Advancement in technology has been instrumental in improving dental material technology which has resulted in the development of better fabrication methods for complete denture bases. Conventionally, there have been various methods used to fabricate the complete dentures. Many years ago, materials such as bones, wood, ivory, porcelain and some polymers were used in the fabrication of full dentures. Polymethacrylate (PMMA) is the last of the materials from which more than 90% of all dentures were made before the advancements in the development methods. The "age of thermoplastics" which came about in 1910, saw PMMA replace Vulcanite as the preferred material (1). The combination of PMMA with the compression molding technology was instrumental for approximately 7% volumetric and 0.9% linear shrinkage. It was thus necessary for more modifications to further reduce the effects of shrinkages, including the addition of high expansion stone and the placement of a post-palatal seal. Similar views are held by Chintalacheruvu, Balraj, Putchala & Pachalla, (2017).
In spite of the general improvements in the physical properties of PMMA, it had some limitations that hindered its application. The net volumetric shrinkages that characterized it is the main one. There was a continuous presence of residual methacrylate monomer. Furthermore, Candida albicans was noted to associate closely with acrylic resin in the dentures (2). It is worth noting that acrylic teeth has been preferred in the dental profession for their affordable price and the aesthetics associated with them. This is one reason why they were favored for many years despite their shortcomings. On the contrary, they have been known to chemically bond to the acrylic denture bases (4).
As seen above, before the adoption of CAD/CAM technology into removable prosthodontics, the main impediment was that there was no congruence between the denture base and denture bearing tissues which were mainly caused by the shrinkages resulting from resin polymerization. These shrinkages results in distortions hence have an adverse effect on fit and retention (5). The development of CAD/CAM technologies was influenced by the need to find alternatives that could overcome some of these issues. Manufacturers sought to develop alternate materials that would be aesthetically appealing, bond well, have biocompatibility features, take less time and also entail fewer clinical visits by patients (13). A lot of research conducted in the area before that hinted that modern technology would yield more accurate results, especially with the inclusion of digital precision techniques. The popularity of the digital CAD/CAM technologies was also influenced by the fact that there is reduced instances of clinical mistakes owing to the high levels of accuracy brought about by the technologies (14).
In addition to Avadent and Dentca which are pioneer companies in the domain, the popularity of CAD/CAM technologies has seen other technologies entering the scene. McLaughlin, Ramos & Dickinson, (2017) suggest that CAD/CAM improved patient outcomes, especially when compared to traditional techniques such as compression-molded dentures. They further agree that CAD/CAM has the most accurate adaptation and low instances of a misfit.
Rapid prototyping techniques, also referred to as Solid Freeform Fabrication, have been applied in the development of complex 3D models in medicine and recently in dentistry. According to Sun & Zhang, (2013), the advent of 3D imaging, CAD and rapid prototyping has fastened the process of fabricating the complete dentures. Firstly, the master models are digitized using a 3D scanner. Secondly, the wax is modelled and designed using a CAD software before it is finally fabricated (7). There are two main CAD/CAM techniques used:
Milling/subtractive manufacturing/computer-aided manufacturing: in this technique, the prosthesis is milled from a solid block of material and by the 3D design generated by the software. Since the completed dentures are generated from prepolymerized acrylic resins, and the process involves high pressure and heat, chances of polymerization shrinkages are reduced, porosity is nearly eliminated, and the adherence of Candida albicans to the denture base is diminished (8).
3D Printing (Additive manufacturing): the process involves layering to replicate the 3D model generated by the CAD software physically. Once the model is generated, it is segmented and continuously layered until completion (9).
Overview of CAD/CAM
CAD/CAM makes it easy to manufacture prostheses using oral data from patients. Dental impressions are generated using an intraoral scanner which allows for the 3D modelling of a tooth of a given patient. CAD/CAM systems are made up of three main components:
A data acquisition unit- which is responsible for collecting data from the area of preparation. As mentioned above, intraoral scanners and other image acquisition systems help collect data regarding the source which is applied in the generation of digital impressions.
Software- software for the design of virtual restorations is used which makes it easy it easy to compute the milling parameters. CAD software used include 3Shape Dental System and AvaDent digital dentures.
A computerized milling device- this machine is used for restoration from a solid block of material through the milling process or additive practices such as 3D printing (9).
A significant aspect of CAD/CAM techniques is that it dramatically reduces the number of appointments that patients have to make as compared to conventional practices. Only two meetings are required for the patients to get their complete denture sets. The impressions, the jaw relations, tooth mold, maxillary anterior tooth positions, and other tasks are completed in the first appointment which saves time for the actual fabrication of the dentures (11).
After the intraoral scans are conducted, the resulting files are fed into appropriate software where superimposition and analysis follow. The models are then analyzed, and the unnecessary and inaccurate parts of the 3D model are eliminated. The CAM technology, also called subtractive manufacturing, makes use of the computerized numerical control (CNC) machining. The baseplates and the dentitions are fabricated differently, with close adherence to the teeth sizes and dimensions (12).
Regarding printing, after the patient's oral data is collected using the intraoral scanner, the STL files are processed and analyzed to generate final prototypes. The final models are fed into a 3D printer which commences the printing process. After that, the teeth are arranged and tested (17).
Overly, it is essential to note that CAD/CAM techniques have been instrumental in revolutionizing how complete dentures are made. In addition to reducing the number of patient visits and appointments, the process has reduced the time spent during fabrication. As the process is heavily data-centered, the data and images collected can be saved digitally for future use (3).
Comparison between printed and milled denture
As mentioned above, milling is mainly a subtractive technique where the final model is derived from a block of material such as resin while the printing process works the way up, beginning with the model. The main advantage of subtractive technologies is that it is available for all the types of materials, with wax and resins being some of the most popular materials used. In the process of milling, there is always the risk of overmilling especially when the thinnest parts are smaller than the smallest bur. At the same time, the process of milling can be expensive, especially when glass-ceramic blocks and such units are used.
Printing, on the other hand, is mainly an additive technology where the models are built using computerized systems and machines which makes the model more accurate and superior as compared to those generated using milling techniques. Printing makes it easy to reproduce the more delicate details of the models such as the anatomy and the undercuts (15). The process has been known to be more economical as compared to milling. As the process is fast and efficient, it is easier for mass production of units and also makes it easier to produce components such as facial prosthesis (16). While it is quite hard to generate and reproduce complex shapes using a milling technique, it is quickly done using printing. Furthermore, it is faster and designed without wastes. The main disadvantage of printing is that it cannot work with materials such as ceramics and titanium (9).
Generally, CAD/CAM approaches are superior to the traditional methods used previously. In addition to increasing accuracy and completeness of the completed dentures, the process costs less and takes a shorter time (20). Furthermore, the modern process resolves the traditional problem of shrinkages as mentioned above (3).
As seen from the discussion above, the application of 3D printing presents a good opportunity to enhance the delivery of services in the dentistry profession further. The many advantages that the techniques hold over the milling process and the conventional methods makes it better poised to be replicated across the board. This also implies that it is worth noting pursuing it in this project.
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2. Baba N. Materials and Processes for CAD/CAM Complete Denture Fabrication. Current Oral Health Reports. 2016;3(3):203-208.
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4. Bilgin M, Erdem A, Aglarci O, Dilber E. Fabricating Complete Dentures with CAD/CAM and RP Technologies. Journal of Prosthodontics. 2015;24(7):576-579.
5. Steinmassl O, Dumfahrt H, Grunert I, Steinmassl P. CAD/CAM produces dentures with improved fit. Clinical Oral Investigations. 2018;.
6. Chintalacheruvu V, Balraj R, Putchala L, Pachalla S. Evaluation of three different processing techniques in the fabrication of complete dentures. Journal of International Society of Preventive and Community Dentistry. 2017;7(7):18.
7. Sun J, Zhang F. The Application of Rapid Prototyping in Prosthodontics. Journal of Prosthodontics. 2012;21(8):641-644.
8. Janeva N, Kovacevska G, Janev E. Complete Dentures Fabricated with CAD/CAM Technology and a Traditional Clinical Recording Method. Open Access Macedonian Journal of Medical Sciences. 2017;5(6):785.
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11. Han W, Li Y, Zhang Y, lv Y, Zhang Y, Hu P et al. Design and fabrication of complete dentures using CAD/CAM technology. Medicine. 2017;96(1):e5435.
12. Jeong Y, Lee W, Lee K. Accuracy evaluation of dental models manufactured by CAD/CAM milling method and 3D printing method. The Journal of Advanced Prosthodontics. 2018;10(3):245.
13. Dawood A, Marti B, Sauret-Jackson V, Darwood A. 3D printing in dentistry. British Dental Journal. 2015;219(11):521-529.
14. Nejatian T, Sefat F, Johnson T. Impact of Packing and Processing Technique on Mechanical Properties of Acrylic Denture Base Materials. Materials. 2015;8(5):2093-2109.
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