Abstract
Objectives
In order to improve the short-comings of glass ionomers such as polishability and esthetics while preserving their excellent clinical bonding effectiveness, nanofiller technology has been introduced in a paste-paste resin-modified glass ionomer (nano-filled RMGI, Ketac Nano, KN, 3 M ESPE). One objective of this study was to investigate if the introduction of nanotechnology had any significant effect on the setting reaction of the nanoionomer compared to a control RMGI, Vitremer (VM, 3 M ESPE). Another objective was to determine the mechanism of bonding of KN in combination with its primer (KNP) to the tooth.
Methods
Fourier-Transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses were performed on KN and VM during the setting of the GIs. FTIR and XPS were also used to study the reaction of the primer of KN (KNP) with hydroxyapatite (HAP). Shear adhesion to dentin and enamel was measured with KN and compared with several RMGIs and one conventional glass ionomers (CGI). The interfaces were examined with scanning electron microscopy (SEM).
Results
FTIR data show that KN undergoes both acid-base and methacrylate setting reactions of classical RMGIs. XPS and FTIR studies of the interaction KNP with HAP shows the formation of calcium-polycarboxylate bond. Shear adhesion and failure mode of KN to enamel and dentin were similar to the other RMGIs and CGI. SEM images of KN with tooth structure showed a tight interface with a thin but distinct layer of 2–3 microns attributed to the primer. This was also observed for VM but not for the other three materials.
Conclusions
KN showed two setting reactions expected in true RMGIs. The adhesion with dentin and enamel was similar to other glass-ionomers. The formation of calcium-polycarboxylate was also evident. This chemical bonding is a significant factor in the excellent long-term adhesion of these materials.
1
Introduction
In spite of the superior adhesive properties and sustained fluoride-releasing characteristics of classical glass ionomers their clinical applications have been limited since their esthetic qualities are not as excellent as those of modern-day composites. In recent years optical properties of many materials have been enhanced by incorporating nanoparticles of 5–100 nm size. The use of nanotechnology in restorative dental composites has greatly improved their esthetics without compromising their mechanical properties and function . Hence, with the aim of improving the esthetic properties of glass ionomers, nanotechnology has been introduced to develop a novel nano-filled resin-modified glass-ionomers (RMGI) restorative material and commercialized as Ketac™ Nano (3M™ ESPE™, St. Paul, MN, also known as Ketac™ N100 in some regions). In this class of RMGI restorative material, called a “nanoionomer”, the combination of acid-reactive fluoroaluminosilicate glass (FAS glass) and non-reactive nanofillers provides unique characteristics not previously found in traditional conventional or resin-modified GIs . The nanoionomer system utilizes a novel hybrid filler system consisting of synthetic nanomeric and nanocluster surface modified particles of zirconia and silica along with a modified FAS glass filler. For ease-of-use it is formulated as a two-paste system that is delivered either through Clicker™ mixing device and hand-spatulated, or through a direct intraorally deliverable automixing capsule (Quick Mix). An aqueous polycarboxylic acid containing light-cured primer is employed to treat the tooth surface prior to placement and curing of this restorative. While conceptually simple, a viable and successful nanoionomer technical solution was not achieved by simply combining conventional FAS glass particles and nanofillers in a two paste system. The nanoparticles used in composites are inherently incompatible in aqueous solutions and give rise to visually opaque formulations. To overcome this critical limitation and achieve the unique combination of clinically desirable properties, the recently introduced nanoionomer system [Ketac™ Nano (KN)] utilizes a novel hybrid filler system consisting of synthetic nanomeric and nanocluster particles of tailored refractive index and surface functionality along with unique FAS glass filler. A recent publication has shown that the partial replacement of the traditional FAS glass with methacrylate surface-modified nanofillers significantly improved the polish and abrasion resistance of the nanoionomer KN as well as the fluoride release and recharge behavior comparable to those exhibited by typical conventional and RMGIs . However, there is little information in the scientific literature on how the setting mechanism and interfacial behavior of the paste-paste nanoionomer compares with those of classical powder-liquid types of conventional and resin-modified glass-ionomers.
The reactive fluoroaluminosilicate glass in the nanoionomer was comminuted to very fine microparticles and then selectively surface-treated to provide a reactive surface area that is at least three times that of the traditional fluoroaluminosilicate glass in the RMGI, Vitremer™ (VM). Furthermore, unlike nanocomposites wherein the nanoparticles are treated with primarily hydrophobic silanes that bind to the resin matrix, in KN the surfaces of the nanofillers were treated with a mixture of silanes to maintain an optimum hydrophilic/hydrophobic balance that would facilitate water-mediated ion transport . However, it is not immediately evident if these bonded nanoparticles in a two-paste system would influence the setting reactions of the nanoionomer compared to other GIs. With that in mind one of the objectives of this investigation was to see if this technological approach of creating the nanoionomer KN would allow substantial water-mediated acid-base reaction to progress during the setting of nanoionomer comparable to the classical RMGI VM. The evaluation of acid-base reaction kinetics by FTIR has been shown to be an effective tool for the study of conventional glass-ionomers CGI . The FTIR technique has also been effectively used to characterize the setting reactions of classical powder-liquid RMGIs, e.g. Vitremer™ (VM) and Fuji ® II LC (FIILC) whereby it has been demonstrated that significant acid-base glass ionomers setting as well as polymerization of methacrylate groups occur in these materials. Hence, the first objective of the present investigation was to perform an FTIR study in which the setting reactions of the paste-paste nanoionomer, KN, were followed progressively with time after photocuring and compare it to that of VM.
The ionic reaction that takes place in the glass-ionomers is also believed to be the reason why they behave as adhesive bioactive materials . In these water-based materials, migration and exchange of ions such as fluoride, calcium, strontium, etc. take place from and into the material. This ion migration is said to occur across the interface of the bulk material and the tooth structure . Several studies have shown that both CGI and RMGI materials have clinical bonding effectiveness comparable to that of the best performing three-step etch-and-rinse adhesives . Some studies have also shown that several RMGIs exhibit superior and more predictable adhesion to tooth structure in comparison to other adhesive strategies (e.g. etch-and-rinse adhesives with three or two steps; self-etch adhesives with two or one steps) although their in vitro measured bond strengths may be lower than pure resin-based adhesives . The chemical bonding of some RMGI liners to HAP crystals has been demonstrated by X-ray photoelectron spectroscopy (XPS) while the ability of these materials to bond micromechanically and form hybrid layers was demonstrated by FE-SEM and confocal microscopy studies . However, such mechanistic studies have not been reported on RMGI filling materials. Nor has the effect of incorporating nanoparticles and nanoclusters in the nanoionomer on its interfacial adhesion behavior been studied in detail. Commercial RMGI filling materials require the use of a pretreatment of the tooth surface prior to the application of the restorative. In some instances the cavity surface is conditioned with a polyacrylic acid solution followed by washing and drying. In other RMGIs the pretreatment (identified by some manufacturers as a primer or a self-conditioner) is provided by the application of a polycarboxylic acid containing solution which remains on the cavity surface and is set through light activation . For KN, an aqueous primer called the Ketac Nano Primer (KNP) is applied to the cavity surface for 20 s and then set in place by light. This material contains a methacrylated polycarboxylic acid (MAP). The Ketac Nano Primer (KNP) is conceptually similar to, but slightly different in formulation from the primer used in VM. A recent publication has shown that the KNP did not affect the fluoride release from or recharge behavior of the bulk of the KN restorative . A TEM study of the interface of KN/KNP/tooth structure has been recently reported ; however, details of the adhesion mechanism on a molecular scale have not been published. In particular it is not known if ionic bonding which is so crucial to the successful clinical bonding of CGIs and RMGIs also plays a significant role for the adhesion of the nanoionomer KN. Thus, a second objective of this investigation was to see if the introduction of nanotechnology in an RMGI had any significant effect on the adhesion mechanism of the nanoionomer KN, particularly on its ability to bond via an ionic bond to the HAP mineral of tooth structure. Since in clinical use the KNP primer is applied to the tooth and cured prior to the placement of the restorative material the adhesion mechanism was elucidated by analyzing the reaction of this component with HAP by FTIR and X-ray photoelectron spectroscopy (XPS). By studying the interaction of the key components to pure HAP, the effects of chemical bonding of RMGI to the HAP crystals in enamel and dentin can be isolated via eliminating the many confounding factors and biological variability in tooth structure. Additionally, the shear bond strengths of the KN primer/restorative combination to enamel and dentin were measured and compared with the values obtained for other conventional and RMGIs in conjunction with their respective pre-treatment agents. Following the adhesion studies the interfacial microstructure was characterized via scanning electron microscopy (SEMs of Ketac Nano, Fuji Filling LC, Fuji II LC, Vitremer and Fuji IX in Section 2 ).
In summary, the scientific hypotheses tested in this investigation were:
- (1)
The unique combination of high surface area FAS glass and synthetic nanofillers with tailored hydrophilic surface treatment in KN would provide an acid–base GI reaction in addition to the photopolymerization setting reaction. The qualitative assessment of acid–base reaction would be similar to that of a classical RMGI, VM.
- (2)
The nanoionomer KN/KNP primer combination would bond to hydroxyapatite mineral via an ionic reaction. The interfacial adhesion performance and mechanism would be essentially similar to that of other GI filling materials studied.
2
Materials and method
The materials used in this investigation are shown in Table 1 along with the manufacturer details and batch numbers.
| Material | Manufacturer | Chemistry | Batch no. |
|---|---|---|---|
| Ketac™ Nano RMGI (KN) | 3 M ESPE | Paste/paste RMGI Filling: methacrylated polyacid, water, FAS glass, nanofillers, methacrylated monomers, initiators | AF7AG |
| Ketac™ Nano Primer (KNP) | 3 M ESPE | RMGI Primer: methacrylated polyacid, water, methacrylated monomers, initiators | 7AC |
| Methacrylated Polyalkenoic acid (MAP) | 3 M ESPE | Methacrylated-polyalkenoic acid copolymer | MFG |
| Calcium salt of MAP (Ca-MAP) | 3 M ESPE | Calcium salt of methacrylated-polyalkenoic acid copolymer | As described in experimental section |
| Hydroxyapatite disks | Clarkson Chromatography | Hydroxyapatite | 290703 |
| Vitremer RMGI (VM) | 3 M ESPE | Powder/Liquid RMGI Filling: methacrylated polyacid, water, FAS glass, methacrylated monomers, initiators | L 20040701 2007-06/L4CT 2007-07 |
| Fuji II LC (FIILC) | GC | Powder/Liquid RMGI Filling: polyacid, water, FAS glass, methacrylated monomers, initiators | L0506021 2007-06 |
| Fuji IX (FIX) | GC | Powder/Liquid CGI Filling: polyacid, water, FAS glass | L 0411251 2007-11 |
| Fuji Filling LC (FFLC) | GC | Paste/paste RMGI Filling: polyacid, water, FAS glass, methacrylated monomers, initiators | L 0602081 2008-02 |
2.1
FTIR studies
2.1.1
Determination of the mechanism of setting reactions
FTIR spectra were collected with a Magna IR Spectrophotometer 550 (Thermo Fisher Scientific Co, Madison, WI). The two pastes of KN were extruded through the supplied dispenser and mixed according to manufacturer’s instructions, applied to a KBr disk and pressed between KBr disks to obtain a thin film. Initial spectra were taken at 2 and 4 min to establish a profile prior to light-curing. The sample was then light-cured for 20 s with an XL3000 Dental Curing Light (3 M ESPE, with a nominal intensity of 460 mW/cm 2 ). Spectra were recorded in transmission mode at 6–60 min every 5 min and every hour over a 24 h period. For comparison a similar study was undertaken on VM. The powder and liquid of VM were dispensed on a mixing pad in the weight ratio of 2.5:1 and mixed according to the instructions for use (IFU). The mixed material was applied onto a KBr disk and pressed between KBr disks as for KN and light-cured per IFU.
2.1.2
Determination of the mechanism of interaction of KNP with HAP powder
The spectrum of the KNP on a polyethylene IR card (3 M) was recorded in transmission mode. The spectrum of the HAP powder was recorded in the diffuse reflectance mode (DRIFT) with a KBr powder reference. For the DRIFT specimens the standard Kubelka-Munk transformation was applied using the software package of the computerized FTIR spectrometer. A portion of the HAP powder was thoroughly mixed with the KNP liquid in the ratio of 1:1 by weight, stirred for 2 h, applied to a KBr disk and pressed between KBr disks to obtain a thin film; the spectrum was recorded in transmission mode. Interfacial interactions were determined via digital subtraction of (HAP + KNP)-KNP using the software package supplied with the FTIR spectrometer.
2.2
Shear adhesion studies to dentin and enamel
Five bovine teeth per each material were potted in acrylics and ground to enamel or dentin using 120 and 320 grit SiC paper. The surfaces were treated according to manufacturer’s instructions using a conditioner or primer. PTFE molds 5 mm wide and 2 mm deep were filled and cured per manufacturer’s instructions. All samples were put in 37 °C/90%RH chamber for 20 min, then in de-ionized (DI) water at 37 °C for 24 h before breaking the bonds. The specimens were tested using an Instron ® Universal Testing System (Norwood, MA) in shear wire-loop mode at 1 mm/min speed. Maximum force and macroscopic mode of failure (observed optically) were recorded and bond strength calculated.
2.3
Scanning electron microscopy studies
Additional bovine dentin and enamel surfaces were prepared as described in the adhesion section. PTFE molds 7 mm wide and 0.5 mm deep were placed on the surfaces and filled with the restorative materials of interest. Compositions were cured per manufacturer’s instructions and samples were conditioned for 20 min in 37 °C/90%RH chamber and then put in 37 °C DI water for few days. Subsequently, the molds were removed and the teeth were cut parallel to their labial surface using an Isomet saw. The cut sections were about 1.5 mm thick and were then scribed from top and bottom to define a fracture along the diameter of restorative material. The samples were fractured using needle-nose pliers.
The fractured tooth samples were mounted on an aluminum stub and were examined using an FEI XL30 Environmental Scanning Electron Microscope (ESEM) without the addition of any conductive coating. The use of a “high pressure” SEM, allowed for direct examination of non-conductive as well as water-containing samples which would have otherwise resulted in cracking and led to artifacts under high vacuum. This ESEM was operated at 15 keV with a spot size of 3 (gun emission current of 80 μA), and chamber pressure of 1.0 Torr at ambient temperature. The sample was mounted on a stub with a 45° angle; the stage was titled 35° for an overall tilt of 80°. The digital photomicrographs created were the product of secondary electron imaging (SEI), a technique used to image surface morphology of a sample. All micrographs were taken at a viewing angle just off normal to the fractured face. Images were captured at magnifications of including 1500× of the interface between the filling material and the tooth structure of interest (dentin or enamel). A length marker was present in the lower portion of each micrograph, should any dimension measurement be necessary.
2.4
XPS study of Interfacial Interactions KNP with HAP Powder
XPS data were acquired using a Kratos AXIS Ultra DLD spectrometer (Kratos Analytical LTD., Manchester, UK) with a monochromated Al-Kα X-ray source that provided a binding energy resolution better than 0.5 eV. All compositional measurements were acquired at a photoelectron take-off angle of 90° in “hybrid mode” (take-off angle is defined as the angle between the sample surface and the axis of the XPS analyzer lens). A low-energy electron flood gun was used to minimize surface charging. The typical X-ray spot size was 700 μm × 300 μm. For each sample, an initial compositional survey scan was acquired using pass energy of 160 eV. High-resolution C1s spectra were also acquired using pass energy of 20 eV and were charge-referenced to the C1s hydrocarbon peak set to 285.0 eV. Two or more replicates of each sample were analyzed and averaged to obtain the reported atomic percent (at.%) values. Data analysis was performed with Vision Processing data reduction software (Kratos Analytical Ltd.) and CasaXPS (Casa Software Ltd.). Elemental concentrations were calculated, and analyzed via one-way ANOVA and Tukey’s T -test with a confidence level of 95% or more ( p < 0.05).
The primer of KN (designated here as KNP) was applied to hydroxyapatite disks ( N = 3, 2 spots each) (Clarkson Chromatography Products, South Williamsport, PA) and allowed to react for 1 h. Three disks were used and each disk received two spots. After 1 h reaction time the coated disks were sonicated in ultrapure 18 mΩ water (U18W) to remove excess KNP and dried under nitrogen gas. XPS spectra were collected on the HAP disks before and after the treatment procedure. The XPS spectra of untreated HAP and of a dried film of KNP were also recorded as control specimens. Two reference samples were used. The first was that of methacrylated-polyalkenoic acid copolymer that is used in formulating the KNP. This group was designated as MAP and it is the same copolymer used in Vitrebond™ RMGI liquid (see Table 1 for details). The second reference sample was a synthetic calcium-polyalkenoic acid salt which was previously prepared by adding a 46% aqueous solution of calcium chloride to a 10% aqueous solution of MAP, on proportions to ensure an excess of calcium. The resulting thick, gelatinous precipitate was decanted, rinsed three times with deionized water followed by drying at 80 °C. This material was referred to as Ca-MAP.
2
Materials and method
The materials used in this investigation are shown in Table 1 along with the manufacturer details and batch numbers.
| Material | Manufacturer | Chemistry | Batch no. |
|---|---|---|---|
| Ketac™ Nano RMGI (KN) | 3 M ESPE | Paste/paste RMGI Filling: methacrylated polyacid, water, FAS glass, nanofillers, methacrylated monomers, initiators | AF7AG |
| Ketac™ Nano Primer (KNP) | 3 M ESPE | RMGI Primer: methacrylated polyacid, water, methacrylated monomers, initiators | 7AC |
| Methacrylated Polyalkenoic acid (MAP) | 3 M ESPE | Methacrylated-polyalkenoic acid copolymer | MFG |
| Calcium salt of MAP (Ca-MAP) | 3 M ESPE | Calcium salt of methacrylated-polyalkenoic acid copolymer | As described in experimental section |
| Hydroxyapatite disks | Clarkson Chromatography | Hydroxyapatite | 290703 |
| Vitremer RMGI (VM) | 3 M ESPE | Powder/Liquid RMGI Filling: methacrylated polyacid, water, FAS glass, methacrylated monomers, initiators | L 20040701 2007-06/L4CT 2007-07 |
| Fuji II LC (FIILC) | GC | Powder/Liquid RMGI Filling: polyacid, water, FAS glass, methacrylated monomers, initiators | L0506021 2007-06 |
| Fuji IX (FIX) | GC | Powder/Liquid CGI Filling: polyacid, water, FAS glass | L 0411251 2007-11 |
| Fuji Filling LC (FFLC) | GC | Paste/paste RMGI Filling: polyacid, water, FAS glass, methacrylated monomers, initiators | L 0602081 2008-02 |
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