Background: Accurate root position is integral for successful orthodontic treatment. Current methods of monitoring root position are either inaccurate, exhibit poor resolution, or use relatively large amount of radiation relative to the benefits for the patient. The purpose of this study was to present an approach that can monitor root position during orthodontic treatment with minimal radiation. Methods: Cone-beam computed tomography (CBCT) scans were taken for a patient at pre-treatment and at a dedicated reset appointment. An extra-oral laser scan of a poured up cast was taken at the reset appointment. An expected root position (ERP) setup, an approximation of the root position at the reset appointment, was generated using the pre-treatment CBCT scan and reset appointment cast. The ERP setup was compared to the CBCT scan taken at the reset appointment which served as the control. Color displacement maps were generated to measure any differences between the expected and true root positions. Results: Color map displacement analysis after indirect superimposition found displacement differences of 0.021 mm ± 0.396 mm for the maxillary roots and 0.079 mm ± 0.499 mm for the mandibular roots. Conclusions: This approach was demonstrated in a patient at the reset appointment to have the potential to accurately monitor root positions during treatment in three dimensions without the need for additional radiographs. Background Proper root placement is important for satisfactory The goal of orthodontic treatment is to move the teeth periodontal health, restorative treatment, and proper into a stable, esthetic, and functional occlusion with occlusal function. Prior studies have found that if roots every crown and root positioned ideally in three dimen- of adjacent teeth are placed in close proximity to one sions. To achieve this optimal occlusion, orthodontists another, periodontal or restorative treatment may be often follow Andrews’ six keys to normal occlusion . compromised [8, 9]. Root proximity and the shape of the While four of Andrews’ keys (molar relationship, rota- crowns are potential causes for a poorly shaped gingival tions, spaces, and occlusal plane) are guided by crown embrasure . Root proximity in which the adjacent position, his other two keys (mesiodistal angulation and roots are 1.0 mm or less apart has been shown to result buccolingual inclination) depend on both crown and in jeopardized health of the interproximal space, hori- root position. Root position plays a role in the mesiodis- zontal bone loss, and more rapid periodontal breakdown tal angulation and buccolingual inclination because of [11–15]. Furthermore, other studies have demonstrated variations in crown morphologies, inconsistencies in that proper root placement and parallelism are critical crown-root angulations, and when a crown is short rela- to distribute occlusal forces and to produce proper tive to root length [2–7]. occlusal and incisal function [2, 16]. Accurate bracket placement facilitates tooth move- * Correspondence: Robert.Lee3@ucsf.edu ment into normal occlusion and minimizes the amount Division of Orthodontics, University of California San Francisco, 707 of required wire bending . However, it is difficult to Parnassus Ave. d3000, San Francisco, CA 94143, USA attain dependably accurate placement of all brackets at Full list of author information is available at the end of the article © The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Lee et al. Progress in Orthodontics (2018) 19:15 Page 2 of 9 the initial bonding. To correct for improperly placed to have the potential to monitor root position at any stage brackets that have resulted in improper crown and root of orthodontic treatment using a single pre-treatment positions, the practitioner may either make adjustments CBCT scan [31, 32]. The generated ERP setup was in the archwire or reposition the bracket. Carlson and reported to be an approximation of the root position at Johnson described an efficient treatment process of the orthodontic stage of interest, and it was demonstrated using a single dedicated reset appointment to correct to be accurate in an ex vivo typodont model and clinically any bracket-positioning errors after the initial leveling in one patient at post-treatment via color displacement and aligning of the teeth . At this dedicated reset maps. The ERP approach has not been demonstrated appointment, both clinical and radiographic examination during orthodontic treatment where it has the potential to were performed to assess the position of the crown and aid with bracket repositioning, especially at a designated roots, and one reason to reposition a bracket would be reset appointment. The purpose of this study was to intro- to address a root parallelism problem. However, an duce the first application of generating an ERP setup accurate radiographic technique controlling beam angu- during orthodontic treatment at the reset appointment. lation or using volumetric imaging is required to assess root parallelism. Materials and methods Traditionally, panoramic radiographs have been used This retrospective study was approved by the Committee to monitor and finalize root positions in orthodontic on Human Research of the University of California, San treatment. In a 2008 Journal of Clinical Orthodontics Francisco (UCSF). We obtained clinical records of a (JCO) survey of American orthodontists, 67.4% of single patient who underwent treatment at the UCSF respondents reported that they took progress panoramic Division of Orthodontics and had casts and CBCT scans radiographs and 80.1% of respondents reported that they taken at pre-treatment and at the reset appointment. took post-treatment panoramic radiographs to assess The patient was an 18-year-old Asian male with a root position . However, panoramic radiographs have skeletal and dental class III malocclusion treated with been found to be inaccurate in depicting root position comprehensive orthodontic treatment and orthognathic through numerous studies which have found that pano- surgery (Fig. 1). The reset appointment was performed ramic radiographs have distortions because of the non- prior to orthognathic surgery. orthogonal X-ray beams directed at the teeth [20–23]. Segmentations of teeth from CBCT scans taken at Furthermore, prior studies have determined that radio- pre-treatment and at the reset appointment were graphic techniques should be able to depict root angula- performed using the Anatomodel modeling service tions with an accuracy of 2.5° in either direction to be (Anatomage, San Jose, CA). The cast taken at the reset considered clinically acceptable, yet panoramic radio- appointment was scanned using an Ortho Insight graphs depict 53–73% of root angulations outside of this (MotionView Software, LLC, Hixson, TN) extra-oral clinically acceptable range [21–24]. A more accurate laser scanner. These laser-scanned crowns were approach for evaluating root placement, especially at the segmented and exported as PLY files using the Ortho reset appointment, will facilitate finalizing root positions. Insight software. The pre-treatment segmented CBCT Cone-beam computed tomography (CBCT) is another teeth obtained from the Anatomodel modeling service radiographic technique that is becoming increasingly more were superimposed using 3-matic software (version 9.0; common to use in orthodontics. In contrast with pano- Materialise, Leuven, Belgium) onto their respective ramic radiographs, CBCT scans have been found to accur- laser-scanned crowns yielding the ERP setup at the reset ately depict true root angulations and inclinations in three appointment (Fig. 2). This superimposition process first dimensions and show dentofacial structures in a 1:1 ratio used the N points registration function to approximate [20, 25–28]. Compared to panoramic radiographs, CBCT the position of the crown of the pre-treatment CBCT scans expose patients to higher levels of radiation. Multiple tooth onto its respective laser-scanned crown by select- CBCT scans to continually monitor root position may not ing three matching points on both crowns. Any gross be recommended clinically, especially in children [27–29]. errors in crown and root mesiodistal angulation and While CBCT technology has improved, resulting in buccolingual inclination observed on the CBCT teeth decreased radiation dosage, practitioners are always recom- after N points registration were corrected using the mended to follow the As Low as Reasonably Achievable translation and rotation functions while roughly match- (ALARA) principle and avoid exposing patients to radiation ing the alignment of the long axes of the CBCT teeth when possible . Thus, a technique that can accurately and laser-scanned crowns. The final part of this super- monitor root position in three dimensions while also imposition process utilized a global registration function reduce radiation exposure to patients is desirable. which consisted of an iterative closest point algorithm. Recently, a new methodology, which generates an A color displacement map between the crowns of the “expected root position” (ERP) setup, was demonstrated pre-treatment CBCT teeth and the extra-oral laser scan Lee et al. Progress in Orthodontics (2018) 19:15 Page 3 of 9 Fig. 1 Clinical photographs of orthodontic treatment at pre-treatment (top) and at the reset appointment (bottom) of the reset appointment cast was generated to validate superimposed reset appointment CBCT teeth and the accuracy of the superimposition. extra-oral laser scan of the reset appointment cast was gen- To validate the accuracy of the ERP setup, indirect erated to validate the accuracy of the superimposition. superimposition was performed as described in literature The ERP setup and reset appointment CBCT teeth [31, 32]. This was accomplished by superimposing the com- were cut roughly at the cemento-enamel junction (CEJ) bined crowns of the CBCT teeth taken at the reset appoint- separating the roots and crowns. Color displacement ment (Fig. 3a) onto the crowns of the same extra-oral laser maps were generated to study two scenarios: (1) super- scan of the reset appointment cast used to generate the imposition of the ERP setup and reset appointment ERP setup (Fig. 3b). After this superimposition process, the CBCT crowns and (2) superimposition of the ERP setup crowns of the reset appointment CBCT teeth and the ERP and reset appointment CBCT roots. All of the color setup were in the same position in three dimensions displacement maps in this study were generated with the (Fig. 3c). After removing the reset appointment laser scan same parameters. Displacement within a 0.75-mm range (Fig. 3d) from the three-dimensional viewport, the ERP was presented as green. Inward displacement greater setup and true position of the roots depicted by the reset than 0.75 mm of the laser-scanned crowns compared to appointment CBCT scan were now indirectly superimposed the CBCT crowns is represented as blue and outward with each other (Fig. 3e). A color displacement map of the displacement greater than 0.75 mm as red. Fig. 2 Generation of the ERP setup. The teeth from the pre-treatment CBCT scan are segmented and isolated. The reset appointment cast is scanned with an extra-oral laser scanner and individualized. The pre-treatment CBCT teeth are superimposed onto the reset appointment crowns yielding the ERP setup Lee et al. Progress in Orthodontics (2018) 19:15 Page 4 of 9 Fig. 3 Indirect superimposition process: a reset appointment CBCT teeth; b reset appointment laser scan model with superimposed pre-treatment CBCT scan yielding the ERP setup; c reset appointment laser scan with both the superimposed pre-treatment and reset appointment CBCT teeth; d removal of the laser scan model from the viewport; e remaining ERP setup and reset appointment CBCT teeth indirectly superimposed allowing for comparison of the expected and true root positions Results 0.371 mm with a maximum of 1.400 mm and mandibular To verify accurate direct superimposition between the displacement of 0.203 mm ± 0.438 mm with a maximum pre-treatment CBCT crowns onto the reset appointment of 1.848 mm (Fig. 7,Table 1). Color displacement map laser scan crowns during generation of the ERP setup, a after indirect superimposition of the ERP setup roots with color displacement was generated. The color displace- the reset appointment CBCT roots showed maxillary ment map found that there was a maxillary displacement displacement of 0.021 mm ± 0.396 mm with a maximum of 0.087 mm ± 0.328 mm with a maximum of 1.363 mm of 1.429 mm and mandibular displacement of 0.079 mm ± and mandibular displacement of 0.071 mm ± 0.382 mm 0.499 mm with a maximum of 1.786 mm (Fig. 8,Table 1). with maximum of 1.398 mm (Fig. 4, Table 1). Direct superimposition between the reset appointment Discussion CBCT crowns and reset appointment laser scan crowns Proper root position is necessary for successful ortho- was also verified to be accurate through a color displace- dontic treatment that is stable, functional, and esthetic. ment map. The color displacement map showed maxillary Typically, the primary focus during orthodontic treatment displacement of 0.146 mm ± 0.349 mm with a maximum of is on crown position rather than root position because 1.269 mm and mandibular displacement of 0.289 mm ± roots are not clinically visible and generally not directly 0.508 mm with a maximum of 1.999 mm (Fig. 5,Table 1). involved with esthetics and occlusion [5, 7, 16]. Root After indirect superimposition, the ERP setup was position plays a role in periodontal health, restorative qualitatively compared to the reset appointment CBCT treatment, and occlusal function [2, 8–16]. Radiographs scan which served as the control. Figure 6 shows different often reveal crown alignment errors in teeth with poor viewpoints of the indirectly superimposed setups with the root angulation. Furthermore, the American Board of reset appointment CBCT virtual model semi-transparent. Orthodontics (ABO) recommends assessing root On a qualitative visual inspection, the root position parallelism and deducts points if the roots of adjacent generated by the ERP setup shows minimal differences teeth are not parallel with each other or if they come in compared to the true root positions depicted by the reset contact with each other . The ABO recommends appointment CBCT roots. use of panoramic radiographs to monitor root align- Color displacement map after indirect superimposition ment even though previous reports and the ABO have of the ERP setup crowns with the reset appointment CBCT acknowledged that panoramic radiographs do not crowns showed maxillary displacement of 0.098 mm ± accurately depict root position [21–23]. Thus, a new Lee et al. Progress in Orthodontics (2018) 19:15 Page 5 of 9 Fig. 4 Verification of accurate crown superimposition during ERP setup generation after direct superimposition between the crowns of the pre-treatment CBCT teeth and the reset appointment laser scan. a Color displacement maps comparing the crown positions of the pre-treatment CBCT crowns and reset appointment laser scan crowns. Green areas indicate 0.0 mm displacement; blue and red areas indicate equal to or greater than 0.75 mm. b, c Histograms showing the distribution of displacements between crowns of the pre-treatment CBCT scan and reset appointment laser scan in the maxillary arch and mandibular arch approach that can accurately monitor root position laser scans of poured up casts are comparable with would be desirable. intra-oral scans [34–38]. However, the accuracy of the This study obtained digital models of the crowns via digital model obtained from the extra-oral laser scan is laser scans of poured up casts from the reset appoint- dependent on an accurate impression and model ment. Previous reports have found that the accuracy of pouring process. Therefore, to validate the accuracy of Table 1 Color displacement map analysis Analysis type Mean displacement (mm) Standard deviation (mm) Maximum displacement (mm) Pre-treatment CBCT crowns vs reset appointment laser scan crowns Maxillary crowns 0.087 0.328 1.363 Mandibular crowns 0.071 0.382 1.398 Reset appointment CBCT crowns vs reset appointment laser scan crowns Maxillary crowns 0.146 0.349 1.269 Mandibular crowns 0.289 0.508 1.999 ERP crowns vs reset appointment CBCT crowns Maxillary crowns 0.098 0.371 1.4 Mandibular crowns 0.203 0.438 1.848 ERP roots vs reset appointment CBCT roots Maxillary roots 0.021 0.396 1.429 Mandibular roots 0.079 0.499 1.786 Lee et al. Progress in Orthodontics (2018) 19:15 Page 6 of 9 Fig. 5 Verification of accurate crown superimposition after direct superimposition between the crowns of the reset appointment CBCT teeth and the reset appointment laser scan. a Color displacement maps comparing the crown positions of the reset appointment CBCT crowns and reset appointment laser scan crowns. Green areas indicate 0.0 mm displacement; blue and red areas indicate equal to or greater than 0.75 mm. b, c Histograms showing the distribution of displacements between crowns of the reset appointment CBCT scan and reset appointment laser scan in the maxillary arch and mandibular arch the extra-oral laser scan used in this study, direct super- of superimposed ERP setup and reset CBCT scan crowns imposition comparing both the pre-treatment and in which minimal differences were found. The blue and reset appointment CBCT scan crowns to the reset red spots on the color maps were noted, indicating appointment laser scan crowns was performed. Color regions of displacement greater than 0.75 mm which displacement maps found minimal differences for appears primarily due to the presence of brackets and both superimpositions. bands for the reset appointment CBCT scan. Sources of To assess the accuracy of the root position depicted by error in determining root form (size and shape) include the ERP setup, it was compared against the reset ap- noise, voxel size, contrast variance, and segmentation pointment CBCT scan which reflects the true root pos- accuracy . Occlusal anatomy is also often difficult to ition. To minimize error in the analysis, an indirect capture with threshold segmentation when the patient is superimposition process was performed in which the in occlusion. A potential solution to this would be to ERP setup and reset appointment CBCT scan were both have the patient bite into a thin piece of wax during the superimposed onto the same reset appointment model CBCT scan to create a small separation between the laser scan. The indirect superimposition process is only upper and lower teeth allowing for easier segmentation applied for research purposes to use the reset appoint- of the occlusal anatomy. Another potential solution ment CBCT scan as a control. In a clinical setting, would be to use a low-dose spiral CT scan, rather than a assessment of root position at the reset appointment CBCT scan, since it has been shown to generate using the ERP setup approach may eliminate the need high-quality images for orthodontic diagnosis without a for panoramic or CBCT imaging. significant increase of radiation to patients . How- The accuracy of the indirect superimposition process ever, even with the presence of brackets and bands, was validated through color displacement map analysis which also add noise during CBCT image acquisition, Lee et al. Progress in Orthodontics (2018) 19:15 Page 7 of 9 Fig. 6 Qualitative comparison of the ERP setup (multicolored teeth) and the reset appointment CBCT teeth (transparent gray) after indirect superimposition Fig. 7 Verification of accurate crown superimposition after indirect superimposition of the ERP setup and reset appointment CBCT scan. a Color displacement maps comparing the crown positions of the ERP setup and reset appointment CBCT scan. Green areas indicate 0.0 mm displacement; blue and red areas indicate equal to or greater than 0.75 mm. b, c Histograms showing the distribution of displacements between crowns of the ERP setup and reset appointment CBCT scan in the maxillary arch and mandibular arch Lee et al. Progress in Orthodontics (2018) 19:15 Page 8 of 9 Fig. 8 Measurement of displacements between the roots after indirect superimposition of the ERP setup and reset appointment CBCT scan. a Color displacement maps comparing the root positions of the ERP setup and reset appointment CBCT scan. Green areas indicate 0.0 mm displacement; blue and red areas indicate equal to or greater than 0.75 mm. b, c Histograms showing the distribution of displacements between roots of the ERP setup and reset appointment CBCT scan in the maxillary arch and mandibular arch and some operator error during the threshold segmenta- which was previously the most time-consuming step. The tion process, the ERP setup still was able to depict simi- superimposition process needed for each individual tooth is lar root position to the reset appointment CBCT scan. still a time-consuming step. However, intra-oral scan This approach to generate an ERP setup was previ- technology applies superimposition functions to stitch nu- ously demonstrated in an ex vivo typodont model and at merous snapshots of teeth together. Potentially in the fu- post-treatment [31, 32]. This study was the first to dem- ture, intra-oral scanning technology may also be able to onstrate this methodology during treatment to facilitate stitch the threshold segmentation of pre-treatment CBCT the correction of any root position errors. While radio- scan, obtained from the third party-vendor, in real time. graphs at the reset appointment may still be needed to Another limitation of this approach is that any change to monitor root resorption and pathology, this study the crown after the pre-treatment CBCT scan, such as a demonstrated that the ERP setup can be used, not just large restoration or crown, may make it difficult or impos- at the reset appointment, but at any time during treat- sible to perform the crown superimposition. If the crown ment since the presence of bands and brackets does not superimposition cannot be performed, then the ERP setup appear to affect the accuracy of the ERP setup. This for the tooth with the changed anatomy would not be finding has clinical implications for practitioners who do possible to generate. Furthermore, teeth with restorations not use a reset appointment in their treatment workflow larger than two surfaces may also be difficult to segment because this demonstrates that they would be able to out of the CBCT scan and could also potentially result in generate an ERP setup at any time during orthodontic an inaccurate model of the tooth leading to unreliable treatment when they desire to evaluate root position. In crown superimposition. addition, the ERP setup could potentially be generated at later appointments to monitor the root positions and to correct any root position errors that may not have been Conclusion fully corrected in the reset appointment without any further radiation to the patient. Thus, this protocol may reduce the 1. We have demonstrated the potential clinical use of number of radiographic procedures recommended. the expected root position (ERP) approach to The main limitation of this methodology is that it is cur- evaluate root position during orthodontic treatment rently too time consuming for use in a clinical setting, without the need for additional radiation after a though technology has improved the speed of this approach pre-treatment CBCT scan. since the previous report of this method. Third-party 2. The bands and brackets during orthodontic vendors now exist that can perform the pre-treatment treatment did not appear to affect the accuracy of CBCT scan threshold segmentation for the practitioner the ERP setup. Lee et al. Progress in Orthodontics (2018) 19:15 Page 9 of 9 Funding 14. Ritchey B, Orban B. The crests of the interdental alveolar septa. J Periodontol. This study was supported by the American Association of Orthodontists 1953;24(2):75–87. 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Keim RG, Gottlieb EL, Nelson AH, Vogels DS. 2008 JCO study of orthodontic read and approved the final manuscript. diagnosis and treatment procedures, part 1: results and trends. J Clin Orthod. 2008;42(11):625–40. Ethics approval and consent to participate 20. Lagravère MO, Carey J, Toogood RW, Major PW. Three-dimensional accuracy This retrospective study was approved by the Committee on Human of measurements made with software on cone-beam computed Research of the University of California, San Francisco (10-00564). tomography images. Am J Orthod Dentofac Orthop. 2008;134(1):112–6. 21. Mckee IW, Glover KE, Williamson PC, Lam EW, Heo G, Major PW. The effect Consent for publication of vertical and horizontal head positioning in panoramic radiography on The patient provided consent for the publication which has been uploaded. mesiodistal tooth angulations. Angle Orthod. 2001;71(6):442–51. 22. Garcia-Figueroa MA, Raboud DW, Lam EW, Heo G, Major PW. 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Published: Jun 4, 2018
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