How a Healthy Shoulder Works
Your shoulder is the most moveable joint in the body. It is made up of three bones:
- the collarbone, or clavicle
- the shoulder blade, or scapula
- and the upper arm bone, or humerus
There are also two important joints that allow for movement:
- The acromioclavicular joint connects the upper part of the shoulder blade (the acromion) to the collarbone.
- The glenohumeral joint, also known as the shoulder joint, is a ball-and-socket joint that connects the upper arm to the shoulder blade. This joint allows free movement of the arm so that it can rotate in a circular fashion.
Although the shoulder is the most moveable joint in the body, it is unstable because the ball (the humerus) is larger than the socket (the glenoid) that holds it. To maintain stability, the bones of the shoulder are held in place by muscles, tendons, and ligaments. Tendons are tough cords of tissue that attach muscles to bone, and ligaments attach bones to each other for stability.
The rotator cuff is made up of four muscles and their tendons, which act to hold the upper arm (humerus) to the socket of the shoulder (glenoid fossa). The rotator cuff also provides mobility and strength to the shoulder joint. Two sac-like structures, called bursae, allow smooth gliding between the bone, muscle, and tendon. They also cushion and protect the rotator-cuff structures from the upper part of the scapula (the acromion).
What causes shoulder pain?
According to the AAOS about 23,000 people have shoulder replacement surgery each year. This compares to more that 700,000 Americans a year who have knee and hip replacement surgery. Shoulder problems may arise because of injury to the soft tissues of the shoulder, overuse or underuse of the shoulder, or even because of damage to the tissues.
Shoulder problems result in pain, which may be localized to the joint or travel to areas around the shoulder or down the arm. Damage to the shoulder joint may result in instability of the joint, and pain is often felt when raising the arm or when soft tissues are trapped between the bones (impingement). Impingement is particularly common in sports activities that involve repetitive overhead arm motions, such as pitching baseballs.
You may have a shoulder injury if:
- Your shoulder is stiff and doesn’t allow full normal movement.
- Your shoulder lacks strength to perform your daily activities.
- Your shoulder feels as if it’s slipping out of place (upper arm bone “popping” or a feeling that your arm is sliding out of the shoulder socket).
Another common cause of shoulder pain is arthritis. The most common type of arthritis is osteoarthritis (OA) — sometimes called degenerative arthritis because it is a “wearing out” condition involving the breakdown of cartilage in the joints. OA can occur without a shoulder injury, but this seldom happens since the shoulder is not a weight-bearing joint like the knee or hip. Instead, shoulder OA commonly occurs many years following a shoulder injury, such as a dislocation, that has led to joint instability and damage, allowing OA to develop.