Patients diagnosed with stage III or IV periodontitis who exhibit mandibular first or second molars with Class II, Subclass A, or B buccal and/or lingual mandibular furcation defects (horizontal probing depth of ≥ 3 mm ) will be recruited. Patients diagnosed with stage III or IV periodontitis who exhibit maxillary first or second molars with Class II, Subclass A, or B buccal furcation defects (horizontal probing depth of ≥ 3 mm ) if furcation involvement on the palatal side (mesiopalatal and distopalatal) does not exceed Class 1 will be recruited. This study will be a prospective, randomized, single-blinded, controlled clinical trial with a split-mouth design. Patients will be recruited from the clinics of the Riga Stradiņš University Institute of Stomatology of Riga, Latvia. Before the baseline treatment, all patients who meet the inclusion criteria will be given detailed oral hygiene instructions and motivation. Patients' furcation defects in each quadrant will be randomly allocated at a 1:1 ratio to either the application of Oscillating Chitosan Brush with Enamel Matrix Derivatives (test group) or Oscillating Chitosan Brush alone (control group). Non-surgical periodontal treatment of furcation defects on the test side will be performed through the application of Oscillating Chitosan Brush with Enamel Matrix Derivatives and on the control side with Oscillating Chitosan Brush alone. The re-evaluation of periodontal status will be performed 12 weeks following the baseline treatment. Biofilm samples and gingival crevicular fluid (GCF) samples will be taken at the baseline and at 4 weeks.