Normally shoulder anatomy delicately balances the humeral head into the cup of the shoulder (the glenoid) with boney and soft tissue stabilization. A good portion of this stability comes from the rotator cuff which centers the humeral head into the joint as it appears in this X-ray.
The tendons of these muscles can tear however, and this suction effect can be lost resulting in increasing superior (or upwards) migration of the humeral head. This inhibits the deltoid from have an appropriate pull or vector on the humerus.
As the head moves up out of the cup and sits underneath a portion of the shoulder blade (the acromion of the scapula). Once this occurs, rotator cuff repair is widely seen as untenable, therefore another option must be utilized.
This effectively destabilizes the joint and can often lead to patients having problems raising their arm because the humeral head is not being compressed into the cup (the glenoid. In these cases, often due to irreparable rotator cuff tears from either cuff tear arthropathy, massive chronic tears, or even failed attempts at repair, a reverse shoulder arthroplasty is often indicated.
The biomechanical principles of reverse shoulder arthroplasty more or less creates a stable fulcrum for your deltoid to lift your arm above your head.
Dr. Schwartz has trained with French pioneers that have popularized this surgery in the United States as well as an American pioneer that has challenged the way we think about this surgery. Please contact Dr. Schwartz if you have any further questions!