There are two main types of dislocation: subcoracoid (anterior), and subglenoid (anteroinferior). These comprise 90-95% of all shoulder dislocations. Other rarer dislocations include subclavicular, intrathoracic, posterior and luxatio erecta (or true inferior).
Dislocations may also involve bony injuries (such as Hill-Sachs lesions, bony Bankart lesions or fractures of the glenoid neck, clavicle, humeral head or greater tuberosity), ligamentous injuries, and neurovascular damage. Click here to review associated injuries.
Subcoracoid (anterior) dislocation
In a subcoracoid dislocation, the humeral head sits anterior and medial to the glenoid, just inferior to the coracoid, as seen in the pictures below . This type of dislocation occurs in about 60% of cases.
Subglenoid (anteroinferior) dislocation
In a subglenoid dislocation the humeral head sits inferior and slightly anterior to the glenoid, as seen in the pictures below. Note, as above, that the humeral head has also travelled medially. This type of dislocation occurs in approximately 30% of cases.
What are the changes that occur when the humeral head moves out of the glenoid fossa (i.e. dislocates)?
In a dislocation the stability of the joint is altered, as the humeral head sits in either a subcoracoid or subglenoid position. As we can see from the pictures above, to relocate the humeral head, it either needs to move posterior and laterally (if subcoracoid dislocation) or superior and laterally (if subglenoid dislocation).
What is stopping the relocation?
The humeral head wants to be relocated in the glenoid fossa. However this is prevented by both static (joint capsule, lip of labrum) and dynamic forces (rotator cuff and other muscles).
How can we overcome these obstacles, and relocate the humeral head?
This can occur by manipulating the humerus or manipulating the scapula. Click here for relocation techniques.
External rotation of the humeral head presents a greater articular surface superiorly to the inferior border of the glenoid fossa. This allows it to roll past the labral rim.
Finding the “zero position” – the point at which all the muscles traversing the glenohumeral joint line up to have the least rotatory and translation forces. This allows the opposing articular surfaces of the humeral head and glenoid rim to slide over eachother without difficulty, facilitating relocation.
Alternatively, the scapula can be rotated so that the glenoid points inferiorly, which presents an easier path for the returning head. This is achieved by directing the tip of the scapula towards the spine or by shrugging the shoulders. Click here to review how the scapula can move.
Which muscles contribute most to the dynamic forces when the shoulder is dislocated?
Dislocation of the glenohumeral joint places tension on several muscles of the shoulder. Spasm and tightening of both the long head of biceps (which traverses over the top of the capsule) and the subscapularis muscles are thought to play the major roles in preventing reduction.
This is reflected in the success of relocation methods that involve reduction of muscle spasm, as in the Cunningham,or Kocher’s methods or by aligning the joint into Zero Position, such the Milch and Scapular Manipulation methods.
What are the other types of dislocation?
Other rarer dislocations include; posterior (4-10%), luxatio erecta (true inferior dislocation), subclavicular and intrathoracic.