Abstract
Aim:
This study delves into how different printing orientations following polishing and coating affect surface roughness and color stability, aiming to refine fabrication methods and enhance patient satisfaction.
Study Setting and Design:
Twenty-seven polymethyl methacrylate (PMMA) resin samples were designed with dimensions of 10 mm × 2 mm and were 3D printed with three different printing orientations (0°, 45°, and 90°), followed by support removal and polishing. After the evaluation, the samples were coated with the layer of PMMA resin.
Materials and Methods:
Standard tessellation language files as the basis for sample preparation were generated using the Autodesk Meshmixer software and exported to 3D printer for the AM of the specimens. The samples underwent assessment for surface roughness and color stability, forming the three groups with different printing orientations. Subsequently, each sample was coated with unpolymerized PMMA resin and polymerized with ultraviolet light, forming the other three groups following coating. After this process, the samples were reevaluated for surface properties, forming the basis of the study’s six distinct groups.
Statistical Analyses Used:
The data was tabulated and evaluated for the mean and the standard deviation.
Results:
Upon statistical analysis, the surface roughness of 3D printed polished PMMA resin polished and coated PMMA resin for all printing orientations (P < 0.001). The surface roughness was highest at a 45° angle, followed by 90°, with the lowest roughness observed at 0°. Similarly, in polished and coated PMMA resin, roughness was significantly higher at 45° compared to 0° and 90° but no significant difference between 0° and 90° (P > 0.05). For the color stability parameter, color change (∆E) values showed no significant difference for the three printing orientations (P > 0.05) and among the six study groups.
Conclusions:
Different printing orientation markedly affects surface roughness and color stability, while coating with PMMA resin had significantly reduced the surface roughness without a significant impact on color stability. Thus, meticulous selection of printing orientation is essential for achieving desired surface properties.
Keywords: Three-dimensional printing, color stability, surface roughness, ultraviolet polymerization
INTRODUCTION
The printing orientation stands out as a crucial variable impacting additive manufacturing (AM) outcomes, necessitating precise adjustment during three-dimensional (3D) printing for optimal results. It dictates the layer-by-layer build direction, creating a surface geometry that significantly influences the surface’s characteristics. In this way, 3D printing orientation contributes greatly to denture surface properties.[1] The application of coatings and mechanical or chemical polishing techniques are frequently used to decrease surface roughness, thereby lessen bacteria adherence on denture surfaces.[2] A surface roughness of 0.2 µm is recognized as the standard for clinical acceptability value, based on several studies.[3,4,5] Diverse surface treatments aimed at reducing roughness may impact liquid sorption, consequently affecting the resin’s color stability.[1]
Numerous previous studies have evaluated the surface roughness and color stability of 3D-printed denture base resins using a fixed printing orientation, primarily after polishing. However, limited attention has been given to the evaluation of surface roughness and color stability of 3D-printed denture base resin when printed at three varying printing orientations, i.e., 0°, 45°, and 90° following polishing and coating.
The present study investigated the impact of three different printing orientations on the surface roughness and color stability of 3D-printed resin, i.e., 0°, 45°, and 90°, subsequent to polishing and coating with a thin layer of unpolymerized resin. The null hypothesis dictates that the surface roughness and color stability of the 3D-printed polymethyl methacrylate (PMMA) resin following polishing and coating are not influenced by the different printing orientations used for the fabrication.
MATERIALS AND METHODS
Specimen preparation
To evaluate the surface roughness and color stability, a total of 27 3D-printed PMMA resin specimens with nine in each printing orientation (0°, 45°, and 90°) were created as per the American Dental Association (ADA) Specification no. 12 and had dimensions of 10 mm by 2 mm.[6] The standard tessellation language files for the preparation of the samples were generated using the Autodesk Meshmixer software (version 3.5.474, USA) and exported to 3D printer (Ackuretta FreeShape 120, Taiwan) for the AM of the specimens [Figure 1].
Figure 1.
Standard tessellation language file formation using Autodesk Meshmixer software
3D Accuprint denture resin (D-TECH, India) was used for the fabrication of the specimens with 3D printer (Ackuretta FreeShape 120). The samples were fabricated with three printing orientations (0° [Figure 2a], 45° [Figure 2b], and 90° [Figure 2c]) and a 100 µm layer thickness. An ultraviolet (UV) light-polymerization unit was used to polymerize the material as instructed by the manufacturer.
Figure 2.
(a) Standard tessellation language (STL) file with 0° printing orientation. (b) STL file with 45° printing orientation. (c) STL file with 90° printing orientation
Using low-speed rotary instruments (Marathon M4 Micromotor, Saeyang, Korea), the support structures were removed with tungsten carbide (Dentsply Sirona) and acrylic trimming burs (pink and white cherry stone, Dentsply). An ultrasonic bath with 99% isopropanol was used to clean the 3D-printed samples, with two cycles of 2 min each, to remove the residual surface monomer and then polymerized in the UV polymerization unit (Formlabs, China). To give each specimen a polished finish, the axial walls and bottom of the sample were ground using SiC paper (grit 800, China), and following that, the samples were washed with water.
Digital calipers (Baker Steel, India) were used to measure the diameter and thickness of all the nine specimens printed using different orientations.
Surface evaluation
The precoating surface roughness evaluation was performed on the finished surface of all the specimens. A profilometer (Bruker Dektak XT Profilometer with Vision 64 software, USA) was used, and the roughness average (Ra) was measured by scanning 5 times, each with a threshold of 0.25 mm.
The precoating evaluation for the color stability was done using spectrophotometer (UV-Vis Spectrophotometer, V-670, Tokyo, Japan). Each sample was dried thoroughly by blotting with tissue paper before color analysis. It was placed in the viewport of the spectrophotometer and L, a and b values for each sample were generated.
Then, the coating procedure was performed using the same unpolymerized resin used in the fabrication of the specimens with a fine bristle brush (series 69 F6, Camlin). Using a bristle brush in both directions, the resin was applied in thin layers until the visible staircase effect was filled [Figure 3]. The coating over the specimen was polymerized using the UV polymerization unit. The postcoating evaluation for the surface roughness and color stability was performed for all the specimens.
Figure 3.
Coating of the samples with liquid polymethyl methacrylate (D-TECH) resin in bidirectional strokes
Spectrophotometers utilize the color systems established by Munsell and the Commission Internationale de l’Éclairage (CIE). This study utilized the CIE L, a, and b* system, which is recommended by the ADA, and according to it, all the colors in nature are based on three colors: red, blue, and green. In this system, L indicates the lightness of the sample, a represents the green–red axis (−a = green; +a = red), and b* corresponds to the blue–yellow axis (−b = blue; +b = yellow).
The following equation was used to measure the change in color: ΔE = ([ΔL]2+ [Δa]2+ [Δb]2)1/2 Where ΔL = L (after coating) − L (before coating), Δa = a (after coating) − a (before coating), Δb = b (after coating) − b (before coating). The National Bureau of Standards has quantified the levels of color change (ΔE).
Statistical analysis
The data were electronically tabulated and evaluated as mean and standard deviation, and the one-way ANOVA test, post hoc Tukey tests, and independent t-test were utilized to identify variations across groups (P < 0.05). The normality verification was done using the Shapiro–Wilk test, and the result showed that the data were normally distributed among the various groups. The statistical analyses were carried out through statistical software(IBM SPSS statistics for windows, v23; IBM corp).
RESULTS
The surface roughness analysis revealed that for each angulation, the surface roughness value of 3D-printed polished PMMA resin material was significantly greater than that of 3D-printed polished and coated PMMA resin material (P < 0.001). The mean surface roughness of the specimens between the study groups was compared as shown in Table 1 and Graph 1.
Table 1.
Comparison of surface roughness between three-dimensional-printed polished polymethyl methacrylate resin and three-dimensional-printed polished and coated polymethyl methacrylate resin in each angulation
| Angle (°) | Polished PMMA, mean±SD | Polished + coated PMMA, mean±SD | Difference | P |
|---|---|---|---|---|
| 0 | 0.28±0.10 | 0.03±0.01 | 0.25 | <0.001* |
| 45 | 10.27±0.31 | 0.21±0.06 | 10.06 | <0.001* |
| 90 | 4.17±0.45 | 0.05±0.01 | 4.13 | <0.001* |
*A significant difference at P≤0.05. Independent t-test. SD: Standard deviation, PMMA: Polymethyl methacrylate
Graph 1.
Comparison of surface roughness between three-dimensional (3D)-printed polished polymethyl methacrylate (PMMA) resin and 3D-printed polished and coated PMMA resin in each. PMMA: Polymethyl methacrylate
Table 2 reveals that in 3D-printed polished PMMA resin material, the surface roughness value at 45° was significantly greater than surface roughness values at 0° and 90° angulations (P < 0.001). Furthermore, the surface roughness value at 90° was significantly greater than the surface roughness value at 0° angulation (P < 0.001).
Table 2.
Comparison of surface roughness between three different angulations in three-dimensional-printed polished polymethyl methacrylate resin material
| Angulations (°) | Mean±SD | P | Pairwise comparisons |
|---|---|---|---|
| 0 | 0.28±0.10 | <0.001* | 0° versus 45°: <0.001* |
| 45 | 10.27±0.31 | 0° versus 90°: <0.001* | |
| 90 | 4.17±0.45 | 45° versus 90°: <0.001* |
*A significant difference at P≤0.05. One-way ANOVA test, Post hoc Tukey test. SD: Standard deviation
Table 3 reveals that in 3D-printed polished and coated PMMA resin material, the surface roughness value at 45° was significantly greater than surface roughness values at 0° and 90° angulations (P < 0.001). However, no significant difference was observed in the surface roughness value at 0° and 90° angulations (P > 0.05).
Table 3.
Comparison of surface roughness between three different angulations in three-dimensional-printed polished and coated polymethyl methacrylate resin material
| Angulations (°) | Mean±SD | P | Pairwise comparisons |
|---|---|---|---|
| 0 | 0.03±0.01 | <0.001* | 0° versus 45°: <0.001* |
| 45 | 0.21±0.06 | 0° versus 90°: 0.720Ϯ | |
| 90 | 0.05±0.01 | 45° versus 90°: <0.001* |
*A significant difference at P≤0.05, ϮNonsignificant difference. One-way ANOVA test, Post hoc Tukey test. SD: Standard deviation
Table 4 reveals the effect of the polishing and coating on the color stability, where there was a nonsignificant difference in the ∆E values among three different printing orientations (P > 0.05), following the coating procedure was performed on the specimens as shown in Graph 2.
Table 4.
Comparison of △E values of three angulations
| Angulations | Mean±SD | P | Pairwise comparisons |
|---|---|---|---|
| 0 | 5.09±3.19 | 0.128 | 0° versus 45°: 0.108 |
| 45 | 9.33±6.04 | 0° versus 90°: 0.574 | |
| 90 | 7.13±2.78 | 45° versus 90°: 0.524 |
One-way ANOVA test, Post hoc Tukey test
Graph 2.
Comparison of delta E value of three angulations
DISCUSSION
This study aimed to evaluate the effect of printing orientation on the surface roughness and color stability of 3D-printed PMMA resin, following coating with a thin layer of unpolymerized PMMA resin, used for the formation of the specimens. Shim et al. investigated the effects of printing orientation – horizontal, oblique, and vertical – and found that the orientation significantly influences the properties of the printed resin.[1]
Coating a layer of this resin significantly reduced the surface roughness, while it had no significant difference on the color stability of the PMMA resin.
The surface roughness of the resin influences the discoloration, water sorption, and hygiene,[7] while color stability affects the esthetics of the prosthesis fabricated with PMMA resin that altogether related to the longevity and maintenance of the prosthesis.[8]
The result obtained for surface roughness of samples fabricated with 45° orientation coated with PMMA resin was 0.21 µm; this agrees with the study conducted by Kraemer Fernandez et al., who stated that the surface roughness reduced to Ra = 0.16 ± 0.04 µm for the 3D-printed samples fabricated with 45° printing orientation following coating with the liquid resin. This may be due to the layer-by-layer deposition in a stepwise orientation, forming repeated angled ridges on the specimen’s surface.[3]
The current study discovered that typical surface features were produced by printing orientation. Recurring craters were seen on the surfaces produced by printing at 0°, angled ridges were seen at 45°, and irregular, amorphous shapes were seen at 90°. Compared to the other groups, the 45° printing group’s surface roughness was statistically higher. The unique features between the printing layers of the 45° orientation group could be the cause of the variances. This printed group’s layers were connected sequentially, and the step edges between the layers could have resulted in problems, thus causing surface roughness. The study yielded similar outcomes, concluded by Shim et al., that the surface roughness was maximum for the 45° orientation, followed by 90° and 0°.[1]
To eliminate the drawbacks of the earlier approach and to diminish the surface roughness, coating materials could be used for additively produced denture bases and as an alternative to conventional polishing.[3,9] Feng et al. discussed that the application of coatings could potentially fill in micropores and fissures, compensating for surface defects left after polymerization, thus leading to modified surface properties.[10]
In light of the results of this study and the arguments made in previously published studies, the coating method that has been recommended could be used in place of the traditionally polished specimens. Moreover, the use of fluid denture resin for coating application shortens production times and eliminates the need for glazing material purchases, which promotes more effective digital workflow from a clinical and financial standpoint.[4]
The threshold for clinically acceptable color change in dental materials is typically set at ∆E*a*b ≤3.32. The ∆E* values represent the extent of color change in the prostheses following simulated coating. A ∆E* value below 1 is regarded as visually imperceptible. Values ranging from 1 to 3.3 are considered clinically acceptable, with the color change being detectable only by trained observers. However, when the ∆E* value exceeds 3.3, the color alteration is deemed clinically unacceptable and becomes easily noticeable to the untrained eye.[11]
The present study showed a nonsignificant difference of ∆E values among the three printing orientations following coating of the specimens. The ∆E value for all the three-printing orientation was more than 3.3, which dictates that the color stability of the 3D-printed PMMA resin is weak as compared to other materials used in the fabrication of the prosthesis.
The present study showed similar results to that of a study conducted by Lee et al. to compare the influence of printing layer orientation on the color stability and stain resistance of a 3D-printed resin material. They concluded that the layer printing orientation influenced the color stability and stain resistance of the 3D-printed resin, wherein the ΔE values in the 0° subgroups were lower than those in the 45° and 90° subgroups.[12]
Fontes et al. in a study concluded that color stability can be affected by various factors such as water absorption, chemical reactivity, surface roughness, and the surface energy of a material.[13]
Zuo et al. concluded that coating can effectively minimize the water absorption and solubility of the resin while also significantly enhancing the color stability of the denture base resin.[14]
In the present study, coating of the prosthesis with PMMA resin, fabricated with different printing orientations, may be used as a proposed procedure to improve the color stability. The main constraint on this research is the difference in form and size between the study specimens and the prosthesis used in a clinical setting.
The shape of the object to be printed affects the surface roughness; the correlation between the shape of the object to be printed and the printing angulation on surface roughness and the thickness of the coated unpolymerized resin layer are considered limitations and topics for future study. Subsequent research with 3D-printed prostheses manufactured with other printing orientations and increased sample size are required to confirm the clinical significance of these findings.
CONCLUSIONS
Different printing orientations had affected the surface features of the 3D-printed PMMA resin, and the following conclusions can be made:
The surface roughness value of 3D-printed polished PMMA resin material was significantly greater than that of polished and coated PMMA resin material
The specimens with the highest surface roughness were those printed at 45°, followed by 90° and 0° for polished specimens
While there was no discernible difference between polished and coated specimens printed at 90° and 0°, specimens produced at 45° showed the maximum surface roughness
No statistically significant difference was observed in the ∆E values, among three different printing orientations (P > 0.05), following the coating procedure.
Moreover, thus, the application of coatings along with mechanical or chemical polishing techniques can be used to decrease surface roughness, thereby decreasing bacteria adherence on denture surfaces.[2]
Ethical committee decision letter toward application and protocol number ACDS/IEC/117/2022.
Conflicts of interest
There are no conflicts of interest.
Funding Statement
Nil.
REFERENCES
- 1.Shim JS, Kim JE, Jeong SH, Choi YJ, Ryu JJ. Printing accuracy, mechanical properties, surface characteristics, and microbial adhesion of 3D-printed resins with various printing orientations. J Prosthet Dent. 2020;124:468–75. doi: 10.1016/j.prosdent.2019.05.034. [DOI] [PubMed] [Google Scholar]
- 2.Köroğlu A, Sahin O, Dede DÖ, Yilmaz B. Effect of different surface treatment methods on the surface roughness and color stability of interim prosthodontic materials. J Prosthet Dent. 2016;115:447–55. doi: 10.1016/j.prosdent.2015.10.005. [DOI] [PubMed] [Google Scholar]
- 3.Kraemer Fernandez P, Unkovskiy A, Benkendorff V, Klink A, Spintzyk S. Surface characteristics of milled and 3D printed denture base materials following polishing and coating: An in-vitro study. Materials (Basel) 2020;13:305. doi: 10.3390/ma13153305. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 4.Quirynen M, van der Mei HC, Bollen CM, Schotte A, Marechal M, Doornbusch GI, et al. An in vivo study of the influence of the surface roughness of implants on the microbiology of supra- and subgingival plaque. J Dent Res. 1993;72:1304–9. doi: 10.1177/00220345930720090801. [DOI] [PubMed] [Google Scholar]
- 5.Bollen CM, Papaioanno W, Van Eldere J, Schepers E, Quirynen M, van Steenberghe D. The influence of abutment surface roughness on plaque accumulation and peri-implant mucositis. Clin Oral Implants Res. 1996;7:201–11. doi: 10.1034/j.1600-0501.1996.070302.x. [DOI] [PubMed] [Google Scholar]
- 6.Revised American Dental Association specification no. 12 for denture base polymers. J Am Dent Assoc. 1975;90:451–8. doi: 10.14219/jada.archive.1975.0069. [DOI] [PubMed] [Google Scholar]
- 7.Yoshijima Y, Murakami K, Kayama S, Liu D, Hirota K, Ichikawa T, et al. Effect of substrate surface hydrophobicity on the adherence of yeast and hyphal Candida. Mycoses. 2010;53:221–6. doi: 10.1111/j.1439-0507.2009.01694.x. [DOI] [PubMed] [Google Scholar]
- 8.McCord JF. Contemporary techniques for denture fabrication. J Prosthodont. 2009;18:106–11. doi: 10.1111/j.1532-849X.2009.00439.x. [DOI] [PubMed] [Google Scholar]
- 9.El Samahy MM, Abdelhamid AM, El Shabrawy SM, Hanno KI. Evaluation of physicomechanical properties of milled versus 3D-printed denture base resins: A comparative in vitro study. J Prosthet Dent. 2023;129:797.e1–7. doi: 10.1016/j.prosdent.2023.03.017. [DOI] [PubMed] [Google Scholar]
- 10.Feng D, Gong H, Zhang J, Guo X, Yan M, Zhu S. Effects of antibacterial coating on monomer exudation and the mechanical properties of denture base resins. J Prosthet Dent. 2017;117:171–7. doi: 10.1016/j.prosdent.2016.05.009. [DOI] [PubMed] [Google Scholar]
- 11.Vichi A, Ferrari M, Davidson CL. Color and opacity variations in three different resin-based composite products after water aging. Dent Mater. 2004;20:530–4. doi: 10.1016/j.dental.2002.11.001. [DOI] [PubMed] [Google Scholar]
- 12.Lee EH, Ahn JS, Lim YJ, Kwon HB, Kim MJ. Effect of layer thickness and printing orientation on the color stability and stainability of a 3D-printed resin material. J Prosthet Dent. 2022;127:784.e1–7. doi: 10.1016/j.prosdent.2022.01.024. [DOI] [PubMed] [Google Scholar]
- 13.Fontes ST, Fernández MR, de Moura CM, Meireles SS. Color stability of a nanofill composite: Effect of different immersion media. J Appl Oral Sci. 2009;17:388–91. doi: 10.1590/S1678-77572009000500007. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 14.Zuo W, Feng D, Song A, Gong H, Zhu S. Effects of organic-inorganic hybrid coating on the color stability of denture base resins. J Prosthet Dent. 2016;115:103–8. doi: 10.1016/j.prosdent.2015.07.008. [DOI] [PubMed] [Google Scholar]