Completely edentulous individuals experience functional, phonetic, esthetic, social, and psychological problems.​ [ 1] ​ Removable complete dentures and implant-supported prostheses have been the standard treatment for the rehabilitation of edentulous jaws for many years, and polymethyl methacrylate (PMMA) denture bases have been used in the fabrication of removable complete dentures and implant-supported overdentures.​ [2, 3] ​ PMMA resin has been used in dentistry since the mid-20th century and is considered standard in the production of removable prostheses​ [4, 5] because of its processing simplicity, color stability, optical properties, compatibility, lightness, and low cost.​ [5, 6]

Acrylic resin denture bases are produced by polymerizing liquid monomer and powder polymer mixed in appropriate proportions. Various methods such as chemical activation, visible light activation, hot water, or microwave energy have been used for the polymerization of acrylic resin​ .[1, 7–9] ​ However, not all the monomers can be polymerized, and unreacted monomers called residual monomers remain in the denture base, [10–13] ​negatively affecting physical properties such as surface characteristics, dimensional stability, water absorption, and tissue compatibility.​ [1, 11, 12, 14, 15] Moreover, the leakage of residual monomer from the denture to soft tissue or its presence in saliva can lead to problems that include allergic reactions, stomatitis, oral ulcerations, and burning sensations.​ [6, 16, 17]

With the advancement of digital technologies, the use of computer-aided design and computer-aided manufacturing (CAD-CAM) technology has greatly facilitated the production of complete dentures. Two main digital methods have been used in the production of complete dentures: CAD-CAM milling and 3-dimensional (3D) printing.​ [5, 18, 19]

CAD-CAM milled denture bases are obtained by milling or grinding prepolymerized PMMA disks that have been fabricated under high pressure.​ [20, 21] ​ Complete dentures milled from these prepolymerized disks have been reported to exhibit better material properties and relatively lower levels of residual monomer than complete dentures produced with conventional heat-polymerized PMMA.​ [22, 23]

Denture bases produced from 3D printing use liquid photosensitive resins that are polymerized in layers under a UV or visible light source. In 3D printing production, there is less material waste compared with CAD-CAM milling, and multiple dentures of complex designs can be produced simultaneously. The reduced cost of 3D printers compared with milling devices has also enabled the widespread use of 3D printed dentures.​ [24]

The concentration of residual monomer in the prosthesis depends on the degree and efficiency of polymerization. [25, 26] In addition to the negative effects of the amount of residual monomer on the physical and mechanical properties of the prosthesis, its release into saliva may lead to tissue irritation, burning sensations, stomatitis, allergic reactions, edema, and ulceration of the oral mucosa.​ [27, 28] ​ The null hypothesis was that no significant difference in residual monomer release would be found among acrylic resin denture bases fabricated using different manufacturing techniques and at different time intervals.

The acrylic resin specimens used in the study were produced using 4 different production methods: conventional heat polymerization, injection molding, CAD-CAM milling, and 3D printing. The sample size was determined as n = 10 based on similar studies.​ [18, 19] ​Disk-shaped specimens measuring 10 mm in diameter and 3 mm in thickness were designed using a computer software program (3D Sprint; 3D Systems Inc) in standard triangular language (STL) format (Fig. 1.A-B).

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Figure 1.B. Discs placed inside block in CAD program software (Millbox; CIMSystem).

For the conventionally heat polymerized specimens, the STL design was used to mill wax patterns from a wax block using a CAD-CAM milling machine (Arum 5X-500; Arum Europe) under water cooling. The prepared wax patterns were placed in a flask. The flask was kept in boiling water for 10 minutes and then opened to eliminate the melted wax. Heat-polymerized PMMA resin (SR Triplex Hot; Ivoclar AG) was prepared according to the manufacturer’s instructions by mixing 23.4 g of powder with 10 mL of liquid for approximately 1 minute. The dough mixture was packed into the negative space, flask was closed, and pressure was applied using a hydraulic press at 20 MPa for 15 minutes. The flask was then placed in room-temperature water, which was gradually heated to 100°C and maintained at boiling temperature for 45 minutes to complete polymerization(Fig. 2.A.).

For the injection-molded specimens, previously prepared wax patterns were placed into the special metallic flask of the injector system (IvoBase; Ivoclar AG) and connected via injection channels. The flask was immersed in 90°C water for 5 to 8 minutes to eliminate the wax. The acrylic material (SR-IvoBase; Ivoclar AG) was prepared using a capsule containing 20 g of powder and 30 mL of liquid; the liquid was poured into the powder compartment and mixed with a spatula for 20 to 30 seconds until a homogeneous dough was obtained. The capsule (IvoBase Denture; Ivoclar AG) was attached to the flask, placed in the injector, and the P2–IvoBase High Impact program was selected. The resin dough was injected into the mold at 6 atm pressure for 5 minutes, during which polymerization occurred simultaneously with the temperature rise inside the device (Fig. 2.B).

For the CAD-CAM milling method, the specimen design was created using CAD software (Millbox; CIMSystem). A 25-mm-thick prepolymerized PMMA disk (Yamahachi Denture Base; Yamakin Co) was used for fabrication. The milling process was performed under water cooling conditions using a 5-axis milling machine (Arum 5X-500; Arum Europe) (Fig. 2C).

Specimens were also produced with denture resin (NextDent Denture 3D; 3D Systems Inc) using digital light processing (DLP) 3D printing technology (NextDent 5100; 3D Systems Inc). The STL files were imported into the printer software (3D Sprint; 3D Systems Inc), and the designs were oriented at a 30-degree angle on the build platform with a layer thickness of 50 µm. After printing, the specimens were cleaned in 96% isopropyl alcohol and then post-polymerized under a light-emitting diode (LED) light at 405 nm wavelength (Fig. 2D).

All specimens were polished by a single dental laboratory technician using 600-grit silicon carbide paper under water cooling. Polishing was completed with pumice and a felt disk, and the specimens were stored in a dark and dry environment until analysis. After the production process, each disk was placed in a glass tube with an airtight lid containing 1 mL of artificial saliva solution with a pH of 6.8. The artificial saliva solution was changed daily to mimic the oral environment. Throughout the experiment, the tubes were kept in an incubator (Nuve EN55; Nuve) at 37 ± 1°C under constant humidity and temperature conditions.

To determine the amount of monomer released by the specimens into the artificial saliva at different times, artificial saliva samples were obtained on days 1, 2, 3, 7, and 15. The determination of residual monomers in the collected specimens was performed using a high-performance liquid chromatography (HPLC) device (Agilent 1100; Agilent Technologies) located at the Scientific and Technological Research Center of Inonu University. A C18 column with dimensions of 250×4.6 mm×5 µm was used in the HPLC device. A methanol-water (80:20) solution was used as the mobile phase, and the flow rate was set to 1.0 mL/minute. The injection volume was determined as 10 µL, and measurements were made using a diode array detector (DAD). Absorbance was measured at 210 nm. All solutions were prepared at HPLC grade (Table 1). Characteristic peaks of MMA monomer were observed at 4.6 minutes. A 5-point standard calibration curve was created with concentrations of 5, 10, 25, 50, and 100 ppm. MMA concentrations in the specimens were calculated using the obtained linear equation (Fig. 3).

The data obtained from the study were analyzed using a software program (R version 4.1.1; R Programming Language). The Kruskal–Wallis H test was applied for the comparison of data that did not show a normal distribution, whereas one-way analysis of variance (ANOVA) was used for data exhibiting a normal distribution (α = .05) .

(HPLC: High performance liquid chromatography)

The resulting chromatograms showed the characteristic peak areas and signal intensities measured in milliabsorbance units (mAUs) for each specimen. The peak area values ​​obtained from each specimen were converted to residual monomer content in ppm using a previously prepared calibration curve. The HPLC revealed that residual methyl methacrylate monomer was present in all the denture bases. The chromatogram graphs of the specimens for days 1, 2, 3, 7, and 15 are shown in Figs. 3 to 7.

In the comparisons among groups, the mean residual monomer release on the first day was 22.24 ppm in Group CH, 20.91 ppm in Group IM, 21.05 ppm in Group CM, and 21.99 ppm in Group 3D. A statistically significant difference was observed among the groups regarding residual monomer release on the second day (P < .001). The mean values were 16.93 ppm for Group CH, 14.36 ppm for Group IM, 19.77 ppm for Group CM, and 18.10 ppm for Group 3D. A statistically significant difference was also found among the groups on the third day (P < .001), with median values of 12.08 ppm in Group CH, 3.76 ppm in Group IM, 14.40 ppm in Group CM, and 0.48 ppm in Group 3D. On the seventh day, statistically significant differences persisted among the groups (P < .001), with median values of 0 ppm in Groups M, E, and C, and 0.11 ppm in Group 3D.

In the within-group comparisons, a statistically significant difference was found among the median residual monomer release values over time in Group CH (P < .001). The median value was 22.6 ppm on the first day, 17.43 ppm on the second day, 12.08 ppm on the third day, and 0 ppm on the seventh day. In Group IM, significant differences were also observed over time (P < .001), with median values of 20.85 ppm on the first day, 14.44 ppm on the second day, 3.76 ppm on the third day, and 0 ppm on the seventh day. Similarly, Group CM showed statistically significant differences between time points (P < .001), with median values of 21.3 ppm on the first day, 20.03 ppm on the second day, 14.4 ppm on the third day, and 0 ppm on the seventh day. In Group 3D, statistically significant differences were also identified across time points (P < .001), with mean values of 21.99 ppm on the first day, 18.10 ppm on the second day, 0.39 ppm on the third day, and 0.11 ppm on the seventh day (Tables 2, 3).

^xOne-Way Analysis of Variance (ANOVA); ^yKruskal–Wallis H Test; ^a−cNo significant difference between groups with same letter. ^A−D No significant difference between time points with same letter.

CH, conventional heat-polymerized; CM, CAD-CAM milling; IM, injection molding; SD, standard deviation; 3D, 3D printed.

CH, conventional heat-polymerized; CM, CAD-CAM milling; IM, injection molding; SD, standard deviation; 3D, 3D

The null hypothesis that no significant difference in the residual monomer release would be found among acrylic resin denture bases fabricated using different manufacturing techniques and at different time intervals was rejected because of the significant differences in the groups. The results demonstrated that the manufacturing technique had a significant effect on residual monomer release. Residual monomers released from these denture base materials exhibit cytotoxic effects, which may lead to mucosal irritation and tissue sensitization.​[29–31] The authors are unaware of previous published research comparing the residual methyl methacrylate concentrations among denture bases fabricated using the injection molding, 3D printing, and CAD-CAM milling techniques.​[5] ​In the present study, when the 15-day cumulative residual monomer release was evaluated, the conventional heat-polymerized method demonstrated a higher amount of residual monomer release than the CAD-CAM milled, injection-molded, and 3D printed methods, although no statistically significant difference was observed in residual monomer release among the groups on the first day. (P > .05)

The findings of the present study were consistent with previous research investigating residual monomer release from denture base materials fabricated using different processing techniques. Tuna et al​[7] reported no significant difference in residual monomer release between conventional and injection-molded methods within the first 24 hours. Similarly, Steinmassl et al​[19] reported no statistically significant variation over a 7-day period in CAD-CAM-milled removable complete dentures when compared with conventionally heat-polymerized dentures. In contrast, some studies have demonstrated differing outcomes. Engler et al​[32] reported a statistically significant difference in residual monomer release between conventional and CAD-CAM dentures on the seventh day, using UV spectrophotometry for analysis. Ayman et al​²¹​ likewise identified a difference after 7 days when comparing the same fabrication techniques; however, their analysis was performed using gas chromatography (GC) rather than high-performance liquid chromatography (HPLC), which may have influenced the reliability and comparability of their findings. Furthermore, Srinivasan et al​[5] reported a statistically significant difference in monomer release within the first 24 hours between CAD-CAM-milled and 3D printed denture bases. Conversely, Al-Otaibi et al​[17] reported no significant difference among conventional, CAD-CAM-milled, and 3D printed materials at pH 6.5. In the present study, however, the 3D printed group exhibited a lower level of residual monomer release than the other 2 groups. This variation may be attributed to differences in the material brands, resin formulations, or processing parameters used across studies.

Limitations of this study included that the research was conducted under in vitro conditions; therefore, the findings may have limited generalizability to clinical settings. In the actual oral environment, biological and environmental factors—such as salivary flow rate, buffering capacity, pH fluctuations, thermal changes, and individual dietary habits—can significantly influence residual monomer release. Moreover, only 1 brand of material was used for each fabrication technique, which restricted the assessment of possible variations among different resin formulations or processing parameters provided by various manufacturers. Consequently, future studies should be conducted in vivo, with larger sample sizes and including multiple material brands and manufacturing systems, to enhance the clinical relevance and validity of the findings.

Based on the findings of this in vitro study, the following conclusions were drawn;

The clinical significance of this study lies in showing that injection-molded and 3D-printed denture base materials release markedly lower residual monomer, indicating improved biocompatibility. The peak release within the first 24 hours highlights the benefit of pre-delivery water immersion in reducing mucosal irritation.