Subjects, study design and setting
Data was collected within the Hamburg City Health Study (HCHS), which is a prospective, long-term, ongoing population-based cohort study. This research platform was developed to expand knowledge about risk and prognostic factors of common chronic diseases. The involved random sample contains 10,000 participants of the general population of Hamburg, Germany, of which 6209 completed a full periodontal examination and were therefore included in the analysis. At sampling, participants were between 45 and 74 years of age. This sample took part in an extensive baseline assessment at one dedicated study center [17]. The institutional review board of the Medical Association of Hamburg approved the study protocol (PV5131). It was registered at ClinicalTrial.gov (NCT03934957). Participants were randomly selected via the residents' registration office and the response rate was 28%. This manuscript was prepared according to the STROBE guidelines [18].
Assessment of education
Education level was classified according to the International Standard Classification of Education 2011 (ISCED 2011) and established by the United Nations Educational, Scientific and Cultural Organization (UNESCO) [19]. Eight levels of education are covered by this instrument: (0) Early childhood education, (1) Primary Education, (2) Lower secondary education, (3) Upper secondary education, (4) Post-secondary Non-Tertiary, (5) Short-cycle tertiary education, (6) Bachelor’s or equivalent level, (7) Master’s or equivalent level, (8) Doctoral or equivalent. For analyses, all participants were categorized in “low (0–2), medium (3–4) or high (5–8)” education.
Assessment of dental variables
Certified study nurses performed the dental examination, which included: diagnosis of periodontitis with a standardized periodontal probe (CP-15 UNC SE, Hu-friedy, Chicago, USA) and a full mouth – six sites protocol, excluding the third molars. Periodontal parameters obtained were: 1) probing depths, 2) bleeding on probing (BOP), and 3) gingival recession. Oral hygiene was assessed via the oral plaque-index (PI). Additionally, the respective clinical attachment loss (CAL) was calculated for every tooth. The severity grading (none/mild, moderate, severe) of periodontitis was based on the classification of Eke & Page [20]:
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(1)
Mild periodontitis: ≥ two interproximal sites with clinical attachment loss ≥ 3 mm, and ≥ two interproximal sites with probing depths ≥ 4 mm (not on the same tooth) or one site with probing depths ≥ 5 mm.
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(2)
Moderate periodontitis: ≥ two interproximal sites with clinical attachment loss ≥ 4 mm (not on the same tooth), or ≥ two interproximal sites with probing depths ≥ 5 mm (not on the same tooth).
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(3)
Severe periodontitis: ≥ two interproximal sites with clinical attachment loss ≥ 6 mm (not on the same tooth) and ≥ one interproximal site with probing depths ≥ 5 mm
Subsequently, the DMFT (D = decayed, M = missing, F = filled, T = teeth) was calculated. Participants requiring endocarditis prophylaxis were excluded from dental examination.
Assessment of additional variables
The migration status was assessed with a self-administered questionnaire. Participants were asked about their own and their parents' place of birth. The answers were transferred into a binary variable (born in Germany/ born in a different country). Migration status was further classified into three categories: immigrated = participants were born outside of Germany and immigrated themselves; migration background = participants were born in Germany, but at least one parent was not born in Germany; no migration background = participants and both parents were born in Germany. Additionally, German language skills were conducted via self-assessment with a 5 point Likert-scale (very good – very poor). Additional variables were assessed at baseline: age (years) and sex (male/female) as well as cardiovascular risk factors: BMI (kg/m2), smoking yes/no, diabetes (positive self-disclosure, taking medication of the A10 group (Anatomical Therapeutic Chemical Classification System (ATC-Code)), fasting glucose (> 126 mg/dl), not fasting glucose (> 200 mg/dl)), coronary artery disease (CAD), and hypertension. Blood samples were obtained for biomarker analysis (high-sensitive C-reactive protein (hs-CRP) and Interleukin 6 (IL-6)) and stored at -80 °C at the HCHS Biobank. Further, plasma samples were analyzed using established enzyme-linked immunosorbent assays (ELISA).
Statistical analyses
In descriptive analyses, continuous variables are presented with their medians and interquartile ranges (IQR). Similarly, absolute numbers (n) and percentages (%) are presented for categorical variables. Descriptive analyses were presented for all variables stratified by the grading of periodontitis (none/mild, moderate and severe) and differences within groups were tested using the chi-squared test or Kruskal–Wallis test. Ordinal logistic regression models were conducted with the outcome variable “periodontitis severity” and the exposure variable “education”. Models with adjustments for relevant confounders (age, sex, history of ever smoking, diabetes, hypertension, migration status, and education) were applied based on prior research and clinical rationale. A p-value of < 0.05 was considered statistically significant. Statistical analyses were performed using R software, version 4.1.0.