Inclusion Criteria:  1) diagnosis of severe chronic periodontitis (corresponding to grade III or IV according to the actual classification); 2) good general health; 3) full-mouth plaque score (FMPS) and full-mouth bleeding score (FMBS) < 20%; 4) at least two contralateral sextants with residual PDs of > 5 mm and persisting bleeding on probing (BoP) at posterior natural teeth 3 months after the completion of cause-related therapy  Exclusion Criteria:  1) pregnancy and lactation; 2) smoking > 10 cigarettes/day; 3) intake of antibiotics in the previous 6 months. In addition, teeth with degree II or III mobility, horizontal bone loss higher than 1/3 of the root length or designed as abutment for prosthetic rehabilitation were also excluded from the study.