Sites will be treated by ultrasonic debridement of the pocket, performed using a piezoceramic ultrasonic device. For the purpose, a Tip will be connected to the Handpiece Led (5 minutes/pocket) . Then, it will be followed by the subgingival use of erythritol powder applied by the use of Handpiece connected to an airflow unit 2x5 seconds/pocket. At this time the subgingival delivery of metronidazole gel will be provided. After instrumentation, patients will rinse with chlorhexidine 0.20% 3 times/day for 2 weeks.