This study used data acquired from KNHANES VII, which was a cross-sectional and nationally representative survey conducted by the Korea Centers for Disease Control and Prevention (KCDC) between 2016 and 2018. This study is based on the data from the Seventh Korea National Health and Nutrition Examination Survey (KNHANES VII: 2016–2018). KNHANES VII is a national cross-sectional survey conducted by the Korean Disease Control and Prevention Agency (KDCA) The survey protocols and secondary use of data were approved by the Institutional Review Board of the KDCA (IRB No. 2018-01-03-P-A). All the study participants provided written informed consent.
The sampling protocol used was a complex, stratified, multistage probability cluster survey of a representative sample of the non-institutionalized civilian population of Korea. A total of 12,689 participants (8,708 males and 3,981 females), aged 19 years or older, completed the KNHANES VII. Individuals without data on periodontitis, age, and sex were excluded from the analysis. From all the data collected by the KNHANES VII, we used the data on socio-demographic characteristics (age, sex, household income, and level of education), oral health-related variables (toothbrushing, interdental brush use, dental floss use, dental clinic visits, and CPI), oral and systematic medical factor variables (smoking, diabetes mellitus [DM], hypercholesterolemia, hypertension, and body mass index (BMI)), and the number of prosthetic crowns.
Periodontal status was evaluated using the CPI probe developed by the WHO . A CPI probe that met the 1997 WHO guidelines  was used on ten index teeth, two molars in each posterior sextant, and the upper right and lower left central incisors at six sites per tooth (mesiobuccal, midbuccal, distobuccal, mesiolingual, midlingual, and distolingual). The periodontal pocket depth was measured. Probing was done by dentists trained in calibration. The five recorded CPI scores were as follows: CPI 0, normal; CPI 1, gingival bleeding; CPI 2, presence of gingival calculus; CPI 3, shallow periodontal pocket (> 3.5 mm and ≤ 5.5 mm), and CPI 4, deep periodontal pocket (> 5.5 mm). Periodontitis was defined as a CPI score of 3 or 4. Participants were classified into two groups: non-periodontitis and periodontitis group.
Number of crowned teeth
Evaluation of the number of crowns examined by the WHO oral health examination method  was based on the treatment of all surfaces of the tooth. Crowns with and without caries were included in the analysis. The number of crowns in the anterior and posterior regions was divided, taking into account the clinical characteristics of each region. The anterior teeth were categorized into two groups based on the median number of crowns, and the molars were classified into one, or, two-three, four-five, or six according to the interquartile range.
The covariates of this study were the following major sociodemographic factors: sex, age, household income, and education . Household income was classified as < 25% (the lowest quartile group), 25–49%, 50–74%, and 75–100% (the highest quartile group). Education level was classified into four groups based on the Korean education system: below primary school, middle school, high school, and college or higher education. The oral health variables were toothbrushing, interdental brushing, dental flossing, and dental clinic visits. The systematic medical factor variables were smoking, DM, hypercholesterolemia, hypertension, and BMI. Participants were categorized into two groups based on their smoking experience: “non-smoker” and “current or past smoker.” With respect to DM, participants were classified into three groups: normal, impaired fasting glucose, and diabetic. The systemic medical factors included in the analysis were DM, hypercholesterolemia, hypertension, and obesity. With respect to DM, participants were classified into three groups defined as: a fasting plasma glucose level ≥ 126 mg/dL, a previous diagnosis of diabetes by a physician, or current use of anti-diabetic agents or insulin. Hypercholesterolemia was defined as a total plasma cholesterol level of ≥ 240 mg/dL or current use of cholesterol-lowering agents. Hypertension was defined as an average SBP/DBP ≥ 140/90 mmHg or the use of antihypertensive agents. Based on the WHO redefined criteria for obesity in the Asia-Pacific region, obesity was defined as a BMI of ≥ 25 kg/m2.
Data were analyzed using SPSS version 23.0 (SPSS, Chicago, IL). All data were weighted for statistical analyses to account for the complex multistage, stratified, and unequally weighted or clustered sampling design of the KNHANES VII. Appropriate sample weighting factors were selected as specified for each national dataset. The chi-square test and independent t-test were used to compare the characteristics of subjects in the periodontitis and non-periodontitis groups. Multivariate logistic regression analyses were used to identify associations between the number of prosthetic crown teeth and periodontitis after adjusting for potential confounders. Regression model 1 was adjusted for age and sex. Household income and level of education were added to regression model 2. Oral health variables (toothbrushing, interdental brushing, dental flossing, and dental clinic visits) were added to regression model 3. Systematic medical factor variables (smoking, DM, hypercholesterolemia, hypertension, and BMI) were added to regression model 4. Other multivariate logistic regression analyses were performed to identify the modifiers, such as sex and age, after adjusting for potential confounders in model 4. P < 0.05 was considered statistically significant.