Shoulder

OVERVIEW

• The shoulder serves to connect the upper extremity to the trunk.
• The muscles acting here are often denervated in brachial plexus and proximal peripheral nerve injury patterns.
• With dysfunction at the shoulder, the upper extremity cannot be properly positioned in space to perform normal functions even if normal hand or other distal function is preserved.
• Restoration of shoulder movement must be a priority in peripheral nerve injury patients.

Structures of the Shoulder

• Bones:
  • Thorax (including sternum, ribs, spinal vertebrae, etc.)
  • Clavicle
  • Scapula
  • Humerus
• Joints:
  • Scapulothoracic
  • Acromioclavicular joint
  • Glenohumeral joint
  • Sternoclavicular

Movements of the Shoulder

Movement at the shoulder is complex:

• The shoulder glenohumeral joint where the key motion occurs is dependent on normal movement at the shoulder girdle (which is made up of the scapula/clavicle and their connection to the thoracic). 
• Dysfunction of scapular motion (which can be seen in various patterns of brachial plexus and peripheral nerve injury) can greatly impede shoulder, and thereby upper extremity, movement.

Key movements:

• Flexion and extension are movements about a coronal axis.
• Abduction and adduction are movements about a sagittal axis.
• Horizontal abduction and horizontal adduction are movements in a transverse plane about a longitudinal axis.
• Internal/Medial rotation and external/lateral rotation are movements about a longitudinal axis through the humerus.

Scapular Movement (in turn allows normal glenohumeral joint movement):

• Abduction and adduction
• Upward (lateral) rotation and downward (medial) rotation
• Anterior tilt and posterior tilt
• Elevation and depression

Glenohumeral Joint Movement:

• Anterior flexion and posterior extension
• Abduction and adduction
• Horizontal abduction and horizontal adduction
• Lateral/external and medial/internal rotation
• Circumduction

Muscle Name	Innervation	Action
Major Muscles
Trapezius	Spinal accessory nerve	Helps move/stabilize the scapula to allow normal shoulder motion especially shoulder abduction beyond 90º.
Serratus anterior	Long thoracic nerve	Abduction (protraction) of the scapula. Responsible for assisting with shoulder flexion especially beyond 90º of flexion.
Rhomboid Major and Minor	Dorsal scapular nerve	Scapular adduction or retraction, elevation, and downward rotation.
Subclavius	Subclavius nerve	Depresses and stabilizes the clavicle.
Pectoralis Major	Lateral and medial pectoral nerve	Adducts and medially rotates the humerus.
Pectoralis minor	Medial pectoral nerve	Thrusts shoulder forward; tilts scapula anteriorly; aids respiration.
Levator Scapulae	Cervical (C3, C4) nerve, dorsal scapular nerve	Elevates the scapula.
Teres Major	Subscapular nerve	Medially rotates, adducts, and extends the shoulder joint.
Latissimus Dorsi	Thoracodorsal nerve	Abducts, extends and internally rotates the shoulder.
Deltoid (Anterior Fibers, Middle Fibers, Posterior Fibers)	Axillary nerve	Shoulder joint abduction, flexion (anterior fibers), extension (posterior fibers), and  (depending on patient position) internal (patient supine) and external rotation (patient prone).
Coracobrachialis	Musculocutaneous nerve	Flexes and adducts the shoulder joint.
Rotator Cuff Muscles
Supraspinatus	Suprascapular nerve	Abduction of the shoulder.
Infraspinatus	Suprascapular nerve	Externally rotates the shoulder.
Teres Minor	Axillary Nerve	Externally rotates the shoulder.
Subscapularis	Upper subscapular nerve, lower subscapular nerve	Medially rotates the shoulder joint; stabilizes the head of the humerus.