A regular check-up has detected crowded teeth on the front lower jaw of a 15-year-old boy. The gums in this area are slightly edematous and hyperemic with a cyanotic tint. The gingival margin has a torusal thickening. Dental calculus is observed on the teeth. The Schiller Pisarev test is positive. X-ray shows that in the front lower jaw the contours of the cortical plate are fuzzy on the apices of the interdental septa, though the cortical plate itself is intact. Make the diagnosis: