TEM characterization of a silorane composite bonded to enamel/dentin

Abstract

Objectives

The low-shrinking composite composed of combined siloxane–oxirane technology (Filtek Silorane, 3M ESPE, Seefeld, Germany) required the development of a specific adhesive (Silorane System Adhesive, 3M ESPE), in particular because of the high hydrophobicity of the silorane composite. The purpose of this study was to characterize the interfacial ultra-structure at enamel and dentin using transmission electron microscopy (TEM).

Methods

Non-demineralized/demineralized 70–90 nm sections were prepared following common TEM specimen processing procedures.

Results

TEM revealed a typical twofold build-up of the adhesive resin, resulting in a total adhesive layer thickness of 10–20 μm. At bur-cut enamel, a tight interface without distinct dissolution of hydroxyapatite was observed. At bur-cut dentin, a relatively thin hybrid layer of maximum a few hundreds of nanometer was formed without clear surface demineralization. No clear resin tags were formed. At fractured dentin, the interaction appeared very superficial (100–200 nm). Distinct resin tags were formed due to the absence of smear plugs. Silver-nitrate infiltration showed a varying pattern of both spot- and cluster-like appearance of nano-leakage. Traces of Ag were typically detected along some part of the enamel–adhesive interface and/or between the two adhesive resin layers. Substantially more Ag-infiltration was observed along the dentin–adhesive interface of bur-cut dentin, as compared to that of fractured dentin.

Conclusions

The nano-interaction of Silorane System Adhesive should be attributed to its relatively high pH of 2.7. The obtained tight interface at both enamel and dentin indicates that the two-step self-etch adhesive effectively bridged the hydrophilic tooth substrate with the hydrophobic silorane composite.

Introduction

Recently, a new class of low-shrinking composites based on silorane technology (Filtek Silorane, 3M ESPE, Seefeld, Germany) was introduced. The silorane resin replaces the conventionally used methacrylate resin matrix within conventional dental composites, thereby providing lower polymerization shrinkage as well as better hydrolytic stability . Since long, reduced polymerization shrinkage is a highly desired property of composites in order to avoid primary clinically problems that are typically associated with polymerization shrinkage stress, such as there are adhesive-tooth de-bonding, post-operative sensitivity, restoration marginal discoloration and defects, caries recurrence, enamel cracks, etc. .

As the resin matrix of the silorane composite significantly differs from that of conventional methacrylate-based composites, a new adhesive needed to be designed and developed to enable bonding of the silorane composite to tooth enamel and dentin. Filtek Silorane therefore comes with a two-step self-etch adhesive, called Silorane System Adhesive (3M ESPE, SSA). It still possesses features of conventional methacrylate adhesives, especially with regard to its bonding mechanism to tooth tissue, while adaptation was needed, especially to make it compatible with the highly hydrophobic silorane matrix. The adhesive somewhat differs from a typical two-step self-etch adhesive, since it involves the application of two resin solutions, of which the first one (Silorane System Adhesive – Self-etch Primer or SSA-Primer) is rather hydrophilic to bond to tooth tissue and the second solution (Silorane System Adhesive – Bond or SSA-Bond) is on the contrary quite hydrophobic in order to adequately bridge the hydrophilic tooth substrate with the hydrophobic silorane composite. For this reason, each resin solution needs to be light-cured separately.

The purpose of this study was to characterize ultra-morphologically the interface complex of the low-shrinking silorane composite Filtek Silorane (3M ESPE) bonded to enamel and dentin using the two-step self-etch adhesive Silorane System Adhesive. For this, transmission electron microscopy (TEM) is the method of choice because of its high structural resolution and relatively low artifact incidence.

Materials and methods

Six non-carious human third molars were stored in 0.5% chloramine solution at 4 °C and used within one month after extraction. The teeth were randomly divided into 3 groups (‘bur-cut enamel’, ‘bur-cut dentin’ or ‘fractured dentin’; see below). All teeth were mounted in gypsum blocks in order to ease manipulation. For the enamel specimens, lingual and buccal enamel was flattened using a medium-grit (100 μm) diamond bur (842, Komet, Lemgo, Germany) in a water-cooled high-speed contra-angle handpiece mounted in the MicroSpecimen Former (The University of Iowa, Iowa, IA, USA). For bur-cut dentin specimens, the occlusal third of the crown was removed at the level of mid-coronal dentin using a slow-speed diamond saw (Isomet 1000, Buehler, lake Bluff, IL, USA), after which a standard smear layer was produced by the medium-grit (100 μm) diamond bur, which was also used to prepare the enamel specimens. From the last two teeth, a shallow 1–2 mm deep groove was cut circumferentially around the tooth at the level of mid-coronal dentin, after which the coronal part was fractured using a forceps to produce a fractured dentin surface free of smear debris. All dentin surfaces were carefully verified for absence of enamel and/or pulp tissue using a stereo-microscope (Wild M5A, Wild Heerbrugg AG, Heerbrugg, Switzerland). The adhesive Silorane System Adhesive (3M ESPE, SSA) with the silorane composite Filtek Silorane (3M ESPE) was then applied, strictly according to the manufacturer’s instructions ( Table 1 ). To appraise the ultra-morphological structure of the silorane composite, TEM specimens of two conventional micro-hybrid resin composites (Clearfil AP-X, Kuraray, Okayama, Japan; Filtek Z100, 3M ESPE) were prepared using a silicon mold. Light-curing was performed using an Optilux 500 (Demetron/Kerr, Danbury, CT, USA) device with a light output not less than 600 mW/cm 2 . After bonding procedures, specimens were stored for 1 day in tap water at 37 °C.

Table 1
Materials used.
Materials Composition (wt%) Application
Silorane System Adhesive – Self-Etch Primer
Lot. 292319
pH = 2.7
3M ESPE, Seefeld, Germany
15–25% 2-hydroxyethyl methacrylate (HEMA); 15–25% bisphenol a diglycidyl ether dimethacrylate (BIS-GMA); 10–15% water; 10–15% ethanol; 5–15% phosphoric acid–methacryloxy–hexylesters; 8–12% silane treated silica; 5–10% 1,6-hexanediol dimethacrylate; <5% copolymer of acrylic and itaconic acid; <5% (dimethylamino) ethyl methacrylate; <3% dl -camphorquinone; <3% phosphine oxide (1) Shake the bottle briefly before dosing so that the primer becomes less viscous.
(2) Place one drop of primer into the dosing well, then close the dosing well to protect the primer from light and prevent the evaporation of the solvent.
(3) Apply the primer to the entire surface of the cavity and massage over the entire area for 15 s.
(4) Use a gentle stream of air until the primer is spread to an even film and does not move any longer.
(5) Cure the primer for 10 s.
Silorane System Adhesive – Bond
Lot. 292274
3M ESPE
70–80% substituted dimethacrylate; 5–10% silane treated silica; 5–10% triethylene glycol dimethacrylate (TEGDMA); <5% phosphoric acid–methacryloxy–hexylesters; <3% dl -camphorquinone; <3% 1,6-hexanediol dimethacrylate (1) Shake bottle briefly before dosing so that the bond becomes less viscous.
(2) Place one drop of bond in the dosing well and close the dosing well to protect the bond from light.
(3) Apply the bond to the entire area of the cavity.
(4) Use a gentle stream of air until the bond is spread to an even film and does not move any longer.
(5) Cure the bond for 10 s.
Filtek™ Silorane
Lot. 203905, Shade A3
3M ESPE
5–15% 3,4-epoxycyclohexylethylcyclo-polymethylsiloxane; 5–15% bis-3,4-epoxycyclohexylethyl-phenylmethylsilane; 50–70% silanized quartz;10–20% yttriumfluoride; camphorquinone (1) The thickness of the individual increments must not exceed 2.5 mm.
(2) Cure the filling material for 40 s.

The specimens were processed for TEM according to the procedure described in detail before by Van Meerbeek et al. . Non-demineralized and lab-demineralized ultra-thin sections were cut (Ultracut UCT, Leica, Vienna, Austria) and examined unstained and positively stained (3% uranyl acetate for 12 min/lead citrate for 13 min) using TEM (JEM-1200EX II, JEOL, Tokyo, Japan). After observation of the resin–enamel interface sections, the TEM grids were additionally exposed for 5 s to 0.1N HCl, and subsequently carefully rinsed with distilled water to remove all dissolved mineral components following a method described before by Hanning et al. . Doing so, the same spots could be imaged by TEM before and after decalcification. In order to reveal potential porous zones in the interface complex, additional specimens were immersed in a 50 wt% ammoniacal silver nitrate solution according to a nanoleakage-detection protocol previously described by Tay et al. .

Materials and methods

Six non-carious human third molars were stored in 0.5% chloramine solution at 4 °C and used within one month after extraction. The teeth were randomly divided into 3 groups (‘bur-cut enamel’, ‘bur-cut dentin’ or ‘fractured dentin’; see below). All teeth were mounted in gypsum blocks in order to ease manipulation. For the enamel specimens, lingual and buccal enamel was flattened using a medium-grit (100 μm) diamond bur (842, Komet, Lemgo, Germany) in a water-cooled high-speed contra-angle handpiece mounted in the MicroSpecimen Former (The University of Iowa, Iowa, IA, USA). For bur-cut dentin specimens, the occlusal third of the crown was removed at the level of mid-coronal dentin using a slow-speed diamond saw (Isomet 1000, Buehler, lake Bluff, IL, USA), after which a standard smear layer was produced by the medium-grit (100 μm) diamond bur, which was also used to prepare the enamel specimens. From the last two teeth, a shallow 1–2 mm deep groove was cut circumferentially around the tooth at the level of mid-coronal dentin, after which the coronal part was fractured using a forceps to produce a fractured dentin surface free of smear debris. All dentin surfaces were carefully verified for absence of enamel and/or pulp tissue using a stereo-microscope (Wild M5A, Wild Heerbrugg AG, Heerbrugg, Switzerland). The adhesive Silorane System Adhesive (3M ESPE, SSA) with the silorane composite Filtek Silorane (3M ESPE) was then applied, strictly according to the manufacturer’s instructions ( Table 1 ). To appraise the ultra-morphological structure of the silorane composite, TEM specimens of two conventional micro-hybrid resin composites (Clearfil AP-X, Kuraray, Okayama, Japan; Filtek Z100, 3M ESPE) were prepared using a silicon mold. Light-curing was performed using an Optilux 500 (Demetron/Kerr, Danbury, CT, USA) device with a light output not less than 600 mW/cm 2 . After bonding procedures, specimens were stored for 1 day in tap water at 37 °C.

Table 1
Materials used.
Materials Composition (wt%) Application
Silorane System Adhesive – Self-Etch Primer
Lot. 292319
pH = 2.7
3M ESPE, Seefeld, Germany
15–25% 2-hydroxyethyl methacrylate (HEMA); 15–25% bisphenol a diglycidyl ether dimethacrylate (BIS-GMA); 10–15% water; 10–15% ethanol; 5–15% phosphoric acid–methacryloxy–hexylesters; 8–12% silane treated silica; 5–10% 1,6-hexanediol dimethacrylate; <5% copolymer of acrylic and itaconic acid; <5% (dimethylamino) ethyl methacrylate; <3% dl -camphorquinone; <3% phosphine oxide (1) Shake the bottle briefly before dosing so that the primer becomes less viscous.
(2) Place one drop of primer into the dosing well, then close the dosing well to protect the primer from light and prevent the evaporation of the solvent.
(3) Apply the primer to the entire surface of the cavity and massage over the entire area for 15 s.
(4) Use a gentle stream of air until the primer is spread to an even film and does not move any longer.
(5) Cure the primer for 10 s.
Silorane System Adhesive – Bond
Lot. 292274
3M ESPE
70–80% substituted dimethacrylate; 5–10% silane treated silica; 5–10% triethylene glycol dimethacrylate (TEGDMA); <5% phosphoric acid–methacryloxy–hexylesters; <3% dl -camphorquinone; <3% 1,6-hexanediol dimethacrylate (1) Shake bottle briefly before dosing so that the bond becomes less viscous.
(2) Place one drop of bond in the dosing well and close the dosing well to protect the bond from light.
(3) Apply the bond to the entire area of the cavity.
(4) Use a gentle stream of air until the bond is spread to an even film and does not move any longer.
(5) Cure the bond for 10 s.
Filtek™ Silorane
Lot. 203905, Shade A3
3M ESPE
5–15% 3,4-epoxycyclohexylethylcyclo-polymethylsiloxane; 5–15% bis-3,4-epoxycyclohexylethyl-phenylmethylsilane; 50–70% silanized quartz;10–20% yttriumfluoride; camphorquinone (1) The thickness of the individual increments must not exceed 2.5 mm.
(2) Cure the filling material for 40 s.

The specimens were processed for TEM according to the procedure described in detail before by Van Meerbeek et al. . Non-demineralized and lab-demineralized ultra-thin sections were cut (Ultracut UCT, Leica, Vienna, Austria) and examined unstained and positively stained (3% uranyl acetate for 12 min/lead citrate for 13 min) using TEM (JEM-1200EX II, JEOL, Tokyo, Japan). After observation of the resin–enamel interface sections, the TEM grids were additionally exposed for 5 s to 0.1N HCl, and subsequently carefully rinsed with distilled water to remove all dissolved mineral components following a method described before by Hanning et al. . Doing so, the same spots could be imaged by TEM before and after decalcification. In order to reveal potential porous zones in the interface complex, additional specimens were immersed in a 50 wt% ammoniacal silver nitrate solution according to a nanoleakage-detection protocol previously described by Tay et al. .

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Nov 30, 2017 | Posted by in Dental Materials | Comments Off on TEM characterization of a silorane composite bonded to enamel/dentin
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