by Jennifer Trizuto, MPT
You probably don’t spend much time thinking about your shoulders. If you did, you might come to realize the important role they play in your everyday life, helping you dress, eat, drive, type, and care for your family. But for the most part, people don’t really notice how much they need their shoulders until something goes wrong. Then it becomes clear how necessary it is to care for them.
The shoulder is truly an amazing joint. It can move your arms in any direction in front of you, to your sides, over your head, and even behind you. When you shake hands, throw a ball, comb your hair, or reach for something on a high shelf, you are making use of your shoulders’ extraordinary range of motion. But with this versatility comes a greater risk of injury and pain. In fact, problems with the shoulders are relatively common. In 2003, 1.37 million Americans sought help for shoulder problems. People can develop shoulder pain from all sorts of activities, including household chores, heavy lifting on the job, and recreational sports.
Why are shoulder problems so common? To answer that question, you first need to understand how the shoulder is designed.
The shoulder is the meeting place of three bones: the arm bone, or humerus, which is shaped like a ball at the top; the collarbone, or clavicle, which is at the front of the shoulder; and the shoulder blade, or scapula, which is in the back of the shoulder. These three bones actually create three different joints, all of which play a role in shoulder movement. The main joint — the one most people are referring to when they talk about the “shoulder joint” — is called the glenohumeral joint. Like many joints in the body, the glenohumeral joint is a ball-and-socket joint. The “ball” at the end of the humerus fits into a concave “socket” in the scapula. As with all mobile joints in the body, the ends of the bones in the shoulder are covered by a layer of slippery cartilage, making the motion of the joint smooth and easy.
When you think of a ball-and-socket joint, you probably imagine a deep socket at one end with a ball that fits snugly inside it on the other. The hip exemplifies this type of ball-and-socket joint. The shoulder, however, is different. It has a shallower socket than the hip and looks more like a golf ball on a tee. The benefit of the shallower socket is mobility. If you compare the mobility of your hip with that of your shoulder, the difference is clear. What the hip lacks in mobility, however, it makes up for in stability.
The shoulder, while very mobile, is much less stable than the hip. Just as it is easy for a golf ball to slip off a tee, it is relatively easy for the ball of the humerus to slip out of its socket — in other words, to dislocate. The reason that your shoulders aren’t “slipping off the tee” every day is that you have many supporting structures in the shoulder. One type of supporting structure is the ligaments in the shoulders, which connect the various bones in the joint. Another supportive soft tissue structure is the joint capsule, which is a kind of envelope around the joint that holds everything inside. Within the capsule is fluid called synovial fluid that helps keep the joint nourished and moving smoothly. The ligaments also help to support the capsule.
Two other contributors to joint stability are tendons and muscles. Tendons are tough cords of tissue at the ends of muscles that attach the muscle to the bone and help the joint move. The most well-known shoulder muscles are those of the rotator cuff. The rotator cuff is a group of four muscles and several tendons that surround the top of the humerus and help keep it in the socket of the scapula. The rotator cuff is among the most commonly injured parts of the shoulder.
Another important component of the shoulder is the bursae — small, fluid-filled sacs that create a cushion for the tendons to glide on, helping to decrease friction in the joint. Bursae are found in most joints and save the tendons a great deal of wear and tear.
It should now be clear how complex the shoulder is. In part because of the shoulder’s wide range of movement, many of the structures described above can run into problems. For example, the ligaments can loosen, decreasing joint stability and increasing the risk that you will dislocate your shoulder. The joint capsule itself can become inflamed and stiffen, a condition called adhesive capsulitis, or “frozen shoulder.” Weakness in the muscles and tendons can cause tendon inflammation, known as tendinitis. The tendons and muscles can also wear out and even tear (rotator cuff tears are especially common). The bursae can become inflamed and swollen, causing pain.
The shoulder may be injured during sports or other recreational activities, or by heavy lifting. But often the cause of shoulder pain is the gradual degeneration of the soft tissues around the joint, including the rotator cuff muscles, that comes with age. According to the National Institute of Arthritis and Musculoskeletal and Skin Diseases, people are much more likely to have rotator cuff problems after age 60. Regularly participating in activities that involve a lot of overhead reaching, including house painting, washing windows, and sports such as tennis, can increase the likelihood of developing shoulder pain at younger ages.
Another potential problem is degeneration of the cartilage that lines the ends of the bones in the joints. When the cartilage wears away, the resulting condition is called osteoarthritis (OA), a common cause of pain in the shoulder. Nobody really knows why people develop shoulder OA, although it is thought that the shoulder’s mobility and the prevalence of activities that put stress on the shoulder are involved.
Last Reviewed on February 23, 2011
Jennifer Trizuto is a senior physical therapist at Mills Health Center in San Mateo, California, and has worked with people with arthritis since 1994.
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