The durability of phosphoric acid promoted bioglass–dentin interaction layer

Abstract

Objectives

Phosphoric acid-Bioglass 45S5 paste can create an interaction layer formed of calcium-phosphate crystals on the dentin surface. In this study, the efficiency of decreasing the dentin permeability exerted by the interaction layer formed between bioglass and dentin was compared to a resin-containing oxalate desensitizing agent (MS Coat One) and a resin-free oxalate desensitizing agent (Super Seal).

Methods

Dentin permeability was measured before/after a brushing abrasion challenge, followed by examining the top and the fractured dentin surfaces with a field emission scanning electron microscope. Moreover, the chemical nature of the compounds formed on top of the dentin surface was examined using the field emission scanning electron microscope (FE-SEM) equipped with an energy-dispersive X-ray spectroscope (EDS), and the crystalline structures of the dentinal surfaces were examined by X-ray diffraction (XRD).

Results

The results showed that application of 45S5 bioglass paste to dentin was able to occlude patent dentinal tubule orifices with a layer of calcium-phosphate crystals, while the oxalate containing agents were able to form small crystals which were found in dentinal tubule orifices and scattered along the superficial parts of the dentinal tubule lumen. The brushing-abrasion challenge significantly increased the permeability of dentin treated by Super Seal and MS Coat One, while these challenges had no significant effect on the dentin permeability of specimens treated with 45S5 bioglass paste.

Significance

The new technique provided better durability than two products available on the market. Moreover, our previous research showed the biocompatibility of using this technique on dental pulp cells, suggesting that this technique can aid in treating dentin hypersensitivity cases.

Introduction

Dentin hypersensitivity is one of the major challenges in dental practice . The hydrodynamic theory hypothesized that the increase in the flow of dentinal tubular fluid, which has patent orifices, activates the nerves situated at the inner ends of the dentinal tubules or in the outer layers of the pulp. Fluid flow through these tubules varies with the fourth power of the dentinal-tubules radii according to Poiseuille’s law , thus any partial reduction of the functional radii of the dentinal tubules will lead to significant reduction in the fluid flow, with consequent reduction in dentin hypersensitivity. Many agents have been employed for treating dentin hypersensitivity by occluding the dentinal tubules, however most of them showed only a temporary effect in the clinical situation because they were gradually removed by food friction, and change in the pH in the oral cavity .

A new aid for treating dentin hypersensitivity was proposed, dependent on the interaction of 45S5 bioglass with phosphoric acid, that resulted in the formation of a calcium-phosphate rich layer named as the “Interaction Layer” on the top of dentin surfaces, moreover, there was evidence of good biocompatibility when using this technique on dentin . However, there was little information regarding the durability of using the aforementioned technique under simulated oral conditions. In this study the brushing abrasion durability of this technique was compared to two oxalate-containing dentin desensitizing agents by examining the top dentin surfaces after application of the agents, and the interface between dentin and the tested materials, using the field emission scanning electron microscope (FE-SEM) equipped with an energy-dispersive X-ray spectroscope (EDS); moreover, the crystalline structures of the dentinal surfaces were examined by X-ray diffraction (XRD) before and after the brushing-abrasion challenge. Dentin permeability before and after the brushing-abrasion challenge was tested. The hypothesis adopted in this experiment was that the interaction layer formed on top of the dentin surface would show better resistance to the brushing-abrasion challenge when compared to the commercially available materials.

Materials and methods

Dentin specimen preparation

One hundred and ten extracted non-carious third molars were used following the guidelines approved by Tokyo Medical and Dental University Ethical Committee. The enamel on the buccal surface and the superficial dentin of each tooth were removed using a slow speed saw (Isomet, Buehler, Lake Bluff, IL, USA). Eighty 0.5-mm-thick dentin discs were prepared from dentin just above the highest part of the pulp chamber. All the dentin surfaces were ultrasonicated for 30 s, etched with 0.5% M EDTA (pH 7.4) for 2 min, and then rinsed with air/water for 30 s . The teeth were randomly divided into four groups where Group I specimens served as controls. The resin-free oxalate containing desensitizing agent (Super Seal, Phoenix Dental, Fenton, MI, USA) and that with resin (MS Coat One, Sun Medical, Shiga, Japan) were applied in Groups II and III, respectively. Application of 45S5 bioglass was in Group IV ( Table 1 ).

Table 1
Materials used in study.
Materials Compositions Procedures
Super Seal (Phoenix Dental, Fenton, MI, USA) Oxalic acid, potassium salt, water Apply directly
MS Coat One (Sun Medical, Tokyo, Japan) Copolymer with salfonic acid group, oxalic acid, water Apply directly
45S5 Bioglass (NovaMin® Technology, USA) SiO 2 (45 wt%), Na 2 O (24.5 wt%), CaO (24.4 wt%), P 2 O 5 (6 wt%) Mix 0.1 g of 45S5 bioglass to 0.2 ml of 50% phosphoric acid
Clearfil SE Bond (Kuraray Medical, Tokyo, Japan) Bond: MDP, HEMA, MFM, PI, accelerators, initiators, microfillers Apply adhesive, gently air dry, light cure (10 s)
HEMA, 2-hydroxyethyl methacrylate; bis-GMA, bisphenyl glycidyl methacrylate; MDP, 10-methacryloxydecyl dihydrogen phosphate; MFM, multifunctional methacrylate; PI, photoinitiator; CA, catalysis.

Tested materials application

The two oxalate desensitizing agents were applied to the dentin surfaces according to the manufacturers’ instructions ( Table 1 ). Powder of 45S5 bioglass (NovaMin®, 5 μm average particle, NovaMin Technology, USA) composed of Na 2 O, CaO, P 2 O 5 , and SiO 2 was mixed with 50% phosphoric acid to form a gel, which was applied as a thin layer on the dentin specimens of Group IV. The phosphoric acid solution was prepared by diluting phosphoric acid (85 wt%, Wako Chemicals, Osaka, Japan) in distilled water with 1:1 volume ratio. A layer of a bonding agent (SE Bond, Kuraray Medical, Tokyo, Japan) was applied on the bioglass gel, then light cured ( Table 1 ). All of the specimens were stored in artificial saliva (1.0 mM CaCl 2 , 3.0 mM KH 2 PO 4 , 100 mM acetate, 100 mM NaCl, 0.02% NaN 3 ; pH 6.3) for 24 h . After storage, the thin layer of the bonding agent was gently removed from the bioglass gel using a sharp excavator followed by applying a strong air/water stream for 30 s.

Dentin permeability analysis

The dentin permeability of all groups ( n = 10) was measured using an in vitro fluid-transport system before and after the brushing-abrasion challenge. The system ( Fig. 1 A) was composed of a reservoir having phosphate-buffered saline (PBS, pH 7.0, Wako Pure Chemical, Osaka, Japan) solution which was placed between a tank of compressed nitrogen gas and a split-chamber device . A pressure of 0.070 MPa was used to force the PBS through polyethylene tubing to the split-chamber holding the dentin disk. A 25 μl micropipette connected to the polyethylene tubing was used to insert an air bubble into the tubing. The rate of air bubble movement was monitored using a millimeter ruler and the rate of the air bubble progression through the tubing was recorded every 2 min over a 6 min interval. The rate of dentin permeability was measured for each specimen at baseline and after application of the tested materials. Therefore each disc served as its own control . This value represented 100% permeability, which represented the baseline permeability for the disk. After application of the tested materials, the dentin permeability was re-assessed and the percent of dentin permeability reduction was calculated.

Fig. 1
Dentin permeability measurement and brushing abrasion challenge. (A) Scheme of the apparatus and the split-chamber device used to measure dentin permeability. The movement of the air bubble towards the split chamber represents the rate of fluid filtering across the dentin specimens. (B) Scheme of the brushing abrasion apparatus.

Brushing-abrasion challenge

Ten specimens which had their permeability measured were attached to an acrylic resin block and then exposed to 6000 cycles of brushing-abrasion challenge ( Fig. 1 B) using a tabletop robot (TT-C3-1-2020-10B, IAI Co, Shizuoka, Japan). The brushing-abrasion challenge was performed using a nylon bristle toothbrush (PROSPEC adult hard, GC, Tokyo, Japan.) attached to a lever arm conducting back and forth motions (load of 250 gf, 150 mm/stroke, 100 stroke/min) under wet conditions. The dentin permeability of specimens was re-measured after the brushing-abrasion challenge and the percentage of permeability reduction was calculated.

FE-SEM surface examination for dentinal orifices closure

Five dentin specimens were selected before/after the durability challenge in Group I (control group). Five dentin specimens were selected directly after applying the tested materials and another five specimens were selected after performing the durability and conductance experiment from Groups II, III, and IV. The specimens were gold coated and then evaluated using a field emission scanning electron microscope FE-SEM (S-4500, Hitachi High-technologies, Hitachinaka, Japan).

Cryofracture examination

Five dentin specimens were selected before/after the durability challenge in Group I (control group). Five dentin specimens were selected directly after applying the tested materials and another five specimens were selected after performing the durability and conductance experiment from Groups II, III, and IV. A slit was made along the pulpal side of each dentin disk using the Isomet saw under water cooling to facilitate cryofracture of the disk into two halves . The specimens were gold-sputtered and examined along their fractured edges using FE-SEM.

XRD examination of the dentin surfaces

In order to detect the type of crystals formed on the dentinal surfaces of Groups II, III, and IV five dentin specimens were selected directly after applying the tested materials and another five selected after performing the durability and conductance experiments. Five specimens from each group were examined by X-ray diffractometry (RAD IIA, Rigaku Denki, Tokyo, Japan) with CuKα radiation of 40 KV and Ni filter. The diffraction intensities were measured 4 times for each specimen by a scanning technique in the range 2 = 10–60° by a 0.1° step for 2 s/point.

FE-SEM/EDS interface examination

The same specimens of Groups II, III, and IV tested by XRD where examined to detect the chemical characterization of chemical compounds formed on dentinal surfaces. The specimens were dehydrated in ascending concentration of ethanol and then embedded in Epoxy resin (Epoxicure resin, Buehler, Lake Bluff, IL, USA). After curing of the resin, the specimens were sectioned perpendicular to the interface to give 1.5-mm-thick slabs. The cut surfaces were polished, gold-coated, and then examined by FE-SEM/EDS (S-4500, Hitachi, Hitachinaka, Japan). Line scans were performed across the treated dentinal surfaces with an EDS attachment for the following elements: phosphate, calcium for all groups; oxygen and carbon in Groups II and III; silica in Group IV.

Statistical analysis

The Mann–Whitney test ( p < 0.05) was used to compare the effects of using the different tested materials on the dentin permeability of all groups before/after the durability challenge, whenever there was a significant difference, so the Steel–Dwass test was applied ( p < 0.05). Wilcoxon Signed-Rank was used to compare the effect of the brushing-abrasion test on each material tested.

Materials and methods

Dentin specimen preparation

One hundred and ten extracted non-carious third molars were used following the guidelines approved by Tokyo Medical and Dental University Ethical Committee. The enamel on the buccal surface and the superficial dentin of each tooth were removed using a slow speed saw (Isomet, Buehler, Lake Bluff, IL, USA). Eighty 0.5-mm-thick dentin discs were prepared from dentin just above the highest part of the pulp chamber. All the dentin surfaces were ultrasonicated for 30 s, etched with 0.5% M EDTA (pH 7.4) for 2 min, and then rinsed with air/water for 30 s . The teeth were randomly divided into four groups where Group I specimens served as controls. The resin-free oxalate containing desensitizing agent (Super Seal, Phoenix Dental, Fenton, MI, USA) and that with resin (MS Coat One, Sun Medical, Shiga, Japan) were applied in Groups II and III, respectively. Application of 45S5 bioglass was in Group IV ( Table 1 ).

Table 1
Materials used in study.
Materials Compositions Procedures
Super Seal (Phoenix Dental, Fenton, MI, USA) Oxalic acid, potassium salt, water Apply directly
MS Coat One (Sun Medical, Tokyo, Japan) Copolymer with salfonic acid group, oxalic acid, water Apply directly
45S5 Bioglass (NovaMin® Technology, USA) SiO 2 (45 wt%), Na 2 O (24.5 wt%), CaO (24.4 wt%), P 2 O 5 (6 wt%) Mix 0.1 g of 45S5 bioglass to 0.2 ml of 50% phosphoric acid
Clearfil SE Bond (Kuraray Medical, Tokyo, Japan) Bond: MDP, HEMA, MFM, PI, accelerators, initiators, microfillers Apply adhesive, gently air dry, light cure (10 s)
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Nov 25, 2017 | Posted by in Dental Materials | Comments Off on The durability of phosphoric acid promoted bioglass–dentin interaction layer
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