Material and methods

This retrospective study was based on a sample of 16 growing subjects (ages, 9-13 years) comprising 8 subjects treated with rapid maxillary expansion for crossbite correction and 8 subjects treated with the Herbst appliance for the correction of Class II malocclusion. CBCT scans (0.4-mm voxel size; 16 × 22-cm field of view) of all subjects were already available at 2 times with at least 6 months between them, before treatment (T1) and after treatment (T2), taken with an i-Cat machine (Imaging Sciences International, Hatfield, Pa).

This study was approved by the University of Michigan institutional review board (HUM00095895).

After converting the DICOM files to “guys image processing lab (gipl)” files using ITK-SNAP open-source software ( http://www.itksnap.org ), the 3D image analysis procedures followed these steps.

• 1.

  Approximation of T1 and T2 scans. The T1 and T2 gipl files were approximated manually by the same observer (A.C.O.R.) with a best fit of the maxillary outlines in 3D multiplanar cross-sections using open-source software (Slicer version 4.3.1; http://www.slicer.org ).

• 2.

  Construction of 3D volumetric label maps of the maxilla (segmentation). The construction of 3D volumetric label maps for the T1 and T2 scans was performed with ITK-SNAP software. The automatic segmentation procedures in ITK-SNAP use active contour methods to compute feature images based on the CBCT images’ gray level intensities and boundaries. The threshold was adjusted scan by scan, since the ITK-SNAP permits adjustment of the parameters for automatic detection of intensities and boundaries and allows interactive editing of the contours by the user. The anatomic structures that were segmented for reference (regions of reference) indicated to the software where it should look for corresponding voxels. The segmentations were also used to build 3D surface mesh models (.stl) that were loaded into software (VECTRA Analysis Module, version 3.7.6; Canfield Scientific, Fairfield, NJ) to generate the landmark coordinates and the distances between landmarks.

• 3.

  Placement of landmarks on the 3D volumetric label maps at T2. One observer (A.C.O.R.) labeled 6 landmarks in all T2 models in different regions of the maxilla to eliminate errors of pitch, roll, and yaw, and also to prevent any landmark identification errors ( Fig 1 ). One label not used in the 3D volumetric label maps was used to label the landmarks in the following regions: zygomatic processes of the maxilla on both sides, buccal surfaces of the maxillary first molars on both sides, anterior nasal spine, and proximal contact points between the maxillary central incisors. The landmarks were labeled in 2 consecutive slices using the paintbrush tool. The same 3D volumetric label maps, labeled with the landmarks, were used by both observers (A.C.O.R., M.R.G.) for the registration procedures to prevent errors caused by segmentation or landmark placement.

  

  Images showing 3D models at T2 with the 6 prelabeled landmarks used to obtain the measurements for comparison between the registration methods and between observers.

• 4.

  Clipping (cropping) of the masks for each registration region. The 3D volumetric label maps at T2, prelabeled with the landmarks, were cropped by 2 calibrated observers (A.C.O.R., M.R.G.). They were trained and calibrated to perform the cropping using a set of ten 3D volumetric label maps not included in this study. The procedures of cropping and registration were performed at 2 times with a 3-month interval between the registrations by the same 2 observers working independently. Two regions of reference (mask) were defined for the voxel-based registration procedures ( Figs 2 and 3 ): (1) the maxillary (MAX) region of reference included the maxillary bone cropped inferiorly at the dentoalveolar processes; superiorly, the regions above the plane through the right and left orbitale points; bilaterally, the zygomatic processes at orbitale points; and posteriorly at a plane through the distal surface of the second molars; and (2) the “Björk-inspired” palate and infrazygomatic region of reference (PIZ) had different posterior and anterior limits (respectively, the planes passing through the distal aspects of the first molars and the canines).

  

  Images of the cropping to define the MAX region of interest (mask shown in blue ) used as the reference for the voxel-based MAX registration method: A, B, and C, superior, inferior, and lateral limits of the mask, respectively ( red refers to regions that will be excluded or cropped); D, final mask for MAX registration.

  

  Images of the cropping to define the PIZ region of interest (mask shown in blue ) used as the reference for the voxel-based PIZ registration: A, B, and C, superior, inferior, and lateral limits of the mask, respectively ( red refers to regions that will be excluded); D, final mask for PIZ registration.

• 5.

  Voxel-based registration procedures. These procedures used the anatomic structures described above as masks for reference, indicating to the software in which areas it should look for corresponding voxels to register the T2 scan (with landmarks prelabeled with a different label) in relation to the T1 scan. After the user had selected the region of reference for registration, a fully automated voxel-based registration method was performed with the Slicer software. The software compares the gray-level values voxel by voxel, within the region of reference, in 2 CBCT images (T1 and T2) maximizing mutual information to compute the rotation and translation parameters between them. Because the sample consisted of growing patients, the scans at the 2 times had different sizes. For this reason, a fully automated voxel-wise rigid growing registration method (that takes into account that the images have different sizes but applies only 6 degrees of freedom of rotation and translation to the T2 scan) was performed. The registrations were voxel based on the region of reference, and the software generated (as an output file) the 3D volumetric labeled maps registered over T1. The 3D mesh surface models were generated from those output files. Then the 3D volumetric label maps (prelabeled with landmarks) at T2 resulting from the registration based on the MAX and PIZ regions of registration, performed twice independently by the 2 observers, were saved as 3D surface mesh models (.stl files) with landmarks already placed, using the Slicer software.

• 6.

  Landmark-based quantitative assessments in VAM software (VECTRA Analysis Module, version 3.7.6). The 3D surface mesh models (.stl files) at T2 with landmarks already placed, registered by the 2 regions of reference and cropped by the 2 calibrated observers at the 2 times, were loaded in the VECTRA software. It generated the coordinates for each landmark and the Euclidean distances between corresponding landmarks. These values were statistically analyzed in 3 ways to assess the intraoberver and interobserver reproducibility values and the consistency of the regions of reference. Figure 4 displays the flowchart of the study methodology.

  

  Flowchart of the study methodology. The light blue box on the left shows the procedures performed by observer 1 ( Obs1 ), and on the right by observer 2 ( Obs2 ). Both observers used the same maxillary surface model shown in cyan that was constructed for each CBCT scan at T2 and prelabeled with landmarks. Maxillary surface models shown in yellow indicate the registration with the MAX reference and, in models in green , the registration with the PIZ as the reference. The red arrows indicate the measurements for comparison between the 2 methods, and the blue arrows indicate the interobserver assessments.

• 7.

  Color-coded assessment. Interactive visual analytic evaluations of surface differences were performed by graphic displays of color-coded maps and semitransparent overlays for visual intraobserver and interobserver comparisons and to compare the 2 regions of reference.

Results

Table I shows the descriptive statistics for the measurements with the MAX and PIZ registration regions for both observers and both times. The means of the Euclidean distances between the T2 .stl models after registrations were small at all landmarks for all combinations tested (all means were smaller than 0.5 mm). When we considered the standard deviations, all differences between the models registered by the 2 regions were 1.0 mm or less.

The statistics to test the concordance (ICC) between regions of reference for registration and for intraobserver and interobserver reproducibility values showed excellent consistency (>0.99).

The consistency between regions of registration, and between intraobserver and interobserver reproducibility values, is shown in Table II . Using limits of agreement of Bland and Altman, one would expect that 95% of the differences between corresponding coordinates for all 6 corresponding landmarks obtained from the registrations in this study would be within the range of −0.82 to 0.77 mm. Figure 5 gives 1 example of consistency between the 2 regions used for registration ( Fig 5 , A ) and intraobserver and interobserver agreement values ( Fig 5 , B and C , respectively) for landmark 6.

Table II

Measurement of the consistency between regions of registration and intraobserver and interobservers reproducibility

Types of comparisons	Landmark 1		Landmark 2		Landmark 3		Landmark 4		Landmark 5		Landmark 6
	Mean	LoA	Mean	LoA	Mean	LoA	Mean	LoA	Mean	LoA	Mean	LoA
Between regions of registration
MAX Obs1 × PIZ Obs1 R1	0.08	−0.38, 0.54	0.04	−0.51, 0.59	0.05	−0.48, 0.57	0.05	−0.45, 0.55	0.01	−0.37, 0.39	0.04	−0.50, 0.59
MAX Obs1 × PIZ Obs1 R2	0.01	−0.37, 0.39	0.02	−0.65, 0.60	0.00	−0.42, 0.43	0.03	−0.54, 0.61	0.04	−034, 0.42	0.06	−0.47, 0.58
MAX Obs2 × PIZ Obs2 R1	0.04	−0.39, 0.46	−0.06	−0.66, 0.53	−0.01	−0.46, 0.43	−0.03	−0.59, 0.52	0.06	−0.34, 0.46	0.04	−0.44, 0.53
MAX Obs2 × PIZ Obs2 R2	0.05	−0.51, 0.60	0.01	−0.53, 0.55	0.01	−0.47, 0.50	0.01	−0.55, 0.57	0.00	−0.52, 0.52	0.09	−0.48, 0.67
Intraobserver
MAX R1 × MAX R2 Obs1	0.03	−0.38, 0.43	0.04	−0.37, 0.45	0.00	−0.48, 0.48	−0.03	−0.43, 0.36	0.01	−0.38, 0.40	−0.03	−0.44, 0.39
PIZ R1 × PIZ R2 Obs1	−0.02	−0.47, 0.43	−0.02	−0.53, 0.48	−0.03	−0.54, 0.47	−0.05	−0.46, 0.36	0.04	−0.27, 0.36	−0.01	−0.34, 0.32
MAX R1 × MAX R2 Obs2	0.01	−0.46, 0.48	0.00	−0.52, 0.52	0.02	−0.49, 0.52	−0.03	−0.67, 0.62	0.04	−0.36, 0.43	0.00	−0.42, 0.41
PIZ R1 × PIZ R2 Obs2	0.06	−0.50, 0.62	0.07	−0.65, 0.80	0.05	−0.55, 0.64	0.02	−0.50, 0.54	−0.03	−0.52, 0.47	0.05	−0.51, 0.60
Interobserver
MAX Obs1 × MAX Obs2 R1	0.04	−0.44, 0.52	0.04	−0.46, 0.54	0.00	−0.51, 0.50	−0.01	−0.53, 0.50	0.00	−0.36, 0.37	0.04	−0.50, 0.59
MAX Obs1 × MAX Obs2 R2	0.03	−0.36, 0.42	0.00	−0.48, 0.48	0.01	−0.58, 0.60	−0.01	−0.62, 0.60	0.03	−0.41, 0.47	−0.01	−0.43, 0.41
PIZ Obs1 × PIZ Obs2 R1	−0.04	−0.63, 0.55	−0.07	−0.82, 0.67	−0.06	−0.57, 0.46	−0.10	−0.66, 0.46	0.06	−0.33, 0.45	−0.03	−0.58, 0.53
PIZ Obs1 × PIZ Obs2 R2	0.04	−0.52, 0.60	0.03	−0.71, 0.77	0.02	−0.59, 0.63	−0.03	−0.56, 0.49	−0.02	−0.41, 0.38	0.03	−0.55, 0.61

Only gold members can continue reading. Log In or Register to continue

Share this:

• Click to share on Twitter (Opens in new window)
• Click to share on Facebook (Opens in new window)

Related

Related posts:

1. Linking orthodontic treatment and caries management for high-risk adolescents
2. Bone and cartilage changes after injection of botulinum toxin
3. Racial disparities in orthodontic service use for Medicaid-enrolled children: An evaluation of the Washington Medicaid program
4. Multiple linear regression analysis
5. Diagnosis and conservative treatment of skeletal Class III malocclusion with anterior crossbite and asymmetric maxillary crowding
6. User acceptance of a touchless sterile system to control virtual orthodontic study models

Stay updated, free dental videos. Join our Telegram channel

Join