Calcific tendinitis (also calcific/calcifying/calcified/calcareous tenonitis/tendonitis/tendinopathy, tendinosis calcarea, hydroxyapatite deposition disease and calcific periarthritis), a form of tendinitis, is a disorder characterized by deposits of hydroxyapatite (a crystalline calcium phosphate) in any tendon of the body, but most commonly in the tendons of the rotator cuff (shoulder), causing pain and inflammation. The condition is related to and may cause frozen shoulder. The etiology is unknown.