Sumita B. Mitra

Long-term mechanical properties of glass ionomers

OBJECTIVE Several methacrylate/glass ionomer hybrid materials are now available for clinical use as restorative filling materials. However, the long-term resistance of these materials to physical degradation in the humid oral condition is not known. The objective of this investigation was to determine the mechanical properties, e.g., ultimate compressive strength and diametral tensile strength, of several glass ionomer materials as a function of time after aging in water at oral temperature. METHODS Eight glass ionomer filling materials indicated for restorative or core build-up applications were studied. Three conventional glass ionomers, two metal-containing conventional glass ionomers and three methacrylate-modified systems were included in the study. Cured specimens of each were aged in distilled water at 37 degrees C for 24 h, 1 wk, 4 wk, 12 wk, 24 wk and 52 wk. RESULTS Like the conventional glass ionomers, the methacrylate-modified glass ionomers of this study, with one exception, did not exhibit a decrease in compressive strength, modulus and diametral tensile strength as a result of prolonged storage in water at oral temperature. Some differences among the various groups were apparent. The compressive strengths of the conventional glass ionomers were lower than the methacrylate-modified system, except for one material, Fuji II (GC Dental Corp.), of the former group. A significant difference in the compressive strength was seen between the encapsulated and hand-mixed versions of the same commercial brand product. The compressive modulus was higher and the diametral tensile strength was lower for the conventional systems indicating that, as a group, these materials are more brittle than the methacrylate-modified hybrid ionomers. With the exception of VariGlass VLC (L.D. Caulk), most of the materials studied showed little decrease in mechanical properties after aging in water for 52 wk. SIGNIFICANCE These materials could, therefore, be indicated for use in applications where they are in contact with oral fluids under physiological conditions.

Long-term adhesion and mechanism of bonding of a paste-liquid resin-modified glass-ionomer

OBJECTIVES The contribution of chemical bonding of the polycarboxylic acid in classical powder/liquid conventional glass ionomers (GI) and resin-modified glass-ionomers (RMGI) has been attributed to the excellent long-term bond strengths and clinical retention. RMGIs have been recently introduced as paste/liquid systems for convenience of clinical usage. The objective of this study was to investigate the long-term bond strengths and mechanism of adhesion of paste-liquid RMGI in order to ascertain whether similar characteristics are retained. METHODS Long-term shear adhesion to dentin and enamel was measured on two paste-liquid RMGIs and one powder/liquid RMGI. Scanning electron microscopy (SEM), Fourier-transformed infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses were carried out on the paste-liquid RMGI Vitrebond Plus (VBP) and compared with the classical powder/liquid RMGI Vitrebond (VB). RESULTS VBP maintains adhesion to dentin and enamel over long times; its long-term adhesive performance is equivalent to VB. FTIR data confirm that VBP exhibits the carboxylate crosslinking reaction of a true glass ionomer. SEM images show evidence of micromechanical bonding at the interface between VBP and the tooth. XPS and FTIR data show that the methacrylated copolyalkenoic acid component present in VB and VBP chemically bonds to the calcium in HAP. SIGNIFICANCE The new paste-liquid RMGI liner, VBP, shows equivalent adhesion to its powder-liquid predecessor, VB. The adhesion mechanism was attributed to micromechanical and chemical bonding. This chemical bond is a significant factor in the excellent long-term adhesion of these materials.

Mechanisms of setting reactions and interfacial behavior of a nano-filled resin-modified glass ionomer

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, 3M 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, 3M 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.

Nanoparticles for Dental Materials: Synthesis, Analysis, and Applications

Publisher Summary This chapter explores use of nanoparticles in creation of dental materials. Use of nanoparticles has become very popular in the design and development of many dental materials, since they can provide a unique combination of properties. By far, the largest application has been in dental composites, although several unique adhesive systems containing nanoparticles have also been commercialized. Since nanoparticles have dimensions well below the wavelength of visible light, they cannot scatter that particular light, resulting in inability to detect the particles by naked eye. This property has tremendous implications for controlling the optical properties of materials containing these particles. Due to their extremely small size, they have a high surface area to volume ratio. Thus, the precise control of the chemical composition of the surface of nanoparticles becomes a prerequisite to the reliability and reproducibility of the nanoparticles. The use of nanoparticle technology allows the formulation of dental materials with high translucency, excellent initial polish as well as retention of gloss while maintaining mechanical properties and wear resistance equivalent to other clinically proven materials. Several clinical investigations have also documented the excellent performance of these materials in a variety of oral restorations.

Fluoride release and dentin caries inhibition adjacent to resin-modified glass-ionomer luting cements

The objectives of this investigation were to compare the fluoride release and caries inhibition ability of resin-modified glass ionomer (RMGI) luting cements. Methods: Materials used for dentin caries inhibition were 3M ESPE RelyX(superscript TM) Luting Plus Cement (REP) paste-paste RMGI, RelyX(superscript TM) Luting Cement (RLC) powder-liquid RMGI and Filtek(superscript TM) Z250 composite/Adper(superscript TM) Single Bond adhesive (ZSB). Rectangular slots (6×2×1 mm) were prepared in 24 dentin blocks cut from 8 bovine roots, and filled with ZSB, RLC, and REP. The specimens were immersed in 10 ml acetic acid solution pH 5.0 (37℃, 3 wks) to create artificial dentin lesion, then sectioned into 400 μm slices and subjected to microradiography. Mineral loss (△Z) was calculated from mineral profiles at 0.5 and 1.0 mm from the material margin, and statistically analyzed (ANOVA, Fishers, p＜0.05). Fluoride release from cured cement discs (20×1 mm; n=8) in distilled water was measured after 1, 7, 14, 28, 90, 180, and 365 day using F-specific electrode and TISAB buffer. A conventional GIC, Ketac(superscript TM) Cem, was used as a control for this experiment. Results: Z250 is not fluoridated and did not exhibit F release. The sustained F release from REP and REC were comparable to each other and higher than from the control conventional GIC. Both RMGIs showed pronounced inhibition zones in dentin adjacent to cement margin. △Z values at 0.5 mm and 1.0 mm from RLP and RLC were significantly less compared to ZSB. △Z values were not significantly different in RLP and RLC groups at 1.0 mm. Conclusion: Both the powder-liquid and paste-paste RMGIs, RelyX Luting and RelyX Luting Plus cements respectively, released comparable amounts of fluoride, and, in contrast to the composite, demonstrated inhibition zones at the adjacent dentin when subjected to in vitro demineralization. Both cements inhibited mineral loss in dentin compared to the composite up to 1.0 mm adjacent to the bonded interface.