Introduction

Conventional complete dentures have been used for centuries and the methods were developed over the years to ensure we get optimum results. However, multiple limitations and disadvantages still exist despite all the efforts to improve them such as the material of choice for conventional dentures. Poly methyl methacrylate (PMMA)1 resins gives superior aesthetics as well as ease of processing and repair, however, on the other hand PMMA also exhibits high polymerization shrinkage which consequently affects the accuracy of the denture. Another reported drawback of PMMA is the suitability for microbial adhesion which leads to infections. Other disadvantages was also reported in the literature including allergy from the leaching residual monomers and mechanical properties that decline overtime especially in the presence of saliva.2 CAD/CAM on the other hand provided dentistry with a new concept in constructing dentures by milling a pre polymerised resin blank with prefabricated or milled teeth bonded to the base. Yet, CAD/CAM denture also have some limitations. The main disadvantage of CAD/CAM is that it wastes a large portion of the blank untouched which eventually gets discarded. Also, the monochromatic feature negatively impact the aesthetics which can be improved by a layering system or by using polychromatic teeth.3 Recently, 3D printing appears to be a revolutionary accepted new technique which might resolve multiple issue that dentists face during the construction of dentures. This technique provides the operator with freedom of designing. Also, it allows rapid customization and utilization of complex geometry in the dental practice. Moreover, 3D printing limit manual labour in dental laboratories by automated manufacturing. High patient satisfaction rate was reported when 3D printing technology was used. Clinically, less pain and ulcer lesions were formed with 3D printed dentures in comparison with conventional ones.4 As the technique is additive, all the printed material is sufficiently utilized with limited waste unlike in CAD/CAM technology. In 3D printing there is a flexibility to use multiple materials to be printed which allows dentist to be creative in constructing the most favorable denture. This can be beneficial to overcome PMMA’s wear weakness by printing cusp tips in metal and combining them with the 3D printed denture.5 The available 3D printing systems for complete removable dental prostheses are FotoDenta denture (Dentamid, Germany) and Dentca 3D Printed Denture (Dentca, USA). The limited resolution and reproducibility of the available printers along with their technical constraints have so far posed obstacles in such manufacturing methods of dental restorations.6 The emerging AM technology is modifying the clinical and laboratory processes of fabricating removable prostheses. The purpose of this paper is to review available literature on 3D printed complete dentures in terms of novel biomaterials, fabrication techniques and workflow, clinical performance, and patient satisfaction.

Method

The methodology included applying a search strategy, defining inclusion and exclusion criteria, and retrieving studies; selecting studies; extracting relevant data; and forming tables to summarize the results. Searches of PubMed, Ovid Medline Scopus, and Embase databases were performed to gather literature published between 2010 and 2022. The search terms used were “Denture” [Mesh] OR “Removable Dental Prostheses” OR “Removable Denture” OR “Complete Denture” AND “CADCAM” [Mesh] OR “CAD/CAM” OR “CAD-CAM” OR “Computer Aided Design and Computer Aided Manufacturing” AND “Milled” [Mesh] AND “3D Printed” OR “Printed” AND “Digital Denture” [Mesh]. The inclusion criteria for selection were articles written in English published between 2010 and 2020 on 3D printed dentures, clinical studies and in vitro studies, technique articles that reported workflow, clinical complications or quality assessment with 3D printed dentures. Exclusion criteria included any articles that failed to involve items described in the inclusion criteria or any article that described repetitive data from another included article was excluded. Additionally, articles on 3D printed removable partial dentures (RPD) or partial dental prostheses (PRDP) were also excluded. The search strategy for this review involved 3 stages: reviewing titles, abstracts, and final selection of articles for full text analysis. Articles selected from the database search were sorted independently by 2 reviewers, and any differences in selection were discussed until a consensus was reached. Upon the reviewers’ agreement, articles that did not meet the predetermined inclusion criteria were excluded. Abstracts of the articles selected at the second stage were independently evaluated by the same reviewers, and articles selected for final analysis were obtained in full text. At the third and final stage, the full text of the obtained articles was analyzed.

Results

Currently, there is a very limited number of in-vitro studies evaluating the materials’ properties and accuracy for Anadioti et al. BMC Oral Health (2020) of 9 3D printing dentures, denture bases and denture teeth. The accuracy of fit between denture base and mucosal tissue is key for the retention of CRDP and long-term success of the prosthesis.7 Milled CRDPs have been shown to have accurate adaptation compared to conventionally processed dentures . With regards to 3DP dentures, an in-vitro study quantitatively compared their tissue surface adaption against the traditional manual method. A wax pattern of a maxillary complete denture was made on a standard edentulous plaster cast using highprecision 3D wax-printing technology (CAD&3DP), and the fit between the wax pattern and the cast was evaluated quantitatively.8 There was no statistically significant difference observed between CAD&3DP group and manual manufacturing group for measurements of deviation between the denture tissue surface and the plaster cast model. Therefore, this study suggested that the use of 3D printing manufacturing to fabricate CRDP for try-in appointments when restoring edentulous jaws, appeared to be clinically acceptable.A shortcoming of PMMA is the great susceptibility to microbial colonization from the highly contaminated oral environment. Several studies recognized the incorporation of approximately 5% weight of TiO2 into acrylic denture base structure has an antibacterial effect. This amount however can cause internal decomposition and weaken the material. 3D printing manufacturing of digital dentures allows the development of new biomaterials with improved properties.9 This issue was addressed by Totu et al., by using of a nanocomposite PMMA 0.4% TiO2 nanoparticles, which inhibit the growth of Candida Scotti strain in standard conditions, according to the toxicity control method (DHA). With 3D printing, the build direction (layer orientation) affects the mechanical properties of the dental restorative material. This is due to the nature of incremental layers in additive manufacturing technology, which may initiate crack propagation and result in a structural failure of the printed material. In an in vitro study, layer orientation was found to affect the compressive strength of 3D-printed composite material. The material printed vertically with the load perpendicular to the layer orientation exhibits a higher compressive strength than a material printed horizontally. Also, it is important to understand that the bond between the layers is weaker than that within the layer. This is explained by the amount of residual stresses and porosities that accumulate during UV polymerization and material shrinkage.10 Choi et al. compared fracture toughness and flexural bond strength between three types of denture-base resins (DBRs), heat cure, CAD-milled, and 3D printed, and four different types of commercial denture teeth (Unfilled PMMA, double cross-linked PMMA, PMMA with nanofillers and 3D printed resin teeth). All specimens were surface treated, bonded, and processed according to manufacturer’s instructions.11 A 4-point bend test, using the chevron-notched beam method, was performed. The results revealed that teeth bonded to heat-cured denture-base resins produced the highest fracture toughness. Teeth bonded to CAD/CAM and 3D printed DBRs showed significantly lower bond strength. The study suggested that despite the increasing popularity of CAD-milled and 3D printed materials, heat-cured denture base resins still produce the highest bond strength to various types of denture teeth. 3D printing of denture teeth is a novel method and uses newly developed materials.12 DENTCA, for example, developed a resin material specifically for 3D printing denture teeth. Chung et al. compared chipping and indirect tensile fracture resistance of 3D printed denture teeth to prefabricated resin denture teeth. 3D printed resin teeth had comparable fracture resistance to some of the conventional prefabricated denture teeth. Cha et al. evaluated the wear resistance of 3D printed resin denture tooth opposing zirconia and metal. The wear behavior of the printed denture tooth resin was comparable to that of prefabricated denture teeth. With regards to improving wear resistance, other authors have incorporated designing of metal functional cusps separate from the denture.13 The separated functional cusp file was printed in metal, and the metal cusp was bonded to the PMMA resin-printed denture base.

With a projected increase of edentulous patients in the future, the need for CRDPs as a treatment modality is recognized. For computer-engineered removable prostheses, a worthwhile consideration is their economic implications as compared to traditional approaches.14 The decrease in chairside, laboratory and overall working time for dentists, technicians and patients is the main factor when assessing cost versus time. The time saved by less human involvement, virtual teeth arrangement, and ability to effortlessly store and reproduce prostheses should be viewed against the increased cost of milling machines and accrual of additional required equipment such as intraoral scanner or proprietary custom trays. A study that was designed in an academic setting stated that despite the initially much higher costs of the materials used to fabricate the digital denture protocol, overall it was determined to be a less costly method of producing CRDP in terms of clinical chairside time and laboratory costs.15 Meanwhile, tabletop 3D printers are much less expensive than a milling center and could be afforded by individual dentists and dental laboratories to offset some of the costs that are currently preventing broad digital denture implementation. When discussing reducing time to improve efficiency, it is important to note that quality should not be compromised. A clinical step considered for elimination with the digital workflow is the try-in appointment. Although digital systems allow for virtual evaluation of the esthetic analysis, this has not yet been proven to be a reliable and adequate replacement for the clinical evaluation of esthetics and phonetics with the patient’s input. Compromising on the esthetic evaluation will lead to patient dissatisfaction and/or remakes.16 Therefore, most published reports recommend clinical try-in for a reliable evaluation. This adds more to the cost and time but may, ultimately, provide better results. When in-house 3D printed denture fabrication becomes streamlined with a two-appointment process that includes reliable virtual esthetic and functional assessment, the amount of waste from conventional or milled workflows will be decreased. The ability to treat edentulism locally but also in lower-income areas or nations, where skilled dental technicians are scarce, will increase and the contribution to public health will be significant considering the comorbidities associated with edentulism.17 Finally, when reflecting on the transformation of dental education with the incorporation of blending learning as well as the generational inclination of today’s dental students with technology, there are initial studies that place the education of digital dentures high in the dental student preferences in a predoctoral setting . The study showed that the digital process was equally effective and more time-efficient option than the conventional process of prosthesis fabrication in the predoctoral program. Another cross-section study that compared predoctoral to postdoctoral students fabricating digital dentures showed that the mean number of appointments needed to insert the prostheses at the predoctoral level was 2.33 (95% CI 2.13–2.53) and 2.45 (95% CI 2.15–2.76) at the postdoctoral resident level, both higher than the 2-appointment solution claimed by the company.18 The reasons for the additional appointments to insert the prostheses were consistent with other reports and included esthetic or phonetic patient dissatisfaction, lack of retention, incorrect occlusal vertical dimension or centric relation as well as operator required teeth try-in evaluation.19 An online survey sent to all of the 50 program directors of postdoctoral prosthodontics programs across the United States revealed that all program directors were aware of current trends in complete denture fabrication using CAD/CAM technology but only 10% or less of complete denture cases are currently processed using the CAD/CAM technology, at either the post- or predoctoral levels . However, plans to add digital denture fabrication into their curricula within the next 1 to 4 years were stated in their responses.

Conclusion

Current innovations and developments in digital dentistry have successfully led to the fabrication of removable dental prostheses using CAD/CAM technologies. 3D printing has the potential to modernize and streamline the denture fabrication techniques, materials and workflows. Current limitations include elimination of try-in appointment without reliable virtual esthetic evaluation, lack of retention with printed polymers requiring reline for clinical acceptability, inability to balanced occlusion that may compromise denture stability or potentially affect bone resorption and long-term color instability that leads to esthetic deterioration. Presently recommended usages for 3D printed complete dentures are interim or immediate dentures as well as custom tray or record base fabrication for conventional workflows. Well designed clinical studies are needed to scientifically prove the claimed advantages of this technology.