1. The shoulder joint consists of three joints.
The glenohumeral joint is where the top of the arm bone (humeral head) meets the glenoid portion of the shoulder blade. The acromioclavicular joint is where the collarbone (clavicle) meets the acromion portion of the shoulder blade. The scapulothoracic joint is where the shoulder blade (scapula) meets the rib cage (thorax).
2. A shoulder dislocation involves the glenohumeral joint.
Whereas the hip is a ball-and-socket joint, the glenohumeral joint is more like a ball (humeral head) on a plate (glenoid). A shoulder dislocation occurs when the ball slides off the plate. Most dislocations are anterior, with the ball sliding off the front of the plate. Rarely are dislocations posterior or inferior. The labrum is an O-ring of cartilage that encircles the glenoid. It provides some added depth and stability to the shoulder, and it acts as a bumper preventing dislocation. When the ball slides off the plate, the labrum is often torn off and flattened. This makes it easier to subsequently re-dislocate. Also, the back of the ball bangs into the sharp front edge of the plate, creating a dent known as a Hill-Sachs lesion. Over time, with multiple dislocations, the Hill-Sachs lesion can become quite large. This causes the ball to become less round and more flattened in the back. As a result, the ball slides off the plate more easily. And once it has slid off, the Hill-Sachs lesion can "engage" the front of the plate, making the shoulder very difficult to pop back in.
3. A shoulder separation involves the acromioclavicular (AC) joint.
In contrast to a shoulder dislocation, a shoulder separation involves the AC joint. Depending on the severity and grade of injury, certain ligaments are either stretched or torn. An injury to this joint usually causes a bump to appear, which is the end of the collarbone popping up. Most minor AC separations can be treated with a sling for comfort, followed by a return to activity once the pain subsides. Higher grade injuries may require surgery to restore the shape and function of the shoulder.
4. The rotator cuff helps keep the ball on the plate.
The rotator cuff is a group of four muscles attached to the shoulder blade--the subscapularis, infraspinatus, teres minor and supraspinatus. They become tendons and surround the humeral head. Together, the rotator cuff tendons provide additional stability to the joint. They can become torn either traumatically or with wear and tear over time.
5. Impingement involves the subacromial space.
Between the rotator cuff and the acromion lies the subacromial bursa in the subacromial space. Think of the bursa as a fluid-filled balloon sitting between the tendon and bone above it. Without it, the tendon would eventually be rubbed away by the bone from the arm being lifted. Sometimes the space is tight, from scar tissue, inflammation (bursitis) or bone on the under surface of the acromion. These things can impinge upon the rotator cuff tendons as the arm is raised, causing pain or limitations of motion.
