What Structures are Injured After a Shoulder Dislocation?
When a shoulder dislocates, the labrum typically tears away from the glenoid (socket). The labrum is a “gasket” type of cartilage which is attached around the entire edge of the glenoid. The shoulder joint allows for the largest range of motion of any joint in the body. It does so because the glenoid is a shallow socket and shoulder stability is provided primarily by the labrum and attached ligaments. By comparison, the hip joint has a deep bony socket and stability is provided primarily by the bone.
With an anterior shoulder dislocation, the labrum tears off the front of the socket. The back of the humeral head also gets impacted on the front side of the socket, causing an impaction fracture called a Hill Sachs defect in the humeral head. In some cases, the edge of the bone socket may even chip or fracture (bony Bankart fracture). In someone who has repeated dislocations, over time the humeral head slowly wears away the cartilage and bone from the front edge of the socket, causing the shoulder to be even more unstable as more bone is lost from the edge of the socket. The shoulder joint is like a golf ball on a golf tee. The ball is held in place due to the concavity of the golf tee and surrounding ligaments, but if you crack the edge of the golf tee, the shoulder becomes even more unstable as bony congruency is critical to stability.
Unfortunately, a labral tear or a bony Bankart fracture will not heal in the correct position. Therefore, if you have sustained one dislocation, you are at higher risk of having yet another dislocation in the future, especially if you are young, male and play contact sports. With each additional dislocation, more damage can occur to the labrum, ligaments and bone. There is also a correlation with the number of shoulder dislocations and the development of future shoulder arthritis. It can get to the point where people even start to dislocate their shoulder simply putting on a shirt or while sleeping. These patients often come to surgery to stabilize their shoulder.