Steven R. Armstrong

The influence of water storage and C-factor on the dentin–resin composite microtensile bond strength and debond pathway utilizing a filled and unfilled adhesive resin

OBJECTIVE To test the elastic wall concept utilizing adhesive resins of varying stiffness in a low- and high-C-factor cavity design after short- and long-term water storage. METHODS A flat and box-shaped cavity was restored on occlusal dentin with a resin composite using a filled and unfilled adhesive resin from which microtensile specimens with a 0.5mm(2) cross-sectional area were formed. After storage for 30- and 150-days the microtensile bond strength (muTBS) was determined in a Zwick materials testing machine and the subsequent debond pathway was examined under scanning electron microscopy. Fishers exact test was used to determine differences in joint and substrate failure modes and a Weibull regression model with gamma frailties was used to test for differences between failure distributions. Tests for three-way and two-way interactions were also completed for storage time, C-factor and adhesive. All tests were at 95% confidence levels. RESULTS The characteristic strength (TBS degrees ) for the Optibond FL adhesive applied on a flat cavity was 47.57 and 20.90MPa and a box-shaped cavity was 49.26 and 17.49MPa for short- and long-term storage, respectively, while the corresponding TBS degrees for the unfilled Optibond adhesive on the flat cavity design was 36.93 and 32.68MPa and in a box-shaped cavity was 32.84 and 15.46MPa. Combining all groups according to storage time revealed a three-fold increase in the debond pathway including the bottom of the hybrid layer. SIGNIFICANCE Evidence suggests that the durability of the bonded joint is threatened by hydrolysis and the most susceptible region is the bottom half of the hybrid layer and in low C-factor cavity designs a more flexible adhesive resin liner was more durable.

Finite element analysis and bond strength of a glass post to intraradicular dentin: Comparison between microtensile and push-out tests

OBJECTIVE This study tested the hypothesis that the stress distribution and bond strength of glass posts to intraradicular dentin is influenced by the mechanical testing methodology. METHODS Thirty single rooted endodontically treated teeth were prepared for luting of tapered fiber-glass posts (Reforpost, Angelus, Londrina, PR, Brazil) with a conventional adhesive system and resin luting cement (Adper Scotchbond Multi-purpose, Rely X ARC, 3M ESPE, St. Paul, MN, USA). The teeth were randomly divided (n=10 per group) into micro-push-out (Mpo), hourglass- (Mh) and rectangular stick-shaped (Ms) microtensile testing groups before sectioning each root into five 1-mm-thick specimens. During specimen preparation for microTBS testing 46/50 stick and 4/50 hourglass specimens prematurely failed; therefore, the Ms group could not be included in the mechanical testing. The remaining specimens were tested at 0.5 mm/min until bond failure. Stress distribution within each specimen type for the three mechanical test methods was analyzed by finite element analysis (FEA). Qualitative analyses were carried out through Von Mises, XY and Sy criterion. RESULTS Mpo and Mh had a mean microTBS of 11.89+/-6.55 and 14.98+/-12.72 MPa, respectively, which was not significantly different (p=0.1311). The push-out test demonstrated a more homogenous stress distribution by FEA and less variability in mechanical testing. SIGNIFICANCE Therefore, the recommended testing method for determining the bond strength of glass posts to intraradicular dentin is by Mpo.

Degree of conversion of Filtek Silorane Adhesive System and Clearfil SE Bond within the hybrid and adhesive layer: An in situ Raman analysis

OBJECTIVES To examine the degree of conversion (DC) of the adhesive interfaces created by Filtek Silorane Adhesive and Clearfil SE Bond using micro-Raman spectroscopy. METHODS The adhesives were applied on human dentin in accordance with manufacturers instructions. Specimens were cut to expose the bonded interfaces to the micro-Raman beam (Ranishaw InVia; laser wl 785 nm). Raman spectra were collected along the dentin/self-etching primer/adhesive interface at 1 microm intervals. The relative intensities of bands associated with mineral (P-O functional group at 960 cm(-1)) and adhesive (C-C-O group at 605 cm(-1)) components within the bonded interface were used to detect monomer penetration into the dentin matrix and to calculate the degree of conversion (C=C at 1640 cm(-1) as reaction peak, C-C-O at 605 cm(-1) as reference peak). Data were statistically analyzed with two-way ANOVA. RESULTS DC of Filtek Silorane Adhesive was 69+/-7% in the adhesive layer, increasing (p<0.05) to 93+/-5% in the primer and 92+/-9% in the hybrid layer. Clearfil SE Bond showed a DC of 83+/-3% in the hybrid and 85+/-3% in the adhesive layer. Thus, Filtek Silorane Adhesive showed a higher DC than Clearfil SE Bond in the hybrid layer (p<0.05), but a lower DC in the adhesive (p<0.05). SIGNIFICANCE As high DC is a fundamental pre-requisite for the stability of the bond over time, this study supports the hypothesis that optimal stability of Filtek Silorane Adhesive can be obtained. However, further research is needed to investigate the mechanical properties of the hybrid layer created by Filtek Silorane Adhesive and its long-term stability.

Effect of hybrid layer on fracture toughness of adhesively bonded dentin–resin composite joint

OBJECTIVES Micromechanical retention from the hybrid layer is generally believed to be the mechanism of adhesion of current generation dentin bonding agents. The purpose of this investigation is to evaluate the interfacial fracture toughness of a commercial dentin bonding agent with and without this hybrid layer. METHODS Ten extracted molars (AB2) were flattened on the occlusal surface, All-Bond 2 Universal Adhesive System (Bisco) was applied according to manufacturers directions and a resin composite (Prodigy, Kerr) crown was formed. Another group of ten molars (AB2Cl) was handled identically with the exception of a 1 min gentle scrubbing application of 5.25% sodium hypochlorite after acid etching to remove the acid-exposed collagen. Plane-strain chevron-notch short bar fracture toughness specimens were fabricated from all 20 composite crowns and tested according to ASTM E1304-89. Each group was tested to failure in tensile mode at 0.1 mm min-1 and the maximum load at failure was used to determine plane-strain fracture toughness (KQvM). Weibull parameters were calculated and fracture probability distributions were tested for significant difference at the 95% confidence level. Scanning electron microscopy was employed on broken specimens (18/20) to describe the failure mode. RESULTS Weibull distributions were not significantly different with characteristic plane-strain fracture toughness from maximum load (KQvM0) of 0.97 MPa m1/2 and 0.81 MPa m1/2 and a Weibull modulus of 4.7 and 3.9, respectively, for AB2 and AB2Cl. All AB2 samples failed within the adhesive joint, while the AB2Cl crack propagated from the interphase of adhesive resin and dentin to 1-2 microns into dentin. SIGNIFICANCE Under the conditions of this study, the presence of collagen did not contribute to a significantly stronger bonded joint. Interfacial fracture toughness evaluation of the dentin-resin composite bimaterial interface shows promise for future investigations.

Microtensile Specimen Attachment and Shape—Finite Element Analysis

Microtensile bond strength values are influenced by specimen shape and attachment method to the gripping device during testing. We hypothesized that stress distribution inside the testing specimen is affected by microtensile specimen shape and attachment method. Rectangular, hourglass-, and dumbbell-shaped specimens, all with a 1 mm2 cross-sectional testing region, were modeled as indirect ceramic restorations luted to dentin. Three specimen attachments were investigated: (1) posterior surface; (2) posterior, superior, and lateral surfaces; and (3) all surfaces. Qualitative and quantitative analyses were carried out according to von Mises’ criteria. Stress analysis showed a direct correlation between attachment modes and stress distribution, with shear stresses observed in models with less surface attachment. Increasing the number of faces for specimen attachment to the metallic gripping device resulted in a more homogeneous and regular distribution of stress, with tensile stress concentrated at the adhesive interface. Dumbbell-shaped specimens showed improved stress distribution compared with rectangular and hourglass-shaped specimens.

Correlation between microtensile bond strength data and clinical outcome of Class V restorations

OBJECTIVE To determine if the results of resin-dentin microtensile bond strength (μTBS) is correlated with the outcome parameters of clinical studies on non-retentive Class V restorations. METHODS Resin-dentin μTBS data were obtained from one test center; the in vitro tests were all performed by the same operator. The μTBS testing was performed 8 h after bonding and after 6 months of storing the specimens in water. Pre-test failures (PTFs) of specimens were included in the analysis, attributing them a value of 1MPa. Prospective clinical studies on cervical restorations (Class V) with an observation period of at least 18 months were searched in the literature. The clinical outcome variables were retention loss, marginal discoloration and marginal integrity. Furthermore, an index was formulated to be better able to compare the laboratory and clinical results. Estimates of adhesive effects in a linear mixed model were used to summarize the clinical performance of each adhesive between 12 and 36 months. Spearman correlations between these clinical performances and the μTBS values were calculated subsequently. RESULTS Thirty-six clinical studies with 15 adhesive/restorative systems for which μTBS data were also available were included in the statistical analysis. In general 3-step and 2-step etch-and-rinse systems showed higher bond strength values than the 2-step/3-step self-etching systems, which, however, produced higher values than the 1-step self-etching and the resin modified glass ionomer systems. Prolonged water storage of specimens resulted in a significant decrease of the mean bond strength values in 5 adhesive systems (Wilcoxon, p<0.05). There was a significant correlation between μTBS values both after 8 h and 6 months of storage and marginal discoloration (r=0.54 and r=0.67, respectively). However, the same correlation was not found between μTBS values and the retention rate, clinical index or marginal integrity. SIGNIFICANCE As μTBS data of adhesive systems, especially after water storage for 6 months, showed a good correlation with marginal discoloration in short-term clinical Class V restorations, longitudinal clinical trials should explore whether early marginal staining is predictive for future retention loss in non-carious cervical restorations.

Association between microtensile bond strength and leakage in the indirect resin composite/dentin adhesively bonded joint.

PURPOSE Develop a methodology to correlate microtensile bond strength (microTBS) and leakage within the same site of a specimen and evaluate the predominate debond and leakage pathways. MATERIALS AND METHODS A Class II (MO) slot cavity was prepared in 37 extracted human molars and restored with indirect resin composite restorations. The restored teeth were thermocycled 300X between 5 and 55 degrees C and then stained with silver. Two specimens per tooth were obtained by diamond saw sectioning for measurement of leakage (microm) and microtensile bond strength (MPa) at the gingival wall and then tested for association with regression analysis. Scanning electron microscopy techniques and energy dispersive spectroscopy (EDS) were used to evaluate the debond and leakage pathways. RESULTS Association between leakage and bond strength was not significant (t=1.31, p=0.198). All 74 samples debonded within the joint, predominantly at the hybrid layer-adhesive resin region with only one specimen including a small portion of resin cement in the debond pathway. Secondary and backscatter imaging and EDS analysis showed leakage in all samples that involved the hybrid layer. CONCLUSION This study demonstrates a feasible method for evaluating leakage and bond strength at the joint interface within the same specimen.

Effect of specimen gripping device, geometry and fixation method on microtensile bond strength, failure mode and stress distribution: Laboratory and finite element analyses

OBJECTIVES Innumerous modifications have been proposed for the microtensile test since its introduction; however, testing parameters are not often well described and wide variations in bond strength are commonly reported. The aim of this study was to evaluate the effect of the test specimens gripping device, specimen geometry and fixation method on microtensile bond strength, failure mode, and stress distribution when using an etch-and-rinse 2-step adhesive system bonded to human dentin. METHODS Resin-based composite bonded to occlusal dentin from 21 human molars was used to fabricate dumbbell- and stick-shaped test specimens which were divided into three groups: Di - dumbbell-specimens placed in a Dircks device; GeS - stick-specimens gripped in a Geraldelis device with Superglue; GeZ - stick-specimens gripped in a Geraldelis device with Zapit. Specimens were tested to failure in tensile mode and the failure mode was examined under stereomicroscopy and fracture initiation sites were verified by scanning electron microscopy and energy dispersive X-ray spectroscopy. Three-dimensional models of each device/specimen were created and finite element calculations were performed. RESULTS The effect of the gripping devices on the bond strength was not significant, unless the bond test areas were normalized. The failure mode was influenced by the type of device. Dircks device was less sensitive to human error than Geraldelis, and produced a more uniform stress distribution at the dumbbell specimen adhesive layer than did the Geraldelis device at the stick layer. SIGNIFICANCE Microtensile testing parameters can directly influence the results and consequently inter-study comparisons.

Spotlight on bond strength testing--unraveling the complexities.

The variability in methods and outcomes of bond strength testing reports is well documented in the dental literature. Many studies lack important information, which impairs the ability to reproduce them as well as to compare them to other studies in the literature. In order to critically discuss the important issues around bond strength testing methods, and to move closer to at least standardizing the reporting of such studies, the Academy of Dental Materials held a conference in 2009 entitled Adhesion in Dentistry-Analyzing Bond Strength Testing Methods, Variables, and Outcomes. Short synopses of the presentations are presented in this article. This article also provides a list of the variables that should be reported in bond strength studies, regardless of testing methods, to be used by authors conducting future studies, as well as journal reviewers and editors. The goal is to provide guidance and a rationale for what should be included in a study so that reporting might be more standardized and to enhance the possibility that more meaningful comparisons and conclusions may be drawn across studies. The table lists detailed descriptions covering all aspects of testing procedures, including variables related to tooth substrate, restorative material, specimen preparation, pre-testing conditions, testing methods, data reporting and analysis.

Effects of oxalate on dentin bond after 3-month simulated pulpal pressure

OBJECTIVES Application of an acidic oxalate solution forms calcium oxalate within dentinal tubules and has been used to desensitize dentin and may also improve performance by reducing internal dentin wetness during bonding. The hypothesis tested was that oxalate restriction of dentinal fluid transudation when using an etch-and-rinse two-step adhesive will improve microtensile bond strength (muTBS) and reduce nanoleakage. METHODS Occlusal dentin of 60 human molars were bonded (Adper Single Bond Plus, 3M ESPE) while one-half of each tooth received either a liquid (SuperSeal, Phoenix Dental) or gel (BisBlock, Bisco) oxalate treatment after acid etching. The restored teeth were placed under pulpal pressure for 3 months before forming cylindrical dumbbell specimens for muTBS and failure pathway determination. Additional teeth were prepared and stored in a similar manner for transmission electron microscopy (TEM) examination of nanoleakage after tracer immersion. RESULTS The mean bond strength in the group with oxalate liquid and the control group was 27.06+/-7.14 and 36.18+/-9.07 MPa, respectively, and for the gel form of oxalate was 25.34+/-13.09 and 33.09+/-14.25 MPa, respectively. The control groups were significantly stronger than either oxalate group using t-tests (liquid p<0.00001; gel p=0.0032) or Weibull (liquid p=0.0002; gel p=0.0029) statistics. Oxalate groups also demonstrated more adhesive failure modes and nanoleakage. CONCLUSIONS Under the conditions of this study, the application of oxalate with an etch-and-rinse two-step bonding system produced significantly lower long-term muTBS and enhanced nanoleakage.