Because normality and homoscedasticity were not obtained for each clinical parameter between the two groups, a statistical study was conducted using the nonparametric test (Mann–Whitney U test) for the difference test between the two groups. PPD was recognized to be significantly greater (p < 0.01) in the HP group, but there were no significant differences in mPI and BoP between the two groups. Therefore, we focused on the PPD parameter, in which a significant difference was found between the two groups, and analyzed correlations between PPD and patient characteristics (age, number of teeth, number of extracted teeth, maintenance period). Significant correlation coefficients were observed for the number of extracted teeth in the H group and for all characteristics in the HP group (Table 5).

We analyzed the correlation between PPD and other parameters in each group using Spearman’s rank correlation coefficient. We found a weak correlation between PPD and BoP for the H group (r _s  = 0.401, p < 0.05). In the HP group, weak correlations were found between PPD and mPI (r _s  = 0.228, p < 0.05) and between PPD and BoP (r _s  = 0.241, p < 0.05) (Table 6).