Effect of phosphate group addition on the properties of denture base resins.

STATEMENT OF PROBLEM: Acrylic resins are prone to microbial adherence, especially by Candida albicans. Surface-charged resins alter the ionic interaction between the denture resin and Candida hyphae, and these resins are being developed as a means to reduce microbial colonization on the denture surface. PURPOSE: The purpose of this study was to investigate the physical and mechanical properties of phosphate-containing polymethyl methacrylate resins for their suitability as a denture material. MATERIAL AND METHODS: Using PMMA with cross-linker (Lucitone 199) as a control, 4 experimental groups containing various levels of phosphate with and without cross-linker were generated. The properties examined were impact strength, fracture toughness, wettability (contact angle), and resin bonding ability to denture teeth. Impact strength was tested in the Izod configuration (n=16), and fracture toughness (n=13) was measured using the single-edge notched bend test. Wettability was determined by calculating the contact angle of water on the material surface (n=12), while ISO 1567 was used for bonding ability (n=12). The data were analyzed by 1- and 2-way ANOVA (alpha=.05). RESULTS: A trend of increased hydrophilicity, as indicated by lower contact angle, was observed with increased concentrations of phosphate. With regard to the other properties, no significant differences were found when compared with the control acrylic resin. CONCLUSIONS: No adverse physical effect due to the addition of a phosphate-containing monomer was found in the acrylic denture resins. Additional mechanical and physical properties, biocompatibility, and clinical efficacy studies are needed to confirm the in vivo anti-Candida activity of these novel resins.

Physical properties of denture base resins potentially resistant to Candida adhesion.

PURPOSE: The addition of anionic charge on denture base resins has been shown to inhibit Candida albicans adhesion and to facilitate adsorption of salivary defense molecules. The aim of this study was to evaluate the physical properties of a modified denture base resin for denture fabrication. MATERIALS AND METHODS: Specimens made from heat polymerizing resin Lucitone 199 were used as the control group. The two experimental groups, E-10 and E-20, had 10% and 20%, respectively, of the monomer substituted with an experimental phosphate-containing monomer. Flexural strength and modulus, water sorption, solubility, and color stability tests were conducted to ensure compliance with ADA specification No. 12. Water diffusion coefficient into the resins and stainability were also assessed. ANOVA and Scheffé tests were performed for statistical significance. RESULTS: There was an overall decline in all properties with the addition of the experimental phosphate compound. The flexural strength and modulus, water sorption and solubility for E-10, as well as the control were, however, within the ADA specifications. The diffusion coefficients were significantly different (p < 0.05) for the three groups. Staining and color specimens showed no significant difference (p > 0.05) among the three groups. CONCLUSIONS: Within the limitations of this study, the physical properties of the phosphate denture base resin at 10% should be suitable for denture fabrication based on the properties assessed.

A comparison of bond strength between direct- and indirect-bonding methods.

The purpose of this study was to evaluate and compare the shear bond strength and the sites of bond failure for brackets bonded to teeth, using two indirect-bonding material protocols and a direct-bonding technique. Sixty extracted human premolars were collected and randomly divided into three groups. The direct-bonded group (group 1) used a light-cured adhesive and primer (Transbond XT). One indirect-bonded group (group 2) consisted of a chemical-cured primer (Sondhi Rapid Set) and light-cured adhesive (Transbond XT), whereas the other group (group 3) used a light-cured primer (Orthosolo) and adhesive (Enlight LV). Forty hours after bonding, the samples were debonded. Mean shear bond strengths were 16.27, 13.83, and 14.76 MPa for groups 1, 2, and 3, respectively. A one-way analysis of variance showed no significant difference in mean bond strength between groups (P = .21). Furthermore, a Weibull analysis showed all three groups tested provided over a 90% survival rate at normal masticatory and orthodontic force levels. For each tooth, an Adhesive Remnant Index (ARI) score was determined. Group 2 was found to have a significantly lower ARI score (P < .05) compared with groups 1 and 3. In addition, Pearson correlation coefficients indicated no strong correlation between bond strength and ARI score within or across all groups.

The effect of repeated torque and salivary contamination on the preload of slotted gold implant prosthetic screws.

STATEMENT OF PROBLEM: Screw loosening and/or fracture is common and has been attributed to many factors, including improper torque and preload. Purpose. This study evaluated the effect of repeated preload torque and salivary contamination on the preload of the slotted gold implant prosthetic screw. MATERIAL AND METHODS: Fifteen slotted gold prosthetic screws were tested in an unused stack of components consisting of an implant, an abutment cylinder, and a screw and gold cylinder. A custom load frame and load cell and associated electronics were used. The component stack was lubricated with human saliva. Each screw was tightened to 10 Ncm with a hand-held torque wrench fitted with strain gauge electronics and then removed 10 times. Preload values at the first (Group 1X), fifth (Group 5X), and tenth (Group 10X) repetition were measured. Repeated-measures 1-way analysis of variance (P<.05) and paired comparisons were used to analyze the data. RESULTS: The mean preload value increased from Group 1X (184.3 N +/- 28.9) to Group 5X (202.5 N +/- 27.7) to Group 10X (220.2 N +/- 29.0). The differences among these groups were significant (P=.004). CONCLUSION: Within the limitations of this study, higher preload was achieved after the repeated use of a saliva-lubricated gold prosthetic retaining screw.

The effect of repeated torque on the ultimate tensile strength of slotted gold prosthetic screws.

STATEMENT OF PROBLEM: Repeated torque may alter the mechanical properties and fracture resistance of certain prosthetic retaining screws. PURPOSE: This study evaluated the effect of repeated torque and salivary contamination on the ultimate tensile strength of one type of slotted gold prosthetic retaining screw. MATERIAL AND METHODS: Forty-five slotted gold prosthetic screws from the same manufacturer (Implant Innovations) were divided randomly into 9 groups of 5 screws each, with group 0X being an unused control. Groups 1XL, 5XL, 10XL, and 20XL were lubricated with human saliva, tightened to 10 Ncm, and removed 1, 5, 10, and 20 times, respectively. This procedure was performed in an unused stack of implant components consisting of an implant, an abutment cylinder and screw, and a gold cylinder. Groups 1XN, 5XN, 10XN, and 20XN were not lubricated. A custom load cell, load frame, and strain gauge torque wrench with associated electronics were used to apply torque; ultimate tensile strength was measured on a universal testing machine. Fracture load values were analyzed against torque cycles with 1-way analysis of variance and a Tukey test (P<.05). Differences between the 2 conditions (lubricated and nonlubricated) were characterized with Student's t test (P<.05). RESULTS: Mean fracture load values ranged from 97.6 +/- 2.2 kg (group 0X) to 102.0 +/- 2.1 kg (group 5XN). Analysis of variance and Tukey's test revealed no significant differences between the lubricated and nonlubricated conditions for all groups and no significant reduction of tensile strength. For 2 of the nonlubricated groups, 5XN and 20XN, a significant increase in fracture load values was recorded (P=.02 and P=.03, respectively). CONCLUSION: Within the limitations of this study, the results suggest that the slotted gold prosthetic screw tested can be tightened and removed up to 20 times without any effect on its ultimate tensile strength. The use of human saliva as a lubricant during torquing also had no apparent effect on ultimate tensile strength.

Delayed linear expansion of improved dental stone.

STATEMENT OF PROBLEMS: Specification 25 of the American Dental Association (ADA) states that final setting expansion measurements of improved dental stone are made 2 hours after mixing. In a limited pilot study on selected type IV and V dental stones, measurements showed delayed linear expansion or early contraction continuing up to 120 hours. PURPOSE: The objectives of this in vitro study were to measure the linear setting expansion of 6 ADA type IV and V dental stones, to compare their expansion at 2 hours relative to ADA Specification 25, and to characterize expansion changes up to 120 hours. MATERIAL AND METHODS: The improved dental stones tested in this study were Die-Keen, Jade Stone, Vel-Mix, ResinRock, Fuji Rock, and Silky Rock. Mixing was performed in accordance with a standardized technique and the manufacturers' instructions. The mixes were poured into an electronic expansion-measuring device, and setting expansion values were continuously measured and recorded for 120 hours. The collected data were subjected to a 2-way repeated-measures multivariate analysis of variance at alpha=.05. Planned comparisons between the 2-hour experimental and 2-hour manufacturer expansion values were made at the 95% confidence interval. RESULTS: All stones showed higher mean linear expansion values at 120 hours than at 2 hours. Expansion was essentially complete at 96 hours for all stones except ResinRock. Die-Keen exhibited the highest total mean setting expansion (0.35% +/- 0.013%), and Silky Rock exhibited the lowest mean linear expansion (0.14% +/- 0.008%) at 120 hours. Multivariate analysis of variance revealed significant interactions between stone and time (P<.0001). Mean expansion values at 2 hours were comparable to those stated by the manufacturer except for Die-Keen and Jade Stone. CONCLUSION: Within the parameters of this study, all dental stones tested exhibited continued expansion while setting under clinical conditions. The amount of expansion varied significantly among the materials, with 22% to 71% of the expansion occurring after 2 hours. Die-Keen exhibited the highest total mean expansion but had the least amount of expansion after 2 hours; both Die-Keen and Silky Rock remained stable after 72 hours.

Defects in hexed gold prosthetic screws: A metallographic and tensile analysis.

STATEMENT OF PROBLEM: Prosthetic gold screw fracture remains a clinical problem in implant prosthodontics. PURPOSE: This study examined hexed gold prosthetic screws for internal defects and determined the effect of these defects on tensile strength. The microstructure, microhardness, and major constituents of the alloys also were determined. MATERIAL AND METHODS: Four intact hexed gold prosthetic screws, 1 from each of 2 different lots from 2 manufacturers (Implant Innovations and Nobel Biocare), were examined with standard metallographic techniques for defects, microstructure, microhardness, and major alloy constituents. Thirty-six screws, 9 from each of the 2 different lots of both manufacturers, were subjected to tensile testing to determine fracture load values. Analysis of variance and Tukey tests were used to identify differences between manufacturers and lots (P<.05). The fracture sites were examined retrospectively with a scanning electron microscope to identify defects that could have contributed to failure. The mode of fracture was characterized. RESULTS: There were no significant defects in the screws tested, but differences were observed in the microstructure, microhardness, alloy composition, and fracture load values for both manufacturers. Screws from the 2 manufacturers demonstrated distinctly different metallurgical characteristics, which highlighted differences in the manufacturing processes. Fracture load values ranged from 850 +/- 20 N to 1093 +/- 64 N. A significant difference was noted for mean fracture load values for the different lots of Implant Innovations screws (P<.05). Ductile fracture was the mode of failure. CONCLUSION: The results of this study suggest that variability in the physical properties of similar hexed gold prosthetic screws made by different manufacturers, as well as different lots from the same manufacturer, may affect clinical success.