Novel Experimental In-Office Bleaching Gels Containing Co-Doped Titanium Dioxide Nanoparticles

The present study reports on the development and testing of novel bleaching agents containing co-doped metaloxide nanoparticles (NP; 0%, 5%, 10% v/w) and hydrogen peroxide (HP, 0%, 6%, 15%, and 35%). Bovine blocks (n = 200, A = 36 mm2) were obtained and randomly distributed into experimental groups (n = 10/group). NPs were incorporated into gels before bleaching (3 sessions, 7 days apart, 30 min/session, irradiated with violet light-LT). Color changes (ΔE00, ΔWID), mineral content (CO32−, PO43−), and topography were assessed (spectrophotometer, ATR-FTIR, and AFM) before and after bleaching procedures (14 days). Metabolic status and three-dimensional components of non-disrupted Streptococcus mutans biofilms were investigated using a multimode reader and confocal microscopy. The results indicate that ΔE00 and ΔWID significantly increased with NPs’ concentrations and LT. The enamel’s mineral ratio was adversely impacted by HP, but alterations were less pronounced when using NP-containing gels. The enamel’s topography was not damaged by the bleaching protocols tested. The bioluminescence results show that bleaching protocols do not render latent antibacterial properties to enamel, and the confocal microscopy results demonstrate that the 3-dimensional distribution of the components was affected by the protocols. The proposed nanotechnology improved the bleaching efficacy of experimental materials independent of hydrogen peroxide or irradiation and did not adversely impact the enamel’s surface properties or its chemical content.

Size-resolved spatial distribution analysis of aerosols with or without the utilization of a novel aerosol containment device in dental settings

The coronavirus disease 2019 pandemic has imposed unprecedented occupational challenges for healthcare professionals. In dentistry, handheld instruments such as air and electric handpieces, ultrasonic scalers, and air/water syringes are capable of generating aerosols, droplets, and splatter, thereby exposing dental professionals to airborne contaminants such as viruses, bacteria, and fungi. The objective of the present study was to determine the spatial distribution of aerosols by size (0.30 to 20.00 μm) and the efficacy of a novel aerosol containment device (ACD) in a large operatory room with 12 dental chairs. Real-time portable laser aerosol spectrometers were used to measure the size-resolved number concentration of aerosols generated by a collision nebulizer. Results reported demonstrate that aerosol number concentrations significantly decreased as a function of distance with or without the utilization of the ACD. The ACD was able to efficiently decrease (up to 8.56-fold) the number and size distribution of particles in a large dental clinic. The novel device demonstrated higher efficiency for particles shown to contain the highest levels of severe acute respiratory syndrome coronavirus 2 in Chinese hospitals, thereby showing great promise to potentially decrease the spreading of nosocomial pathogens in dental settings.

Assessment of Streptococcus mutans biofilms on orthodontic adhesives over 7 days

INTRODUCTION

The purpose of this study was to compare the metabolism of Streptococcus mutans biofilms after 1-7 days of growth on different orthodontic adhesives.

METHODS

Specimens of 6 commercial orthodontic adhesives were fabricated in custom-made molds and polymerized using a light-emitting diode light-curing unit. Bioluminescent S mutans (UA159:JM10) biofilms were grown on ultraviolet-sterilized specimens for 1, 3, 5, and 7 days (n = 18 biofilms/d/product) in anaerobic conditions at 37°C. The metabolism of biofilms (relative luminescence unit [RLU]) was measured 0, 2, 4, and 6 minutes after exposure to D-luciferin solution using a microplate reader. A linear mixed-effects model was used to analyze the logarithm of RLU (log RLU). The model included fixed effects of products, days, and minutes. Tukey-Kramer post-hoc tests were then performed on the significant predictors of log RLU (α = 0.05).

RESULTS

Days (P <0.0001) and minutes (P <0.0001) were independent predictors of log RLU, but the products were not (P = 0.5869). After adjusting for minutes, the log RLU was analyzed with a post-hoc test, and all differences between days were significant with the exceptions of day 3 from day 5 (P = 0.0731) and day 5 from day 7 (P = 0.8802). After adjusting for day, log RLU was analyzed with a post-hoc test and all differences in minutes were significant.

CONCLUSIONS

No significant differences in the metabolism of S mutans biofilms were observed among the 6 orthodontic adhesives. Biofilms that were grown for 3 days demonstrated the highest levels of biofilm metabolism as evidenced by higher mean log RLU values relative to 1, 5, and 7-day growth durations.

Sorption, solubility and cytotoxicity of novel antibacterial nanofilled dental adhesive resins

Dental adhesives hydrolyze in the mouth. This study investigated the water sorption (SOR), solubility (SOL) and cytotoxicity (CYTO) of experimental adhesives containing nitrogen-doped titanium dioxide nanoparticles (N_TiO₂). Specimens (n = 15/group [SOR, SOL]; n = 10/group [CYTO]) of unaltered Clearfil SE Protect (CSP), OptiBond Solo Plus (OSP), Adper Scotchbond (ASB) and experimental adhesives (OSP + 25% or 30% of N_TiO₂) were fabricated, desiccated (37 °C) and tested for SOR and SOL according to ISO Specification 4049 (2009). CYTO specimens were UV-sterilized (8 J/cm²) and monomer extracted in growth medium (1, 3 or 7 days). Human pulp cells were isolated and seeded (0.5 × 10⁴) for MTT assay. SOR and SOL data was analyzed using GLM and SNK (α = 0.05) and CYTO data was analyzed with Kruskal–Wallis and SNK tests (α = 0.05). SOR and SOL values ranged from 25.80 μg/mm³ (30% N_TiO₂) to 28.01 μg/mm³ (OSP) and 23.88 μg/mm³ (30% N_TiO₂) to 25.39 μg/mm³ (25% N_TiO₂). CYTO results indicated that pulp cells exposed to experimental materials displayed comparable viabilities (p > 0.05) to those of OSP. Experimental materials displayed comparable SOR, SOL and CYTO values (p > 0.05) when compared to unaltered materials. N_TiO₂ incorporation have not adversely impacted SOR, SOL and CYTO properties of unaltered adhesives.

Pairing orthodontic adhesive resins and light-curing units for optimal degree of conversion

OBJECTIVES

The purpose of this study was to identify the combination of orthodontic adhesive resins and light-emitting diode (LED) light-curing units (LCUs) that result in the highest degree of resin conversion (DC). The hypothesis tested was that pairing orthodontic resins with LCUs from the same manufacturer produces higher DC versus unpaired resins and LCUs.

METHODS

Pre-polymerization spectra of adhesive resins (Transbond XT [3M Unitek], BracePaste [American Orthodontics] or Opal Bond MV [Opal by Ultradent]) were obtained at oral temperature (37°C) using an attenuated total reflectance (ATR) diamond crystal (Golden Gate, Specac) in a Fourier-transform infrared (FTIR) spectrometer (Nicolet IS50). Resin specimens were polymerized using various LCUs (Ortholux Luminous [3M Unitek], Blue Ray 3 [American Orthodontics], or VALO Ortho Cordless [Ultradent Products, Inc.]) before obtaining post-polymerization infrared (IR); spectra. Twelve LCU-resin combinations were tested (n = 20/combination), half with a bracket present (Mini Diamond Twin, Ormco) and half without. DC values (%) were calculated using the two-frequency method and tangent-baseline technique. Data for each resin were statistically analyzed using General Linear Models and Student-Newman-Keuls post hoc tests (α = 0.05).

RESULTS

Statistically significant differences were found within each resin for the groups without brackets present (P < 0.0001), the groups with brackets present (P < 0.0001), and the groups with brackets when compared with the manufacturer-paired group without a bracket (P < 0.0001).

CONCLUSIONS

Adhesive resins and LCUs from the same manufacturer did not consistently result in statistically higher mean DC values than unpaired combinations. Metal brackets do not uniformly reduce the degree of conversion of adhesive resins when assessed using IR spectrometry.

Antimicrobial, biocompatibility, and physicochemical properties of novel adhesive methacrylate dental monomers

For the advancement of Class V restoratives, our goal was to evaluate the physicochemical and mechanical properties, antimicrobial functionality, and cytotoxic potential of novel antimicrobial copolymers. 5-Carboxy-N-(2-(methacryloyloxy)ethyl)-N,N-dimethylpentan-1-aminium bromide (AMadh1) and 10-carboxy-N-(2-(methacryloyloxy)ethyl)-N,N-dimethyldecan-1-aminium bromide (AMadh2) were incorporated into light-curable urethane dimethacrylate, polyethylene glycol–extended urethane dimethacrylate, ethyl 2-(hydroxymethyl) acrylate resin (UPE resin). In the AMadhs-UPE resin, the hydrophobic/hydrophilic balance, degree of vinyl conversion, flexural strength, elastic modulus, and shear bond strength were assessed. Antimicrobial properties were measured using Streptococcus mutans (planktonic and biofilm). Cytotoxicity was tested using human gingival fibroblasts and mouse connective tissue fibroblasts (ATCC® CCL-1™) exposed to two-fold serial dilutions (≤10.6 mmol/L AMadh1 or ≤8.8 mmol/L AMadh2). At 10% mass of AMadh, the attained degree of vinyl conversion values (AMadh1 = 90.1% and AMadh2 = 88.5%) were not statistically different from the UPE resin (88.1%). At both AMadh levels, the flexural strength was reduced in a dose-dependent manner. Elastic modulus and contact angle were not significantly affected by AMadh1. Variations in elastic modulus and contact angle were observed with AMadh2; however, this does not disqualify it in future design of Class V restoratives. Compared to UPE resin, AMadh1-UPE and AMadh2-UPE (10% mass) copolymers reduced S. mutans biofilm 4.2- and 1.6-fold, respectively (p ≤ 0.006). In direct contact with human gingival fibroblasts or ATCC CCL-1 cells, at biologically relevant concentrations, the AMadhs did not adversely affect cell viability or their metabolic activity. This effort addresses a significant oral health issue associated with elderly populations. Its successful completion is expected to yield dental restoratives with well-controlled biofunction.

Physicochemical, Mechanical, and Antimicrobial Properties of Novel Dental Polymers Containing Quaternary Ammonium and Trimethoxysilyl Functionalities

The aims of this study were to evaluate the physicochemical and mechanical properties, antimicrobial (AM) functionality, and cytotoxic potential of novel dental polymers containing quaternary ammonium and trimethoxysilyl functionalities (e.g., N-(2-(methacryloyloxy)ethyl)-N,N-dimethyl-3-(trimethoxysilyl)propan-1-aminium iodide (AMsil1) and N-(2-(methacryloyloxy)ethyl)-N,N-dimethyl-11-(trimethoxysilyl)undecan-1-aminium bromide (AMsil2)). AMsil1 or AMsil2 were incorporated into light-cured (camphorquinone + ethyl-4-N,N-dimethylamino benzoate) urethane dimethacrylate (UDMA)/polyethylene glycol-extended UDMA/ethyl 2-(hydroxymethyl)acrylate (EHMA) resins (hereafter, UPE resin) at 10 or 20 mass %. Cytotoxic potential was assessed by measuring viability and metabolic activity of immortalized mouse connective tissue and human gingival fibroblasts in direct contact with monomers. AMsil-UPE resins were evaluated for wettability by contact angle measurements and degree of vinyl conversion (DVC) by near infra-red spectroscopy analyses. Mechanical property evaluations entailed flexural strength (FS) and elastic modulus (E) testing of copolymer specimens. The AM properties were assessed using Streptococcus mutans (planktonic and biofilm forms) and Porphyromonas gingivalis biofilm. Neither AMsil exhibited significant toxicity in direct contact with cells at biologically relevant concentrations. Addition of AMsils made the UPE resin more hydrophilic. DVC values for the AMsil-UPE copolymers were 2%-31% lower than that attained in the UPE resin control. The mechanical properties (FS and E) of AMsil-UPE specimens were reduced (11%-57%) compared to the control. Compared to UPE resin, AMsil1-UPE and AMsil2-UPE (10% mass) copolymers reduced S. mutans biofilm 4.7- and 1.7-fold, respectively (p ≤ 0.005). Although not statistically different, P. gingivalis biofilm biomass on AMsil1-UPE and AM AMsil2-UPE copolymer disks were lower (71% and 85%, respectively) than that observed with a commercial AM dental material. In conclusion, the AM function of new monomers is not inundated by their toxicity towards cells. Despite the reduction in mechanical properties of the AMsil-UPE copolymers, AMsil2 is a good candidate for incorporation into multifunctional composites due to the favorable overall hydrophilicity of the resins and the satisfactory DVC values attained upon light polymerization of AMsil-containing UDMA/PEG-U/EHMA copolymers.

Flexural properties of rectangular nickel-titanium orthodontic wires when used as ribbon archwires

OBJECTIVE

To compare the flexural properties of rectangular nickel-titanium (Ni-Ti) orthodontic wires in occlusoapical and faciolingual orientations using a standardized test method.

MATERIALS AND METHODS

Twenty-two rectangular Ni-Ti wire groups were tested in occlusoapical (ribbon) orientation: eight conventional Ni-Ti products, five superelastic Ni-Ti products, and nine thermal Ni-Ti products (n = 10 per group). Six products of thermal Ni-Ti wire were tested in faciolingual (edgewise) orientation. A three-point bending test was performed to measure deactivation force at 3.0-, 2.0-, 1.0-, and 0.5-mm deflections of each rectangular wire at 37.0 ± 0.5°C. Analysis of variance and post hoc Student-Newman-Keuls tests were used to compare the mean values of the different groups (α = .05).

RESULTS

The ranges of deactivation forces varied greatly with different kinds, sizes, products, and deflections of Ni-Ti wires. One product of conventional and superelastic Ni-Ti wires had steeper force-deflection curves. Four products had similarly shaped flat force-deflection curves, whereas the sixth product had a moderately steep force-deflection curve. Thermal Ni-Ti wires had smaller deactivation forces ranging from 0.773 N (78.8 g) to 2.475 N (252.4 g) between deflections of 1.0 and 0.5 mm, whereas wider ranges of force from 3.371 N (343.7 g) to 9.343 N (952.7 g) were predominantly found among conventional Ni-Ti wires between deflections of 3.0 and 2.0 mm.

CONCLUSIONS

Clinicians should critically select archwires for use in the occlusoapical orientation not only based on Ni-Ti wire type, size (0.022 × 0.016-in or 0.025 × 0.017-in), and product but also with deactivation deflections from 0.5 and 1.0 mm to obtain light forces in the occlusoapical orientation.

Concurrent quantification of cellular and extracellular components of biofilms

Confocal laser scanning microscopy (CLSM) is a powerful tool for investigation of biofilms. Very few investigations have successfully quantified concurrent distribution of more than two components within biofilms because: 1) selection of fluorescent dyes having minimal spectral overlap is complicated, and 2) quantification of multiple fluorochromes poses a multifactorial problem.

OBJECTIVES

Report a methodology to quantify and compare concurrent 3-dimensional distributions of three cellular/extracellular components of biofilms grown on relevant substrates.

METHODS

The method consists of distinct, interconnected steps involving biofilm growth, staining, CLSM imaging, biofilm structural analysis and visualization, and statistical analysis of structural parameters. Biofilms of Streptococcus mutans (strain UA159) were grown for 48 hr on sterile specimens of Point 4 and TPH(3) resin composites. Specimens were subsequently immersed for 60 sec in either Biotène PBF (BIO) or Listerine Total Care (LTO) mouthwashes, or water (control group; n=5/group). Biofilms were stained with fluorochromes for extracellular polymeric substances, proteins and nucleic acids before imaging with CLSM. Biofilm structural parameters calculated using ISA3D image analysis software were biovolume and mean biofilm thickness. Mixed models statistical analyses compared structural parameters between mouthwash and control groups (SAS software; α=0.05). Volocity software permitted visualization of 3D distributions of overlaid biofilm components (fluorochromes).

RESULTS

Mouthwash BIO produced biofilm structures that differed significantly from the control (p<0.05) on both resin composites, whereas LTO did not produce differences (p>0.05) on either product.

CONCLUSIONS

This methodology efficiently and successfully quantified and compared concurrent 3D distributions of three major components within S. mutans biofilms on relevant substrates, thus overcoming two challenges to simultaneous assessment of biofilm components. This method can also be used to determine the efficacy of antibacterial/antifouling agents against multiple biofilm components, as shown using mouthwashes. Furthermore, this method has broad application because it facilitates comparison of 3D structures/architecture of biofilms in a variety of disciplines.

Dental education in a flat world: advocating for increased global collaboration and standardization

Globalization is a broad term referring to the increasing connectivity, integration, and interdependence of economies, societies, technologies, cultures, and political and ecological spheres across the world. This position paper was developed by a working group of the 2007 American Dental Education Association (ADEA) Leadership Institute. The authors explore the effect that globalization has had on dentistry and dental education to date and hypothesize what dental education could look like in the years ahead. While the paper is written from a North American perspective, some of the authors bring international expertise and experience to the topic of global dental education in a flat world. Specific issues and barriers addressed in this position paper include variations in accreditation and licensure requirements in dental education throughout the world; the historical development of dental education models (odontology and stomatology) and the need for congruency of these models in the global environment; the competency-based model of education and its relevance to development and implementation of global dental competencies; and the slow adoption of technological advances in dental education for promoting collaborations and encouraging resource sharing among countries. These challenges are discussed as they affect the implementation of a standardized global dental education that can lead to improved access to oral health care services and better oral and overall health for the citizens of the world.

Antibacterial activity of chitosan-based matrices on oral pathogens

Chitosan is a well sought-after polysaccharide in biomedical applications due to its biocompatibility, biodegradability to non-toxic substances, and ease of fabrication into various configurations. However, alterations in the anti-bacterial properties of chitosan in various forms is not completely understood. The objective of this study was to evaluate the anti-bacterial properties of chitosan matrices in different configurations against two pathogens-Gram-positive Streptococcus mutans and Gram-negative Actinobacillus actinomycetemcomitans. Two-dimensional (2-D) membranes and three-dimensional (3-D) porous scaffolds were synthesized by air drying and controlled-rate freeze drying. Matrices were suspended in bacterial broths with or without lysozyme (enzyme that degrades chitosan). Influences of pore size, blending with Polycaprolactone (PCL, a synthetic polymer), and neutralization process on bacterial proliferation were studied. Transient changes in optical density of the broth, adhesion characteristics, viability, and contact-dependent bacterial activity were assessed. 3-D porous scaffolds were more effective in reducing the proliferation of S. mutans in suspension than 2-D membranes. However, no significant differences were observed on the proliferation of A. actinomycetemcomitans. Presence of lysozyme significantly increased the antibacterial activity of chitosan against A. actinomycetemcomitans. Pore size did not affect the proliferation kinetics of either species, with or without lysozyme. NaOH neutralization of chitosan increased bacterial adhesion whereas ethanol neutralization inhibited adhesion without lowering proliferation. Mat culture tests indicated that chitosan does not allow proliferation on its surface and it loses antibacterial activity upon blending with PCL. Results suggest that the chemical and structural characteristics of chitosan-based matrices can be manipulated to influence the interaction of different bacterial species.

Microleakage in the proximal walls of direct and indirect posterior resin slot restorations

This study compared the degree of microleakage in the proximal walls of direct and indirect resin slot restorations in relation to the types of dentin bonding systems and the location of gingival margins. Two Class II slot preparations were prepared and restored in each of 60 extracted human molars using direct (Filtek Supreme) and indirect (Tescera ATL) restorative resin materials. Various types of dentin bonding systems, including self-etching (OneStep Plus/Tyrian SPE, iBond, Xeno III) and etch and rinse systems (All-Bond 2, Prime & Bond NT) were used to restore the prepared teeth. The gingival proximal wall was placed apical to the cementoenamel junction (CEJ) in 1 proximal box and coronal to the CEJ in the other. The specimens were stained and evaluated for microleakage using a digital imaging and analysis system. Significant differences were found in the degree of microleakage observed in the various restorative groups. In general, the group restored with indirect resin had less microleakage than the direct resin groups. Factors, such as type of dentin bonding system and location of gingival margins, exert a substantial influence on the degree of microleakage that occurred along the walls of proximal resin restorations.

Deactivation force-deflection behavior of multistranded stainless steel wires

This investigation measured the deactivation (unloading) force-deflection behavior of selected multistranded stainless steel orthodontic wires. The guidelines from the revised American National Standards Institute/American Dental Association Specification No. 32 for orthodontic wires (type 2, nonlinear elasticity) were used to perform a 3-point bending test to obtain data for the deactivation force levels at 3, 2, 1, and 0.5 mm, and permanent deformation values. The study tested 20 wire designs: triple-stranded twisted wires, 5- and 6-stranded coaxial wires, and 8- and 9-stranded braided rectangular wires of selected cross-sections. Twenty specimens were tested for each wire design. The specification guidelines for testing provided a means for obtaining comparative unloading force delivery data for the deflections tested. Multistranded wires of significantly different cross-section, weave, and strand number fell within relatively narrow ranges of force delivery at the 0.5-mm (1.7-42.7 grams-force [gmf]) and 1-mm (31.1-134.2 gmf) deflections and wider ranges of force levels at the 2-mm (57.0-320.5 gmf) and 3-mm (72.5-451.8 gmf) deflections. All wires exhibited some permanent deformation, but none averaged greater than 0.66 mm. The testing also showed that multistranded wires of significantly different design exhibit similar force levels. The guidelines of the revised Specification No. 32 yielded deactivation data with regard to force delivery and provided comparisons of the wire designs tested.

A radiographic recall evaluation of 894 endodontic cases treated in a dental school setting

A radiographic recall evaluation of 894 endodontic cases treated by dental students at the University of Oklahoma College of Dentistry was conducted by faculty. The purpose of this study was to determine the success of standardized nonsurgical endodontic treatments by using radiographic techniques of determination and to compare the results to similar studies at other dental schools. Cases were labeled as successful, acceptable, questionable, and failing. Observations included time since treatment, age, gender, and tooth type. The combined percentage of successful and acceptable cases was 91.05%. Differences in percentage of success among recall time intervals were statistically significant (p < 0.01), as were the percentages of success between tooth types treated (p < 0.05). Success rate was not affected by age or gender (p > 0.05). The overall rate of successful and acceptable cases in this study was found to compare favorably with those from recall studies conducted at other dental schools.

Effect of cyclical lateral forces on microleakage in cervical resin composite restorations

OBJECTIVE

The aim of this study was to assess the effect on microleakage of cyclic lateral fatigue forces in different types of cervical resin composite restorations.

METHOD AND MATERIALS

V-notch cavities were prepared at the cementoenamel junction in 60 extracted human premolars and restored with a bonding system along with one of the following resin composites: hybrid resin composite, microfilled resin composite, and flowable resin composite (20 each). Ten specimens from each group of 20 were subjected to 8,400 cycles of lateral fatigue forces (44 N) at the occlusal half of the clinical crown in a machine specifically developed for this purpose. The other 10 specimens of each group served as controls, representing the microleakage that occurred without lateral fatigue forces. All specimens were stained with a 50% silver nitrate solution and sectioned sagittally. A scoring system of 0 to 6 was used to record the amount of die penetration along the restoration-tooth interface.

RESULTS

There was leakage in both the fatigued specimens and the nonfatigued control specimens. There were significant differences in the mean values of microleakage for the restorative materials studied. In the nonfatigued groups, hybrid resin specimens exhibited significantly more microleakage at the dentin-resin interface than did microfilled or flowable resin specimens. In the fatigued groups, hybrid resin specimens exhibited significantly more leakage at the dentin-resin interface than did microfilled resin specimens.

CONCLUSION

The least elastic resin composite tested, the hybrid material, generally showed the most leakage. This suggests that the elasticity of these resin composite restorative materials may be a factor in the amount of microleakage in cervical restorations.

Fluoride concentrations in bottled water

Bottled water use and lack of adequate labeling have given rise to questions regarding fluoride intake. This study examined fluoride concentration of commercially available bottled water products, determined if significant differences existed among the products, and explored packaging date as a variable on the concentration of fluoride. The values were compared to the ADA guideline for optimally fluoridated water and the American Academy of Pediatric Dentistry recommended schedule for fluoride supplementation. Five commercial brands were chosen for the study. Three bottles of each brand, each with different batch dates, were purchased. Following calibration, six tests were conducted on each bottle using the Hach DR-100 Colorimeter System for fluoride analysis and the values were recorded. Data were analyzed using a two-tailed ANOVA and Tukey's HSD Test (p = 0.05). The results showed that significant differences in fluoride concentrations existed among the five brands. For three brands, significant differences existed between the batches. All products had fluoride concentrations that fell below the ADA accepted standards for optimally fluoridated water. Due to the significant variability in fluoride concentration between products and batches, dental providers should advise their patients that bottled water might not contain optimal levels of fluoride.

The effect of saliva on shear bond strengths of hydrophilic bonding systems

Failure of orthodontic bonded attachments and brackets is mostly attributed to contamination of the enamel surface. To overcome this problem, materials have been developed that purportedly overcome the moisture and contaminants present in the oral environment. This study compared the shear bond strengths of 2 lightcured hydrophilic bonding systems, Transbond XT with MIP (3M/Unitek, Monrovia, Calif) and Assure (Reliance Orthodontics, Itasca, Ill) with a hydrophobic bonding system, Transbond XT with XT primer (3M/Unitek). Comparison tests were conducted under 4 enamel surface conditions: (1) etched and dried; (2) etched and moistened with artificial saliva; (3) etched, primed, and moistened with artificial saliva; and (4) etched, primed, moistened with artificial saliva, and reprimed. In addition, an adhesive remnant index score was used to determine the amount of adhesive remaining on the tooth. Stainless steel brackets with mesh-backed pads (n = 144) were bonded to bovine teeth. Bond strength was then tested in shear using an Instron mechanical testing instrument. There were significant differences in the bond strengths among the products (P <.05), within surface treatments (P <.05), and among the different bonding materials in combination with various surface treatments (P <.05). Treatments 1 and 4 showed the highest mean bond strengths adhesive remnant index scores, whereas treatments 2 and 3 showed the lowest mean bond strengths and scores.

Effect of toothbrushing on elemental release from dental casting alloys

PURPOSE

This study assessed the effect of toothbrushing on the elemental release from common dental casting alloys.

MATERIALS AND METHODS

A Au-Pt high-noble alloy, a Au-Pd high-noble alloy, a Pd-Cu-Ga noble alloy, and a Ni-Cr base-metal alloy were placed into a biological medium for 1 week to allow equilibration of elemental release, then brushed for 30 minutes at 90 strokes/min under 200 g of force with a soft toothbrush. The brushing was performed in neutral saline or lactic acid at pH 4 with or without toothpaste. The alloys were then transferred back into a biological medium for 1 week. Atomic absorption spectroscopy was used to measure the mass lost from each alloy during and in the week after brushing. Mass loss was expressed as micrograms per square centimeter of alloy surface, and the various brushing conditions were compared by analysis of variance and Tukey multiple comparison intervals (alpha = 0.05).

RESULTS

During brushing, element release increased significantly over nonbrushed controls for all alloys except the Ni-Cr. Toothpastes, acidic solutions, or combinations of these conditions with toothbrushing further enhanced elemental release, but Ni release from the Ni-Cr alloy increased the most (30-fold). In the week after brushing, brushing alone increased elemental release only from the Ni-Cr alloy. However, toothpastes and acidic solutions increased elemental release for all alloys significantly, except for the Au-Pd alloy. In general, elemental release during brushing was far greater than in the week after brushing.

CONCLUSIONS

Under the conditions of this study, toothbrushing increased the elemental release from dental casting alloys.

Elimination, via high-rate laser dilatometry, of structural relaxation during thermal expansion measurement of dental porcelains

OBJECTIVES

Thermal expansion measurement of glassy materials is complicated by thermal history effects. Excess volume--trapped in quenched dental porcelains after firing--collapses via structural relaxation on first slow heating during conventional dilatometry, making the thermal expansion coefficient (alpha) obtained on first heating unreliable. The purpose of this study was to determine whether porcelain thermal expansion measurement at high thermal rates could minimize the influence of thermal history.

METHODS

Eight thermal expansion specimens each of six body porcelains and the Component No. 1 (leucite-containing) frit prepared according to the patent by Weinstein et al. (US Patent No. 3,052,982) were subjected to three heat-cool conventional dilatometry runs at 3 degrees C/min, while eight thermal expansion specimens of each porcelain were reserved as untreated controls. Eight hollow, cylindrical specimens of the same brands were subjected to three heat-cool laser dilatometer thermal expansion runs at 600 degrees C/min, while eight cylindrical specimens of each porcelain were reserved as untreated controls. Thermal expansion data (25-500 degrees C) of all specimens were subjected to repeated measures analysis of variance.

RESULTS

The alpha obtained on first slow heating was significantly lower than values for succeeding slow heat and cool runs in all porcelains (P < 0.001). High-rate alpha obtained on first heating was not significantly different from values of succeeding heat and cool runs in all porcelains (P > 0.05).

SIGNIFICANCE

Conventional dilatometer measurements demonstrated occurrence of structural relaxation, as evidenced by the significant difference in the first heating and subsequent runs. High-rate laser dilatometry eliminated structural relaxation, thereby providing a thermal expansion measurement that is free of interference from thermal history effects.

Effect of pH on element release from dental casting alloys

STATEMENT OF PROBLEM

Dental casting alloys are subjected to transient acidity in the oral environment, yet most studies have not investigated the effects of these transient environments on elemental release from alloys. Elemental release is important because it plays a significant role in alloy biocompatibility.

PURPOSE

It was hypothesized that acidic environments would increase elemental release from dental alloys during exposure and after the acidic environment was removed. This hypothesis was based on the known increase in release of nickel from nickel-based alloys in an acidic environment.

MATERIAL AND METHODS

High-noble, noble, and base metal casting alloys were exposed for 30 minutes to solutions with pH ranging from 1 to 7. Elemental release of representative elements was measured by means of atomic absorption spectrometry during the exposure and in the week after the exposure. This release was compared with elemental release in the week before the exposure.

RESULTS

High-noble and noble alloys were resistant to acidic environments. A pH of 4 did not increase elemental release during or after exposure. A pH 1 environment slightly elevated release of Ag, Cu, and Pd in some alloys. However, a Ni-based alloy released large amounts of Ni during the acidic exposure of pH 1 or 4, and more importantly, in the week after the exposure as well. Increased time of exposure to acid did not alter elemental release from noble or high-noble alloys, but markedly increased release from the Ni-based alloy.

CONCLUSIONS

Transient exposure of casting alloys to an acidic oral environment is likely to significantly increase elemental release from Ni-based alloys, but not from high-noble or noble alloys.

Fracture toughness and load relaxation of dentin bonding resin systems

OBJECTIVES

The fracture toughness (KIC) and load relaxation of four dentin bonding resins were determined to characterize some of the mechanical properties of these materials after polymerization.

METHODS

A total of 40 single-edge notch bar specimens were fabricated, 10 each of four commercially available brands, and subjected to three-point bending until fracture, as described in ASTM Standard E399-83 (1991a). The critical stress intensification factor, KIC, was derived for each specimen and compared by analysis of variance and Scheffé's multiple comparisons test (p < 0.01). To study the load relaxation characteristics, five rectangular specimens (without notches) of each brand were subjected to three-point loading until a predetermined limiting load value was reached. The test load was allowed to relax for 4 min, after which the specimen was unloaded to the zero load condition, and the load was allowed to build up on its own accord for 3 min. Load relaxation values were measured from the chart, and the mean percent load drop was calculated. The load relaxation data were compared using analysis of variance and Scheffé's multiple comparisons test (p < 0.05).

RESULTS

The fracture toughness (KIC) values of the four adhesive resins studied in this investigation ranged from 0.37-0.94 MPa.m0.5 and were statistically different from each other (p < 0.001). The load relaxation values were found to be greatest within the first 0.5 min, with the total load relaxation of the four bonding agents ranging from 16%-30%. Two of the materials studied showed significantly different short-term load relaxation behavior than the other two resins (p < 0.05).

SIGNIFICANCE

Bonding agents can be implicated as one of the factors that weaken the interface between the dentin and the composite restorative material. These materials are capable of a rapid short-term response, demonstrating significant load relaxation in the first 0.5 min after loading.

Potential interference of leucite crystallization during porcelain thermal expansion measurement

OBJECTIVES

The elevated temperatures and slow heating and cooling rates associated with a thermal expansion run provide a thermal treatment comparable to those previously shown to produce leucite crystallization in dental PFM porcelains. The purpose of this study was to determine whether the exposure of dental porcelains to the time-temperature profile of a thermal expansion run could result in leucite crystallization, a potential interference with the accurate determination of the thermal expansion coefficient.

METHODS

A total of 112 thermal expansion specimens were prepared, 16 each of six commercially available body porcelains, as well as the "Component No. 1" (leucite-containing) frit of the Weinstein et al. (1962) patent. In a randomized design, the leucite contents of specimens subjected to a 3 degrees C/min thermal expansion run were compared to those of untreated controls. The leucite content of each specimen was determined via quantitative X-ray powder diffraction using an Al2O3 internal standard. One-tailed t-tests were applied to determine whether the dilatometer thermal treatment produced a statistically significant increase in leucite content.

RESULTS

The heat treatment afforded by a 3 degrees C/min dilatometer run was not sufficient to induce measurable leucite crystallization in any of the porcelains examined in this study.

SIGNIFICANCE

The crystallization of leucite during a dilatometer heating run can be ruled out as a possible interference with the determination of the coefficient of thermal expansion of dental porcelain.