The combined interaction of four elements produces lesions of the rotator cuff, commonly known as impingement. The elements are: vascular, degenerative, traumatic, and mechanical or anatomic factors. The elements are interrelated, and each affects the tendons in a manner that contributes to tendon weakening. It is unlikely that any one element is solely responsible for cuff lesions; the nature of each lesion is determined by the factors that predominate in that individual case. The net result is degeneration of the tendons.