## 4 Discussion
### CBCT-Analysis  
In the present study, the results were exclusively based on measuring of the distance between the CEJ or a restauration at the level of the CEJ to the alveolar bone crest (ABC) by linear measurements in CBCT data. Thus, the study only investigated radiographic findings representing a momentary assessment providing no information about medical history of patients, previous history of teeth (i.e., orthodontic treatment) or even the progression of periodontitis. The study did not report bone loss, which is defined by degradation of bone over a period of time, but only described the bone level. No clinical parameters like the well-accepted surrogates assessing periodontal inflammation like bleeding on probing (BoP), or pocket depths (PD) were assessed and no conventional diagnosis and grading of periodontitis was performed. The study must be regarded as a radiographically comparative observational cross sectional study evaluating a possible association between a root canal filling and the corresponding alveolar bone crest level. This may be a limitation of the study but at the same time it represents a strength, because radiographic measurements are not affected by clinical parameters that may also have an impact on the accuracy and reliability of periodontal probing--influenced by numerous factors like periodontal health, probing force, type of periodontal probe, and probing site [21]. However, the prevalence of periodontal disease based on alveolar bone loss or alveolar bone level can be accurately and reliably evaluated even from non-standardized radiographs [22; 23] independent of numerous accompanying and influencing factors like the thickness, width, and angulation of the alveolar crest; thickness of lingual and facial alveolar bone plates; presence of fenestration and dehiscence; tooth alignment in the jaw; tooth and root trunk anatomy; tooth position within the alveolar process; proximity with another tooth surface [24].  
Despite the inherent bias in the selected cohort--individuals were subjected to CBCT analysis because of special needs and may be expected to have better health awareness than the general population--the use of CBCT in the present study should overcome the limitations of the two-dimensional intraoral radiographs with its superimposing artefacts due to the summation of adjacent structures or the distortions caused by different angulations of the central X-ray may influence the linear radiographic measurement of the distance CEJ to ABC [20]. In general, intraoral radiographs tend to underestimate the extent of alveolar bone loss as compared to intrasurgical measurements [19; 25]. Similarly, also CBCT analysis may underestimate the bone loss compared to clinical situations [26]. In contrast to this finding, it may be speculated that thin cortical bone margins (often the labial/buccal margins) are difficult to assess in the presence of artefacts. Hence, the bone level may also be underestimated (or a possible bone loss overestimated) [27]. Characteristically CBCT-associated limitations are scattering and moving artefacts or beam hardening [28]. The realistically achievable spatial resolution for images offering no visible blurring or moving artefacts is about 0.25-0.5 mm in-vivo [29]. Nevertheless, CBCT analysis is a reliable diagnostic tool for assessing bone level, and thus also allows appropriate periodontal treatment planning, asactual bone level correlates with the clinical situation well [15; 17; 30; 31]. Even its suitability for assessing the cemento-enamel junction of teeth under clinical conditions has been proven [32]. However, CBCT analysis does not represent the golden standard in routine radiographic examination of patients affected with periodontal diseases. This also applies to endodontic diagnostics, with CBCT examinations suggested only for specific indications, when clinical examination and conventional imaging modalities are not conclusive [33; 34; 35].