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Analysis of compressive strength of occlusal splints manufactured with three liquid resins at three angles of orientation on 3D printer

The aim of this study is to analyze the compressive strength of occlusal splints manufactured with three different liquid resins in a 3D printer and in three angles of orientation. The resins used here are (n=12): Resilab Clear (Wilcos do Brasil, Petrópolis, RJ, Brazil), Prizma Smart Print Bio (Makertech Labs, Tatuí, SP, Brazil), and Cosmos Splint (Yller Biomateriais, Pelotas, RS, Brazil); each resin group are divided into three subgroups according to orientation of manufacture: 0, 45 and 90 degrees. A dental manikin was scanned and the file was used to manufacture a steel hemiarch model. This model was used to design the occlusal splints and as a basis for the tests. The splints were designed with flat occlusal surface and minimum thickness of 2 mm. The compressive test was performed with constant force of 200N, velocity of 0.5 mm/min, along the entire occlusal surface until fracture. Results show no difference between the resins, regardless of orientation of manufacture. The orientation showed no intragroup effect for resins Prizma Smart Print Bio and Resilab Clear; the resin Research, Society and Development, v. 11, n. 3, e40811326820, 2022 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v11i3.26820 2 Cosmos showed larger compressive strength for the samples manufactured at 45 degrees than at 0 degree. The 90-degree samples were intermediary and showed no difference from other angles. This study shows that the three resins had similar behavior in terms of compressive strength, except for the intragroup effect of orientation of Cosmos Splint resin, where plates manufactured at 0 degree performed worse than those at 45 degrees.


Introduction
The occlusal splint is a removable device used to improve the temporomandibular joint stability and enable a functional occlusion, which reorganizes the neuromuscular reflex activity (Maia, 2014). These devices are also used to protect the teeth of patients with bruxism from wear and excessive occlusal forces, and to minimize the symptoms of temporomandibular disorders.
They can also be used to test a new occlusal dimension of patients in need of extensive prosthetic treatment. (Reymus & Stawaczyk, 2020) Traditional manufacturing techniques of occlusal splints require several laboratory steps, which increases the odds of errors and distortions. In the last years, new technologies enabled the manufacturing of digitized occlusal splints that improve and simplify treatment (Vasques, 2018). This manufacturing process can either be subtractive, with the use of milling procedure, or additive, with the use of 3D printing. (Reymus et al., 2020).
The additive method of manufacturing was first described in 2013 and used stereolithography (SLA). Nowadays, SLA and DLP (Digital Light Processing) are commonly used technologies in the manufacturing of occlusal devices and both techniques use liquid resin immediately polymerized with laser (SLA) or light beam in wide areas (DLP) (Reymus & Stawarczyk, 2020).
In 3D printing, the angle of orientation may affect the reproducibility of the test specimens in terms of roughness and strength (Unkovskiy et al., 2018).
Regardless of the manufacturing process, the material used in the occlusal device must meet the mechanical requirements to allow its clinical use and must be resistant to occlusal forces of up to 770 N and to the impact of clenching and grinding of teeth.

Methods
Samples calculation was run on G*Power 3.1.9.4. The groups were defined with 12 samples for each resin type and 4 samples for each angle. Therefore, each subgroup was composed of 4 test specimens for each angle, totaling 36 occlusal splints.
The test specimens were manufactured using DLP technology in a MoonRay 3D printer (SprintRay Inc. Los Angeles-CA,  The test specimens were designed as hemiarches; the splint's occlusal surface is flat and encompasses the region from the upper right second molar to the upper right first premolar. The splints measure 35mm X 12mm X 5mm and have minimum thickness of 2 mm, considering the longest cuspid (Vasques, 2018).
The test specimens were manufactured in three different angles in the 3D printer; 0 degree (4 test specimens; Figure 2    After printing, the excess resin was removed with 2 isopropyl alcohol (Isopropanol, Votorantim, São Paulo -Brazil) baths (Berli et al., 2020): the first bath for 3 minutes, followed by a second bath for 2 minutes, replacing the alcohol between procedures.

Results
The Kruskal-Wallis' test showed no significant difference between resins Resilab Clear (Wilcos), Prizma Smart Print Bio (Makertech Labs) and Cosmos Splint (Yller) used in the occlusal splints, regardless of angle of orientation, as seen in Table 1 and Graph 1.

Discussion
Studies show that 3D printed occlusal splints are a safe choice because, compared with conventional and milled splints (Prpic et al., 2019), they are more precise and present good compressive strength, and can be used in the clinical practice (Tahayeri et al., 2018). Other studies also show good precision and accuracy of the digital light processing (DLP) technique in the manufacture of occlusal splints (Brown et al., 2018;Kim et al., 2018;Sherman et al., 2020;Emir et al., 2021). Based on the mentioned results, the present study used the DLP technology as printing method.
Studies show that the orientation of printing influences strength and mechanical properties. Some of these studies show larger strength values for the vertical orientation (Väyrynen et al., 2016;Kebler et al., 2021;Nold et al., 2021).  Marcel et al., 2020). However, the methods used in the mentioned study are different from the methods used here.
The present study aimed to analyze the compressive strength of occlusal splints manufactured with three different resins in 3D printer in three different manufacture orientations: 0º, 45º and 90º degrees. Unkovskiy et al. (2018) used a similar method, but the test specimens were designed as bars and the compressive test was done in three points. In the present study, the test specimens were designed as plates on top of a dental model and the compressive test was done along the entire test specimens.
Here, results show that the printing orientation can affect the structure of splints manufactured with Cosmos Splint resin; Unkovskiy et al. (2018) results also show that the angle of manufacture influences the strength of the 3D-printed material.
Regarding angle of manufacture, Shim et al. (2020) show that a 90-degree orientation produces the smallest error rates for length and the 45-degree orientation, the largest error rates for thickness; flexural strength increases according to 90 <45<0. Unkovskiy et al. (2021) obtained better mechanical properties with 0 degree. Rubayo et al. (2020) also obtained better results with 45 and 0 degree, corroborating the present study's findings; however, the 90-degree test specimens required less support and used a smaller amount of material in the manufacturing. Studies that tested flexural strength obtained larger values for the orientation of 45 degrees (Hada et al., 2020;Gao et al., 2021;Grymak et al., 2020).
In the present study and on Hirai et al. (2017) and Vasques, 2018, the occlusal splints were designed with 2 mm of thickness from the longest cuspid. Kurt et al. (2012) designed the occlusal splints with 3 mm of thickness to quantify the wearing on different plates.

Conclusion
The three resins showed the same behavior regarding compressive strength, except for the intragroup effect of orientation with the Cosmos resin, where splints manufactured at 0 degree performed worse than those manufactured at 45 degrees.
More studies using other methodologies, other resins and different angulation orientations are needed to prove the best effectiveness when making plates in 3D Printer.