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Short Paper Proc. of Int. Conf. on Advances in Industrial and Production Engineering 2012

Dental Crown Manufacturing using Stereolithography Method Shruti S. Bammani a, 1, Pranav R. Birajdarb, 1 and Shriniwas S Metanc, 1 1

Dept. of Mechanical Engineering, Nagesh Karajagi Orchid College of Engineering and Technology Solapur, Maharashtra, India. a shruti.bammani@gmail.com,bprnvbirajdar@gmail.com,cmetana@rediffmail.com II. LITERATURE REVIEW

Abstract—Dental crown by conventional methods greatly relies on the quality and accuracy of the final product which depends mostly on the technician’s subjective judgment. Also Conventional method is manually operated which involves many complex procedures, and is a time-consuming and laborintensive job with no quantitative design and manufacturing information preserved for future retrieval. In this paper, Rapid Prototyping an additive manufacturing method is proposed for the design of custom dental crown. It is applied to modify the morphology and design of the dental crown which are able to generate a 3D object layer by layer directly from a CAD file with the most powerful and versatile RP technique known as Stereo lithography (SLA), which is opposite to that of conventional manufacturing processes. It has the highest fabrication accuracy and an increasing number of materials, which can be processed, are becoming available. This paper gives an overview regarding the conventional crown manufacturing and presents a more effective solution: the Rapid Prototyping Method which is a more sophisticated and advanced process. The proposed method provides solid evidence that computer-aided design and manufacturing technologies may become a new avenue for custom-made dental crown design, analysis, and production in the 21st century.

For 20 years, exciting new developments in dental materials and computer technology have led to the success of contemporary dental computer-aided design /computeraided manufacturing technology [4].Fuster-Torres M.A. in his study used CAD/CAM systems in implant dentistry, especially emphasizing implant abutments and surgical templates manufacturing. He published this in English at Medline and Scopus databases, introducing “dental CAD/ CAM”, “implants abutments” and “surgical guide CAD/ CAM” [5]. J.P. Kruth in his work explained in detail regarding the Medical and dental applications. He stressed the advantage of the evolution by using Selective Laser Sintering (SLS) / or Selective Laser Melting (SLM), not only for plastic devices like visual anatomical models or one-time surgical guides, but also for functional implants or prostheses with long-term consistency made from a biocompatible metal. Dental applications are very suitable for processing by means of RP&M due to their complex geometries, low volume and strong individualization. The manufacturing of multiple unique parts in a single production run enables mass customization. Moreover, computer controlled production corresponds to the global trend of digitizing the fabrication of medical and dental parts [6].

Keywords—Rapid Prototyping (RP), Stereolithography (SLA), anterior crown, posterior crown, dental implant.

I. INTRODUCTION Rapid Prototyping (RP) is an additive processes that fabricates parts layer-by-layer also known as Layer Manufacturing (LM). They are capable of creating parts with small internal cavities and complex geometries. The process for RP and manufacturing basically consists of three steps: form the cross sections of the object to be manufactured, lay the cross sections layer by layer, and combine the layers [1]. RP&M method was first presented at the AUTOFACT show in Detroit (U.S.A.) in November 1987, by 3D Systems, Inc. At that time, the process was very inaccurate and the choice of materials was limited. Over about twenty years, many RP processes have been developed to produce various parts , for instance Laminated Object Manufacturing (LOM), Fused Deposition Modeling (FDM), Ink Jet Printing , Three Dimensional Printing (3DP), Selective Laser Sintering (SLS), Solid Ground Curing (SGC) and Stereolithography (SLA) [2] ,[3].

A. Basic Process of Rapid Prototyping Common to all the different techniques of RP, this is the basic approach adopted, which can be described as follows:

Fig. 1. Rapid Manufacturing Process

Short Paper Proc. of Int. Conf. on Advances in Industrial and Production Engineering 2012 1. Create a CAD model of the design. 2. Convert the CAD model to STL format. 3. Slice the STL file into thin cross-sectional layers. 4. Construct the model one layer atop another. 5. Clean and finish the model [7], [8].

1. Anterior Crowns: Anterior crown are made for longer, thinner teeth located in the front of the mouth, which include the lower and upper incisors and cuspids in both primary dentition and secondary dentition [13]. 2. Posterior Crowns: Posterior crowns are made for thicker teeth located in the back of the mouth, which include the lower and upper molars in primary dentition and premolars and molars in secondary dentition [14].

B. Need of Dental Crowns: 1. To protect a weak tooth ( from decay) from breaking or to hold together parts of a cracked tooth. 2. To cover and support a tooth with a large filling when there isn’t a lot of tooth left. 3. To cover severely discolored teeth. 4. To cover a dental implant. 5. To restore an already broken tooth or a tooth that has been severely worn down.

III. MANUFACTURING OF DENTAL CROWNS For a crown restoration, metals and ceramics are the used because they have good hardness, strength, stiffness, durability, corrosion resistance, and bio-compatibility. It is a delicate job and needs accuracy. There are two techniques for preparing dental restorations which are detailed below:

C. Stereolithography Stereolithography (SLA) is the most widely used rapid prototyping technology. It was the first rapid prototyping process, introduced in 1988 by 3D Systems, Inc., based on work by inventor Charles Hull. It is an additive manufacturing process in which a liquid photo curable resin acrylate material is used. SLA uses a highly focused UV laser to trace out successive cross-sections of a three-dimensional object in a vat of liquid photosensitive polymer [9], [10].

A. Conventional Manufacturing There are 9 steps that are followed by most companies when producing a dental crown using the conventional approach. Although slight differences occur between manufacturers, the same 9 steps are usually the same. Common conventional approach for crown fabrication is as follows: 1. Initial Preparation: Before beginning, the dentist may need to prepare the area where the crown is to be installed which requires the removal of 2-3 millimeters of tooth structure from the four sides and the biting edges. 2. Impression: The manufacturing process begins with a preliminary impression of the patient’s mouth, which is usually carried out by placing a viscous liquid material like impression plaster (Fig 4a) into the mouth in a customized dental impression tray. The material then sets to become an elastic solid and when removed, provides a detailed and stable reproduction of teeth. This impression is used to prepare a diagnostic cast [15]. 3. Cast: The next step is to prepare another type of plaster, known as model plaster which is harder than the impression plaster. Once again the plaster is mixed with the appropriate quantity of water. Then the slurry is poured into the impression mold. In this way a positive model of the tooth can be made which is known as a cast, used by the dentist for study purposes. 4. Mold: Once an appropriate preliminary cast has been obtained, the mold is usually made from gypsum, a stone-like product which will produce an accurate mold of the patient’s teeth. The final mold is inspected before using it to manufacture the teeth. 5. Macro Mold: Macro model is an exact replication of the desired crown which is carved out using Horus dental carving wax (Fig 4b). A digital caliper rule (Mitutoyo Absolute Dogmatic) is used to obtain the measurements of the length, width and width of tooth necks of crown in Mesiodistal (MD) and Buccolingual (BL) directions. The obtained measurements are magnified to 6X to establish the proportion between the macro models and the natural teeth (6:1) and then the model is made. In this phase, only the crown is carved [16], [17].

Fig. 2. Schematic of SLA process

As the laser traces the layer, the polymer solidifies and the excess areas are left as liquid. When a layer is completed, a leveling blade is moved across the surface to smooth it before depositing the next layer (Fig 2). The platform is lowered by a distance equal to the layer thickness (typically 0.0030.002 in), and a subsequent layer is formed on top of the previously completed layers. This process of tracing and smoothing is repeated until the build is complete. Once complete, the part is elevated above the vat and drained. In many cases, a final cure is given by placing the part in a UV oven. After the final cure, surfaces are polished, sanded and finished [11]. D. Types of Dental Crown: A dental crown is a type of dental restoration which completely caps or encircles a tooth or dental implant. Crowns are often needed when a large cavity threatens the ongoing health of a tooth [12].They are typically bonded to the tooth using a dental cement. They can be classified according to their positions as following: © 2012 AMAE DOI: 02.AIPE.2012.2.1

Short Paper Proc. of Int. Conf. on Advances in Industrial and Production Engineering 2012 may require minor grinding and smoothening of its surface to ensure it fits correctly [19], [20]. B.Stereolithography Method: 1. Examine and Preparation: First, your Dentist examines the tooth and prepares your tooth for the restoration by removing decayed and weakened tooth tissue. 2. 3D Impression: The next step in the process is to take a silicone impression (Fig 5a). The casts produced from this impression are used to prepare the initial model of malocclusion. 3. 3D Scanning: Noncontact scanning of the initial model is performed with a high-resolution optical 3D scanner (Fig 5b).The 3D scanner examines the model from various perspectives to create a complete 3D representation, with a resolution of at least 0.01 mm. The result is a surface consisting of many thousands of minute triangles that can be turned, observed, and processed on a computer with CAD software. 4. Computer Aided Design (CAD) Technology: CAD software converts the scan into a three dimensional virtual model of your prepared tooth (Fig 5c). CAD techniques have been developed to build surgical guides in an attempt to improve precision of implant placement [21]. 5. Digital Design Framework: The next step of the procedure is the automated design of the framework based on the computed position of the implants and the captured geometry of the tooth arrangement. This design strategy is implemented in a software module by translating the design rules to computer tools. An important issue during the design step is the manipulation of the complex dataflow. The different and large data files of the complex medical shape have to be combined to one geometrical model in a convenient way without any mistakes (Fig 4d).

Fig. 3a-3h. Conventional manufacturing steps

After carving, the wax is smoothed using a wax carver and silk stockings. 6. Investment: The finished wax-up of the crown is then placed for investing (Fig 4c). The wax model is invested in a sort of plaster and connected to a short plastic stick, called a “sprue former”, which will stick out of the investing plaster. Then it is placed in a furnace, which will completely burn off the wax and plastic and a hollow within the investment material remains, known as an “investment pattern. This pattern is then placed in a simple centrifuge where pennyweights of metal are melted down and rapidly shot in the investment pattern, through the sprue former, and into the hollow of the crown [16].Later the cast metal solidifies in this void and is finished down to fit on the working model (Fig 4d). 7. Porcelain Fabrication: Metal to Porcelain fabrication is a bit more involved than that of a full metal crown. Here the restoration must fit and function properly and should also blend in with the other natural teeth. Consideration of shades, contours, and texture, as well as the preparation design, working space, and type of materials to be used is made (Fig 4e). To hide the metal, a thin layer of opaque porcelain is applied and then baked on in a furnace (Fig 4f). 8. Furnace: Here the crown is fired in the furnace to somewhere around 1700 degrees Fahrenheit (depending on the type of material). This baked crown can now be fit to the model and adjusted if need [18].Later it is given correction build-up and after which it is fitted to the model and then once again fired in the furnace to achieve a nice finished glaze(Fig 4g). 9. Installation: After the crown has been successfully completed, it is ready for installation (Fig 4h). The prosethedontist applies cement to the inside of the crown surface and then fits it into place over the tooth. The crown

Fig. 4a-4h. Steps of additive manufacturing.

Short Paper Proc. of Int. Conf. on Advances in Industrial and Production Engineering 2012 6. Design Restoration: When the design of the restoration is complete (Fig 5e), the CAD software transforms the virtual model into a specific set of commands. These in turn drive the SLA unit, which fabricates the designed restoration (Fig 5f). A dental restoration must fit its abutment within a 50 ìm range. 7. SLA Method: Here the computer design of the dental restoration generates a path much like a cutting tool and instead of cutting, the system sinters material along the path, building a part from a “bath” of ceramic or metal powder and adding material continually until the complex part is complete. No excess material remains and the dental crown is manufactured in the unit (Fig 5g). 8. Installation: After the crown has been completely manufactured in SLA unit, it is taken out of the machine and is ready for installation (Fig 5h).

manufacturing. A successful attempt has been made to include some important factors which prove the RP technique to be more efficient and accurate than the conventional method. REFERENCES [1] Terry Wohlers, “New Developments and Trends in Product Design, Prototyping, Tooling, and Reverse Engineering”, Wohlers Associates, Inc, 2000, USA. [2] Chaput C, Chartier T, “Fabrication of ceramics by Stereolithography”, in RTejournal - Forum für Rapid Technologie, vol.4, Iss.1, 2007, pp 1. [3] Dr. A. Dolenc, “An Overview of Rapid Prototyping Technologies In Manufacturing”, Institute of Industrial Automation, Helsinki University of Technology, July 1994, pp 5. [4] Kruth, J.P., Mercelis, P., Van Vaerenbergh, J., Froyen, L. & Rombouts, M., “Advances in Selective Laser Sintering”, Invited keynote paper. Proc. of the 1st Int. Conf. on Advanced Research in Virtual and Rapid Prototyping (VRAP2003), Leiria, 1-4 October 2003, pp 59-70. [5] Fuster-Torres M. A., Albalat-Estela S., Alcañiz-Raya M., Peñarrocha-Diago M., “Cad / Cam Dental Systems in Implant Dentistry: Update”, Med Oral Patol Oral Cir Bucal, vol. 14, iss. 3, March 2009, pp141-145. [6] Estafan D., Dussetschleger F., Agosta C., Reich S., “Scanning electron microscope evaluation of CEREC II and CEREC III inlays” in Gen Dent., vol. 51, iss. 5, Sept-Oct 2003, pp 450454. [7] Richard Bibb, John Winder,”A review of the issues surrounding three-dimensional computed tomography techniques,” in Radiography, vol. 16, iss. 1, February 2010, pp 78-83. [8] Miltiades A. Boboulos, “Cad-Cam & Rapid Prototyping Application Evaluation”, Ventus Publishing ApS, pp 134-135. [9] S.Kumawat, A.Gawali, “Application of Cad in Rapid Prototyping Technology”, in Journal of Engineering Research and Studies, Vol.2, Iss.4, Oct-Dec 2011, pp 3. [10] Muhammad Enamul Hoque, “Rapid Prototyping Technology Principles and Functional Requirements”, InTech Publishers, Sept 2011, pp 2. [11] Yongnian Yan et al., “Rapid Prototyping and Manufacturing Technology: Principle, Representative Technics,Applications, and Development Trends” in Tsinghua Science &Technology, vol. 14, iss 1, June 2009, pp 1-12. [12] http://www.custompartnet.com/wu/stereolithography [13] http://www.webmd.com/oral-health/dental-crowns [14] http://www.biologyonline.org/dictionary/Anterior_teeth [15] http://www.biologyonline.org/dictionary/Posterior_teeth [16] http://en.wikipedia.org/wiki/Dental_impression [17] S.SiéssereI, M. Vitti, L. Gustavo de Sousa, M. Semprini, S. Cecílio Hallak Regalo, “Educational Material of Dental Anatomy Applied to Study the Morphology of Permanent Teeth”, in Brazilian Dental Journal, vol.15, no.3, Sept. /Dec. 2004, pp 238-242. [18] http://en.wikipedia.org/wiki/Crown_(dentistry) [19] http://www.bothelldentallab.com/ [20] http://www.enotes.com/science/dental crown [21] Sarment D.P., Sukovic P., Clinthorne N., “Accuracy of implant placement with a stereolithographic surgical guide”, in The International Journal of Oral & Maxillofacial Implants, vol. 18, July 2003, pp 571-577.

IV. DISCUSSION The Conventional process is time-consuming, imprecise, and requires materials that might not provide the best durability or aesthetic appearance. Frequent calibration and constant adjustment has to be done which takes time and decreases daily production. Stereolithography process is basically selected because of its highest fabrication accuracy which eventually makes the dental crown more comfortable and fits precisely. This method is more efficient than the conventional method in many ways. Curves, holes and more complex shapes are easier and more accurately reproduced and the object is produced exactly as designed without waste. Process time and material costs are reduced and there is no harmful dust or fumes are created to harm the working environment. This process requires very less training time. One can print 50 to 80 units in 56 minutes in high quality mode. CONCLUSION A brief description of RP technology is given and a comparative study between conventional manufacturing and manufacturing using Stereo lithography (SLA) is explained in detail with dental crown as an example. It is observed that Stereo lithography is fast and effective method which saves time and money, allows speed delivery and is therefore one of the better alternatives for dental crown manufacturing. In medical applications tolerances in manufacturing is one of the important aspects and by SLA we can control tolerances within 0.005mm/mm. Especially for products that require very high precision or surface finish stereo lithography system is one of the best option. The description of various stages of dental restoration manufacturing by both of these methods proves that SLA method is more effective for dental crown