Evaluation of the Bond Strength of Denture Base Resins to Acrylic Resin Teeth: Effect of Thermocycling

Purpose: The purpose of this study was to evaluate the thermocycling effects and shear bond strength of acrylic resin teeth to denture base resins.
Materials and Methods: Three acrylic teeth (Biotone, Trilux, Ivoclar) were chosen for bonding to four denture base resins: microwave-polymerized (Acron MC), heat-polymerized (Lucitone 550 and QC-20), and light-polymerized (Versyo.bond). Twenty specimens were produced for each denture base/acrylic tooth combination and were divided into two groups (n = 10): without thermocycling (control groups) and thermocycled groups submitted to 5000 cycles between 4 and 60°C. Shear strength tests (MPa) were performed with a universal testing machine at a crosshead speed of 1 mm/min. Statistical analysis of the results was carried out with three-way ANOVA and Bonferroni's multiple comparisons post hoc analysis for test groups (α= 0.05).
Results: The shear bond strengths of Lucitone/Biotone, Lucitone/Trilux, and Versyo/Ivoclar specimens were significantly decreased by thermocycling, compared with the corresponding control groups ( p < 0.05). The means of Acron/Ivoclar and Lucitone/Ivoclar specimens increased after thermocycling ( p < 0.05). The highest mean shear bond strength value was observed with Lucitone/Biotone in the control group (14.54 MPa) and the lowest with QC-20/Trilux in the thermocycled group (3.69 MPa).
Conclusion: Some acrylic tooth/denture base resin combinations can be more affected by thermocycling; effects vary based upon the materials used.     

Effect of Surface Preparation Using Ethyl Acetate on the Shear Bond Strength of Repair Resin to Denture Base Resin

Purpose : This study evaluated the effect of using ethyl acetate as a surface preparation agent on the shear bond strength of repair resin to denture base resin.
Materials and Methods : The flat surfaces of a heat-processed denture base resin were prepared with one of the following: (1) without preparation, (2) 60-second application of ethyl acetate, (3) 120-second application of ethyl acetate, (4) 180-second application of ethyl acetate, and (5) 5-second application of dichloromethane. An autopolymerizing repair resin was applied. The specimens were then immersed in 37°C distilled water for 24 hours. The specimens in groups 1, 3, and 5 were thermocycled up to 10,000 times in water between 5 and 55°C with a 1-minute dwell time at each temperature. The shear bond strengths were determined at a crosshead speed of 1.0 mm/min (n = 10). The morphological changes in the repair surfaces after preparation were observed with a scanning electron microscope.
Results : The shear bond strengths of groups 3 and 5 were significantly higher than the other groups before thermocycling ( p < 0.05). The shear bond strengths of group 3 were significantly lower than those of group 5 after thermocycling ( p < 0.05). The scanning electron microscope (SEM) views showed that the dissolution progressed deeper with the preparation duration.
Conclusions : The 120-second surface application of ethyl acetate enhanced the shear bond strength between the repair resin and the denture base resin, although the bond durability was inferior to that of the conventional surface preparation.     

Effect of Disinfectants on the Hardness and Roughness of Reline Acrylic Resins

PURPOSE: Potential effects on hardness and roughness of a necessary and effective disinfecting regimen (1% sodium hypocholorite and 4% chlorhexidine) were investigated for two hard chairside reline resins versus a heat-polymerizing denture base acrylic resin. MATERIALS AND METHODS: Two standard hard chairside reliners (Kooliner and Duraliner II), one heat-treated chairside reliner (Duraliner II +10 minutes in water at 55 degrees C), and one standard denture base material (Lucitone 550) were exposed to two disinfecting solutions (1% sodium hypochlorite; 4% chlorhexidine gluconate), and tested for two surface properties [Vickers hardness number (VHN, kg/mm(2)); Roughness (Ra, microm)] for different times and conditions (1 hour after production, after 48 hours at 37 +/- 2 degrees C in water, after two disinfection cycles, after 7 days in disinfection solutions, after 7 days in water only). For each experimental condition, eight specimens were made from each material. Data were analyzed by analysis of variance followed by Tukey's test, and Student's t-test (p= 0.05). RESULTS: For Kooliner (from 6.2 +/- 0.3 to 6.5 +/- 0.5 VHN) and Lucitone 550 (from 16.5 +/- 0.4 to 18.4 +/- 1.7 VHN), no significant changes in hardness were observed either after the disinfection or after 7 days of immersion, regardless of the disinfectant solution used. For Duraliner II (from 4.0 +/- 0.1 to 4.2 +/- 0.1 VHN), with and without heat treatment, a small but significant increase in hardness was observed for the specimens immersed in the disinfectant solutions for 7 days (from 4.3 +/- 0.2 to 4.8 +/- 0.5 VHN). All materials showed no significant change in roughness (Kooliner: from 0.13 +/- 0.05 to 0.48 +/- 0.24 microm; Duraliner II, with and without heat treatment: from 0.15 +/- 0.04 to 0.29 +/- 0.07 microm; Lucitone 550: from 0.44 +/- 0.19 to 0.49 +/- 0.15 microm) after disinfection and after storage in water for 7 days. CONCLUSIONS: The disinfectant solutions, 1% sodium hypochlorite and 4% chlorhexidine gluconate, caused no apparent damage on hardness and roughness of the materials evaluated.     

A Review of Fiber-Reinforced Denture Base Resins

Purpose One method of reinforcing denture base material is to use fiber composite reinforcement. Different types of fibers, such as glass (GF), carbon/graphite, aramid, and ultrahigh-modulus polyethylene (UHMP) fibers have been tested for this purpose. Materials and Methods This article reviews the studies conducted on the fiber-reinforced denture base resin systems. Results The literature has reported that the fiber concentration and its adhesion to polymer matrix influences the transverse strength of the fiber composite. The highest transverse strength value (265 MPa) with polymethyl methacrylate (PMMA) was obtained by incorporating 58 wt% GF into the resin. UHMP fibers incorporated into PMMA resin yielded the highest impact strength value (134 kJm^-2) of the fiber-PMMA composites. Conclusions Despite the improved mechanical properties of fiber-reinforced denture materials, further research is required to show the clinical usefulness of the fiber reinforcement.     

Shear Bond Strength of Two Chemically Different Denture Base Polymers to Reline Materials

Purpose: This study evaluated the shear bond strengths of light-polymerized urethane dimethacrylate (Eclipse) and heat-polymerized polymethylmethacrylate (Meliodent) denture base polymers to intraoral and laboratory-processed reline materials.
Materials and Methods: Thirty disks measuring 15 mm diameter and 2 mm thick were prepared for each denture base material following the manufacturers' recommendation. They were relined with Meliodent RR, Kooliner, and Secure reline materials after 1 month of water immersion. Ten additional Eclipse specimens were relined using the same Eclipse resin. A shear bond test was carried out on an Instron machine at a crosshead speed of 1.0 mm/min 24 hours after relining. Data were analyzed using two-way and one-way ANOVAs and post hoc Dunnett's T3 test ( p = 0.05). The nature of failure was analyzed under a stereomicroscope. The effect of dichloromethane adhesive on the two denture polymer surfaces and the failed interfaces of mixed and adhesive failures were analyzed under a SEM (scanning electron microscope).
Results: Two-way ANOVA showed significant differences in the shear bond strength values as a function of the denture base polymers, reline materials, and their interaction ( p < 0.05). One-way ANOVA showed significant differences in shear bond strength values among denture base-reline combinations ( p < 0.05) except for Meliodent-Kooliner and Eclipse-Meliodent RR relines. Meliodent showed the highest shear bond strength value when relined with Meliodent RR (14.5 ± 0.5 MPa), and Eclipse showed the highest value with Eclipse relining (11.4 ± 0.6 MPa). Meliodent denture base showed adhesive, cohesive, and mixed failure, while all Eclipse showed adhesive failure with various reline materials.
Conclusion: The two chemically different denture base polymers showed different shear bond strength values to corresponding reline materials.     

Effect of Microwave Disinfection Procedures on Torsional Bond Strengths of Two Hard Chairside Denture Reline Materials

PURPOSE: This study evaluated the potential effects of denture base resin water storage time and an effective denture disinfection method (microwave irradiation at 650 W for 6 minutes) on the torsional bond strength between two hard chairside reline resins (GC Reline and New Truliner) and one heat-polymerizing denture base acrylic resin (Lucitone 199). MATERIALS AND METHODS: Cylindrical (30 x 3.9 mm) denture base specimens (n= 160) were stored in water at 37 degrees C (2 or 30 days) before bonding. A section (3.0 mm) was removed from the center of the specimens, surfaces prepared, and the reline materials packed into the space. After polymerization, specimens were divided into four groups (n= 10): Group 1 (G1)--tests performed after bonding; Group 2 (G2)--specimens immersed in water (200 ml) and irradiated twice (650 W for 6 minutes); Group 3 (G3)--specimens irradiated daily until seven cycles of disinfection; Group 4 (G4)-specimens immersed in water (37 degrees C) for 7 days. Specimens were submitted to a torsional test (0.1 Nm/min), and the torsional strengths (MPa) and the mode of failure were recorded. Data from each reline material were analyzed by a two-way analysis of variance, followed by Neuman-Keuls test (p= 0.05). RESULTS: For both Lucitone 199 water storage periods, before bonding to GC Reline resin, the mean torsional strengths of G2 (2 days--138 MPa; 30 days--132 MPa), G3 (2 days--126 MPa; 30 days--130 MPa), and G4 (2 days--130 MPa; 30 days--137 MPa) were significantly higher (p < 0.05) than G1 (2 days--108 MPa; 30 days--115 MPa). Similar results were found for Lucitone 199 specimens bonded to New Truliner resin, with G1 specimens (2 days-73 MPa; 30 days--71 MPa) exhibiting significantly lower mean torsional bond strength (p < 0.05) than G2 (2 day--86 MPa; 30 days--90 MPa), G3 (2 days--82 MPa; 30 days--82 MPa), and G4 specimens (2 days--78 MPa; 30 days--79 MPa). The adhesion of both materials was not affected by water storage time of Lucitone 199 (p > 0.05). GC reline showed a mixed mode of failure (adhesive/cohesive) and New Truliner failed adhesively. CONCLUSIONS: Up to seven microwave disinfection cycles did not decrease the torsional bond strengths between the hard reline resins, GC Reline and New Truliner to the denture base resin Lucitone 199. The effect of additional disinfection cycles on reline material may be clinically significant and requires further study.     

Bond Strength and Degree of Infiltration between Acrylic Resin Denture Liner after Immersion in Effervescent Denture Cleanser

Purpose: The purpose of this study was to investigate the effect of sodium perborate on the bond strength and degree of infiltration between acrylic resin/resilient denture liners. Materials and Methods: Three denture liners (Elite Soft, Mucopren Soft, Kooliner) were investigated. Twenty specimens (83 × 10 × 10 mm³) of each material were made by processing the denture liners against two polymerized PMMA blocks. Ten specimens for each material were stored in artificial saliva at 37°C (control group: TBS1), and the other ten specimens were stored in artificial saliva at 37°C combined with sodium perborate (experimental group: TBS2). All specimens were placed under tension until failure in a Universal Testing Machine at a crosshead speed of 5 mm/min after 7 (T7) and 60 (T60) days (n = 5). Failure strength (MPa) was recorded, and mode of failure was characterized as cohesive, adhesive, or cohesive/adhesive. For the infiltration tests, ten circular specimens (14‐mm diameter × 2‐mm thick) of each material were stored in artificial saliva and 0.5% methylene blue at 37°C (control group: I1), and ten specimens were stored in artificial saliva and 0.5% methylene blue at 37°C combined with daily immersions for 5 minutes in an effervescent solution of sodium perborate (experimental group: I2). The degree of infiltration was obtained through photographs and using Software Image Tool after 120 days. Results: For Kooliner, the statistical test did not show a significant difference in the bond strength due to the influence of the immersion period or to the use of sodium perborate. Elite Soft presented a significant increase in the average tension in T7 and in T60 in both TBS1 and TBS2. Inversely, the Mucopren suffered a significant decrease in the tension value in the same period as the TBS1 group as well as in the TBS2. The infiltration percentage was analyzed with the Kruskal–Wallis test (26.18; p < 0.05), which indicated significant differences between the compared averages for the groups. Comparing the averages of materials, the statistical test did not show significant differences between the control (I1) and experimental (I2) groups after 120 days. Conclusions: The use of sodium perborate did not promote significant alterations in the evaluated properties. Kooliner presented the best results.     

A Study on Effect of Surface Treatments on the Shear Bond Strength between Composite Resin and Acrylic Resin Denture Teeth

Visible light-cured composite resins have become popular in prosthetic dentistry for the replacement of fractured/debonded denture teeth, making composite denture teeth on partial denture metal frameworks, esthetic modification of denture teeth to harmonize with the characteristics of adjacent natural teeth, remodelling of worn occlusal surfaces of posterior denture teeth etc. However, the researches published on the bond strength between VLC composite resins and acrylic resin denture teeth is very limited. The purpose of this study is to investigate the effect of five different methods of surface treatments on acrylic resin teeth on the shear bond strength between light activated composite resin and acrylic resin denture teeth. Ninety cylindrical sticks of acrylic resin with denture teeth mounted atop were prepared. Various treatments were done upon the acrylic resin teeth surfaces. The samples were divided into six groups, containing 15 samples each. Over all the treated and untreated surfaces of all groups, light-cured composite resin was applied. The shear strengths were measured in a Universal Testing Machine using a knife-edge shear test. Data were analyzed using one way analysis of variance (ANOVA) and mean values were compared by the F test. Application of bonding agent with prior treatment of methyl methacrylate on the acrylic resin denture teeth resulted in maximum bond strength with composite resin.     

Reinforcement of Denture Base Resin with Glass Fillers

PURPOSE: This study evaluated the effect of short glass fibers on the transverse strength of a heat-polymerized acrylic resin denture base material. MATERIALS AND METHODS: Four groups of specimens (n = 10) were fabricated according to the ISO standard for the transverse strength test. E-glass fibers were triturated to produce short fibers of different lengths. Specimens for Group 1 (control) were made of unfilled polymethylmethacrylate (PMMA). For group 2, the PMMA powder was modified with 0.1 g of dry glass fibers. For group 3, the PMMA powder was modified with 0.1 g of silanized glass fibers. For group 4, the PMMA powder was modified with 0.2 g of silanized glass fibers. A three-point loading test was used to determine the transverse strength of the tested specimens. The fracture surface of each specimen was evaluated using SEM. RESULTS: The addition of untreated glass fibers increased the transverse strength by 11% but produced some porosity in the polymeric matrix. The addition of silane-treated glass fibers increased the transverse strength of PMMA by 28% for group 3 and by 26% for group 4, and produced a dense structure for the polymer-fiber composite. CONCLUSION: The transverse strength of PMMA can be slightly increased by the addition of short glass fibers.     

Incorporation of Antimicrobial Macromolecules in Acrylic Denture Base Resins: A Research Composition and Update

Contemporary research in acrylic denture base materials focuses on the development of a novel poly(methyl methacrylate) (PMMA) resin with antimicrobial properties. Although PMMA resin has fulfilled all the requirements of an ideal denture base material, its susceptibility to microbial colonization in the oral environment is a formidable concern to clinicians. Many mechanisms including the absence of ionic charge in the methyl methacrylate resins, hydrophobic interactions, electrostatic interactions, and mechanical attachment have been found to contribute to the formation of biofilm. The present article outlines the basic categories of potential antimicrobial polymer (polymeric biocides) formulations (modified PMMA resins) and considers their applicability, biological status, and usage potential over the coming years.     

Strength of Denture Base Resins Repaired with Auto- and Visible Light-Polymerized Materials

Purpose: Clinicians are still confused about the choice of repair method, which depends on factors such as the length of time required for processing, the mechanical strength of the repaired material, and the effect of stress concentration in the acrylic resins before the repair. The aim was to determine the impact and flexural strength characteristics, such as stress at yield, Young's modulus, and displacement at yield of denture base resins fractured and repaired by three methods using heat‐, auto‐, and visible light‐polymerized acrylic resins. Material and Methods: For impact and flexural strength tests, 18 rectangular specimens measuring 50 × 6 × 4 mm³ and 64 × 10 × 3.3 mm³, respectively, were processed using Impact 2000, Lucitone 550, Impact 1500, and QC‐20 acrylic resins. Fracture tests were performed according to ISO1567:1999. Afterward, all fractured specimens were stored in distilled water at 37°C for 7 days, and then repaired with (1) the same acrylic resin used for specimen fabrication (n = 6), (2) an autopolymerized acrylic resin (TruRepair, n = 6), and (3) a visible light acrylic resin (Versyo.com, n = 6). The repaired specimens were again submitted to the same fracture tests, and the failures were classified as adhesive or cohesive. Data from all mechanical tests after repair by the different methods were submitted to two‐way ANOVA, and mean values were compared by the Tukey test. Results: All acrylic resins showed adhesive fractures after impact and flexural strength tests. Differences (p < 0.05) were found among repair methods for all acrylic resins studied, with the exception of displacement at yield, which showed similar values for repairs with auto‐ and visible light‐polymerized acrylic resins. The highest values for impact strength, stress, and displacement at yield were obtained when the repair was made with the same resin the specimen was made of. Conclusion: Denture base acrylic resins repaired with the same resin they were made of showed greater fracture strength.     

The Effect of Chemical Surface Treatments on the Flexural Strength of Repaired Acrylic Denture Base Resin

Purpose: To investigate the effect of the selected chemical surface treatment agents on the flexural strength of heat‐polymerized acrylic resin repaired with autopolymerized acrylic resin. Materials and Methods: Ninety heat‐polymerized acrylic resin specimens (Meliodent) were prepared according to ISO1567 and randomly divided into nine groups: positive and negative control groups (groups I and II), and seven experimental groups (groups III to IX). Specimens in groups II to IX were cut in the middle and beveled 45°. Group III was then treated with methyl methacrylate (the liquid part of Unifast TRAD) for 180 seconds. Group IV was treated with Rebase II adhesive according to the manufacturer's instructions. Groups V to IX were treated with methyl formate, methyl acetate, and a mixture of methyl formate–methyl acetate at various concentrations (75:25, 50:50, 25:75% v/v, respectively) for 15 seconds. They were then repaired with autopolymerized acrylic resin (Unifast TRAD). A three‐point loading test was performed using a universal testing machine. One‐way ANOVA and post hoc Tukey's analysis at p < 0.05 were used for statistical comparison. Failure analysis was then recorded for each specimen. The morphological changes in untreated and treated specimens were observed by scanning electron microscopy. Results: The flexural strengths of groups III to IX were significantly higher than that of group II (p < 0.05). The flexural strengths of groups IV to IX showed no significant difference among them (p > 0.05). All specimens in groups V to IX showed 100% cohesive failure, while groups II, III, and IV showed cohesive failure of 10%, 60%, and 60%, respectively. From scanning electron micrographs, the application of methyl formate, methyl acetate, and a mixture of methyl formate–methyl acetate solutions on heat‐polymerized acrylic resin resulted in a 3D honeycomb appearance, while specimens treated with methyl methacrylate and Rebase II adhesive developed shallow pits and small crest patterns, respectively. Conclusion: Treating surfaces with methyl formate, methyl acetate, and a mixture of methyl formate–methyl acetate solutions significantly enhanced the flexural strength of heat‐polymerized acrylic denture base resin that had been repaired with autopolymerized acrylic resin.     

Shear Bond Strength of Denture Teeth to Heat‐ and Light‐Polymerized Denture Base Resin

Purpose: To evaluate the shear bond strengths of highly cross‐linked denture teeth bonded to heat‐polymerized poly(methyl methacrylate) (PMMA) or a light‐polymerized urethane dimethacrylate (UDMA) denture base resin with or without a diatoric and with or without an acrylate bonding agent. Materials and Methods: The denture base resins tested were Lucitone 199 (heat‐polymerized PMMA) and Eclipse (light‐polymerized UDMA). One hundred sixty mandibular central incisor denture teeth were divided into four groups (n = 40): group 1: ground surface as control; group 2: ground surface with diatoric; group 3: ground surface with bonding agent; group 4: ground surface with bonding agent and diatoric. Half of each group (n = 20) was processed with either heat‐ or light‐polymerized resin. All specimens were treated with thermocycling for 1000 cycles, alternating between 5 and 55°C with a dwell time of 30 seconds. Half the specimens in each group were treated with cyclic loading at 22 N for 14,400 cycles at 1.5 Hz. All specimens were tested with shear load to failure. Data were analyzed with student's t‐test, 2‐ and 3‐way ANOVA, and Dunnett's T3 method (p < 0.05). Results: Statistical analysis demonstrated no significant effect on shear bond strength from cyclic loading. For the Lucitone 199 (L) specimens, mean shear bond strengths and standard deviations were (N) 66.5 ± 28.4, 72.7 ± 31.5, 80.6 ± 17.1, and 76.9 ± 21.9 for groups 1L, 2L, 3L, and 4L, respectively. For the Eclipse (E) specimens, mean shear bond strengths and standard deviations were (N) 3.7 ± 1.2, 7.3 ± 3.3, 90.0 ± 20.7, and 94.2 ± 17.8 for groups 1E, 2E, 3E, and 4E, respectively. No statistically significant differences in shear bond strengths were noted for the Lucitone 199 groups (p= 0.11). Eclipse shear bond strengths were significantly higher in groups 3E and 4E than in groups 1E and 2E (p≤ 0.05). In a 3‐way ANOVA for groups 3 and 4, the shear bond strengths for the Eclipse specimens were significantly higher than the Lucitone 199 specimens (p= 0.01). Conclusions: When evaluating the shear bond strength of IPN denture teeth to denture base resins, specimens using an acrylate bonding agent with the Eclipse (light‐polymerized) resin yielded significantly higher shear bond strengths than all of the Lucitone 199 groups and the Eclipse resin groups without a bonding agent.