International Journal of Prosthodontics and Restorative Dentistry
Volume 13 | Issue 3 | Year 2023

Evaluation of the Linear Dimensional Stability of CAD/CAM Milled, 3-D Printed, and Heat-cured Denture Base Polymers Subjected to Thermocycling and Immersion in Artificial Saliva: An In vitro study

Mariya Dimitrova1https://orcid.org/0000-0003-2444-2471, Rada Kazakova2https://orcid.org/0000-0003-2872-6850, Angelina Vlahova3https://orcid.org/0000-0002-4794-2324

1–3Department of Prosthetic Dentistry, Faculty of Dental Medicine, Medical University of Plovdiv, Plovdiv, Bulgaria

Corresponding Author: Mariya Dimitrova, Department of Prosthetic Dentistry, Faculty of Dental Medicine, Medical University of Plovdiv, Plovdiv, Bulgaria, Phone: +359889648514, e-mail: dimitrovamarria@gmail.com

Received on: 27 July 2023; Accepted on: 29 August 2023; Published on: 29 September 2023


Purpose: The purpose of the study was to evaluate and compare the linear dimensional stability of three types of denture base polymers—computer-aided designing/computer-aided manufacturing (CAD/CAM) resin, three-dimensional (3D) printed resin, and heat-cured polymethylmethacrylate (PMMA), subjected to immersion in artificial saliva and thermocycling.

Materials and methods: A total of 300 test specimens were fabricated and divided into six groups (n = 50 each); [groups I and IV—CAD/CAM resin (Ivotion, Ivoclar Vivadent, Liechtenstein), groups II and V—3D printed resin (NextDent, 3D Systems, Netherlands), groups III and VI—PMMA denture base resin (Vertex BasiQ, 3D Systems, Netherlands)]. The dimensions of groups I, II, and III were measured with an accuracy of 0.02 mm with a digital caliper (Wireman, Brighton, United Kingdom), then they were immersed in artificial saliva for three intervals (7, 14, and 1 month), and measured after each period. Groups IV, V, and VI were subjected to thermocycling for 100 hours (5,000 cycles) between water baths of 5 and 55oC, then immersed in artificial saliva for the same intervals and measured again. The obtained data were submitted to a one-way analysis of variance (ANOVA) and the mean values were compared by the Tukey test.

Results: The results obtained indicated that the distinctions in linear stability among denture base polymers manufactured via 3D printing, CAD/CAM milling, and conventional methods are most noticeable following a 7-day immersion in artificial saliva. During this period, the 3D-printed dental resins exhibited higher average values. Over the course of 14 days and 1 month, the various groups demonstrated similar mean values, while the heat-cured conventional PMMA displayed greater linear stability.

Conclusion: To summarize, during the initial week of the study, 3D-printed denture base materials displayed more significant linear alterations, whereas CAD/CAM milled, and traditional resins showcased better resistance to dimensional changes throughout the entire test duration. It’s important to note that substantial linear changes were observed in all groups after undergoing thermocycling. In the subsequent two observation periods, both CAD/CAM milled, and 3D-printed dental resins exhibited lower linear stability compared to conventional PMMA.

How to cite this article: Dimitrova M, Kazakova R, Vlahova A. Evaluation of the Linear Dimensional Stability of CAD/CAM Milled, 3-D Printed, and Heat-cured Denture Base Polymers Subjected to Thermocycling and Immersion in Artificial Saliva: An In vitro study. Int J Prosthodont Restor Dent 2023;13(3):177–183.

Source of support: University Grant - DPDP - 01/2022 of Medical University of Plovdiv, Plovdiv, Bulgaria.

Conflict of interest: None

Keywords: Computer-aided designing/computer-aided manufacturing, Denture base materials, Linear stability, Removable dentures, Three-dimensional printing


Full edentulism presents a significant health concern, necessitating prosthetic intervention involving artificial replacements such as removable dentures to ensure a healthy way of life.1 All variations of dental polymers yield satisfactory esthetic and functional outcomes; they are easily workable and demonstrate reasonable stability and resilience to the challenges posed by the oral environment.2 Despite their numerous merits, denture base polymers exhibit certain limitations—they experience substantial dimensional and visual alterations within the oral environment after a specific duration, resulting in contraction and changes in coloration.3 Factors such as water absorption, extended usage, and the consumption of pigmented foods and beverages negatively affect the integrity of removable prosthetic restorations. This contributes to discomfort during chewing, a decline in esthetic attributes and as time goes on, patient discontentment with the prosthetic treatment.4

The most common type of denture base polymer is polymethylmethacrylate (PMMA), which is used in the conventional method of heat polymerization.5 Five clinical sessions are usually required for the fabrication of conventional dentures. The first and second sessions are used to collect primary and final impressions, respectively and the third session is used to collect jaw relation. The fourth and fifth sessions are then set aside for try-in and insertion.4 Heat-cured dentures necessitate a significant amount of work from both the dentist and the dental technician. Conversely, digital dentures necessitate 2–4 appointments for finalization, contingent upon the specific system employed.6

Several printing techniques exist, each possessing distinct merits and drawbacks. Regrettably, an aspect common to the more efficient and effective methodologies is the elevated expense related to materials and upkeep, often compounded by intricate postprocessing requirements and, in certain instances, demanding considerations for health and safety.7

With the progression of contemporary technologies, the realm of three-dimensional (3D) printing broadens the potential for creating removable dentures while significantly economizing time and effort for dental practitioners and technicians.8 This novel additive manufacturing process relies on stereolithography and encompasses methodologies for constructing items layer by layer.7

Subtractive manufacturing, also referred to as “milling,” has an extensive history within dental medicine. The introduction of computer-aided design/computer-aided manufacturing (CAD/CAM) has unveiled captivating fresh avenues in the field of removable prosthodontics.9 Subtractive manufacturing entails material removal to shape an object. Contemporary dental technology now incorporates CAD/CAM techniques for milling crown copings and bridge frameworks.10 The approach to producing pucks stands out as a pivotal element in conferring superior attributes to CAD/CAM restorations compared to other systems and techniques.8 In modern dentistry, there is familiarity with materials crafted to align with CAD/CAM processes, supplanting traditional precious metal casting alloys that have witnessed substantial price surges in recent times.11

A single bicolored milling disk integrates high-quality cross-linked PMMA tooth material with a premium denture base material, streamlining the rapid and reliable production process.12 This disk eliminates the need for time-consuming manual tooth-bonding procedures. The transition from the tooth to the base sections of the milling disk is facilitated by the 3D dental arch structure, ensuring a smooth and uniform connection.13 By utilizing CAD/CAM techniques, challenging materials can be employed, and the labor-intensive traditional crafting methods can be avoided. This shift allows dental technicians to channel their manual dexterity into the more imaginative facets of the manufacturing process. However, it’s important to note that milling technology comes with drawbacks such as significant material wastage during processing and high equipment expenses.14

Rapid prototyping and milling technologies are exerting a substantial influence across all domains of dentistry.15 The capabilities of 3D printing permit the precise fabrication of intricate, unique shapes derived from digital data, using an array of materials, either on a local scale or within industrial hubs. The advent of additive manufacturing is reshaping the clinical and laboratory processes involved in crafting removable dentures. Presently, almost all items created for patients can be produced using a 3D printer, although no single technology can comprehensively address all patient requirements.16

The objective of the present investigation was to assess the linear dimensional stability of three denture base materials generated through three distinct methodologies (CAD/CAM milling, 3D printing, and heat-cured denture base resin) when subjected to placement in artificial saliva and simulated aging. To evaluate the impact of treatment and storage conditions on the measured parameters, the subsequent hypotheses were examined—H0—immersing in artificial saliva and simulated aging will not exert a substantial effect on the dimensions of the test specimens. H1—immersing in artificial saliva and simulated aging will significantly impact the dimensions of the test specimens.


The current in vitro study was conducted in the CAD/CAM Center of Dental Medicine, Research Institute, Department of Prosthetic Dentistry, Faculty of Dental Medicine, Medical University–Plovdiv, and it was granted by “University Grant: DPDP–01/2022.”

A total of 300 test samples, distributed across six groups (with 50 specimens each), were produced in the form of a rectangular parallelepiped, featuring dimensions of 20 by 20 mm for both width and length and a cross-sectional diameter of 3 mm (depicted in Fig. 1). The design of the specimens in terms of shape and size adhered to predefined criteria, accomplished using nonparametric software free CAD version 0.19 (from Free CAD Software, Ulm, Germany). Subsequently, a standard tessellation language (STL) file was generated to fulfil this purpose. The required quantity of test samples essential for the study was determined using a priori power analysis, employing G*Power 3.1 (developed by Universität Düsseldorf, Germany).

Fig. 1: Design of the test specimens from the software 3D viewer

Six groups of test specimens of different denture base polymers were fabricated with each group having n = 50 specimens. The groups I and IV test specimens were made from CAD/CAM resin disks for subtractive manufacturing (Ivotion, Ivotion Denture System, Ivoclar Vivadent, Schaan, Liechtenstein) (Figs 2 and 3). A wax model of the specimen was subjected to scanning, with the resulting scan data saved in the STL format. This data was then transferred to the CAM software, specifically the PrograMill CAM software developed by Ivoclar Vivadent based in Schaan, Liechtenstein. Subsequently, an automated CAM was employed to mill the designated specimens from prepolymerized CAD/CAM resin disks composed entirely of PMMA (100% by weight) from the Ivotion System by Ivoclar Vivadent in Liechtenstein. This milling procedure operated through a subtractive method. During the milling process, burrs with a maximum diameter of 2.5 mm and a 5-axis configuration were utilized to achieve precise and intricate details. The milling was carried out under wet conditions to prevent excessive heat buildup, following the manufacturer’s guidelines.17

Fig. 2: The 3D-printed experimental specimens on the platform of the 3D-printer NextDent 5100 (NextDent, 3D Systems, Soesterberg, Netherlands)

Fig. 3: PMMA heat-cured experimental samples

The groups II and V test samples were made by the 3D printing method, using NextDent 3D Denture+ (NextDent, 3D Systems, Soesterberg, Netherlands), with a pregenerated STL file from the software. The STL file format, which represents the geometry of an object in the form of triangles, is a standard for transferring 3D information to 3D printers. Dental resin, a photosensitive polymerizable material based on PMMA, exists in a liquid state. It is introduced into the tray positioned at the bottom of the 3D printer. Following the import of the digital file and the positioning of 3D renditions of the prototype specimens, the printing process can be initiated within the 3D printer software. The printing specifications included a layer thickness of 100 μm and a printing orientation of 45°. Subsequent to the completion of 3D printing, the test samples were positioned on a platform situated at the printer’s top.18 Additional treatment was applied to the 3D printed samples, involving the removal of unpolymerized material from the surface. This cleaning process entailed immersing the samples in a container of isopropyl alcohol for a duration of 10 minutes. For the 3D printed components, this procedure typically spans around 6 minutes, with the alcohol diluted using distilled water in a proportion of 70% isopropyl alcohol to 30% distilled water. Following the cleaning step, the test specimens were immersed in glycerin for a further 45 minutes in a final polymerization oven. This immersion aimed to facilitate the reaction of any remaining monomers.19

Groups III and VI samples were crafted using heat-cured PMMA material Vertex BasiQ 20 (Vertex Dental, 3D Systems, Soesterberg, Netherlands) through the conventional technique of heat-polymerization executed within a water bath at 100°C. The 3D printer employed to generate the wax prototypes of the test specimens, characterized by predetermined shapes and dimensions, was the NextDent 5100 DLP 3D Printer (NextDent, 3D Systems, Soesterberg, Netherlands). For the production of test specimens using thermosetting plastic, the polymer was first weighed and then mixed with the monomer in a weight ratio of 2:1. This amalgamation occurred in a clean porcelain vessel at room temperature, as stipulated in the manufacturer’s instructions. Approximately 15 minutes later, the dough-like consistency was achieved, at which point the plastic was positioned within cuvettes and pressed in a hydraulic press (Sirio P400, from Sirio Dental, Milan, Italy) at a pressure of 100 kg/cm² and this pressure was gradually released until reaching the final phase. Following this, the specimens were placed in a water bath at 100°C for 20 minutes to conclude the heat polymerization process. After the polymerization cycle was finalized and gradual cooling to room temperature was carried out, the surfaces of the test specimens were treated with 70% ethyl alcohol and subsequently dried.20

The prepared test specimens from groups I, II, and III were immersed in artificial saliva, with the aim of maximally reproducing the environment in the oral cavity (Fig. 4). The artificial saliva was prepared according to a standard recipe by a specialist at the Department of Chemical Sciences, Medical College, Medical University–Plovdiv, Bulgaria.21

Fig. 4: CAD/CAM milled test samples, immersed in artificial saliva

To conduct the study, reference points were placed on the test specimens using a plastic cutter. The points were uniformly spaced at 1.5 mm from the periphery of the test specimen at the four ends of the surface of the test specimens, in order to read the results. Each of the investigated plastic test specimens was numbered to facilitate the tracking of the obtained measurement results. After that, the test specimens from the two types of plastic were placed in glass containers, in which 100 mL of artificial saliva was added, at room temperature (23.5°C) and with a regulated pH (pH = 6.8). The temperature was constant and was recorded using a digital thermometer and the alkaline-acid environment using pH test strips for saliva.22

At specific time intervals, linear alterations were measured (on the 7th day—referred to as T1, the 14th day—referred to as T2, and the 1st month—referred to as T3). After being taken out of the artificial saliva containers, the test specimens were promptly dried using a paper towel and measured using a digital caliper called Wireman (manufactured by Wireman, Brighton, United Kingdom) with a precision of 0.02 mm (Fig. 5). The collected outcomes underwent analysis and were displayed using statistical methods, which are presented in tables.

Fig. 5: Measurement of test specimens with a digital caliper

Groups IV, V, and VI underwent simulated aging through exposure to artificial conditions. The samples from each group were stored in distilled water at 37°C (depicted in Fig. 6). A process of thermocycling was carried out involving 5,000 cycles, alternating between temperatures of 5 and 55°C, with a dwell time of 30 seconds during each transition. This replicates the temperature fluctuations experienced in the oral environment over a span of 5 years.

Fig. 6: Specimens placed in the thermocycler

The gathered data were subjected to statistical analysis using Statistical Package for the Social Sciences (SPSS) software (IBM Corp., version 26.0, Released 2019, IBM SPSS Statistics, Windows, Armonk, New York: IBM Corp). The data were subjected to a one-way analysis of variance (ANOVA), and the mean values were compared using the Tukey test (with a significance level of α = 0.05).


Artificial aging through thermocycling and immersion in artificial saliva significantly influenced the linear dimensions for the three tested denture base materials in the current study. Descriptive analysis showed that the mean values for the three studied groups of materials were close, at a 95% confidence interval (Table 1). One-way ANOVA was applied, which aims to find whether the value of linear changes depends on the type of material (α < 0.05) (Table 2).

Table 1: Descriptive analysis values for the investigated denture base materials (groups I, II, and III—after immersion, groups IV, V, and VI—after thermocycling)
Denture base material Number of specimens Mean value Standard deviation Standard error 95% interval of confidentiality Minimal Maximum
Lower Upper
NextDent denture 3D+ Group I 50 20.285 0.221 0.018 20.248 20.322 19.790 20.670
Ivotion base Group II 50 20.021 0.082 0.006 20.107 20.135 19.950 20.270
Vertex BasiQ 20 Group III 50 20.203 0.185 0.011 20.181 20.225 19.790 20.670
NextDent denture 3D+ Group IV 50 20.234 0.215 0.009 20.226 20.286 19.670 20.490
Ivotion base Group V 50 20.010 0.060 0.004 19.987 20.089 19.550 20.280
Vertex BasiQ 20 Group VI 50 20.197 0.176 0.007 20.036 20.226 19.380 20.570
Table 2: One-way ANOVA analysis for interaction between tested groups
Sum of squares Degrees of freedom Sum of mean values F р
Between the groups 4.918 3 1.639 96.185 <0.01*
In the groups 4.704 276 0.017
Total 9.622 279

*p < 0.05 is significant

The obtained p-value is <0.01, therefore there is a strong correlation between the linear changes and the type of material from which the samples are made. Conventional resin performed better and had smaller mean change values (20.02171, p <0.01), while 3D-printed resin changes were larger (20.28558, p <0.01).

Through one-way ANOVA, we determine whether there is a correlation between the value of the linear changes and the time spent in artificial saliva. The obtained p-value is <0.01 (very low value), therefore there is a strong correlation between the linear changes and the residence time of the test specimens in artificial saliva.

A Tukey test was applied, which compares each period with the others before and after immersion in artificial saliva (the control group—0 days in artificial saliva, group I—NextDent, group II—Ivotion Base, group III—Vertex) (Table 3). All p-values = 0.05, thus once again confirming that the residence time in artificial saliva is relevant and is a determinant of the linear changes. It is obvious that in the observed samples for the first 7 days, the changes are the largest, after which for the remaining periods of time they increase uniformly at a reduced rate. The changes are greater in the test specimens made of resin by the 3D-printing method for the three immersion periods.

Table 3: Tukey test for multiple comparisons between the different periods of immersion in artificial saliva
Groups (J) V2 Mean values (I-J) Standard deviation р 95% interval of confidentiality
Lower Upper
Control group 7 days −0.229 0.022 <0.01* −0.286 −0.172
14 days −0.291 0.022 <0.01* −0.348 −0.234
30 days −0.349 0.022 <0.01* −0.406 −0.292
After 7 days Control group 0.229 0.022 <0.01* 0.172 0.286
14 days −0.061 0.022 0.028* −0.118 −0.004
30 days −0.119 0.022 <0.01* −0.176 −0.062
After 14 days Control group 0.291 0.022 <0.01* 0.234 0.348
7 days 0.061 0.022 0.028* 0.004 0.118
30 days −0.058 0.022 0.045* −0.115 −0.001
After 30 days Control group 0.349 0.022 <0.01* 0.292 0.406
7 days 0.119 0.022 <0.01* 0.062 0.176
14 days 0.058 0.022 0.045* 0.001 0.115

*The difference in means is significant at the 0.05 significance level

Figure 7 illustrates the contrast between the examined groups prior to and after undergoing thermocycling. The NextDent group displays notably more pronounced mean values, with greater linear shifts observed after the thermocycling process. The Ivotion base group follows with comparable alterations, while the PMMA Vertex group exhibits the least average values.

Fig. 7: Comparison of the tested materials before and after thermocycling


Within this investigation, three distinct varieties of denture base materials were categorized into six groups, each of which underwent varying storage conditions over three different durations.

Immersion in artificial saliva had a significant influence on the linear stability of the 3D printed and CAD/CAM milled denture base polymers after the first 7 days of the study. Artificial aging had a strong correlation with the linear changes of the three types of materials after immersion in artificial saliva for 1 month.

In their survey, Gad et al.23 investigated two groups of heat-polymerizing and four groups of self-polymerizing resin by comparing their linear changes after being in distilled water. The results coincide with those of the author’s collective and show that over time the materials expand their linear dimensions, the changes occurring during the first reporting period being statistically significant. As noted by Alp et al.,24 the stress stemming from thermal shrinkage dissipates shortly after the resin is extracted from the mold, whereas stress due to polymerization shrinkage dissipates gradually. Their findings indicated that thermal shrinkage stress operates as an immediate mechanical effect, while the stress arising from polymerization occurs at the molecular level, engaging polymer chains. In the current study, there was the least expansion in the PMMA test specimens, in comparison with the other two types of denture base materials.

Other authors have found that most types of denture base resin shrink during the polymerization process.25 However, when immersed in water, they expand, which was concluded as well from the present study results.26 Greater shrinkage has been reported during processing than when stored in water for up to 90 days.27 Other researchers reported that there were significant changes in the linear dimensions of removable denture material that were determined between the two phases—before and after thermocycling.28

According to other authors, after storage in water for 30 days, test specimens demonstrated changes in linear dimensions, but the reported values were not statistically significant.29 In contrast, the results of the present study had higher mean values than those reported by Venus et al.30 Discrepancies in the observed alterations between our investigation and these previous studies could be attributed to variations in experimental setups or diverse storage conditions employed.31

In the study by Al Helal et al.,32 volumetric and linear changes were compared in three groups of materials—conventional, 3D-printed, and CAD/CAM resin. It was found that resin made by the CAD/CAM method showed better volumetric and linear stability over the time of examination compared to the other two groups. The results were statistically significant, which is similar to other studies.33 In the present study, PMMA specimens showed better linear stability, compared to the CAD/CAM, and 3D-printed test specimens. Therefore, in the present study, the H1 study hypothesis was accepted.

The constraints of the carried out study can be succinctly summarized. Due to the high level of control in this experimental investigation, outcomes are likely to be specific and consistently relevant. This method allows for swift assessment of the linear stability after immersion in artificial saliva, facilitating quicker evaluation of the dental resin types compared to alternative validation approaches. Furthermore, the data can be manipulated to appear favorable, yet given the vast disparity between the controlled laboratory setting and the real clinical environment, achieving positive outcomes outside of experimental research remains improbable. Given the conflicting outcomes observed within the restricted pool of studies, it is imperative to conduct additional research aimed at uncovering the mechanical and physical attributes of the novel 3D-printed denture base materials.


Considering the restrictions of this present study, it can be deduced that a robust association exists between the linear modifications and the submersion of test samples in synthetic saliva. Both thermocycling and the storage environment notably lead to a marked decline in the linear steadfastness of all the assessed substances. Notably, PMMA displayed minimal reduction subsequent to simulated aging, irrespective of the duration of storage. The linear adjustments following thermocycling were most noticeable in the case of the 3D-printed test specimens. Linear alterations across all polymer types were more conspicuous during the initial week and subsequently escalated in direct proportion to the elapsed time.


Mariya Dimitrova https://orcid.org/0000-0003-2444-2471

Rada Kazakova https://orcid.org/0000-0003-2872-6850

Angelina Vlahova https://orcid.org/0000-0002-4794-2324


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