# Comparison of CAD/CAM and Conventional Denture Base Resins: A Systematic Review
## 1 Introduction  
Due to anatomical, physiological or financial restrictions, complete dentures (CDs) still represent the only treatment available for the majority of dedentulous patients. Since the 1950s, complete dentures have been primarily designed and fabricated by hand, using conventional methods that follow well-defined clinical protocols and laboratory techniques. To produce a set of complete dentures, dedentulous patients must have at least five clinical appointments with their dentist to (1) make preliminary and (2) final impressions to make a replica of the oral tissues (gypsum cast), (3) record the maxillomandibular relationship and select the appropriate denture teeth, (4) test the trial wax prostheses and (5) insert the complete prostheses. Once the trial wax dentures are deemed appropriate, the laboratory steps to make the final prostheses begin. The gypsum cast and the trial wax replica are flasked in more gypsum to produce the external surface form. Once all the gypsum is solidified, the trial wax is removed using hot water, producing a mold cavity. The resin to fabricate the final denture is then introduced into the mold cavity within the flask by either pressure or injection. Each technique has advantages and disadvantages. The quality of the prosthetic resin material obtained is strongly influenced by the steps involved in handling the resin. To date, acrylic resin polymethylmethacrylate (PMMA) is the most popular denture base material [1]. PMMA resin best meets all the specifications for an ideal denture base material. Its advantages range from easy repair to good appearance and reasonable cost. It has simple handling characteristics and polymerization is initiated by mixing polymethylmethacrylate (polymer) and methyl methacrylate (monomer) [2]. The polymerization of this type of PMMA resin is completed by application of heat. When this resin is used for dental prostheses, it shows linear distortions by contraction with a theoretical shrinkage of 6%. When using the conventional technique, internal stresses are created during the final heat application during polymerization. These stresses are released when the prosthesis is removed from the gypsum mold and flask, resulting in twisting, bending and dimensional variations of the denture bases compared to the cast. Clinically, these deformations result in a loss of accuracy and retention of the complete dentures. There is also a loss of precision in the occlusal relationship as compared to the wax trial prosthesis [3]. Furthermore, the chemical reaction between the monomer and the polymer is never complete. The residual monomer that remains after the polymerization process can alter the mechanical properties of the resin [4]. The released monomer is also suspected of being responsible for allergic or irritating chemical reactions such as burning sensations in the mouth, stomatitis, edema or ulceration of the oral mucosa [5]. With the development of digital technology, computer aided design and manufacture (CAD/CAM) techniques are increasingly popular in dentistry. Thirty years after the first articles on digital removable prostheses, many systems now exist to both reduce the number of patient appointments needed and the laboratory time needed for prosthesis fabrication [6; 7; 8; 9]. The process of manufacturing CDs with computer-assisted technology involves the acquisition of clinical information and the digital design of prostheses on computer software. CDs can be produced using an additive (3D printing) or subtractive (milling) process. The most commonly used method at present is the subtractive method [10; 11; 12; 13]. In the subtractive method, the prostheses are milled from proprietary resin discs that are polymerized under high pressure and high temperature [14]. CAD/CAM techniques for denture fabrication have many clinical and laboratory advantages [15; 16; 17; 18]. The computer-aided design and manufacture of complete prostheses simplifies the laboratory protocol, stores data and reduces the number of appointments required.