The shoulder is the most complex joint in the body. It is made up of three bones, the scapula (shoulder blade), the humerus (upper arm) and the clavicle (collar bone). There are four articulations, the AC (acromioclavicular) joint, SC (sternoclavicular) joint, GH (glenohumeral) joint and the scapulo-thoracic articulation. Finally, there are 13 muscles including the four of the rotator cuff. The rotator cuff muscles move our shoulders and also keep it centered in the socket. The rotator cuff muscles are the supraspinatus, whose primary function is to lift your hand over your head, the infraspinatus and teres minor both of which are external rotator’s of the shoulder and the largest muscle of the group is the subscapularis which is the primary internal rotator of the shoulder.
The shoulder is the least stable joint in the body. I liken the shoulder joint to a golf ball sitting on a golf tee. It is a very unstable system. When you tilt the golf ball just a little the ball will roll right off the tee. Why doesn’t our shoulder dislocate throughout the day with activities of daily living? Because there’s a complex system of passive and dynamic stabilizers to keep the ball (humeral head) centered on the socket (glenoid) no matter what position we put our arms in.
What do I mean by passive stabilizer? A passive stabilizer is a structure that keeps the shoulder from dislocating based upon the stabilizer’s shape, structure or location. One example is that the shape or curvature of the socket (glenoid) is the same as the much larger ball (humeral head). Furthermore, a thin film of joint fluid helps the ball stick to the socket. Next there is a rubber-like gasket or washer attached to the periphery of the glenoid called the labrum. The labrum is composed of cartilage and turns the socket into a suction cup. Injury to the labrum of the shoulder can cause painful clicking or worse – instability of the ball in the socket resulting in dislocation. Labrum injuries are associated with shoulder dislocations, overhead throwing or racket sports, and even just the aging process. Most patients over the age of 30 will have a labrum tear of some sort – SLAP tears, posterior labrum tears, degenerative tears, etc. Finally, the joint capsule is another passive stabilizer that acts like a shrink wrap around the shoulder joint. This shrink wrap creates negative pressure in the joint that sucks the ball (humeral head) into the socket (glenoid). Thickening in specific areas of the capsule are referred to as the glenohumeral ligaments. At extreme rotation the ligaments tighten and help to keep the shoulder centered in the joint.
What about the dynamic stabilizers? Dynamic stabilizers prevent the shoulder from dislocating by actively contributing. For example, the most important dynamic stabilizers are the 4 muscles of the rotator cuff. When working together in a normal fashion, they compress or hold the humeral head in the center of the glenoid no matter what position we put our hand in. Weakness due to tearing, inflammation or strains of the rotator cuff muscles will cause the humeral head to move around in the socket more than normal. This will not only cause pain, it can lead to a complete dislocation of the shoulder.
As you can see, the shoulder is a very complex joint. This is why almost everyone gets some sort of shoulder pain in their lifetime!