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Comparison of Fitness of Digital Denture Bases Produced Using CAD
Int J Clin Prev Dent 2023;19(4):51-56
Published online December 31, 2023;  https://doi.org/10.15236/ijcpd.2023.19.4.51
© 2023 International Journal of Clinical Preventive Dentistry.

Nam-Joong Kim1, Do-Hun Song2

1Department of Dental Technology & Science, Shinhan University, Uijeongbu, 2Best Dental Lab, Uijeongbu, Korea
Correspondence to: Nam-Joong Kim
E-mail: wnj120@hanmail.net
https://orcid.org/0000-0001-6334-6402
Received October 15, 2023; Revised December 4, 2023; Accepted December 20, 2023.
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Abstract
Objective: The purpose of this study is to compare the suitability of a denture base by manufacturing a denture base designed using three methods: milling PMMA, 3D printing, and injecting resin and thermal polymerization after burying milled wax.
Methods: Prepare 3 sets of 11 standard models to compare the fit, and design a denture base for each model using the Dental CAD program. After 3D printing, PMMA milling, and wax milling, the designed denture bases are manufactured by three methods, 11 each. To check the gap between the model and the base, apply silicone to the inner surface of the denture base and press it with constant pressure. When it is cured, it is scanned once again with the silicone present, and the thickness of the silicone is measured by merging it with the first model. The measured values are analyzed through a statistical program (SPSS).
Results: Looking at the average value of the gap between each measurement area according to the manufacturing method, the 3D printing method showed the smallest overall value. In the right alveolar line, the milling method showed the largest gap, and in the center of the palate, PMMA milling showed the largest gap. Also, the curing method showed the highest value in the left alveolar line, and there was no statistically significant difference between the manufacturing methods in the center of the palate and the left alveolar line.
Conclusion: 3D printing showed the best fit for denture bases manufactured using CAD/CAM system, but the gap in the center of the palate was larger than the left and right alveolar lines in all three methods.
Keywords : fitness, denture base, wax milling, 3D printing
Introduction

Dental CAD/CAM systems began to be used in the inlay and crown areas in the 1980s, along with the advancement of dental technology and materials in the 20th century. Since the 1990s, the area has expanded to include complete denture production, and active research is ongoing to this day [1].

The manufacturing process of complete dentures is a very long and complex process and requires a lot of skill from dentists, dental hygienists, and dental technicians [2]. Manu-facturing digital complete dentures using CAD is the same as the previous method, including taking an impression of the patient using an oral scanner or impression material, producing a base image and occlusal material, and recording the patient’s occlusion. Afterwards, the occlusal material and model are scanned using a scanner and arranged using a dental CAD program. Within the program, the occlusion is checked using an articulator, and when the alignment is completed, it is transferred to the CAM program and the denture base and teeth are printed separately [3]. The denture base and teeth that were printed separately are attached using resin, light-polymerized, and then polished to complete the process.

Compared to the traditional complete denture production method, digital complete denture production uses a computer program to enable design with the operator’s knowledge, and can be expanded at a high rate, enabling precise work. In addition, most tasks done by hand can be solved on a computer, and modifications and repetitions are possible. It has advantages such as ease and simplification of the pore process, possibility of remanufacturing using stored data, reduced number of visits to the hospital, and reduced shrinkage during the polymerization process. However, compared to the existing analog production method, there is a limitation in that it lacks evaluation of the suitability of the prosthesis and consideration of the long-term prognosis [4]. Improving the quality of dentures directly affects patient satisfaction and treatment success [5]. In particular, retention is important for complete dentures to function stably in the patient’s mouth [6]. Recently, much research has been conducted on denture manufacturing using CAD/CAM, but research using denture bases is lacking [7]. Although there are many comparative studies on cutting and additive manufacturing, there is also a lack of research comparing suitability depending on the manufacturing method.

The denture base supports the artificial teeth and at the same time transmits and distributes the occlusal pressure generated in the oral cavity to the tissues [8]. There are three ways to create a digital denture base. The first is a cutting processing method that produces dentures by cutting polymerized PMMA disks. The second is a lamination method that forms a three-dimensional structure by stacking photocurable liquid resin layer by layer. The third method is to cut the wax disk and then temper it. All three have something in common: they produce results using resin, but there are differences in their production methods.

The first method of cutting the PMMA disk is in a polymerized and compressed state, which has excellent mechanical strength due to fewer air bubbles or internal defects. It is cut after the polymerization of the resin has been completed, so there is no shrinkage due to polymerization, resulting in excellent accuracy. However, it consumes a lot of material and has the disadvantage of not being able to manufacture dentures that are too large depending on the size of the original plate or equipment, and if the undercut is severe, processing may be difficult [9].

The second method of forming a three-dimensional layered structure using a 3D printer is to photopolymerize the liquid containing additives by layering them one by one using a laser or light. Depending on the light source and module, various 3D printers such as SLA, DLP, and LCD are used. Compared to cutting processing, even shapes with severe undercuts can be processed without problems, and because material is consumed only in necessary areas, there is less material waste. Additionally, it has the advantage that the output is relatively precise and the surface roughness is smooth [10]. However, unlike cutting processing, because polymerization is performed using light, shrinkage may occur during the polymerization process, additional cleaning and curing processes are required after printing, and there may be errors in the results depending on the post-processing [9].

The third method of cutting and polymerizing wax can be said to be a combination of digital and analog methods. By cutting denture bases designed in CAD with wax, time and strain on the processing machine can be reduced. Afterwards, the process of embedding the denture and replacing it with resin is the same as the previous method. Denture base resins, which are widely used to date, are mainly polymerized into denture base resins by heating or chemically activating a mixture of monomer, a solution monomer, and poly methyl methacrylate, a powder polymer, at room temperature [11].

Dentures can achieve the best retention when the denture base fits the patient’s oral tissue precisely, and the patient feels more comfortable and chewing efficiency is improved, which increases satisfaction with dentures [12]. Accordingly, in this study, to evaluate digital dentures, which are increasingly being used in clinical practice, a denture base designed using CAD was milled with PMMA, 3D printed, and injected with resin after investing the milled wax. We would like to compare the suitability of denture bases, which play an important role in the completeness of dentures, by manufacturing them using three methods, including heat polymerization.

Materials and Methods

1. Model making

In this experiment, a denture base was manufactured using a maxillary edentulous model mold (H3-402, Nissin dental, Japan). Carbide gypsum (Snowrock, DKmungyo, Korea) was mixed and injected according to the manufacturer’s instructions, and then dried for more than 30 minutes. As shown in Figure 1, three sets of a total of 11 models were prepared, and each model was numbered. The PMMA milling model was designated as M1-M11, the 3D printing model was designated as P1-P11, and the heat polymerization model after wax milling was designated as W1-W11.

Figure 1. Master model reproduced from standard upper mold. PMMA milling models (A), 3D printing models (B), wax milling and curing models (C).

2. Scan and design

Thirty-three maxillary edentulous cemented carbide models were individually scanned with a scanner (E2, 3shape, Denmark) (Figure 2, 3). The design was done using a dental design program (3shape, Copenhagen K, Denmark), and the order was as follows.

Figure 2. Scanning each model with scanner.
Figure 3. The scanned model.

First, an order form was filled out with the number appropriate for each model. Since this experiment only compares the suitability of the denture base, there is no need to print the teeth, so it was designed using the [Device] - [Custom Impression Tray] function, which allows designing only the base with a constant thickness.

Next, set the insertion direction considering the path and block out the undercut portion. The thickness of the denture base was uniformly applied at 2 mm, and the amount of inner relief was set at 0.1 mm (Figure 4).

Figure 4. Design denture base.

3. Fabrication of denture base

1) Wax milling

After calculating using a milling program (hyperdent, Germany), the wax block was cut using a wet mill (Craft-5x, DOF, Korea) (Figure 5). A base was made to bury the printed wax denture base and then trimmed. After completion of burial, heat was applied using a steamer (Warmer, Hubdentech, Korea) for 15 minutes and the wax was removed. Then, resin (Rapid simplified, Vertex, Netherlands) was injected and polymerized for 40 minutes. Afterwards, the plaster on the inside was removed and the surface was trimmed with a bur to fit the model.

Figure 5. Milling machine used in this study and milled wax block.
2) 3D printing

The designed denture base was printed using an LCD 3D printer (A1sd, Sindoh, Korea). Supporters were attached and set values were given using a slicing program (A desktop, Sindoh, Korea), and denture base resin for 3D printing (Tera Harz TDD-80, Graphy, Korea) was used as the material. The supporter was attached at a 90 degree angle, which minimized post-processing and had the highest success rate for printing. Afterwards, it was washed with isopropyl alcohol and then light-polymerized for 3 minutes using a light-polymerizer (MP300, Veltz, Korea).

3) PMMA milling

As with wax milling, calculations were made using a milling program, and then the PMMA block was cut using a wet milling machine.

4. Adaptation

To measure the gap between the model and the denture base, silicone material (Fit checker advanced, GC, Japan) was mixed and applied between the model and the denture base. It was fitted by applying a static load for 3 minutes and 30 seconds at a pressure of 4 kgf (32.9 N) while the silicone was curing.

5. Measurement and analysis

Each denture base was removed from the model and scanned again with silicone present. The measurement locations were selected at three points at the rear of the palate where the denture base is most likely to float: the left alveolar ridge (L), the center of the palate (M), and the right alveolar ridge (R). Afterwards, it was merged with the first scanned model and the thickness of the silicon was measured using the [Cross- section view] function.

Statistical analysis was performed using SPSS 22.0 Ko (SPSS 22, IBM, USA). One-way ANOVA was performed to compare the means of the experimental results. As a post hoc test, multiple comparisons were made using Tukey HSD, and the significance level was set at 0.05.

Results

The results of the test comparing the suitability of denture bases produced using a digital system are shown in Table 1. Looking at the average value of each measurement area according to the production method, the gap of the denture base produced by 3D printing was measured to be the smallest overall. Looking at the average value measured at the right alveolar ridge, the denture base made by milling showed the largest gap at 0.141, and the minimum and maximum values also showed the largest deviations of 0.014 and 0.295. In the center of the palate (M), the average value of the denture base made by milling was the highest at 0.571, which was the largest average value among all measurements. The minimum value was 0.406 for the denture base made by 3D printing, and the maximum value was 0.708 for the denture base made by milling. Looking at the average value at the left alveolar ridge, the denture base made of curing showed the highest value at 0.120, the maximum value was 0.275 in the denture base made by milling, and the minimum value was 0.011 in the denture base made by curing.

Table 1 . Denture base fit measurement results (gap) (n=11, unit: mm)

Measuring pointManufacturing methodM±SDMinMaxp
Right (R)Milling (M)0.141±.121.014.295p<0.05
3D printing (P)0.078±.056.016.182
Curing (W)0.095±.077.023.257
Middle (M)*Milling (M)0.571±.065.501.708
3D printing (P)0.406±.065.311.530
Curing (W)0.508±.101.325.686
Left (L)*Milling (M)0.094±.076.030.275
3D printing (P)0.073±.034.031.143
Curing (W)0.120±.086.011.251

*No statistically significant difference.



There was no statistically significant difference between the production methods in the center of the palate and the left alveolar ridge, and the gap in the center of the palate was the largest for all three methods compared to the right ridge and left ridge.

Discussion

Recently, as interest in digital dentures has increased, much research is being conducted. Manufacturing dentures using digital technology has the advantages of reducing the number of patient visits, reducing processing errors using computers, and allowing stored information to be used infinitely regardless of the number of times [13].

The causes of shrinkage and stress that occur during the polymerization process of denture base resin are well known. The density of MMA changes from 0.94 g/cm2 to 1.19 g/cm2 when polymerized, and this change in density is accompanied by a 21% volumetric shrinkage. However, when mixed at a typical ratio of heat polymerization resin, only about 1/3 of the mixture is MMA, so the volumetric shrinkage during polymerization is about 7%. In addition to volumetric shrinkage, linear shrinkage must be considered, which greatly affects denture fit. Volumetric shrinkage is about 7%, and linear shrinkage is about 2%. However, the actually measured shrinkage in the posterior part of the maxillary denture base is reported to be less than 1% [14]. As a result, it is believed that a difference in suitability occurred due to shrinkage in the method of milling and heat polymerizing the wax block. Yoon [15] say that printing denture bases in a 90-degree direction shows the highest reproducibility.

Comparing the three methods in terms of manufacturing efficiency, the WAX milling followed by heat polymerization method is considered inefficient for digital manufacturing. Instead, it has the advantage of lower block and bur costs because it does not use a PMMA block. The method of milling PMMA shows excellent results in terms of reproducibility, but is considered to be more inefficient than 3D printing in that it takes an average of 3 hours to manufacture and consumes a lot of burs. 3D printing time is determined by the printing height, and printing at 90 degrees, which takes the longest, took about 1 hour and 30 minutes. However, since the time to print one item and the time to print three items simultaneously are the same, it is considered to be much more efficient than the other two methods in terms of time. The disadvantage is that printing failures can often occur and the price of resin is expensive.

In this study, PMMA milling was expected to have the highest suitability before fabricating the denture base. However, the experimental results showed that 3D printing had the highest suitability. When designing the denture base using a design program, the same 0.1 mm space was given to all denture bases. Then, a gap of at least 0.1 mm or more must be measured in the three manufacturing methods, but 3D printing shows a gap of less than 0.1. This is believed to be because the three settings of equipment, materials, and output program were applied to the model to suit the model due to the results being at a level where suitability evaluation was impossible at the time of initial printing. In a study where the same design had to be compared using different printing methods, the need to adjust the settings for the output was judged to be a limitation in the comparison process.

Although it served as a limitation in this study, I think the advantage of digitally manufacturing dental prosthesis is that the fit can be adjusted by adjusting the settings. It is believed that if you can accurately understand and actively handle all three of the output equipment, program, and materials, you can significantly increase the accuracy of the prosthesis. In the future, it is believed that detailed research will be needed on setting values that can output CAD-designed files as identically as possible without volumetric deformation.

Conclusion

In this study, to evaluate digital dentures, which are increasingly being used in clinical practice, denture bases designed using CAD were milled using PMMA, 3D printed, buried the milled wax, and then injected with resin. The following results were obtained by comparing the suitability of denture bases, which play an important role in the completeness of dentures, by producing them using three methods, including heat polymerization.

Looking at the average value of the gap in each measurement area according to the production method, the one produced by 3D printing appeared to be the smallest overall. The milling method showed the largest gap at the right alveolar line, and the PMMA milling method showed the largest gap at the center of the palate. In addition, the heat polymerization method showed the highest value in the left alveolar line area, and there was no statistically significant difference between the production methods in the center of the palate and the left alveolar line area.

3D printing showed the best fit for denture bases produced using the CAD/CAM system, but in all three methods, the gap was measured to be larger in the center of the palate than in the left and right alveolar ridges, and digitally produced prosthetics require printing equipment. It is believed that adjusting the settings of the material and program will have a significant impact on suitability.

Conflict of Interest

No potential conflict of interest relevant to this article was reported.

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