Classification of Periodontitis The American Academy of Periodontology's (AAP, 2017) classification of periodontal diseases and conditions distinguishes 3 forms of periodontitis: Necrotizing periodontitis Periodontitis as a direct manifestation of systemic disease Periodontitis Other periodontal conditions included in the AAP designations are periodontal abscesses, periodontitis associated with endodontic lesions, developmental or acquired deformities and conditions, and peri-implant diseases.