## 4 Discussion  
In our previous study, we investigated the serum inflammatory biomarker levels associated with apical periodontitis and found that successful endodontic treatment reduces the systemic inflammatory burden and thus reduces the risk for CVD development [16]. Salivary sampling can be used to detect diagnostic and prognostic biomarkers for both local and systemic inflammation and offers a non-invasive, low-cost and easy alternative to serum sampling for both patients and researchers [19; 20]. The present prospective longitudinal study assessed salivary inflammatory biomarkers' levels in the same cohort of patients as our previous study [16], offering an opportunity to determine changes in local biomarkers associated with apical periodontitis and endodontic treatment and to  
Figure 3: Correlation plot between salivary and serum levels of inflammatory markers: (**a**) Correlation between levels of FGF-23 at baseline; (**b**) Correlation between levels of ICAM-1 at 1-year post-treatment follow-up; (**c**) Correlation between levels of TNF-\(\alpha\) at 2-year post-treatment follow-up; (**d**) Correlation between levels of ICAM-1 at 2-year post-treatment follow-up; (**e**) Correlation between levels of E-selectin at 2-year post-treatment follow-up. investigate any correlation of salivary markers with systemic serum levels. We found that levels of salivary biomarkers were much higher compared to the serum levels, which is probably due to local diffusion of inflammatory cytokines produced in apical periodontitis lesions. Furthermore, we saw that, at the 1-year review appointment, the salivary levels of most of the biomarkers except hs-CRP were still raised compared to the baseline values, whereas in our previous study [16], most of the serum levels did decline at the 1-year review. This may be due to the local influence of healing, potentiating these levels in saliva when the serum levels have already reduced. High levels of these cytokines, MMPs and vascular adhesion molecules in saliva detected at review appointments could be potentially due to their activity in periapical healing by regeneration and bone remodelling [21]. This is consistent with other studies where levels of interleukins and MMPs were raised in saliva and gingival circular fluid (GCF) in patients with chronic periodontitis [22; 23]. One of the limitations of this study is that GCF was not investigated. This can provide more accurate information about the local inflammation and answer why some markers in saliva were still higher after endodontic treatment when serum markers had already reduced. Investigating correlations of GCF biomarkers with serum and salivary biomarkers levels can provide information about the reliability of salivary biomarkers for endodontic treatment. The one-year recall rate was 76.9%, which was reduced at 2-year post-treatment review due to COVID-19 restrictions. It has been found that, in England, almost 19 million dental appointments have been missed in the NHS due to the restrictions [24]. However, the recall rate of our study was comparable to similar outcome studies [25; 26]. The overall treatment outcome when considering the healed and healing cases using periapical radiographs was 92% at 1-year review, which reached 100% at 2-year post-treatment follow-up. This emphasises that care should be taken when assessing the outcome, as the European Society of Endodontics recommends follow-up of patients with root-treated teeth for up to four years post-treatment [27]. The pre-operative levels of hs-CRP in the apical periodontitis group were significantly higher at baseline when compared to the control group. At 1-year post-treatment follow-up, hs-CRP salivary levels were reduced compared to pre-operative levels. CRP is an acute phase protein produced by the liver and induces the production of pro-inflammatory cytokines [28; 29]. CRP levels could potentially be used as a diagnostic marker for the prediction of CVDs and myocardial infarction [10; 30].