2024 Volume 66 Issue 2 Pages 120-124
Purpose: To evaluate the flexural properties of repaired poly(methylmethacrylate) (PMMA) denture base materials for computer-aided design/computer-aided manufacturing (CAD-CAM) and to compare them with heat-activated polymerized PMMA.
Methods: A total of 288 specimens (65 × 10 × 2.5 mm) were prepared using both CAD-CAM and conventional blocks and repaired using autopolymerizing and visible-light polymerizing (VLC) materials. Microwave energy, water storage and hydroflask polymerization were applied as additional post-polymerization cycles after the repair process. The flexural strength (FS) of the specimens was evaluated using the three-point bending test. Data were evaluated statistically using 2-way ANOVA followed by Bonferroni’s correction to determine the significance of differences between the groups (P ≤ 0.05).
Results: The FS of the denture base materials for CAD-CAM was significantly higher than that for the heat-activated group (P ≤ 0.05). The FS was significantly highest when microwave energy was used for the post-polymerization cycle. The FS values for all groups repaired with VLC resin were significantly lower than for the autopolymerization group (P ≤ 0.05).
Conclusion: The flexural properties of denture base materials for CAD-CAM repaired using autopolymerizing acrylic resins can recover by 50-70%. Additional post-polymerization cycles for autopolymerizing repair resin can be suggested to improve the clinical service properties of repaired dentures.
In recent years, the computer-aided design/computer aided manufacturing (CAD-CAM) technique has been used successfully for the production of complete denture applications.
Dentures fabricated using CAD-CAM have some advantages over those made conventionally in that they offer a better denture base fit due to absence of polymerization shrinkage [1,2,3], a minimal amount of residual monomer [4,5] and minimized porosity [6,7] because pre-polymerized blocks are used [8,9]. This reduces the number of patient appointments and cost [10], minimizes microorganism contamination [11], and simplifies repeat production of the dentures in case of complications [12,13], thus improving both patient and dentist satisfaction [14], standardization, and quality control [15,16].
Fracture is one of the most common problems of complete dentures [17,18,19,20], with functional and financial repercussions for both patient and dentist. Denture repair is considered to be the easiest and most cost-effective method for restoring both esthetics and function in the short term [21,22,23].
Although many previous studies have investigated the flexural and physical properties of denture base materials [24,25,26,27,28] used for the CAD-CAM technique, only a few [29,30] have focused on the repair properties of milled. Polymethyl methacrylate (PMMA) materials. This may be because such materials allow easy digital re-fabrication within a short period, and have a low likelihood of fracture due to their strong mechanical properties [21,22,23]. Repair of a denture fabricated using CAD-CAM might be as important as that for conventional materials in view of the functional and esthetic inconvenience of creating a new denture, as well as the additional cost.
Repair materials such as autopolymerizing and VLC resins, or repair techniques that require only basic equipment and application, such as bench-curing [31,32,33,34,35], visible light curing [35,36,37,38,39,40] or hydroflask curing [32,33,38] can be useful and preferable in everyday prosthodontic practice. The application of additional post-polymerization cycles for autopolymerizing acrylic repair resin designed to improve the success and longevity of repaired conventional denture base materials to the repair of CAD-CAM denture base materials would yield useful additional scientific data. On the other hand, it might be clinically sensible to test a VLC repair material based on urethane dimethacrylate (UDMA) within a uniform study protocol [31,33,34,35].
Polymerization methods using autopolymerizing PMMA result in incomplete conversion of monomer to polymer, leaving a certain amount of residual MMA. For reduction of this residual monomer, additional post-polymerization steps such as immersion in water up to 65°C [32,36,39] or application of microwave energy [33,40] have been suggested. Application of additional cycles to restore the original strength of the denture base after repair has also been considered [32,33,36,38,39].
To date, no reported study has investigated the repair of milled PMMA using autopolymerizing resins with additional post-polymerization cycles as well as the use of VLC resin. Therefore, the aim of the present study was to evaluate the flexural strength of denture base materials subjected to the CAD-CAM technique after repair with autopolymerizing and VLC resin with additional post-polymerization cycles in comparison with repaired heat-activated polymerized poly(methylmethacrylate) (PMMA) denture base material.
The null hypothesis (H0) was that the type of resin used for repair and additional post-polymerization cycles would affect the flexural strength of the repaired milled and conventional denture base materials.
Pre-polymerized PMMA-based denture base pucks of a standard size (98 mm diameter and 25 mm thickness) manufactured for the CAD-CAM technique by 3 different manufacturers were included in the study (Table 1). The pucks were milled into specimens measuring 65 × 10 × 2.5 mm for flexural testing.
As a control group, specimens with the same dimensions were fabricated using a conventional heat-activated polymerized PMMA denture base material (Paladent 20, Heraus Kulzer GmBH&Co. Hanau, Germany) (Table 1). Acrylic resin was mixed using a powder/liquid ratio of 23.4 g/10 mL in accordance with the manufacturer’s recommendations. After homogeneously mixing the resin for 60 s at room temperature (23 ± 2°C) and waiting for 15 min of working time, the mixture was poured into gypsum molds. For the polymerization process, metal flasks were placed in a thermostatically controlled water bath (Kavo Elektrotechnisches Werk GmbH, Biberach Germany) at room temperature and then the water was heated to 74°C in accordance with the manufacturer’s instructions. After holding this temperature for 30 min, it was raised to 100°C and maintained for 30 min. The flasks were then allowed to cool to room temperature in the water bath. After deflasking, the resin specimens were removed, and any excess resin was trimmed off with a handpiece and tungsten carbide bur.
| Manufacturer | Code | Polymerization Type | |
|---|---|---|---|
| Bilkim Pmma Blank | Bilkim Co. Ltd., İzmir, Turkey | P1 | prepolymerized puck |
| Smile-cam | Pressing Dental Srl San Marino, İtaly | P2 | prepolymerized puck |
| M-pm disc | Merz Dental GmbH, Lütjenburg, Germany | P3 | prepolymerized puck |
| Paladent 20 | Heraus Kulzer GmbH, Hanau, Germany | H | Heat-activated polymerization powder and liquid |
To simulate the clinical repair process, the intact specimens were first embedded in plaster molds. After setting, each specimen and its corresponding mold was recorded with a number and then the specimens were removed from the mold (Fig. 1).
To mimic denture fracture, the specimens were divided into 2 equal pieces with a tungsten carbide bur (Rapidy Microbur, Bredent GmbH, Senden, Germany) at a rotation speed of 2,000 rpm.
To adjust the repair gap to 2 mm with a 45-degree bevel, guide marks were drawn on the specimen surfaces to facilitate milling 2 mm from the top and 7 mm from the bottom (Fig. 2). The repair surfaces of all specimens were milled with a carbide bur (Frank Dental, Gmund am Tegernsee, Germany) at 1,000 rpm and then smoothed under running tap water using 200 and 400 grit sandpaper (Waterproof silicon carbide paper, English Abrasives Ltd., London, UK). The definitive dimensions of each specimen were checked with a digital caliper (Absolute Digimatic Caliper, Mitutoyo, Kawasaki, Japan). After the definitive dimensions of the specimens had been confirmed, each specimen pair was placed back in the corresponding mold spaces.
Methyl methacrylate (MMA)-based monomer liquid (Meliodent Rapid Repair, Selfcure Denture Base Material, Heraus Kulzer GmbH) was applied to the prepared repair surfaces using a brush, followed by a 180-s pause [24]. For autopolymerizing repair resin, repair surfaces were first air-dried and then the autopolymerizing PMMA-based repair resin (Meliodent Rapid Repair Repair, Self-cure Denture Base Material, Heraus Kulzer GmbH) was packed into the repair gap using a spatula after it had been mixed at room temperature at a powder/liquid ratio of 5/3.5 by weight and waiting for 5 min, in accordance with manufacturer’s recommendations. The specimens were removed from the repair molds after allowing autopolymerization for 15 min at room temperature. Four additional post-polymerization cycles were applied to the specimens after autopolymerization as follows:
O1: Repair with autopolymerizing acrylic resin without any additional post-polymerization cycle.
O2: The repaired specimens were additionally kept in a thermostatically controlled water bath (Kavo Elektrotechnisches Werk GmbH) at 60°C for 30 min.
O3: The repaired specimens were additionally kept in a hydroflask (Kavo Elektrotechnisches Werk GmbH) containing water at 40°C under a pressure of 2.5 bar for 15 min.
O4: The repaired specimens were irradiated using microwave energy at 500 W for 3 min in a domestic-type microwave oven (Inox Microwave Oven FRN-SMG-0004, Samsung, Malaysia) at 23,000 MHz frequency and 1,100 W power output.
After each additional post-polymerization cycle, the specimens were allowed to cool to room temperature.
For VLC repairs, UDMA-based material (Eclipse Prosthetic Resin, Dentsply Int., New York, NY, USA) with a paste-like consistency was used. For repair procedures with VLC resin, specimens in the repair molds were first heated in an oven (Eclipse Conditioning Oven, Dentsply Sirona Int., Ontario, Canada) for 2 min at 55°C to facilitate adaptation of the repair resin to the mold cavity before application. Immediately after heating, the plaster molds were removed from the oven and a bonding agent (Prime & Bond Universal Simple, Dentsply Sirona GmbH, Konstanz, Germany) was applied to the repair surfaces of the specimens using a brush. After continuous air-drying of the bonding for 30 s, the bonded surfaces were light-polymerized with a hand-held light polymerization device (SmartLite max LED curing light, model: 644050, Dentsply International, Milford, DE, USA) for 20 s. After this stage, pieces of dough-like material of sufficient size were cut with a spatula and adapted to the repair cavity with finger pressure. Air barrier coating (Eclipse Air Barrier Coating, Dentsply Sirona Inc.) was applied with a brush so that oxygen in the air would not affect the polymerization reaction of the repair resin, and then polymerization was conducted for 10 min using a light curing device (Eclipse Junior, Dentsply Sirona, Inc., New York, NY, USA). After light-curing, the specimens were cooled to room temperature. The air barrier coating on the specimens was then removed by rinsing in tap water. No additional cycle was applied to the repair resin.
A sample size of 12 was selected on the basis of power analysis (G*Power program, (Power and sample size, HyLown Consulting LLC, Atlanta, GA). At least 5 specimens were required to detect differences with an alpha error of 5% and a power of 80% (d: 0.861 and standard deviation: 6). For each CAD-CAM and conventional acrylic resin and for each post-polymerization cycle, 12 specimens were fabricated for the repairs with autopolymerizing resin, giving a total of 192. For VLC repair group 12 specimens were fabricated for each of CAD-CAM and conventional acrylic resin. As as control group, 12 intact specimens for each PMMA denture block were prepared. A total of 288 specimens were tested in this study.
The flexural strength of the specimens was measured using the 3-point bending test according to ISO-1567 [27] on a universal testing machine (ModDental Universal Testing Device, Esetron, Ankara, Turkey) at a crosshead speed of 5.0 mm/min. The specimens were supported on jigs with a diameter of 3.2 mm and the span length was 50 mm (Fig. 3). The load applied was measured by a load cell attached to the crosshead of the machine via a data acquisition system connected to a computer. The load was applied vertically from the top center surface of the specimen until failure occurred. The maximum force and maximum bending for each specimen were recorded. Flexural strength values were calculated using the load and the values of bending against the load. The flexural strength of each specimen was calculated according to the following formula [28]:
FS = 3FL/(2bd2)
Where FS is the flexural strength (MPa), F is the load or force at which fracture occurred (N), L is the span of the specimen between the supports (50 mm), b is the width (10 mm), and d is the thickness of the specimen (2.5 mm).
After the 3-point bending test, a further fracture analysis was performed to evaluate the mode of failure. Fractured surfaces were examined using an optical microscope (Eclipse 200; Nikon, Tokyo, Japan) at ×50 magnification and categorized according to the percentage of retained repair material on the repaired surfaces as adhesive (up to 25%), mixed (25-75%) or cohesive (more than 75%), as indicated by Mariatos et al. [40].
The STATA 16 (StataCorp LP, College Station, TX, USA) package program was used to analyze the data. Descriptive statistics were calculated for flexural strength as the arithmetic mean (AM) ± standard deviation (SD). Two-way analysis of variance (ANOVA) was used to study the effects of repair and denture base materials, and their interaction on the flexural strength, followed by one-way ANOVA in cases where at least one of the group averages was found to be significantly different from the others. Simple effects analysis was performed by applying Bonferroni’s adjustment to analyze the interactions with a confidence level of 0.05.
Results of the Levene test supported the null hypothesis that the variance of the dependent variable was equal (Levene statistic: 1.545; df1: 23, df2: 264; P: 0.056). The statistical evaluation of the flexural strength values for the experimental groups with 2-way ANOVA and arithmetic mean values are presented in Tables 2 and 3, respectively. The comparison of flexural strength for denture base and repair material denture base was statistically significant (P ≤ 0.001) (Table 2).
No significant differences were observed between the flexural strength values of the intact CAD-CAM denture base materials (P1, P2 and P3 groups) (P > 0.05). The flexural strength values in the intact conventional heat-activated polymerized group were significantly lower than those in the intact CAD-CAM groups (P ≤ 0.05) (Table 3).
For repairs with autopolymerizing and VLC resins, no significant differences in flexural strength values were observed between any of the groups (P > 0.05). In general, the mean flexural strength values of specimens repaired with VLC were lower than those of the specimens repaired with autopolymerizing resin (Table 3).
For additional post-polymerization cycles, hydroflasking significantly provided the highest values of flexural strength for all the repaired groups relative to the values for specimens subjected to only bench-curing (P ≤ 0.05). The additional post-polymerization cycles involving water storage and microwaving of autopolymerizing repair resin increased the mean flexural strength values of all the repaired specimens but not to a significant degree (P > 0.05) (Table 3).
Table 4 shows the various percentages of mode of failure for each repair material.
| Source | Sum of squares | sd | Mean square | F | P |
|---|---|---|---|---|---|
| Repair material | 117845.276 | 5 | 23569.055 | 269.06 | <0.001 |
| Denture base material | 4208.996 | 3 | 1402.999 | 16.017 | <0.001 |
| Repair material *denture base material | 6153.317 | 15 | 410.221 | 4.683 | <0.001 |
| Error | 23125.509 | 264 | 87.597 | ||
| Total | 767761.666 | 288 |
P values for terms in the model: repair material: P < 0.001; prosthetic base material P < 0.001; repair material *prosthetic base material: P < 0.001
| Intact | Autopolymerizing resin repair | VLC repair | ||||
|---|---|---|---|---|---|---|
| O1 | O2 | O3 | O4 | L | ||
| P1 | 88.08 ± 1.14 a,A | 41.99 ± 4.09 c,A | 52.22 ± 1.48 bc,A | 52.14 ± 3.08 bc,A | 60.65 ± 1.57 b,A | 10.22 ± 0.75 d,A |
| P2 | 84.38 ± 2.27 a,AB | 39.08 ± 4.28 c,A | 55.4 ± 2.64 b,A | 50.49 ± 2.92 b,A | 53.89 ± 2.34 b,A | 11.94 ± 0.8 d,A |
| P3 | 76.75 ± 3.02 a,B | 35.03 ± 5.32 c,A | 37.69 ± 3.01 c,B | 32.04 ± 2.81 c,B | 60.0 ± 2.72 b,A | 12.68 ± 1.0 d,A |
| H | 64.47 ± 1.31 a,C | 34.28 ± 2.97 c,A | 45.77 ± 3.05 b,AB | 48.57 ± 2.3 b,A | 52.78 ± 2.98 b,A | 9.82 ± 0.71 d,A |
a-d: different letters on the same line represent statistically significant differences (P ≤ 0.05); A-C: different letters in the same column represent statistically significant differences (P ≤ 0.05)
| Autopolymerizing | VLC | |||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| P1O1 | P2O1 | P3O1 | HO1 | P1O2 | P2O2 | P3O2 | HO2 | P1O3 | P2O3 | P3O3 | HO3 | P1O4 | P2O4 | P3O4 | HO4 | P1L | P2L | P3L | HL | |
| Cohesive | 92 | 42 | 25 | 92 | 100 | 17 | 8 | 92 | 92 | 50 | 8 | 100 | 92 | 100 | 100 | 100 | 0 | 0 | 0 | 0 |
| Adhesive | 0 | 16 | 25 | 0 | 0 | 17 | 42 | 0 | 0 | 0 | 17 | 0 | 0 | 0 | 0 | 0 | 100 | 83 | 83 | 92 |
| Mixed | 8 | 42 | 50 | 8 | 0 | 66 | 50 | 8 | 8 | 50 | 75 | 0 | 8 | 0 | 0 | 0 | 0 | 17 | 17 | 8 |
The null hypothesis that the repair resin type and additional post-polymerization cycles would affect the flexural strength of the repaired CAD-CAM and conventional denture base materials was accepted. The significant difference in the flexural strength of autopolymerizing and VLC resin used for repair and the significant differences between the values obtained after post-polymerization showed the same trend for both milled and conventionally heat-activated polymerized denture base materials. The null hypothesis (H0) was that the repair resin type and additional post-polymerization cycles would affect the flexural strength of the repaired milled and conventional denture base materials was accepted.
Denture base materials manufactured for the CAD-CAM technique showed different flexural properties depending on the manufacturer. Those manufactured for CAD-CAM showed superior flexural properties than the conventional heat-activated polymerized denture base material.
One of the most important advantages of dentures made using CAD-CAM is that the data is stored digitally, facilitating ease of production of duplicate prostheses in the event of denture fracture or loss, without the need for rehearsal sessions [25]. Until CAD-CAM became possible, the aim of fractured denture repair was to complete the repair in a short time while preserving esthetics and function. Other patient-related factors, such as time and cost of obtaining a new digitally fabricated denture, should also be considered. Saponaro et al. [24] compared the clinical performance and postoperative complications of complete dentures produced using CAD-CAM and those fabricated conventionally and reported that the most common postoperative complications were poor retention, incorrect determination of the vertical dimension of occlusion, and incorrect recording of centric relationships. The present finding that the overall flexural strength of intact specimens fabricated using CAD-CAM was significantly higher than that of conventionally heat-activated polymerized specimens confirmed the results of previous studies [26,27]. The improved properties of these materials may be due to the use of pre-polymerized pucks manufactured under high pressure [8,9], lower polymerization shrinkage [1,3] and minimal residual monomer content [4,5]. To date, there have been no scientific or in vivo data related to the mechanical failure of digitally fabricated removable dentures. Although intra-oral denture failure is unlikely to be a significant factor with the CAD-CAM technique, denture fracture due to extra-oral failure such as that resulting from accidents [21,23] can occur at any time. As the use of CAD-CAM-fabricated denture base materials becomes more widespread, further data on both intra- and extra-oral failures are expected.
The main goal of denture repair is to achieve long-term bond strength between the repair material and the denture base, with restoration of the original strength. The preparation and design of repair surfaces are of great importance for the success of repair. The shape of the interface between two fractured pieces is also clinically significant. Repair surfaces and edges can be prepared in various ways to provide a better joint prior. The preparation of either 45° angled or rounded repair surfaces has been reported to increase the interface joint area, thereby shifting the interfacial tension model towards shear stress rather than damage stress [21,22]. Mahajan et al. [34] investigated the effect of repair surface design on flexural strength and found that rounded surfaces showed higher values than flat and step-shaped repair surfaces. By strengthening the repair with glass fiber, Vasthare et al. [35] stated that a 45-degree angled repair yielded higher values than a butt-joint design. In the present study, it was decided to use a 45- degree angled repair surface design for ease of preparation and better distribution of negative stresses.
Vallittu et al. [23] stated that wetting of the repair surfaces with MMA for 180 s resulted in dissolution of the PMMA surface, creating better adhesion between the repair resin and the adhering surface. In the present study, application of MMA for 180 s was chosen considering that this application time can be also suitable for denture base materials subjected to CAD-CAM, as it can increase both the adhesion surface and the mechanical properties. Since the denture pucks used in the CAD-CAM technique are pre-polymerized, future studies of other alternative shorter or longer MMA application periods that provide a dissolution layer on the PMMA surface may be practical for repair.
Repair resins using autopolymerization and VLC provide the quickest delivery of a repaired denture in a single session using basic equipment. Repairs using heat-activated polymerization are not preferred due to the need for more complicated equipment such as flasking and a prolonged working time. Pfeiffer et al. [31] compared the post-repair flexural strength and modulus of elasticity of hypoallergenic base materials and stated that repairs with VLC satisfied the standards before and after repair. On the other hand, Bural et al. [32] reported superior mechanical properties for autopolymerizing repair resin than for VLC resin, while Cilingir et al. [33] suggested that VLC resin could be an alternative repair material. In the present study, however, it was suggested that repairs with VLC resin were not recommended because of the high probability of recurrent denture base fracture, in view of the poor flexural properties of both conventional and CAD-CAM denture base materials with a minimum adhesive failure rate of 83% (Table 3). To compare the interaction between denture pucks regarding the pre-polymerized structure and both types of repair resin, it was decided to investigate autopolymerizing and VLC repair resins. Bural et al. [32] stated that additional post-polymerization cycles such as soaking in water at 60°C were simple and effective for clinical practice. Yunus et al. [38] reported the effect of microwave energy on the bending strength and residual monomer levels of autopolymerizing repair material. Polyzois et al. [37] indicated that microwave application as a post-polymerization cycle increased the fracture load by 22%, which was similar to the present findings. The application of additional post-polymerization cycles increased the mechanical values of all the denture base materials investigated in the present study. It was possible to restore the flexural properties by 50-70% when denture base materials manufactured for the CAD-CAM technique were repaired with autopolymerizing acrylic resin. Additional post-polymerization cycles for repairs with autopolymerizing acrylic resin also had a positive effect on the flexural properties of denture base materials manufactured for CAD-CAM. Digital remanufacturing of a denture base may be a safer method considering the risk of fracture recurrence, since the repaired denture bases prepared with the CAD-CAM technique did not show the same flexural strength as the intact form. The use of autopolymerizing acrylic resin as a repair material as well post-polymerization cycles resulted in better adhesive properties, indicating a general trend for a decrease in the adhesive mode of fracture (Table 4).
To improve the properties of adhesion between PMMA and autopolymerizing repair material, few studies [37,40] have indicated an overall positive effect of mechanical and chemical surface treatments on the repair bond strength of milled PMMA. Perhaps it would be more helpful if mechanical and chemical surface treatments, as well as post-polymerization cycles, could be more fully applied to increase the mechanical properties of repaired milled PMMA materials due to their dense resin matrix structure.
Repair with VLC resin is not recommended because of the increased risk of recurrent denture base fracture, as it showed poor adhesive and flexural properties for both conventional and CAD-CAM denture base materials. To date, no available study has investigated the repair process for milled denture base materials using VLC repair resin. Besides the investigation of surface treatments for milled PMMA, there is a need to develop a bonding agent when a VLC material is preferred. In addition, the combined use of varied MMA surface application before repair can also be an issue for CAD-CAM materials.
It should be noted that in vitro studies for predicting the success of a material or technique in clinical use have been limited. The present study was limited by the use of a simple rectangular specimen instead of a complex prosthesis design, as well as the lack of longer water absorption or thermal cycling.
If a successful repair is performed, fracture should not recur. In cases where it is easy to access the patient’s digital data and the cost is affordable, reconstruction of a prosthesis in a short time using the CAD-CAM technique may be more advantageous and beneficial for the patient. Opting for a quick repair process should only be considered as a short-term solution.
Digital re-fabrication instead of denture base repair may be a safer option considering the risk of fracture recurrence, since repaired denture bases prepared with the CAD-CAM technique did not show the same flexural strength as the intact form. Therefore, re-fabrication instead of repair should be suggested to the patient when a fracture of either a digitally or conventionally fabricated denture occurs.
The authors have no conflicts of interest to declare.
This study was funded by Scientific Research Projects Coordination Unit of Istanbul University. Project number: TDK-2018-29607.
ŞÖ: sebnem_oztk@hotmail.com, https://orcid.org/0000-0001-8579-7570
CBA*: cbural@istanbul.edu.tr, https://orcid.org/0000-0003-2684-5506
The authors would like to thank Associate Professor Dr Çagatay Dayan for sharing ideas regarding statistical interpretation.
