Anatomically, the humeral head is by nature retroverted, the post instability does not necessarily require trauma involving a large range of motion (unlike previous dislocations often consecutive to a ‘thwarted arm’)
Seizures of epilepsy type grand mal (commitiale crisis). Following a sudden and intense contraction of the muscles with tendency to wind the arms in internal rotation, the joint is dislocated towards the back. The associated amnesia of the crisis may mask the notion of trauma. The trauma is so violent that it can also be a dislocation fracture.
The anatomical disorders of orientation of the bony parts (exaggeration of the retroversion of the humeral head or presence of a retroversion of the glenoid).
Constitutional or pathological joint hyperlaxity (Ehlers Danlos syndrome)
We must distinguish between recurrent involuntary instability initially linked to trauma (see above) and reproducible, reproducible forms present since childhood that do not require medical care.
Radiographically from the front there is a superposition of the humeral head and the glenoid of the scapula, in profile dislocation is evident with a head located behind the glenoid.
In emergency in someone very painful the technical realization of good quality x-rays can be problematic, so the diagnosis can be difficult or inapparent especially when there is only a simple bone overlay.
Maneuvers to reduce posterior dislocation are different from those of anterior dislocation, often requiring general anesthesia. The movement is less. Immobilization requires neutralization of the natural retroversion of the humeral head, otherwise it will be recurrence in the form of a neutral rotation cushion.
Like any shoulder instability, the reinforcement of the stabilizing muscles of the glenohumeral joint most often makes it possible to limit the recurrent character. Rehabilitation is therefore fundamental.
Surgery may be indicated in recurrent forms resistant to medical treatment, the technique of which varies from case to case and surgeons: