3D assessment of mandibular growth based on image registration: a feasibility study in a rabbit model

A comparison of voxel- and surface-based cone-beam computed tomography mandibular superimposition in adult orthodontic patients

OBJECTIVE

To evaluate the accuracy, reliability, and efficiency of voxel- and surface-based registrations for cone-beam computed tomography (CBCT) mandibular superimposition in adult orthodontic patients.

METHODS

Pre- and post-orthodontic treatment CBCT scans of 27 adult patients were obtained. Voxel- and surface-based CBCT mandibular superimpositions were performed using the mandibular basal bone as a reference. The accuracy of the two methods was evaluated using the absolute mean distance measured. The time that was required to perform the measurements using these methods was also compared. Statistical differences were determined using paired t-tests, and inter-observer reliability was assessed by intraclass correlation coefficients (ICCs).

RESULTS

The absolute mean distance on seven mandible surface areas between voxel- and surface-based registrations was similar but not significantly different. ICC values of the surface-based registration were 0.918 to 0.990, which were slightly lower than those of voxel-based registration that ranged from 0.984 to 0.996. The time required for voxel-based registration and surface-based registration was 44.6 ± 2.5 s and 252.3 ± 7.1 s, respectively.

CONCLUSIONS

Both methods are accurate and reliable and not significantly different from each other. However, voxel-based registration is more efficient than surface-based registration for CBCT mandibular superimposition.

3D superimposition of craniofacial imaging-The utility of multicentre collaborations

Clinical applications of 3D image registration and superimposition have contributed to better understanding growth changes and clinical outcomes. The use of 3D dental and craniofacial imaging in dentistry requires validate image analysis methods for improved diagnosis, treatment planning, navigation and assessment of treatment response. Volumetric 3D images, such as cone-beam computed tomography, can now be superimposed by voxels, surfaces or landmarks. Regardless of the image modality or the software tools, the concepts of regions or points of reference affect all quantitative of qualitative assessments. This study reviews current state of the art in 3D image analysis including 3D superimpositions relative to the cranial base and different regional superimpositions, the development of open source and commercial tools for 3D analysis, how this technology has increased clinical research collaborations from centres all around the globe, some insight on how to incorporate artificial intelligence for big data analysis and progress towards personalized orthodontics.

Comparison and reproducibility of 2 regions of reference for maxillary regional registration with cone-beam computed tomography

INTRODUCTION

The aims of this study were to evaluate the differences between 2 regions of maxillary voxel-based registration and to test the reproducibility of the registration.

METHODS

Three-dimensional models were built for before-treatment (T1) and after-treatment (T2) based on cone-beam computed tomography images from 16 growing subjects. Landmarks were labeled in all T2 models of the maxilla, and voxel-based registrations were performed independently by 2 observers at 2 times using 2 reference regions. The first region, the maxillary region, included the maxillary bone clipped inferiorly at the dentoalveolar processes, superiorly at the plane passing through the right and left orbitale points, laterally at the zygomatic processes through the orbitale point, and posteriorly at a plane passing through the distal surface of the second molars. In the second region, the palate and infrazygomatic region had different posterior and anterior limits (at the plane passing through the distal aspects of the first molars and the canines, respectively). The differences between the registration regions were measured by comparing the distances between corresponding landmarks in the T2 registered models and comparing the corresponding x, y, and z coordinates from corresponding landmarks. Statistical analysis of the differences between the T2 surface models was performed by evaluating the means and standard deviations of the distances between landmarks and by testing the agreement between coordinates from corresponding landmarks (intraclass correlation coefficient and Bland-Altman method).

RESULTS

The means of the differences between landmarks from the palate and infrazygomatic region to the maxillary region 3-dimensional surface models at T2 for all regions of reference, times of registrations, and observer combinations were smaller than 0.5 mm. The intraclass correlation coefficient and the Bland-Altman plots indicated adequate concordance.

CONCLUSIONS

The 2 regions of regional maxillary registration showed similar results and adequate intraobserver and interobserver reproducibility values.

Congenital and acquired mandibular asymmetry: Mapping growth and remodeling in 3 dimensions

INTRODUCTION

Disordered craniofacial development frequently results in definitive facial asymmetries that can significantly impact a person's social and functional well-being. The mandible plays a prominent role in defining facial symmetry and, as an active region of growth, commonly acquires asymmetric features. Additionally, syndromic mandibular asymmetry characterizes craniofacial microsomia (CFM), the second most prevalent congenital craniofacial anomaly (1:3000 to 1:5000 live births) after cleft lip and palate. We hypothesized that asymmetric rates of mandibular growth occur in the context of syndromic and acquired facial asymmetries.

METHODS

To test this hypothesis, a spherical harmonic-based shape correspondence algorithm was applied to quantify and characterize asymmetries in mandibular growth and remodeling in 3 groups during adolescence. Longitudinal time points were automatically registered, and regions of the condyle and posterior ramus were selected for growth quantification. The first group (n = 9) had a diagnosis of CFM, limited to Pruzansky-Kaban type I or IIA mandibular deformities. The second group (n = 10) consisted of subjects with asymmetric, nonsyndromic dentofacial asymmetry requiring surgical intervention. A control group (n = 10) of symmetric patients was selected for comparison. A linear mixed model was used for the statistical comparison of growth asymmetry between the groups.

RESULTS

Initial mandibular shape and symmetry displayed distinct signatures in the 3 groups (P <0.001), with the greatest asymmetries in the condyle and ramus. Similarly, mandibular growth had unique patterns in the groups. The dentofacial asymmetry group was characterized by significant asymmetry in condylar and posterior ramal remodeling with growth (P <0.001). The CFM group was characterized by asymmetric growth of the posterior ramus (P <0.001) but relatively symmetric growth of the condyles (P = 0.47).

CONCLUSIONS

Forms of CFM are characterized by active and variable growth of the dysplastic side, which has a distinct pattern from other disorders of mandibular growth.

3D Mandibular Superimposition: Comparison of Regions of Reference for Voxel-Based Registration

Introduction

The aim was to evaluate three regions of reference (Björk, Modified Björk and mandibular Body) for mandibular registration testing them in a patients’ CBCT sample.

Methods

Mandibular 3D volumetric label maps were built from CBCTs taken before (T1) and after treatment (T2) in a sample of 16 growing subjects and labeled with eight landmarks. Registrations of T1 and T2 images relative to the different regions of reference were performed, and 3D surface models were generated. Seven mandibular dimensions were measured separately for each time-point (T1 and T2) in relation to a stable reference structure (lingual cortical of symphysis), and the T2-T1 differences were calculated. These differences were compared to differences measured between the superimposed T2 (generated from different regions of reference: Björk, Modified Björk and Mandibular Body) over T1 surface models. ICC and the Bland-Altman method tested the agreement of the changes obtained by nonsuperimposition measurements from the patients’ sample, and changes between the overlapped surfaces after registration using the different regions of reference.

Results

The Björk region of reference (or mask) did work properly only in 2 of 16 patients. Evaluating the two other masks (Modified Björk and Mandibular body) on patients’ scans registration, the concordance and agreement of the changes obtained from superimpositions (registered T2 over T1) compared to results obtained from non superimposed T1 and T2 separately, indicated that Mandibular Body mask displayed more consistent results.

Conclusions

The mandibular body mask (mandible without teeth, alveolar bone, rami and condyles) is a reliable reference for 3D regional registration.

Implant-based three-dimensional superimposition of the growing mandible in a rabbit model

BACKGROUND

The reliable assessment of craniofacial morphological changes during growth requires invariant regions for image registration. As these regions have not yet been identified in three dimensions, intra-osseous implants are required as fiducial markers for the reliable assessment of three-dimensional (3D) mandibular growth changes. The objective of this study was to develop an animal model for the assessment of the 3D morphological changes of the mandible during growth, using implants as fiducial markers.

MATERIALS AND METHODS

Titanium implants were placed in the body of the mandible of six New Zealand White rabbits. Cone beam computed tomography (CBCT) scans were taken 1-week following implant placement and after an additional 8-weeks of growth. Segmentations of CBCT images were exported into custom-made scripts, implant centroids were identified, implant stability during growth calculated, and the segmented mandibles were registered on the implant centroids.

RESULTS

The buccal cortical bone of the body of the mandible was stable during growth and suitable for fiducial marker placement. Bilateral implants resulted in more accurate rigid registration of the growing rabbit mandible than only unilateral implants. 3D mandibular growth changes were visualised by means of semi-transparencies.

CONCLUSIONS

This animal model appears to be feasible for the assessment of the 3D morphological changes occurring during mandibular growth. To the best of our knowledge this is the first time that the implant superimposition method has been combined with 3D imaging to accurately reveal mandibular growth changes.

A Novel Restraining Device for Small Animal Imaging Exams: Validation in Rabbits

Objective. To develop, validate, and patent a Restraining Device for Small Animal Imaging Exams (RDSAIE) that allows exams to be comfortably conducted without risks to animals and professionals. Methods. A RDSAIE with a mobile cover and shelf was built with transparent acrylic material. A total of six anesthetized rabbits were used to perform the following imaging exams of the skull: Cone Beam Computed Tomography, Magnetic Resonance Imaging, and Scintigraphy. Results. The device showed great functionality and full visibility of the animal behavior, which remained fully stabilized and immobilized in either the horizontal or vertical position without the need for a person to remain in the test room to assist them. The procedures were performed without difficulty, and images of good resolution and without artifacts were obtained. Conclusion. The RDSAIE is comfortable, safe, efficient, and ergonomic. It allows the easy placement of animals in different body positions, including the vertical, the maintenance of postural stability, and full visibility. It may be constructed for animals heavier than 4 kg and it is adaptable for translational studies in anima nobile.