Study design and participants
This cross-sectional survey was conducted at the North Medical Center Kyoto Prefectural University of Medicine, from March 2022 to January 2023. A total of 261 patients (male, 112; females, 149) at the Department of Oral and Maxillofacial Surgery volunteered to be included in the study. Data from radiographic examinations, questionnaires (see Supplementary Note S1 online), and medical records were collected and used for the study. Written informed consent for the publication of any potentially identifiable data or images included in this study was obtained from all subjects and/or their legal guardian(s) prior to conducting any research activity. The study was approved by the Ethical Committee of the Kyoto Prefectural University of Medicine (approved number: ERB-C-2265). All experiments were performed in accordance with the Declaration of Helsinki.
A detailed flow diagram of the inclusion and exclusion process is illustrated in Supplementary Fig. S4. The exclusion criteria in this study included the following: provision of inconsistent responses in the questionnaire (N = 2); patients who underwent radiotherapy for the jaw (N = 3); those who have been diagnosed with osteoporosis and received medications to treat (N = 13); and those who presented with periodontal bone loss (> two-thirds of the root length; N = 4), apical radiolucency (> 3 mm in diameter; N = 4), missing teeth in the mandibular anterior region (N = 7), and extensive mandibular torus in the intraoral radiographs (N = 3). Based on these criteria, 225 patients were enrolled and divided into three mutually exclusive groups: 25–49, 50–74, and > 75 years old. The age categorization was performed according to the previous literatures on bone health indices17,22,23.
Dental radiographic image acquisition and measurements of the alveolar BMD value
Digital periapical radiographs of the mandibular incisor region were acquired for research purposes using intraoral X-ray equipment (maxiX Type2®, J. MORITA MFG. Corp., Kyoto, Japan) with an imaging plate system (Digora Optime®, Soredex, Orion Corp., Helsinki, Finland) under the following conditions: tube voltage, 60 kV; electrical current, 7 mA, and exposure time, 0.13 s. The DentalSCOPE, version 1.0.2 (Media Corp., Tokyo, Japan, https://md-scope.com/products/dental/index.html [provided in Japanese only]) system was used to evaluate the alveolar-BMD (al-BMD). Under certification by the national health institute in the authors’ nation, an X-ray beam indicator (Fig. 1a,b) was designed to built-in special reference objects which comprised 20% (calibrated to 500 mg/cm2), 60% (1000 mg/cm2), and 100% (1500 mg/cm2) calcium carbonate. The mg/cm2 is a convenient unit to express the value of al-BMD determined from two-dimensional X-ray images. After inputting IP-based digital X-ray images, the computer-aided detection software automatically created a calibration line based on the image densities of the reference phantoms and converted each image density of the arbitrary region into a BMD value. A 12–14 mm2 rectangular area, 3 mm away (in the vertical direction) from the central incisor root apex, was selected as the region of interest (Fig. 1c) due to lower incidence of anatomical variance, such as torus mandibularis24 or idiopathic osteosclerosis25 for analysis. Three measurements were taken for each subject blindly every 1 week by single well-trained examiner, and the median value was recorded as the representative data.
Questionnaire survey
The questionnaire used in this study (see Supplementary Note S1 online) included two items about the individual’s basic demographic information: Q1. What is your sex (male/ female), Q2. What is your age (in years)? Two questions regarding osteoporosis diagnosis and treatment were also provided: Q3. Have you ever been diagnosed with osteoporosis by a physician? (yes/no), Q4. For those who answered “yes” to the previous question; Are you currently or have you ever taken any of the following medications for osteoporosis, such as bisphosphonates, denosumab, teriparatide, estrogens, raloxifene, and calcitonin? Additionally, women answered the following question: Q5. What is your menopausal status (premenopausal/ postmenopausal)?
Statistical analysis
First, based on the Shapiro–Wilk test, each age- and sex-stratified group showed a significant departure from normality distribution of the al-BMD (p = 0.006–0.032). The Levene’s test for homogeneity of variance was statistically insignificant (F = 0.851, p = 0.403), indicating the equal variance. The al-BMD values were therefore compared using the nonparametric Mann–Whitney U test or Kruskal–Wallis test, followed by the pairwise Wilcox test as post-hoc analysis. All statistical analyses were performed using the R software package, version 4.1.0 (R Foundation for Statistical Computing, Vienna, Austria). A significance level of < 0.05 was considered significant.
Approval for human experiments
Informed consent was obtained from all subjects and/or their legal guardian(s) for study participation.
