The shoulder trades stability for range: it moves more than any other joint. A large ball sits on a small, shallow socket and is held there mainly by muscles.
Ball and socket
The head of the humerus meets the glenoid of the scapula. A ring of fibrocartilage, the labrum, deepens the shallow glenoid a little.
Rotator cuff
Four muscles wrap the joint and keep the humeral head centered: supraspinatus, infraspinatus, teres minor and subscapularis. Their tendons can become inflamed or tear, especially with overhead work and age.
The rest of the shoulder girdle
The acromioclavicular (AC) joint links the clavicle to the acromion, and the sternoclavicular joint links the clavicle to the sternum. The scapula also glides on the chest wall, adding to arm elevation.
Dislocation
Most dislocations are anterior, with the humeral head pushed forward and down, often by a fall on an outstretched, externally rotated arm. The axillary nerve can be stretched, causing numbness over the outer shoulder.
| Muscle | Main action | Quick test |
|---|---|---|
| Supraspinatus | Starts abduction | Empty can test |
| Infraspinatus | External rotation | Resisted external rotation |
| Teres minor | External rotation | Resisted external rotation |
| Subscapularis | Internal rotation | Lift-off or belly press test |
At the bedside
In a suspected dislocation, check sensation over the outer upper arm and the distal pulses before and after any reduction. Test each rotator cuff muscle against resistance to find which tendon is weak or painful.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).