Participants with periodontitis and presence of bilateral intrabony defect will be randomised to have one side of the mouth undergo root surface debridement using hand instruments and ultrasonic scaler alone, while the contralateral side will receive the same treatment, with the addition of the Er:YAG laser. Outcomes studies will be periodontal clinical parameters at 3 months, 6 months, 9 months and 12 months after treatment, radiographic parameters at 12 months, patient-reported parameters and laboratory analyses.

The study is a Double-masked randomised controlled trial with split-mouth design.

Participants with periodontitis will be randomised to have one side of the mouth undergo root surface debridement using hand instruments and ultrasonic scaler alone, while the contralateral side will receive the same treatment, with the addition of the Er:YAG laser. Outcomes studies will be periodontal clinical parameters at 3 months, 6 months, 9 months and 12 months after treatment, radiographic parameters at 12 months, patient-reported parameters and laboratory analyses.

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

Randomisation for the site receiving the additional test procedure (laser) will be carried out between baseline and visit 2. The randomisation will be carried out by Postgraduate Students independent of the study. Random permuted block sizes of 4 will be employed. The centralised randomisation service 'Sealed envelope' will be used for randomisation and to ensure allocation concealment. A sealed envelope will be enclosed with the patient's notes by personnel not directly involved in the study. The therapist will only be informed about treatment allocation by opening the envelope following completion of non-surgical therapy (when the laser will need to be employed on the test site). No minimization or stratification is planned. The randomisation code will only be revealed after statistical analysis. The randomisation will be done by a different postgraduate student who will be masked and is not involved in the study.