Anatomy of the Shoulder
The shoulder joint, where the bones in the shoulder meet, is the body's most mobile joint. It allows for a large range of movement, much of it central to everyday activity. Being capable of that range of movement, however, makes the shoulder joint inherently unstable. Tendons, ligaments, muscles and the glenoid labrum are in place to compensate for that instability. Still, shoulder-joint injuries are not uncommon, and should be evaluated by a medical professional to determine their severity and whether treatment is required.
Bones and Joints in the Shoulder
The shoulder comprises three bones: the clavicle, humerus and scapula, more commonly known as the collarbone, upper arm bone and shoulder blade, respectively. There are also three joints in the shoulder responsible for movement, including:
Glenohumeral Joint (GHJ)
The glenohumeral joint connects the ball of the humerus to the socket in the scapula, allowing the arm to rotate and be moved away from the body. It provides most of the motion in the shoulder, and is commonly thought of as the "shoulder joint." As with almost any joint in the body, articular cartilage covers the surfaces where the ball and socket meet. White, shiny, rubbery and slippery, it enables the surfaces of the joints to slide against one another without causing damage.
Because of the shallowness of the socket, the glenohumeral joint has surrounding structures to support it and hold everything in place. These structures form a protective capsule designed to cushion and lubricate the joint. Injury to the GHJ is often caused by falling on an outstretched arm, a forced twisting of the arm or a blow to the shoulder.
Acromioclavicular Joint (ACJ)
The acromioclavicular joint is formed by the connection of the clavicle to the highest point of the scapula, the acromium. This joint allows the arm to be raised above the head. Injuries to this area, called separations, are usually the result of falling on the shoulder and tearing ligaments, which are soft-tissue structures that connect bones to one another. Injury to the ACJ is often caused by a fall that is broken by an outstretched hand.
Sternoclavicular Joint (SCJ)
The sternoclavicular joint connects the clavicle and the uppermost part of the breastbone, and provides most of the arm's ability to rotate. It is the only bony joint linking the bones of the arm and shoulder to the main part of the skeleton. Strong ligaments surround it to keep it stable. Injury to this joint is fairly uncommon, usually occurring from a blow to, or fall on, the joint, or from a direct hit to the clavicle or breastbone. Many SCJ injuries are caused by car accidents.
Ligaments in the Shoulder
The ligaments in the shoulder provide it with support and keep it from dislocating. One important ligament in the shoulder is the glenoid labrum, which is dense, fibrous tissue that runs around the socket cavity. It makes the shallow socket cavity deeper, providing the shoulder with more stability. Glenoid labrum injuries can result from repetitive shoulder movement or acute trauma.
Muscles and Tendons in the Shoulder
Tendons attach muscles to bones, and when muscles pull on those tendons, bones move. There are important muscles and tendons in the shoulder that help with its stability. They include the rotator cuff tendons, which attach to the deeper rotator cuff muscles, the main muscles used in conjunction with the GHJ. The rotator cuff is made up of four muscles that connect the scapula to the humerus. These muscles pull the head of the humerus into the scapula so that they are fitted tightly against one another, stabilizing the GHJ. The rotator cuff also helps with arm movement, particularly rotation. The rotator cuff is subject to repetitive-motion injury, and is often a problem for athletes who throw, swim or play tennis.
The biceps muscle is found at the front of the upper arm. It has two tendons that attach it to the bones of the shoulder and one tendon that attaches it to the radius bone at the elbow. Biceps tendon injuries become more common with age as tendons become less elastic.
Another muscle in the shoulder is the deltoid, the outer layer of the shoulder muscle. It makes the shoulder look rounded. The largest, strongest shoulder muscle, it is essential for raising the arm. Injury to the deltoid often results from heavy lifting.