Experimental Design This is a prospective, randomized and controlled clinical trial designed to evaluate the clinical and radiographic outcomes 12 months after two treatment modalities of IBDs: test sites ( TS) will be treated by a combination of PRF and ABB; control sites (CS) will receive the combined treatment of collagen membrane + ABB. The patients enrolled will exhibit unfavorable IBDs and all experimental sites will be accessed with a simplified papilla preservation flap (SPPF) procedure. The filling material (ABB) will be common to both treatments: therefore, the investigation focused on the addictive effect of PRF used instead of COLLAGEN MEMBRANE. Collagen membrane is chosen as the active control (AC) as it is currently considered as the "gold standard" among the biological agents for the treatment of IBDs.

A non-inferiority trial (NIT) is designed to verify whether to use PRF instead of collagen membrane in association with ABB can lead to not inferior therapeutic results, reducing the treatment's cost and allowing the clinicians to use autogenous materials only.

Study Population 62 patients (33 males and 29 females) aged 42 to 64 years (mean: 53±12) seeking treatment at the Unit of Periodontology of the "G. D'Annunzio" University of Chieti- Pescara, Italy, and affected by moderate-to-severe chronic periodontitis, will be selected for the study. The inclusion criteria are: 1) no systemic diseases; 2) no medications affecting periodontal status during the previous 6 months; 3) not pregnant or lactating; 4) non-smoker; and 5) the following dental and periodontal factors: a full-mouth plaque score (FMPS)25 and a full-mouth bleeding score (FMBS)26 \< 20% at the time of surgery, no periodontal therapy in the 2 previous years, no inadequate endodontic treatment at the experimental sites, no dental mobility, ≥ 20 teeth, exhibit vertical bone loss detected by radiographic examination ( ACL (alveolar crest level) - (BD bottom of the defect) distance = BDD (Bone defect depth ) ≥ 4 mm and a probing pocket depth (PPD) ≥ 5mm when evaluated 12 weeks after non- surgical therapy \[ scaling and root planing ( SRP)\]. Only predominantly 1-, combined 1- and 2-, 2-walls defects or teeth with a defect angle ≥ 36° were considered in this study (unfavorable IBDs). To be included in this category, the IBDs should not have a 3-wall component \>20% of the total defect depth. The architecture of the defect had to be confirmed by observation during the surgical intervention. Each patient will participate in the study with a single experimental site. In the case that a patient will have more than one IBD with clinical features that could be eligible for study, the most severe defect will be chosen. The participants volunteered for the study after they will receive verbal and written information and signed a consent form approved by the Ethical Committee of the G. D'Annunzio University of Chieti - medical faculty. The study protocol is in accordance with the Declaration of Helsinki of 1975, revised in Tokyo in 2004. The study was performed from February 2013 to December 2016. Four months before the surgical treatment, all 44 patients will undergo SRP by ultrasonic instruments II and hand curettes¶ and motivational instructions on oral home care.