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실제 교정치료결과 비교를 통한 3D 디지털 치아모형 셋업의 정확성 평가

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INTRODUCTION

Successful orthodontic treatments are based on com- prehensive diagnosis and treatment plans. The basic in- formation for correct diagnoses can be obtained through

interviews, clinical findings, dental cast models, 3D as- sessments of the face (if possible), and radiographs.1 The scope of orthodontic treatment has expanded to include adult patients who may undergo restorative and prostho- dontic procedures, thereby increasing the need for com- prehensive assessments using multiple diagnostic tools for prediction of orthodontic treatment outcomes.

Prediction of orthodontic teeth movement is usually performed by an occlusogram2 or by manual setup of a plaster cast model.3,4,5 The setup process of the patient’s dental cast model plays an especially important role in

실제 교정치료결과 비교를 통한 3D 디지털 치아모형 셋업의 정확성 평가

국민건강보험 일산병원 치과

김정훈

Accuracy of 3-dimensional (3D) digital teeth setups in comparison with actual orthodontic treatment results: A pilot study

Jung Hoon Kim

Department Orthodontics National Health Insurance Service Ilsan Hospital Goyang, Korea

Objectives: The accuracy of digital teeth setups created using a 3-dimensional (3D) simulation program was evaluated by comparing them with actual orthodontic treatment results. The characteristics and limitations of such digital teeth setups were also assessed.

Patients and Methods: Patients who underwent orthodontic treatment at NHIS Ilsan hospital were included. The initial and final 3D digital cast models were constructed from dental cast scan data, while the 3D digital teeth setup was fabricated from the initial 3D digital image. The accuracy of the 3D digital setup was analyzed by comparing 14 linear measurements in the 3D digital setup image and the final 3D image, using the Wilcoxon signed rank test.

Results: The study sample included 17 patients (10 non-extraction cases; 7 extraction cases). In the non-extraction cases, the lower inter first premolar width, and the upper and lower arch lengths showed statistically significant intergroup differences (P <

.05), with smaller values in the digital setup. In the extraction cases, the upper and lower inter first molar width, and the upper and lower inter second molar width showed statistically significant intergroup differences (P < .05), with smaller values in the digital setup.

Conclusions: Digital teeth setup using the 3D simulation program complemented the disadvantages of the conventional manual setup, and had an adequate accuracy to allow prediction of orthodontic treatment results. However, in cases involving wider tooth movement (such as extraction cases), data on predictable tooth movement at the posterior part of the setup will be needed.

Key Words: Digital teeth setups, Digital cast model, 3-dimensional simulation

책임저자: 김정훈

10444 경기도 고양시 일산동구 일산로 100 국민건강보험 일산병원 치과

전화: (031)900-0620, 팩스: 0303-3448-7107 E-mail : jhoonkim@nhimc.or.kr

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establishing the treatment plan. However, the conven- tional setup method using plaster cast models has dis- advantages like the need for a laboratory procedure and a physical space to store plaster cast models.

To address these disadvantages, many com- puter-assisted systems have been introduced recently and have been found to show successful results and help clinicians obtain important information in a short peri- od of time. In particular, the development of 3D scan- ners has allowed the use the digital cast models as new diagnostic tools replacing the plaster cast models.1,6,7,8 These digital cast models offer advantages such as ease of storage and retrieval, ease of interoffice trans- ferability, and possibly equal or better diagnostic capa- bilities compared to conventional plaster cast models.1,9 Digital cast models also show high levels of accuracy and reproducibility in clinical trials,10,11 and digital and man- ual setups with digital and plaster cast models can lead to similar measurements for intra-arch and interarch oc- clusal variables.7

The development of 3-dimensional simulation pro- grams has enabled clinicians to perform digital setup of digital cast models for visualization of treatment ob- jectives and helped predict treatment progress and estab- lish a precise treatment plan. To apply the digital teeth setup in predicting treatment progress and establishing a treatment plan, it is necessary to determine whether this setup can accurately reflect the actual results of ortho- dontic treatment. Therefore, this study aimed to evaluate the accuracy of digital teeth setups using a 3D simulation program by comparing them with the actual orthodontic treatment results, and to identify the characteristics and limitations of these digital teeth setups. The null hypoth- esis was that the digital teeth setup would be similar to actual orthodontic treatment outcomes.

MATERIAL and METHODS

This was a retrospective and patient-oriented re- search study. The study sample included consecutively selected patients who underwent orthodontic treatment

at NHIS Ilsan hospital from January 2010 through January 2014. The following inclusion and exclusion cri- teria were applied to the study sample. We included pa- tients who were older than 18 years; were treated with MBT prescription brackets of a 0.022″ × 0.028″ slot; had completed orthodontic treatment without extraction or with extraction of maxillary and mandibular first pre- molars; and had class I molar and canine key after treatment. We excluded patients who were still growing, and those who had undergone orthodontic treatment in past; underwent extraction of only maxilla or only man- dible; had no satisfying class I molar and canine key af- ter treatment; showed congenitally missing teeth; man- ifested severe alveolar bone loss requiring periodontal treatment; presented with systemic disease or craniofa- cial syndrome; or had undergone orthognathic surgery.

This study was approved by the Institutional Review Board of OOO hospital (2017-01-025). Because of the retrospective nature of this study, the institutional review board waived the requirement for written informed pa- tient consent.

Each patient’s initial and final cast models were trans- formed into a 3D digital model using a light scanner (RexcanDS2; SOLUTIONIX, Seoul, Korea). The status and occlusion of the digital cast model were finally con- firmed by the orthodontist. All digital teeth setups were obtained by 1 orthodontist, each with more than 3 years of clinical experience, using Mimics (MaterialiseTM NV, Leuven, Belgium). Orthodontic treatment was performed by 4 clinicians and all patients were treated using Victory series MBT bracket (3M Unitek™, Monrovia, Calif) or Clarity MBT bracket (3M Unitek™, Monrovia, Calif).

Each tooth on the digital cast model was separated us- ing Mimics® 14.0, and the digital teeth setup was ini- tiated by predicting the orthodontic treatment, which yielded the digital teeth setup cast model (Figure 1). In the extraction case, rotation control and crowding relief were carried out first, followed by En-masse retraction.

At this time, the amount of anterior and posterior teeth movement was based on the findings reported by Park et al.12

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The maxillary and mandibular planes, which pass through the midpoint of the maxillary and mandibular central incisors and both contact points of the first and second molars, were determined, and then the final oc- clusion plane was determined by the median plane of these 2 planes. The sagittal plane was defined as a plane passing through the midpoint of the maxillary and man- dibular central incisor and the midpoint of both contact points of the first and second molars and perpendicular to the occlusal plane. The coronal plane was defined as a plane perpendicular to the occlusal plane and sagittal plane. The definitions of the reference planes and land- marks are described in Figure 2. Then, the linear dis- tance was measured using Rapidform XOV2 software (INUS Technology, Seoul, Korea).

Fourteen linear distance measurements were obtained in the digital teeth setup cast model and the final cast model: upper and lower inter-canine widths (UICW and LICW), upper and lower inter first premolar widths (UIP1W and LIP1W), upper and lower inter second pre- molar widths (UIP2W and LIP2W), upper and lower inter first molar widths (UIM1W and LIM1W), upper and lower inter second molar widths (UIM2W and LIM2W), upper and lower arch lengths (UAL and LAL), and overjet/over- bite (OJ and OB). Canine and premolar widths were measured at the buccal cusp tip and the molar width at the central fossa, and arch length was measured from the midpoint of the maxillary and mandibular central in- cisor to the line connecting the projected points to the occlusal plane of both contact points of the first and second molars (Figure 3).

One examiner repeated all measurements twice at 1-month intervals. The intra-examiner reliability was evaluated with intraclass correlation coefficients (ICCs).

To evaluate the difference between the digital teeth set- up cast model group and the final cast model group, the Wilcoxon signed rank test was used to compare the mean values of measurements between the 2 groups. Statistical analysis was performed using SPSS 23.0 (IBM Corp, Armonk, NY); a P value less than .05 considered statisti- cally significant.

RESULTS

The subjects were 17 patients (10 non-extraction and 7 extraction cases; mean age, 25.60 ± 7.72 years in the non-extraction cases and 20.57 ± 2.07 years in the ex- traction cases). The average duration of the total treat- ment course was approximately 16.40 ± 4.97 months in the non-extraction cases and 30.71 ± 8.52 months in the extraction cases.

The ICC values showed excellent reproducibility for all measurements, with an intra-examiner reliability of 0.942 (95% confidence interval [CI], 0.829-0.998) in non-extraction cases and 0.970 (95% confidence interval [CI], 0.871-0.996) in extraction cases.

In non-extraction cases, all measurements in the digi- tal setup, except UICW, UIM1W, LIM1W, and OB, showed smaller values than the actual treatment results.

Three measurements (LIP1W, UAL, and LAL) showed Fig. 1. Digital teeth setup procedure: A. separation of each tooth; B. alignment; C. En-masse retraction of the maxillary anterior segment; D. En-masse retraction of the mandibular anterior segment; E. completion of digital teeth setup.

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Fig. 2. Decision regarding reference planes: A, B. landmark for setting the occlusal plane: the midpoint of maxillary and mandibular central incisor and both contact points of the first and second molars; C. occlusal plane; D. sagittal plane; E.

coronal plane.

Fig. 3. Measurements of linear distance: UICW and LICW, upper and lower inter-canine width; UIP1W and LIP1W, upper and lower inter first premolar width; UIP2W and LIP2W, upper and lower inter second premolar width; UIM1W and LIM1W, upper and lower inter first molar width; UIM2W and LIM2W, upper and lower inter second molar; UAL and LAL, upper and lower arch length; OJ and OB, overjet/overbite.

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statistically significant differences between the 2 groups (P < .05), with smaller values in the digital setup.

In extraction cases, all measurements in the digital setup, except UICW, UAL, and OB, showed smaller val- ues than the actual treatment results. Four measure- ments (UIM1W, UIM2W, LIM1W, and LIM2W) showed statistically significant differences between the 2 groups (P < .05), with smaller values in the digital setup (Table1, 2).

DISCUSSION

This study aimed to evaluate the accuracy of digital teeth setups created using 3D simulation programs by comparing them with actual orthodontic treatment re- sults, and to identify the characteristics and limitations of these digital teeth setups. The null hypothesis was that digital teeth setups would be similar to actual or-

thodontic treatment outcomes. The findings showed statistically significant intergroup differences in three measurements (LIP1W, UAL, and LAL) in non-extraction cases, with smaller values in the digital setup, and four measurements (UIM1W, UIM2W, LIM1W, and LIM2W) in the extraction cases, with smaller values in the digital setup.

A common method for comparing dental cast models is assessment of teeth displacement by superimposition of the hard palate anterior palatal rugae, which is a rela- tively stable reference point after orthodontic treatment and growth.13,14 However, the loss of the palatal rugae during the dental sectioning process prevented super- imposition of these structures for the setup cast model.7 Therefore, in this study, we compared 2 digital cast models using measurements of the model without superimposition.

It has been shown that there are no significant differ-

non-extraction Simulation Setup Model Final Model

N Mean SD Mean SD P value

UICW 10 35.53 2.28 35.34 1.60 0.508

UIP1W 10 42.97 2.16 43.75 2.12 0.333

UIP2W 10 49.71 1.78 49.99 0.92 0.721

UIM1W 10 49.00 2.23 48.65 0.94 0.575

UIM2W 10 53.93 3.03 54.01 2.06 0.799

LICW 10 26.70 1.70 27.32 1.47 0.114

LIP1W 10 35.02 1.77 36.13 1.42 0.037*

LIP2W 10 41.42 1.47 41.88 1.18 0.203

LIM1W 10 43.41 1.34 42.78 1.04 0.093

LIM2W 10 48.42 2.52 48.55 2.29 0.721

UAL 10 35.96 1.69 36.94 2.24 0.028*

LAL 10 32.04 1.48 33.26 1.96 0.009§

OJ 10 2.68 0.46 3.19 0.82 0.059

OB 10 1.52 0.44 1.37 0.61 0.646

Simulation Setup Model; preoperative digital simulation setup using Mimics software, Final Model; postoperative digital dental cast.

The Wilcoxon signed rank test was used to compare the mean values of the measurements between the 2 groups. Upper and lower inter-canine width (UICW and LICW), upper and lower inter first premolar width (UIP1W and LIP1W), upper and lower inter second premolar width (UIP2W and LIP2W), upper and lower inter first molar width (UIM1W and LIM1W), upper and lower inter second molar width (UIM2W and LIM2W), upper and lower arch length (UAL and LAL), and overjet/overbite (OJ and OB).

*P < .05; §P <.01.

Table 1. Comparison of arch dimensions between the simulation setup models and final models in non-extraction case.

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ences between the measurements of digital cast model images obtained via laser scanners or CBCT scanners us- ing 3D programs and the measurements of plaster cast models using digital calipers.15,16 In particular, Rossini et al17 and Fleming et al18 reported that there were no sig- nificant differences between digital cast model images and plaster cast models in transverse dimension meas- urements, and the two models showed good agreement in arch length, overjet, and overbite.

In non-extraction cases, the digital teeth setup is sim- ilar to the actual treatment results in transverse meas- urements, because the setup is mainly composed of ro- tation control and crowding relief. However, the upper and lower arch lengths in the digital setup were less than the actual treatment results. Im et al7 reported that since digital cast model data obtained via laser scanners were stored as sterolithographic files and these data only con- tained positional information of the surface, it was diffi- cult to form the proximal surface of the tooth when the teeth were sectioned in the digital cast model for setup,

so the mesiodistal width of the tooth from the digital cast model was less than that of the real tooth. In our study, the digital cast models were obtained from laser scanner (RexcanDS2, SOLUTIONIX, Seoul, Korea) and the data were also stored as sterolithographic files. For the same reason, the mesiodistal width of the teeth from the digital cast model was less than that of real teeth, so the maxillary and mandibular arch lengths of the digital setup appeared to be less than the actual treatment results.

In extraction cases, after rotation control and crowd- ing relief, the anterior segment and posterior segment show movements in the sequence of the En-masse retraction. In such cases, there will be discontinuities caused by the difference between intercanine and inter- premolar widths, and rotation of the posterior segment is required in order to recover these discontinuities. This may cause differences between the width of the digital setup and the actual results in the posterior region.

When the posterior segment is rotated around the sec-

extraction Simulation Setup Model Final Model

N Mean SD Mean SD P value

UICW 7 36.07 1.44 35.84 1.81 0.553

UIP2W 7 44.29 1.86 44.98 1.31 0.063

UIM1W 7 42.88 2.22 45.90 2.53 0.018*

UIM2W 7 48.08 2.58 51.73 3.12 0.018*

LICW 7 27.26 1.32 27.49 0.70 0.735

LIP2W 7 35.40 1.32 36.10 0.99 0.499

LIM1W 7 36.80 2.23 39.19 2.58 0.018*

LIM2W 7 42.23 1.88 45.57 2.66 0.018*

UAL 7 31.08 1.45 30.87 1.26 0.398

LAL 7 26.43 0.94 27.21 1.38 0.310

OJ 7 3.05 0.39 3.21 0.82 0.345

OB 7 2.07 0.55 1.99 0.50 0.735

Simulation Setup Model; preoperative digital simulation setup using Mimics software, Final Model; postoperative digital dental cast.

The Wilcoxon signed rank test was used to compare the mean values of the measurements between the 2 groups. Upper and lower inter-canine width (UICW and LICW), upper and lower inter first premolar width (UIP1W and LIP1W), upper and lower inter second premolar width (UIP2W and LIP2W), upper and lower inter first molar width (UIM1W and LIM1W), upper and lower inter second molar width (UIM2W and LIM2W), upper and lower arch length (UAL and LAL), and overjet/overbite (OJ and OB).

*P < .05; §P <.01.

Table 2. Comparison of arch dimensions between the simulation setup models and final models in extraction case.

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ond premolar, the first molar and second molar tend to move to the lingual side to form a proper arch, so that the intermolar width of the digital setup will be less than the actual treatment results.

In the digital setup, teeth movement is performed without considering the limitation of teeth movement due to the anatomical structure.in the posterior molar area. The mandibular molars are inclined lingually due to masticatory muscle activity and function. That is, the mandibular posterior teeth area receives buccal loads due to masticatory function, so the buccal cortical bone in the molar area is thick and there is less need for strong support on the lingual side; thus, the lingual cort- ical bone appears to be thinner in the molar area.19 This may act as a limit to the lingual movements of the man- dibular molar, especially the root, in the actual treatment. However, during digital setup, these anatomi- cal limitations are not considered because the system considers the occlusion by crown movement only, which leads to more lingual movement than the actual treat- ment results in the mandibular posterior area. Thus, the intermolar width in the digital setup is less than actual treatment results. To solve this problem, a computed to- mography (CT) image is taken and a digital cast model is superimposed on the CT image to reconstruct the im- age,20 and the setup is performed considering the crown, the root, and the alveolar bone. Using this approach, the limitation in teeth movement due to the anatomical structure can be predicted and the setup can be close to the actual treatment results.

Digital setup is influenced by the practitioner’s sub- jective decisions.1,21 Even though a single clinician per- formed all digital setup procedures, the actual treatment was carried out by several other clinicians, and this could be one reason for the differences between digital setups and actual treatment results. There may also be inaccuracies due to inadequate digital experience among clinicians21 and errors that may occur during the setup process.7

Because the digital cast model is stored in the form of files, the digital setup procedure can be conducted on a

personal computer, which facilitates communication among other clinicians because only the simple exchange of files is necessary. Moreover, many laboratory proce- dures which were necessary for the conventional plaster model setup can be omitted in the digital setup.1,20

However, there are still limitations. In this study, the plaster cast model was converted into a digital image by laser scanning and used for setup. In digital dentistry, the plaster model also shows obstacles. To remove the plaster model, some authors used micro CT after taking the rubber impression, but that approach also needed an impression process. The intraoral digital scanner can re- move the need for this process. However, development of full arch digital impressions using intraoral digital scanners has not been popularized yet, and the accuracy of intraoral scanners is questionable. There are also oth- er problem associated with the time required to make an impression and the comfort levels of the patients with the conventional procedure to make impressions.22,23,24 These problem could be overcome by the development of intraoral digital scanners, and this will eliminate the need for impression taking and storage of plaster models. However, this is also a new technology that clinicians and dental technicians may not be familiar with, so more training is needed.

Digital teeth setup using 3D simulation programs complemented the disadvantages of the conventional manual setup and showed accuracy that can be used to predict orthodontic treatment results. However, in the cases with large amounts of tooth movement, such as extraction cases, data on predictable tooth movement at the posterior part of the setup will be needed, and with appropriate training along with improved hardware and software development, digital dentistry will have un- limited possibilities. Future prospective clinical trials in- volving a larger sample would be helpful for evi- dence-based decision making related to digital setup.

CONCLUSIONS

Digital teeth setup using the 3D simulation program

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showed adequate accuracy to allow prediction of ortho- dontic treatment results. However, in cases involving treatment with large amounts of tooth movement, such as in extraction cases, data on predictable tooth move- ment at the posterior part of the setup will be needed.

Further study involving a larger sample would be helpful for evidence-based decision making related to digital setup.

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diagnostic accuracy and surface registration analysis.

Am J Orthop Dentofacial Orthop. 2013;144:831-7.

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Orthop. 2006;129: 794-803.

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23. Goracci C, Franchi L, Vichi A, Ferrari M. Accuracy, reli- ability, and efficiency of intraoral scanners for full-arch impressions: a systematic review of the clinical evidence. Eur J Orthod. 2016;38: 422-28.

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수치

Fig.  3.  Measurements  of  linear  distance:  UICW  and  LICW,  upper  and  lower  inter-canine  width;  UIP1W  and  LIP1W,  upper and lower inter first premolar width; UIP2W and LIP2W, upper and lower inter second premolar width; UIM1W  and LIM1W,  upper
Table 1. Comparison of arch dimensions between the simulation setup models and final models in non-extraction case.
Table 2. Comparison of arch dimensions between the simulation setup models and final models in extraction case.

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