1. Introduction
The use of occlusal devices is regarded as an evidence-based therapeutic option for the management of temporomandibular disorder symptoms [
1]. Bruxism, myalgia of the temporomandibular muscles, and arthralgia of the temporomandibular joints are among the indications for using occlusal splints. The materials used to make such devices should be strong enough to bear significant occlusal forces [
2]. Typically, acrylic resin that has been auto-, heat-, or light-polymerized is used to make conventional occlusal devices [
3]. Recently, additive (3D-printing) techniques have been made possible by manufacturing methods assisted by CAD-CAM technology [
4].
Dental 3D printers employ stereolithography (SLA), polyjet (triple-jetting technology), fusion deposition modeling printing, digital light processing, and selective laser sintering [
4,
5]. SLA printing uses ultraviolet lasers to sculpt resin. In this method, the printing plate travels down in small increments, and the liquid polymer is subjected to an ultraviolet laser that cures a cross section layer by layer. This process is repeated until a dental model is produced [
6].
Polyjet 3D printing is comparable to inkjet printing, except that the printer sprays layers of curable liquid photopolymer onto a construction platform [
7]. More material is placed immediately on the preceding layer when the construction platform goes down one layer. This is repeated until the form is printed [
8]. Triple-jetting technology allows 3D printing of complex things with different materials. Triple jetting uses three printheads to extrude various materials simultaneously, unlike single-nozzle 3D printing. This new method deposits model, support, and contrasting materials precisely and simultaneously. The printheads layer materials to generate intricate shapes and geometries. This approach is useful for prototyping, functioning pieces, and designs that require a mix of materials for mechanical, aesthetic, or functional features. The triple-jetting method allows for more flexibility and efficiency in 3D printing personalized goods [
9].
A powerful laser is used in the additive manufacturing technique known as SLS (Selective Laser Sintering) to selectively fuse powdered materials, layer by layer, to produce three-dimensional things out of a variety of materials, including plastics, metals, and ceramics [
5].
Recent mechanical property studies of printable denture-based resins have been helpful. The PMMA samples had the highest mean surface roughness, Vickers hardness number, and flexural strength but the lowest contact angle. The samples printed using the ASIGA resin had the highest average contact angle and smoothest average surface, while the Dentona 3D printer had the highest mean impact force. NextDent materials had the highest mean bending modulus and the lowest mean Vickers hardness number, flexural strength, and impact strength [
10].
In a separate study on 3D-printed materials, the printed group had the lowest roughness before polishing, the lowest bacterial adherence after 90 min, and superior flexural qualities except for strength. Polished PMMAs had similar surface roughness and microbial adherence [
11].
Flexural testing showed that neither the IvoBase printed group nor Vertex ThermoSens specimens fractured under load, while other groups had flexural strength ratings of 71.7 ± 7.4 MPa to 111.9 ± 4.3 MPa. Flexural strength and surface hardness varied from 67.13 ± 10.64 MPa to 145.66 ± 2.22 MPa, indicating significant differences between the tested materials. CAD/CAM and polyamide had the highest flexural strength, while a third group and polyamide had the lowest. Flexural strength was lowest in 3D-printed materials [
12].
Antibacterial 3D-printed materials have been enhanced using various methods. Stereolithography-printed aluminum nitride composites had good dispersion and antibacterial properties, reducing colony-forming units by 70%. Aluminum nitride-reinforced PMMA resins also had good mechanical properties, with a 12 percent loss of ultimate strength for ceramic fractions of 15 percent and the potential for further strengthening through conventional post-curing [
13].
Three-dimensional-printed materials may have lower mechanical properties than heat-cured PMMA, according to some studies. ZrO
2NPs in 3D-printed resins increased flexure strength, impact strength, and hardness (
p < 0.05) but not surface roughness or elastic modulus. Three-dimensional-printed resins with ZrO
2NPs had better mechanical properties than heat-polymerized acrylic resin. The new nano-composite denture-based resins may be clinically applicable [
14].
Considering these advancements, the present study aimed to investigate the mechanical properties of 3D-printing materials and compare them with the conventional polymethylmethacrylate. Specifically, we evaluated and compared the compression, flexural, and tensile properties of samples manufactured using these two approaches. Additionally, scanning electron microscopy was employed to examine the surface characteristics of the investigated samples. By elucidating the mechanical performance of 3D-printed occlusal splints, this research contributes to our understanding of their potential applications in clinical practice.
4. Discussion
Regarding the compression, our results concluded that the control material (PMMA) has a statistically significant higher Young’s modulus of compression and tensile strength (p < 0.05). Concerning the flexural tests, statistically significant and higher results were shown to be better for the control samples (PMMA) for load at break. Maximum bending stress at maximum load (MPa) is statistically more significant for the 3D-printed samples. Young’s modulus of tensile testing (MPa) is statistically significant for the control samples. The 3D-printed samples had statistically significant values for elongation at break.
The results of our study cannot clearly conclude which of the material behaves better, as the control behaves better in some conditions and the 3D printed using the polyjet technique in other conditions. The polyjet technique has brought some improvements in the splint quality, and it also inherently has other advantages, such as reproducibility of the technique, fast learning curve for the technician, scalability, less residual material and lower toxicity, uniformity of the product, and mechanical proprieties that are the same in all the thicknesses of the splint.
A review of the literature has shown similar results regarding the mechanical features of the polyjet materials having better flexural strength [
18].
To summarize the results of our study, PMMA shows better compression results with higher values, whereas the 3D-printed polyjet material presents better tensile and flexural mechanical results. This anisotropy is in accordance with other studies, which find that modifying the force direction majorly influences the outcome of the mechanical testing [
19].
Additionally, considering the technical aspect of the 3D-printing polyjet technique, the mechanical proprieties differ due to the placement and orientation on the XYZ on the printing tray, according to some studies [
20]. Some papers suggest that the tensile proprieties of the material, when printed along the Z-axis, are improved [
21].
Other parameters, such as printing mode and type of finish, can be modified in the printing process to enhance some proprieties, such as the tensile strength [
22]. Additionally, postprocessing of polishing can also significantly modify the results of the mechanical testing [
23].
Figure 10 shows a compact microstructure generated by PMMA particles diffusing toward one another to form strong connection necks because of the photopolymerization process in the PMMA sample prior to the flexural test. However, at 500× magnification, many pores are seen that are polyhedral in shape and range in size from 80 to 200 nm. In
Figure 10, a pore detail shown at 1000× magnification shows the strong bonding of the photopolymerized PMMA particles. The formation of tiny PMMA filaments on the pores’ surface is evidence of the consolidation process’ effectiveness. The neck between two previous PMMA particles can be seen in the middle-lower half of the image at a high magnification level of 5000×, demonstrating the material diffusion from left to right. The image’s upper left side demonstrates a tendency for pores to form. Because of the energy being dissipated through the pore walls, which enhances the material tenacity, this complex structure may be exceedingly resilient.
SEM microscopy was also used to analyze the fracture microstructure following the flexural strength test. The results are shown in
Figure 10. Complex solicitations, such as lateral compression on the upper side and extension on the lower side of the testing specimen, take place. The energy is lost through the pores and produces intricate micro solicitations over the PMMA necks inside the substance. Due to the breaking line’s progressive elongation observable at low magnification, the failure first manifests itself on the lower side. Average magnification demonstrates interior pore rupture and PMMA neck failure close to the surface. It happens when necks elongate past the point of greatest resistance, as seen at extreme magnification (5000×). On the middle-lower side of the image, a broken neck can be seen, with the fracture margin looking like a “bent plastic sheet” due to the intense elongation effort.
On the plus side, the compression determines the densification of material, and the holes decrease, increasing the overall resistance. The failure, which started on the specimen’s lower side, gradually spreads through the material until it reaches the dense layer, which suddenly becomes exposed during the elongation effort and completely breaks the sample, as seen in the right side of the SEM image at low magnification in
Figure 10. As a result, the PMMA sample’s overall behavior shows good tenacity but just average flexural strength. The addition of a modest amount of micro-sized filler material may help the situation.
The printed PMMA sample was generated layer-by-layer by adding material that was locally photopolymerized. Thus, the SEM image at low magnification (100×) in
Figure 11 presents a mean layer thickness of about 350 µm. Successive layers are bonded together by a transition zone of about 50 µm. Moderate magnification (×500) reveals a compact structure of the PMMA inside of the printed layers having a relative dendritic aspect and a low number of pores. Only a few rounded pores are observed ranging from 10 to 30 µm in diameter. This compact structure inside of the printed layers assures good cohesion of the material. Unfortunately, the high magnification SEM images (1000× and 5000×) in
Figure 11 indicate a fine-grained structure of the layer transition zone, which is a sign of a lower cohesion due to the partial lack of photopolymerization. The presence of small, rounded PMMA grains of about 5 µm might be sensitive to certain effort dissipation through the material.
The flexural strength of the material was tested perpendicular to the inter-layer transition zones, as observed at low magnification (100×) in
Figure 11, and the fracture aspect is complex depending on the microstructural aspects. Therefore, the average magnification (500×) in
Figure 11 reveals the lower part of the sample that supports the elongation effort in the upper side of the image. The transition zones are very tensioned and elongated, proving their behavior as failure promoters, circumstances better observed at higher magnifications (1000× and 5000×). Since they disintegrated, the failure is further propagated into the printed layers. The compressed area situated on the lower side of the image reveals good preservation of the microstructural aspects due to the compression effort, and the failure occurs simultaneously in both layers and transition zones.
SEM investigation proves that lower values obtained for the flexural strength of printed PMMA compared to the bulk samples are generated by the behavior of the transition zones between printed layers. There might be an improvement in the quality of the material with more control of the parameters allowing a better adhesion between printed layers.
Limitations
This study was conducted with some constraints regarding the sample size and the number of products to be compared. Ideally, without any space and physical constraints, more samples in different forms should be used.
Only one type of printing direction was used, and this can be a limitation due to the increased anisotropy of the mechanical proprieties in such cases. Additionally, no tweaking of other proprieties of the printing process was performed, such as surface finish or speed of printing.
Assessing materials with the purpose of dental use must consider the oral environment, which is complex. Aging the materials modifies the proprieties and can drastically change the proprieties and the behavior for the foreseen period of use of the appliance [
24].
Only one type of printer and one type of material were used.
Some studies suggest that there is no statistical significance in different 3D-printed materials using another type of printing method (low-force stereolithography) when assessing mechanical proprieties [
25,
26].
Further studies should, therefore, be performed to investigate whether triple-jetting technology could be suitable for occlusal retainers after conventional [
27], lingual [
28], or aligner-based orthodontic treatments [
29]. Additionally, adding different nanofillers [
30] to the compound could enhance some proprieties, as seen in previous studies [
10,
11,
12,
13,
14,
31,
32].
5. Conclusions
The findings of this study demonstrate that 3D-printed materials, particularly those produced using triple-jetting technology, offer a promising and viable solution for clinical applications. Our research revealed favorable mechanical properties of the 3D-printed material, indicating its potential to replace the conventional heat-cured resin (PMMA) in various applications.
However, further exploration and understanding of 3D-printing protocols and processes are warranted to enhance the mechanical properties of the printed materials. Improvements in these areas will contribute to the optimization of 3D-printed occlusal splints for clinical use.
Additionally, while polyjet technology is still under development, future research endeavors can expand its capabilities and explore its potential in the field of orthodontic occlusal splint manufacturing. The ease of use and comparable mechanical properties of polyjet technology make it an appealing avenue for further investigation.
In conclusion, the present study highlights the significant potential of 3D-printed materials in clinical practice. PMMA showed better compression results with higher values, whereas the 3D-printed polyjet material presented better tensile and flexural mechanical results. However, ongoing research and advancements in 3D-printing protocols and technologies are crucial to unlocking the full range of benefits and improving the mechanical properties of these materials. By capitalizing on these developments, the field of orthodontics can continue to progress towards more effective and efficient occlusal splint manufacturing techniques.