Stability of the Shoulder Joint
The shoulder joint sacrifices stability for mobility. Despite this, several factors maintain its integrity.
Factors Maintaining Stability
1. Coracoacromial Arch (Secondary Socket)
- Formed by the coracoid process + coracoacromial ligament + acromion process
- Provides a protective roof over the humeral head
- Prevents superior displacement of the humeral head
2. Rotator Cuff (Musculotendinous Cuff)
- SITS muscles (Supraspinatus, Infraspinatus, Teres minor, Subscapularis)
- Tendons blend with the capsule and compress the humeral head into the glenoid
- Reinforces the capsule on all sides except inferiorly
- The inferior gap explains why dislocations are primarily subglenoid
3. Glenoid Labrum
- Fibrocartilaginous rim deepens the shallow glenoid cavity
- Increases the surface area for articulation
- Provides mechanical resistance to translation of the humeral head
4. Long Head of Biceps Brachii
- Intracapsular but extrasynovial
- Arises from supraglenoid tubercle; passes above the head of humerus
- Acts as a dynamic stabiliser, preventing upward displacement
5. Long Head of Triceps Brachii
- Arises from infraglenoid tubercle
- Provides some inferior stability
6. Atmospheric Pressure
- The negative intra-articular pressure acts as a suction mechanism
- Contributes to passive stability of the joint
7. Surrounding Muscles
- Deltoid, pectoralis major, latissimus dorsi, teres major, and coracobrachialis all contribute to dynamic stability by compressing the humeral head
Weakest Point
The capsule is least supported inferiorly (no rotator cuff reinforcement). The axillary nerve lies in close proximity here.
Primary dislocations are therefore subglenoid (inferiorly). Clinically described as anterior or posterior depending on the final resting position of the humeral head relative to the infraglenoid tubercle.
Clinical Implication
The inverse relationship between mobility and stability means the shoulder is:
- Most commonly dislocated joint in the body
- Prone to recurrent dislocations
- Susceptible to instability, degenerative changes, and rotator cuff disorders — especially in individuals performing repetitive overhead activities