Voxel-based superimposition of Cone Beam CT scans for orthodontic and craniofacial follow-up: Overview and clinical implementation

New Assessment Method of Alveolar Bone Grafting Using Automatic Registration and AI-based Segmentation

OBJECTIVE

The aim of this Technical note is to present a new assessment method of alveolar bone grafts after secondary alveolar bone grafting using automatic registration and artificial intelligence (AI)-based segmentation.

METHODS

A total of 7 Japanese patients (4 men and 3 women) with a unilateral cleft lip, alveolus, and/or palate, who underwent secondary alveolar bone grafting between March 2021 and August 2022 were evaluated. Computed tomography (CT) analyses were performed about 1 month before the surgery, and 1 day and 6 months after the surgery. All CT images were imported into a 3-dimensional analysis workstation. CT images from after the surgery were superimposed onto CT images from before the surgery, by automatic rigid image registration. The segmentation of bone tissues was automatically performed by the AI-based function. Grafted bone was extracted by subtraction of the bone tissue after the surgery from the bone tissue before the surgery. The volumes and Hounsfield units (HUs) of the grafted bones were calculated. The intraclass correlation coefficient (ICC) was reviewed to assess inter-rater reliability.

RESULTS

The ICCs (2,1) of the volumes and HUs measured by the observers immediately after the surgery were 0.95 and 0.99, respectively. On the other hand, the ICCs (2,1) of the volumes and HUs measured by the observers 6 months after the surgery were 0.81 and 0.57, respectively.

CONCLUSIONS

Our new assessment method enables simple and quick evaluation of residual grafted bone after secondary alveolar bone grafting and demonstrated relatively high inter-rater reliability.

Three-Dimensional Changes of Condylar Position After Bimaxillary Surgery to Correct Skeletal III Malocclusion: Cone Beam Computed Tomography Voxel-Based Superimposition Analysis

BACKGROUND

The stability of the condylar position within the first 3 months after orthognathic surgery in patients with skeletal Class III malocclusion is crucial for subsequent orthodontic treatment.

PURPOSE

The purpose was to compare condylar positional changes 3 months after bimaxillary surgery to correct skeletal Class III deformities, utilizing the conventional two-dimensional combined with cone beam computed tomography voxel-based superimposition analysis.

STUDY DESIGN, SETTING, SAMPLE

This retrospective cohort study included skeletal Class III patients undergoing bimaxillary orthognathic surgery at Khon Kaen University, Thailand, in 2020. Exclusion criteria were patients with temporomandibular disorder symptoms, facial asymmetry prior to the surgery, or surgical complications.

EXPOSURE VARIABLE

The exposure variable was the timing, both preoperatively and 3 months postoperatively. This 3-month postoperative time point was chosen because complete healing of the bimaxillary surgery is expected by then, without any potential influence of condylar changes resulting from subsequent orthodontic treatment.

MAIN OUTCOME VARIABLES

The outcome variables are condylar position measurement, joint space (mm), and axial condylar angle (degrees).

COVARIATES

Demographics (age, sex) and surgical details (direction and amount of movement) were collected as covariates.

ANALYSES

Statistical analysis of condylar positional changes and correlations was performed using paired t-test and linear correlation (P value < .05), respectively.

RESULTS

The sample included 11 subjects (22 condyles), with a mean age of 24 ± 5.24 years. Both two-dimensional measurements and voxel-based three-dimensional superimposition showed significant changes in condylar position 3 months after bimaxillary surgery: inferior (0.45 ± 0.26 mm, P < .001), posterior (0.46 ± 0.39 mm, P = .003), lateral (0.38 ± 0.42 mm, P = .01) displacement, and inward rotation (5.21 ± 2.54°, P < .001). No significant correlation was found between jaw movement distance and condylar changes.

CONCLUSION AND RELEVANCE

To our knowledge, this is the first study to report measures of condylar changes at 3 months, when complete healing of the osteotomies would be expected, using Le Fort I osteotomy for maxillary advancement combined with bilateral sagittal split ramus osteotomy for mandibular setback. These changes are small in magnitude and may be of little relevance to patient care.

Accuracy and reliability of mandibular digital model superimposition based on the morphological characteristics of vessels in extraction adult patients

BACKGROUND

This study aimed to validate the availability of superimposing full-color mandibular digital models (DMs) by the morphological characteristics of vessels in extraction adult patients.

METHODS

Twenty-eight adult patients were included, and their DMs were superimposed with pre- and posttreatment cone beam computed tomography (CBCT) and the morphological characteristics of lingual vessels. The measurements of each tooth were compared under the same coordinate system.

RESULTS

The ICC results displayed exceptional agreement in intra- and interrater assessments, with scores exceeding 0.891 in the crown for intrarater agreement and scores surpassing 0.888 in the crown for interrater agreement. Furthermore, no statistically significant differences were found in the 2 superimposition methods (P > 0.05).

CONCLUSION

The morphological characteristics of vessels under the mucogingival junction in the lingual side of mandible of are stable enough for the superimposition of mandibular DMs in the adult patients undergo orthodontic treatment with premolars extraction.

Radiological evaluation of inferior alveolar nerve displacement after removal of impacted mandibular third molars prior to sagittal split osteotomy

INTRODUCTION

We hypothesize that the removal of mandibular third molars (M3) 6 months prior to a bilateral sagittal split osteotomy (BSSO) could allow the displacement of the inferior alveolar nerve (IAN) in a favorable lingual position. This study aimed to radiographically compare the position of IAN before and after M3 removal in patients with Class II malocclusion.

MATERIALS AND METHOD

The CBCT images of 30 randomly selected patients (mean age 15.5 years, 19 females and 11 males) were segmented regarding the mandibular bone and the IAN canal. Mandibles were then superimposed and compared using 3D slicer (www.slicer.org). An orthonormal system was constructed, and the coordinates of IAN were assessed in the x- (horizontal axis), y- (depth axis), and z- (vertical axis) directions.

RESULTS

The mean changes in x- and z-values were 0.37 %, -0.09 % for the right IAN, 0.07 %, and -0.10 % for the left IAN, respectively. Y-axis was the dimension the most impacted by the M3 removal with a mean variation of -11.96 % for the right IAN, and 0.45 % for the left nerve (p1=0.74 and p2=0.04, respectively). Three patients presented a change in the IAN position superior to 1 mm on at least one coordinate axis. We observed a more important change in x-values of the right IAN in male than in female (p = 0.04), and no significant modifications regarding the other dimensions. Finally, there was no correlation between the age of the patients and the changes in IAN position.

CONCLUSION

This study confirms the absence of influence of mandibular third molar removal on the inferior alveolar nerve route prior to BSSO.

Accuracy of Palatal Orthodontic Mini-Implants Placed Using Fully Digital Planned Insertion Guides: A Cadaver Study

Digital workflows have become integral in orthodontic diagnosis and therapy, reducing risk factors and chair time with one-visit protocols. This study assessed the transfer accuracy of fully digital planned insertion guides for orthodontic mini-implants (OMIs) compared with freehanded insertion. Cone-beam computed tomography (CBCT) datasets and intraoral surface scans of 32 cadaver maxillae were used to place 64 miniscrews in the anterior palate. Three groups were formed, two using printed insertion guides (A and B) and one with freehand insertion (C). Group A used commercially available customized surgical templates and Group B in-house planned and fabricated insertion guides. Postoperative CBCT datasets were superimposed with the planning model, and accuracy measurements were performed using orthodontic software. Statistical differences were found for transverse angular deviations (4.81° in A vs. 12.66° in B and 5.02° in C, p = 0.003) and sagittal angular deviations (2.26° in A vs. 2.20° in B and 5.34° in C, p = 0.007). However, accurate insertion depth was not achieved in either guide group; Group A insertion was too shallow (-0.17 mm), whereas Group B insertion was deeper (+0.65 mm) than planned. Outsourcing the planning and fabrication of computer-aided design and computer-aided manufacturing insertion guides may be beneficial for certain indications; particularly, in this study, commercial templates demonstrated superior accuracy than our in-house-fabricated insertion guides.

Natural reference structures for three-dimensional maxillary regional superimposition in growing patients

BACKGROUND

Assessment of growth-related or treatment-related changes in the maxilla requires a reliable method of superimposition. Such methods are well established for two-dimensional (2D) cephalometric images but not yet for three-dimensions (3D). The aims of this study were to identify natural reference structures (NRS) for the maxilla in growing patients in 3D, opportunistically using orthodontic mini-screws as reference; and to test the applicability of the proposed NRS for maxillary superimposition by assessing the concordance of this approach with Björk's 'stable reference structures' in lateral projection.

METHODS

The stability of the mini-screws was tested on longitudinal pairs of pre- and post-orthodontic cone-beam computed tomography (CBCT) images by measuring the distance changes between screws. After verifying the stability of the mini-screws, rigid registration was performed for aligning the stable mini-screws. Then, non-rigid registration was used to establish the dense voxel-correspondence among CBCT images and calculate the displacement of each voxel belonging to the maxilla relative to the mini-screws. The displacement vectors were transformed to a standardized maxillary template to categorize the stability of the internal structures statistically. Those voxels that displaced less relative to the mini-screws were considered as the natural reference structures (NRS) for the maxilla. Test samples included another dataset of longitudinal CBCT scans. They were used to evaluate the applicability of the proposed NRS for maxillary superimposition. We assessed whether aligning the maxilla with proposed NRS is in concordance with the maxillary internal reference structures superimposition in the traditional 2D lateral view as suggested by Björk. This was quantitively assessed by comparing the mean sagittal and vertical tooth movements for both superimposition methods.

RESULTS

The stability of the mini-screws was tested on 10 pairs of pre- and post-orthodontic cone-beam computed tomography (CBCT) images (T1: 12.9 ± 0.8 yrs, T2: 14.8 ± 0.7 yrs). Both the loaded and the unloaded mini-screws were shown to be stable during orthodontic treatment, which indicates that they can be used as reference points. By analyzing the deformation map of the maxilla, we confirmed that the infraorbital rims, maxilla around the piriform foramen, the infrazygomatic crest and the hard palate (palatal vault more than  1 cm distal to incisor foramen except the palatal suture) were stable during growth. Another dataset of longitudinal CBCT scans (T1: 12.2 ± 0.63 yrs, T2: 15.2 ± 0.96 yrs) was used to assess the concordance of this approach with Björk's 'stable reference structures'. The movement of the maxillary first molar and central incisor showed no statistically significant difference when superimposing the test images with the proposed NRS or with the classic Björk maxillary superimposition in the lateral view.

CONCLUSIONS

The infraorbital rims, maxilla around the piriform foramen, the infrazygomatic crest and the hard palate (palatal vault more than 1 cm posterior to incisal foramen except the palatal suture) were identified as stable regions in the maxilla. These stable structures can be used for maxillary superimposition in 3D and generate comparable results to Björk superimposition in the lateral view.

Applications of three-dimensional imaging techniques in craniomaxillofacial surgery: A literature review

Three-dimensional (3D) imaging technologies are increasingly used in craniomaxillofacial (CMF) surgery, especially to enable clinicians to get an effective approach and obtain better treatment results during different preoperative and postoperative phases, namely during image acquisition and diagnosis, virtual surgical planning (VSP), actual surgery, and treatment outcome assessment. The article presents an overview of 3D imaging technologies used in the aforementioned phases of the most common CMF surgery. We searched for relevant studies on 3D imaging applications in CMF surgery published over the past 10 years in the PubMed, ProQuest (Medline), Web of Science, Science Direct, Clinical Key, and Embase databases. A total of 2094 articles were found, of which 712 were relevant. An additional 26 manually searched articles were included in the analysis. The findings of the review demonstrated that 3D imaging technology is becoming increasingly popular in clinical practice and an essential tool for plastic surgeons. This review provides information that will help researchers and clinicians consider the use of 3D imaging techniques in CMF surgery to improve the quality of surgical procedures and achieve satisfactory treatment outcomes.

Three-Dimensional Analysis of Bone Volume Change at Donor Sites in Mandibular Body Bone Block Grafts by a Computer-Assisted Automatic Registration Method: A Retrospective Study

This study aimed to evaluate the bone volume change at donor sites in patients who received mandibular body bone block grafts using intensity-based automatic image registration. A retrospective study was conducted with 32 patients who received mandibular bone block grafts between 2017 and 2019 at the Pusan National University Dental Hospital. Cone-beam computed tomography (CBCT) images were obtained before surgery (T0), 1 day after surgery (T1), and 4 months after surgery (T2). Scattered artefacts were removed by manual segmentation. The T0 image was used as the reference image for registration of T1 and T2 images using intensity-based registration. A total of 32 donor sites were analyzed three-dimensionally. The volume and pixel value of the bones were measured and analyzed. The mean regenerated bone volume rate on follow-up images (T2) was 34.87% ± 17.11%. However, no statistically significant differences of regenerated bone volume were noted among the four areas of the donor site (upper anterior, upper posterior, lower anterior, and lower posterior). The mean pixel value rate of the follow-up images (T2) was 78.99% ± 16.9% compared with that of T1, which was statistically significant (p < 0.05). Intensity-based registration with histogram matching showed that newly generated bone is generally qualitatively and quantitatively poorer than the original bone, thus revealing the feasibility of pixel value to evaluate bone quality in CBCT images. Considering the bone mass recovered in this study, 4 months may not be sufficient for a second harvesting, and a longer period of follow-up is required.

A new protocol to accurately track long-term orthodontic tooth movement and support patient-specific numerical modeling

Numerical simulation of long-term orthodontic tooth movement based on Finite Element Analysis (FEA) could help clinicians to plan more efficient and mechanically sound treatments. However, most of FEA studies assume idealized loading conditions and lack experimental calibration or validation. The goal of this paper is to propose a novel clinical protocol to accurately track orthodontic tooth displacement in three-dimensions (3D) and provide 3D models that may support FEA. Our protocol uses an initial cone beam computed tomography (CBCT) scan and several intra-oral scans (IOS) to generate 3D models of the maxillary bone and teeth ready for use in FEA. The protocol was applied to monitor the canine retraction of a patient during seven months. A second CBCT scan was performed at the end of the study for validation purposes. In order to ease FEA, a frictionless and statically determinate lingual device for maxillary canine retraction was designed. Numerical simulations were set up using the 3D models provided by our protocol to show the relevance of our proposal. Comparison of numerical and clinical results highlights the suitability of this protocol to support patient-specific FEA.