Inclusion Criteria:  Age ≥ 18 years (age 18-60 years), both genders Good compliance and good oral hygiene Systemically healthy Isolated intrabony defect of more than 3 mm depth, combined with more than 6 mm probing depth and attachment loss (stage III/IV periodontitis) The area of the intrabony defect should not exceed the lingual surface area of the root The morphology of the intrabony defect will be detected during the operation and finally determined whether the patient would be enrolled in the trial. The associated tooth should either maintain normal pulp vitality or should have undergone root canal therapy for at least 6 months before.  Exclusion Criteria:  Full-mouth plaque index ≥ 20% (Ainamo & Bay 1975) Full-mouth sulcus bleeding index ≥ 15% (Mühlemann & Son 1971) Smoking Systemic diseases with compromised healing potential of infectious diseases Drugs affecting periodontal health / healing Pregnant and lactating females Previous periodontal surgery in the examined area Affected teeth with 3° mobility Furcation involvement Acute periapical inflammation