The present study explored the relationship between different types of caries and periodontal disease severity in middle-aged and elderly people in a nationally sample in China. We discuss the relationship between the two diseases by age group because the risk factors, susceptibility to both diseases, and especially the number and causes of missing teeth in different age groups are different. Because bivariate analyses cannot exclude confounders related to periodontitis, ordered logistic regression models were used. We found that different types of caries in middle-aged people and overall caries and the caries types involving the root surface in elderly people were significantly associated with periodontal disease severity. To the best of our knowledge, this study is the first to analyse the relationship between caries and periodontal disease in a national sample of Chinese adults.
There are some potential limitations in this study that should be addressed. First, due to the cross-sectional data used in this study, the causal relationship between caries and periodontitis cannot be determined. Second, we were obligated to identify periodontitis using CAL instead of definitions of periodontitis, such as that described by the Centers for Disease Control and Prevention and the American Academy of Periodontology (CDC–AAP). Third, the exclusion criteria of fewer than 20 teeth may have rendered the age group of elderly people less representative. Fourth, people who could be treated for periodontal disease could be a confounder. Fifth, the present study only considered decayed and filled teeth as a caries experience, which may not reflect the actual caries experience as this definition excludes missing teeth due to caries. Sixth, pairing individuals or teeth in this type of research is ideal, which is a significant limitation of this study.
The criteria commonly used to define periodontitis include the probing depth (PD) and CAL. The PD reflects the current situation of periodontitis, and CAL reflects the long-term accumulation of periodontal damage. Because caries and periodontitis are long-term chronic destructive diseases, the past cumulative lifetime experience of periodontitis is reflected by CAL measurements. Therefore, it is recommended to use this measure to analyse the risk factors of periodontitis in epidemiological studies [23].
Previous studies concerning the relationship between caries and periodontitis have rarely clearly described whether the caries studied was crown caries, root caries, or mixed type caries. In recent years, two articles clearly defined the types of caries studied, i.e., one study investigated crown/root caries [1], and the other study investigated crown caries [7]. Some studies have noted that when we encounter type B caries, we do not know whether to classify it as crown caries or root caries. This issue is a measurement issue more than a diagnostic issue [24]. The relationship between caries and periodontitis in different locations of caries may be different; thus, we divided the types of caries in this way to further understand the relationship between caries and periodontitis.
We found that all four caries types (ABC/A/B/C) were positively correlated with periodontitis in the middle-aged group, and three caries types (ABC/B/C) were positively correlated with periodontitis in the elderly group. Previously, few articles discussed the relationship between the two diseases according to different age groups. Some studies report that a positive correlation exists between caries and periodontal disease severity in adults. Al Qobaly et al. found that individuals aged 35 years or older with periodontitis had a higher risk of coronal and root caries in England, Northern Ireland and Wales [1]. Hyman et al. reported that untreated coronal caries surfaces in US individuals aged 20 years or older were positively correlated with the mean CAL [7]. Mattila et al. reported a positive association between PD and the DT index in Finnish adults aged 30 years and older [6]. Strauss et al. reported a positive association between periodontitis and the DT index among Chilean adults aged between 35 and 44 years [2]. This positive association could be explained by the common socioeconomic status they share. The socioeconomic status seems to influence relevant health behaviours, such as diet, oral hygiene, smoking and patterns of seeking professional prevention or treatment [15]. In our analysis, gender, area, smoking status, tooth brushing frequency and household income per capita were significantly associated with periodontitis in both age groups. To the best of our knowledge, both caries and periodontal diseases are bacterial infectious diseases. Although the microbiological profiles of the two diseases are different, poor oral hygiene is considered a main common risk factor, which could provide enough fermentable carbohydrates for bacterial reproduction and subsequently increase the prevalence of the two diseases [1, 15]. In addition, smoking has been proven to be another important health behaviour that is a common risk factor for both diseases [15]. Furthermore, it has been demonstrated that host genetics plays a moderate role in contributing to the susceptibility to both caries and periodontal disease. Although no common genetic genes have been found, pleiotropy (one gene influences two or more seemingly unrelated phenotypic traits) in caries and periodontal disease may exist. Hence, regarding the role of genetics, more analyses of pleiotropy are needed to unravel the mechanism and better understand the genetic association between caries and periodontal disease accumulating in the same person [15].
Type A caries were shown to have no relationship, but types B and C were positively associated with periodontitis in the elderly group in our study. In the middle-aged group, the DFT index increased with the aggravation of periodontitis, while in the elderly, the group with the most severe CAL (≥6 mm) had the lowest DFT. Tooth loss may be a cause. With an increase in age, the tooth loss rate of the elderly significantly increases, and the most frequent occurrence of tooth extraction due to periodontitis should be in the group with CAL≥6 mm. However, because of the nature of this cross-sectional study, we could not investigate the specific reasons for tooth extraction. Carious lesions of both types B and C involve the root surface. Periodontal attachment loss could lead to the exposure of the root surface. As a result of poor oral hygiene, root caries commonly presents as a progressive lesion in patients with periodontitis [25]. The positive correlation between types B and C caries and periodontitis is consistent with a recent systemic review. Root caries incidence or increment has been reported to be associated with patients having periodontal disease [17].
Taken together, to draw a conclusion regarding the association between caries and periodontitis, longitudinal studies are needed. Because the susceptibility to the two diseases could change with ageing. Future research should be conducted in different age groups using a similar methodology such that the results could be compared. It has been reported that using toothpicks is a risk factor and that using dental floss is a protective factor for CAL in the 35- to 44-year-old group in China [21]. This finding is probably because the wrongful use of toothpicks could increase the risk of periodontal damage [21]. In this study, we could observe that in both age groups, nearly half of the participants use toothpicks, and only a few people use dental floss. Additionally, the brushing rate in both age groups was high, close to 90%; however, whether the correct brushing method was adopted was unclear. Hence, to address these two common oral diseases, public health policies should be adopted to promote public awareness of oral health care, develop good oral hygiene habits, such as mastering correct tooth-brushing methods, regular flossing, oral examinations and timely dental scaling.