Healing response to non-surgical periodontal therapy: Time-varying effect of smoking

Objectives: To compare the time-varying effect of non-surgical periodontal therapy on the reduction in probing pocket depth (PPD) between smokers and non-smokers using a 4-level multilevel model. Methods: 40 patients, 20 smokers and 20 non-smokers, underwent non-surgical periodontal therapy over a 12 month period. The follow-up changes from baseline in PPD for the 594 initially diseased sites (baseline PPD ≥ 5 mm) were analyzed. In order to investigate the time-varying effects of the independent variables on the healing response, a 4-level regression model with subject as level 4, tooth as level 3, site as level 2 and time as level 1 was adopted. Results: The 4-level model showed that two factors had time-varying effects. Non-smokers had significantly greater reduction in PPD than smokers, particularly in the 6-month (0.77mm) and 12-month (0.73mm) than 3-month follow-up (0.35mm, p < 0.05). Lower full-mouth % plaque at baseline was associated with a significantly greater reduction in PPD only in the 12-month follow-up (p < 0.05). Three other factors showed constant effect over the follow-up period. Posterior teeth, lingual surfaces and sites with plaque at baseline had significantly lower reductions in PPD throughout the follow-up period (p<0.05). Conclusions: A 4-level multilevel model, with time as the lowest level, can provide statistical inference on the time-varying effects of the independent variables on the healing responses following non-surgical periodontal therapy. Smokers showed a time-varying less favourable PPD reduction at deep sites after non-surgical periodontal therapy.