This is a pilot prospective cohort study to assess the efficacy of minimally-invasive non-surgical periodontal therapy (MINST) for the treatment of teeth with unfavourable periodontal prognosis. 20 patients who have teeth with unfavourable periodontal prognosis (due to bone loss to the apex or furcation involvement grade II C) will receive minimally-invasive non-surgical treatment (MINST) and will be reassessed up to 12 months after treatment.

Twenty patients with advanced periodontitis who have teeth with unfavourable periodontal prognosis (due to bone loss to the apex or furcation involvement grade II C) will be entered in this study, conducted at Guy's Hospital in London, UK. The study has a prospective cohort design, so following baseline assessment, all patients will receive minimally-invasive non-surgical treatment (MINST). A total of 10 visits are planned over a 12-13 months period including initial treatment (MINST) and follow-up appointments.

The primary outcomes are radiographic defect depth (for teeth with bone loss to the apex, n=10) and furcation involvement (for teeth with furcation involvement degree II C, n=10). Paired t-test will be used to detect significant changes between baseline and 12 months for the primary outcomes using the patient as unit of analysis. Additional outcomes will be probing pocket depths (PPD), clinical attachment level (CAL), levels of inflammatory markers and growth factors in gingival crevicular fluid (GCF) and saliva and bacterial detection in periodontal pockets

Inclusion Criteria:

• Age 18-70
• Diagnosis of Periodontitis stage III or IV (grade A, B or C) (Tonetti et al. 2018)
• Presence of:
• N=10: Maxillary or mandibular molars with:

A. Furcation involvement degree II C based on Hamp et al. 1975 horizontal classification and Tarnow & Fletcher 1984 vertical classification (with concomitant furcation involvement (FI) not exceeding degree II on the other aspects of the same tooth) B. gingival recession ≤ 2mm on furcation site C. mobility maximum degree 2

• N=10: 'Teeth with bone loss to the apex' (also defined as 'hopeless' by Cortellini et al. 2011), defined as having:

A. radiographic bone loss to the apex or beyond the apex on at least one inter-dental aspect B. presence of severe attachment loss (>10 mm) at the same inter-proximal site/s and at least at one of either the buccal or lingual aspect (Cortellini et al. 2011) C. lack of function because of hypermobility, and/or lack of chewing strength, and/or pain, and/or recurrent periodontal infection/abscesses (Cortellini et al. 2011) D. positive response to vitality testing with electric pulp test (EPT) or previous endodontic treatment considered satisfactory, as judged by the examining clinician

- Willing to sign informed consent to take part in the study

Exclusion Criteria:

• Smoking (any current or in past 5 years)
• Medical history including diabetes or hepatic or renal disease, or other serious medical conditions or transmittable disease
• History of conditions requiring prophylactic antibiotic coverage prior to invasive dental procedures
• Anti-inflammatory or anticoagulant therapy during the month preceding the baseline exam
• Systemic antibiotic therapy during the 3 months preceding the baseline exam
• History of alcohol or drug abuse, vii) Self-reported pregnancy or lactation
• Other severe acute or chronic medical or psychiatric condition or laboratory abnormality that according to the investigator may increase the risk associated with trial participation
• Periodontal treatment to the study site within the last 12 months