Impact of Fabrication Techniques and Polishing Procedures on Surface Roughness of Denture Base Resins

Summary

This article presents a protocol to evaluate the effects of different manufacturing methods (heat-polymerized PMMA, CAD-CAM milled PMMA, and 3D-printed resin) and polishing techniques (600, 800, and 1000 grit silicon carbide abrasive papers) on the surface roughness (Ra) of resin base materials used for complete dentures.

Abstract

This study aimed to assess the impact of various fabrication techniques and polishing procedures on the surface roughness (Ra) of resin-based materials used in the fabrication of complete dentures. A total of 90 specimens were produced from three different resin materials: heat-polymerized polymethyl methacrylate (PMMA) resin, CAD-CAM milled PMMA resin, and 3D-printed resin (n = 30). Each specimen measured 10 mm in diameter and 2 mm in height. The surface roughness (Ra) values of the specimens were initially determined using a contact profilometer following fabrication. Subsequently, each group of specimens was polished with 600-, 800-, and 1000-grit silicon carbide abrasive papers under running water. A second measurement of the surface roughness (Ra) values was then performed. The data were analyzed statistically using the Kruskal-Wallis test, Mann-Whitney U test, Wilcoxon signed-rank test, and paired samples t-test (p = 0.05). A statistically significant difference was identified between the groups in terms of surface roughness (Ra) prior to the polishing process (p < 0.001). However, no statistically significant difference was observed between the milled and heat-polymerized PMMA base materials following the polishing process. The 3D-printed specimens showed the most notable improvement in surface roughness due to the polishing process. Nevertheless, their surface roughness remained statistically significantly higher compared to the other samples, both before and after polishing (p < 0.001). The fabrication method of complete denture base materials was observed to influence surface roughness. The surface roughness values of the base materials fabricated using the 3D printing method were higher compared to those fabricated with milled and heat-polymerized PMMA resin, both before and after polishing.

Introduction

The restoration of edentulous areas is most commonly achieved through the use of partial or complete removable dentures, which serve as an important alternative in cases where implant-supported fixed prostheses are not feasible due to anatomical factors or patient-related conditions such as economic constraints or systemic illnesses¹. The base materials employed in these prostheses are typically resins containing polymethyl methacrylate (PMMA). PMMA is a cost-effective material valued for its ease of processing, repairability, and polishability². It also demonstrates favorable physicochemical properties and satisfactory esthetic outcomes³. Various fabrication methods, such as fluid resin pouring and mold-filling techniques like compression and injection molding, have been employed to produce removable dentures from PMMA resin.

Among traditional methods, the most commonly used fabrication technique is compression molding, also known as the flask press method. It involves placing the resin material into a mold within a flask, followed by pressing it under pressure to fill the mold and achieve the desired shape. The flask pack press method, which has been in use for many years, offers advantages such as ease of application and low cost. However, it also has certain disadvantages, including the requirement for manual labor and time-consuming steps in laboratory procedures, susceptibility to human error, the risk of failing to achieve a homogeneous structure during the mixing and processing of the resin, and polymerization shrinkage. However, with the advent of computer-aided design manufacturing (CAD/CAM) technologies, subtractive manufacturing techniques such as milling have also been employed for their production⁴. Studies have demonstrated that denture base materials produced using the milling technique possess greater flexural strength and base adaptation than those fabricated using conventional methods^5,6. These improvements can be attributed to the elevated pressure and temperature levels applied during the fabrication of milled PMMA disks, which ultimately yields a more compact material with a reduced number of voids^7,8,9.

The research into the physical properties of materials produced via subtractive manufacturing in dentistry has revealed a number of advantages, including an improved fit, greater durability, and enhanced dimensional stability^5,10,11,12. Nevertheless, considerable disadvantages have been identified, including the generation of substantial quantities of waste during milling and the high costs associated with this process¹³. In order to address these challenges, as well as the polymerization shrinkage observed in conventionally fabricated denture bases, additive manufacturing methods, in particular three-dimensional (3D) printing, have emerged as a viable alternative. 3D-printed denture base materials offer a number of advantages, including streamlined production processes, enhanced dimensional stability, and minimal material waste, which positions them as a promising alternative manufacturing method^8,14,15. Nevertheless, it is hypothesized that denture bases produced via 3D printing may display a higher propensity for discoloration in comparison to those manufactured through conventional or milling techniques¹⁶. Such discoloration could have implications for the long-term aesthetic appeal and patient satisfaction, warranting further investigation into the material composition and surface treatments employed in 3D-printed denture bases. One of the primary causes of discoloration of 3D printed materials is their inherently rough surface. Denture bases with rough surfaces are more susceptible to staining and discoloration. Furthermore, surface roughness provides an environment conducive to biofilm accumulation, increasing the adherence of microorganisms such as Candida albicans. This microbial accumulation is dangerous for both oral hygiene and overall health, highlighting the importance of optimizing the surface smoothness of denture base materials^17,18,19.

The increased surface roughness observed in denture bases produced via 3D printing, as compared to those fabricated using conventional heat-cured or milled methods, can be attributed to the inherent characteristics of the manufacturing process. 3D printing relies on a layer-by-layer fabrication technique, where each layer leaves microscopic traces on the surface, contributing to surface irregularities^14,17. This effect becomes more pronounced with lower-resolution printers, further exacerbating the surface roughness⁴. Additionally, the photopolymer resins utilized in 3D printing undergo light-induced polymerization, which may not achieve complete polymerization in some areas, leading to surface imperfections^2,15. Inadequate polymerization or insufficient post-processing can further compound this issue³. Furthermore, the nature of photopolymer resins and the rapid polymerization reactions involved can impact material homogeneity, thereby compromising surface smoothness^5,13. In contrast, the subtractive milling technique removes material from a pre-fabricated block, resulting in a more uniform and smoother surface due to the high precision of milling burs and the continuous cutting process^16,11. Lastly, the post-processing steps required in 3D printing, such as sanding and polishing, may not always be performed with adequate rigor, leaving residual surface irregularities^8,10. Collectively, these factors explain the increased surface roughness associated with 3D-printed denture bases. However, advancements in printer resolution, material optimization, and more effective post-processing protocols hold promise for mitigating these surface deficiencies⁹.

3D printing technology may also present challenges, such as the "stair-stepping phenomenon," particularly evident on curved surfaces. This issue arises when the printed surface lacks smoothness and instead exhibits a layered, step-like structure rather than a smooth finish, which can negatively impact the color stability of the materials used in aesthetically critical regions^20,21. A variety of techniques have been proposed for the reduction of surface roughness in denture bases. These include mechanical polishing with water sandpaper, the application of specialized chemical agents, and a combination of both approaches^17,22,23,24.

Despite the existence of numerous studies that have compared the properties of removable denture bases, there has been a paucity of detailed investigation into surface roughness, a key factor contributing to discoloration, across different fabrication methods. The objective of this study is to assess the influence of contemporary denture-based fabrication techniques and mechanical polishing procedures on surface roughness. The initial null hypothesis to be tested is that there is no discernible difference in the surface roughness of denture base materials produced by 3D printing, milling, or conventional methods. The second null hypothesis is that mechanical polishing has no effect on the surface roughness of denture base materials.

Protocol

The details of the reagents, equipment, and software used are listed in the Table of Materials.

1. Sample preparation

1. Production of heat-polymerized PMMA discs
   1. Create a wax model with dimensions of 2 mm height and 10 mm width. Pour molten wax into a 2 mm high and 10 mm wide metal ring and allow it to cool. Once solidified, remove it from the ring to obtain a 2 mm x 10 mm wax model.
   2. Pour plaster into the lower part of a two-part flask.
   3. Place the prepared wax model into the plaster so that it is half-embedded. Before filling the upper part of the flask with plaster, apply a separating fluid (see Table of Materials).
      NOTE: Prevent the two layers of plaster from sticking together.
   4. Close the upper part of the flask and pour plaster over it to secure the top of the wax model. Once the plaster has fully set, heat the flask to allow the wax model to melt, then remove it from the mold.
      NOTE: A cavity will form in place of the wax, allowing the acrylic material to be poured into this space.
   5. Mix the liquid and powder of heat-polymerized acrylic in a ratio of 22.5 g of powder to 10 mL of liquid. Pack the acrylic into the cavity within the flask.
   6. After placing the acrylic material into the mold, subject the flask to polymerization in boiling water at 100 °C for 45 min.
   7. After the polymerization is complete, open the flask and carefully remove the acrylic disc from the plaster. Clean the acrylic disc from any plaster residues using steam and rinse it with distilled water.
2. Production of CAD-CAM milled polymethyl methacrylate resin discs
   1. Use designing software (see Table of Materials) to design a disc with a height of 2 mm and a width of 10 mm.
   2. Place the 98.5 mm/25 mm CAD/CAM milled PMMA disc into the milling unit. Position the 2 mm x 10 mm design onto the disc in the software, ensuring a 4 mm gap for the milling tool.
   3. Separate the acrylic discs from the block using a sharp carbide bur. Clean the acrylic discs from any residues using steam and rinse with distilled water.
3. Production of 3D-printed polymethyl methacrylate resin discs
   1. Export the 2 mm height and 10 mm width disc design from the design software in standard tessellation language (STL) format and import it into the 3D printer's software.
   2. Place the support structures on the disc surfaces at a 45-degree angle. Select a 0.5 mm micron layer thickness that is recommended for removable dentures. Set the printing speed to 20-30 mm/s.
   3. Open the cover of the printer. Insert the full denture resin. Close the cover of the printer.
   4. Press the approve button for the build time that will appear on the screen. The screen will display the message: "Confirm build area is clear. Start building now?" Press the Yes button.
      NOTE: After 3D printing is complete, the printed prosthetic or dental parts are typically semi-cured. At this stage, the material has not yet achieved its full mechanical properties and may still have soft areas.
   5. Before post-curing, clean the discs with isopropyl alcohol (IPA) for 20 min to remove excess resin and achieve a smoother finish.
   6. Place the discs into a post-curing unit that uses an ultraviolet (UV) light source. This device emits UV light at a specific wavelength, ensuring the uniform hardening of the material within 30 min.
      NOTE: The curing unit typically emits 360-degree UV light.
   7. Separate the produced discs from the print supports using a sharp carbide bur. Clean the acrylic discs from any residues using steam and rinse with distilled water.

2. Measurement of surface roughness

NOTE: Perform surface roughness measurements of the samples both before and after the polishing process.

1. Calibration of the profilometer
   1. Press and hold the Power button to turn on the device. Once the main screen appears, press the Start button.
      NOTE: The scanner tip will open with the message "Returning."
   2. Open the calibration panel without touching the gray area and position it under the scanner tip with the text facing the user.
      NOTE: Place the scanner tip on the gray matte area.
   3. Press the Menu/Enter button on the control panel to initiate calibration. Select the Calib Measurement option and press the Start button.
      NOTE: After calibration is completed, press the red button twice to return to the previous menu and press the blue button to open the main menu.
   4. Adjust the settings for surface roughness readout to cover 0.5 mm, with a cutoff value of 0.8 mm, at a speed of 0.25 mm/s and a resolution of 0.01 µm.
2. Surface roughness measurement of the samples
   1. Place the sample on the panel so that its surface touches the scanner tip.
      NOTE: If contact is not established, a red warning will appear on the screen. No measurement will be taken unless the contact is made and the indicator turns blue.
   2. Once the scanner tip completes the surface scan, save the numerical data displayed on the screen to an Excel file.
      NOTE: Measure each sample three times and record the values. After completing the measurements, power off the device by pressing and holding the Power button when the screen goes dark and then pressing the Start button once.

3. Polishing procedure

1. Place a 600-grit silicon carbide abrasive paper onto the Grinder/Polisher machine.
2. Turn on the machine's water supply. Apply the samples to the rotating abrasive paper for 10 s, ensuring the entire measured surface makes contact.
3. Repeat the process sequentially with 800- and 1000-grit silicon carbide abrasive papers, using a fresh sheet for each sample. Clean the acrylic disc from any residues using steam and rinse with distilled water.

4. Statistical analysis

1. Perform statistical analyses.
2. Apply the Kruskal-Wallis test and pairwise Mann-Whitney U test (with Bonferroni correction) to determine any significant differences between the groups.
3. Consider a p-value below 0.05 as statistically significant.

Results

The measurement of surface roughness values in the study groups before the polishing procedure yielded the following values: 2.13 (IQR 0.84) for the HP group, 4.21 (2.73) for the 3D-printed group, and 0.99 (0.54) for the ML group. After the mechanical polishing procedure, a decrease in surface roughness values was observed in all groups, with measurement of SR values post-polishing yielding the following outcomes: 0.29 (0.06) for the HP group, 0.41 (0.05) for the 3D-printed group, and 0.31 (0.06) for the ML group. Althou...

Discussion

In this study, the impact of different fabrication techniques and polishing procedures on the surface roughness (Ra) of resin-based materials used in the fabrication of complete dentures was thoroughly assessed. The statistical analysis revealed significant differences in surface roughness values across all groups, with the samples produced via 3D printing exhibiting the highest roughness values, both before and after polishing. Mechanical polishing resulted in an effective reduction in surface roughness values....

Materials

Name	Company	Catalog Number	Comments
3-dimensionally printed resin	Dentabase, Asiga, Australia	https://www.asiga.com/materials-dental/	complete denture material
Asiga Composer Software	Asiga, Australia	https://www.asiga.com/software-composer/	3D Printer software
CAD-CAM milled polymethyl methacrylate resin	M-PM Disc, Merz Dental, Germany	A2: SKU 1019085	complete denture material
Curing unit	Lilivis, Huvitz, South Korea	https://www.medicalexpo.com/prod/huvitz/product-80194-1066733.html	3D light curing
Exocad software	Align Technology, Germany	https://exocad.com/company/about-us/	desing software
Grinder/Polisher machine	Buehler Inc, Phoenix Beta, Germany		polishing
Milled Unit	Dentifa PRO2,IFA Machinery Design Engineering Services Industry and Trade Ltd. Co., Turkey	http://www.dentifa.com/	Milling of the CAD-CAM milled polymethyl methacrylate resin discs
Polimerized polymethyl methacrylate resin	Probase, Ivoclar, Liechtenstein	https://www.ivoclar.com/en_us/products/removable-prosthetics/probase-hot-cold	complete denture material
Profilometer	Surftest SJ-210, Mitutoyo, Japan	178-561-12A	surface roughness measurement
Separating agent	Ivoclar Vivadent Separating Fluid	https://www.ivoclar.com/en_li/products/removable-prosthetics/probase-hot-cold	separating agent
SPSS28 software	IBM Corp., Armork, NY,USA	https://www.ibm.com/spss	statistical analyses