**4. Discussion**

The main spectral changes in the tissues of teeth with periodontitis were identified in this work. The etiology of these changes is connected to the formation of calculus on the surface of teeth which, in turn, results in structural changes to all tooth tissues with periodontitis. These changes occur due to

the presence of a water–mineral metabolism disorder in the tooth tissues (intensive substitution of hydroxide ion OH by apatite ions CO3<sup>2</sup>− in the structure) or collagen synthesis disorder. Similar changes occur in bone tissues with osteoporosis, as we presented earlier in [25].

The spectral changes found in this work that occur in periodontitis do not occur in other widespread dental diseases (e.g., caries). We previously carried out studies [27] that showed a decrease in the concentration of ions (PO4)<sup>3</sup>− in caries, as also shown in [28].

Diagnosing the spectral changes in tooth enamel, as well as developing an algorithm to identify enamel with periodontitis, will allow at-risk patients to be identified and treated with hydroxyapatite. The sensitivity and specificity values of the developed algorithm were 95.5% and 95.3%, respectively.

The received results are a prerequisite for creating an express device for the noninvasive (in vivo) assessment of periodontitis, based on changes in tooth enamel spectral values. These studies have already been carried out in vivo in this work and have shown good results, similar to the results of in vitro studies.

**Author Contributions:** Conceptualization, E.T. and L.V.; methodology, P.T. and L.V.; software, O.F. and P.T.; validation, P.T. and L.V.; formal analysis, P.T. and O.F.; investigation, O.F., M.Z. and I.B.; resources, E.T., L.V., M.Z. and I.B.; data curation, M.Z. and I.B.; writing—original draft preparation, E.T.; writing—review and editing, P.T. and L.V.; visualization, O.F. and P.T.; supervision, E.T., L.V. and I.V.B.; project administration, E.T.; All authors have read and agreed to the published version of the manuscript.

**Funding:** The study was made with financial support from Samara University (www.ssau.ru).

**Conflicts of Interest:** The authors declare no conflict of interest.
