The primary aim of the study is to evaluate the efficacy of conservative surgery with or without papilla preservation technique in the treatment of deep residual pockets ≥ 6 mm in stage III-IV periodontitis patients after cause related therapy (step II) in terms of endpoints of therapy and other secondary outcomes at different time intervals (1, 3 and 5 years). Moreover, the study aims to evaluate the stability of the results obtained for the different surgical approaches over a 5 years follow-up period.