Biomedical subjects

D H Pashley

Publications and source records attributed to D H Pashley.

At least 73 records · Page 4Linked to original sources

PubMed2001

Relationship between bond strengths and nanoleakage: evaluation of a new assessment method.

PURPOSE: To test if there was a correlation between resin-dentin bond strengths and nanoleakage of total etch and self-etching primer bonding systems. MATERIALS AND METHODS: Ten extracted third molars were ground flat with 600 grit SIC paper under running water to expose middle dentin. Clearfil Liner Bond 2V or Single Bond was applied to the dentin surface according to the manufacturers' instructions. A crown was built-up with Clearfil AP-X resin-based composite, and the specimens stored in water for 24 hrs at 37 degrees C. The bonded assemblies were cut mesiodistally perpendicular to the interface into approximately 0.7 mm thick slabs, and trimmed for microtensile bond strength testing. Alternate slices from each tooth were either entirely double-coated with nail varnish leaving a 0.5 mm window around the interface, or left uncoated to be immersed in water for 13 hrs as the control. The varnished (experimental) slabs were immersed in 50% AgNO3 for 1 hr, and then immersed in a photo developing solution for 12 hrs. All specimens were subjected to microtensile bond test at a crosshead speed of 1 mm/min. After debonding, the specimens were embedded in epoxy resin and observed under a confocal laser scanning microscope for determination of lateral silver penetration within the interface. Silver penetration was further analyzed under the SEM. Data were analyzed with one- and two-way ANOVA, Fisher's PLSD test (P < 0.05), and regression analysis. RESULTS: The bond strengths of Clearfil Liner Bond 2V and Single Bond were not significantly different (P > 0.05); however the bond strengths of the specimens that were soaked in 50% AgNO3 for 1 hr significantly increased (P < 0.01). No correlation was observed between bond strengths and nanoleakage.

Acid Etching, Dental↗

PubMed2001

The use of oxalate to reduce dentin permeability under adhesive restorations.

PURPOSE: To test a novel approach to reducing dentin permeability that localizes occlusion of dentin tubules by calcium oxalate crystals to the subsurface without lowering resin bond strengths to oxalate-treated dentin surfaces. MATERIALS AND METHODS: Flat dentin surfaces of extracted human third molars were etched with a mass fraction of 32% phosphoric acid (Bisco) for 15 s and rinsed. Half of the surface was treated with a potassium oxalate gel (mass fraction of 3% monopotassium monohydrogen oxalate) for 2 mins. The entire surface was then moist bonded with either One Step (OS) or Scotchbond Multi-Purpose (SB) adhesive systems. A resin-based composite (Z-100) buildup was made over the entire surface. After storage for 24 hrs in distilled water, the teeth were longitudinally sectioned to separate the oxalate-treated half from the untreated (control) half. Each half was serially sectioned into several 0.7 mm thick slabs which were then trimmed to an hour-glass shape to reduce the bonded area to approximately 0.8 mm2 and tested in tension. Pre- and post-treatment hydraulic conductance (Lp) of dentin was determined using dentin discs with a standard protocol. SEM images were obtained to examine the effects of treatment both on the dentin surface and along the tubules beneath the surface. The bonded interface was also examined by SEM. RESULTS: Treating etched dentin discs with the oxalate gel caused significant reduction of the Lp of dentin (80% reduction, P < 0.05). There were no statistically significant differences between bond strengths of oxalate-treated and untreated surfaces for either adhesive system. OS resulted in a bond strength of 25.8 +/- 9.2 MPa to untreated and 27.8 +/- 8.9 MPa to oxalate treated surfaces (P > 0.05). SB showed bond strengths of 22.9 +/- 7.9 MPa and 22.9 +/- 9.6 MPa to untreated and treated surfaces, respectively (P > 0.05). SEM images showed that the application of potassium oxalate gel on etched dentin resulted in the formation of crystals inside the tubules rather than on the surface. Examination of the bonded interfaces demonstrated that the crystal formation inside the tubules did not compromise the formation of a typical hybrid layer on the top of dentin surfaces. Resin monomers penetrated into the tubules filling the spaces around the crystals forming resin tags with a jagged-like feature.

Analysis of Variance↗

PubMed2001

Effect of different conditioning protocols on adhesion of a GIC to dentin.

PURPOSE: This study examined the ultrastructure and microtensile bond strength (microTBS) of a restorative glass-ionomer cement (GIC; Dentsply) to sound dentin that was conditioned with various techniques. MATERIALS AND METHODS: Dentin surfaces from extracted human third molars were abraded with 180-grit SiC paper. Five groups of three teeth each were prepared: C - no acid pretreatment (control); P - 10% polyacrylic acid (PAA) for 10 s, no rinsing; R - 10% PAA for 20 s, rinsed; K - 25% PAA for 25 s, rinsed; and H -32% phosphoric acid for 15 s, rinsed. TEM was performed on a bonded specimen from each group, using unstained, undemineralized sections. GIC buildups were made on the remaining teeth, and after storage at 100% humidity for 24 h, the teeth were sectioned for microTBS and SEM evaluation. RESULTS: TEM revealed the presence of a structure known as the intermediate layer in all groups. This layer contains metallic salts contributed by both the GIC and dentin. In group C, this layer was restricted to the smear layer. In groups P and R, intermediate layers could be found above partially demineralized zones within the intertubular dentin. In groups conditioned with more aggressive protocols (K and H), the intermediate layers shifted downward to reside within the superficial portions of completely demineralized collagen. Group C had statistically lower microTBS (p < 0.05), while the other groups were not significantly different from each other. SEM revealed adhesive failures along the dentin surface in group C and mixed failures in the other groups. CONCLUSION: The lower microTBS observed in the control group reflects the weakness of the smear layer attachment to dentin. The higher microTBS in the other groups probably represent the cohesive strength of GIC under tension, rather than its true adhesive strength to dentin. Acid pretreatment dissolves the smear layer, creates a zone of partially demineralized dentin, and allows the PAA to interact with dentin via the intermediate layer. Overly aggressive conditioning renders the dentinal tubules patent, and leaves deeper demineralized dentin that does not form part of the intermediate layer.

Acid Etching, Dental↗

PubMed2001

Effect of delayed activation of light-cured resin composites on bonding of all-in-one adhesives.

PURPOSE: This study examined the effect of delayed activation of light-cured resin composites on the microtensile bond strengths of two all-in-one adhesives to sound dentin. MATERIALS AND METHODS: Flat dentin surfaces prepared from caries-free third molars were bonded with either Prompt L-Pop (ESPE) or One-Up Bond F (Tokuyama). Each adhesive was divided into 6 subgroups containing three teeth each. A hybrid composite was left to contact the cured adhesive surfaces for 0, 2.5, 5, 10 or 20 min before light activation. In the 6th subgroup, the cured adhesive was covered with a layer of nonacidic bonding resin before contacting the composite for 20 min. Composite-dentin beams, with average cross-sectional areas of 0.85 mm2 were prepared for microtensile bond testing. Representative fractured beams from the 0, 10- and 20-min delay subgroups were prepared for SEM examination. Additional specimens were prepared in the 0- and 20-min delay subgroups by replacing the hybrid composite with either a flowable composite or a composite liner to facilitate TEM preparation and examination. RESULTS: Kruskal-Wallis ANOVA and Dunn's multiple comparison tests showed that significant differences existed among the subgroups of each adhesive. Regression analyses showed that for each adhesive, there was an exponential decline in mean bond strength with increasing delay in light activation, and the correlation was highly significant (r = -0.99, p < 0.005 for Prompt L-Pop; r = -0.96, p < 0.01 for One-Up Bond F). SEM examination of fractured interfaces in the immediately light-cured subgroups revealed the absence of voids within the fractured composite. However, voids were consistently observed in the 10 min- and 20 min-delay subgroups. They corresponded to soapsuds-like blisters that were observed with TEM in the 20 min-delay subgroups. CONCLUSION: Adverse interactions between acidic adhesive resin monomers and tertiary amines in light-cured composites normally do not occur because of the fast rate of free-radical generation in photochemical redox reactions. However, such interactions can occur in all-in-one adhesives on prolonged contact of light-cured composites with the cured adhesive layer.

Composite Resins↗

PubMed2001

Tensile strength of human dentin as a function of tubule orientation and density.

PURPOSE: This study investigated the ultimate tensile strength (UTS) of human dentin as a function of tubule orientation and density. MATERIALS AND METHODS: Slabs of dentin (ca 0.7 mm thick) were obtained from human third molars by either transversally or longitudinally sectioning the crowns with an Isomet saw. The slabs were gently trimmed to reduce the central area of the coronal dentin to a cross-sectional area of approximately 0.5 mm2. The longitudinally sectioned slabs were either trimmed from the mesial and distal sides or from the occlusal and pulpal aspects to permit the tensile load to be applied either parallel or perpendicular to the tubule orientation. The transversally sectioned specimens were obtained at several distances from the pulp and were used to evaluate the effects of tubule density. The trimmed specimens were tested in tension on a Kratos testing machine at 0.5 mm/min. After failure, the UTS of each was calculated and expressed in MPa. The fractured ends of the transversally sectioned specimens were viewed under SEM to calculate the number of tubules per mm2 at the site of fracture, and its relation with the UTS was investigated by regression analysis. RESULTS: The UTS of dentin is higher when the load is applied perpendicular to the tubule orientation (80 +/- 13 MPa) than when applied parallel to tubule orientation (58 +/- 11 MPa, p < 0.05). There was a tendency for dentin to be weaker as the number of tubules at the site of fracture increased, although this relationship was not statistically significant (R2 = 0.051, p > 0.05). CONCLUSION: The UTS of dentin is dependent on the tubule direction. Dentin tends to be weaker as the number of tubules per area increases.

Dental Stress Analysis↗

PubMed2001

The influence of tubule density and area of solid dentin on bond strength of two adhesive systems to dentin.

PURPOSE: To determine the correlation between the tubule density (TD) and the area occupied by solid dentin (ASD) with the bond strength of one conventional and one self-etching adhesive system to dentin. MATERIALS AND METHODS: The crown of extracted human third molars was transversally sectioned with a diamond saw to expose either superficial, middle, or deep dentin. The three groups of dentin surfaces were randomly divided and bonded with either Clearfil Liner Bond 2V (LB) or Prime & Bond 2.1 (PB) adhesive systems according to manufacturer's directions. Resin composite buildup crowns (10.0 mm high) were incrementally constructed on the bonded surfaces and the teeth stored in water at 37 degrees C. After 24 h of storage, the teeth were vertically, serially sectioned in both x and y directions to obtain several bonded sticks of approximately 0.7 mm2 cross-sectional area. Each stick was tested in tension in a EMIC DL-500 tester at 0.5 mm/min until failure. After testing, the dentin side of the fractured specimen was gently abraded with a 1000-grit SiC paper, etched with 37% phosphoric acid for 15 s and allowed to air dry. SEM micrographs at 1000x and 4000x magnification were taken to permit calculation of the TD (number of tubules/mm2) and ASD (% of total area) at the site of fracture. Correlation between TD and ASD with the bond strength data was performed by linear regression. All statistical analysis was done with alpha = 0.05. RESULTS: Overall bond strength (MPa) for LB was 26.0 +/- 10.2, and 42.6 +/- 15.2 for PB. There was a significant direct relationship between bond strength and ASD for both materials (r2 = 0.20, p < 0.05 and r2 = 0.66, p < 0.01, respectively for LB and PB). PB bond strength dropped significantly as the TD increased (r2 = 0.63, p < 0.05), while LB was not sensitive to TD (r2 = 0.05, p > 0.05). Mean bond strength of PB was significantly higher than LB for both superficial and middle dentin (p < 0.05), while there was no significant difference for deep dentin (p > 0.05). CONCLUSION: Regional variations in TD and ASD may modify bond strength of both conventional and self-etching adhesive systems. Bonding sites with larger ASD seem to yield higher bond strengths regardless of the type of adhesive system used.

Acetone↗

PubMed2000

Effect of dentin desensitizers and cementing agents on retention of full crowns using standardized crown preparations.

STATEMENT OF PROBLEM: Past research has not controlled preparation surface area when examining the influence of dentin desensitizers on the retentive strength of cemented cast crowns, leading to inconsistent results. PURPOSE: This research controlled crown preparation surface area and evaluated the effect of various dentin desensitizers and conventional cementing agents on the in vitro retentive strength of cast crowns. METHODS AND MATERIAL: Freshly extracted human molars were prepared for a standardized crown preparation (26 degrees total convergence, 4 mm axial height) with a custom-made pantograph. Dentin desensitizers included none (control), a polymerizable material (All-Bond 2), and a nonpolymerizable desensitizer (Gluma Desensitizer). Cementing agents included zinc phosphate (Fleck's), glass ionomer (Ketac-Cem), resin-modified glass ionomer (Fuji II), and resin cement (Panavia 21). Twelve teeth were prepared for each test condition (144 teeth total). Individual castings were made from a base metal alloy (Rexillium III). Crowns were removed after storage at 26 degrees C for 48 hours at 100% relative humidity using a universal testing machine at a crosshead speed of 1.27 mm/min. The proportion of cement retained on the tooth and casting after debonding was quantified according to treatment. Statistical treatment included 1- and 2-way ANOVAs, followed by the Tukey-Kramer post hoc test at a preset alpha of 0.05.Results. Resin cement exhibited the highest retentive strength and all dentin treatments resulted in significantly different retentive values (All-Bond 2 (5.68 +/- 0.70 MPa) > control (4.67 +/- 0.48 MPa) > Gluma (4.12 +/- 0.37 MPa)). Retention of resin-modified glass ionomer was between the resin cement and glass ionomer groups: All-Bond 2 (3.46 +/- 0.26 MPa) > Gluma (2.81 +/- 0.15 MPa) = control (2.96 +/- 0.18 MPa). Conventional glass ionomer values were between those of Fuji Plus and zinc phosphate groups: All Bond 2 (2.23 +/- 0. 20 MPa) = control (2.36 +/- 0.20 MPa) > Gluma (1.98 +/- 0.23 MPa). Zinc phosphate had the lowest retention values: control (1.68 +/- 0. 08 MPa) > Gluma (0.81 +/- 0.11 MPa) > All-Bond 2 (0.67 +/- 0.14 MPa). The majority of cement was retained on the debonded tooth surface versus the casting, with the exception of zinc phosphate when used with dentin pretreatments. CONCLUSION: Controlled crown surface areas reduced the variation in strength values permitting high discrimination among retention values of desensitizer/cement combinations. In all but 1 combination, Gluma desensitizer significantly decreased crown retention. With resin cement and resin-modified glass ionomer, use of All-Bond 2 desensitizer significantly increased crown retention values.

Analysis of Variance↗

PubMed2000

Permeability of demineralized dentin to HEMA.

OBJECTIVES: The objectives of this study were to develop a macromodel of the demineralized layer of dentin that is formed after acid-etching, and to determine if the amount of HEMA uptake was responsive to air drying. The hypothesis that was tested was that HEMA uptake by demineralized dentin depends upon the degree of expansion of the collagen fibril network. METHODS: Dentin cubes (2 x 2 x 2 mm) were prepared in coronal dentin from extracted unerupted human third molars. They were incubated in 100% HEMA for up to 1000 min and then removed and blotted free of excess adherent HEMA. The HEMA taken up by each cube was extracted in 2 ml of water for 1 hr with shaking. This was repeated and the extracts combined. HEMA was quantitated spectrophometrically. The dentin cubes were then demineralized in 0.5 M EDTA for 10 days and the HEMA uptake remeasured at 1, 10, 100 and 1000 min. Then the cubes were air-dried and the HEMA uptake remeasured. After re-expanding the specimens in water, the cubes were stiffened in ascending concentrations of acetone of 100%, then allowed to air dry again in a more expanded state, and HEMA uptake was remeasured. Finally, after rehydration, the dentin cubes were "acid-etched" for 1 or 10 min with 37 wt% phosphoric acid and HEMA uptake remeasured. RESULTS: Before demineralization, the dentin took up little HEMA (ca. 4.8 x 10(-7) moles min-3) compared to after demineralization when the uptake for 10, 100 and 1000 min was 27.4, 43.8 and 51.4 x 10(-3) moles mm-3, respectively. Acid etching for 1 or 10 min had no effect on HEMA uptake. Air drying produced a 72% volumetric shrinkage but a 97% reduction in uptake. When the demineralized matrix was stiffened in acetone prior to air-drying, the volumetric shrinkage was only 27% and the HEMA uptake only fell 16% compared to the wet, fully expanded condition. SIGNIFICANCE: The results support the hypothesis that the uptake of HEMA by demineralized dentin depends on the degree of expansion of the dentin matrix.

Acid Etching, Dental↗

PubMed2000

Effect of region and dentin perfusion on bond strengths of resin-modified glass ionomer cements.

OBJECTIVES: The purpose of this study was to evaluate the hypothesis that regional differences and pulpal pressure would significantly affect bond strengths of resin-modified glass ionomer cements to dentin. METHODS: Twenty-six extracted caries-free human third molars were ground to expose middle dentin and were randomly divided into two groups for bonding: no pulpal pressure and pulpal pressure of 15cm H(2)O. Fuji II LC, Vitremer, or Photac-Fil Quick were applied to the previously pre-treated surfaces and light-cured as recommended by the manufacturers. After immersion in water at 37 degrees C for 24h, the teeth were sectioned to 0.7mm thick slabs, divided into pulp horn, center, and peripheral regions according to visual criteria, and trimmed along the bonded interface for microtensile bond test to a cross-sectional area of 1mm(2). The slabs were subjected to tensile forces and the data analyzed using ANOVA and Fisher's PLSD at the 95% level of confidence. RESULTS: For Fuji II LC, bond strengths to the pulp horn regions were significantly lower than those to the other regions independent of pulpal pressure (p<0.05). On the other hand, regional bond strengths were not observed for Vitremer and Photac-fil Quick 0.05). CONCLUSIONS: Pulpal pressure had a stronger influence on bond strengths and failure modes of resin-modified glass ionomers than regional differences of the substrate.

Analysis of Variance↗

PubMed2000

An ultrastructural study of the application of dentine adhesives to acid-conditioned sclerotic dentine.

OBJECTIVE: This in vitro study examined the ultrastructure of resin-infiltrated sclerotic dentine following the application of a self-etching primer, with or without the adjunctive use of phosphoric acid pre-conditioning. MATERIALS AND METHODS: Non-carious, natural cervical sclerotic lesions were hand-cleaned with a slurry of pumice and chlorhexidine and bonded without further cavity preparation. One group was bonded using Clearfil Liner Bond 2V (Kuraray Co. Ltd, Osaka, Japan) alone. Specimens from the other group were pre-conditioned with K-etchant (40% phosphoric acid gel, Kuraray) prior to the application of the same self-etching primer. Artificially prepared wedge-shaped lesions were also made in sound bicuspids, bonded using the two methods, and used as controls for the two groups. For SEM examination, each specimen was cryofractured into two halves through a pre-formed slit made on the lingual surface, after the respective conditioning treatment. Different locations within the lesions were examined after rinsing of the phosphoric acid/self-etching primer and specimen dehydration. For TEM investigation, specimens were bonded with the adhesive. Both demineralised and undemineralised ultrathin sections were prepared from the occlusal, gingival and deepest part of the wedge-shaped bonded lesions following specimen fixation, dehydration and resin embedding. RESULTS: A hypermineralised surface layer was present on the surface of etched sclerotic dentine. This layer was thicker in the deepest part of the natural lesions, where bacterial colonisation of the lesion surface was also apparent. Both treatment protocols were unable to effectively dissolve sclerotic casts that occluded the dentinal tubules. Depending upon the thickness of the surface layers at different locations in the natural lesion, self-etching primer treatment alone resulted in reduction of the thickness of the authentic hybrid layer (i.e. hybridised intertubular dentine). This was also true of phosphoric acid pre-conditioning along the deepest part of the natural lesions. Within this region, intertubular dentine completely devoid of an authentic hybrid layer could be seen in both treatment groups. Resin tags were also sparsely observed in such regions. CONCLUSIONS: Adhesive strategies that rely mostly on micromechanical retention alone may be compromised by the sporadic absence of the hybrid layer and resin tags in sclerotic dentine. Based on the ultrastructural features presented, it is further speculated that adaptive strategies such as removal of the surface layers and extended etching time may not be completely effective in improving bonding efficacy in highly sclerotic dentine. Interdisciplinary research should be continued to develop alternative procedures for bonding resins equally well to sound and sclerotic dentine.

Acid Etching, Dental↗

PubMed2000

Effects of acids and additives on the susceptibility of human dentine to denaturation.

The purpose of this study was to evaluate the efficacy of a number of additives to acid conditioners at reducing the denaturation of dentine collagen. Dentine collagen is normally not very susceptible to trypsin attack. After denaturation, however, it becomes more susceptible to the action of trypsin. Slabs of human dentine were dipped in water or acidic conditioners (10% citric acid or 37% phosphoric acid) containing no additives, 5 or 10% NaCl, 3 or 6% ferric chloride or 50% hydroxyethylmethacrylate (HEMA) for 15 or 60 s, followed by rinsing. The slabs were then exposed to trypsin for 24 h to solubilize any denatured collagen. The solubilized collagen was hydrolysed to liberate hydroxyproline that was quantitated spectrophotometrically. The amount of hydroxyproline (HOP) liberated was indicative of the amount of dentine collagen that was denatured by the test solutions. The only additive that consistently reduced HOP release was 50% HEMA, and this only occurred in the 60 s exposure group. Thus, the use of salt additives to acidic conditioners has little protective effect.

Acid Etching, Dental↗

PubMed2000

Gallium diffusion in human root dentin: quantitative measurements by pulsed Nd:YAG laser ablation combined with an inductively coupled plasma mass spectrometer.

OBJECTIVE: The purpose of this work was to determine if gallium nitrate placed in human root canals would diffuse across root dentin and reach concentrations high enough to inhibit osteoclasts (approximately 10(-4) M). BACKGROUND DATA: External root resorption by osteoclasts is a common sequela of dental trauma. If not detected and treated, it can lead to the loss of a tooth. Gallium has recently been reported to inhibit osteoclastic resorption in vitro. METHODS: Roots were cleaned and shaped using standard endodontic procedures and the tips sealed with cyanoacrylate cement. The root canal space was filled with an aqueous solution of 1.0 M gallium nitrate chelated with 1.0 M sodium citrate buffer (pH 7.2). The roots were then sectioned longitudinally into two equal halves. Each half was fixed to a translation stage that moved at a constant rate beneath a frequency-quadrupled Nd-YAG laser (266 nm) laser that was used to sample the concentration of 43Ca, 69Ga, and 71Ga by laser ablation across the thickness of root dentin to the periodontal surface. The plume of ablated dentin was swept into an inductively heated plasma chamber by argon gas and hence into a mass spectrometer. RESULTS: Quantitative analyses of the distribution of gallium showed it was highest adjacent to the root canal space and fell as more peripheral sites were sampled but then rose slightly at the external boundary of the root which is covered with a thin layer of atubular cementum. CONCLUSIONS: Even the lowest concentrations of gallium found in peripheral root dentin exceeded the 10(-4) M concentration required to inhibit osteoclastic activity. This simple endodontic treatment should undergo clinical trials to determine its efficacy in vivo. The laser ablation, inductively coupled mass spectrometry method is a powerful analytic tool for measuring spatial distribution of materials in mineralized tissues.

Dental Pulp Cavity↗

PubMed2000

H+-induced tension development in demineralized dentin matrix.

Recent atomic force microscopy studies have shown that acid-etching dentin causes the surface to recede. The purpose of this work was to test the hypothesis that acidic solutions can cause contraction of demineralized dentin matrix. Small beams of dentin were cut from extracted human third molars. The central region of each specimen was completely demineralized in EDTA. Specimens held in a tester were immersed in acidic solutions, and the amount of tension was recorded. Test variables included gauge length, cross-sectional area, pre-strain, and pH. The results showed that immersion of demineralized dentin in acidic solutions caused tension that was directly related both to H+ concentration at pH < 2 and to pre-strain. The contractile stress development (ca. 0.2-0.4 MPa) was sufficient to cause a collapse of demineralized dentin matrix. The mechanism for this effect probably involves H+-induced conformation changes in the collagen matrix.

Acid Etching, Dental↗

PubMed2000

Effects of prism orientation on tensile strength of enamel.

PURPOSE: This study tested the hypothesis that the tensile strength of enamel varies according to prismatic orientation. MATERIALS AND METHODS: Eight extracted, caries-free human third molars were thoroughly cleaned with pumice and water. The entire enamel surface was conditioned with 37% phosphoric acid for 30 s, air dried, and bonded with Single Bond adhesive system. Several layers of composite (Z-100) were incrementally applied to the crown to build up a "cube-like" resin structure approximately 5 mm thick covering the entire crown of the teeth. The teeth were stored for 24 h in water at 37 degrees C, and then the crown was serially, vertically sectioned in a mesio-distal direction to obtain several slices approximately 0.7 mm thick. The slices were sectioned into halves, and each half was gently trimmed from both sides with a diamond bur to reduce the cross-sectional area to a "neck" located in enamel, either on the external slope or functional slope of the cusps. The specimens were trimmed to permit testing of enamel with its prisms either oriented parallel (PL) or perpendicular (PD) to the applied load. The trimmed specimens were glued to the fixtures of a Vitrodyne tester and stressed in tension at 0.5 mm/min. SEM images were taken from the fractured surfaces to examine the site of failure and confirm the prism orientation. RESULTS: Mean tensile strength of enamel was 24.7 +/- 9.6 MPa (n = 22) for PL and 11.4 +/- 6.3 MPa (n = 22) for PD oriented enamel prisms (t value = -5.45, p < 0.05). There was no significant difference between specimens originating from different slopes of the cusps (p > 0.05). CONCLUSION: The results showed that tensile strength of enamel is dependent on the prismatic orientation.

Acid Etching, Dental↗

PubMed2000

Mechanical disruption of dentin collagen fibrils during resin-dentin bond testing.

PURPOSE: To determine if collagen fibrils on the dentin side of failed resin-dentin interfaces undergo mechanical disruption during microtensile bond testing. MATERIALS AND METHODS: Extracted, caries-free human third molars were divided into four groups. The occlusal enamel was removed, leaving a flat dentin surface for bonding. Resin composite buildups were made after the acid-conditioned dentin was bonded with either Single Bond (S) or One-Step (O), and using either moist bonding (M) or air drying for 5 s (D). After storage in water for 24 h, the teeth were vertically sectioned into an array of 0.9 x 0.9 mm resin composite-dentin beams. They were stressed to failure using the nontrimming version of the microtensile bond test. Fractured dentin and resin composite sides of representative beams from each group that exhibited adhesive failures under light microscopy examination were prepared for scanning (SEM) and transmission electron microscopy (TEM). RESULTS: A two-way ANOVA showed that moist bond strengths were significantly higher than those made to dry dentin (M > D; p < 0.001), but that there was no difference between the adhesives (S vs O; p = 0.547). SEM analysis showed the presence of loose collagen fibrils within fractured hybrid layers in the dry groups, but not in the moist groups. TEM examination of the dry-bonded groups revealed collagen fibrils that were thinner and exhibited abnormally wide interfibrillar spaces when hybrid layers were intact. Within dry-bonded fractured hybrid layers, broad mechanical disruption zones could be seen, consisting of fibrils that were devoid of cross banding, defibrillation of the subfibrillar architecture, and gross disaggregation into microfibrils. In the moist-bonded groups, only short mechanical disruption zones were found along the torn edges of the collagen fibrils. The rest of the fibrils beyond the fracture site were intact and retained their periodicity. Mechanical testing of demineralized matrices yielded a maximum modulus of elasticity of 43.9 +/- 6.1 MPa. CONCLUSION: We speculate that adhesive resin has a protective function for demineralized collagen in well-infiltrated hybrid layers. We propose that both the collagen and resin contribute to load sharing during stress application until the final moment of rupture. On the other hand, collagen fibrils in poorly infiltrated hybrid layers, being unsupported by resin, undergo various degrees of irreversible mechanical disruption depending on how the stress is dissipated. The collagen fibril network has a much lower modulus of elasticity compared to those of resin-infiltrated fibrils or the demineralized dentin.

Acid Etching, Dental↗

PubMed2000

Long-term mechanical properties of EDTA-demineralized dentin matrix.

PURPOSE: To determine the long-term ultimate tensile strength (UTS) and modulus of elasticity (E) of EDTA-demineralized human dentin after storage in PBS (phosphate buffered saline) for up to 48 months. MATERIALS AND METHODS: Dentin beams measuring 0.7 x 0.7 x 8.0 mm were prepared from the crowns of extracted human third molars. The ends of the beams were covered with resin composite and demineralized for 6 days in 0.5 mol/L EDTA (pH = 7.0). Demineralized control specimens were subjected to tensile testing at 0.5 mm/min after 24 hours of immersion in PBS solution. Experimental specimens were stored in PBS at room temperature (25 degrees C) and tested after 18 and 48 months. The maximum load at failure and the load/displacement curves were used to calculate the UTS as a function of the cross-sectional area and E, respectively. The results were analyzed by ANOVA and Student Neuman Keuls tests. TEM micrographs were obtained from the control specimens and from those that were stored and tested after 48 months. RESULTS: No significant changes in either UTS or E were observed in the specimens after long-term storage. The 48-month values were not statistically significantly different from the values obtained at 24 hours (p > 0.05). TEM images revealed a normal, intact structure of the collagen fibrils with no signs of degradation or denaturation. CONCLUSION: Long-term storage of EDTA-demineralized human dentin in PBS solution did not cause any significant reduction of its mechanical properties. The structure of the collagen fibrils as observed by TEM appeared normal and did not seem affected by long-term storage.

Analysis of Variance↗

PubMed2000

An ultrastructural study of the influence of acidity of self-etching primers and smear layer thickness on bonding to intact dentin.

PURPOSE: The objectives of this study were (1) to determine the depth of demineralization into intact dentin using several self-etching primer systems with different pH values, and (2) to evaluate whether hybridization of intact dentin in Clearfil SE Bond may be affected by variation in the thickness of the smear layers. MATERIALS AND METHODS: Dentin disks were created from mid-coronal dentin in extracted, human third molars. Three self-etching primer systems (Clearfil Liner Bond II, Liner Bond 2V, and SE Bond) were applied separately to these disks to evaluate how deep self-etching systems penetrate through smear layers into intact dentin. Dentin treated with All-Bond 2 using the "no-etch" technique was used as a control group. In the second part of the study, dentin disks with different smear-layer thicknesses were produced. The cryofractured control group was devoid of a smear layer. The experimental teeth were ground with 60-, 180-, or 600-grit SiC paper and bonded using SE Bond. Dentin disks were bonded together and examined with TEM. RESULTS: All-Bond 2 did not etch beyond the smear layer. The three self-etching primers etched beyond the smear layer to form true hybrid layers within intact dentin. This layer was thickest with Liner Bond 2 (ca 1.2 to 1.4 microns), but very thin (0.5 micron) using both Liner Bond 2V and SE Bond. Application of SE Bond to dentin of different surface roughness produced hybridized smear layers of variable thickness. However, the thickness of the underlying true hybrid remained consistent for the four groups (ca 0.4 to 0.5 micron). CONCLUSION: Self-etching primers create thin hybrid layers that incorporate the smear layer. The suspicion that thick smear layers may interfere with the diffusion of self-etching primers into the underlying intact dentin was not confirmed.

Acid Etching, Dental↗

PubMed2000

Effect of smear layers on the bonding of a self-etching primer to dentin.

PURPOSE: The objective of this work was to evaluate the effect of the absence and presence of smear layers on bonds made to dentin using a self-etching primer system, Clearfil SE Bond. MATERIALS AND METHODS: Dentin surfaces with different smear layer thickness were created from mid-coronal sound dentin in extracted, human third molars. The control group was cryofractured to create a bonding surface that was devoid of a smear layer. The experimental teeth were ground with wet 60-, 180- or 600-grit SiC paper. They were bonded using SE Bond, followed by resin composite buildups. After 1 day, bonded specimens were sectioned into multiple 1- x 1-mm beams. Microtensile bond strengths were determined and the results analyzed with ANOVA and the Student Neuman Keuls test. Fractographic study of cross sections of failed interfaces from the dentin side of representative beams was performed using both SEM and TEM. RESULTS: SE Bond produced high bond strengths (ca 50 MPa) to both smear layer-free and smear layer-covered dentin. SEM examination was inadequate to define the exact nature of interfacial failures. TEM observations demonstrated a thin (ca 400 to 500 nm) hybrid layer in the fractured dentin and thicker (ca 1 to 4 microns) hybrid layers on smear layer-covered dentin. This included a thick, hybridized smear layer and a thin, underlying true hybrid layer in the intact dentin. Separation of the two hybrid layers was not evident in interfacial failures. CONCLUSION: Self-etching primers create thin hybrid layers that incorporate the smear layer. This study shows that formation of true hybrid layers occurs irrespective of smear layer thickness and that both hybrid layers may function as a unit during loading without separation. identification of secondary cracks from TEM fractographic analysis exemplifies the complex reaction to tensile stresses in multilayered joint systems that comprise materials of variable compliance.

Acid Etching, Dental↗