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PubMed2004

Fatigue resistance and stiffness of glass fiber-reinforced urethane dimethacrylate composite.

STATEMENT OF PROBLEM: Retentive properties of cast metal clasps decrease over time because of metal fatigue. Novel fiber-reinforced composite materials are purported to have increased fatigue resistance compared with metals and may offer a solution to the problem of metal fatigue. PURPOSE: The aim of this study was to investigate the fatigue resistance and stiffness of E-glass fiber-reinforced composite. MATERIAL AND METHODS: Twelve cylindrical fiber-reinforced composite test cylinders (2 mm in diameter and 60 mm in length) were made from light-polymerized urethane dimethacrylate monomer with unidirectional, single-stranded, polymer preimpregnated E-glass fiber reinforcement. Six cylinders were stored in dry conditions and 6 in distilled water for 30 days before testing. Fatigue resistance was measured by a constant-deflection fatigue test with 1 mm of deflection across a specimen span of 11 mm for a maximum of 150,000 loading cycles. The resistance of the cylinder against deflection was measured (N) and the mean values of the force were compared by 1-way analysis of variance (alpha = .05). The flexural modulus (GPa) was calculated for the dry and water-stored cylinders for the first loading cycle. Scanning electron microscopy was used to assess the distribution of the fibers, and the volume percent of fibers and polymer were assessed by combustion analysis. RESULTS: The test cylinders did not fracture due to fatigue following 150,000 loading cycles. Flexural modulus at the first loading cycle was 18.9 (+/- 2.9) GPa and 17.5 (+/- 1.7) GPa for the dry and water-stored cylinders, respectively. The mean force required to cause the first 1-mm deflection was 33.5 (+/- 5.2) N and 37.7 (+/- 3.6) N for the dry and water stored cylinders, respectively; however, the differences were not significant. After 150,000 cycles the mean force to cause 1-mm deflection was significantly reduced to 23.4 (+/- 8.5) N and 13.1 (+/- 3.5) N, respectively (P < .0001). Scanning electron microscopy highlighted fiber- and polymer-rich areas within the specimens and indicated that individual fibers were well impregnated with resin. The combustion analysis studies identified the fiber content to be 35.9 vol%. CONCLUSION: The results of this study suggest that the fatigue resistance of the fiber-reinforced material examined was increased; however, the reduction in flexural modulus of fiber-reinforced composites may restrict their use where high rigidity is required, such as in removable partial denture clasps.

Air↗
PubMed2004

The effect of fiber reinforcement on the fracture toughness and flexural strength of provisional restorative resins.

STATEMENT OF PROBLEM: Fracture of provisional restorations is of concern, especially with long-span fixed partial dentures or areas of heavy occlusal stress. A number of different techniques for reinforcement of provisional restorations have been suggested; however, the effect of these techniques is largely unclear. PURPOSE: The aim of this study was to determine the fracture toughness and flexural strength of different types of provisional restoration resins reinforced with different commercially available fibers. MATERIAL AND METHODS: A total of 105 specimens were prepared in this study for each test; compact tensile specimens for the fracture toughness test and rectangular specimens for the flexural strength test. The specimens were divided into 3 groups according to the type of resin used, Jet, Trim, or Temphase (n=35), and then each group was divided into 7 subgroups (n=5) according to the type of fiber reinforcement, Construct, Fibrestick, Ribbond normal, Ribbond THM, Ribbond triaxial, or Fibrenet. Unreinforced specimens served as the control. Specimens were loaded in a universal testing machine until fracture. The mean fracture toughness (MPa.m(1/2)) and mean flexural strength (MPa) were compared by 1-way analysis of variance, followed by the Tukey standardized range test (alpha=.05). RESULTS: Fibrestick and Construct reinforcements showed a significant increase (P<.0001) in mean fracture toughness over unreinforced controls for all resins tested. Fibrestick increased the polymethyl methacrylate from 1.25+/-0.06 MPa.m(1/2) to 2.74+/-0.12 MPa.m(1/2); polyethyl methacrylate from 0.67+/-0.07 MPa.m(1/2) to 1.64+/-0.13 MPa.m(1/2); and bis-acryl from 0.87+/-0.05 MPa.m(1/2) to 1.39+/-0.11 MPa.m(1/2). Construct increased polymethyl methacrylate to 2.59+/-0.28 MPa.m(1/2); polyethyl methacrylate to 1.53+/-0.22 MPa.m(1/2); and bis-acryl to 1.30+/-0.13 MPa.m(1/2); however, there was no significant difference between Fibrestick and Construct reinforcements in the degree of reinforcement. Similarly the mean flexural strength values were significantly increased by different combinations of fiber and resin (P<.0001). CONCLUSION: The addition of fibers to provisional resin increased both fracture toughness and flexural strength.

Analysis of Variance↗
PubMed2004

Repair strength of autopolymerizing, microwave, and conventional heat-polymerized acrylic resins.

STATEMENT OF PROBLEM: Acrylic resin dentures are susceptible to fracture after clinical use. The repair procedure should be time saving, strong, and should not affect dimensional accuracy. PURPOSE: This study evaluated the transverse repair strength of a conventional heat-polymerized (Lucitone 199, "L") and a microwave-polymerized (Acron MC, "A") acrylic resin that were repaired with these same resins and with an autopolymerizing acrylic resin (Acron MC/R, "AR"). MATERIAL AND METHODS: Twenty-four rectangular specimens (65.0 x 10.0 x 3.3 mm) of L or A and 6 of AR were manufactured according to ISO/FDI 1567 and stored in distilled water at 37 degrees C for 7 days. Eighteen specimens of L or A were selected randomly. Six specimens of each material remained intact (control), each 6 were sectioned in the middle to create a 10-mm gap and repaired with the materials L, A, or AR. After an additional 7 days of storage at 37 degrees C, the transverse strength (MPa) of the repaired and intact specimens was measured using a 3-point bending test. A 2-way ANOVA and a Tukey HSD test were performed to identify significant differences (alpha=.05). The nature of the failure was noted as adhesive, cohesive, or mixed. RESULTS: The intact and repaired specimens showed similar transverse strength values (MPa), except for A-intact (80.6 +/- 2.9), which was significantly stronger (P<.05) than the other materials tested as well as the repaired groups (P<.05). No differences were detected among the repaired groups. Repaired specimens exhibited 3 types of failures: adhesive (interface), cohesive (only in the repair material), and mixed (interface and repair material), with incidences of 2.8, 25, and 72.2%, respectively. CONCLUSION: The autopolymerizing resin exhibited a repair strength similar to those found for the conventional heat- and microwave-polymerized materials.

Acrylic Resins↗
PubMed2004

Evaluation of load transfer characteristics of five different implants in compact bone at different load levels by finite elements analysis.

STATEMENT OF PROBLEM: The external contour of an implant and the magnitude of occlusal loading can have significant effects on the load transfer characteristics and may result in different bone failure rates for different implant systems. PURPOSE: The goal of this study was to investigate the effects of external geometry and occlusal load magnitude on bone failure modes for 5 commercially available dental implant systems. MATERIAL AND METHODS: Five different implant systems; Ankylos, Astra, Bicon, ITI, and Nobel Biocare, comparable in size, but different in thread profile and crest module shapes, were compared using the finite element method. Type II bone quality was approximated and complete osseous integration was assumed. Occlusal loads of varying magnitudes (0 to 2000 N) were applied on the abutments supporting single tooth restorations at 11.3 degrees from the vertical axis with a 1-mm offset. Total overloaded bone area, where tensile and compressive normal stresses fell outside of the recommended limits of 100 and 170 MPa, respectively, was investigated for different load levels. RESULTS: For moderate levels of occlusal loads up to 300 N, the compact bone was not overloaded by any of the implant systems. At the extreme end of the occlusal load range (1000 N or more) the overloading characteristics of implants may be dependent on geometric shape. CONCLUSION: In general, overloading occurs near the superior region of compact bone, in compression, and it is primarily caused by the normal and lateral components of the occlusal load. At the region of intersection of compact and trabecular bone, overloading occurs in tension due to the vertical component of the occlusal load. For excessive forces greater than 1000 N, the overloaded areas of the bone varied considerably among 5 different implants systems evaluated.

Alveolar Bone Loss↗
PubMed2005

Bending properties of strengthened Ti-6Al-7Nb alloy major connectors compared to Co-Cr alloy major connectors.

STATEMENT OF PROBLEM: Although Ti-6Al-7Nb alloy has several excellent mechanical properties, its poor rigidity has limited its clinical use as a material for the fabrication of a removable partial denture (RPD) major connector. PURPOSE: The purpose of this study was to develop and evaluate strengthening designs of Ti-6Al-7Nb RPD major connectors in an effort to increase rigidity. MATERIAL AND METHODS: Four strengthening designs of Ti-6Al-7Nb alloy major connectors were developed using finite element analysis modeling: wide, thick, thick at the middle, and thick at the anterior and posterior borders. The designs had similar rigidity values to a conventional Co-Cr alloy major connector, as measured by the maximum deformation when a simulated load was applied. Next, 30 Kennedy Class II maxillary RPD specimens, using 6 different major connector designs (n = 5), were fabricated. These 6 designs included Ti-6Al-7Nb alloy RPDs with 4 different strengthening designs, a Ti-6Al-7Nb alloy RPD without strengthening designs, and a conventional Co-Cr alloy RPD as a control. The rigidity of the RPDs was evaluated by measuring strains on the major connector and force on the intaglio surface of a denture base under a 30 N loading condition. The data obtained from the 6 different RPD designs were compared using a repeated measures analysis of variance and the Bonferroni correction (alpha=.05). RESULTS: The strains on the major connectors and the force on the denture bases measured from the Ti-6Al-7Nb RPD without strengthening designs were significantly larger than those of the standard Co-Cr alloy RPD. The strains measured from the 4 strengthening designs were not significantly larger than those of the Co-Cr alloy RPD. CONCLUSION: The results suggest that the strengthening designs tested may improve the rigidity of the Ti-6Al-7Nb alloy major connectors and, hence, may promote clinical application for RPDs.

Chromium Alloys↗
PubMed2005

Flexural strength and moduli of hypoallergenic denture base materials.

STATEMENT OF PROBLEM: Hypoallergenic denture base materials show no residual methyl methacrylate (MMA) or significantly lower residual MMA monomer content compared to polymethyl methacrylate-based (PMMA) heat-polymerizing acrylic resin. There is insufficient knowledge of the mechanical properties of hypoallergenic denture base materials to warrant their use in place of PMMA-based acrylic resins for patients with allergic reaction to MMA. PURPOSE: This in vitro study compared flexural strength and flexural modulus of 4 hypoallergenic denture base materials with flexural strength/modulus of a PMMA heat-polymerizing acrylic resin. MATERIAL AND METHODS: The following denture base resins were examined: Sinomer (heat-polymerized, modified methacrylate), Polyan (thermoplastic, modified methacrylate), Promysan (thermoplastic, enterephthalate-based), Microbase (microwave-polymerized, polyurethane-based), and Paladon 65 (heat-polymerized, methacrylate, control group). Specimens of each material were tested for flexural strength and flexural modulus (MPa, n = 5) according to ISO 1567:1999. The data were analyzed with 1-way analysis of variance and the Bonferroni-Dunn multiple comparisons post hoc analysis for each test variable (alpha=.05). RESULTS: Flexural strength of Microbase (67.2 +/- 5.3 MPa) was significantly lower than Paladon 65 (78.6 +/- 5.5 MPa, P <.0001). Flexural strength of Polyan (79.7 +/- 4.2 MPa, P =.599), Promysan (83.5 +/- 3.8 MPa, P =.412), and Sinomer (72.3 +/- 2.1 MPa, P =.015) did not differ significantly from the control group. Significantly lower flexural modulus was obtained from Sinomer (1720 +/- 30 MPa, P =.0007) compared to the PMMA control group (2050 +/- 40 MPa), whereas the flexural modulus of Promysan (2350 +/- 170 MPa, P =.0005) was significantly higher than the PMMA material. Microbase (2100 +/- 210 MPa, P =.373) and Polyan (2070 +/- 60 MPa, P =.577) exhibited flexural modulus similar to the PMMA material. The tested denture base materials fulfilled the requirements regarding flexural strength (>65 MPa). With the exception of Sinomer, the tested denture base resins passed the requirements of ISO 1567 regarding flexural modulus (>2000 MPa). CONCLUSION: Flexural modulus of Promysan was significantly higher than the PMMA material. Microbase and Sinomer exhibited significantly lower flexural strength and flexural modulus, respectively, than PMMA. The other groups did not differ significantly from the control group.

Acrylic Resins↗
PubMed2005

Flexural strength of autopolymerizing denture reline resins with microwave postpolymerization treatment.

STATEMENT OF PROBLEM: Microwave postpolymerization has been suggested as a method to improve the mechanical strength of repaired denture base materials. However, the effect of microwave heating on the flexural strength of the autopolymerizing denture reline resins has not been investigated. PURPOSE: This study analyzed the effect of microwave postpolymerization on the flexural strength of 4 autopolymerizing reline resins (Duraliner II, Kooliner, Ufi Gel Hard, and Tokuso Rebase Fast) and 1 heat-polymerized resin (Lucitone 550). MATERIAL AND METHODS: For each material, 80 specimens (64 x 10 x 3.3 mm) were polymerized according to the manufacturer's instructions and divided into 10 groups (n = 8). Control group specimens remained as processed. Before testing, the specimens were subjected to postpolymerization in a microwave oven using different power (500, 550, or 650 W) and time (3, 4, or 5 minutes) settings. Load measurements (newtons) were made at a crosshead speed of 5 mm/min using a 3-point bending device with a span of 50 mm. The flexural strength values were calculated in MPa. Data analyses included 3-way and 2-way analysis of variance and the Tukey Honestly Significant Difference test (alpha = .05). RESULTS: The flexural strengths of resins Duraliner II and Kooliner were significantly increased (P = .0015 and P = .0046, respectively) with the application of microwave irradiation using different time/power combinations. The materials Lucitone 550, Tokuso Rebase Fast, and Ufi Gel Hard demonstrated no significant strength improvement compared to the corresponding control. Only after microwave postpolymerization irradiation for 3 minutes at 550 W did Lucitone 550 show significantly higher flexural strength (P =.001) than Tokuso Rebase Fast and Ufi Gel Hard relining resins. CONCLUSION: Microwave postpolymerization irradiation can be an effective method for increasing the flexural strength of Duraliner II (at 650 W) and Kooliner (at 550 W and 650 W for 5 minutes).

Acrylic Resins↗
PubMed2005

Flexural strength of a layered zirconia and porcelain dental all-ceramic system.

STATEMENT OF PROBLEM: New processing techniques have facilitated the use of zirconia core materials in all-ceramic dental prostheses. Zirconia has many potential advantages compared to existing core materials; however, its performance when layered with porcelain has not been evaluated. PURPOSE: This study investigated the strength of a wide variety of layered zirconia and porcelain beams to determine whether the inclusion of zirconia cores results in improved strength. MATERIAL AND METHODS: Eight types of layered or simple zirconia and porcelain beams (n = 10), approximately fixed partial denture-size, were made of a tetragonal polycrystalline zirconium dioxide partially stabilized with yttria core (Lava System Frame) and a feldspathic dental porcelain (Lava Ceram veneer ceramic). Elastic moduli of the materials were measured using an acoustic method. Maximum force and modulus of rupture were determined using 3-point flexural testing and a universal testing machine. Descriptive statistical methods were used. RESULTS: Beams with porcelain tensile surfaces recorded mean tensile strengths or moduli of rupture from 77 to 85 MPa, whereas beams with zirconia tensile surfaces recorded moduli of rupture almost an order of magnitude higher, 636 to 786 MPa. The elastic moduli of the porcelain and zirconia materials were 71 and 224 GPa, respectively. Crack propagation following initial tensile cracking often involved the porcelain-zirconia interface, as well as bulk porcelain and zirconia. CONCLUSION: The layered zirconia-porcelain system tested recorded substantially higher moduli of rupture than have been previously reported for other layered all-ceramic systems.

Dental Porcelain↗
PubMed2005

Effect of surface treatment on bond strength of low-fusing porcelain to commercially pure titanium.

STATEMENT OF PROBLEM: Due to the pronounced oxidative nature of titanium at high temperatures, an excessively thick layer of TiO(2) may form on the surface. This oxide layer could adversely affect titanium-porcelain bonding. PURPOSE: The purpose of this study was to investigate the effect of bonding agent and surface treatment using airborne-particle abrasion and hydrochloric acid on the bond strength between a low-fusing porcelain and commercially pure cast titanium. MATERIAL AND METHODS: A casting unit was used to cast 60 specimens of commercially pure titanium (25.0 x 3.0 x 0.5 mm). The specimens were equally divided into 3 groups. The first group received no surface treatment and served as the control, the second group was subjected to airborne-particle abrasion, and the third group was treated with hydrochloric acid. The specimens in each group were further divided into 2 subgroups of 10 each. Ten specimens were treated with bonding agent (Noritake), and 10 specimens were not treated with bonding agent. Low-fusing porcelain (Noritake) was fired onto the surface of the specimens. A universal testing machine was used to perform the 3-point bending test. The titanium-ceramic interfaces were subjected to scanning electron microscopic analysis. The bond failure data (MPa) were analyzed with a 2-way analysis of variance and Tukey multiple range tests (alpha=.05). Four specimens from each group were selected for scanning electron microscopic examination. RESULTS: The debonding test showed that surface treatment with airborne-particle abrasion followed by application of a bonding agent resulted in the strongest (35.60 +/- 8.15 MPa) titanium-ceramic bond (P<.001), followed by airborne-particle abrasion alone (25.6 +/- 5.4 MPa) and bonding agent alone (24.7 +/- 6.3 MPa). Hydrochloric acid surface treatment provided no beneficial effect to the titanium-ceramic bond strength compared to untreated specimens (P=.975). The photomicrographs of the titanium surface after debonding demonstrated residual porcelain retained on the metal surface for all groups. CONCLUSIONS: Surface treatment using either airborne-particle abrasion or bonding agent alone enhanced the bond strength of cast commercially pure titanium to low-fusing porcelain. The combination of airborne-particle abrasion and bonding agent provided the greatest improvement in titanium-ceramic bond strength. Titanium surface treatment with hydrochloric acid, with or without bonding agent, produced values that were not statistically different than the control.

Acid Etching, Dental↗
PubMed2005

An evaluation of the effects of handpiece speed, abrasive characteristics, and polishing load on the flexural strength of polished ceramics.

STATEMENT OF PROBLEM: Many studies on the strengthening effects of grinding and polishing, as well as heat treatment on ceramics, are not well standardized or use commercially available industrial polishing systems. The reported effectiveness of these strengthening mechanisms on ceramics may not be applicable to clinical dentistry. PURPOSE: The purpose of this study was to evaluate the effects of controlled polishing on the flexural strength of dental ceramics by using a custom-made machine that applied standardized loads and speeds that coincided with the mean loads and speeds used by experienced prosthodontists. MATERIAL AND METHODS: A total of 140 aluminous dental ceramic bar-shaped specimens (Vitadur Alpha Enamel) measuring 1.5 x 2.0 x 25 mm were fabricated and divided into 12 groups (for most groups, n=10). Specimens were untreated, polished with different polishing systems, polished at different speeds, ground and autoglazed, polished and autoglazed, autoglazed and polished, polished with loose (paste) and bonded abrasives, or overglazed. Simulated clinical polishing was performed on the ceramic specimens by using a customized polishing apparatus that allowed independent control over the relevant polishing parameters (abrasive hardness, applied load, linear speed, rotational velocity, and wheel stiffness). Flexural strength (MPa) was measured with a 4-point bending test, and subjective surface roughness was assessed with scanning electron microscopy. Autoglazing was performed at various stages of the polishing sequence to determine the effects of polishing on surface stresses. Mean values, standard deviations, independent-sample t tests, 1-way and 2-way analyses of variance, Dunnett t tests and Kruskal-Wallis tests were applied to the data (alpha=.05). RESULTS: Under a clinical load of 0.6 N for a coarse polishing wheel, 1.0 N for a medium polishing wheel, and 1.3 N for a fine polishing wheel, a linear speed of 499 mm/min, and a rotational velocity of 10,000 rpm, the use of clinical polishing instruments did not affect the flexural strength of the aluminous ceramics studied (P=.274). At higher rotational velocity (20,000 rpm), specimens polished with the diamond polishing system produced statistically weaker specimens compared with those that had been polished at 10,000 rpm (P=.019). Autoglazing treatment of the diamond-polished specimens did not reverse the strength degradation (P=.125). Conversely, diamond polishing of the autoglazed specimens resulted in significant flexural strength reduction (P=.029). Fine-diamond-bonded abrasive significantly reduced flexural strength (P=.025). CONCLUSIONS: Simulated clinical polishing at 10,000 rpm did not appear to substantially strengthen or weaken the ceramic specimens. Polishing at 20,000 rpm reduced flexural strength of the ceramic bars.

Aluminum Oxide↗
PubMed2005

Effect of soldering on the metal-ceramic bond strength of an Ni-Cr base alloy.

STATEMENT OF PROBLEM: Although soldering is a common laboratory procedure, the use of soldering alloys may adversely affect metal-ceramic bond strength and potentially decrease the longevity of metal-ceramic restorations. PURPOSE: The purpose of this study was to investigate the effect of soldering on metal-ceramic bond strength of a representative Ni-Cr base metal alloy. MATERIAL AND METHODS: Twenty-eight rectangular (25 x 3 x 0.5 mm) Ni-based alloy (Wiron 99) specimens were equally divided into soldering (S) and reference (R) groups. Soldering group specimens were covered with a 0.1-mm layer of the appropriate solder (Wiron-Lot) and reduced by 0.1 mm on the opposite side. Five specimens of each group were used for the measurement of surface roughness parameter (R(z)) and hardness, and 3 were used for measurement of the modulus of elasticity. Six specimens of each group were covered with porcelain (Ceramco 3) and subjected to a 3-point bending test for evaluation of the metal-ceramic bond strength according to the ISO 9693 specification. The data from surface roughness, hardness, modulus of elasticity, and metal-ceramic bond strength were analyzed statistically, using independent t tests (alpha=.05). RESULTS: Statistical analysis of the R(z) surface roughness parameter (S: 3.4 +/- 0.3 mum; R: 3.7 +/- 0.7 microm; P=.07) and bond strength (S: 46 +/- 3 MPa; R: 40 +/- 5 MPa; P=.057) failed to reveal any significant difference between the 2 groups. The specimens of the soldering group demonstrated significantly lower values both in hardness (S: 128 +/- 11 VHN; R: 217 +/- 4 VHN; P<.001) and in modulus of elasticity (S: 135 +/- 4 GPa; R: 183 +/- 6 GPa; P=.035) than the reference group. CONCLUSION: Under the conditions of the present study, the addition of solder to the base metal alloy did not affect the metal-ceramic bond strength.

Chromium Alloys↗
PubMed2005

Influence of polymerization mode on flexural properties of esthetic resin luting agents.

STATEMENT OF PROBLEM: Dual-polymerized esthetic resin luting agents have become popular. However, it is not clear whether the autopolymerized versions of such products have adequate strength to meet clinical requirements. PURPOSE: The aim of this study was to investigate the effect of different polymerization modes on the flexural properties of esthetic resin luting agents. MATERIAL AND METHODS: Ten esthetic resin luting agents were studied. Materials with 3 polymerization modes (dual, auto, and light) were Appeal, Calibra, Illusion, Lute-It, and Variolink-II; those with 2 polymerization modes (dual and auto) were Bistite-II-DC, Cement-It-C&B, Clearfil-DC-Cement, Linkmax, and Nexus2-Dual-Syringe. Five flexural strength specimens (2 x 2 x 25 mm) were made for every available polymerization mode for each material. The specimens were stored in distilled water for 24 hours at 37 degrees C. The specimens were then tested for flexural strength (MPa) and flexural modulus (GPa) using the 3-point bending method on a universal material testing machine at a cross-head speed of 0.5 mm/min. Data were analyzed by Kruskal-Wallis and Mann-Whitney tests (alpha=.05). RESULTS: Flexural strength values were the highest for dual-polymerized Nexus2-Dual-Syringe (155 MPa), whereas the values were lowest for autopolymerized Bistite-II-DC (56 MPa) and light-polymerized Appeal (63 MPa). Flexural moduli values ranged from 4.3 to 10.0 GPa. The polymerization mode and luting agent influenced flexural strength and modulus significantly (P<.05). CONCLUSION: Dual-polymerized resin luting agents had higher or equal flexural strength compared to the autopolymerized mode. All the groups tested passed the flexural strength requirement of the ISO 4049 specification.

Dental Stress Analysis↗
PubMed2006

The evaluation of microleakage and bond strength of a silicone-based resilient liner following denture base surface pretreatment.

STATEMENT OF PROBLEM: The failure of adhesion between a silicone-based resilient liner and a denture base is a significant clinical problem. PURPOSE: The purpose of this study was to examine the effects of denture base resin surface pretreatments with different chemical etchants preceding the silicone-based resilient liner application on microleakage and bond strength. The initial effects of chemical etchants on the denture base resin in terms of microstructural changes and flexural strength were also examined. MATERIAL AND METHODS: Forty-two polymethyl methacrylate (PMMA) denture base resin (Meliodent) specimens consisting of 2 plates measuring 30 x 30 x 2 mm were prepared and divided into 7 groups (n = 6). Specimen groups were treated by immersion in acetone for 30 (A30) or 45 (A45) seconds, methyl methacrylate monomer for 180 (M180) seconds, and methylene chloride for 5 (MC5), 15 (MC15) or 30 (MC30) seconds. Group C had no surface treatment and served as the control. Subsequently, an adhesive (Mollosil) and a silicone-based resilient denture liner (Mollosil) were applied to the treated surfaces, and all specimens were immersed in the radiotracer solution (thalium-201 chloride) for 24 hours. Tracer activity (x-ray counts), as a parameter of microleakage, was measured using a gamma camera. For bond-strength measurement, 84 rectangular PMMA specimens (10 x 10 x 40 mm) were surface-smoothed for bonding and treated with the different chemical etchants using the same previously described group configurations. The adhesive and the silicone-based denture liner were applied to the treated surfaces. Tensile bond-strength (MPa) was measured in a universal testing machine. Flexural strength measurement was performed with 49 PMMA specimens (65 x 10 x 3.3 mm according to ISO standard 1567) in 7 groups (n = 7), with 1 flat surface of each treated with 1 of the chemical etchants preceding adhesive application. The flexural strength (MPa) was measured using a 3-point bending test in a universal testing machine. The data were analyzed by 1-way analysis of variance and the Tukey HSD test (alpha = .05). RESULTS: Significant differences were found between the groups in terms of microleakage (P < .0001). The lowest microleakage was observed in group M180 (30,000 x-ray counts) and the highest in the control group (44,000 x-ray counts). The mean bond strength to PMMA resin ranged from 1.44 to 2.22 MPa. All treated specimens showed significantly higher bond strength than controls (P < .01). The flexural strength values all significantly differed (P < .05). All experimental specimens that had chemical surface treatments showed lower flexural strength than controls (P < .05). CONCLUSION: Treating the denture base resin surface with chemical etchants increased the bond strength of silicone-based resilient denture liner to denture base and decreased the microleakage between the 2 materials. Considering the results of both tests together, the use of methyl methacrylate monomer for 180 seconds was found to be the most effective chemical treatment.

Acid Etching, Dental↗
PubMed2006

Effect of silica coating and silanization on flexural and composite-resin bond strengths of zirconia posts: An in vitro study.

STATEMENT OF PROBLEM: The bond strength of composite resin cores to the smooth surface of prefabricated zirconia-based ceramic posts is problematic because it might not be sufficient to ensure stability of the post-and-core system. It is also not clear whether any alteration of the post surface to provide additional mechanical or chemical retention would compromise flexural strength of the posts. PURPOSE: This study tested the effect of a tribochemical treatment on the bond strength (BS) between zirconia-based ceramic posts and composite resin and on the flexural strength (FS) of the posts. MATERIAL AND METHODS: For the BS test, 2 groups of specimens (n = 10) were prepared as follows: In the experimental (E) group, zirconia-based ceramic posts (Cosmopost), 21 mm long and 1.7 mm in diameter, were treated with a tribochemical silica coating and silanization system (Rocatec). For the control (C) group, zirconia-based ceramic posts were used as supplied by the manufacturer. For the 2 groups, posts were centered and vertically positioned in a metal mold, and composite resin (Tetric Ceram) was polymerized around the posts to form rings (6 mm diameter x 4 mm height). All specimens were positioned in a universal testing machine, and a load was applied axially on the protruding heads of posts in a shear push-out mode test until bond failure (MPa). Fracture mode was assessed as cohesive, cohesive/adhesive, or adhesive. Following this, the FS of the posts was measured in MPa using a 3-point bending test. Data were recorded and statistically analyzed using the Student t test for both tests (alpha = .01). RESULTS: Mean fracture load values +/- SDs for the BS test were 28.1 +/- 2.3 MPa and 8.9 +/- 3.97 MPa for the E and C groups, respectively. The mean FS values were 1544.9 +/- 214.1 MPa and 844.8 +/- 50.8 MPa for the E and C groups, respectively. The tribochemically treated posts exhibited significantly higher values for both bond strength to composite resin and resistance to fracture when compared to posts in the control group (P < .001). Fracture analysis showed 100% cohesive/adhesive fractures for group E specimens and 100% adhesive fractures for group C specimens. CONCLUSIONS: The use of a tribochemical silica coating process and silanization on zirconia-based ceramic posts increased both the bond strength to composite resin as well as the fracture strength of posts.

Adhesiveness↗
PubMed2006

Evaluation of welded titanium joints used with cantilevered implant-supported prostheses.

STATEMENT OF PROBLEM: Early failure of laser-welded titanium implant frameworks in clinical practice has prompted an investigation of the strength and durability of welded cantilevered titanium sections. PURPOSE: The purpose of this study was to determine the effect that the use of filler wire in laser welding of titanium cantilever frameworks had on the flexural strength and fatigue resistance of the welded joints. MATERIAL AND METHODS: Sixty titanium implant-supported frameworks with 12-mm cantilevers were fabricated in 4 groups (n=15), using 3 different laser welding protocols with 0, 1, and 2 weld passes with filler wire, and 1 conventional tungsten inert gas welding method. The volume of filler wire used (mean volumes 0, 1.7, 3.4, and 8.3 mm(3)) was determined by measurement of the length of wire before and after welding each joint. Ten frameworks from each group were tested for ultimate flexural strength by loading the cantilevers 10 mm from the abutment. The remaining 5 frameworks from each group were similarly tested under a simulated masticatory load of 200 N until failure, or to 1 million cycles. A 2-way analysis of variance was used to examine the flexural strengths, and log-rank statistics were applied to cyclic test data (alpha=.05). RESULTS: There were significant differences between the 4 groups for ultimate flexural strength (P<.001) and resistance to cyclic loading (P=.002). The volume of filler wire added was a significant predictor of ultimate flexural strength (P=.03), and was a borderline determinant of the number of cycles to failure at 200 N (P=.05). Each laser weld pass with filler wire roughly doubled the ultimate flexural strength and fatigue resistance of the joint relative to the previous weld. Tungsten inert gas welding with efficient argon shielding deposited the most filler wire and produced the strongest and most fatigue-resistant joints. CONCLUSION: The ultimate flexural strength and fatigue resistance of cantilevered joints in laser-welded titanium prostheses are improved by the use of filler wire. Tungsten inert gas welding with efficient argon shielding can be used in situations when a high-strength joint is required.

Analysis of Variance↗
PubMed2006

Influence of thermal and mechanical stresses on the strength of intact and relined denture bases.

STATEMENT OF PROBLEM: Denture bases may become increasingly weaker as a result of thermal stress and flexural cyclic loading. Information regarding this potential problem and its relationship to the denture base reline is limited. PURPOSE: This study evaluated the influence of thermal and mechanical stresses on the strength of intact and relined denture bases. MATERIAL AND METHODS: Twenty-eight microwave-polymerized (Acron MC) intact denture bases were prepared in the shape of a 3-mm-thick maxillary denture. Additionally, fifty-six 2-mm-thick denture bases were relined with 1 mm of autopolymerizing resin (Tokuyama Rebase Fast II or New Truliner) (n=28). Intact and relined specimens were divided into 4 groups (n=7) as follows: without stress (control); a mechanical stress at 0.8 Hz for 10,000 cycles; 5000 thermal cycles between 5 degrees C and 55 degrees C; or a combination thermo-mechanical stress. The specimens were vertically loaded in compression with a rounded rod at 5 mm/min until failure, using a universal testing machine. Data on maximum fracture load (N), deflection at fracture (%), and fracture energy (N.mm) were analyzed by 2-way analysis of variance and Student-Newman-Keuls tests (alpha=.05). RESULTS: The strength of the denture bases relined with New Truliner was not significantly affected by any of the experimental conditions, but comparing the control groups, New Truliner exhibited the lowest maximum fracture load values. The maximum fracture load of intact denture bases (P=.002) and those relined with Tokuyama Rebase Fast II (P=.01) showed a significant decrease after thermal stress. Additionally, cyclic loading significantly decreased the maximum fracture load (P<.001), deflection at fracture (P=.025), and fracture energy (P<.001) of intact denture bases and those relined with Tokuyama Rebase (P values of .002, .039, and .001, respectively). CONCLUSION: Thermal and mechanical stresses exert deleterious effects on the strength of intact and/or relined denture bases, which vary according to the relining material used.

Acrylic Resins↗
PubMed2006

In vitro evaluation of low-temperature aging effects and finishing procedures on the flexural strength and structural stability of Y-TZP dental ceramics.

STATEMENT OF PROBLEM: Yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) ceramic material has been reported to possess exceptionally high flexural strength; however, the strength of Y-TZP ceramics can be affected by surface treatments performed by dental laboratory technicians and by intraoral conditions. PURPOSE: This study investigated the influence of low-temperature-degradation (LTD) treatment, airborne-particle abrasion, and polishing on the flexural strength and structural stability of a Y-TZP ceramic material. MATERIAL AND METHODS: By sectioning zirconia ceramic blocks (Vita In-Ceram YZ blocks) into 25 x 4 x 2-mm bars, 310 specimens were fabricated and divided into 9 groups: Group C, as-sintered (control); group B24h, boiled in water for 24 hours; group B7d, boiled in water for 7 days; group H6h, stored in humidified air at 250 degrees C for 6 hours; group H24h, stored in humidified air at 250 degrees C for 24 hours; group H7d, stored in humidified air at 250 degrees C for 7 days; group P, polished; group A, airborne-particle abraded; and group AB, airborne-particle abraded and boiled for 7 days. The flexural strengths (MPa) of all specimens were determined by using a 3-point bending test. The variability was analyzed by using Weibull statistics. Pairwise differences among the 9 groups were evaluated by using confidence intervals (95%) for scale and shape parameters. From every block used, 8 specimens were obtained and every specimen was marked according to the location of its origin from the original block. Surfaces of the specimens were evaluated by using scanning electron microscopy (SEM) at various magnifications. X-ray diffraction analysis was performed to identify possible tetragonal-to-monoclinic phase transformations. Energy dispersive spectroscopy (EDS) was used to obtain information on the chemical composition. RESULTS: Aging or "finishing" treatments had no significant negative effects on flexural strengths. Mean flexural strengths ranged from 796.7 to 950.2 MPa. Group A exhibited the highest mean flexural strength (950.2 MPa). Weibull modulus values ranged between 5.6 and 9.3. Tetragonal-to-monoclinic phase transformation was detected for a specimen boiled for 24 hours. More tetragonal-to-monoclinic phase transformation was detected for specimens boiled for 7 days and stored in humidified air at 250 degrees C for 7 days. The SEM examination of fractured surfaces revealed sintering defects, and EDS analysis showed less yttria concentration on Y-TZP specimens that were boiled for 7 days. CONCLUSION: Within the limitations of this study, the results suggested that the LTD procedures and the polishing treatment used did not reduce the flexural strengths of zirconia bars. Airborne-particle abrasion increased the flexural strength of specimens. CLINICAL IMPLICATIONS: Although polishing, airborne-particle abrasion, and various low-temperature degradation (LTD) treatments did not significantly degrade the strength of the Y-TZP ceramic material, LTD resulted in loss of yttria. Over time, gradual dissolution of yttria could decrease the tetragonal-phase stability and long-term clinical serviceability of this dental ceramic material.

Crystallography, X-Ray↗
PubMed2006

Viscoelastic behavior and fracture toughness of six glass-ionomer cements.

STATEMENT OF PROBLEM: Viscoelastic behavior can influence the fracture properties of glass ionomers, which is of clinical relevance. Glass-ionomer cements can display viscoelastic behavior, defined as having displacement rate- or strain rate-dependent mechanical properties. Understanding and describing the viscoelastic behavior of glass ionomers is important to understanding their clinical behavior. PURPOSE: The purpose of this study was to evaluate the viscoelastic behavior of 6 glass-ionomer cements and determine whether there was a correlation to fracture toughness. MATERIAL AND METHODS: Three conventional glass-ionomer cements (alpha-Silver, alpha-Fil, and Ketac-Molar) and 3 resin-modified glass-ionomer cements (Vitremer, Fuji II LC, and Photac-Fil Quick) were evaluated using measurements of compressive strength (CS), flexural strength (FS), and diametral tensile strength (DTS) at displacement rates of 0.5, 1.0, 1.5, and 2.0 mm/min. The CS and DTS specimens were cured in glass tubes and cut to 4 x 6-mm and 4 x 2-mm disk-shaped specimens, respectively. The FS specimens were cured in bar molds (2 x 2 x 15 mm). The fracture toughness (FT) specimens were cured in a minicompact mold to obtain precracked specimens. The mechanical testing results were compared statistically using generalized linear model/analysis of covariance and the Ryan-Einot-Gabriel-Welsch multiple range test at the alpha=.05 level. RESULTS: For all 3 mechanical properties, there was a displacement-rate dependence on the mechanical property. However, there were no differences in the displacement-rate dependence based on the type of material-conventional glass ionomer or resin-modified glass ionomer-for any of the mechanical properties. Only for FS test was there a significant difference based on the brand of material. There was no statistical difference in FT among the glass-ionomer cements tested, although the resin-modified glass ionomers tended to display higher FT. CONCLUSION: A larger sample size and a much wider range of crosshead speeds are necessary to support a correlation between viscoelastic behavior and FT.

Analysis of Variance↗