1

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.

2

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.

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. .

2

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.

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. .