The study group (DS group) consisted of 40 white patients, selected according to the following inclusion criteria: age between 12 and 30 years; genetically confirmed diagnosis of DS; definitively erupted teeth; availability of three-dimensional cone-beam computed tomography (CBCT) images of acceptable quality of the maxilla; no history of surgical procedures in the maxillofacial area (with the exception of strictly dental procedures); and no previous orthodontic/orthopedic treatment or severe maxillary trauma.
The control group included 40 nonsyndromic individuals matched for age and sex. In addition to the selection criteria applied to the study group, we excluded those individuals with systemic disease that could affect orofacial development or dental/skeletal maturity directly or as a result of their treatment.
All images used in this study were from individuals with DS who had undergone upper and lower jaw CBCT in a previous study to evaluate atlantoaxial instability. The control group underwent CBCT mainly for the presurgical assessment of unerupted third molars, the root resorption assessment of periapical lesions, the assessment of temporomandibular (TMJ) disorders and cross-sectional imaging prior to dental implant placement. All images from the DS and control groups were retrieved from the archive of the Radiology Unit of the Faculty of Medicine and Dentistry at the University of Santiago de Compostela in Spain. All participants or, as applicable, their legal guardians signed an informed consent to authorize the use of images for teaching or research purposes. These radiological studies were performed in accordance with the radiation protection principles of “As Low As Reasonably Achievable (ALARA)” and following the guidelines of the SEDENTEXCT Guideline Development Panel, Radiation Protection No. 172: Cone Beam CT for Dental and Maxillofacial Radiology, Evidence Based Guidelines 2012 (www.sedentexct.eu). The study was approved by the ethics committee of the University of Santiago de Compostela.
All images employed in the study were obtained usnig an I-CAT® scanner (Imaging Sciences International, Hatfield, PA). The protocol for acquiring and manipulating the images has been previously described in detail12. All measurements were performed using open-source OsiriX image processing software (Pixmeo, Geneva, Switzerland; www.osirix-viewer.com).
To perform the measurements in the vestibular-palatine (VP) direction based on the axial slices of the original CBCT images, we performed a panoramic reconstruction of the superior maxilla using the Dental3DPlugin® plugin. Using this panoramic reconstruction, we selected the areas of interest in both quadrants of the superior maxilla, which were the interradicular spaces from the distal wall of the canine to the mesial of the second molar. To associate data with a specific tooth, we employed the World Health Organization’s two-digit notation system (also known as ISO 3950 notation)13. From each area of interest, we obtained an image of a 1-mm thick cross-section with which we measured the distance between the external surfaces of the vestibular and palatal cortical bones of the superior maxilla, to depths of 3, 6 and 9 mm from the alveolar ridge (Fig. 1). We performed a total of 24 VP measurements per patient.
To perform the measurements in the mesiodistal (MD) direction, we used the same panoramic reconstruction of the superior maxilla described in the previous paragraph. Using this image, we selected the areas of interest and determined their interradicular distance to depths of 3, 6 and 9 mm from the alveolar ridge (Fig. 1), until a total of 24 MD measurements per patient had been completed.
In the event of missing teeth in the area of interest, we skipped the measurements in the corresponding interradicular spaces. In the event of pneumatization of the maxillary sinus with invasion of the interradicular space, the VP measurement was performed from the vestibular cortical bone of the superior maxilla to the vestibular wall of the sinus. When the dental roots that delimited an area of interest were shorter than 9 mm, we assigned the value of the maximum MD distance obtained in this area of interest to the interradicular distance corresponding to this depth from the alveolar ridge.
Once the VP and MD measurements had been performed, we delimited the ideal areas for the insertion of orthodontic miniscrews. To this end and based on the study by Martinelli et al.14, which recommended miniscrews with a diameter of 1.5–2.3 mm and a length of 6–8 mm, we established a minimum reference length of 6 mm in the VP direction and 2 mm in the MD direction. In each location, we defined 3 types of areas depending on the percentage of individuals who satisfied the criteria of minimum dimensions established as the reference: “safety area”, if at least 75% of the individuals met the criteria; “secondary area”, when the percentage was between 51% and 74%; and “risk area”, when the percentage did not exceed 50%.
To determine the bone density, we simulated the placement of a miniscrew in each of the interradicular locations of interest. To this end, we drew a rectangle (6 mm long by 1.5 mm wide) at a depth of 6 mm from the bone crest on the cross-sections of the superior maxilla obtained from the original CBCT images.
The statistical analysis was performed with the free R software (version 2.12.0, R Core Team, Vienna, Austria). The alveolar measurements MD and VP were recorded in millimeters and the density measurements were recorded using grayscale values. To evaluate intraobserver reliability, we randomly selected images from 10 participants (5 from the DS group and 5 from the control group) and a single trained observer performed all measurements during the study on 2 occasions with a 6-week interval. The reproducibility was evaluated using the intraclass correlation coefficient (ICC, 0.94; 95% confidence interval, 0.82–0.99). The significance for the differences between 2 sites within a single group (DS or control) was evaluated by performing multiple comparisons in a linear mixed model. We employed logistic regression to compare the values obtained from each site between the DS and control groups. Some of the variables did not follow a normal distribution. Therefore, to analyze the differences in the alveolar measurements evaluated according to sex in the DS and control groups, we employed the Wilcoxon test for independent samples. To analyze the differences in the measurements evaluated according to age, we calculated Spearman’s correlation coefficient. A p-value < 0.05 was considered statistically significant.
The study was approved by the Institutional Review Board of the University of Santiago de Compostela (USC), Spain.
No specific informed consent was required as all participants or, as appropriate, their legal representatives had signed an informed consent to authorize the use of images for teaching or research purposes.