Dental caries and periodontal diseases are the most common chronic diseases caused by bacterial colonization (Biofilm) on the tooth surfaces, leading to tooth loss [18, 19]. Oral health affects general health, whereby cytokines and microbial products released in response to periapical and periodontal infections accelerate the development of systemic diseases by causing inflammation in distant organs [20,21,22]. Studies have reported that periodontal diseases accelerate the development of arteriosclerotic heart disease and diabetes [5, 23], Alzheimer’s disease , and respiratory diseases . Since periodontal diseases and endodontic apical periodontitis have similar microbiology, their negative effects on general health are also similar [26, 27].
This study used RBL classification as a symptom of periodontal disease, APGS classification as a symptom of periapical disease, and PPDC, which provides information about the pathophysiology of dental caries, as an indicator of oral hygiene. Using the DD Stg, which was developed based on these three markers, this study determined the patients’ oral and dental health status. Periodontal status is evaluated using CAL, PPD and GR levels  and the amount of destruction in alveolar bone that support teeth radiologically . RBL measurement can be used in cases, where it is not possible to perform CAL, PPD and GR clinically . In previous studies, RBL has been generally calculated using a ruler, where the amount of lost or remaining bone was given as a percentage of the root or tooth length . In recent years, RBL measurement on panoramic radiographs has been performed using methods that automatically detect bone loss such as CAD based on deep learning and the convolution neural network (CNN) [16, 17, 28]. We successfully used the APGS classification, which we created based on the PAI, in the evaluation of AP in our previous studies [7, 8]. This classification was also compatible with the markers associated with inflammatory process (IL-6, hsCRP and PAPP-A).
This study found that the groups with high dental damage had higher CD and NDC, and that the NMT value was quite different in the group with the most severe dental damage. These results suggest that the DD Stg system based on APGS, RBL and PPDC can be used as an important indicator of dental health. In the past, many similar scales and scoring have been developed as an indicator of oral and dental status . The most used ones are the Total Dental Index , the Modified Total Dental Index [31, 32], the Dental Asymptotic Score , the Brief Oral Health Status Examination, and the Global Oral Health Scale. All of these classifications require intraoral examination and measurements. However, today, there is a need for classifications to show dental health status in a more practical and comprehensive way. The classification developed in this study, which was planned to achieve this aim, was based on more rational radiographic data. Therefore, this classification does not require a direct contact with patients during the COVID-19 pandemic, and has a practical and easy algorithm that provides broader information about dental health status. Since the study group had a risk of COVID-19 transmission, the study was conducted retrospectively using radiographs and dental records. Therefore, it did not include intraoral examination of the patients. We think that this classification will be useful in determining the oral and dental health status in patients with infectious diseases such as COVID-19. According to the classification developed in this study, there was a significant correlation between the DD Stg and the morbidity and mortality of COVID-19 disease. Namely, the rate of mortality and chronic disease due to COVID-19 was significantly higher in patients in the DD Stg 2 and 3. This result is consistent with those in studies reporting that the rate of mortality due to COVID-19 disease is higher in patients with chronic diseases [34, 35]. In addition, the presence of a relationship between immune system weakness and morbidity/mortality due to COVID-19 disease [36, 37] suggests that there may be an indirect relationship between DD Stg and immune system.
Oral mucosa is a potential entry route for the SARS-CoV-2 virus that causes COVID-19 disease . The presence of ACE2, which is the cellular entry receptor of SARS-CoV-2, in oral mucosa tissues, tongue and gum epithelium supports that oral cavity is an entry point of the virus . There may be an increase in ACE2 receptors in people with poor oral health [39, 40]. This is why oral hygiene and dental health status are important in combating the COVID-19 pandemic. Therefore, oropharyngeal colonization of viruses and bacteria can be prevented and the risk of respiratory complications due to these pathogens can be reduced by improving oral health. The fact that improving oral health reduces both occurrence and progression of respiratory tract diseases especially in the elderly population and intensive care patients  supports this view.
This study found that patients in the DD Stg 3, who had the worst dental indicators, had higher mean age compared to the others. This result suggests that those with poor dental status were mostly from the elderly population. This was actually an expected result. Because immune system weakens and general body functions deteriorate by aging. Parallel to this, a deterioration in oral and dental health is also expected. Moreover, numerical and dimensional increase in dental caries occurs due to the net mineral loss as a result of increased demineralization, decreased remineralization, insufficiency in bacterial defense and chronic malnutrition along with aging. In addition, the balanced population of oral microorganisms deteriorates due to aging, changing into a population that produces acid or survives in an acid environment [42, 43]. The elderly population is also at risk of developing serious complications related to COVID-19 [44, 45]. All these explain why patients in the DD Stg 3 with poor dental health were mostly the elderly. Therefore, it is important to pay special attention to the population aged 50 years and above in terms of preserving oral and dental health. Because oral and dental health is directly related to the maintenance of healthy nutrition. Nutrition, on the other hand, is a vital activity related to both the supply of all mineral, micro and macro substances for physical regeneration and the immune system [46, 47]. The fact that patients in the DD Stg 3 were older and had higher rate of mortality due to COVID-19 disease confirms these evaluations.
Higher rates of hospitalization in patients with higher DD Stg suggest a correlation between the stage of dental damage and the severity of COVID-19 disease. This may be because individual resistance to viral diseases such as COVID-19 is dependent on general health status, including both immune system and dental health status. Because Scannapieco et al. reported a relationship between infectious respiratory diseases and oral hygiene . The present study also found that COVID-19 symptoms were more observed in those with higher dental damage and the mortality rate was very higher in patients with more severe dental damage (e.g., those in the DD Stg 3), which also supports these researchers. However, it should not be forgotten that dental status, which can be an indicator of oral hygiene, is also associated with behavioral conditions, in other words, it is multi-factorial. Thus, oral hygiene and dental status may also be indicators of resistance to viral diseases such as COVID-19, in relation to general health status.
All independent variables (NHC, SAC and CD) except for NDC, which were created as an expression of the dental health status and were found to affect the prediction of DD staging, were parameters related to the course of COVID-19 disease. The reason why these parameters are related to dental health status can be explained by the pathophysiological mechanism of COVID-19 disease. That is to say, not only the local exposure of the teeth lies behind dental diseases. In a wider scope, immunological mechanisms involving the body’s defense system against viruses and bacteria, biochemical reactions in regeneration or remineralization involving cofactors such as calcium, phosphate, ferrous, magnesium, organic (vitamins) or metalloorganic complexes and macronutrients, and physiological processes involving both macro and microcirculatory system that enable fluid and nutrient exchange can play a role in the development of dental diseases [48,49,50]. These parameters are also related to chronic diseases, hospitalization time, and disease symptoms.