The rotator cuff is a group of muscles and tendons surrounding the shoulder joint. A rotator cuff injury can cause an ache in the shoulder, which can worsen when you sleep on the involved side.

A rotator cuff tear is a common cause of pain and disability among adults. Each year, almost 2 million people in the United States visit their doctors because of a rotator cuff problem.

A torn rotator cuff will weaken your shoulder. This means that many daily activities, like combing your hair or getting dressed, may become painful and difficult to do.

Your shoulder is made up of three bones: your upper arm bone (humerus), your shoulder blade (scapula), and your collarbone (clavicle). The shoulder is a ball-and-socket joint: the ball, or head, of your upper arm bone fits into a shallow socket in your shoulder blade.

Your arm is kept in your shoulder socket by your rotator cuff. The rotator cuff is a group of four muscles that come together as tendons to form a covering around the head of the humerus. The rotator cuff attaches the humerus to the shoulder blade and helps to lift and rotate your arm.

The rotator cuff tendons cover the head of the humerus (upper arm bone), helping you to raise and rotate your arm.

There is a lubricating sac called a bursa between the rotator cuff and the bone on top of your shoulder (acromion). The bursa allows the rotator cuff tendons to glide freely when you move your arm. When the rotator cuff tendons are injured or damaged, this bursa can also become inflamed and painful.

When one or more of the rotator cuff tendons is torn, the tendon no longer fully attaches to the head of the humerus.

In most rotator cuff tears, the tendon is torn away from the bone.

Most tears occur in the supraspinatus tendon, but other parts of the rotator cuff may also be involved.

In many cases, torn tendons begin by fraying. As the damage progresses, the tendon can completely tear, sometimes with lifting a heavy object.

There are different types of tears.

• Partial tear. This type of tear is also called an incomplete tear. It damages the tendon, but does not completely sever it.
• Full-thickness tear. This type of tear is also called a complete tear. It separates all of the tendon from the bone. With a full-thickness tear, there is basically a hole in the tendon.