dental specialty concerned with the prevention, diagnosis, and treatment of functional and structural diseases of the periodontal membrane and related tissues that surround and support the teeth. Degeneration or inflammation of these tissues can be caused by various systemic or local diseases or by poor oral hygiene. In some cases the cause is not established. Most commonly, periodontal diseases are caused by hardened bacteria, called bacterial plaque, which adheres to teeth and destroys periodontal tissue. The most prevalent periodontal disease is periodontitis, inflammation of the periodontium. If untreated, periodontitis leads to resorption of the portion of the jawbone that supports the teeth.