Impingement occurs when inflammation, bone spurs, or fluid buildup causes a rotator cuff tendon to be squeezed and rub against a bone.
Every time you raise your arm above your head, the upper tendon in the rotator cuff (supraspinatus tendon) and the bursa that lies on top of the tendon glide on the underside of the upper end of your shoulder blade (acromion).
The underside of the acromion may be rough 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 wear against the underside of the acromion.
Tiny tears may develop.
Then, the tendons in the rotator cuff can no longer balance the upward pull of the shoulder muscle (deltoid) on the head of the upper arm bone. This can lead to further damage to the rotator cuff tendon. The weaker the tendon becomes, the more likely it is to get partially or completely torn.