How the Shoulder Works
Of all the joints in the body, the shoulder is the joint most prone to dislocation. It can dislocate as a result of accidents, whether from involvement in a motor vehicle collision, sporting activity, or from a fall from a standing position onto an outstretched arm. Recovery from a dislocated shoulder can take months to regain normal function.
A little over a year ago, I dislocated my shoulder in an accident. I will describe here what I now know about dislocation from a patient's viewpoint. Before doing so, I have a few general things to say about healthy joints.
Shoulder and Other Joints
The shoulder is one of the most complicated joints in the human body. Many of the other joints, such as elbows and fingertip joints, are like simple hinges. They can only bend in one direction and straighten out (Fig. 1a). The knee has a little more movement. In addition to bending and straightening out, knees can to some extent rotate. However, in comparison to other joints, the shoulder is relatively complex in its structure and this gives it greater movement capabilities.The rounded top of the upper arm bone, the humerus, fits into a shallow socket located in the upper part of the scapula, just above its shoulder blade (Fig. 1b).
When working properly, this ball-and-socket arrangement allows the arm to move in most directions, including an arc of almost 360°.
The hip joint has a similar ball-and-socket structure. Hips can also dislocate as a result of trauma such as from water skiing accidents, but hips are more stable than shoulders and tend to dislocate less frequently.
What is a dislocated shoulder?
The arm is normally held in the shoulder socket by the soft tissue capsule which fits over the joint like a sock (Fig. 1c, left). It is also held together and stabilised by fibrous ligaments that lie within the capsule, by the muscles and tendons that rotate the arm. A dislocated shoulder is an upper arm out of its socket (Fig. 1c, right).
Instability is usually defined as a clinical syndrome which occurs when a shoulder is loose enough to produce symptoms. It can refer to either outright dislocation where the upper arm bone comes out of the socket or to a more subtle slipping of the humeral head within the socket, a condition known as subluxation. Click here for a detailed description of normal shoulder structure and what happens during dislocation (section 2).
What causes the shoulder to dislocate?
Shoulders can dislocate when a strong force, such as a traumatic injury, abnormally stretches the ligaments and tendons, causing the ball-shaped end of the humerus to pop out of its socket. A minority of people have shoulders that can sublux or even dislocate spontaneously. However, almost 95% of shoulder dislocations result from either a forceful collision or from a sudden wrenching movement as may occur during sport, from falling onto an outstretched arm, and from motor vehicle collision. In my case, I fell from a bicycle onto my arm after a slight bump from a passing car that came too close.