Voxel-based superimposition of serial craniofacial cone-beam computed tomographies for facial soft tissue assessment: Reproducibility and segmentation effects

A Critical Evaluation of Image Superimposition in Dentistry

Image superimposition currently serves as an essential tool for evaluating the effectiveness of treatment by overlaying 2 or more images taken at different time intervals. Superimposition has proved to be an integral diagnostic tool in terms of assessing procedural accuracy, growth patterns, possible changes in extraoral soft tissue, and overall direction of teeth displacement. Several superimposition protocols have been proposed in literature. Traditionally, superimposition was done on 2-dimensional lateral cephalometric radiographs. However, this approach has its limitations. The rise of 3-dimensional (3D) imaging, including computed tomography and cone beam computed tomography, has introduced more advanced diagnostic tools. These technologies enable the creation of volumetric 3D images, which can be used for 3D superimposition to evaluate changes in the dentocraniofacial area. The primary techniques for 3D superimposition include landmark-based methods, surface-based methods, and the more recent voxel-based superimposition. By incorporating soft tissue data, the development of a virtual 3D patient is possible, offering substantial benefits in the field of dental and craniofacial medicine. Moreover, 4-dimensional superimposition, which captures the dynamics of temporomandibular joint and mandibular movements, is shifting research toward the study of dynamic, rather than static, virtual patients. Additionally, the advent of artificial intelligence is revolutionizing superimposition by enabling automated processes, which are poised to transform clinical and research practices. Here, we explore the recent advancements in superimposition techniques and their potential implications in dentocraniofacial analysis.

Lip and incisor changes in patients with different ethnicities treated with extraction versus nonextraction: A cone-beam computed tomography study

BACKGROUND

This study used cone-beam computed tomography (CBCT) via voxel-based superimposition to evaluate lip and incisor changes after orthodontic treatment with four premolar extractions (Ext) versus nonextraction (Non-Ext) among African American (AA) and White (W) patients.

METHODS

A total of 240 CBCTs of 120 adolescent orthodontic patients with Class I skeletal/dental relationships were included. Patients were initially divided according to treatment, and then each group was subdivided according to patients' ethnicity (Ext/W = 30, Ext/AA = 30, Non-Ext/W = 30, and Non-Ext/AA = 30). CBCTs were imported into Invivo6 for voxel-based superimposition. Lip and incisor measurements were recorded. Independent t tests and two-way ANOVA were used for statistical assessment.

RESULTS

The Non-Ext/W group had a greater increase in all outcome variables compared with the Non-Ext/AA group, with a significant change in volume and position of upper (UL) and lower lips (LL), inclination of upper (U1) and lower incisors (L1), and position of U1. The Ext/AA group had a greater decrease in all measured outcomes compared with the Ext/W group, with significant change in inclination of U1 and L1, and position and volume of LL. W patients had more crowding than AA patients in both treatment approaches. Retraction ratios of 6.5:1 and 2.1:1 were recorded between the U1 and UL positions, and L1 and LL positions, respectively. There were no direct interaction effects between ethnicity and treatment, nor were there any significant effects of ethnicity after controlling for the covariates.

CONCLUSIONS

Ethnicity alone has no impact on incisor and lip position after treatment. However, ethnicity in the form of initial presentation of malocclusion can have a significant influence.

Profile line accuracy in cephalometric radiographs

INTRODUCTION

This study investigates the accuracy of facial soft-tissue profile lines in lateral cephalometric radiographs by comparing them to true profile lines derived from 3-dimensional photographs.

METHODS

This prospective methodological study was performed on preexisting records of 100 orthodontic patients. The true profiles were obtained by defining the true midsagittal plane through best-fit approximation of mirrored 3-dimensional surface models. Two curves were drawn on each profile image, and landmarks and sliding semilandmarks were placed on them. This resulted in 2 profile landmark configurations per patient, which were superimposed using Procrustes superimposition. The Procrustes distances between corresponding landmarks were used as a metric to assess the accuracy of the cephalometric profile line, as compared with the true reference.

RESULTS

On average, there were small statistically significant differences between the cephalometric and the actual profile lines (100,000 permutations; P = 0.031; median interlandmark distance, 0.84 mm). However, when assessing individual patients, the cephalometric profile line deviated significantly from the true profile, with 40% of the distances between corresponding landmarks being >1 mm and 10% being >2 mm. There were no differences between the sexes or between younger and older patients (aged 8.0-12.5 vs 12.5-55.0 years). However, there were small differences between 2 x-ray devices (median, 0.18 mm; P <0.001), which often exceeded 1 mm at the soft-tissue nasion area, probably because of the cephalostat.

CONCLUSIONS

On average, the lateral cephalometric radiographs might provide an adequate representation of the facial profile, but when individual patients are considered, there is often a clinically significant error. Thus, lateral cephalograms should be used with caution to evaluate the facial soft-tissue profile.

Accuracy assessment between computer-guided surgery planning and actual tooth position during tooth autotransplantation

BACKGROUND/AIM

This study aims to evaluate the precision and efficacy of utilizing computer-aided design (CAD) in combination with three-dimensional printing technology for tooth transplantation.

MATERIAL AND METHODS

This study analysed 50 transplanted teeth from 48 patients who underwent tooth transplantation surgery with the aid of CAD and positional guides. A consistent coordinate system was established using preoperative and postoperative cone-beam computed tomography images. Linear displacements and angular deviations were calculated by identifying key regions in both virtual designs and actual transplanted teeth. Additionally, an analysis was conducted to explore potential factors influencing these deviations.

RESULTS

The mean cervical deviation, apical deviation, and angular deviation among the 50 transplanted teeth were 1.16 ± 0.57 mm, 1.80 ± 0.94 mm, and 6.82 ± 3.14°, respectively. Cervical deviation was significantly smaller than apical deviation. No significant difference in deviation was observed among different recipient socket locations, holding true for both single-root, and multi-root teeth. However, a significant difference was noted in apical deviation between single-root and multi-root teeth. Our analysis identified a correlation between apical deviation and root length, leading to the development of a prediction model: Apical deviation = 0.1390 × (root length) + 0.2791.

CONCLUSIONS

The postoperative position of the donor teeth shows discrepancies compared to preoperative simulation when utilizing CAD and 3D printed templates during autotransplantation procedures. Continual refinement of preoperative design is a crucial endeavour.

Three-dimensional video recordings: Accuracy, reliability, clinical and research guidelines - Reliability assessment of a 4D camera

OBJECTIVES

In addition to studying facial anatomy, stereophotogrammetry is an efficient diagnostic tool for assessing facial expressions through 3D video recordings. Current technology produces high-quality recordings but also generates extremely excessive data. Here, we compare various recording speeds for three standardized movements using the 3dMDface camera system, to assess its accuracy and reliability.

MATERIALS AND METHODS

A linear and two circular movements were performed using a 3D-printed cube mounted on a robotic arm. All movements were recorded initially at 60 fps (frames/second) and then at 30 and 15 fps. Recording accuracy was tested with best-fit superimpositions of consecutive frames of the 3D cube and calculation of the Mean Absolute Distance (MAD). The reliability of the recordings were tested with evaluation of the inter- and intra-examiner error.

RESULTS

The accuracy of movement recordings was excellent at all speeds (60, 30 and 15 fps), with variability in MAD values consistently being less than 1 mm. The reliability of the camera recordings was excellent at all recording speeds.

CONCLUSIONS

This study demonstrated that 3D recordings of facial expressions can be performed at 30 or even at 15 fps without significant loss of information. This considerably reduces the amount of produced data facilitating further processing and analyses.

The use of CBCT in orthodontics with special focus on upper airway analysis in patients with sleep-disordered breathing

Applications of cone-beam CT (CBCT) in orthodontics have been increasingly discussed and evaluated in science and practice over the last two decades. The present work provides a comprehensive summary of current consolidated practice guidelines, cutting-edge innovative applications, and future outlooks about potential use of CBCT in orthodontics with a special focus on upper airway analysis in patients with sleep-disordered breathing. The present scoping review reveals that clinical applications of CBCT in orthodontics are broadly supported by evidence for the diagnosis of dental anomalies, temporomandibular joint disorders, and craniofacial malformations. On the other hand, CBCT imaging for upper airway analysis-including soft tissue diagnosis and airway morphology-needs further validation in order to provide better understanding regarding which diagnostic questions it can be expected to answer. Internationally recognized guidelines for CBCT use in orthodontics are existent, and similar ones should be developed to provide clear indications about the appropriate use of CBCT for upper airway assessment, including a list of specific clinical questions justifying its prescription.

Long-term evaluation of soft-tissue changes after miniscrew-assisted and conventional rapid palatal expansion using voxel-based superimposition of cone-beam computed tomography scans

INTRODUCTION

This study aimed to evaluate the soft-tissue changes in the long-term after miniscrew-assisted rapid palatal expansion (MARPE) and conventional rapid palatal expansion (RPE) appliances compared with a matched control group using voxel-based superimposition of cone-beam computed tomography (CBCT) scans.

METHODS

A total of 180 CBCTs for 60 patients at 3-time points were evaluated: pretreatment (T1), postexpansion (T2), and posttreatment (T3) for 3 groups: (1) MARPE, (2) RPE, and (3) controls (time-period T1 to T3: MARPE, 2 years 8 months; RPE, 2 years 9 months; control, 2 years 7 months). The voxel-based superimposition technique was used to superimpose the CBCT scans, after which the soft-tissue surfaces were extracted from the superimposed T1-CBCT, T2-CBCT, and T3-CBCT scans. Nine landmarks were identified on the CBCT scans: nasion, A-point, pogonion, right and left alar base, right and left zygoma, and right and left gonion. The coordinates of the 9 parameters were obtained in the x-axis, y-axis, and z-axis for the CBCT scans and subjected to statistical analyses. The changes in the soft-tissue surfaces were also evaluated by color-coded maps for short-term (T2) and long-term (T3) changes. The mean changes from T1 to T2 and T1 to T3 were tested against no change within the groups by paired t test, and the mean changes among the 3 groups were compared with analysis of variance F test with Tukey's Honest significant difference used for adjusting P values for multiple testing.

RESULTS

In the short term, both MARPE and RPE led to a significant downward movement of pogonion, left gonion, and lateral movement of the right and left alar base compared with controls at T2 (P <0.05). In addition, MARPE led to a significant downward movement of right gonion than controls at T2 (P <0.05). Moreover, RPE led to a significant downward and forward movement of A-point and downward movement of the right and left alar base than controls at T2 (P <0.05). However, in the long-term, there were no significant differences in the soft-tissue changes among the MARPE, RPE, and control groups.

CONCLUSIONS

MARPE and RPE do not lead to significant soft-tissue changes in the long term when compared with controls.

Effect of hydration on the anatomical form of human dry skulls

In radiology research soft tissues are often simulated on bone specimens using liquid materials such as water, or gel-like materials, such as ballistic gel. This study aimed to test the effect of hydration on the anatomical form of dry craniofacial bone specimens. Sixteen human dry skulls and 16 mandibles were scanned with an industrial scanner in dry conditions and after water embedding. Ten skulls were also embedded for different time periods (5 or 15 min). The subsequent 3D surface models were best-fit superimposed and compared by calculating mean absolute distances between them at various measurement areas. There was a significant, primarily enlargement effect of hydration on the anatomical form of dry skeletal specimens as detected after water embedding for a short time period. The effect was smaller in dry skulls (median 0.20 mm, IQR 0.17 mm) and larger in mandibles (median 0.56 mm, IQR 0.57 mm). The effect of different water embedding times was negligible. Based on the present findings, we suggest to shortly hydrate the skeletal specimens prior to reference model acquisition so that they are comparable to hydrated specimens when liquid materials are used as soft-tissue simulants for various radiologic research purposes.

Intraoral Scanners for In Vivo 3D Imaging of the Gingiva and the Alveolar Process

This study aimed to assess the reliability of two intraoral surface scanners for the representation of the alveolar process in vivo. Complete maxillary scans (CS 3600, Carestream and TRIOS 3, 3Shape) were repeatedly obtained from 13 fully dentate individuals. Scanner precision and agreement were tested using 3D surface superimpositions on the following reference areas: the buccal front teeth area, the entire dental arch, the entire alveolar process, or single teeth by applying an iterative closest point algorithm. Following each superimposition, the mean absolute distance (MAD) between predefined 3D model surfaces was calculated. Outcomes were analyzed through non-parametric statistics and the visualization of color-coded distance maps. When superimpositions were performed on the alveolar process, the median scanner precision was below 0.05 mm, with statistically significant but negligible differences between scanners. The agreement between the scanners was approximately 0.06 mm. When single-tooth superimpositions were used to assess the precision of adjacent alveolar soft-tissue surfaces, the median error was 0.028 mm, and there was higher agreement between the scanners. The in vivo reliability of the intraoral scanners in the alveolar surface area was high overall. Single-tooth superimpositions should be preferred for the optimal assessment of neighboring alveolar surface areas relative to the dentition.

Precision of a Hand-Held 3D Surface Scanner in Dry and Wet Skeletal Surfaces: An Ex Vivo Study

Three-dimensional surface scans of skeletal structures have various clinical and research applications in medicine, anthropology, and other relevant fields. The aim of this study was to test the precision of a widely used hand-held surface scanner and the associated software's 3D model generation-error in both dry and wet skeletal surfaces. Ten human dry skulls and ten mandibles (dry and wet conditions) were scanned twice with an industrial scanner (Artec Space Spider) by one operator. Following a best-fit superimposition of corresponding surface model pairs, the mean absolute distance (MAD) between them was calculated on ten anatomical regions on the skulls and six on the mandibles. The software's 3D model generation process was repeated for the same scan of four dry skulls and four mandibles (wet and dry conditions), and the results were compared in a similar manner. The median scanner precision was 31 μm for the skulls and 25 μm for the mandibles in dry conditions, whereas in wet conditions it was slightly lower at 40 μm for the mandibles. The 3D model generation-error was negligible (range: 5-10 μm). The Artec Space Spider scanner exhibits very high precision in the scanning of dry and wet skeletal surfaces.

Novel Anterior Cranial Base Area for Voxel-Based Superimposition of Craniofacial CBCTs

A standard method to assess changes in craniofacial morphology over time is through the superimposition of serial patient images. This study evaluated the reliability of a novel anterior cranial base reference area, principally including stable midline structures (EMACB) after an early age, and compared it to the total anterior cranial base (TACB) and an area including only midline structures (MACB). Fifteen pairs of pre-existing serial CBCT images acquired from growing patients were superimposed with all techniques by applying a best-fit registration algorithm of corresponding voxel intensities (Dolphin 3D software). The research outcomes were the reproducibility of each technique and the agreement between them in skeletal change detection, as well as their validity. The TACB and EMACB methods were valid, since the superimposed midline ACB structures consistently showed adequate overlap. They also presented perfect overall reproducibility (median error < 0.01 mm) and agreement (median difference < 0.01 mm). MACB showed reduced validity, higher errors, and a moderate agreement to the TACB. Thus, the EMACB method performed efficiently and mainly included the stable midline ACB structures during growth. Based on the technical, anatomical, and biological principles applied when superimposing serial 3D data to assess craniofacial changes, we recommend the EMACB method as the method of choice to fulfil this purpose.

Reliability of Different Anterior Cranial Base Reference Areas for Voxel-Based Superimposition

The study aimed to evaluate the reliability and reproducibility and compare the outcomes of two 3D voxel-based superimposition techniques for craniofacial CBCT images, using anterior cranial base areas of different extent as references. Fifteen preexisting pairs of serial CBCTs (initial age: 11.7 ± 0.6 years; interval: 1.7 ± 0.4 years) were superimposed on total anterior cranial base (TACB) or middle anterior cranial base (MACB) structures through the Dolphin 3D software. The overlap of the reference structures was assessed visually to indicate reliability. All superimpositions were repeated by the same investigator. Outcomes were compared to assess the agreement between the two methods. Reliability was perfect for the TACB and moderate for the MACB method (p = 0.044). Both areas showed good overall reproducibility, though in individual cases there were notable differences for MACB superimpositions, ranging from −1.84 to 1.64 mm (TACB range: −0.48 to 0.31 mm). The overall agreement in the detected T0/T1 changes was also good, though it was significantly reduced for individual measurements (median < 0.01 mm, IQR: 0.46 mm, range: −2.81 to 0.73 mm). In conclusion, the voxel-based superimposition on TACB was more reliable and showed higher reproducibility than the superimposition on MACB. Thus, the extended anterior cranial base area is recommended for the assessment of craniofacial changes.

Valid 3D surface superimposition references to assess facial changes during growth

Currently, the primary techniques applied for the assessment of facial changes over time utilize 2D images. However, this approach has important limitations related to the dimensional reduction and the accuracy of the used data. 3D facial photography has been recently introduced as a risk-free alternative that overcomes these limitations. However, the proper reference areas that should be used to superimpose serial 3D facial images of growing individuals are not yet known. Here, we tested various 3D facial photo superimposition reference areas and compared their outcomes to those of a standard anterior cranial base superimposition technique. We found that a small rectangular area on the forehead plus an area including the middle part of the nose and the lower wall of the orbital foramen provided comparable results to the standard technique and showed adequate reproducibility. Other reference areas that have been used so far in the literature were less reliable. Within the limitations of the study, a valid superimposition reference area for serial 3D facial images of growing individuals is suggested. The method has potential to greatly expand the possibilities of this highly informative, risk free, and easily obtained 3D tool for the assessment of facial changes in growing individuals.