Mechanical properties of urethane and bis-acryl interim resin materials.

STATEMENT OF PROBLEM: The fracture or dislodgement of an interim prosthesis may result in additional appointments and could negatively affect the outcome of a fixed prosthodontic procedure. PURPOSE: The purpose of this study was to evaluate the flexural strength (FS), flexural modulus (FM), work-of-fracture (WOF), and Weibull parameters of 4 bis-acryl (Protemp Plus, Integrity, Turbo Temp 2, Temphase Fast-set) and 2 urethane (NuForm and Tuff-Temp) interim resins after being stored in distilled water for 1 hour and 24 hours. MATERIAL AND METHODS: Rectangular beam specimens (25 × 2 × 2 mm) of each resin (n=23) were prepared and stored in distilled water at 37°C. Then 3-point bending tests were performed on 1-hour and 24-hour (wet) and 24-hour (dry and wet) specimens in a universal testing machine at a cross-head speed of 1.0 mm/min. Stress/strain curves were generated and flexural strength, flexural modulus, and WOF values were calculated. Data were subjected to 3-way ANOVA and the Tukey-Kramer pairwise comparison test (α=.05). FS data were also subjected to a 2-parameter Weibull analysis. RESULTS: The 24-hour (wet) mean flexural strength and flexural modulus values were significantly higher than 1-hour values. The urethane Tuff-Temp, chemically and dual-polymerized, showed significantly higher (P<.05) 1-hour FS and FM, while the bis-acryl Protemp Plus showed significantly greater flexural strength and work-of-fracture than the other resins after 24 hours of wet storage. Protemp Plus, NuForm, and Turbo Temp 2 did not decrease in flexural strength and flexural modulus after 24 hours in dry and wet storage conditions. The Weibull modulus values of the flexural strengths of the interim resins at 1 hour and 24 hours ranged from 8.4 to 26.8. CONCLUSIONS: Postgelation polymerization plays an important role in the substantial increase in the flexural strength and flexural moduli of the interim resins between 1 and 24 hours. A wide variation in the mechanical properties of the interim resin materials was noted but was, for the most part, not category specific.

Relative fracture toughness of bis-acryl interim resin materials.

STATEMENT OF PROBLEM: Fracture of the interim partial denture may cause patient discomfort and result in unplanned appointments and expense. At present, limited information is available concerning the fracture toughness of bis-acryl interim resins, especially immediately following fabrication. PURPOSE: The purpose of this investigation was to evaluate the relative fracture toughness and Weibull parameters of 4 commercial chemically polymerized (Protemp Garant 3, Perfectemp II, Integrity, Temphase) and 1 dual polymerizing (Luxatemp Solar) bis-acryl interim resin materials with various filler loading after 1 hour and 24 hours. MATERIAL AND METHODS: Disk-shaped, mini-compact test specimens (8.20 × 1.85 mm thick, n=18) with introduced pre-cracks were prepared in a pre-heated split-mold and maintained at 37°C for 5 minutes during polymerization to simulate clinical conditions. Polymerization temperature was measured using an infrared digital thermometer. Specimens were stored in distilled water at 37°C after which the peak load to fracture was recorded at 1 and 24 hours and the fracture toughness (K(1c)) was calculated. An overall Weibull analysis of the fracture toughness was performed incorporating the fracture toughness data with factors polymerization method, time of testing and filler load of the resin. Weibull analysis was performed at (α =.05). Fracture surfaces of representative specimens were examined using scanning electron microscopy. RESULTS: The overall Weibull analysis results showed significant differences (P<.001) in fracture toughness between the times of testing (1 and 24 hours), polymerization method, and as a function of filler loading. The results of the individual Weibull survival analyses showed a significant increase in Weibull Characteristic Strength (σo) values between 1 and 24 hours for all materials. The Weibull moduli (m) for the interim resins ranged between 5.8 and 10.3. SEM analysis of fractured surfaces revealed that crack propagation occurred primarily through the resin matrix between filler particles. CONCLUSIONS: Post-gelation polymerization has an important role in determining the fracture properties of both chemical and dual-polymerized bis-acryl interim materials within the first 24 hours. For the materials tested, the higher the filler load, the lower the fracture toughness.

Synthesis and evaluation of modified urethane dimethacrylate resins with reduced water sorption and solubility.

OBJECTIVES: New aliphatic and aromatic urethane dimethacrylate monomers containing pendant phenyl methoxy or ethyl substituents were synthesized in order to reduce the water sorption and solubility of urethane dimethacrylate systems. Selected properties including flexural strength, flexural modulus, water sorption and solubility, and water contact angle were evaluated. Hoy's solubility parameters were also calculated to rank copolymer hydrophilicity. METHODS: Filled (20%) composite resins were formulated with each of the newly synthesized dimethacrylates as well as the commercially available urethane dimethacrylate monomer, UDMA. Flexural strength, flexural modulus, water sorption and solubility of the urethane composites were evaluated after light-cured specimens were immersed in water for seven days. Water contact angles were measured on the surface of each material. Data were analyzed using ANOVA and Ryan-Einot-Gabriel-Welsch multiple range tests (alpha=0.05). RESULTS: A significant reduction of nearly 30% and 40% in water uptake was observed with composite polymers containing pendant ethyl and phenyl methoxy groups, respectively, compared to UDMA (p<0.05). Urethane copolymers containing pendant ethyl groups also showed a significant reduction in water solubility (p<0.05). A positive correlation was found between contact angle and water sorption as well as Hoy's delta(h) for hydrogen bonding forces. SIGNIFICANCE: The results of this study indicate that the incorporation of pendant hydrophobic substituents within the monomer backbone may be an effective method in reducing the water sorption and water solubility of urethane based dimethacrylate systems. The use of Hoy's solubility parameters to determine the relative hydrophilicity of a polymer may be limited by its three-dimensional chemical structure.

Effect of mixing methods on the physical properties of dental stones.

OBJECTIVES: This in vitro comparative study evaluated the effect of different stone mixing methods on material properties of four dental stones. Two ADA type IV stones (Silky-Rock and Snap-Stone), one type V high expansion stone (Die Keen), and one recently introduced type V specialty stone (HandiMix) were chosen for this study. METHODS: Forty cylindrical specimens (25 mm x 12.5 mm) were cast for each of the nine stone sub-groups and bench dried at 23+/-2 degrees C for 1 and 24h. Specimens were then tested in an Instron in tensile and compression modes at crosshead speeds of 0.5 and 1.0 mm/min, respectively. Four rectangular-shaped specimens (30 mm x 15 mm x 15 mm) of each stone type were cast and bench dried for 48 h. Knoop microhardness measurements were obtained from defined areas on each specimen for surface hardness testing using 200 g load and 20s dwell time. A 12.6 mm(2) area was then delimited in the center of two sides of each specimen and photographed under low power magnification (40x). The average pore number per area was then determined for each specimen for surface porosity testing. The setting time and setting expansion for each stone type was recorded as well. RESULTS: ANOVA (P<0.001) and Ryan-Einot-Gabriel-Welsh test (P<0.05) showed significant differences between diametral tensile strengths and pore numbers for both stone types and mixing methods. CONCLUSION: Within the limitations of this study, the newly introduced mixing method did not appear to have an effect on the physical properties of HandiMix stone.

Bond strengths of one- and two-step self-etch adhesive systems.

STATEMENT OF PROBLEM: One-bottle self-etching adhesives have been introduced that combine the etchant, primer, and adhesive into a single bottle in an effort to reduce the number of bonding steps. Limited research has been conducted comparing the efficacy of 1-bottle and 2-bottle self-etching adhesives. PURPOSE: This investigation evaluated the microtensile bond strength (MTBS) of 3 one-step self-etching adhesives, 2 two-step self-etching adhesives, and a total-etch adhesive. A secondary objective was to evaluate the effect of an intermediary layer of elastic resin on the MTBS of 2 of the self-etching adhesives. MATERIAL AND METHODS: The MTBS of 3 one-step self-etching adhesives (G-Bond, iBond, and Clearfil S(3)), 2 two-step self-etching adhesives (Clearfil SE and Optibond Solo Plus Self-Etch), and a total-etch adhesive (Prime & Bond NT) was evaluated. Microtensile bond strength (MPa) was determined using composite resin (Herculite XRV) cylinders bonded to the dentin surface of human third molars (n=24) using 6 bonding agents. Each bonded tooth specimen was longitudinally sectioned to produce bar-shaped specimens with a square cross-sectional nominal bond area of 1.4 mm(2) (n=20). Bonded specimens were stored in distilled water at 37 degrees C +/- 2 degrees C for 72 hours, during which time they were subjected to thermal cycling (1500 cycles, in water baths of 5 degrees C and 55 degrees C, for a dwell time of 30 seconds). Testing was performed on a universal testing machine at a crosshead speed of 1.0 mm/min. One-way ANOVA, Ryan-Einot-Gabriel-Welsh multiple range test, and Wald chi square test, each using an overall alpha=.05, were performed on all data. Low-power (x20) microscopic examination of the bond failure was observed for each specimen. RESULTS: The 1-way ANOVA (P<.001) and Ryan-Einot-Gabriel-Welsh multiple range test (P<.05) showed significant differences between the mean MTBS values for several of the bonding agents tested. The Wald chi square test of the Weibull distribution showed significant differences between adhesives (P<.001). CONCLUSION: Of the bonding agents tested, significantly higher MTBS was obtained with the total-etch 2-stage adhesive. The results of this study suggest that there are similar bond strengths between the 1- and 2-bottle self-etch adhesives.

Influence of interface surface conditions on indentation failure of simulated bonded ceramic onlays.

OBJECTIVE: This study aims to evaluate the influence of surface roughness and acid etching on interfacial failure load and fracture initiation site for simulated resin bonded ceramic restorations. METHODS: Layered disks were fabricated using soda lime glass to simulate porcelain and fiberglass to represent dentin. These materials were selected due to their approximate elastic modulus to porcelain or dentin. In addition, soda lime glass allowed for visualization of crack initiation. Glass microscope slides (1.2 mm thick) were sectioned into 1-mm2 specimens. The frosted portions of the slides were used as the rough or sandblasted group and the clear areas were used as the smooth or polished group. All surfaces were either cleaned or etched. Prepared surfaces were divided into eight groups and bonded to the substrates using Optibond Solo Plus (SDS Kerr, Orange, CA) and Nexus resin cement (SDS Kerr). Support substrates were made from 5-mm slices of fiberglass rods (1/2 in. diameter). Vertical loads were applied to the layered disks with a 20-mm spherical indenter at a cross-head speed of 0.05 mm/min. Observations were made using 10x magnification with a video monitor. Load magnitudes were recorded at the time of crack initiation. Weibull parametric survival analysis and the effect likelihood ratio test were performed on the failure initiation load data to determine significant differences at the 0.05 level. RESULTS: Mean failure initiation loads ranged from 273.6 to 341.9N for the rough specimens and from 1056.5 to 2980.2N for the smooth samples. According to the Weibull parametric survival analysis and the effect likelihood ratio test, surface roughness and surface etch significantly influenced failure initiation loads. Smoother surfaces tolerated much greater contact loads before failure initiation. SIGNIFICANCE: The clinical performance of all-ceramic restorations may be improved with smoother internal surfaces. Etching significantly increases survival for restorations with smooth internal surfaces.

Microtensile bond strengths of one-step and self-etching adhesive systems.

The microtensile bond strength of resin composite bonded to human enamel was evaluated utilizing four light-cure bonding agents. Human third molars were embedded in auto-cure acrylic and the buccal surfaces were sequentially abraded to 400 grit. Resin composite cylinders were then bonded using the four bonding systems according to the manufacturer's specifications. Each bonded tooth produced three to four longitudinal sections which were then laterally notched to give a square bond area (approximately 2.25 mm2). Specimens (n=10) were assigned to two groups: Group I was stored in distilled water at 37 degrees +/- 2 degrees C for seven days. Group II was stored in distilled water at 37 degrees +/- 2 degrees C for seven days, during which time it was thermocycled in hot and cold water baths for 1,000 cycles. In addition, a water sorption test was performed on three of the four adhesive systems. The microtensile bond strength of the conventional adhesive Optibond Solo Plus was significantly greater than that of the self-etching adhesives Tyrian SPE and Prompt L-Pop. Adhesive systems that were more hydrophilic tended to show lower bond strengths, especially after thermocycling.

Relative wear of flowable and highly filled composite.

STATEMENT OF PROBLEM: By decreasing the proportion of filler, flowable composites are easy to adapt to small cavity preparations. However, the wear resistance of flowable composites is a clinical concern, as improved wear has been related to increased filler percentage. PURPOSE: The purpose of this study was to compare wear for flowable composites and comparable heavily filled materials. MATERIAL AND METHODS: Wear by attrition and abrasion (microm) of microfilled flowable (Heliomolar Flow) and heavily filled microfill (Heliomolar HB) was measured and compared to a microhybrid flowable (Tetric Flow) and a heavily filled microhybrid (Tetric HB). There were 5 specimens, 2 mm thick and 15 mm in diameter, in each group. The Oregon Health Sciences University oral wear simulator was used to produce abrasive wear and attrition for all specimens. Abrasion and attrition of the substrates were measured using a profilometer. Data were subjected to a 3-way analysis of variance for the 3 main factors: composite type, filler load, and wear type (alpha=.05) RESULTS: Composite wear rates were significantly influenced (P <.01) by both the type of wear and the filler amount. Abrasive wear occurred less than wear by attrition, and the heavily filled composites experienced less wear than the flowable composites. CONCLUSIONS: Results of this in vitro study suggest that highly filled microhybrid composites may have greater wear resistance for contact-supporting posterior restorations. However, in small noncontact restorations where longevity may be dictated by abrasion, flowable microfilled resins may be adequate.

Microleakage and bond strength of sealant to primary enamel comparing air abrasion and acid etch techniques.

PURPOSE: The objective of this study was to compare the effect of air abrasion (KCP 2000), acid etching (37% phosphoric acid), and the combination of both procedures on the shear bond strength and microleakage of a light-cured pit-and-fissure sealant to the enamel of human primary molar teeth. METHODS: Noncarious extracted human primary molars were randomly divided into 4 groups in preparation for enamel bonding. The enamel surface was treated as follows for each group: (1) group 1 (control group); (2) group 2 (acid etch group); (3) group 3 (KCP [Kinetic Cavity Preparation System] group); and (4) group 4 (KCP and acid etch group). Delton, a light-cured pit-and-fissure sealant, was then applied to the occlusal surface after conditioning. The bonded specimens were maintained in distilled water at 37 degrees C+/-2 degrees C for 7 days, after which they were subjected to thermocycling followed by shear bond testing. Microleakage was determined by immersing the prepared teeth in 50% silver nitrate dye followed by sectioning and calculation of dye penetration. RESULTS: The mean shear bond strength of the KCP+acid etch group exhibited nearly 50% higher bond strength than the acid etch group (P<.01). In addition, specimens bonded to enamel conditioned only with acid etch exhibited bond strengths that were nearly twice that of those conditioned with the KCP system alone. No significant difference was noted between the air abrasion and control groups. CONCLUSIONS: In primary teeth, air abrasion combined with acid etching appears to provide the best conditions for enamel treatment prior to sealant placement.

Hardness and degree of conversion of posterior packable composites.

Knoop microhardness and the degree of conversion of three packable composites (SureFil, Alert and Solitaire), a microfil composite (Heliomolar), a microhybrid composite (Herculite) and an indirect laboratory-processed composite (Belleglass) were evaluated as a function of distance from the irradiated surface. Cylindrical specimens (5.0 mm [diameter] x 6.0 mm [length]) of each material were visible light cured for 60 seconds in black-backed Teflon molds and sectioned. Knoop microhardness values were then obtained at 0-, 2- and 4-mm using a 50-gram load and 20 second dwell time. Degree of conversion was determined using Fourier Transform Infrared Spectroscopy. ANOVA (p<0.001) and Tukey Multiple Comparison Test (p<0.05) showed the indirect laboratory-processed composite Belleglass exhibited the highest mean values for both hardness and degree of conversion. Alert exhibited significantly greater hardness than SureFil and Solitaire at 0-mm depth. SureFil had significantly greater hardness than Alert at 4-mm depth. The degree of conversion of SureFil packable composite at 4-mm depth was significantly higher than any of the other direct composites tested.

Fracture toughness of packable and conventional composite materials.

STATEMENT OF PROBLEM: The introduction of packable composite has expanded the choices of materials for the restoration of posterior teeth. Few independent studies are available on the fracture toughness (K(IC)) of the presently available packable composites compared with more conventional composite alternatives. PURPOSE: This investigation evaluated the relative fracture toughness of 3 packable composites, 2 conventional composites, and 1 laboratory-processed composite. MATERIALS AND METHODS: Six composite materials were tested in this study. These included: 3 packable composites (Alert, SureFil and Solitaire), 2 conventional composites (Herculite and Heliomolar), and 1 laboratory-processed composite (Belleglass). K(IC) was determined by preparing 8 mini-compact test specimens (8.2 mm diameter x 2 mm thickness) for each composite in a polytetrafluoroethylene split-mold with introduced precracks created with a razor blade. Specimens were stored in distilled water at 37 degrees +/- 2 degrees C for 7 days. Testing was performed on a universal testing machine at a displacement rate of 0.5 mm/min until fracture. Analysis of variance (P<.0001) and Ryan-Einot-Gabriel-Welsch multiple range tests (P<.05) were performed on all data. RESULTS: The mean fracture toughness of Alert (1.57 Mpa x m(1/2)) was significantly greater than any of the other composites tested. Solitaire, a packable composite, exhibited a mean fracture toughness (0.67 MPa x m(1/2)) that was significantly lower than any of the other materials tested. No significant difference was noted between Belleglass (1.27 MPa x m(1/2)), SureFil (1.25 MPa x m(1/2)) and Herculite (1.16 MPa x m(1/2)). CONCLUSION: Within the limitations of this study, the glass fiber-reinforced packable composite exhibited improved fracture toughness when compared with the other composite materials tested.