Computers in Biology and Medicine 44 (2014) 10–14

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Computer-aided design of tooth preparations for automated development of fixed prosthodontics Fusong Yuan, Yuchun Sun, Yong Wang, Peijun Lv n Center of Digital Dentistry, Peking University School and Hospital of Stomatology & National Engineering Laboratory for Digital and Material Technology of Stomatology & Research Center of Engineering and Technology for Digital Dentistry, Ministry of Health, 22 Zhongguancun Nandajie, Haidian District, Beijing, PR China

art ic l e i nf o

a b s t r a c t

Article history: Received 9 July 2013 Accepted 19 October 2013

Background: This paper introduces a method to digitally design a virtual model of a tooth preparation of the mandibular first molar, by using the commercial three-dimensional (3D) computer-aided design software packages Geomagic and Imageware, and using the model as an input to automatic tooth preparing system. Methods: The procedure included acquisition of 3D data from dentate casts and digital modeling of the shape of the tooth preparation components, such as the margin, occlusal surface, and axial surface. The completed model data were stored as stereolithography (STL) files, which were used in a tooth preparation system to help to plan the trajectory. Meanwhile, the required mathematical models in the design process were introduced. Results: The method was used to make an individualized tooth preparation of the mandibular first molar. The entire process took 15 min. Discussion: Using the method presented, a straightforward 3D shape of a full crown can be obtained to meet clinical needs prior to tooth preparation. & 2013 Elsevier Ltd. All rights reserved.

Keywords: Tooth preparation CAD Mandibular first molar Fixed prosthodontics Automation

1. Introduction Tooth hard tissue disease has been identified as one of the most common dental diseases. It includes mainly dental caries, congenital defects, fractures, fluorosis or acid erosion. Fixed partial dentures are one of the most common repair methods for tooth hard tissue diseases. The ultimate goal of fixed prosthodontics is to recover the esthetic appearance and chewing function of the patient, and precise tooth preparation is a prerequisite for realizing this goal. Smooth and precise tooth preparation can facilitate impression taking, and the fabrication of a precisely fitting restoration contributes to a durable, esthetic, and functional result. At present, there are two methods typically used in clinical tooth preparation, namely grinding by using turbine-driven drills and ablation by erbium-doped yttrium aluminum garnet (Er:YAG) or erbium, chromium-doped yttrium scandium gallium garnet (Er, Cr:YSGG) handheld laser systems. These two methods not only have poor accuracy and efficiency, but also generate mechanical and thermal stress and create micro-cracks of several tens of microns in the enamel [1,2]. Therefore, in order to overcome the drawbacks of the traditional tooth preparing methods, a miniature

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Corresponding author. Tel.: þ 86 10 62188981; fax: þ86 10 62142111. E-mail addresses: [email protected], [email protected] (P. Lv).

0010-4825/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compbiomed.2013.10.019

robotic system for tooth preparation was developed, which can manipulate an ultra-short pulse laser beam to shape a target tooth into a tooth preparation [3]. The advantages of this method compared to the methods currently in use include higher precision and efficiency as well as less mechanical and thermal damage. In addition, the surfaces are better prepared, without significant traces of melting, deformation, or cracking [4,5]. Precise preparation design is crucial and is the first step in the process of automatic tooth preparation. Tooth preparation should follow some basic design principles. First, the tooth preparation design determines the amount of tooth structure removal. Conservation of sound tooth structure helps preserve tooth vitality and reduces dentin-pulp complex injury. Daniel Edelhoff et al. [6,7] introduced a comprehensive tooth preparation design classification system to quantify the amount of tooth structure removed during innovative and conventional preparations in order to preserve sound tooth construction as much as possible. Second, tooth preparation design plays an important role in the stability and fracture resistance of some artificial restorations. El Salam Shakal et al. [8] indicated that the tooth-preparation design strongly influences the bonding strength of resin-bonded fixed partial dentures. An in vitro study verified that a 0.5-mm axial chamfer tooth preparation was not worse than 1 mm to the stability and fracture resistance of Artglass crowns of posterior metal-free Artglass crowns [9]. Third, the design of the tooth

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preparation convergence angle showed a strong effect on the applicability of restorations. Corazza et al. [10] performed finite element analysis(FEA) to analyze the influence of convergence angle design on the stress distribution of all-ceramic full crowns, used the compressive test to register the load to fracture, and reported that crown performances were best when the mesialdistal convergence angle was 201. The applicability of restorations also includes their retention properties. Chan et al. [11] studied the relationship between retention and the marginal seating discrepancy of complete veneer crowns with various preparation convergence designs, and drew the conclusion that the optimal retention and minimum discrepancy of a complete veneer crown can be obtained with tooth preparation convergence angles between 21 and 201. In summary, tooth preparation design has an important impact on the retention and stability of the prosthesis and the health of the natural teeth. To this end, some researchers have assessed the design of tooth preparations for different types of dental restorations. Goodacre et al. [12] developed nine scientific principles of tooth preparation to ensure mechanical, biologic, and esthetic success for tooth preparations of full-crown restorations. Presently, tooth preparation design is performed by dentists, who produce the 3D shape of the tooth preparation based only on their clinical experience and theoretical knowledge. However, in this approach, the intended tooth preparation design in the dentist's mind cannot be used as input data for an automatic tooth preparation system. Computer-aided design (CAD) tooth preparation can solve this problem by providing the precise 3D data required for creating a tooth preparation. At the same time, It may be conducive to communication between patients and dentists or between dental technicians and dentists. Furthermore, ever-greater numbers of educators are used to train dental interns and students. However, there is no unified evaluation standard for tooth preparation. CAD tooth preparation will help to establish an evaluation standard. However, according to the literature, no studies on CAD of tooth preparation for fixed prosthodontics exist at present; therefore, that is the object of this work. In this study, two commercial reverse engineering software packages, Geomagic Studio 2012(3D Systems, Morrisville, NC, USA)

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[13] and Imageware 11(Siemens PLM Software, Plano, TX, USA) [14], were used to complete the CAD process for a tooth preparation.

2. Materials and methods 2.1. 3D data achievement and reconstruction One male patient was selected as the experimental subject. His mandibular first molar underwent root canal therapy and had an amalgam filling. He wanted to replace the filling with a full-crown restoration. The patient voluntarily joined the study and provided informed consent. At first, in order to design the subgingival shoulder, gingival retraction was used around the tooth. After about 15 min, the gingival retraction thread was taken out. Conventional polyether rubber impression material was used to obtain impressions of the patient's upper and lower edentulous jaw; then, super-hard plaster models were made. A commercial noncontact line laser scanner (D700 series, 3Shape A/S, Copenhagen, Denmark) was used to obtain the 3D data for the upper and lower dental cast models and their relations to the jaw. Upper and lower plaster models were fixed in maximum intercuspation with a line laser scanner modeling device fixed and mounted in the scanner work area. The upper and lower models and their jaw relations were scanned, and 3D data were obtained. Then, the upper and lower models were mounted in the same scanner work area and were scanned separately to obtain the 3D data. After the removal of redundant data, noise reduction, smoothing, and resampling were performed using data processing software and the 3D data were saved in STL format (Fig. 1). 2.2. Tooth preparation design Each part of the tooth preparation was designed based on the point cloud data using two commercial reverse engineering software packages, Geomagic and Imageware, to meet the clinical fixed prosthodontic requirements.

Fig. 1. 3D data of the upper and lower dental casts and their relation to the jaw.

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2.2.1. Margin design For an all-ceramic crown restoration, the margin of the tooth preparation typically had a shoulder with a width of 1 mm, which was considered the standard of margin design in this study [15]. The process consists of three steps. First, the outward edge of the margin was extracted along the gingival margin around the involved tooth using the ‘curve draw’ and ‘extract’ tools in the Geomagic software (Fig. 2a). Second, the tooth crown was separated along the outward edge using the ‘trim of curve’ tool in Geomagic (Fig. 2b). Third, the separated crown was scaled to 1 mm in its entirety about its centroid using the ‘scale model’ tool (Fig. 2c). Finally, the gap between the outward edge and the free margin of the scaled crown was filled using the ‘fill holes group’ tool (Fig. 2d). After removing the scaled crown using the ‘select’ and ‘delete’ tools, the margin of the tooth preparation was complete (Fig. 3).

group’ tool in Geomagic. An axial surface with 61 of occlusal convergence was created (Fig. 5).

2.3. Mathematical models required in the design process (Pythagorean proposition application) In the design process of the axial surface with 61 of occlusal convergence, the angle between the Y-axis and the connection of the inward edge of the margin and the edge of the occlusal surface is equal to 61 (Fig. 6). When the edge line of the occlusal surface is determined, the Pythagorean proposition is applied. The distance that the inward edge of the margin is scaled can be calculated using the Pythagorean proposition. The calculation formula is as follows: S ¼ h  tan 61

2.2.2. Design of the edge and location of the occlusal surface First, the inward edge of the tooth preparation was extracted to a free curve using the ‘create from boundary’ and ‘extract’ tools. Second, the inward edge was scaled to 0.758 mm (the height of tooth preparation  tan 61) around its centroid by using the ‘scale model’ tool. The data of the inward edge was imported into Imageware software. Using the ‘sweep surface’ tool in Imageware, a swept surface was created with the inward edge along the Y axis direction. Then, the swept surface was imported into Geomagic. After determining where the initial tooth crown intersected the swept surface with the ‘Boolean’ tool, the occlusal surface was made. With the ‘object mover’ tool in Geomagic, the occlusal surface was designed to fall 2 mm along the Y axis direction (Fig. 4).

2.2.3. Axial surface design The gap between the inward edge of the margin and the outward edge of the occlusal surface was filled using the ‘fill holes

Fig. 3. Margin shaping.

Fig. 2. The design process of a margin. (a) the extraction of the outer edge of the margin; (b) the trim of the tooth crown; (c) scaling the separated tooth crown; and (d) a curved suture.

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Fig. 4. The design process of occlusal surface. (a) Intersection of the swept surface and the tooth crown; (b) occlusal surface shaping; (c) axial movement of the occlusal surface; and (d) the complete design.

Fig. 5. The axial surface of the tooth preparation. (a) The occlusal conditions observed; (b) the buccal observation; (c) the lingual observation; and (d) the occlusal observation.

(S represents the distance of the inward edge of the margin; h represents the height of the tooth preparation.) Therefore, the location where the inward edge of margin is scaled is the projected position of the edge of the occlusal surface along the direction of the Y-axis.

3. Results The tooth preparation of the mandibular first molar was designed easily with the new method. The entire design process took approximately 15 minfor each tooth preparation. Additionally, the completed models were stored as STL files, which will be

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through human-computer interaction. Therefore, further studies are necessary to research automated design software to make tooth preparation design both simpler and faster.

Conflict of interest statement The authors declare no conflict of interest.

Acknowledgment

Fig. 6. A diagram of the longitudinal section of the tooth preparation.

used to transfer the CAD models to an automatic tooth preparation system. 4. Discussion The method based on Geomagic and Imageware software was developed specifically for designing tooth preparations. In this procedure, it takes the clinician about 15 min to design and survey the model electronically. There are several drawbacks to the software that need to be improved. First, the whole tooth preparation design process depends on commercial software (Geomagic and Imageware) and requires human interaction, which reduces efficiency. In the future, special software should be developed to enable an automated design process. The scanned data can be transferred to a tooth preparation system for automatic tooth preparation design and to provide the tooth preparation data. Second, it is impossible to automatically determine the outer edge of the margin in the point cloud, because the program cannot differentiate between points. Therefore, the outer edge of the margin was selected both visually and manually, which could reduce accuracy. Third, the method is only applicable to the design of molar preparations. For anterior and premolar teeth, the axial tooth preparation surface would have different occlusal convergences in the gingival and occlusal portions of the surface. However, through the curved suture, an axial surface with one occlusal convergence was made. Thus, further research is still underway to solve the above problems. 5. Conclusions The mathematical models and methods of the tooth preparation design were analyzed and explained. In practical application, the operation is much simpler than is described here. However, the whole design result needs to be precisely modified using the tools available in the two software packages to obtain a satisfactory shape. Additionally, the entire design process was completed

This study was supported by the Twelfth Five-Year National Key Technologies Research and Development Program of China (Grant no. 2012BAI07B00). References [1] M.F. Ayad, S.F. Rosenstiel, M.M. Hassan, Surface roughness of dentin after tooth preparation with different rotary instrumentation, J. Prosthet. Dent. 75 (2) (1996) 122–128. [2] M. Hossain, Y. Yamada, Y. Nakamura, Y. Murakami, Y. Tamaki, K. Matsumoto, A study on surface roughness and microleakage test in cavities prepared by Er: YAG laser irradiation and etched bur cavities, Lasers Med. Sci. 18 (1) (2003) 25–31. [3] L. Wang, D.X. Wang, L. Ma, Y.R. Zhang, F.S. Yuan, Y.C. Sun, Preliminary experiments of a miniature robotic system for tooth ablation using ultrashort pulsed lasers, J. Intell. Robot. Syst. (2013) (in press). [4] M.S. Bello-Silva, M. Wehner, C. de Paula Eduardo, F. Lampert, R. Poprawe, M. Hermans, M. Esteves-Oliveira, Precise ablation of dental hard tissues with ultra-short pulsed lasers. Preliminary exploratory investigation on adequate laser parameters, Lasers Med. Sci. 28 (1) (2013) 171–184. [5] M. Portillo Muñoz, M.C. Lorenzo Luengo, J.M. Sánchez Llorente, M. Peix Sánchez, A. Albaladejo, A. García, P. Moreno Pedraz, Morphological alterations in dentine after mechanical treatment and ultrashort pulse laser irradiation, Lasers Med. Sci. 27 (1) (2012) 53–58. [6] D. Edelhoff, J.A. Sorensen, Tooth structure removal associated with various preparation designs for posterior teeth, Int. J. Periodontics Restor. Dent. 22 (3) (2002) 241–249. [7] D. Edelhoff, J.A. Sorensen, Tooth structure removal associated with various preparation designs for anterior teeth, J. Prosthet. Dent. 87 (5) (2002) 503–509. [8] M.A. el Salam Shakal, P. Pfeiffer, R.D. Hilgers, Effect of tooth preparation design on bond strengths of resin-bonded prostheses: a pilot study, J. Prosthet. Dent. 77 (3) (1997) 243–249. [9] P. Rammelsberg, G. Eickemeyer, K. Erdelt, P. Pospiech, Fracture resistance of posterior metal-free polymer crowns, J. Prosthet. Dent. 84 (3) (2000) 303–308. [10] P.H. Corazza, S.A. Feitosa, A.L.S. Borges, A. Della Bona, Influence of convergence angle of tooth preparation on the fracture resistance of Y-TZP-based allceramic restorations, Dent. Mater. 29 (3) (2013) 339–347. [11] D.C.N. Chan, A.H. Wilson Jr., P. Barbe, R.J. Cronin Jr., C. Chung, K. Chung, Effect of preparation convergence on retention and seating discrepancy of complete veneer crowns, J. Oral Rehabil. 32 (1) (2005) 58–64. [12] C.J. Goodacre, W.V. Campagni, S.A. Aquilino, Tooth preparations for complete crowns: an art form based on scientific principles, J. Prosthet. Dent. 85 (4) (2001) 363–376. [13] L.B. Zhou, H.T. Shang, L.S. He, B. Bo, G.C. Liu, Y.P. Liu, J.L. Zhao, Accurate reconstruction of discontinuous mandible using a reverse engineering/computer-aided design/rapid prototyping technique: a preliminary clinical study, J. Oral Maxillofac. Surg. 68 (9) (2010) 2115–2121. [14] J. Wu, B. Gao, H. Tan, J. Chen, C.Y. Tang, C.P. Tsui, A feasibility study on laser rapid forming of a complete titanium denture base plate, Lasers Med. Sci. 25 (3) (2010) 309–315. [15] H.L. Feng, J. Xu, Prosthodontics, Peking University Medical Press (2005) 79.

Computer-aided design of tooth preparations for automated development of fixed prosthodontics.

This paper introduces a method to digitally design a virtual model of a tooth preparation of the mandibular first molar, by using the commercial three...
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