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OBJECTIVES: Visible light cure oxirane/polyol resins of Cyracure UVR-6105 with pTHF-250 has been previously shown useful for development of dental composites. This oxirane/polyol (4016) in combination with other oxiranes were formulated into composites (4016E, 4016G and 4016GB) containing 72.9-74.9% quartz filler. The main objective of the study was to evaluate some of the physical properties and the biocompatibility of the composites. RESULTS: PhotoDSC analysis of composites demonstrated twice the enthalphy values of Z100 (31J/g). Composites 4016E and 4016G showed compressive strengths similar to Z100 (337+/-35Mpa), P>0.05. Discs of composite 4016E, containing Epon 825 oxirane (E), and composite 4016G containing Araldite GY 281 oxirane (G) were non-cytotoxic (-) while the composite 4016GB, containing G and Ebecryl 1830 (B), was mildly (+) cytotoxic to L929 cells in the agar diffusion assay. Seven-day extracts of 4016GB composite were cytotoxic while extracts of 4016E and 4016G were less cytotoxic to L929 cells in the MTT assay. Extracts were obtained from 7 day incubations of composite (3 cm(2) surface area/ml) in acetone or ethanol/saline (1:20) at 37 degrees C. All composite extracts were non-mutagenic to Ames strains TA100, TA98, TA97a and TA1535. The overall results with composite 4016GB suggest that leachable components were cytotoxic but non-mutagenic. With the exception of oxirane components, G and E, the oxirane Cyracure UVR-6105 and other components were non-mutagenic. From cytotoxicity studies, the photoinitiator, Sarcat CD 1012, was the most cytotoxic (TC(50)=14 microM) component. Components G (TC(50)=17 microM), E (TC(50)=50 microM) and B (TC(50)=151 microM) were significantly (p < 0.05) more cytotoxic than Cyracure UVR-6105 (1488 microM) and the polyol, pTHF-250 (TC(50)=6072 microM). SIGNIFICANCE: Favorable results obtained with composites 4016G and 4016E indicates that suitable oxirane/polyol formulations can be designed and optimized for development of dental composites with acceptable mechanical properties and biocompatibility. However, leachable analysis of extracts obtained from longer incubation periods is needed before final conclusions could be drawn about the leachability of oxirane components.

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OBJECTIVES: (a) to design, formulate and evaluate prototype primers and a crosslinking agent for use with isocyanatomethacrylate-based comonomer adhesives and (b) to establish correlations between bond strength and solubility parameter differences between the adhesives and etched dentin, and the permeability coefficients of the adhesives. METHODS: Equimolar mixtures of 2-isocyanatoethyl methacrylate (IEM) and a methacrylate comonomer were formulated with tri-n-butyl borane oxide (TBBO) as the free radical initiator to have cure times of 6-10 min. Shear bond strengths to dentin were determined for each adhesive mixture (n = 7) using standard testing protocols. Shear bond strengths for the three systems were also determined after application of "reactive primers" to the dentin surface. The "reactive primers" contained 10-20 parts by weight of the respective comonomer mixture and 3.5 parts by weight TBBO in acetone. Solubility parameters difference values (delta delta) and permeability coefficients (P) were approximated for each adhesive system and correlated with shear bond strength values. Additionally, a crosslinking agent was prepared by bulk reaction of an equimolar mixture containing IEM and a methacrylate comonomer. The effects of crosslinker addition on: (a) the setting time of IEM; and (b) the setting times and initiator requirements of selected IEM/comonomer mixtures were determined. RESULTS: Shear bond strength values (MPa): IEM/HEMA 13.6 +/- 2.0 (no primer), 20.1 +/- 2.0 (with primer); IEM/HETMA 9.3 +/- 3.3 (no primer), 20.8 +/- 8.1 (with primer); IEM/AAEMA 13.6 +/- 1.9 (no primer), 17.3 +/- 3.2 (with primer). Also, approximated permeability coefficients showed a significant correlation (r = +0.867, p < 0.001) with shear bond strength values. Crosslinker addition studies with IEM/4-META: (a) at 5-9 mol% reduced the setting time of IEM polymerization by 79%; and (b) at 6 mol% reduced initiator level requirements 60-70% to achieve a comparable setting time, and decreased setting times by ca. 75% for a given initiator level with selected IEM/methacrylate adhesive systems. SIGNIFICANCE: The shear bond strengths of isocyanatomethacrylate-based dental adhesives can be enhanced by using reactive primers; their setting times and initiator requirements can be improved using a dimethacrylate crosslinker. Approximated permeability coefficients may be useful as indicators of bonding performance for dentin adhesive systems.

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In development of photopolymerized expanding monomers with epoxy resin systems, there is a need for reactive expanding monomers that exert a good biocompatibility profile. The objective of this study was to evaluate the in vitro toxicology of new spiroorthocarbonates designed to be expanding monomers. The expanding monomers investigated were: trans/trans-2,3,8,9-di(tetramethylene)-1,5,7,11-tetraoxaspiro[5,5] undecane (DTM-TOSU), 5,5-diethyl-19-oxadispiro-[1,3-dioxane-2,2'-1,3-dioxane-5',4'-bicy clo[4.1.0]heptane] (DECHE-TOSU); 3,9-diethyl-3,9-dipropionyloxy methyl-1,5,7,11-tetraoxaspiro[5.5]undecane (DEDPM-TOSU); and 3,9-diethyl-3,9-diacetoxy methyl-1,5,7,11-tetraoxaspiro[5.5]undecane (DAMDE-TOSU). The in vitro toxicology of these monomers measured their cytotoxicity and mutagenicity potential. Succinic dehydrogenase (SDH) activity in the MTT assay was used to assess the toxic dose that kills 50% of cells (TC50) for all the monomers. Their mutagenic potential was measured in the Ames Salmonella assay with and without metabolic activation. Two solvents, DMSO and acetone, were used to validate effects. Appropriate controls included the solvents alone. All the expanding monomers in this study were less cytotoxic than BISGMA (p < 0.01), a commercial component of dental restoratives. The relative cytotoxicity of the expanding monomers in DMSO was defined in the following order: DTM-TOSU (more toxic) > DECHE-TOSU > DEDPM-TOSU > DAMDE-TOSU. Each was significantly different from the other (p < 0.05). Overall, the TC50 values of all expanding monomers were significantly greater in DMSO than in acetone (p < 0.05). However, for BISGMA this trend was opposite. For mutagenicity results, the expanding monomers were non-mutagenic and there was no solvent effect on this outcome. The non-mutagenicity and low cytotoxicity profile of these expanding monomers suggests their potential for development of biocompatible non-shrinking composites.

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The GIAO-SCF method for calculating isotropic nuclear magnetic shielding values has been utilized to explain certain features in the 1H-NMR spectrum of 2-methylene-8,8-dimethyl-1,4,6,10-tetraoxaspiro[4.5] decane. Population distributions of the low-energy conformers based on their ab initio energies were used to produce weighting factors for the individual calculated shielding values to calculate the weighted average of the shielding values for a complete set of conformers. The differences in 1H chemical shifts between the hydrogens of the two methyl groups and between the axial and equatorial hydrogens in 2-methylene-8,8-dimethyl-1,4,6,10-tetraoxaspiro[4.5] decane were shown to be due to energy differences between the chair and boat orientations of the six-membered ring and contribution from a twist-boat conformation. Results suggest a hypothesis that intramolecular differences in chemical shift might be calculated to a greater degree of accuracy than chemical shifts calculated relative to a standard.

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Experiments have recently been completed to explore the development of isocyanatoacrylate copolymers as new dental adhesives. A main goal of this work was to test the utility of solubility parameter differences between the candidate adhesives and etched dentin as a predictor of relative bond strength. All candidate adhesive mixtures contained 2-isocyanatoethyl methacrylate (IEM), a selected amount of tri-n-butylborane oxide (TBBO) initiator, and one of 13 methacrylate comonomers. Reactivity ratios were computed for comonomer pairs as indicators of relative reactivity. The concentration of TBBO was optimized for each comonomer mixture to obtain working times of 2-6 min and setting times of 6-10 min. The solubility parameter difference Deltadelta (J/cm(3))(1/2) was calculated for each test mixture with respect to an etched dentin substrate, as an approximation of wetting ability. Using standard techniques for shear bond strength evaluation, mean shear bond strength values ranging between 7-15.5 MPa were obtained for comonomer adhesives in bonding Z-100 composite to treated dentin. Shear bond strength values showed a good correlation (r = -0.612, P

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OBJECTIVE: The objective of this study was to evaluate the effect of adding a spiroorthocarbonate (SOC) or a polyol on the cytotoxicity of epoxy-based dental resins. METHODS: Resins contained one of the epoxies: diglycidyl ether Bisphenol A (GY-6004); 3,4-epoxycyclohexanemethyl-3,4-epoxycyclohexane carboxylate (UVR-6105); vinyl cyclohexane dioxide (ERL-4206) or the three-epoxy mixture (Epoxy-M). The SOC was t/t-2,3,8,9-di(tetramethylene)-1,5,7,11-tetraoxaspiro[5.5]undecane (SOC). The polyols were polytetrahydrofuran (p-THF-250) and polycaprolactone triol (TONE-301). The photoinitiator (4-octylphenyl)phenyliodonium hexafluoroantimonate and camphorquinone were used for light curing the resins. Four types of resins (epoxy, SOC/epoxy, polyol/epoxy and SOC/polyol/epoxy) were evaluated for cytotoxicity as solids in the agar diffusion assay and as aqueous extracts in the MTT assay using L929 cells. RESULTS: In agar diffusion analysis, ERL-4206 and UVR-6105 resins were severely cytotoxic (+3), but the addition of SOC changed them to non-cytotoxic (-). Addition of 1-3% SOC changed Epoxy-M from mild (+) to non-cytotoxic. Adding SOC changed GY-6004 from moderate (+2) to mild (-) cytotoxicity. Generally, addition of SOC did not change cytotoxicity when added to polyol/epoxy combinations. Either polyol produced resins with reduced cytotoxicity when added to UVR-6105, but the opposite occurred when added to Epoxy-M resins. In MTT analysis, percent cell survival from 100 microliters resin extracts were statistically compared (ANOVA, p < 0.05). Epoxy-M and GY-6004 resin extracts were significantly less cytotoxic than UVR-6105 and ERL-4206 resin extracts were. Overall, the SOC component reduced the cytotoxicity of all SOC/epoxy combinations, except SOC/ERL-4206, which was significantly more cytotoxic than ERL-4206 resin extract. This may be the result of cell fixative effects observed for SOC/ERL-4206 in agar diffusion analysis. Addition of SOC produced significantly less cytotoxic SOC/polyol/Epoxy-M resins when compared to its non-SOC counterpart. The contrary result was obtained with SOC/polyol/UVR-6105 resin combinations. Consistent with agar diffusion results, adding polyol significantly decreased cytotoxicity of UVR-6105 resins. The cytotoxicity of these resins may be related to the 50% cytotoxicity (TC50) of their components as leachates. The TC50 values of the individual components were compared to BISGMA. Polyols, epoxy monomers, SOC monomer and camphorquinone were significantly (p < 0.05) less cytotoxic than BISGMA. SIGNIFICANCE: Addition of SOCs and polyols in the formulation of epoxy-based resins may contribute to development of biocompatible dental composites.

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The principal aim of this study was to investigate the relationships between the solubility parameters of ectched dentin, and adhesive primer solutions and adhesive bond strength. Solubility parameters characterize the molecular interactions which determine physical properties such as wetting, and thus can serve as tools to aid development of polymeric adhesives and interpenetrating polymer networks. If an adhesive monomer has a solubility parameter close to that of a polymer substrate, then the monomer may act as a solvent for the polymer and penetrate below the surface. Subsequent polymerization of the monomer may then produce an interpenetrating network, thus adhering without necessarily forming primary chemical bonds to the substrate. The dentin substrate considered in this study was abraded dentin treated with ethylenediaminetetraaceitc acid. Solubility parameters delta pr, delta h, and delta d calculated for the etched dentin substrate were 20.3, 23.6, and 16.0 (J/cm3)1/2, respectively. Solubility parameters of the primers were expressed using Hansen's three-dimensional scheme. The data indicate a correlation between the calculated solubility parameters of the etched dentin, and dentin primers and the resulting bond strengths. The results corroborate the significance of solubility parameter considerations for adhesive bonding to dentin.

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The greatest problem with current dental composite systems is their polymerization shrinkage. Extensive work is being done by many investigators to alleviate this problem. Our approach has been to examine epoxy- and spiro-orthocarbonate (SOC)-based resins. The hypothesis to be tested in this study was that the cure characteristics of experimental visible-light-cured epoxy resin systems are governed by the types and concentrations of co-reactants and activators. Resin samples containing onium salt initiators and a thiozanthone sensitizer were successfully cured by means of either an experimental visible-light irradiation system or a commercially available dental lamp. Test resins consisted of di-epoxies alone or in combination, epoxy mixtures in combination with an SOC, or an epoxy in combination with a caprolactone-derived polyol. Significant findings were as follows: (a) Resins containing the SOC had longer cure times than their counterparts; (b) the optimum ratios of epoxy to polyol for most rapid cure were 50:50 or 60:40 under conditions tested; (c) resins containing TONE 305 polyol generally were faster to cure than those containing no polyol, or TONES 201 or 310; and (d) a resin mixture was found that had a cure time of 1 to 3 min when irradiated with a commercial dental lamp. Based on this exploratory study, it should be possible for clinically relevant cure times to be achieved for visible-light-cured epoxy-based resins by careful manipulation and optimization of key elements.

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The ultimate success of a dentin adhesive bond is dependent in large part on specific conditions at the interface between the tooth and the adhesive. Most current dentin adhesive systems use some sort of pre-treatment to demineralize the first few microns of the dentin surface, leaving a meshwork of collagen into which the adhesive resin can penetrate, infiltrate, and polymerize. The general hypothesis tested in this experiment was that the penetration and distribution of adhesive resin into the demineralized zone are a function of the conditioner used as a pre-treatment for the adhesive application. Four commercially available adhesive systems were modified to incorporate hydroxyethylthiomethacrylate (HETMA), a sulfur-substituted, traceable analogue of 2-hydroxyethylmethacrylate (HEMA), thereby allowing for a qualitative measurement of the amount and distribution of monomer in the treated dentin substrate by energy-dispersive x-ray spectroscopy (EDS) and a quantitative measurement by Auger electron spectroscopy (AES). The dentin pre-treatments investigated were: (1) 10% citric acid/3% ferric chloride, (2) 10% maleic acid, (3) 2.5% nitric acid, and (4) an alcoholic solution of HEMA with a phosphorus acid ester. These pre-treatments were applied to freshly extracted teeth that had been sectioned to expose the dentin and ground to simulate the smeared layer. After the appropriate pre-treatment was applied, a 10% (v/v) solution of HETMA in acetone was applied to the surface, followed by the corresponding adhesive resin, which was then polymerized. The samples were then processed for observation by scanning transmission electron microscopy (STEM), AES, and STEM/EDS analysis. The results indicated significant differences in the ability of HETMA to penetrate the dentin surface conditioned by the four pretreatments investigated here. This study also demonstrated that AES and STEM/EDS could be used in a correlative fashion to determine the distribution of HETMA within or adjacent to the treated dentin surface.

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During the last 12 years, significant progress has been made in the development of dental adhesive systems. Some of the more promising systems are based on multifunctional structures that contain polymerizable vinyl double bonds and reactive isocyanate groups. The utility of compounds with such structures as adhesives arises in part because their isocyanate functionality is available for reaction independently, without compromising the reactivity of the vinyl groups. The hypotheses tested in this investigation were: (1) that the monomer reactivity ratios (r1, r2) for the free-radical-initiated copolymerization of ethyl alpha-isocyanatoacrylate (alpha-EIA) and 2-isocyanatoethyl methacrylate (IEM) with selected vinyl monomers can be determined; (2) that these reactivity ratios can be used to establish Q (reactivity) and e (polarity) values for alpha-EIA and IEM; and (3) that these reactivity parameters can be useful in designing copolymers with controlled compositions for dental adhesive applications. The free-radical copolymerization characteristics of alpha-EIA and IEM were studied. The isocyanate monomers were copolymerized at seven comonomer ratios with n-butyl acrylate (NBA), methyl methacrylate (MMA), and styrene (STY). Reactivity ratios, r1 and r2, were calculated for each of the copolymer systems, giving:IEM (r1) = 0.38 and STY (r2) = 0.44; IEM (r1) = 1.19 and MMA (r2) = 0.84; IEM (r1) = 2.50 and NBA (r2) = 0.40; alpha-EIA (r1) = 2.20 and STY (r2) = 0.06; alpha-EIA (r1) = 7.00 and MMA (r2) = 0.10; and alpha-EIA (r1) = 23.50 and NBA (r2) = 0.04. The Q (reactivity) and e (polarity) values for IEM and alpha-EIA were calculated from r1 and r2 with use of the Alfrey-Price equations, giving, for IEM, Q = 0.89 and e = 0.60, and, for alpha-EIA, Q = 7.64 and e = 0.74. These reactivity parameters are useful for tailoring copolymers with controlled compositions and properties. Based on these calculated reactivity parameters, several copolymers of IEM [for example, IEM/2-hydroxyethyl methacrylate (HEMA)] are currently being prepared and evaluated as adhesives.

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OBJECTIVES: The purpose of this study was to compare a methylthiazole tetrazolium (MTT) dye colorimetric method with the standard 51Cr assay as methods of assessing cytotoxicity of dental materials. METHODS: Two MTT-based colorimetric formats, test tube and 96-well microplate methods, were compared to the 51Cr release assay. A series of eight dental materials were evaluated. Cytotoxicity profiles were determined for each test material. A TC50 value (Toxic Concentration required to kill 50% of the cells) was determined for each biomaterial, and these results were used to make statistical comparisons between the methods. RESULTS: The three methods were statistically correlated (p<0.005) by comparison of the eight samples tested. That is, the same rank in toxicity was given by the two tetrazolium sample formats and the 51Cr method. SIGNIFICANCE: The MTT assay was found to have several advantages in comparison to the current standard 51Cr release assay. Optimized in the 96-well format, complete dose response curves and greater sample comparisons can be made rapidly, making the MTT method more economical in time and cost. Furthermore, the MTT method is based on intracellular biochemical changes, measuring cell viability rather than cell morbidity, and has lower detectable limits than the 51Cr release method. There is also less detector chemical binding interference than encountered in the 51Cr release method.

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In an attempt to compare the morphology of the dentin adhesive interface and the wetting and penetration of the adhesive in relation to the dentin surface, we studied four dentin adhesive systems using scanning transmission electron microscopy (STEM) and energy-dispersive spectroscopy (EDS). 2-Hydroxyethylmethacrylate (HEMA), a monomer common to many commercial dentin adhesive systems, was altered to produce a thiolated analogue (HETMA). Sulfur, traceable by EDS and STEM, was substituted for the oxygen atom in the backbone of the HEMA molecule. The resulting analogue, with solubility parameters and other wetting and physical properties very similar to those of HEMA, was applied to four sets of tooth specimens, each pre-treated with a different primer or etchant. Three separate pre-treatments--nitric acid, maleic acid, and citric acid/ferric chloride--created a demineralized zone approximately 1 to 3 microns thick at the dentin surface. The HETMA was found to permeate freely into this zone when either of the latter two pre-treatments was used. However, the band of dentin that was demineralized by the nitric acid pre-treatment appeared impermeable to the HETMA. The fourth pre-treatment, an alcohol-based solution including the phosphorus acid ester PENTA and HEMA, modified the smear layer of the tooth slightly and did not appear to demineralize the dentin. HETMA applied to the specimens pre-treated with PENTA and HEMA was clearly in intimate contact with the dentin or modified smear layer; however, it did not penetrate or diffuse into these areas. It did flow into the dentinal tubules, as was also evident with each of the other systems. It was concluded that the acid pre-treatment of the dentin greatly influenced the wetting behavior of the dentin adhesive and thus could substantially affect the resultant bond strength of the dentin adhesive systems.

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A major improvement in dental restoratives is possible through the development of biomaterials that do not shrink upon polymerization, hence, avoid leakage and subsequent breakdown. Polymers containing spiroorthocarbonates (SOCs) show promise in this respect, but their toxicology in copolymerized materials has not been explored. In this study, the in vitro toxicology of these materials in homopolymer form and in two trial non-shrinking epoxy co-polymers was evaluated for cytotoxicity and mutagenicity. Cytotoxicity was determined by the MTT test to measure the lethality effect on mouse L929 cells. Mutagenicity was evaluated using the Ames-Salmonella Test. For comparison, commercial composite and adhesive materials as well as several other materials of current interest in dentistry were also evaluated. Epoxy resin samples containing 5% of either T/T SOC or Dp SOC reduced the cytotoxicity (TC50) from approximately 400 to 800 micrograms/200 microliters. The epoxy-spiro copolymers had more favorable TC50 values than the commercial product Super-Bond. They showed TC50 values on the order of 35% greater than Super-Bond and 45% less than Scotchbond 2, the latter two being materials currently used in the clinic. These two comparatives demonstrated dose response curves with lower doses at maximum cell kill values than the spiro materials. The epoxy formulations all showed weak mutagenesis, but this is attributed to the epoxy formulation and not the SOCs. Although considerable toxicology is yet be conducted, these in vitro results suggest that biocompatible copolymer formulations for spiroorthocarbonates are a developmental reality.

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Much attention has been directed toward producing dentinal adhesives that withstand the forces involved during polymerization shrinkage of composite resins. Studies have shown that an effective dentin-adhesive bond depends on the wetting and penetration characteristics of the dentinal adhesive system and the reactivity of the treated dentinal surface. The structure of the collagen in the demineralized dentinal layer also seems to influence the behavior of the bond. Adhesive systems that do not completely denature the fibrous collagen and leave interwoven banded collagen in the demineralized layer produce superior bond strengths (greater than 20 MPa). Other research efforts seek to develop a nonshrinking high-performance polymer for use as a matrix material for dental composite resins. Spiroorthocarbonates that expand during polymerization have been developed. In combination with a three-component epoxy comonomer, matrix resins have been produced that expand during polymerization and whose mechanical properties, water sorption, solubility, and degree of polymerization are acceptable for dental use. Further developments of nonshrinking composite resins and improved dentinal adhesives will greatly increase the longevity of 21st-century composite resin restorations and should significantly ease clinical placement.

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The objective of this work was to develop copolymers of alicyclic spiroorthocarbonates (SOCs) and epoxies that would yield hard non-shrinking matrix resins suitable for formulating dental composites. Several reactant ratio combinations of a four-component SOC/epoxy comonomer system were developed that demonstrated a copolymer expansion of between 0.1 and 0.8% on polymerization. The physical properties of tensile strength (29-48 MPa), water solubility of less than 5 microgram/mm(3), and water sorption of less than 50 microgram/mm(3) for the copolymer containing 5% SOC suggest that these materials have potential as matrix resins for dental composites.

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The objective of this work was to develop polymeric materials that expand slightly upon polymerization and that could potentially be used as matrix resins for dental composites. A series of stereoisomeric alicyclic spiroorthocarbonates (SOC's) that expand when polymerized were synthesized. Three of these SOC racemates were analyzed: cis/cis-, cis/trans- and trans/trans-2,3,8,9-di(tetramethylene)-1,5,7,11-tetraoxaspiro[5.5] undecane. The degrees of expansion, approximately 3.9% and 3.5%, for the cis/cis and trans/trans, were determined by measuring the specific volume of the monomers and polymers in dilute solutions. This method of determining densities and subsequent calculated expansion or shrinkage was validated by duplicating the reported shrinkage of 4-tert-butylphenyl glycidyl ether, styrene, and methyl methacrylate. Based on these data and spectral data obtained using other analytical techniques, these stereoisomeric alicyclic SOC's appear to have potential as nonshrinking polymer or copolymer matrices for dental composites.

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