The commonly used criteria for defining periodontitis are probing depth (PD) and CAL. 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].
DMFT, DT and DFT are the most commonly used indices to evaluate caries status and experience. In people aged 30 years or older, the M component of the DMFT index refers to the loss of teeth caused by any reason. To avoid the overestimation of the true extent of caries, we use the DFT index to analyse caries experience.
Previous studies on the relationship between caries and periodontitis have rarely clearly described whether the caries studied were crown caries, root caries, or mixed type. In recent years, there have been two articles clearly defining the types of caries studied—one was crown/root caries [1], and the other was crown caries [7]. Some studies have pointed out that, when we encounter type B caries, we do not know whether to classify it as crown caries or root caries. This is a measurement issue more than a diagnostic issue [24]. The relationship between caries and periodontitis in different locations for caries may be different, so we divided the types of caries in this way, hoping to further understand the relationship between caries and periodontitis.
We found 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. In the past, few articles have discussed the relationship between the two diseases according to different age groups. There are studies reporting a positive correlation between caries and periodontal disease severity in adults. Al Qobaly et al. found 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 of US individuals aged 20 years or older was positively correlated with mean CAL [7]. Mattila et al. reported a positive association between PD and the DT index in Finnish adults who were 30 years and older [6]. Strauss et al. reported a positive association between periodontitis and the DT index of Chilean adults aged between 35 and 44 years [2]. The positive association could be explained by the common socioeconomic status they shared. Socioeconomic status seems to influence relevant health behaviours, such as diet, oral hygiene, smoking and the patterns of seeking professional prevention or treatment [15]. In our analysis, gender, area, smoking status, tooth brushing frequency and household income per capita showed a significant association with periodontitis in both age groups. As we know, both caries and periodontal diseases are bacterial infectious diseases. Although the microbiological profiles of the two are different, poor oral hygiene is considered one of the main common risk factors, which could provide enough fermentable carbohydrates for bacterial reproduction and subsequently increase the prevalence of the two diseases [1, 15]. In addition, smoking status 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 has a moderate role in contributing to the susceptibility of both caries and periodontal disease. Although no common genetic genes were found, pleiotropy (one gene influences two or more seemingly unrelated phenotypic traits) in caries and periodontal disease may exist. Hence, with regards to the role of genetics, more analysis of pleiotropy is needed to unravel the mechanism to better understand the genetic association of 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 the increase of age, the tooth loss rate of the elderly increased significantly, and the most frequent occurrence of tooth extraction due to periodontitis should be in the group with CAL ≥ 6 mm. But because of the 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 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].
This is the first study to analyse the relationship between caries and periodontal disease in a representative national sample of Chinese adults. We discuss the relationship between the two diseases by age group, because the risk factors, susceptibility of both diseases, and especially the number and causes of missing teeth in different age groups are different. However, there are some potential limitations in this study that should be addressed. First, due to the cross-sectional data of this study, the causal relationship between caries and periodontitis cannot be concluded. 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).