Most often, rotator cuff disorders develop gradually, from aging, continued overuse, repetitive activity, or a combination of these things.

The rotator cuff is a group of four tendons. These tendons connect the main muscles of the shoulder-the supraspinatus, infraspinatus, subscapularis, and teres minor-to the upper arm. The rotator cuff tendons and muscles stabilize the shoulder joint and allow you to raise and rotate your arm. Every time you raise your arm above your head, the upper tendon in the rotator cuff (supraspinatus tendon) and the subacromial bursa, which lies on top, glide on the underside of the upper end of your shoulder blade (acromion). The undersurface of the acromion may be rough or abnormally shaped and rub or scrape (abrade) the bursa and tendon. Activities that require repeated overhead arm movements can result in painful inflammation of the bursa (bursitis) and/or the tendon (tendinitis).

Over time, the tendon may rub against the undersurface of the acromion, causing tiny tears and bleeding. As those tears heal, the original healthy tissue is replaced by scar tissue that is weak, less flexible, and stringy. The gradual scarring of the tendon causes the entire rotator cuff to weaken. Finally, the tendons in the rotator cuff can no longer balance the upward pull of the muscle in your shoulder called the deltoid. This can lead to further damage to the tendon, renewing the cycle of tearing and scar formation. The weaker the tendon becomes, the more susceptible it is to partial or complete tears.

Without treatment, a cycle of inflammation, tearing of tendons, and scar formation may develop, resulting in pain and loss of function. This cycle of inflammation, wear and tear, and limited use can lead to other shoulder problems, such as stiffness or frozen shoulder (adhesive capsulitis).

Rotator cuff tears generally occur when the related tendons become weak from inflammation, scarring, or fraying. Tears often result from slow, progressive damage over time, often without a known injury. In older, less active adults, even simple movements such as lifting a suitcase can cause a tear.

It takes tremendous force to tear a healthy rotator cuff tendon. This may happen while you are playing sports or during an accident or a severe fall.