ring of cartilage that deepens the glenoid fossa
• Rotator Cuff Muscles (SITS): Supraspinatus, Infraspinatus, Teres Minor, Subscapularis
• Capsuloligamentous Complex: Capsules and ligaments surrounding the joint
• Coracohumeral Ligament: Reinforces joint capsule
• Long Head of Biceps: Connects to joint
• Subacromial Space: Space between acromion, humeral head and rotator cuff tendons.

Glenohumeral Ligaments

• SGHL: Superior glenohumeral ligament
• MGHL: Middle glenohumeral ligament
• IGHL: Inferior glenohumeral ligament (complex of anterior and posterior bands, and axillary pouch).

Coracohumeral Ligament

• Anterior and Posterior bands

Glenohumeral Joint Motions

• Flexion/Extension, Abduction/Adduction, Internal/External Rotations
• Cannot be isolated from other shoulder girdle elements

Glenohumeral Joint Stability

• Static restraints (Bony structures, Labrum, Capsuloligamentous structure, Negative joint pressure)
• Dynamic restraints (Rotator Cuff, Biceps, Proprioceptive receptors, Neuromotor coordination)

Kinematics and Biomechanics

• Convex Concave Rules: Describes rolling and gliding motions between joint surfaces.
• Normal Arthrokinematics: How the surfaces of a joint move in relation to one another (glenohumeral joint). Rotation and translation are combined, with joint surfaces trying to maintain humeral head center within the glenoid.

Glenohumeral Roll/Glide

• Describe glenohumeral joint surface movement during shoulder actions.

Glenohumeral Joint Motions: Abnormal Arthrokinematics

• Capsular tightness
• Rotator cuff tears

Clinical Implication

• Clinical Relevance to Subacromial Space: Avoid impingement by appropriately rotating the humerus and upward rotation of scapula to elevate the lateral end of the acromion.
• Primary impingement: Structural stenosis of subacromial space.
• Secondary impingement: Functional stenosis of subacromial space due to abnormal arthrokinematics.

Dynamic Stabilization Mechanisms

• Passive muscle tension
• Compressive muscles from muscle contraction
• Joint motion that results in passive structure tightening.
• Redirection of joint force toward the center of the glenohumeral joint.

Dynamic Stabilization Mechanisms

• Force-length relationships vary due to multiple joints.
• Tension development in agonist and antagonist muscles is important for preventing humeral head dislocation.
• Force couples—two or more forces acting on an object where the forces are equal and opposite.

Force Couples Acting on Glenohumeral Joint

• Anterior/Posterior rotator cuff muscles (transverse plane).
• Deltoid versus inferior rotator cuff muscles (coronal plane)

Force Couples Disruption

Rotator Cuff Muscles

• Supraspinatus
• Infraspinatus
• Teres Minor
• Subscapularis

Rotator Cuff – Dynamic GH Stabilizer

Rotator Cuff Function

• Approximates the humerus to function
• Supraspinatus assists the deltoid in abduction
• Subscapularis, Infraspinatus & Teres minor depress the humeral head

Scapulo humeral rhythm

• Movement relationship between humerus and scapula during arm raising motions.
• 2:1 ratio: for every 3 degrees of shoulder abduction, 2 degrees occur at the GH joint and 1 degree occurs at the SC joint.

Scapulo humeral rhythm Clinical Implications

• Rotator cuff pain in early phase of active abduction
• Impingement pain in mid phase of active abduction
• AC pain in later phase of active abduction

Effects of Poor Scapular Stabilization

• Poor resting posture of the scapula
• Loss of upward or optimal tilt of scapula
• Loss of static stability of the glenohumeral joint
• Inferior translation and internal rotation of humeral head
• Traction ischemia in rotator cuff (especially supraspinatus) muscles
• Capsular laxity