It is common practice to carry out the therapy of periodontal diseases in two phases. In a first, non-surgical phase, bacterial deposits on tooth surfaces (plaque and calculus) are removed using a cleaning method called "scaling and root planing" (SRP). After three to six months the case is evaluated and, if necessary, further treatment is provided, usually taking a more aggressive, surgical approach. Beneficial effects of adjunctive systemic antibiotics on clinical outcomes have been shown repeatedly but specific indications for antibiotics in subgroups of diseased patients, and the optimal timing of antimicrobial therapy, continue to be issues of a long lasting controversy.This study assessed the differential outcomes of periodontal therapy supplemented with amoxicillin-metronidazole during either the non-surgical or the surgical treatment phase.

Single center, randomized placebo controlled crossover clinical trial with a one-year follow-up. Eighty participants with Aggregatibacter actinomycetemcomitans-associated chronic or aggressive periodontitis were randomized into two treatment groups. A: Antibiotics (3/d 500 mg metronidazole plus 375 mg amoxicillin for 7 days) during the first, non-surgical phase of periodontal therapy (T1), and placebo during the second, surgical phase (T2). B: Placebo during T1, and antibiotics during T2. Persistence of sites with a probing depth (PD) >4 mm and bleeding on probing (BOP) was the primary outcome. Evaluations were made three months after T1, as well as 6 and 12 months after T2.

Inclusion Criteria:

• Untreated moderate to advanced periodontitis (at least 4 teeth with a probing pocket depth (PD) >4 mm, clinical attachment loss of at least 2 mm and radiographic evidence of bone loss)
• Presence of at least 12 scorable teeth (not including 3rd molars, teeth with orthodontic appliances, bridges, crowns or implants)
• Aggregatibacter actinomycetemcomitans-positive

Exclusion Criteria:

• Systemic illnesses (i.e. diabetes mellitus, cancer, HIV, bone metabolic diseases or disorders that compromise wound healing, radiation or immunosuppressive therapy)
• Pregnancy or lactation
• Systemic antibiotics taken within the previous two months
• Use of non-steroid anti-inflammatory drugs
• Confirmed or suspected intolerance to 5-nitroimidazole-derivatives or amoxicillin subgingival SRP or surgical periodontal therapy in the last year