Currently, it is generally acknowledged that the PD ≥ 5 mm after 3 months of periodontal non-surgical treatment indicates that the treatment needs to be continued. Regarding this, Tomasi et al.  have put forward the concept of “periodontal pockets closure”, that is, after 3 months of periodontal treatment, the PD < 5 mm is regarded as a good indicator of non-surgical treatment effect. However, for many patients with moderate to severe periodontitis, the deep periodontal pockets perform obvious effect even though FPD > 5 mm after treatment, shows that the concept of “periodontal pockets closure” is not fully consistent with the efficacy of non-surgical periodontal treatment. Hence, in this study, the Δ PD was used to act as the result variable of the model.
The results of null model showed that the ΔPD was significant difference between the site level, tooth level and patient level, and these three levels accounted for 66%, 18%, and 16% of the overall variations, which indicated that the site level had the greatest impact on short-term efficacy of periodontal non-surgical treatment. The results of mixed-effects model showed that the mesial and distal sites showed significantly greater changes in PD after non-surgical periodontal therapy than buccolingual central site at the site level, which was similar to previous studies by D’Aiuto et al. . In addition, other studies have also found that after non-surgical periodontal treatment of medium-depth periodontal pockets, the ΔPD was 1.2 mm, and those of deep periodontal pockets (PD ≥ 6 mm), the PD decreased by an average of 2.4 mm . Similarly, this study also showed a positive correlation between Δ PD and baseline depth, which may be related to the control of inflammation after the treatment, the swollen gums subsided obviously, and the long epithelium formed on the root surface, which made the decrease of PD in deep periodontal pockets more obvious than in shallow periodontal pockets.
At the tooth level, the Δ PD of multi-rooted teeth was smaller than that of single-rooted teeth, which can be explained from three aspects. Firstly, single-rooted teeth were mainly located in the front of the dentition, with high treatment efficiency, and the gingival tissue in the anterior teeth area was thinner than that in the premolars or molars, with better healing degree. Secondly, the anatomical structure of multi-rooted teeth was complex, with complex root canal system, the presence of furcations, enamel pearl and root depression. Shi et al.  found that the furcation involvement seriously affected the reduction of PD in the molar, and the molar was considered to be the tooth with the poor prognosis . Thirdly, the multi-rooted teeth often bear greater masticatory force than the single-rooted teeth. This model also demonstrated that teeth with severe CAL and hypermobility were associated with inferior prognosis, which indicated that the effect of non-surgical treatment on hopeless teeth or questionable teeth is limited and surgical treatment or extraction should be involved to achieve better outcome.
Smoking has been proved to be one of the main risk factors for the occurrence and development of periodontal diseases. Compared with non-smokers, smokers have deeper periodontal pockets and higher CAL, more obvious alveolar bone resorption, more severe gingival recession, and higher risk of teeth loss . In this study, cigarette smoking negatively affects the outcome of non-surgical periodontal therapy: smokers had 0.35 mm less PD reduction than non-smokers, which was similar to the previous study by Bunaes et al. . This may be related to the fact that the depth of periodontal pockets and the level of CAL of smokers were larger than that of non-smokers, and the treatment difficulty was correspondingly increased. In addition, because of the lower degree of tissue inflammation in smokers, during the process of periodontal exploration, the probe penetration was reduced, and the measured CAL was smaller than actual value. Moreover, the ecological environment in deep periodontal pockets of smokers was more difficult to change by simple mechanical debridement. Regarding this, the previous studies have found that after 3 months of periodontal treatment, a significant reduction in red and orange complexes was only observed in non-smokers. After 6 months of treatment, subcolonial bacterial recolonization of pathogenic bacteria was observed only in smokers, indicating that smokers were more likely to reconstruct pathogenic subgingival plaque biofilms than non-smokers .
Furthermore, compliance is another important factor that affected periodontal non-surgical treatment. In this study, after 3 months of treatment, patients in the complete compliance group had significantly higher Δ PD than those in the irregular compliance group. The result demonstrated that good compliance is essential to a successful periodontal treatment. Periodontitis, characterized by its painless property, the awareness of plaque control in patients will decreasing after treatment, so they no longer followed up, until the dysfunction such as more severe bleeding gums and hypermobility in teeth occurred. Lee et al.  found that patients with complete compliance had significantly lower rates of tooth loss during supportive periodontal therapy (SPT) than the patients with irregular compliance. Moreover, Robinson et al.  and Furuta et al.  have found that women had stronger anti-infection ability, and the oral health habits of women were better than men, and the overall prognosis of periodontal treatment of women was better than that of men . However, in this study, the factors of age and gender showed no significant correlation, which may be related to the limited sample size. Meanwhile, the goodness of fit (− 2logL) of the final model was significantly different from that of the null model, which indicated that the difference was significantly improved after all the clinical covariates were added to the model, but there were still unexplained parts. Hence, the remaining research variables can be added to future studies to further explain the model, such as the factors of furcation involvement, pulp status, crown root ratio, direction of bone resorption, crowded degree of dentition, width of attached gingiva, subgingival plaque microorganisms, systemic diseases, and genetics.
The enlightenment of this study for clinicians is that the prognosis of periodontal non-surgical treatment for periodontitis patients with smoking, poor compliance and poor awareness of oral health may be poor. For this group, clinicians should emphasize the necessities of quitting smoking and SPT, strengthen oral hygiene instruction. Moreover, the multi-rooted teeth, teeth with severe TM, BI and CAL often suggest lower PD reduction, especially the buccolingual central site, which requires clinicians to fully consider the above factors when making the treatment plans for initial diagnosis. It is also necessary to focus on these teeth and sites during the long-term follow-up to prevent the further disease progression.