The Shoulder Is Like a Ball-on-a-Plate or a Golf Ball-on-a-Tee
The shoulder is a very mobile joint. The humeral head (top part of the arm bone) is round, while the glenoid portion of the shoulder blade is relatively flat. As a result, we have the ability to move the shoulder in all planes of motion. Think of the ways swimmers warm up their shoulders, whipping them around like propellers. So the shoulder is more like a ball-on-a-plate than a ball-and-socket joint. Another way of picturing this is a golf ball-on-a-tee. The golf ball is perched tenuously on the tee and can fall off rather easily.
Subluxations Are Partial Dislocations
Sometimes, the humeral head will slide partially off the glenoid, but not all the way. This is called a subluxation. Because it does not slide all the way off, it usually will pop right back into place. Dislocations occur when the humeral head slides entirely off the glenoid. The humeral head may get locked out of socket, especially as the muscles spasm.
Shoulder Dislocations Can Happen Anteiror, Posterior or Inferior
The direction of dislocation refers to the direction the humeral head goes. Dislocations can happen from falls or other trauma, such as car accidents. They occur when significant force is placed onto the shoulder, when the joint is in a susceptible position. A particularly precarious position is the abducted-externally rotated position. Think of the position the shoulder is in right before you're about to throw a ball. You can imagine if the arm is externally rotated further (hand placed further behind the head), or the elbow is pushed backwards, the shoulder might pop out the front. Other examples are during football, when you're trying to make an arm tackle, but the other player runs through your tackle. Your arm would be wrenched backwards and may dislocate.
Most Dislocations Are Anterior
Posterior dislocations are rare. They mainly occur in cases of seizures, as the muscle imbalance pulls the arm out the back. The muscles that internally rotate the arm are much stronger than the external rotators. In cases of involuntary muscle contractions, which can also occur with electrocution, the humeral head internally rotates so much that it spins out the back. Rarer still are inferior dislocations. They result in an interesting phenomenon called luxatio erecta, in which the arm is locked into a raised position, such as when you raise your arm to ask a question.
Your Risk for Recurrence Depends on Your Age
The younger you are, the more likely you will re-dislocate. Teenagers have upwards of a 90 percent chance of re-dislocating. This is because of a labral tear or Bankart lesion. In older patients, dislocations tend to cause rotator cuff tears, but usually do not result in repeated dislocations.
