Inclusion Criteria:  Minimum of 20 teeth. Diagnosis of Severe Periodontitis (Diagnosis of Periodontitis stage III or IV (grade A, B or C) (Tonetti et al. 2018)  Presence of at least 2 teeth with ≥ 6 mm pockets and minimum of 3 mm intrabony defect, in opposing quadrants of the upper or lower arch.  Exclusion Criteria:  Inclusion:  Minimum of 20 teeth. Diagnosis of Severe Periodontitis (Diagnosis of Periodontitis stage III or IV (grade A, B or C) (Tonetti et al. 2018)  Presence of at least 2 teeth with ≥ 6 mm pockets and minimum of 3 mm intrabony defect, in opposing quadrants of the upper or lower arch.  Exclusion:  Wearing of a removable appliance/denture Wearing of orthodontic appliances. Current smoking - any history of smoking or vaping within last 5 years Relevant medical history likely to affect periodontal disease or taking relevant medication, including diabetes, as judged by the examining clinician Periodontal treatment (surgical/non-surgical root surface debridement under local anaesthesia) within last 12 months, as judged by the examining clinician History of any antibiotic use within last 3 months. Pregnant patients Teeth with hopeless prognosis indicated for extraction (Cortellini et al 2011) Patients with a non-adequate level of English