Introduction
The purpose of this study was to investigate the registration accuracy between intraoral-scanned crowns and cone-beam computed tomography (CBCT)–scanned crowns in various registration methods.
Methods
The samples consisted of 18 Korean adult patients, whose pretreatment intraoral scans and CBCT images were available. A 3-dimensional (3D) dental model was fabricated using a TRIOS intraoral scanner (3Shape, Copenhagen, Denmark) and the OrthoAnalyzer program (version 1.7.1.4; 3Shape). After the CBCT image was taken, 3D volume rendering was performed to fabricate a 3D dental model using InVivo5 software (version 5.1; Anatomage, San Jose, Calif). Registration of the 3D dental crowns made from intraoral- and CBCT-scanned images was performed with Rapidform 2006 software (Inus Technology, Seoul, Korea) by a single operator. According to registration methods, 3 groups were established: individual-arch-total-registration group, individual-arch-segment-registration group, and bimaxillary-arch-centric-occlusion-registration group (n = 18 per group). After the amounts of shell/shell deviation were obtained, the mixed model analysis of variance and Bonferroni correction were performed.
Results
Although there was no significant difference in the registration accuracy between the individual-arch-total-registration group and individual-arch-segment-registration group, the bimaxillary-arch-centric-occlusion-registration group exhibited the lowest registration accuracy (maxillary and mandibular teeth, all 0.21 mm in the individual-arch-total-registration group; all 0.20 mm in the individual-arch-segment-registration group vs 0.26 mm and 0.25 mm in the bimaxillary-arch-centric-occlusion-registration group; P <0.001). Color-coded visualization charts exhibited that most red spots were localized on the occlusal surface of the posterior teeth in all 3 groups.
Conclusions
When considering the registration accuracy and convenience of the process, the individual-arch-total-registration method can be regarded as an efficient tool when integrating CBCT-scanned crown and intraoral-scanned crown.
Highlights
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When registering intraoral-scanned crown with cone-beam computed tomography (CBCT) scanned crown, it is necessary to investigate which method shows the highest registration accuracy.
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The most commonly used registration methods include individual-arch-total-registration method, individual-arch-segment-registration method, and bimaxillary-arch-centric-occlusion-registration method.
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The order of registration accuracy was individual-arch-segment-registration method, individual-arch-total-registration method, and bimaxillary-arch-centric-occlusion-registration method.
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The amount of difference in registration error between the individual-arch-total-registration method and individual-arch-segment-registration method was only 0.01 mm.
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The individual-arch-total-registration method can be used as an accurate and convenient tool for substituting the dentition of CBCT images with the intraoral-scanned images.
With the recent development of cone-beam computed tomography (CBCT) and intraoral scanning technology, 3-dimensional (3D) digital virtual models and computer-aided design and computer-aided manufacturing are gaining popularity in the fields of orthodontics and orthognathic surgery. Customized orthodontic appliance systems, such as Insignia (Ormco, Orange, Calif) and Elemetrix (Suresmile, Orametrix, Richardson, Tex), have integrated CBCT-scanned and intraoral-scanned images to simultaneously assess the alignment of crowns and roots. ,
The CBCT and intraoral scanning technology can provide complementary information for the 3D reconstruction of a dental image. Although CBCT can represent the crown and root of the dentition and the position and shape of the maxilla and mandible, this modality cannot provide precise occlusal surface and accurate interdigitation because of low scanning resolution and streak artifacts from metal or gold restorations. , By contrast, intraoral scanning can provide precise anatomic morphology of the dentition with accurate interocclusal relationships, and its reproducibility and accuracy are known to be clinically acceptable. The major drawback of intraoral scanning is its inability to exhibit the root.
For these reasons, there have been numerous studies to integrate CBCT-scanned images and intraoral-scanned images, replacing the dentition part of the former with the crown of the latter. Although accurate registration is mandatory for successful integration, there has been no consensus about the registration method.
Three registration methods are available: (1) Registration using the individual maxillary or mandibular arch; Noh et al and Sun et al used the whole maxillary or mandibular arch individually for registration with CBCT images. (2) Registration using the combined bimaxillary arches; Ye et al registered the combined bimaxillary arches maintaining the occlusal relationship onto a CBCT model to measure the registration accuracy of the whole bimaxillary dentition. (3) Registration using the individual tooth; Lee et al and Macchi et al registered 2 modality images using each individual crown despite a time-consuming procedure and laborious work.
Regarding the modalities of obtaining digital dental models, extraoral scanning of plaster models has been used to obtain a 3D digital dental model. However, its accuracy is dependent on several procedures, from impression taking to stone pouring and stone setting. With the recent introduction of precise chair-side intraoral scanners, digital dental models can be fabricated in a direct way.
To find out the most efficient registration method between CBCT-scanned crowns and intraoral-scanned crowns, it is necessary to consider 2 experimental designs: first, use of the direct in-vivo intraoral scan, and second, use of the segmented dentition area or whole bimaxillary dentition instead of individual tooth to improve clinical feasibility. Therefore, the purpose of this study was to investigate the registration accuracy between intraoral-scanned crowns and CBCT-scanned crowns in various registration methods.
Material and methods
The sample consisted of 18 Korean young adult patients (10 men and 8 women; mean age, 18.1 years), who were treated in the Department of Orthodontics, Chonnam National University Dental Hospital in Gwangju, Korea, and whose pretreatment intraoral scans and CBCTs were available. The present study was reviewed and approved by the Institutional Review Board of Chonnam National University Dental Hospital (number CNUDH-2018-006).
The inclusion criteria were as follows: (1) subjects with complete permanent dentition from the second molar to the contralateral second molar in the maxillary and mandibular arches, (2) no more than 2 missing teeth in the maxillary and mandibular arches, respectively, (3) no more than 2 prosthetic crown restorations in the maxillary and mandibular arches, respectively, and (4) subjects who did not have severe crowding (arch length discrepancy < 6 mm) in the maxillary and mandibular arches, respectively.
A power analysis was performed to determine the sample size using a sample size determination program (version 2.0.1; Seoul National University Dental Hospital, registration number 2007-01-122-004453, Seoul, Korea). The mean and standard deviation values of a previous study, which investigated the registration accuracy of the crown, were used. With 0.05 of α error probability and 0.8 of power (1-β error probability), the minimal sample size was 10 ∼ 14 subjects. Therefore, we recruited 18 subjects to allow for drop-outs (there was no drop-out).
Image acquisition, processing, and surface modeling of the intraoral-scanned crown are shown in Figure 1 . After the maxillary and mandibular dental arches of subjects were scanned with an optical intraoral scanner (TRIOS; 3Shape, Copenhagen, Denmark), the buccal surfaces of the maxillary and mandibular posterior teeth in centric occlusion were scanned to obtain the interarch relationship. This entire procedure was performed by a single operator (HHC).
The OrthoAnalyzer software program (version 1.7.1.4; 3Shape) was used to create a stereolithography (STL) file. This STL format images were exported to Rapidform 2006 software (Inus Technology, Seoul, Korea). The intraoral-scanned image was cut off along the gingival margin using the Entity function to allow registration of the clinical crown afterward.
Image acquisition, processing, and surface modeling of the CBCT-scanned crown are shown in Figure 1 . After CBCT images (81 kV; 5 mA; 0.39-mm × 0.39-mm × 0.39-mm voxel size; and field of view 200 mm × 179 mm) (Alphard VEGA; Asahi Roentgen, Kyoto, Japan) were obtained in the upright position and centric occlusion state, the digital imaging and communication in medicine file was exported to InVivo5 software (version 5.1; Anatomage, San Jose, Calif) for 3D volume rendering.
The medical design studio device of InVivo5 software was used to convert the volume-rendered image into the STL format. Individual teeth, including crowns and roots, were segmented from whole-skull images using the ‘Sculpt’ function. The threshold value for segmentation was set to iso-value 900 to minimize the difference between intraoral-scanned images and CBCT images. These whole procedures were performed by a single operator (HMK).
The registration of intraoral-scanned crown with CBCT-scanned crown is shown in Figure 2 . The surface-based registration process was performed with Rapidform 2006 software as follows: First, the initial registration of the 2 images was performed by selecting 3 corresponding points per each image, resulting in a rough alignment. Second, the ‘Fine’ automatic registration function, which uses an iterative closest point algorithm, was used to finalize the matches. These whole procedures were performed by a single operator (SWL).
Three registration groups were established according to registration method. (1) In the individual-arch-total-registration group, the whole intraoral-scanned image of the maxillary dentition was registered with the whole CBCT-scanned image of the maxillary dentition. Then, registration of the mandibular dentition images was performed in the same way. (2) In the individual-arch-segment-registration group, the intraoral-scanned image of the maxillary dentition was registered with the CBCT-scanned image of the maxillary dentition in 3 segments including the anterior teeth, right posterior teeth, and left posterior teeth. Then, registration of the mandibular dentition images was performed in the same way. For comparison, the 3 segments in each arch were re-combined to create the whole maxillary or mandibular arch. (3) In the bimaxillary-arch-centric-occlusion-registration group, the intraoral-scanned image of the bimaxillary arches in centric occlusion bite was registered with the CBCT-scanned image of the bimaxillary arches in centric occlusion bite as a whole. For comparison, the combined bimaxillary arches were separated into the maxillary arch and mandibular arch.
To evaluate registration accuracy, the absolute values of the 3D Euclidean distances between the intraoral-scanned crown and CBCT-scanned crown were computed at all points using the ‘shell/shell deviation’ function of the program. To remove outliers, the threshold value was set at 1 mm. In addition, the distance between the 2 images was visualized by color-mapping, from blue (minimum, 0 mm) to red (maximum, 1.0 mm) ( Fig 3 ).
Statistical analysis
The mixed model analysis of variance and Bonferroni correction were performed using SPSS software (version 12.0; SPSS, Chicago, Ill). P value <0.05 was set as significant.
Results
The registration error of the 3 groups ranged from 0.20 mm to 0.26 mm ( Table I ). Although there was no significant difference in registration accuracy between the individual-arch-total-registration group and individual-arch-segment-registration group, the bimaxillary-arch-centric-occlusion-registration group exhibited the lowest registration accuracy among the 3 groups (maxilla and mandible, 0.21 mm and 0.21 mm in the individual-arch-total-registration group; 0.20 mm and 0.20 mm in the individual-arch-segment-registration group vs 0.26 mm and 0.25 mm in the bimaxillary-arch-centric-occlusion-registration group, P <0.001) ( Table I ). However, there was no significant difference in registration between the maxillary and mandibular teeth in all 3 groups ( Table I ).
| Shell/shell deviation (mm) | Individual-arch-total-registration group | Individual-arch-segment-registration group | Bimaxillary-arch-centric-occlusion-registration group | P value | |||
|---|---|---|---|---|---|---|---|
| Mean | SD | Mean | SD | Mean | SD | ||
| Maxilla | 0.21* | 0.04 | 0.20* | 0.04 | 0.26† | 0.04 | <0.001 |
| Mandible | 0.21* | 0.04 | 0.20* | 0.03 | 0.25† | 0.04 | <0.001 |
Color-coded visualization charts exhibited that most red spots were localized on the occlusal surface of the posterior teeth in all 3 groups ( Fig 3 ).
Results
The registration error of the 3 groups ranged from 0.20 mm to 0.26 mm ( Table I ). Although there was no significant difference in registration accuracy between the individual-arch-total-registration group and individual-arch-segment-registration group, the bimaxillary-arch-centric-occlusion-registration group exhibited the lowest registration accuracy among the 3 groups (maxilla and mandible, 0.21 mm and 0.21 mm in the individual-arch-total-registration group; 0.20 mm and 0.20 mm in the individual-arch-segment-registration group vs 0.26 mm and 0.25 mm in the bimaxillary-arch-centric-occlusion-registration group, P <0.001) ( Table I ). However, there was no significant difference in registration between the maxillary and mandibular teeth in all 3 groups ( Table I ).
| Shell/shell deviation (mm) | Individual-arch-total-registration group | Individual-arch-segment-registration group | Bimaxillary-arch-centric-occlusion-registration group | P value | |||
|---|---|---|---|---|---|---|---|
| Mean | SD | Mean | SD | Mean | SD | ||
| Maxilla | 0.21* | 0.04 | 0.20* | 0.04 | 0.26† | 0.04 | <0.001 |
| Mandible | 0.21* | 0.04 | 0.20* | 0.03 | 0.25† | 0.04 | <0.001 |
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