Subjects and CBCT scan

The study was approved by the necessary ethics committee. The sample for this retrospective study comprised the CBCT files for two dry skulls obtained from the Oral Diagnostic Science Department of Virginia Commonwealth University and secondary data from 15 patients who had undergone either surgical treatment (coronectomy of wisdom teeth and bone grafts) and/or orthodontic treatment at a private practice. The CBCT scans were taken between April 2009 and March 2015 and the patients ranged in age from 27 to 65 years. All of the patients had either full dentitions or were partially edentulous. Inclusion criteria for the human subjects were (1) non-growing patient, with (2) two CBCT scans (T1 and T2) taken using the same machine and with the same voxel size (0.25 mm). Exclusion criteria were (1) same patient with CBCT scans from different machines, (2) CBCT scans with a different voxel size between T1 and T2.

The dry skulls images were acquired with a Kodak Carestream 9300 (Carestream Health Inc., Rochester, NY, USA) and 13.5 × 17 cm FOV, scan time of 11.3 s, set at 85 kVp, 4 mA, and 0.3-mm voxel size. Two images of each dry skull were taken, modifying its position between T1 and T2. These images were used as a gold standard since there was no bony change between T1 and T2. The patient images were acquired with an i-CAT scanner (Imaging Sciences International LLC, Hatfield, PA, USA) and 16 × 13 cm FOV, scan time of 27 s, set at 120 kVp, 8 mA, and isotropic 0.25-mm voxel size. The DICOM (Digital Imaging and Communication in Medicine) files were imported into OnDemand 3D v1.0.10.5261 (Cybermed Inc., Seoul, Korea). The T2 scan was taken between 4 and 24 months (average 12.3 months) after T1.

3D image processing

A summary of the method is given in Fig. 1 . One observer cropped the CBCT files from T1 and T2 to simulate a 10 × 5 cm FOV scan, obtaining a significant amount of the maxillary and mandibular area. The crops were done as shown in Fig. 2 ; this resulted in a total of four images: T1 mandible, T1 maxilla, T2 mandible, and T2 maxilla. The software used allows the clinician to crop in any dimension, and the inferosuperior crops are done precisely by selecting the number of slices that the user wants to keep. In the present study, 200 slices were used to simulate 5 cm of height (200 × 0.25 mm = 5 cm). The software does not allow precise cropping in the anteroposterior dimension, therefore the CBCT scans were approximately 10 cm. The images were saved in the software database.

Flowchart of the method. The blue boxes are steps done using OnDemand 3D and the green boxes are steps done using VAM (Md, mandible; Mx, maxilla; T2S, T2 superimposed).

(A) Full skull before crop. (B) Maxilla cropped, and (C) mandible cropped. Each image was cropped to simulate a 10 × 5 cm CBCT. The maxillary crop included the upper teeth, alveolar process, and part of the zygomatic bone, avoiding the inclusion of the zygomatic arch as a whole. The mandibular crop included the lower teeth, corpus, angle, and part of the ramus.

Two observers (L.K. and A.W.) attempted to perform the regional superimposition independently. For the mandibular superimposition, the cropped mandibular files from T1 and T2 were opened using the ‘fusion’ tab of the software. The fusion module allows the observer to manually move T2 as close as possible to the position of T1 and also allows the observer to do an automatic voxel-based superimposition. The superimposition process took approximately 10–15 s. The software reads the voxels from the whole scan in T1 and tries to match them with a similar area in T2. Although the software had a tool to focus on the voxels of a specific region of interest, this was not needed in the present study. For the maxillary superimposition, the stable areas included in the crop were the zygomatic process of the maxilla and the palate. For the mandible, the stable areas were the symphysis, corpus, and part of the ramus.

After the superimposition had been done ( Fig. 3 ), the T2 file in its new orientation was saved (T2 superimposed, T2S). One observer (L.K.) was responsible for segmenting T1 and T2S mandibular files using the ‘3D picker’ tool inside the ‘3D’ module. All the segmentations were standardized at 381–382 grey levels and the segmented files were exported in STL format (Standard Tessellation Language) using the software parameters of 0.005 reduction error and a smooth of 1. The same steps were done for the maxillary cropped area.

(A) Sagittal and (B) axial views of the maxilla before superimposition, and the same (C) sagittal and (D) axial views after the maxillary superimposition. Note that in the maxillary area, the T1 and T2 images match, while in the mandible (white arrows), they do not.

One observer imported all six STL files (T1 maxilla and mandible and T2S maxilla and mandible for each observer) into VAM (Canfield Scientific, Fairfield, NJ, USA) and performed measurements with the iterative closest point (ICP) technique. The ICP measures the smallest distance between two surfaces, providing the root mean square (RMS). A RMS value smaller than 0.25 mm (the voxel size) and errors for living subjects comparable to those found with the dry skulls were required to prove that the superimposition method is accurate. The aim was to perform the measurements in stable areas not influenced by the alveolar changes. In the maxilla, measurements were made at the lower border of the zygomatic alveolar crest, anterior and posterior to the zygomatic maxillary suture ( Fig. 4 A–C) . In the mandible, the measurements were made at the basal bone of the chin prominence and distal to the mental foramens on both sides ( Fig. 4 D, E).

Areas of interest measured and colour-coded map ranging from 0.4 to −0.4 mm. (A) Lateral view of the maxilla; (B) latero-inferior view of the maxilla; (C) anterior view of the maxilla; (D) lateral view of the mandible; (E) anterior view of the mandible.

To prove that the method is reproducible, the ICP was used to measure the distances between T2S of operator 1 and T2S of operator 2. The RMS value was obtained: the distance between the models should be smaller than 0.25, while 0 would be considered perfect. The measurements were repeated by the same operator after 10 days to ensure reproducibility. The results were exported to an excel spreadsheet.

Statistical analysis

The statistical analyses were done using IBM SPSS Statistics software version 22.0 (IBM Corp., Armonk, NY, USA). Descriptive statistics for the RMS were obtained separately for the dry skulls and the human subjects. Intra-examiner agreement for the measurements was assessed by means of the intra-class correlation coefficient (ICC) and descriptive statistics with mean differences and confidence intervals set at 95%, and included the living subjects and dry skulls. The values for inter-examiner reproducibility are reported individually.