The Shoulder Joint

2 JOINTS

These bones articulate to form 2 joints within the shoulder joint:

 1. Acromioclavicular (AC) joint:

• Plane synovial (gliding) joint between the clavicle and the acromion of the scapula.
  

 2. Glenohumeral joint:

• Ball and socket joint between the humeral head and the glenoid cavity of the scapula.
  
• Large humeral head and shallow glenoid cavity, allows mobility however it is unstable.
• To increase the stability, the labrum, joint capsule (ligaments) and muscles (rotator cuff, deltoid, pectoralis major) surround the glenoid cavity.

* Imagine the glenohumeral joint as a golf ball (ball) on a tee (socket). The golf ball is the humeral head, the golf tee is the glenoid cavity. Just like a golf ball on a tee, the joint is unstable which commonly leads to dislocation * 

4 LIGAMENTS: (REMEMBER: LIGAMENTS = BONE → BONE) 

• The joint capsule is a lax fibrous sheath that covers the shoulder joint.
  • The ligaments (1,2,3) of the shoulder joint are thickenings of the joint capsule that provide joint capsule stability.
• The laxity of the ligaments allows for free movement of the shoulder joint.

There are 4 main ligaments of the shoulder joint:

1. Glenohumeral ligaments:

• Glenoid cavity → Anterior neck of the humerus.
• Stabilises the anterior aspect of the shoulder joint.

2. Coroacohumeral ligament:

• Base of the coracoid process → greater tubercle of the humerus.
• Strengthens and supports anterior and superior part of the joint capsule.

3. Transverse humeral ligament:

• Transverse across from greater tubercle of humerus → lesser tubercle of the humerus.
• Stabilises the tendon of the long head of the bicep muscle.

4. Coracoacromial ligament:

• Coracoid process → overlies the shoulder joint preventing superior shoulder displacement.

4 ROTATOR CUFF MUSCLES:

The rotator cuff muscles make up the main strength for the stability of the shoulder joint. There are four rotator cuff muscles, that work as a group to hold the large head of the humerus in the shallow glenoid cavity:

TIP: * Mnemonic Alert * The SITS muscles:

• S – Supraspinatus = ABDuction
  
• I – Infraspinatus = external rotation
• T – Teres Minor = ADDuction and external rotation
• S – Subscapularis = ADDuction and internal rotation

References

Moore K, Dalley A, Agur A. (2014). Clinically Oriented Anatomy. 7th ed. Philadelphia: Wolters Kluwer.

Teach Me Anatomy. (2017). The Shoulder Joint. [online]. Available at: http://teachmeanatomy.info/upper-limb/joints/shoulder/ [accessed 1 March 2017].

Tortora G, Derrickson B. (2011). Principles of Anatomy and Physiology. 13th ed. Chichester: John Wiley & Sons.

Image References

Humerus.    Clavicle.   Scapula.   Bones of The Shoulder Joint.   Shoulder Joint 1.   Shoulder Joint 2.   Shoulder Joint 3.   Ligaments 1.   Ligaments 2.   Rotator Cuff Muscles.