Anatomy of the Shoulder

1. Bones of the Shoulder Girdle

• Clavicle - the only bony connection between the upper limb and the axial skeleton
• Scapula - a flat, triangular bone with key landmarks:
  • Glenoid cavity (fossa) - shallow, oval depression that articulates with the humeral head
  • Acromion - projects superolaterally, forms roof of the shoulder
  • Coracoid process - projects anteriorly, serves as attachment for muscles and ligaments
  • Spine of scapula - prominent ridge on the posterior surface
  • Scapular notch - transmits the suprascapular nerve (artery passes above the ligament)
• Humerus - the proximal end has:
  • Head - articulates with the glenoid (4:1 surface area ratio - head is much larger)
  • Anatomical neck
  • Surgical neck - common fracture site; the axillary nerve runs here
  • Greater tubercle - insertion of supraspinatus, infraspinatus, teres minor
  • Lesser tubercle - insertion of subscapularis
  • Intertubercular (bicipital) groove - lodges the long head of biceps tendon

2. The Four Joints of the Shoulder Complex

3. The Glenohumeral Joint - Key Features

Glenoid Labrum

Joint Capsule

• Fibrous sheath attaching from the anatomical neck of the humerus to the rim of the glenoid fossa
• Very loose and redundant - the axillary recess forms inferiorly when the arm hangs at rest; this allows full abduction range of motion
• The capsule is thin and unprotected posteriorly (no ligament reinforcement)
• The subtendinous bursa of subscapularis and the subcoracoid bursa communicate with the joint cavity
• The synovial sheath of the long head of biceps also communicates with the joint cavity via the intertubercular groove

4. Ligaments

Glenohumeral Ligaments (reinforce the anterior capsule)

• Superior glenohumeral ligament (SGHL) - prevents inferior subluxation when arm is adducted
• Middle glenohumeral ligament (MGHL) - limits external rotation and anterior translation
• Inferior glenohumeral ligament (IGHL) - the most important stabilizer; its anterior band is the primary restraint against anterior dislocation when the arm is abducted and externally rotated (the "AMBRI/TUBS position")

Coracohumeral Ligament

• Arises from the base of the coracoid process; splits into two bands attaching to the greater and lesser tubercles
• Reinforces the superior capsule; stabilizes the long head of biceps tendon in the rotator interval

Coracoacromial Ligament

• Spans from coracoid process to the undersurface of the acromion
• Together with the acromion and coracoid, forms the coracoacromial arch - a rigid roof over the rotator cuff that protects the humeral head superiorly but is also the site of rotator cuff impingement

Coracoclavicular Ligament

• Connects the clavicle to the coracoid process (two parts: conoid and trapezoid)
• Critical for AC joint stability; rupture = grade III AC separation (Rockwood III)

5. Rotator Cuff (SITS Muscles)

6. Other Muscles Acting on the Shoulder

• Trapezius (upper, middle, lower) - elevates, retracts, and depresses the scapula
• Serratus anterior - protracts the scapula and holds it against the chest wall; innervated by the long thoracic nerve (C5-C7); injury causes "winged scapula"
• Rhomboids (major and minor) - retract and elevate the scapula; innervated by dorsal scapular nerve
• Levator scapulae - elevates and medially rotates the scapula

• Pectoralis major - flexion, adduction, internal rotation
• Latissimus dorsi - extension, adduction, internal rotation
• Teres major - adduction and internal rotation
• Biceps brachii (long head) - assists in shoulder flexion; stabilizes the humeral head by compressing it into the glenoid

7. Bursae

• Subacromial bursa - the largest and most clinically important; lies between the deltoid/acromion and the supraspinatus tendon; inflammation causes subacromial impingement syndrome
• Subdeltoid bursa - often fuses with the subacromial bursa
• Subcoracoid bursa - communicates with the joint cavity
• Subtendinous bursa of subscapularis - communicates with the joint cavity

8. Neurovascular Supply

Nerve Supply

• Axillary nerve (C5-C6) - supplies deltoid and teres minor; vulnerable in shoulder dislocation and surgical neck fractures; tested by sensation over the "regimental badge" area (lateral upper arm)
• Suprascapular nerve (C5-C6) - supplies supraspinatus and infraspinatus; can be compressed at the scapular notch
• Subscapular nerves (upper C5-C6, lower C5-C6) - supply subscapularis; lower subscapular also supplies teres major
• Long thoracic nerve (C5-C7) - supplies serratus anterior; injury = winged scapula

Blood Supply

• Anterior and posterior circumflex humeral arteries - branches of the axillary artery; form an anastomotic ring around the surgical neck
• Suprascapular artery - supplies supraspinatus and infraspinatus
• Thoracoacromial artery - supplies the acromioclavicular region

9. Stabilizers - Summary

10. Range of Motion