Dental composite biodeterioration in the presence of oral Streptococci and extracellular metabolic products.

OBJECTIVE: Secondary caries is a primary cause of early restoration failure. While primary dental caries has been extensively researched, our knowledge about the impact of secondary caries on dental restorations is relatively limited. In this study, we examined how different clinically relevant microbially-influenced environments impact the degradation of nano-filled (FIL) and micro-hybrid (AEL) dental composites. METHODS: Material strength of two commercial dental composites was measured following incubation in aqueous media containing: i) cariogenic (Streptococcus mutans) and non-cariogenic bacteria (Streptococcus sanguinis) grown on sucrose or glucose, ii) abiotic mixtures of artificial saliva and sucrose and glucose fermentation products (volatile fatty acids and ethanol) in proportions known to be produced by these microorganisms, and iii) abiotic mixtures of artificial saliva and esterase, a common oral extracellular enzyme. RESULTS: Nano-filled FIL composite strength decreased in all three types of incubations, while micro-hybrid AEL composite strength only decreased significantly in biotic incubations. The strength of both composites was statistically significantly decreased in all biotic incubations containing both cariogenic and non-cariogenic bacteria beyond that induced by either abiotic mixtures of fermentation products or esterase alone. Finally, there were no statistically significant differences in composite strength decrease among the tested biotic conditions. CONCLUSIONS: The results show that conditions created during the growth of both cariogenic and non-cariogenic oral Streptococci substantially reduce commercial composite strength, and this effect warrants further study to identify the mechanism(s). CLINICAL SIGNIFICANCE: Dental biofilms of oral Streptococci bacteria significantly affect the mechanical strength of dental restorations.

Relationships between composite roughness and Streptococcus mutans biofilm depth under shear in vitro.

OBJECTIVE: To investigate the effect of substrate, surface roughness, and hydraulic residence time (HRT) on Streptococcus mutans biofilms growing on dental composites under conditions relevant to the oral cavity. METHODS: Dental composites were prepared with varying amounts of polishing and incubated in a CDC bioreactor with an approximate shear of 0.4 Pa. S. mutans biofilms developed in the bioreactors fed sucrose or glucose and at 10-h or 40-h HRT for one week. Biofilms were characterized by confocal laser microscopy (CLM). Composite surface roughness was characterized by optical profilometry, and pre- and post-incubation composite surface fine structure and elemental composition were determined using scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS). RESULTS: Polishing had a significant impact on surface roughness, varying by a factor of 15 between the polished samples and the unpolished control. S. mutans biofilms grew statistically significantly thicker on the unpolished composites. Biofilm thickness was greater at shorter 10-h HRT compared to 40-h HRT. In most cases, biofilm thickness was not statistically significantly greater in sucrose-fed bioreactors than in glucose-fed bioreactors. SEM-EDS analysis did not identify any significant change in elemental composition after aging. CONCLUSIONS: Accurate characterization of oral cavity biofilms must consider shear forces and the use of techniques that minimize alteration of the biofilm structure. Under shear, surface smoothness is the most important factor determining S. mutans biofilm thickness followed by HRT, while sucrose presence did not result in significantly greater biofilm thickness. CLINICAL SIGNIFICANCE: The obvious patterns of S. mutans growth along sub-micron scale grooving created by the polishing process suggested that initial biofilm attachment occurred in the shear-protected grooves. These results suggest that fine polishing may help prevent the initial formation of S. mutans biofilms compared to unpolished/coarse polished composites.

Stoichiometric models of sucrose and glucose fermentation by oral streptococci: Implications for free acid formation and enamel demineralization.

OBJECTIVES: The objective of this study is to develop stoichiometric models of sugar fermentation and cell biosynthesis for model cariogenic Streptococcus mutans and non-cariogenic Streptococcus sanguinis to better understand and predict metabolic product formation. METHODS: Streptococcus mutans (strain UA159) and Streptococcus sanguinis (strain DSS-10) were grown separately in bioreactors fed brain heart infusion broth supplemented with either sucrose or glucose at 37 °C. Cell mass concentration and fermentation products were measured at different hydraulic residence times (HRT) to determine cell growth yield. RESULTS: Sucrose growth yields were 0.080 ± 0.0078 g cell/g and 0.18 ± 0.031 g cell/g for S. sanguinis and S. mutans, respectively. For glucose, this reversed, with S. sanguinis having a yield of 0.10 ± 0.0080 g cell/g and S. mutans 0.053 ± 0.0064 g cell/g. Stoichiometric equations to predict free acid concentrations were developed for each test case. Results demonstrate that S. sanguinis produces more free acid at a given pH than S. mutans due to lesser cell yield and production of more acetic acid. Greater amounts of free acid were produced at the shortest HRT of 2.5 hr compared to longer HRTs for both microorganisms and substrates. SIGNIFICANCE: The finding that the non-cariogenic S. sanguinis produces greater amounts of free acids than S. mutans strongly suggests that bacterial physiology and environmental factors affecting substrate/metabolite mass transfer play a much greater role in tooth or enamel/dentin demineralization than acidogenesis. These findings enhance the understanding of fermentation production by oral streptococci and provide useful data for comparing studies under different environmental conditions.

Logistic Regression Analysis of LC-MS/MS Data of Monomers Eluted from Aged Dental Composites: A Supervised Machine-Learning Approach.

Compound identification by database searching that matches experimental with library mass spectra is commonly used in mass spectrometric (MS) data analysis. Vendor software often outputs scores that represent the quality of each spectral match for the identified compounds. However, software-generated identification results can differ drastically depending on the initial search parameters. Machine learning is applied here to provide a statistical evaluation of software-generated compound identification results from experimental tandem MS data. This task was accomplished using the logistic regression algorithm to assign an identification probability value to each identified compound. Logistic regression is usually used for classification, but here it is used to generate identification probabilities without setting a threshold for classification. Liquid chromatography coupled with quadrupole-time-of-flight tandem MS was used to analyze the organic monomers leached from resin-based dental composites in a simulated oral environment. The collected tandem MS data were processed with vendor software, followed by statistical evaluation of these results using logistic regression. The assigned identification probability to each compound provides more confidence in identification beyond solely by database matching. A total of 21 distinct monomers were identified among all samples, including five intact monomers and chemical degradation products of bisphenol A glycidyl methacrylate (BisGMA), oligomers of bisphenol-A ethoxylate methacrylate (BisEMA), triethylene glycol dimethacrylate (TEGDMA), and urethane dimethacrylate (UDMA). The logistic regression model can be used to evaluate any database-matched liquid chromatography-tandem MS result by training a new model using analytical standards of compounds present in a chosen database and then generating identification probabilities for candidates from unknown data using the new model.

The impact of long-term aging in artificial saliva media on resin-based dental composite strength.

Although salivary liquid can degrade constituents in resin-based dental composites in short-term incubations, there is a knowledge gap on how longer-term aging impacts their bulk strength. We address this through extended aging studies with resin-based dental composites in different environments. Two commercial composites (FIL and AEL) were aged aseptically at 37°C in air (A, control), artificial saliva (AS), and esterase enzyme amended AS (EAS). Diametral and pushout strength were measured after periods of 120-180 days. At 120 days, the diametral strength of composites aged in air was 69.9 ± 11.0 and 57.7 ± 3.31 MPa in FIL and AEL, respectively. These were significantly greater compared to composites aged in AS (32.1 ± 7.01 and 46.2 ± 9.38 MPa in FIL and AEL, respectively) or EAS (36.7 ± 8.49 and 43.5 ± 5.51 MPa in FIL and AEL, respectively). In contrast, pushout strength for both composites were smaller in A compared to those aged in AS and EAS, results attributed to AS absorption and polymer expansion. No significant change in either diametral or pushout strength occurred after 120 days. There was no significant difference between aging in AS and EAS, suggesting that esterase did not significantly decrease the bulk material strength to a greater extent than AS under the test conditions. Aqueous diffusivities for the composites ranged from 8.4 to 11 × 10-13  m2 /s, with associated porosities ranging from 0.06% to 0.10%. These results indicate that saturation of a typical dental composite occurs over a time frame of 4-5 months, longer than typical aging studies. Together, the results demonstrate the importance of aging time on composite strength.

The effect of esterase enzyme on aging dental composites.

We measured the push-out and diametral tensile strength of dental restorative composites following aging under environmental conditions relevant to the oral cavity; air (A), artificial saliva (AS), acidified (50 mM CH3 COOH, pH = 4.7) artificial saliva (AS + HAc), and AS with esterase enzyme (AS + ENZ). Cylindrical test specimens (6.3 mm diameter by 5.1 mm long) were prepared by placing 0.3 g of nanofilled composite in an epoxy ring and cured. Twenty samples were aged in each environment for 163-186 days at 37°C. The push-out strengths (mean ± standard error of the mean [SEM], in MPa) for specimens were: A-2.4 ± 0.2, AS-7.3 ± 0.5, AS + HAc-7.2 ± 0.9, and AS + ENZ-6.0 ± 0.6. Following the push-out test, the diametral tensile strength and elasticity were immediately determined. The diametral tensile strengths (mean ± SEM, in MPa) for specimens were: A-54.0 ± 1.6, AS-31.4 ± 1.3, AS + HAc-34.3 ± 1.2, and AS + ENZ-22.5 ± 0.7. The push-out strength was lowest for the A environment due to shrinkage of the composite. The push-out strength increased significantly as water diffused into the specimens (AS and AS + HAc) but decreased significantly in the enzyme environment (AS + ENZ). The diametral tensile strength was highest for specimens in the A environment, which was significantly higher than both the AS and AS + HAc specimens and > 2× higher than the AS + ENZ specimens. The results indicated that a water environment (with or without acid) caused a significant decrease in the mechanical properties of this composite, but the greatest decrease was seen in water with esterase. This is the first study to demonstrate that esterase enzymes affect the bulk strength of a commonly used commercial dental composite. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 2178-2184, 2019.

Factors affecting the internal brain drain of Saudi healthcare professionals.

OBJECTIVE: To investigate factors affecting the internal brain drain of healthcare professionals in the Kingdom of Saudi Arabia. METHODS: A cross-sectional study was conducted using an anonymous self-administered online questionnaire sent to all Saudi students enrolled in healthcare profession programs in North America. The data was collected between January and March 2008 at the University of Illinois at Chicago, Chicago, Illinois, United States of America. Results were analyzed using logistic regression analyses. RESULTS: A total of 377 completed questionnaires were returned. Results revealed that 71% of respondents intended to return to work within the 2 major urban cities Riyadh and Jeddah. Respondents who completed their undergraduate studies in a large city were more likely to work in the same city (odds ratio [OR]=3.2; p=0.000; 95% confidence interval [CI] = 2.0-5.2). Furthermore, 51% of the students were willing to work in a rural area for a 50% or more increase in their salary. Finally, men were more willing to work in a rural area for a financial incentive (OR=2.3; p=0.006, 95% CI = 1.3-4.3). CONCLUSION: This study suggests that realistic financial incentives would probably not suffice to attract Saudi healthcare providers to rural areas. The provision of medical schools in smaller cities and rural areas is predicted to be a more effective method for improving the current maldistribution of healthcare providers.

The effects of environment and cyclic fatigue on the mechanical properties of an indirect composite.

OBJECTIVES: The purpose of this study was to examine the flexure strength (σ) and fracture toughness (K(IC)) of three indirect dental composites (dentin, body, and incisal) with respect to loading (static and cyclic), testing environments (air and water) and ageing (0 (controls) and 6 months in air and water). METHODS: The specimens were 3 mm × 3 mm × 25 mm bars with the fracture toughness specimens having a 0.75 mm notch machined in the midspan. Static testing utilized 15 bars and cyclic testing 25 bars for each testing variable. All bars were tested using three-point loading and the cyclic testing was for 1000 cycles using a staircase approach. RESULTS: For flexure strength and fracture toughness, all specimen groups showed a decrease in mean values when exposed to cyclic loading as compared to the static loading mean and when exposed to ageing compared to the control specimens. ANOVA analysis demonstrated that dentin specimens had higher flexure strength and fracture toughness means than incisal and body specimens and that control and water specimens had higher flexure strength and fracture toughness means than aged and air specimens. CONCLUSIONS: Degradation of these materials, as a result of cyclic loading and ageing in an aqueous environment, appears to be influenced by more than just the processing of the composite.

Aeration prevents methyl mercury production in dental wastewater.

Although research has demonstrated that Hg is methylated in the reducing conditions of the dental clinic wastewater collection system, studies are inconclusive as to whether further methylation occurs in the aeration basin of activated sludge wastewater treatment plant (WWTP) which typically treats this waste. Given the high levels of methyl Hg reported in dental wastewater (DWW), it is important to determine whether additional methylation occurs once it enters the WWTP. To achieve this objective, we incubated DWW under conditions designed to mimic the oxidized conditions of the activated sludge aeration basin in a WWTP. Duplicate bioreactors were charged with raw DWW collected from a 12-chair dental clinic and incubated both with and without aeration. Aeration was continued for 15 days, consistent with the typical mean cell residence time (MCRT) necessary for both heterotrophic carbon oxidation (typically 5-6 days) and nitrification (typically 12-15 days), thus ensuring that incubation time exceeded those for most conceivable MCRTs used in the activated sludge process. Results demonstrate a rapid increase in pH concomitant with an increase in dissolved oxygen (DO) to near saturation in the aerated reactor. The non-aerated reactor remained low or at zero DO due to low surface reaeration coupled with the high levels of organic matter. The rate of mercury methylation increased in the unaearated reactors rapidly upon incubation, reaching highest levels when DO was at the lowest levels during the experiment. In great contrast, methyl mercury levels were much lower and net mercury methylation does not appear to occur at any significant rate under aeration. These results imply that although some mercury methylation may occur in the sewer collection system (or anaerobic digesters), net methylation is unlikely to occur in the aeration basin in activated sludge WWTPs, and thus methyl Hg influent levels from DWW represent an upper bound on effluent levels.

In vitro evaluation of self-etch bonding in orthodontics using cyclic fatigue.

OBJECTIVE: To evaluate the bond strength of a self-etch bonding system using static loading and cyclic fatigue with shear testing. This is a two-part evaluation; the first part will evaluate shear testing, the second part tensile testing. MATERIALS AND METHODS: Bovine teeth (n = 82) were randomly distributed to either a self-etch (Transbond Plus) or total-etch (37% phosphoric-acid + Transbond XT) group. The static shear (SSBS) and cyclic shear (CSBS) bond strengths were measured 24 hours after the bonding of mesh-based brackets. The adhesive remnant index (ARI) and failure sites were evaluated. RESULTS: The mean SSBS was 34.25 ± 9.21 and 24.64 ± 9.42 MPa for the total- and self-etch groups, respectively. CSBS was 24.07 ± 0.65 MPa and 18.92 ± 1.08 MPa for the total- and self-etch groups, respectively. Cyclic loading produced lower bond strengths compared to static testing for both adhesives; the difference was only statistically significant for the total-etch system. Comparison of the two materials showed a statistically significant difference between the two techniques. The total-etch had higher bond strengths than the self-etch bonding system. The samples showed a predominance of ARI scored of 2 and 1, and their bonding failure sites were cohesive within the composite. CONCLUSION: Cyclic loading, simulating occlusal forces, reduces the bond strength of both bonding systems. Even though the self-etch bonding system had lower shear bond strength than the total-etch system, both were still clinically acceptable.

Cyclic loading of notched dental composite specimens.

OBJECTIVE: The purpose of this study was to examine the fracture toughness (K(IC)) of three direct dental composites and one indirect dental composite subject to cyclic loading. METHODS: The composites were a micro-filled (Micronew, Bisco INC., Schaumburg, IL, USA), a hybrid (Renew, Bisco INC.), a nano-filled composite (Filtek Supreme Plus, 3M ESPE, St. Paul, MN, USA) and an indirect dental composite (BelleGlass HP, SDS-Kerr, Orange, CA, USA). Rectangular bar specimens (3 mm x 3 mm x 25 mm) were fabricated, notched, aged (5 months) and cyclic loaded in four different environments, air, water, artificial saliva, and a 50/50 by volume mixture of ethanol and water. Specimens were cyclic loaded for 1, 1000, 10,000, and 100,000 cycles. RESULTS: A 3-way ANOVA (non-aged and aged group, four aging media, four loading cycles) showed a significant difference between non-aged and aged, aging media, and loading cycles. For the control groups as the number of cycles increased, there were no significant differences on the number of cycles completed and fracture toughness, except for Micronew, which showed an increased specimen failure rate and decreased fracture toughness. In the aged groups, cyclic loading in water and artificial saliva did not have a significant effect on BelleGlass HP, Filtek Supreme Plus and Renew for fracture toughness and the number of cycles completed, However for Renew in the 50/50 mixture at 100,000 cycles, Filtek Supreme Plus in air and the 50/50 mixture, and Micronew, there was an increased specimen failure rate and a decreased fracture toughness during cyclic fatigue loading as the number of cycles increased. SIGNIFICANCE: BelleGlass HP displayed the best overall resistance to cyclic loading, followed by Renew and Filtek Supreme Plus, and Micronew.

The use of collagen cross-linking agents to enhance dentin bond strength.

UNLABELLED: Type I collagen is a major component of the hybrid layer, and improvement of its mechanical properties may be advantageous during bonding procedures. OBJECTIVE: To investigate the effect of three different cross-linking agents (Glutaraldehyde [GD], Grape seed extract [GSE], and Genipin [GE]) on the tensile bond strength (TBS) of resin-dentin bonds. MATERIALS AND METHODS: Sixty-four sound human molars were collected and their occlusal surfaces were ground flat to expose dentin. Dentin surfaces were etched using a phosphoric acid and then teeth were randomly divided according to the dentin treatment: Control group (no treatment), 5% GD, 6.5% GSE, or 0.5% GE. Teeth were restored either with One Step Plus or Adper Single Bond Plus adhesive systems and resin composite. After 24 h, teeth were sectioned to produce a cross-sectional surface area of 1.0 mm(2) and tested for tensile bond strength. Data were statistically analyzed using ANOVA and Fisher's PLSD tests (p < 0.05). There was a statistically significant interaction between factors (treatment and adhesive p < 0.001). Treatment affected TBS (p < 0.0001), while no differences were observed between the adhesive systems (p = 0.6961). CONCLUSION: Chemical modification to the dentin matrix promoted by GD and GSE, but not GE, resulted in increased bond strength. The application of selective collagen cross-linkers during adhesive restorative procedures may be a new approach to improve dentin bond strength properties.

Reliability of the adhesive remnant index score system with different magnifications.

OBJECTIVE: To test the hypothesis that adhesive remnant index (ARI) scores show no differences when examined under different magnifications. MATERIALS AND METHODS: The study included 80 upper human premolars. Stainless steel brackets were bonded to the specimens with Transbond XT light cure adhesive and Transbond Plus Self Etch Primer (3M Unitek, Monrovia, Calif, USA). The brackets were debonded 24 hours after bonding with a universal testing machine (LLOYD Instruments, Segensworth, Fareham, England) at a cross head speed of 2.00 mm/min. The adhesive remnant was evaluated after debonding with the naked eye and under 10x and 20x magnification (Mag) using the 4-point scale described by Artun and Bergland. RESULTS: ARI scores were significantly different at different magnifications (P = .018). Scores were not significantly different when evaluated with the naked eye and under 10x Mag (P = .102). Scores were significantly different under 20x Mag and with the naked eye (P = .014); under 20x Mag, score 0 decreased from 12 to 6 and score 2 increased from 14 to 20 and also under 20x Mag and under 10x Mag (P = .046); the under 20x Mag score 1 decreased from 40 to 38 and score 3 increased from 14 to 16. CONCLUSION: ARI scores were significantly different under 20x Mag, where score 0 decreased and score 2 increased compared with the naked eye, while score 1 decreased and score 3 increased compared with 10x Mag.

Fatigue behaviour of dental composite materials.

OBJECTIVE: The intent of the project was to evaluate the fatigue behavior of particle and fiber filled dental composites that are fabricated either directly or indirectly using a notched specimen with respect to the number of cycles until failure. METHODS: The materials were five dental composites, three normally cured in the oral cavity (Restolux, Renew and Filtek Supreme), direct processing, and two laboratory produced (BelleGlass, and Tescera), indirect processing. The specimens were 3mmx3mmx25mm bars with a 0.75 or 1.0mm notch in the mid-span of the bars, polished with 320 grit SiC paper and aged for 6 months in air, distilled water, artificial saliva, and a 50/50 mixture of ethanol and distilled water. Testing was performed with a stress mean range of 5-49MPa, the maximum number of cycles was 100,000, and the number of cycles to failure was recorded. RESULTS: For the specimens that failed, BelleGlass, Restolux, and Tescera were able to withstand a higher cyclic loaded stress than Renew and Filtek. The 50/50 by volume mixture of water and ethanol resulted in the lowest resistance to fatigue for all materials. CONCLUSIONS: Of the dental composite materials investigated, the indirect processed and those with large particle fillers (higher weight percent filler) had better cyclic fatigue resistance than micro- and nano-particle fillers (lower weight percent filler).

Multiaxial analysis of dental composite materials.

Dental composites are subjected to extreme chemical and mechanical conditions in the oral environment, contributing to the degradation and ultimate failure of the material in vivo. The objective of this study is to validate an alternative method of mechanically loading dental composite materials. Confined compression testing more closely represents the complex loading that dental restorations experience in the oral cavity. Dental composites, a nanofilled and a hybrid microfilled, were prepared as cylindrical specimens, light-cured in ring molds of 6061 aluminum, with the ends polished to ensure parallel surfaces. The samples were subjected to confined compression loading to 3, 6, 9, 12, and 15% axial strain. Upon loading, the ring constrains radial expansion of the specimen, generating confinement stresses. A strain gage placed on the outer wall of the aluminum confining ring records hoop strain. Assuming plane stress conditions, the confining stress (sigma(c)) can be calculated at the sample/ring interface. Following mechanical loading, tomographic data was generated using a high-resolution microtomography system developed at beamline 2-BM of the Advanced Photon Source at Argonne National Laboratory. Extraction of the crack and void surfaces present in the material bulk is numerically represented as crack edge/volume (CE/V), and calculated as a fraction of total specimen volume. Initial results indicate that as the strain level increases the CE/V increases. Analysis of the composite specimens under different mechanical loads suggests that microtomography is a useful tool for three-dimensional evaluation of dental composite fracture surfaces.

Characterization of methyl mercury in dental wastewater and correlation with sulfate-reducing bacterial DNA.

Dental wastewater (DWW) was collected over two months from a 12-chair clinic and a single-chair office to identify conditions that may affect Hg methylation. DWW was settled for 24 h and samples were collected from the top and bottom of the supernatant to simulate a range of particles that may escape in-line traps. Total Hg spanned 5 orders of magnitude (0.02-5000 microM), following a log-normal distribution with p10, p50, and p90 concentration values of 0.24, 31 and 4000 microM, respectively; typically well in excess of free aqueous Hg solubility. Methyl Hg was present in high levels (2-270 nM), also following a log-normal distribution with p10, p50, and p90 concentration values of 2.8, 17, and 100 nM, respectively. There were no statistically significant differences (90% CI) in p50 methyl Hg or total Hg between the clinic and office. Methyl Hg was predicted from total Hg data by (+/- 95% CI): Log (Me-Hg) = 0.33 (+/- 0.06) x Log (T-Hg) - 2.27 (+/- 0.13). Total methyl Hg from DWW to U.S. wastewater collection systems is estimated to be 2-5 kg yr(-1). Equilibrium speciation modeling predicted that DWW Hg was primarily in sulfide-Hg complexes, except at high total Hg levels where organo-Hg complexes become significant. DNA extracts amplified by quantitative polymerase chain reaction with primers for total eubacteria and sulfate-reducing bacteria (SRB) indicated that the total eubacterial DNA was composed primarily of SRB, and highly significant correlations were found between methyl Hg and both amplified Desulfobacteraceae (p < 0.0001) and Desulfovibrionacaea DNA (p < 0.00001). Both are known Hg methylators. In marked contrast, there was no significant correlation between methyl Hg and amplified Desulfobulbus DNA, a genus generally not known to methylate Hg at high rates. These results strongly suggest that SRB are implicated in DWW Hg methylation.

Rebonding of orthodontic brackets. Part I, a laboratory and clinical study.

OBJECTIVE: To compare rebonding of orthodontic brackets based on the hypothesis that no difference would be found between the adhesive systems with respect to shear bond strength, mode of failure, and clinical failure rates. MATERIALS AND METHODS: The three adhesive systems included two self-etch primers (Transbond and M-Bond) and a conventional phosphoric acid etch (Rely-a-Bond). The sample size was 20 premolars for each adhesive system. The shear bond strength was tested 24 hours after bracket bonding with the bonding/debonding procedures repeated two times after the first debonding. Bond strength, adhesive remnant index (ARI), and failure sites were evaluated for each debonding. Statistical analysis consisted of a two-way analysis of variance (ANOVA) followed by Scheffè analysis. The clinical portion evaluated 15 patients over a 12-month period. RESULTS: The mean shear bond strengths after the first, second, and third debondings for Rely-a-Bond were 8.4 +/- 1.8, 10.3 +/- 2.4, and 14.1 +/- 3.3 MPa, respectively; for Transbond 11.1 +/- 4.6, 13.6 +/- 4.5, and 12.9 +/- 4.4 MPa, respectively; and for M-Bond 8.7 +/- 2.7, 10.4 +/- 2.4, and 12.4 +/- 3.4 MPa, respectively. After the three debondings the mean shear bond strength increased significantly from the first to the third debonding for Rely-a-Bond and M-bond (P

Rebonding of orthodontic brackets. Part II, an XPS and SEM study.

OBJECTIVE: The hypothesis of this two-part study is that adhesive systems for bonding orthodontic brackets (ie, two self-etch primers [Transbond and M-Bond] and a conventional phosphoric acid etch [Rely-a-Bond]) would show a difference with respect to rebonded enamel surface morphology and chemical composition. MATERIALS AND METHODS: This study examined the enamel surface before and after debonding with scanning electron microscopy and the enamel surface chemical composition for the elements Ca, P, O, F, Si, and C using x-ray photoelectron spectroscopy. RESULTS: The etching of the two self-etch groups is less aggressive and less uniform than that of phosphoric acid. The change in the concentration of C indicated that the separation of the bracket from the enamel surface is at the resin-enamel interface for the phosphoric acid-etched adhesive and a mixed mode involving the enamel-resin-bracket interfaces for the self-etching systems. F release appears to occur for Transbond but not for M-Bond. CONCLUSIONS: The results confirm the original hypothesis that differences in adhesive systems are manifested in less aggressive etches and less adhesive left on the enamel surface for the self-etching adhesive systems.

Bond strength analysis of custom base variables in indirect bonding techniques.

INTRODUCTION: Various methods are used to prepare the cured composite-adhesive interface for orthodontic indirect bonding. The intent of this study was to determine the effect on the shear bond strength of the following variables: use of a filled flowable composite resin as an adhesive, light air-abrasion of the cured composite bracket pad, and wetting the cured composite bracket pad with an unfilled resin. METHODS: The sample of 240 brackets was divided into 2 groups of 120 each. The first group was further divided into 4 groups of 30 each. Brackets were bonded to bovine incisors with a filled flowable composite resin (Filtek, 3M ESPE, St Paul Minn), but the bracket pads were prepared differently in the 4 groups: unfilled resin was applied (Orthosolo, Ormco, Glendora, Calif), the surface was air abraded, the surface was air abraded followed by application of an unfilled resin (Orthosolo), and a control group. A matching sample of 120 brackets was bonded without the flowable composite as an adhesive. The different bracket pad preparations were chosen to represent the various techniques clinicians use in indirect bonding. The shear bond strength was measured on a universal testing machine. RESULTS: Two-way ANOVA analysis showed significant differences in the shear bond strength among the different surface preparations, but not between the use and nonuse of flowable composite. The Scheffé test showed that the mean shear bond strength of the air-abraded surface was significantly higher than all other surface preparations. CONCLUSIONS: Air abrading orthodontic bracket-pad composite surfaces in indirect bonding increased the shear bond strength, whereas the use of flowable composite did not affect bond strengths.

Changes in stiffness of demineralized dentin following application of collagen crosslinkers.

It is thought that increasing the strength of the dentin matrix using crosslinking agents may improve both the strength and the durability of resin-dentin bonds. The purpose of this study was to evaluate the effect of two collagen crosslinking agents (glutaraldehyde, GD and grape seed extract, GSE) on the modulus of elasticity of demineralized dentin. Sound molar fragments were fully demineralized and divided into five groups according to the type and concentration of crosslinking agents: 2.5% GD; 5% GD, 25% GD; 0.65% GSE; 6.5% GSE. Specimens were immersed in their respective solution and tested at baseline, 10 min, 30 min, 1 h, 2 h, 4 h. The elastic modulus of dentin was significantly affected by the treatment (p < 0.01) and exposure time (p < 0.01). There was a statistically significant interaction between the two factors evaluated (treatment vs. time p < 0.01). Mean baselines values varied between 4.8 and 6.2 MPa in water; after 4 h of treatment the values increased between 34.9 and 242.5 MPa, that were treatment time and agent dependent. The use of these collagen crosslinkers to increase the stiffness of demineralized dentin, was both concentration and time dependent.