The clinical procedure to adhesively lute ceramic restorations is complex, by which a simplified application procedure is desirable.
Silane-coupling monomers degrade in water; hydrolysis is avoided when the silane-coupling monomer is added to the cement’s hydrophobic paste. Once mixed, the silane-coupling monomer hydrolyses to chemically interact with the ceramic surface.
The novel silane-containing self-adhesive cement combined efficient silane-coupling ability onto glass ceramics with adequate self-adhesiveness onto dentin.
Hydrofluoric-acid etching followed by silanization is a routine clinical protocol for durable bonding to glass ceramics. Simplifying ceramic-bonding procedures, new technological developments involve the inclusion of a silane coupling agent in a self-adhesive composite cement. To investigate the effectiveness of the incorporated silane coupling agent, shear bond strength (SB) to ceramic and dentin, contact angle of water (CA), transmission electron microscopy (TEM), X-ray diffraction (XRD) and 29 Si nuclear magnetic resonance (NMR) assessments were correlatively conducted.
Materials and Methods
SB to glass ceramic was measured without (‘immediate’) and with (‘aged’) 50K thermocycles upon application of (1) the silane-containing self-adhesive composite cement Panavia SA Cement Universal (‘SAU’), being light-cured: ‘SAU_light’, (2) ‘SAU_chem’: chemically cured SAU, (3) ‘SAP_light’: light-cured Panavia SA Cement Plus (‘SAP’), and (4) ‘SAP_CP’: SAP light-cured after separate silanization using Clearfil Ceramic Primer Plus (‘CP’). CA was also measured on glass ceramic. The cement pastes before and upon mixing were characterized using 29 Si NMR. SB of SAU or SAP onto dentin was measured. Finally, the cement-dentin interface was characterized by TEM and XRD.
The immediate and aged SB to glass ceramic of SAU did not significantly differ from those of SAP_CP, while they were significantly higher than those of SAP. CA of SAU did not significantly differ from that of SAP_CP, but it was significantly higher than CA of SAP. 29 Si NMR revealed siloxane bonds after mixture. SB of SAU and SAP to dentin did not show any significant difference. SEM, TEM and XRD confirmed tight and chemical interaction, respectively.
Incorporating silane in a 10-MDP-based self-adhesive composite cement combined efficient silane-coupling ability at the ceramic surface with effective bonding ability at dentin.
Dental glass ceramics, such as leucite and lithium-disilicate glass ceramics, are nowadays often used to adhesively restore teeth because of the patients’ increasing demand for esthetics and the improved computer-aided-design/computer-aided-manufacturing (CAD/CAM) systems. Strongly upcoming alternative restoratives are resin-based composite CAD/CAM blocks, like they are today in Japan promoted, as restorations made from these blocks are reimbursed by the Japanese health insurance systems to reduce metal consumption .
The mechanisms involved mainly in bonding to dental ceramics and composites are micro-mechanical interlocking and chemical binding . Micro-mechanical interlocking can be obtained by either etching glass-rich ceramics, commonly using hydrofluoric-acid (HF), or by sandblasting, which is preferentially recommended to provide micro-mechanical interlocking at the surface of composite CAD/CAM blocks. Upon HF etching or sandblasting, additional chemical binding to both ceramic and composite CAD/CAM restorations is achieved through silanization using dedicated ceramic primers. They contain silane coupling agents, of which γ-methacryloxypropyl trimethoxysilane (γ-MPTS) is most commonly used . Silane coupling occurs when the alkoxy groups of the bi-functional silane molecule hydrolyse to silanols, upon which monomer adsorption to the ceramic/composite substrate occurs through condensation . It is difficult to maintain the silanol status for a long time, limiting the shelf life of silane primers. Most commercial one-bottle and thus silane-coupling primers are supplied in a water-free solvent, upon which the silane is activated when clinically applied. To avoid having to use the rather caustic HF, even not used in certain countries, and to simplify ceramic-bonding procedures, combined etching and silanization has more recently been made possible with the introduction of Monobond Etch & Prime (Ivoclar Vivadent, Schaan, Liechtenstein) . Previous research showed that silane-containing so-called ‘universal’ adhesives did not have enough silane-coupling effect . The silane-coupling monomer γ-MPTS was found to have undergone condensation in the adhesive bottle because of the high acidic nature of the adhesive solution in the presence of water . Therefore, separate silanization using a ceramic primer remained recommended when using such a silane-containing adhesive.
However, the clinical procedure to adhesively lute ceramic restorations, involving a multi-step surface treatment of both the tooth prep (or core build-up) and the ceramic restoration, remains complex. Therefore, there is a definite demand to simplify this application procedure. Recently, a new silane-containing self-adhesive cement was developed and introduced on the market. The silane-coupling efficiency of this silane-containing self-adhesive cement has not yet (independently) been determined.
In this study, the silane effect of the new silane-containing self-adhesive cement Panavia SA Cement Universal (Kuraray Noritake, Tokyo, Japan; ‘SAU’) and its bonding ability were investigated. The silane effect was tested using a conventional shear bond-strength approach in light- and self-cure mode onto glass ceramic, as compared with that upon luting with a silane-free self-adhesive cement and upon separate silanization with a ceramic primer followed by luting with the silane-free self-adhesive cement. The contact angle of water onto the same glass ceramic exposed to luting agents, with the silane-free self-adhesive cement applied with/without the separate silane primer, was measured to determine surface hydrophobicity/hydrophilicity. Mechanistic research involved characterization of the silane-coupling monomer status in the new cement using nuclear magnetic resonance (NMR). Finally, the bonding efficacy onto dentin was determined using shear bond-strength testing as compared to that of the silane-free self-adhesive cement with/without separate prior silanization, while the resultant cement-dentin interfaces were ultra-morphologically characterized using scanning/transmission electron microscopy (S/TEM). The null hypotheses tested in this study were that (1) the new silane-containing cement does not have a silane-coupling effect onto glass ceramic, (2) the new silane-containing self-adhesive cement bonded less effectively to dentin than the silane-free self-adhesive cement, and both irrespective of light-curing or chemical curing.
Materials and Methods
Shear-bond strength onto leucite glass-ceramic plates
Leucite-based glass-ceramic IPS Empress CAD blocks (Ivoclar Vivadent; Shade A2) were cut into disks 10 × 10 mm wide and 1.0 mm in thickness. The surface was polished using a 15 μm diamond lapping film (Struers, Ballerup, Denmark) to a smooth surface, reducing the potential for mechanical micro-retention. Four different cement protocols were applied: (1) ‘SAU_light’: SAU cured by light, (2) ‘SAU_chem’: SAU chemically cured, (3) ‘SAP_light’: Panavia SA Cement Plus (Kuraray Noritake), a silane-free self-adhesive cement, cured by light, and (4) ‘SAP_CP’: Clearfil Ceramic Primer Plus (Kuraray Noritake; ‘CP’) applied and air-dried prior to the application of SAP, which is light-cured ( Table 1 ). Zirconia cylinder blocks (Tosoh, Tokyo, Japan) with a 3.6-mm diameter were sandblasted using a Shofu High Blaster (Shofu, Kyoto, Japan) with 50 μm alumina particles (Shofu), followed by silanization using Clearfil Ceramic primer Plus (Kuraray Noritake); the blocks were luted using one of the four experimental luting protocols onto the glass-ceramic disks using finger pressure (corresponding to a pressure of about 2.2 MPa) . The cement, except for the SAU_chem experimental group when the cement was allowed to self-cure for 30 min, was light-cured for 40 s from two opposing directions (totaling a 80-s curing time) using F3 mode (high-intensity blue LED mode) of a G-Light Prima II Plus (GC, Tokyo, Japan) light-emitting diode (LED) light-curing unit with a light irradiance of 2000 mW/cm 2 . The specimens were next stored in 37 °C water. For each experimental group, 20 specimens were prepared. Ten specimens were kept for 24 h prior to being subjected to a shear bond-strength test. Another 10 specimens were thermocycled (‘TC’: 60 s of immersion, alternatively, in a 5 °C and 55 °C water bath) for 50,000 cycles before executing the shear bond-strength test using a Shear Bond Tester (Bisco, Schaumburg, IL, USA). Fractured specimens were analyzed using a light microscope (SMZ-10, Nikon, Tokyo, Japan) to assess the fracture pattern. For statistical comparisons of the data, two-way analysis of variance (ANOVA) and Tukey post hoc tests were applied with p < 0.05 considered statistically significant.
|Panavia SA Cement Plus (SAP)||A||Monomer (10-MDP, Bis-GMA, TEGDMA, HEMA, other methacrylate monomer), filler (silanated barium glass filler, silanated colloidal silica), initiator, pigment, others|
|B||Methacrylate monomer, filler (silanated barium glass filler, silanated sodium fluoride), accelerator, pigment, others|
|Panavia SA Universal (SAU)||A||Monomer (10-MDP, Bis-GMA, TEGDMA, HEMA, other methacrylate monomer), filler (silanated barium glass filler, silanated colloidal silica), initiator, pigment, others|
|B||Methacrylate monomer, filler (silanated barium glass filler, aluminium oxide, silanated sodium fluoride), accelerator, pigment, silane coupling agent, others|
|Clearfil Ceramic Primer Plus||Silane coupling agent, monomer (10-MDP), ethanol|
Contact-angle measurement of water onto cement-treated glass ceramic
Glass-ceramic plates were prepared in the same way as described previously for the shear bond-strength test. The ceramic plates were cleaned in an acetone ultrasonic bath, upon which they were treated as follows: (1) SAU applied and after 20 s washed using ethanol, (2) same procedure using SAP, (3) CP applied and air-dried, followed by the application of SAP and after 20 s washed using ethanol. Untreated glass ceramic served as control. The contact angle of distilled water dropped on the treated ceramic plates was measured using contact-angle measurement equipment (SImage AUTO 100, Excimer, Yokohama, Japan). Specimen images were analyzed using a computer program (SESF, Excimer) using an angular dimension tool to measure the contact angle (θ). Right and left angles were measured to obtain a mean θ value. All measurements were done in triplicate, after which the data were analyzed by one-way ANOVA and a Tukey multiple comparison test ( α = 0.05).
NMR analysis of silane reaction in SAU
For NMR, experimental filler- and initiator-free SAU cement Pastes A and B were prepared and provided by Kuraray Noritake. Before measurement, paste B was dissolved in the same volume of d-methanol. Upon mixture of the experimental pastes A and B, the mixed paste was kept for 3 h in atmosphere, upon which they were dissolved in the same volume of d-methanol. Each sample was poured into NMR test glass tubes with a 5-mm diameter and an 8-inch length (Wilmad, Buena, NJ, USA). An 400-MHz NMR spectrometer (Bruker, Tokyo, Japan) was employed to acquire 29 Si NMR spectra at 100.58 MHz in CD 3 CD 2 OD.
Shear-bond strength onto dentin
Thirty extracted non-carious human molars (approved by the Commission for Medical Ethics of Okayama University under the file number #1606-020) were used. The teeth were embedded in epoxy resin (EpoFix, Ballerup, Denmark). Dentin was exposed and the surface was polished using 600-grit SiC paper (WTCC-S, Nihon Kenshi, Fukuyama, Japan). This was followed by one of the following surface treatments: (1) SAU_light, (2) SAU_chem, (3) SAP_light and (4) SAP_chem. Zirconia cylinders, prepared in the same manner as for the bond-strength measurements onto glass-ceramic plates, were luted onto dentin using one of the three experimental luting protocols onto dentin and light-cured for 40 s from two opposing directions (totaling a 80-s curing time) using F3 mode (high-intensity blue LED mode) of G-Light Prima II Plus (Kuraray Noritake) except for the SAU/SAP_chem experimental groups, which were allowed to self-cure during 30 min. Ten specimens were prepared for each experimental group. All specimens were subjected to a shear bond-strength testing protocol after 24-h storage in water at 37 °C. Fractured specimens were analyzed using using Feg-SEM (JSM-6701F, Jeol, Tokyo, Japan) to assess the fracture pattern. For statistical comparisons of data, one-way ANOVA followed by Tukey’s post hoc tests ( α < 0.05) was used with p < 0.05 considered statistically significant.
Thin-film XRD of dentin exposed to composite cements
Dentin flat plates (10 × 8 × 1 mm) were cut from human molar teeth (approved by the Commission for Medical Ethics of Okayama University). The dentin surface was polished using 600-grit SiC-paper (WTCC-S). SAU or SAP was applied on the dentin surface. After 20 s, the samples were washing using ethanol in order to remove the cement. The surface structures of SAU- and SAP-treated dentin (SAU/SAP_D) specimens were examined by thin-film XRD using an X-ray diffractometer (RINT2500, Rigaku, Tokyo, Japan) under 40-kV acceleration and 200-mA current, with the angle of the incident X-ray beam fixed at 1.0° and a scanning time of 0.02°/s for a 2θ scan. Interaction of an experimental 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP) solution, consisting of 15 wt% 10-MDP, 45 wt% ethanol and 40 wt% water, served as reference.
Feg-SEM and TEM of cement-dentin interfaces
Four extracted non-carious human third molars (approved by the Commission for Medical Ethics of Okayama University under the file number #1606-020) were used. After removal of the occlusal crown third using a diamond saw (IsoMet 1000, Buehler, Lake Bluff, IL, USA), the exposed dentin was wet-sanded using 600-grit SiC paper (WTCC-S, Nihon Kenshi) to produce a standard smear layer. The composite cements SAU or SAP were applied onto dentin (SAU_light, SAP_light). The top of the cement was covered with matrix Tape (3 M Oral Care, St. Paul, MN, USA). Each pair of specimens was light-cured for 20 s using the G-Light Prima Plus (GC) light-curing unit. The cement-bonded dentin specimens were then processed for Feg-SEM and TEM using TEM-specimen processing described in detail in previous work . In brief, this specimen processing involved fixation using 2.5 wt% glutaraldehyde (Nacalai Tesque, Kyoto, Japan), staining with 2 wt% osmium (TAAB Laboratories Equipment, Aldermaston, UK), and gradual dehydration in ascending ethanol concentrations prior to embedding in epoxy resin (TAAB Laboratories Equipment). For cross-sectional Feg-SEM observations, specimen cross-sections were prepared using a cross-section polisher (SM-09020CP Cross Section Polisher, JEOL). Subsequently, a thin layer of osmium was deposited on their surfaces (Neo Osmium coater, Meiwa, Osaka, Japan), upon which the specimens were examined with Feg-SEM (JSM-6701F, JEOL) operated at 5 kV and employing an annular semiconductor detector. For TEM, ultrathin sections were cut (Leica EM UC7, Leica, Vienna, Austria) prior to being observed with high-resolution TEM (HR-TEM, 200 kV) utilizing a JEM-2100 microscope (JEOL).
Shear bond strength onto leucite glass-ceramic plates ( Fig. 1 )
When the silane-containing self-adhesive composite cement SAU was light- (SAU_light) or chemically cured (SAU_chem), the resultant bond strength onto leucite glass-ceramic plates was not statistically significantly different from that recorded for the silane-free cement SAP when applied following separate silanization (SAP_CP) ( Fig. 1 a) . Without prior silanization, a significantly lower bond strength was recorded for SAP_light. After long-term thermocycling, SAP_light presented with the significantly lowest bond strength, while no statistically significant difference in bond strength was found between SAU_light, SAU_chem and SAP_CP. Moreover, no significant decrease in bond strength was recorded upon aging among the latter three experimental groups. In contrast, when the silane-free self-adhesive composite cement SAP was applied without prior silanization, nearly no bond strength remained upon long-term TC. Bonded cement-ceramic specimens failed mostly cohesively within the ceramic for SAU_light, SAU_chem and SAP_CP with/without thermocycling, while SAU_light specimens failed at the actual cement-ceramic interface ( Fig. 1 b).