What is the main mechanism of dislocation?
Anteroinferior dislocations classically occur with a combination of abduction, external rotation and extension, similar to the Zero Position described here. A fall onto the outstretched arm transmitting the force to the glenohumeral joint is a typical mechanism involving these movements.
Alternatively the patient can fall sideways and backwards onto the point of the shoulder, forcing the humeral head anteriorly.
Subcoracoid (anterior) dislocation occurs when the arm is in the low to mid range of abduction and externally rotated.
Subglenoid (anteroinferior) dislocations usually result from rapid hyperabduction.
Spontaneous – The forces required to displace the humeral head vary from case to case. In fact recurrent dislocators may require little or no force – dislocating from performing simple tasks such as waving goodbye or putting on a seatbelt. When the dislocation occurs with trivial trauma it is described as a spontaneous dislocation.
The rare ones:
Subclavicular and intrathoracic dislocations involve large lateral to medial forces on the abducted humeral shaft.
Luxatio erectae (true inferior dislocation) usually results from sudden or extreme hyperextension (bodysurfers beware of big waves!).
Posterior dislocations usually result from a forced adduction, internal rotation and flexion of the humerus. They are rare but has an association with seizures and electrocution.