Shoulder Dislocation - Easy Explanation 🦴

What is Shoulder Dislocation?

The shoulder joint is made up of the head of the humerus (the ball) sitting in the glenoid cavity (a shallow cup on the shoulder blade). A dislocation means the ball has popped out of the cup.

3 Types of Shoulder Dislocation

Shoulder Dislocation
  ├── Anterior (Ant.)
  ├── Posterior (Post.)
  └── Inferior (Luxatio Erecta)

1. Anterior Dislocation (Most Common - ~95%)

The ball slips forward out of the cup
Mechanism remembered as AEB:
- A = Abduction (arm raised away from body)
- E = Extension (arm moved backward)
- B = External Rotation (arm twisted outward)
Think: falling with your arm outstretched

2. Posterior Dislocation (Rare - ~5%)

The ball slips backward
Mechanism remembered as PID:
- P = adduction (arm pulled inward toward body)
- I = Internal Rotation (arm twisted inward)
- D = (Direct blow from front)
Classic cause: electric shock or seizure

3. Inferior Dislocation - Luxatio Erecta (Very Rare)

The arm gets stuck pointing straight up above the head
Very dramatic-looking position

Injuries that Happen with Dislocation

Type	Examples
Bony	Hill-Sachs lesion (dent in humerus), Bankart lesion (chip in glenoid)
Soft Tissue	Capsule tear, labrum tear
Tendinous	Rotator cuff tear

Shoulder Joint Anatomy (Simple)

Glenoid cavity + Head of Humerus = Shoulder Joint

The glenoid is a shallow socket - like a golf tee holding a golf ball. This gives great movement BUT poor stability, which is why the shoulder dislocates more than any other joint.

What Keeps the Shoulder Stable?

Static Stabilizers (passive, no muscle needed)

Glenoid labrum - a cartilage rim that deepens the shallow cup
Capsule - a tight bag surrounding the joint
Ligaments - bands holding bones together
Bones themselves

Dynamic Stabilizers (active, muscles contract)

Rotator Cuff - the main muscular stabilizer
Long head of biceps - attaches to the coracoid process and helps hold the ball in

Rotator Cuff - "SITS" Muscles

This is the most important stabilizer. Remembered by the acronym SITS:

Letter	Muscle	Main Action
S	Supraspinatus	Initiates abduction (first 15°)
I	Infraspinatus	External rotation
T	Teres Minor	External rotation
S	Subscapularis	Internal rotation

Think of these 4 muscles as a "cuff" that wraps around the ball and holds it snugly in the socket at all times.

Ligaments (at the bottom of your notes)

The shoulder has 3 main ligaments:

Superior - with Anterior and Posterior bands
Middle
(likely inferior below - cut off in image)

Quick Summary to Remember

Most dislocations = ANTERIOR (arm was out + externally rotated)
Most dangerous nerve injury = Axillary nerve → deltoid paralysis
Best stabilizer = Rotator Cuff (SITS muscles)
Shoulder is most mobile BUT least stable joint in the body

Source: Pye's Surgical Handicraft, 22nd Edition

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