This study confirmed the hypothesis that PD is associated with a smaller PISA than HD, indicating a smaller inflammatory burden, independent of a large number of sociodemographic, lifestyle, laboratory and clinical factors. This result may be due to the fact that HD is better for the removal of small-molecular-weight molecules, such as urea and creatinine, which are not real uremic toxins (Kt/V is higher in patients on HD than in patients on PD), and that PD is better for the removal of mid-sized molecules (uremic toxins). Uremic toxins can lower the capacity of the immune system [16,17,18], which may result in a better immune response and healthier periodontal tissue in PD patients. As all dialysis patients are potential organ recipients, it is of upmost importance that there is no hidden source of inflammation prior to kidney transplantation. A meta-analysis published in 2016  also reported that pretransplant dialysis influences short- and long-term complications after kidney transplantation and that PD may be a better choice of pretransplant dialysis modality than HD.
This study also showed the degree of inflammatory burden in Croatian dialysis patients presented by PISA. After adjusting for all confounders, the mean PISA was 798 mm2 in the HD group and 52 mm2 in the PD group. According to the PISA cut-off values defined by the Centers for Disease Control and Prevention-American Academy of Periodontology for the classification of periodontitis, i.e., that severe periodontitis ranges from 934,71 mm2 to 3274,96 mm2, moderate periodontitis ranges from 521,58 to 790,30 mm2, mild periodontitis ranges from 110,16 and 447,01 mm2, and no periodontitis corresponds to PISA values from 10.22 mm2 to 62.78 mm2 , periodontal treatment is highly needed among dialysis patients in Croatia.
To our knowledge, this is the first study to measure or compare PISA in CKD patients on HD and PD. Some previous studies have compared other periodontal indices among patients on HD and PD. Cross-sectional studies conducted in Brazil, Canada, Turkey, USA and Taiwan have reported that chronic severe periodontitis is significantly more prevalent among patients on HD than among healthy persons, and periodontal disease is comparatively more prevalent and more severe in CKD patients [21,22,23,24,25]. However, PISA provides important advantages over these other periodontal indices. It represents a classification that quantifies the amount of inflamed periodontal tissue and, as such, quantifies the systemic inflammatory burden [7, 12].
Previous studies  have shown higher levels of periodontitis in patients with CKD than in healthy controls, and the disease is most advanced in maintenance HD patients but successively diminished in PD and pre-dialysis CKD patients.
In 2016, Chinese authors  stated that the periodontal status of HD and PD patients was worse than that in healthy controls, but there were no statistically significant differences between the PD and HD groups. However, the average calculus surface index was significantly higher in HD patients than in PD patients. This finding may be related to the alteration in serum phosphorus-calcium in HD patients.
Bayraktar et al.  reported a higher gingival index (GI) in the HD group than in the peritoneal group and higher calculus accumulation in both dialysis groups compared with healthy controls. Brito et al.  concluded that patients on PD had similar CALs to healthy controls. Moreover, according to Thorman et al. , pre-dialysed patients and patients on HD have a higher prevalence of severe periodontitis than healthy controls and patients on PD. Such results can also be explained by the psychological state of patients according to dialysis type and satisfaction with their quality of life. Patients on HD visit the hospital several times a week and are connected to dialysis machines for approximately 4 h. Patients on PD can perform dialysis procedures in their own home or at other clean locations, have more independence and are able to more actively work; and consequently, patients on PD report a better quality of life. In fact, better quality of life and higher satisfaction of patients on PD have been statistically proven by various studies [31, 32]. Some studies have reported high levels of quality of life among patients on PD, although these levels were not statistically significant from the quality of life of other dialysis patients [33,34,35]. It has been proposed that patients on PD exhibit higher motivation and have a more proactive approach regarding their oral hygiene habits, thus leading to a better periodontal state.
This study did not find significant differences in PISA according to the duration of dialysis in the three groups of patients: those on dialysis for less than a year, for 2–3 years and for more than 3 years. These results are in accordance with the study reported by Parkar and Ajithkrishnan , which outlined four subgroups according to the duration of dialysis: less than 3 months, 4–6 months, 7–9 months and 10–12 months. The authors of that study reported no effect of the duration of dialysis on periodontal tissues. A similar study by Marakoglu et al.  also revealed no significant differences in age, gingival index, plaque index or periodontal pocket depth among subgroups of patients on HD for less than 1 year, 1–3 years and more than 3 years. In contrast, Cengiz et al.  compared patients on dialysis for less than 5 years, 5–10 years and more than 10 years and concluded that there were significant increases in plaque index, gingival bleeding and periodontal pocket depth after 5 years and that the difference was statistically more significant after 10 years. These findings suggest that the significant influence of dialysis on periodontal health becomes obvious after 5 years.
Limitations of this study
The first limitation of this study is that the patients were not randomly allocated to receive HD or PD. An attempt was made to control this source of bias by controlling the effect of a larger number of possible confounders. Nevertheless, we were able to control only the included and known variables, while different important unmeasured factors remained uncontrolled. Periodontal status before the initiation of dialysis and the duration of kidney disease are likely to be highly important factors. The cross-sectional design of this study prevented the observation of a temporal sequence between periodontitis and dialysis, and for this reason, it was not possible to make any causal inferences. Second, patients were enrolled in the specialized nephrology ward of a large university teaching hospital in a highly urban area of the country’s capital. It is possible that periodontal disease, dialysis parameters, and their association are different in small, regional hospitals with sparser resources and less educated patients of a lower socioeconomic status. There is no evidence on which to base this claim, but the possibility should be taken into account. Therefore, these findings should not be uncritically generalized to the general Croatian population of patients treated for CKD. Third, the primary outcome was not independently assessed, and the participating investigators were not blinded to the type of dialysis. These factors likely induced bias against the null hypothesis. Therefore, these findings are probably somewhat overoptimistic and should be replicated in properly blinded studies. Fourth, the dosages of monitored therapies were not controlled, whereas only whether or not the patient was treated with a particular drug was monitored. Fifth, a consecutive sample of patients was selected, which might have increased the risk of selection bias.