Evaluation of flexural strength and surface properties of pre-polymerized CAD/CAM PMMA-based polymers used for digital 3D complete dentures

Evaluation of flexural strength and surface properties of pre-polymerized CAD/CAM PMMA-based polymers used for digital 3D complete dentures

MENA Clinical Dentistry

26. January 2019

Mustafa Arslan, Gulce Alp, Ali Zaimoglu, Sema Murat

Purpose: The objectives of this in-vitro study were to evaluate the flexural strength (FS), surface roughness (Ra) and hydrophobicity of PMMA-based CAD/CAM polymers; and to compare the properties of different CAD/CAM PMMA-based polymers and conventional heat-polymerized PMMA following thermal cycling.

Materials and Methods: Twenty rectangular-shaped specimens (64×10×3.3 mm) were fabricated from three CAD/CAM poly (methyl methacrylate) (PMMA)-based polymers [M-PM-Disc (M); AvaDent Puck Disc (A); Pink CAD/CAM Disc (P)] and one conventional heat-polymerized poly (methyl methacrylate) (PMMA) (Promolux® (C)) according to ISO 20795-1:2013 standards. The specimens were divided into 2 subgroups (n = 10), a control and thermo-cycled group. The specimens in the thermo-cycled group were subjected to 5000 thermal cycling procedure (5 -55°C; 30-s dwell times). Ra value was measured by using a profilometer. Contact angle (CA) was assessed with sessile drop method in order to evaluate surface hydrophobicity. In addition, FS of the specimens was tested in a universal testing machine at a crosshead speed of 1.0 mm/min. Surface texture of the materials was assessed using scanning electron microscope (SEM). The data were analyzed using two-way ANOVA, followed by Tukey HSD post-hoc test, (α ˂ .05).

Results: CAD/CAM PMMA-based polymers showed significantly higher FS than conventional heat-polymerized PMMA for each group (P < .001). CAD/CAM PMMA-based polymer P, showed the highest FS whereas conventional PMMA showed the lowest FS before and after thermal cycling (P < .001). There were no significant differences among the Ra values of the tested denture base polymers in the control group (P ˃ .05). In thermal cycling groups, the lowest Ra value was observed for CAD/CAM PMMA-based M polymer (P < .001), whereas A, P CAD/CAM PMMA-based and conventional PMMA-C polymers had similar Ra values (P ˃ .05). Conventional PMMA had significantly lower CA, and consequently higher hydrophilicity compared to CAD/CAM polymers in control group (P < .001). In the thermo-cycled group, CAD/CAM PMMA-based A polymer and conventional PMMA had significantly higher CA, and consequently lower hydrophilicity when compared to M and P CAD/CAM polymers (P < .001). However, no significant differences were found among other materials (P ˃ .05).

Conclusions: The flexural strength and hydrophobicity of CAD/CAM PMMA-based polymers were higher than the conventional heat-polymerized PMMA, whereas CAD/CAM PMMA-based polymers had similar Ra values with conventional PMMA. Thermal cycling had a significant effect on flexural strength and hydrophobicity except for surface roughness of denture base materials.

Introduction

Computer-aided design and computer aided manufacturing (CAD/CAM) has a wide range of applications in dentistry due to its advanced data acquisition and production capability. CAD/CAM technology is routinely used for the fabrication of inlay, onlay, full crown restorations, fixed and removable partial prostheses, removable and fixed implant-supported prostheses, and maxillofacial prosthesis.1

In recent years, CAD/CAM technology has become an alternative to conventional methods in the fabrication of complete dentures. Clinical and laboratory protocols include the combined use of manual and digital procedures. However, fabrication of CAD/CAM digital dentures shortens the number of clinical appointment procedures from five to two.1-4

With the recently increased interest in complete dentures fabricated by using computer-aided technology, a range of CAD/CAM poly (methyl methacrylate) (PMMA)-based polymers was introduced by manufacturers. Currently, the fabrication of CAD/CAM digital dentures is accessible to practitioners from five manufacturers: AvaDent (Global Dental Science LLC), Dentca (Dentca Inc), Ceramill full denture system (Amann Girrbach AG), Weiland digital denture (Ivoclar Vivadent Inc) and Baltic system (BDS; Merz Dental GmbH). CAD/CAM PMMA-based polymers which are utilized for the fabrication of the denture base provide a superior fit and strength in comparison to the conventionally processed base. Also, CAD/CAM PMMA-based polymers undergo no polymerization shrinkage due to milling of blanks and contain less residual monomer. They are also more hydrophobic than conventionally processed acrylic resin which results in a more bio-hygienic denture.1,3-6 Moreover, highly crossed-linked PMMA-based blanks are industrially polymerized under standardized conditions at high temperature and pressure in order to improve mechanical properties.7-10

Denture base polymers should possess adequate mechanical properties in order to prevent denture fracture for successful denture construction and patient satisfaction. Dentures are subjected to flexural stress during mastication which creates internal stresses that cause cyclic deformation of denture base polymer resulting in the crack formation and, eventually, fracture of the denture. Therefore, high flexural strength is crucial to denture wearing success, as alveolar resorption is a gradual, irregular process that leaves tissue-born prostheses unevenly supported.11,12

In addition to mechanical properties, surface roughness and surface free energy are other important factors for denture base materials which affect bacterial plaque retention, discoloration, oral health, and patient satisfaction.13-15 Previous studies,13,14,16-21 indicated that surfaces with a lower surface free energy (hydrophobic) and a lower roughness delay plaque accumulation on polymeric materials.

Denture base materials are routinely exposed to thermal stresses that cause degradation of surface and weaken mechanical properties of materials during consumption of hot and cold foods and beverages in the oral cavity.22-24 Thermal stresses may increase the water absorption of the material by increasing the distance between the polymer chains.25 An increase in temperature causes the water molecules to diffuse into the base materials quicker,26 thus diffused water in the polymer acts as a plasticizer, allowing the chains to slide under chewing forces easily. This phenomenon has a negative impact on the mechanical properties of the polymers. In addition, the effect of thermal cycling on the surface properties of denture base polymer has not been thoroughly investigated.

Moreover, no published information regarding the surface and the mechanical properties of the CAD/CAM PMMA-based polymers is available so far. Therefore, the aim of the present study was to evaluate the flexural strength, surface roughness and hydrophobicity of CAD/CAM PMMA-based polymers before and after thermal cycling.

               For the present study, three null hypotheses were addressed: (1) the flexural strength of CAD/CAM PMMA-based polymers is not different from a conventional heat-polymerized PMMA, (2) the surface roughness and hydrophobicity of CAD/CAM PMMA-based polymers are not different from conventional heat-polymerized PMMA, and (3) thermal cycling does not affect the mechanical and surface properties of denture base resins.

Materials and Methods

               Three commercially available CAD/CAM PMMA-based polymers along with 1 conventional heat-polymerized PMMA, were assessed. Table 1 lists the four denture base resins used, including manufacturer and material information.

Table 1 Tested denture base polymers

Material name Code Polymer type Manufacturer
Promolux [C] Conventional PMMA Merz Dental GmbH
M-PM Disc [M] CAD/CAM PMMA-based polymer Merz Dental GmbH
AvaDent Puck Disc [A] CAD/CAM PMMA-based polymer AvaDent Global Dental Science LLC
Polident Pink CAD/CAM Disc [P] CAD/CAM PMMA-based polymer Polident d.o.o

Specimen preparation

The specimens of each group (n=20) were prepared according to ISO 20795-1:2013 (E) (Dentistry-Base polymers -Part 1: Denture base polymers)27 and were divided into 2 subgroups (n=10), a control and thermo-cycled group. In order to prepare heat-polymerized conventional PMMA specimens, uniform, rectangular prism-shaped wax (Cavex® Holland BV, RW Haarlem, Netherlands) specimens (64(ı)×10(w)×3.3(h) mm) were fabricated using metal matrix. A standard resin denture-making technology was applied according to the manufacturer’s instructions.

In order to prepare standard specimens from CAD/CAM PMMA-based polymers, virtual design of rectangular prisms with dimensions of 64(ı)x10(w)x25(h) mm were made with the help of a computer program (Yenadent CAM 5.1; Yenadent Ltd., Istanbul, Turkey) and then converted into a stereolithography (STL) file. Thereafter, rectangular prism sheets from blanks were milled (Yenadent D14; Yenadent Ltd., Istanbul, Turkey). After the milling process, CAD/CAM polymer sheets were sectioned using a precision cutter (Secotom 10 isomet; Struers A/S, Ballerup, Denmark) in order to obtain 20 standard specimens for each CAD/CAM PMMA-based polymer.

All specimens were polished with silicon carbide papers (grits 120, 400, 600 and 800) to the final dimension. A digital caliper (IP54; ShanTM) was used to measure the final dimensions of the specimens. All specimens were immersed in water at 37ºC for 48±2 hours prior to the thermal cycling procedure. Specimens in thermo-cycled group were subjected to thermal cycling protocol (5000 cycles 5/55°C, 5 min/cycle) by using thermal cycler (MTE-101; Moddental; Esetron Smart Robotecnologies, Ankara, Turkey) in order to simulate a clinical aging process.

Flexural strength testing

For the determination of flexural strength (FS; [MPa]), a three-point bending test was conducted using a universal testing machine (Lloyd LRX; Lloyd Instruments Ltd., Fareham, UK), with Nexygen software computer program (Nexygen-MT; Lloyd Instruments). After mounting the specimens in the testing device apparatus with 50 mm distance between two vertical supports, the vertical load was applied midway between the supports with 5 mm/minute crosshead speed until the specimen was fractured, and the maximum load at fracture was recorded. The FS data (s) was calculated according to the following formula: s = 3Fd/2wh2, where F is the applied load (N) at the highest point of the load–deflection curve, d is the span length (50.0 mm), w is the measured width of the test specimen, and h is the measured thickness of the test specimen. Immediately after flexural testing, fracture pieces were recovered and used to determine surface roughness and hydrophobicity.

Surface roughness measurement

The surface roughness (Ra, μm) was determined at three spots of each specimen (one in central position, and two at the margins) by using a profilometric contact surface measurement device (Perthometer M2; Mahr GmbH, Gottingen, Germany) (with a measurement length of 5.5 mm and 0.5 mm/s) in order to obtain the general surface characteristics of the specimens. Ten specimens were used for each material and for each group. Three readings were conducted for each specimen, and a mean value was calculated.

Contact angle measurement

For the evaluation of surface hydrophobicity, water contact angles (CA) were measured using an automated contact angle measurement device equipped with a video camera and an image analyzer (OCA 15 plus; Dataphysics Instruments GmbH, Filderstadt, Germany). All specimens were cleaned ultrasonically with sterile water for 20 min in order to remove any contaminants. Thereafter, the specimens were dried in an oven at 30°C for 15 min and used for physicochemical characterization by the sessile drop method. For each specimen, three drops of deionized water (12 μm) were analyzed on 10 specimens (30 measurements in total per each material), and the left and the right contact angle of each drop were averaged.

Microscopic evaluation

Two specimens randomly selected from each tested group were gold sputtered and subjected to microscopic surface analyses, using a scanning electron microscope (SEM) (x5000 magnification) (Nova Nanosem 430; FEI); in order to assess the surface topography of the specimens.

Statistical analysis

Data analyses were performed with IBM SPSS Statistics 22 software (IBM SPSS Inc., Chicago, IL, USA). A two-way analysis of variance (ANOVA) was conducted to determine the effects of material and thermal cycling on mechanical and surface properties; and to assess the interaction between the two factors. Post-hoc analysis (Tukey HSD test) was carried out to determine the differences between materials evaluated. Statistical significance was set at P < .05.

Results

The mean flexural strength and mean standard deviation (SD) values of denture base materials are shown in Figure 1. The two-way ANOVA indicated that both material and thermal cycling influenced FS but there was no significant interaction between the two main factors (P < .05). Tukey HSD test determined that CAD/CAM PMMA-based P polymer showed the highest FS whereas conventional PMMA possessed the lowest FS in the control group (P < .001). FS of CAD/CAM PMMA-based A and M polymers were significantly higher (P < .001) than that of conventional PMMA-C, however; no significant differences (P > .05) were observed between M and A CAD/CAM PMMA-based polymers. In thermo-cycled group, CAD/CAM PMMA-based P polymer showed the highest FS, whereas conventional PMMA-C showed the lowest (P < .001). FS of CAD/CAM PMMA-based polymer M was significantly higher (P < .001) than those of CAD/CAM PMMA-based A polymer and conventional PMMA-C. Besides, FS of CAD/CAM PMMA-based A polymer was higher (P < .001) than that of conventional PMMA-C. Moreover, FS of all denture base polymers decreased significantly following thermal cycling (P < .001).

Fig 1 Comparison of mean FS values of four denture base polymers before and after thermal cycling. Different single letters denote statistical difference (* indicates significant difference between two groups).

Figure 2 shows the mean and SD of the Ra values of denture base polymers before and after thermal cycling. The two-way ANOVA indicated that material and thermal cycling had no significant effect (P > .05); however, there was a significant interaction between the material and thermal cycling (P < .001). In the control groups, no significant differences (P > .05) were found among all the materials for Ra value. The highest mean Ra was found for conventional PMMA whereas the lowest Ra was found for CAD/CAM PMMA-based M polymer. In the thermo-cycled group, the Ra value of CAD/CAM PMMA-based M polymer was significantly lower (P < .001) than those of A and P CAD/CAM polymers. However, no significant differences (P > .05) were found among other denture base polymers. The highest mean Ra value was found for P CAD/CAM polymer, while the lowest Ra value was found for CAD/CAM M polymer (P < .001). The post hoc Tukey HSD test revealed no significant differences (P > .05) between the control and the thermal cycling groups for each polymer. Conventional PMMA-C and CAD/CAM M polymer showed reduced Ra values whereas CAD/CAM A and P polymers showed increased Ra values following thermal cycling (P > .05).

Fig 2 Comparison of mean Ra values of four denture base polymers before and after thermal cycling. Different single letters denote statistical difference (*indicates no significant difference between two groups).

The mean contact angle values are presented in Figure 3. The two-way ANOVA indicated that material influenced CA of denture base polymers; in addition, there was also a significant interaction between the material and thermal cycling (P < .05). Tukey HSD test determined that the highest hydrophilicity, characterized by the lowest CA was calculated for conventional PMMA-C in the control groups. Conventional PMMA-C had significantly lower (P < .001) CA, and consequently lower hydrophobicity when compared to M, P and A CAD/CAM PMMA-based polymers. However, no significant differences (P > .05) were found between CAD/CAM PMMA-based M and P polymers. CA of CAD/CAM PMMA-based polymer, A, was also found to be significantly higher (P < .001) than those of conventional PMMA-C and CAD/CAM PMMA-based M and P polymers. In the thermo-cycled groups, CAD/CAM PMMA-based A and P polymers had significantly higher (P < .001) CA, and consequently lower hydrophilicity when compared to M and P CAD/CAM PMMA-based polymers. However, no significant differences (P > .05) were found among other denture base polymers. Furthermore, there were statistical significant differences (P < .001). between the control and thermal cycling groups for each polymer. CA of all the materials in the thermo-cycled group, apart from conventional PMMA-C, were found to be significantly lower (P < .001) than those of control group. On the other hand, thermal cycling increased the contact angle of conventional PMMA significantly (P < .001).

Fig 3 Comparison of mean CA values of four denture base polymers before and after thermal cycling. Different single letters denote statistical difference (*indicates significant difference between two groups).

Representative SEM images of conventional PMMA-C, CAD/CAM PMMA-based polymer M, A and P specimens before (Fig 4) and after thermal cycling are presented in Figure 5.

Fig 4a to d SEM images (×5000 magnification) of denture base resins before thermal cycling. (a) Conventional PMMA [C]; (b) CAD/CAM PMMA- based polymer [M]; (c) CAD/CAM PMMA-based polymer [A]; (d) CAD/ CAM PMMA-based polymer [P].
Fig 5a to d SEM images (×5000 magnification) of denture base resins after thermal cycling. (a) Conventional PMMA [C]; (b) CAD/CAM PMMA- based polymer [M]; (c) CAD/CAM PMMA-based polymer [A]; (d) CAD/ CAM PMMA-based polymer [P].

Discussion

Based on the results obtained, the first null hypothesis which suggested that CAD/CAM PMMA-based polymers exhibit flexural strength in the same range as conventional heat-polymerized PMMA is rejected. On the other hand, the results obtained reveal that CAD/CAM PMMA-based polymers showed a statistically insignificant difference in surface roughness when compared to conventional heat-polymerized PMMA; thus the second null hypothesis is accepted. It was also observed that significant differences (P < .001) occurred in flexural strength and hydrophobicity except for surface roughness of denture base materials before and after thermal cycling. Therefore, the hypothesis which suggested that thermal cycling would not affect the flexural strength, surface roughness or hydrophobicity of denture base materials is partially rejected.

CAD/CAM PMMA-based polymers showed significantly higher (P < .001) flexural strength than conventional PMMA before and after thermal cycling. According to the findings of this study, the polymerization technique that was used to produce CAD/CAM PMMA-based polymer blanks might facilitate higher mechanical properties than those of conventional heat-polymerized PMMA. Similarly, a recent study,9 found that heat polymerization under a high pressure of 250 MPa at a high temperature of 180 ºC, increased the flexural strength of commercially available dental resin composites. It is likely that polymerization under high pressure/high temperature resulted in a reduction of the number and size of defects which led to an increase in flexural strength.28 Moreover, polymerization under high pressure/high-temperature technique increased the degree of polymerization, and reduced residual monomer, which subsequently improved the strength of pre-polymerized blank materials.7-10 On contrary, Murakami et al,11 reported that polymerization under a high pressure of 500 MPa decreased flexural strength and elastic modulus of PMMA denture base resin. In this scenario, reduction in flexural strength might be due to the high pressure which created interfacial stress on the border between filler and matrix that could cause micro-cracks formation on the tensile bottom surface of the specimens. Further studies should focus on the effects of the high pressure/high temperature condition on the mechanical properties of polymers.

CAD/CAM PMMA-based polymer investigated in the current study exhibited the highest strength even after thermal cycling. Although the mechanical properties of commercially available CAD/CAM PMMA-based polymer have not been reported in the literature, this was in agreement with that claimed by manufacturers, which flexural strength of CAD/CAM PMMA-based polymers P (114 MPa) and M (91.5/96.6 MPa) were higher than those of previously reported values for conventional heat-polymerized PMMA (generally ranged between 70-90 MPa).7,29,32,33 According to the findings of this study, all the tested denture base resins passed the requirement of ISO 20795-1:201317 regarding flexural strength before and after thermal cycling (˃ 65 MPa).

In the present in vitro study, thermal cycling was chosen in order to simulate the clinical oral environment in terms of temperature and moisture which cause thermal stress. According to our finding, the flexural strength of all denture base polymers decreased significantly following 5000 thermal cycling. This finding is in agreement with several studies,10,23,24,32,33 which reported that the flexural strength of denture base resins reduced significantly after thermal cycling. The results of the present study disagreed with that of Ayaz et al,29 who showed that thermal cycling did not cause significant adverse effect on the flexural strength of PMMA base resins. In present study, it was observed that surface roughness values of CAD/CAM PMMA-based polymers were insignificantly lower than conventional heat-polymerized PMMA in control groups. In addition, this study demonstrated that the roughness of all resin specimens was at the threshold value or slightly higher than the threshold value (0.2 μm) reported by Bollen et. al.15

Although several previous studies investigated the effect of thermal cycling on the surface and mechanical properties of restorative materials with different polymer structures, only a limited number of studies24,29,32,33-35 examined the effect of thermal cycling on the surface properties of denture base resins. Ayaz et al,29 reported that thermal cycling caused an increase in surface roughness in denture resin materials without statistically significant difference. Conversely, Lira et al,34 reported that thermal cycling significantly affected surface roughness of microwaved resins and microwaved resins showed lower values of roughness after thermal cycling. It was observed that CAD/CAM PMMA-based M polymer and conventional PMMA showed reduced surface roughness values whereas CAD/CAM PMMA-based A and P polymers showed increased surface roughness values after thermal cycling. However, according to the results of this study, changes in roughness values were not statistically significant. The results of this study coincided with those of Ayaz et al,29 and Wieckiewicz et al,35 who found that thermal cycling (5000 cycles) did not cause statistically significant changes in surface roughness values of denture base resins. Inconsistent results among different studies can be explained by variations in the water solubility, surface hardness, microstructures, and chemical configurations of resins studied.36

According to the results obtained from SEM observations, conventional PMMA exhibited more porous surface with multi dots and surface irregularities than CAD/CAM PMMA-based polymers. This finding was supported by the surface roughness results that indicated conventional heat-polymerized PMMA to show rougher surface than CAD/CAM PMMA-based polymers without statistical significance. Additionally, CAD/CAM PMMA-based M polymer showed more homogeneous surface along with randomly located micro pores and pits than other denture base polymers. Scratch lines made by the abrasive polishing papers were also observed in all polymers. Moreover, on SEM evaluation, denture base polymers showed surface characteristics after thermal cycling similar to untreated specimens.

Surface free energy is another important factor which affects bacterial adhesion and biofilm formation on material surfaces and contact angle measurement is a reliable technique used to assess the surface free energy of the materials.13,16,37 In principle, materials having lower surface free energy (hydrophobic) exhibit higher value of water contact angle.37 Previous studies,13,15,16,19-21 indicated a negative correlation between solid hydrophobicity and the number of adhered cells (or biofilm). In the present study, the highest contact angle values for CAD/CAM PMMA-based polymers was determined, while the lowest contact angle values for conventional PMMA (P < .001). These results suggest that CAD/CAM PMMA-based polymers showed more hydrophobic properties causing less bacterial adhesion than conventional PMMA. This might be due to the presence of less residual monomer as a result of the production of the corresponding CAD/CAM PMMA-based polymers under high pressure and temperature or the polarity of the molecules in the polymer matrix.36,38 Additionally, it was observed that CAD/CAM PMMA-based polymer showed a significantly lower (P < .001) contact angle following thermal cycling. Thermal cycling reduced the hydrophobicity of CAD/CAM PMMA-based polymers whereas it increased the hydrophobicity of conventional PMMA. It is likely that thermal cycling process actually reduced residual unpolymerized components of conventional PMMA, which probably enhanced hydrophobic properties.

One limitation of the present study was that the experimental design had limitations in accurately replicating clinical conditions. The researchers also failed to simulate repetitive mechanical stressing which included repeated loads during mastication. Besides, a clinical long-term prospective study designed to evaluate whether if there is a clinical significance between properties and long-term use of materials would be contributive to current knowledge. Assessment of micro-hardness, water solubility and residual monomer content of materials after thermal cycling were not in the scope of the present research. Therefore, surface hardness, solubility properties of polymers and also the effects of the polymerization methods used in the production of the materials should be examined in more detail; in order to clarify the influence of aging procedures on the surface and mechanical properties of denture base resins.

Conclusion

Within the limitations of this in vitro study, the following conclusions are drawn:

  1. CAD/CAM PMMA-based polymers showed significantly higher flexural strength than that of conventional resin before and after thermal cycling. Thermal cycling resulted in decreased flexural strength for all denture base polymers.
  2. There were no significant differences (P < .001) among the surface roughness of tested denture base polymers before thermal cycling. Thermal cycling did not have a significant effect on the surface roughness of denture base polymers.
  3. Conventional PMMA had significantly lower contact angle and consequently higher hydrophilicity than CAD/CAM PMMA-based polymers before thermal cycling. Thermal cycling resulted in a significant decrease in the hydrophobicity of CAD/CAM PMMA-based polymers and a significant increase in the hydrophobicity of conventional PMMA.

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Authors

Dr. Semar Murat

Dr. SEMA MURAT, Besevler Ankara University, semamurat47@yahoo.com.tr, Turkey

Mr. Mustafa Arslan, Prosthodontics, Istanbul Aydin University

Dr. Gulce Alp, Prosthodontics, Okan University

Prof. Ali Zaimoglu, Prosthodontics, Istanbul Aydin University