AS: artificial saliva

CAD/CAM: computer-aided design/computer-aided manufacturing

ISO: International Organization of Standardization

PMMA: polymethyl methacrylate

STL: stereolithography, standard triangle language, standard tessellation language

3D: three dimensional

The increasingly popular CAD/CAM (computer-aided design/computer-aided manufacturing) methods save a lot of effort and provide greater comfort for the patient.^[1,2] They are divided into two main groups – additive and subtractive manufacturing.^[3] With the subtractive method, the denture base is milled from a pre-polymerized resin block. 3D printing or additive manufacturing (AM) is based on stereolithography (SLA) and encompasses techniques that fabricate objects layer by layer.^[4]

Dental resins for removable dentures must be resistant to volume changes under all conditions and not change their dimensions over time. Volumetric changes are expressed in polymerization shrinkage, which is compensated by the significant water sorption of this type of material.‌^[5] This might seriously affect the stability of the denture during chewing and cause the dental resin used to manufacture it to age. Removable dentures are widely preferred by patients who cannot afford more expensive prosthetic restorations, such as implant-supported fixed prostheses.^[6] Dentures have been manufactured by different types of acrylics, including the conventional polymer polymethyl methacrylate (PMMA) and the popular nowadays three-dimensional (3D) printed resins. Water sorption and water solubility often occur because these prosthetic restorations are constantly immersed in saliva and always have interactions with oral fluids.^[7,8]

Alternating processes of imbibition and drying of acrylics lead to internal stresses and fatigue. As a result, dental resins undergo significant dimensional changes. The water diffuses into the dental resin and inflicts a gradual expansion and volume increase, which may cause aging of the material and discomfort during masticatory function.^[9,10] Denture-base resins have low water solubility, which results from the leaching of unreacted monomers and soluble additives into the oral cavity. This is an undesired property and may cause soft tissue reactions.^[10,11]

The mechanical characteristics of PMMA resins for the fabrication of partial and complete removable dentures include satisfactory tensile strength (48-62 mPa) and compressive strength (75 mPa).^[12,13] Light-curing plastics have lower values than these indicators.^[10] Strength qualities are determined depending on several factors, such as the composition of dental resin, degree of polymerization, technology protocol, water sorption, and condition storage. Ideal dental resins should have high impact strength to prevent the risk of breakage when the normal masticatory function is applied. Unmodified acrylics are more fragile, and the addition of plasticizers aims to improve their mechanical qualities.^[14,15]

A simulation represents an imitation of a system of a specific type of process over time. Simulations require the use of artificial models that represent the main characteristics of the selected process, and they are usually computer-based.^[16]3D (three-dimensional) simulations involve the application of a 3D computer graphics process, which requires the production of a mathematical network using a specialized program. This product can be presented as a two-dimensional image through rendering or used to digitally simulate a physical phenomenon.^[17]

Nowadays, there are various software programs for 3D simulations. The specialized program ANSYS is a software package, whose purpose is to solve practical problems in various engineering fields.^[18] It is focused on linear and non-linear equations from solid body mechanics, fluid mechanics, and thermodynamics, as well as issues related to the electromagnetic properties of different materials. The abbreviation ANSYS comes from “ANalysis SYStem” and was founded by Dr. John Swanson, the company developer of ANSYS versions from the first to version 5.1.^[19]

The aim of the current study was to investigate and compare the changes occurring in two types of denture base materials before and after immersion in artificial saliva, subjected to deformation, tension, and stress under 3D-simulated conditions.

Based on the conducted 3D-simulation research on the experimental bodies placed under different conditions: deformation, tension, and stress, respectively for the initial phase after drying, before immersion, and after staying in artificial saliva for 1 month, we performed statistical processing of the data using descriptive analysis and one-way ANOVA analysis (Table 1, 2).

The results of the one-way ANOVA statistical method showed that the standard deviation for Vertex BasiQ 20 was slightly larger but given the upper and lower limits found for the applied strain in both types of materials and the average values, no significant differences were observed. The value for P is greater than 0.05, therefore, the differences in deformation at an applied pressure equal to a force equal to 700 N are not significant.

In the one-way ANOVA, as expected, the differences between the two types of materials were not significant (Table 3, 4). The mean voltage and standard deviation values found are almost identical, the difference being minimal. The p-value is greater than 0.05, that is, the difference is not statistically significant. The deformation of the test body when loaded with a bending moment is represented by the 3D illustration (Fig. 4).

During the stress test in the initial phase, almost equal average values were obtained, establishing a visible but not particularly large difference in the standard of inclination (Tables 5, 6). The lower and upper limits are close in value, and ANOVA analysis confirms that the differences between the two groups of materials are not significant in this case.

In the case of deformation on experimental samples that have been in artificial saliva for 1-month, significant differences in the average values, small differences in the standard deviation, and visibly different upper and lower limits are found according to the type of material (Tables 7, 8).

ANOVA analysis confirmed statistically significant differences in the studied material groups, with the P-value being much less than 0.01. Fig. 5 presents visually a 3D illustration of the deformation under the uniaxial loading of the experimental body.

Fig. 6 presents the data for the average values, because of the comparative simulation study of the two groups of plastics. There was a significant difference between the means (0.79 for NextDent, 0.52 for Vertex), and there were significant differences in the standard deviation between the two types of materials, as well as in the measured lower and upper limits.

The means are almost identical, with a slightly larger difference in the standard deviation. The established upper and lower limits are also almost equal (Tables 9, 10). As expected, the one-way ANOVA analysis confirmed that the differences between the two types of plastics were not significant as far as the applied stress was concerned.

Fig. 7 represents a 3D illustration of stress under uniaxial loading of the test body.

The mean and standard deviation have minimal differences, with close lower and upper limits. The P-value was greater than 0.05, and ANOVA analysis confirmed that differences between materials were not significant in this case (Tables 11, 12).

As a result of the ANOVA analysis, the following results were obtained – there is a difference between the two types of materials in the case where they were kept for 1 month in artificial saliva and a point pressure was applied to them, with a force equal to 1500 N (Fig. 8).

The aim of the current study was to evaluate the dimensional changes of two types of denture base materials, immersed in artificial saliva for different periods, after applying 3D simulated tests for deformation, tension, and stress. The results from the conducted experiments support the null hypothesis – there is no statistically significant difference between the tested samples.

The documented nominal values for deformation, tension, and stress not only align with the findings reported in existing literature^[4,9,12] but also serve as a foundation for the exploration of innovative avenues in the field. Al-Dwairi et al.^[20] illuminate a compelling consideration for practitioners to explore the utilization of plastics in 3D-printed removable dentures, even in the light of their comparatively inferior mechanical properties when compared to PMMA. This underscores the need for a nuanced evaluation that balances material properties with the potential benefits of the 3D printing approach.

Prpić et al.^[21] further expand on this perspective by suggesting that the optimization of 3D-printed plastics could be achieved through strategic modifications or reinforcements with nanoparticles. This strategic enhancement, as proposed, holds the promise of unlocking the full potential of the digital method in denture fabrication, emphasizing the importance of continual refinement in materials and methodologies.

Gad et al.^[22] shed light on the impact strength dynamics of thermosetting plastic compared to unmodified 3D-printed resin. The observed influence of the layer-by-layer printing process and the specific printing angulation offers valuable insights into the intricacies of the manufacturing process. The parallel loading direction applied in both their study and our stress simulation study further establishes a consistent correlation, adding robustness to the collective body of knowledge.^[23,24]

Consistent with the findings of Altarazi et al.^[25], the examination of 3D-printed polymers indicates reduced strain levels in contrast to heat-polymerized PMMA. The discussion expands to highlight the multifaceted impact of residual monomer levels and water absorption during the heat cycle on surface hardness and deformation. These nuanced factors emphasize the importance of meticulous material considerations and processing parameters in the pursuit of optimal outcomes.^[26–29]

The findings in the studies by Aati et al.^[30] and Gad et al.^[31] contribute valuable insights into the potential enhancements achievable through the incorporation of nanoparticles, specifically ZrO₂NPs and SiO₂NPs, respectively. The role of chemical composition, type, and concentration of nanoparticles, as expounded by Hada et al.’s research^[32], further underscores the need for a thorough understanding of these variables in shaping the mechanical properties of 3D-printed materials.

The exploration into the laboratory protocol modifications for polyamide prosthetic base materials^[33] introduces a dimension of process refinement. The discovery of a smoother surface resulting from protocol adjustments suggests a ripple effect on various factors and conditions during the fabrication process. These findings align with those of other researchers who have emphasized the influence of different laboratory methods on the surface texture of tested materials^[34], emphasizing the intricate interplay between methodologies and material outcomes in dental research.

A 3D-simulation study was conducted on two groups of test specimens, and subsequent data processing allows us to draw the following conclusions: the 3D deformation simulations revealed that thermosetting acrylics, which had been immersed in artificial saliva for one month, exhibited a slightly higher resistance. The reported values for both types of dental resin meet acceptable standards for their use in the production of removable dentures, affirming the satisfactory mechanical properties of 3D-printed dental resin. In the bending moment stress simulation study, the average values for the experimental 3D-printed test samples were slightly higher than those recorded for the conventional counterparts. However, these values remain within the limits set by ISO standards for the clinical application of this material.

The authors declare no conflicts of interest.

Conceptualization: M.D.; methodology: M.D., R.R., and A.V.; software: R.R. and A.V.; validation: R.R. and B.C.; formal analysis: R.R. and B.C.; investigation: R.R. and M.D.; resources: M.D. and R.K.; data curation: B.C. and R.K.; writing the original draft preparation: M.D.; writing the review and editing: A.V.; visualization: M.D.; supervision: A.V. and R.R.; project administration: B.C.; funding acquisition: R.K.

The authors have no support to report.

The authors have no funding to report.

The authors have declared that no competing interests exist.