The aim is to evaluate in a prospective, randomized, controlled clinical study the healing of a MIST procedure combined with PRF, or GBT combined with PRF. Clinical, radiological evaluation of the regenerative surgical method and the GBT non-surgical therapy in solitaire deep intrabony periodontal defect.

This clinical study aims to clinically and radiologically evaluate the healing response of the periodontal tissues in there is not opened flap design to access the intrabony defect test 1 group where The pre- surgically prepared Platelet Rich fibrine is added into the previously cleaned defect. In the test 2 group a minimally invasive surgical approach is used to access to the intrabony pocket. After opening a flap the same procedure is used to prepare PRF, and added into the pocket. The pockets in the control Group is only cleaned by guided biofilm therapy. Periodontal clinical parameters measured at baseline and at 6 months serves for secondary. After six months the healing is evaluated with a standardized X-ray holder. The primary outcome value is the delta clinical attachment level (gain or loss) The main interest is how the PRF influence the non-surgical healing. Does it any advance of flap opening procedure in comparison with the GBT, if PRF is used.

Inclusion Criteria:

• solitaire intrabony periodontal defect at least 4 mm

Exclusion Criteria:

• Periodontitis with intrabony defect (Intrabony component ≥ 4mm, defect radiological angulation > 25°,
• The patient is able to comply with the study -related procedures (i.e. good level of oral hygiene, follow-up procedures).
• Patients must not be heavy smokers (<5 cigarettes/day).
• furcation involvement
• Full mouth plaque and bleeding scores (FMPS and FMBS) of <15% (O'Leary et al. 1972).
• The patient is able to fully understand the nature of the study, signed informed consent.