A 16-year-old has 8–10 mm probing depths at tooth #9, 6–7 mm at #3 and #14, and 5 mm at scattered other sites. Plaque scores are 20% with little visible calculus. Radiographs show 3–4 mm of vertical bone loss localized to the incisors and first molars. How is this best classified under the 2017 AAP/EFP scheme?

Correct answer: C. Stage III, Grade C, molar-incisor distribution

• A. Stage II is moderate disease with attachment loss of 3–4 mm and bone loss confined to the coronal third. Probing depths of 8–10 mm with vertical defects are well past that, and Grade A denotes slow or arrested progression, the opposite of this presentation.
• B. The severity is correctly read as Stage III, but Grade B is wrong. Grade B describes destruction roughly commensurate with the biofilm present. Here a minimally plaque-covered 16-year-old has 3–4 mm vertical defects — the mismatch is the whole point of the case. Note also that under the 2017 system the correct extent term for this pattern is molar-incisor distribution, not simply 'localized'.
• C. CORRECT. STAGE III is set by severity and complexity: interdental clinical attachment loss of 5 mm or more, radiographic bone loss extending to the middle third of the root, probing depths of 6 mm or more, and vertical defects of 3 mm or more — with no tooth loss from periodontitis and no need for complex rehabilitation. GRADE C follows from the rate of progression: destruction that is DISPROPORTIONATE to the biofilm present (20% plaque, minimal calculus) in a 16-year-old is the defining Grade C indicator, as is a bone loss percentage to age ratio above 1.0. The MOLAR-INCISOR DISTRIBUTION is the extent descriptor — the pattern formerly named localized aggressive periodontitis, now folded into this system as a distribution rather than a separate disease.
• D. Stage IV requires additional complexity beyond Stage III: five or more teeth lost to periodontitis, masticatory dysfunction, secondary occlusal trauma with mobility of Grade 2 or more, severe ridge defects, bite collapse, drifting or flaring. None of that is described here.

Note: The three descriptors answer three separate questions. STAGE = how bad is it and how hard will it be to treat (I–IV). GRADE = how fast is it moving and why (A slow, B moderate, C rapid; upgraded by smoking of 10 or more cigarettes a day, by HbA1c of 7.0% or higher, or by destruction out of proportion to biofilm). EXTENT = where is it (localized under 30% of teeth, generalized 30% or more, or molar-incisor distribution). A young patient with clean teeth and deep defects is the archetypal Grade C.