4. Etiology and mechanism( Chemico-bacterial theory Proteolytic theory Proteolytic-chelation theory Three primary factors theory Department Of Oral Pathology, Shandong University 4http://1.51.214.13Department Of Oral Pathology, Shandong University 4 http://1.51.214.13 Chemico-bacterial theory Proteolytic theory Proteolytic-chelation theory Three primary factors theory