Hydrofluoric acid on dentin should be avoided

Abstract

Hydrofluoric acid can be used for intra-oral repair of restorations. Contamination of tooth substrate with hydrofluoric acid cannot always be avoided.

Objectives

To investigate the bonding effectiveness to hydrofluoric acid contaminated dentin by, micro-tensile bond strength testing, SEM and TEM.

Methods

For this study, 15 molar teeth were used of which dentin surfaces were subjected to five, different etching procedures. Group A, 37.5% phosphoric acid (Kerr Gel) (control group); group B, 37.5% phosphoric acid followed by 3% hydrofluoric acid (DenMat); group C, 37.5% phosphoric acid, followed by 9.6% hydrofluoric acid (Pulpdent); group D, 3% hydrofluoric acid followed by 37.5%, phosphoric acid; group E, 9.6% hydrofluoric acid followed by 37.5% phosphoric acid. After the bonding procedure (OptiBond FL, Kerr) a composite resin build-up (Clearfil AP-X, Kuraray), was made. After 1 week storage, specimens were prepared for micro-tensile bond testing, SEM- and, TEM-analysis. Data were analyzed using ANOVA and post hoc Tukey’s HSD ( p < 0.05).

Results

In the control group (solely phosphoric acid), the mean μTBS was 53.4 ± 10.6 MPa, which was, significantly higher than any hydrofluoric acid prepared group (group A versus groups B–E, p < 0.001). No, significant differences in μTBS were found between the 3% and 9.6% hydrofluoric acid groups: group B versus group C (13.5 ± 5.5 MPa and 18.7 ± 4.3 MPa, respectively) or group D versus group E (19.9 ± 6.8 MPa and 20.3 ± 4.1 MPa, respectively).

Significance

Due to its adverse effect on the bond strength of composite to dentin, contact of hydrofluoric acid to dentin should be avoided.

Introduction

In line with the minimally invasive dentistry approach, repair of a defective or malfunctioning restoration is the treatment of choice, if possible . Adhesion to porcelain and composite resins can be significantly improved by etching with hydrofluoric acid, as this creates a micro-retentive surface by dissolving in part the glass phase .

The use of hydrofluoric acid is commonly performed as it is the clinical standard procedure to repair defective porcelain restorations. However, hydrofluoric acid should be handled with caution as it is corrosive and a contact poison. Because of its low dissociation constant, it penetrates the tissue more quickly than other mineral acids. Symptoms of exposure to hydrofluoric acid may not be immediately evident as it interferes with nerve function and burns may not initially be painful . Selective intra-oral application of hydrofluoric acid is not always easy and therefore contamination on tooth (substrate) material is sometimes unavoidable.

Exposure of hydrofluoric acid on dentin will not completely remove the smear layer and will produce an amorphous precipitate of fluoride . Thereby, resin-infiltration within dentin is hindered and an adequate resin-impregnated (hybrid) layer can hardly be formed . Both studies focused on morphological changes of the dentinal surface after exposure to hydrofluoric acid, while other implications of such exposure are unknown. Therefore, the purpose of this study was to investigate the bonding effectiveness to hydrofluoric acid contaminated dentin by micro-tensile bond strength (μTBS) testing, scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

Materials and methods

Specimen preparation

Fifteen non-carious human lower third molars were stored in a 0.5% chloramine solution after extraction. All teeth were mounted in gypsum blocks to facilitate specimen manipulation and the occlusal third of the crown was removed using a slow-speed diamond saw (Isomet 1000, Buehler, Lake Bluff, IL, USA). Dentin surfaces were checked with a stereomicroscope (Wild M5A, Heerbrugg, Switzerland) for absence of enamel and pulp tissue. On each tooth, a standard smear layer was produced using a high-speed medium-grit diamond bur (100 μm, No.: 842314014, Komet, Lemgo, Germany) mounted in the MicroSpecimen Former (The University of Iowa, Iowa City, IA, USA). Three dentin surfaces were exposed to each of the five different etching procedures:

  • Group A: Dentin was etched for 15 s with 37.5% phosphoric acid (Kerr Gel, Orange, CA, USA), rinsed for 15 s and air-dried for 5 s (Control).

  • Group B: Dentin was etched for 15 s with 37.5% phosphoric acid, rinsed for 15 s and air-dried for 5 s. Subsequently, dentin was etched for 15 s with 3% hydrofluoric acid (DenMat, Santa Maria, CA, USA), rinsed for 15 s and air-dried for 5 s.

  • Group C: Same as group B, but a 9.6% hydrofluoric acid (Pulpdent Co., Watertown, USA) was used.

  • Group D: Dentin was etched for 15 s with 3% hydrofluoric acid, rinsed for 15 s and air-dried for 5 s. Then dentin was etched for 15 s with 37.5% phosphoric acid, rinsed for 15 s and air-dried for 5 s.

  • Group E: Same as group D, but a 9.6% hydrofluoric acid was used.

After etching, the three-step etch-and-rinse adhesive (OptiBond FL, Kerr) was applied to the dentin surface according to manufacturers’ instructions. Using a silicon mould, a composite (Clearfil AP-X, Kuraray Co., Osaka, Japan) build-up, 5 mm in height, was made in two horizontal increments, each separately polymerized for 20 s. After removing the mould, the build-up was post-cured from buccal and lingual for 20 s each. All light-curing was performed using a LED curing unit (The Cure, Spring Health Products, Norristown, USA; light intensity: >600 mW/cm 2 ).

μTBS-testing

After 1 week of storage in a 0.5% chloramine solution at room temperature, the teeth were sectioned perpendicular to the bonding surface using a diamond saw at slow-speed (Isomet 1000, Buehler Ltd, Lake Bluff, IL, USA) under continuous water cooling to obtain beams of 1.8 mm × 1.8 mm wide. The four central specimens were mounted in the pin-chuck of the MicroSpecimen Former and trimmed at the dentin–composite interface into an hour-glass shape with a bonding surface of approximately 1 mm 2 using a cylindrical fine-grit diamond bur (5835KREF, Komet, Lemgo, Germany) in a high-speed turbine under air/water coolant. Specimens were then fixed to a Ciucchi’s jig with a cyanoacrylate glue (Model Repair II Blue, Dentsply-Sankin, Tochigi, Japan) and stressed at a crosshead speed of 1 mm/min until failure using a universal testing device (LRX, Lloyd, Hampshire, UK) with a load cell of 100 N. μTBS is expressed in MPa as derived from dividing the imposed force (N) at the time of fracture by the bond area (mm 2 ). The occurrence of failure prior to the actual testing was included in the calculation of the mean μTBS as 0 MPa. The highest and the lowest value in each group were excluded. Failure mode was determined at a magnification of 50× using the stereomicroscope (Wild M5A), and recorded as adhesive at interface (interfacial failure), cohesive in resin (including failures within the adhesive layer and/or composite) or cohesive in dentin. One-way analysis of variance (ANOVA) and post hoc Tukey’s HSD multiple comparisons were used to determine statistical differences in μTBS ( p < 0.05).

Feg-SEM fracture analysis

After the initial failure analysis with the stereomicroscope, representative specimens were processed for field-emission-gun scanning electron microscopy (Feg-SEM; Philips XL30, Eindhoven, The Netherlands), using common electron microscopic specimen processing techniques including fixation, dehydration, chemical drying, and gold-sputter coating .

TEM interfacial analysis

For each group, two additional dentin surfaces, prepared similarly as for μTBS-testing, were analyzed with TEM using a standardized preparation protocol, including fixation, dehydration, embedding and diamond-knife ultra-microtomy . Non-demineralized and lab-demineralized (10% formaldehyde-formic acid for 36 h) ultra-thin sections were cut (Ultracut UCT, Leica, Vienna, Austria) and examined unstained and positively stained (5% uranyl acetate for 12 min/saturated lead citrate for 13 min) using TEM (JEM-1200EX II, JEOL, Tokyo, Japan). In order to reveal potential defects, additional specimens immersed in a 50 wt% ammoniac silver nitrate solution, were prepared according to a nanoleakage detection protocol .

Materials and methods

Specimen preparation

Fifteen non-carious human lower third molars were stored in a 0.5% chloramine solution after extraction. All teeth were mounted in gypsum blocks to facilitate specimen manipulation and the occlusal third of the crown was removed using a slow-speed diamond saw (Isomet 1000, Buehler, Lake Bluff, IL, USA). Dentin surfaces were checked with a stereomicroscope (Wild M5A, Heerbrugg, Switzerland) for absence of enamel and pulp tissue. On each tooth, a standard smear layer was produced using a high-speed medium-grit diamond bur (100 μm, No.: 842314014, Komet, Lemgo, Germany) mounted in the MicroSpecimen Former (The University of Iowa, Iowa City, IA, USA). Three dentin surfaces were exposed to each of the five different etching procedures:

  • Group A: Dentin was etched for 15 s with 37.5% phosphoric acid (Kerr Gel, Orange, CA, USA), rinsed for 15 s and air-dried for 5 s (Control).

  • Group B: Dentin was etched for 15 s with 37.5% phosphoric acid, rinsed for 15 s and air-dried for 5 s. Subsequently, dentin was etched for 15 s with 3% hydrofluoric acid (DenMat, Santa Maria, CA, USA), rinsed for 15 s and air-dried for 5 s.

  • Group C: Same as group B, but a 9.6% hydrofluoric acid (Pulpdent Co., Watertown, USA) was used.

  • Group D: Dentin was etched for 15 s with 3% hydrofluoric acid, rinsed for 15 s and air-dried for 5 s. Then dentin was etched for 15 s with 37.5% phosphoric acid, rinsed for 15 s and air-dried for 5 s.

  • Group E: Same as group D, but a 9.6% hydrofluoric acid was used.

After etching, the three-step etch-and-rinse adhesive (OptiBond FL, Kerr) was applied to the dentin surface according to manufacturers’ instructions. Using a silicon mould, a composite (Clearfil AP-X, Kuraray Co., Osaka, Japan) build-up, 5 mm in height, was made in two horizontal increments, each separately polymerized for 20 s. After removing the mould, the build-up was post-cured from buccal and lingual for 20 s each. All light-curing was performed using a LED curing unit (The Cure, Spring Health Products, Norristown, USA; light intensity: >600 mW/cm 2 ).

μTBS-testing

After 1 week of storage in a 0.5% chloramine solution at room temperature, the teeth were sectioned perpendicular to the bonding surface using a diamond saw at slow-speed (Isomet 1000, Buehler Ltd, Lake Bluff, IL, USA) under continuous water cooling to obtain beams of 1.8 mm × 1.8 mm wide. The four central specimens were mounted in the pin-chuck of the MicroSpecimen Former and trimmed at the dentin–composite interface into an hour-glass shape with a bonding surface of approximately 1 mm 2 using a cylindrical fine-grit diamond bur (5835KREF, Komet, Lemgo, Germany) in a high-speed turbine under air/water coolant. Specimens were then fixed to a Ciucchi’s jig with a cyanoacrylate glue (Model Repair II Blue, Dentsply-Sankin, Tochigi, Japan) and stressed at a crosshead speed of 1 mm/min until failure using a universal testing device (LRX, Lloyd, Hampshire, UK) with a load cell of 100 N. μTBS is expressed in MPa as derived from dividing the imposed force (N) at the time of fracture by the bond area (mm 2 ). The occurrence of failure prior to the actual testing was included in the calculation of the mean μTBS as 0 MPa. The highest and the lowest value in each group were excluded. Failure mode was determined at a magnification of 50× using the stereomicroscope (Wild M5A), and recorded as adhesive at interface (interfacial failure), cohesive in resin (including failures within the adhesive layer and/or composite) or cohesive in dentin. One-way analysis of variance (ANOVA) and post hoc Tukey’s HSD multiple comparisons were used to determine statistical differences in μTBS ( p < 0.05).

Feg-SEM fracture analysis

After the initial failure analysis with the stereomicroscope, representative specimens were processed for field-emission-gun scanning electron microscopy (Feg-SEM; Philips XL30, Eindhoven, The Netherlands), using common electron microscopic specimen processing techniques including fixation, dehydration, chemical drying, and gold-sputter coating .

TEM interfacial analysis

For each group, two additional dentin surfaces, prepared similarly as for μTBS-testing, were analyzed with TEM using a standardized preparation protocol, including fixation, dehydration, embedding and diamond-knife ultra-microtomy . Non-demineralized and lab-demineralized (10% formaldehyde-formic acid for 36 h) ultra-thin sections were cut (Ultracut UCT, Leica, Vienna, Austria) and examined unstained and positively stained (5% uranyl acetate for 12 min/saturated lead citrate for 13 min) using TEM (JEM-1200EX II, JEOL, Tokyo, Japan). In order to reveal potential defects, additional specimens immersed in a 50 wt% ammoniac silver nitrate solution, were prepared according to a nanoleakage detection protocol .

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Nov 30, 2017 | Posted by in Dental Materials | Comments Off on Hydrofluoric acid on dentin should be avoided
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