Inclusion criteria:

1. The patient is 18 years of age or older
2. The patient does not have any systemic diseases that may affect the results of the study
3. The patient has a plaque index of 20% or less at the study entry
4. The patient has a history of severe localized or generalized periodontitis (stage III or IV)
5. Completed the first and second steps of periodontal treatment (According to Treatment of stage I-III periodontitis-The EFP S3 level clinical practice guideline) (Sanz M. et al.)
6. Buccal and/or lingual Class 2A and/or 2B furcation involvement (FI) defects on both sides of the Mandibular first and/or second molars after completing the first and second steps of periodontal treatment.
7. Buccal Class 2A and/or 2B furcation involvement (FI) defects on both sides of the maxillary first and/or second molars after completion of the first and second step of periodontal treatment if furcation involvement on the palatal side (mesiopalatal and distopalatal) does not exceed Class 1.
8. Psychological appropriateness
9. Consents to all follow-up visits

Exclusion Criteria:

1. Patients who have systemic diseases that may affect the results of the study
2. Use of medications that may affect study results (anti-osteoporotic drugs, statins, corticosteroids, anticoagulants)
3. Patients who use calcium channel blockers, cyclosporine A or antiepileptic drugs if there are clinically visible hyperplastic changes in the gingival margin.
4. Patients who have received any type of systemic antibiotics in the last six months prior to the start of the study and patients starting antibiotics during the study
5. Patients requiring antibiotic premedication prior to periodontal treatment
6. Oncological disease
7. Chemotherapy and/or radiotherapy (active or history)
8. Pregnancy and breastfeeding
9. Any condition or current treatment which, in the opinion of the investigator and/or consulting physician, may present an unreasonable risk
10. Psychoemotional disorders and depression
11. Use of antipsychotic medication or antidepressants
12. Lack of patient motivation to undertake adequate dental care at home or complete periodontal treatment, patients residing outside Latvia
13. Molars with combined endodontic-periodontal lesions, active endodontic infection
14. Prosthetic factors for molars not allowing clinical measurements
15. Huge restoration or amalgam fillings that could potentially cause fractures or furcation involvement
16. Enamel pearls or filling or crown margins
17. Decay or root resorption
18. Tooth mobility degree 3
19. Molars in which the gingival margin is positioned apically from the entrance into the furcation area
20. Clinical attachment level and or pockets mesial and distal to the furcation involvement defect of 6 or more mm
21. Strong vomiting reflex that would prevent adequate periodontal treatment
22. People close to the study subjects, work colleagues, relatives, etc.