Impingement syndrome occurs when there is inflammation of the rotator cuff tendons and the bursa that surrounds these tendons. Impingement syndrome, or shoulder bursitis, occurs when there is inflammation in the space between the top of the humerus and the acromion (bone at the tip of the shoulder).
Between these bones lies a group of tendons (forming a single combination tendon) known as the rotator cuff, and the bursa that protects them. Impingement syndrome is a collection of symptoms caused by pinching of the rotator cuff and subacromial bursa between the top of the humerus and the acromion (the bone at the tip of the shoulder). In many individuals with this problem, the shape of their bones is such that they have less space than most others. Therefore, small thickenings of the tendons or bursa can cause symptoms.
Normally, the rotator cuff slides freely within this space. However, in some instances this space becomes too narrow for normal motion, and the tendons and bursa become inflamed. Inflammation leads to thickening of the tendons and bursa, and contributes to the loss of space. Eventually, this space becomes too narrow to accommodate the tendons and the bursa, and every time these structures move between the bones they are pinched; thus impingement.