Outside-the-(cavity-prep)-box thinking.

Direct placement restorative materials must interface with tooth structures that are often compromised by caries or trauma. The material must seal the interface while providing sufficient strength and wear resistance to assure function of the tooth for, ideally, the lifetime of the patient. Needed are direct restorative materials that are less technique-sensitive than current resin-based composite systems while having improved properties. The ideal material could be successfully used in areas of the world with limited infrastructure. Advances in our understanding of the interface between the restoration adhesive system and the stages of carious dentin can be used to promote remineralization. Application of fracture mechanics to adhesion at the tooth-restoration interface can provide insights for improvement. Research in polymer systems suggests alternatives to current composite resin matrix systems to overcome technique sensitivity, while advances in nano- and mesoparticle reinforcement and alignment in composite systems can increase material strength, toughness, and wear resistance, foreshadowing dental application.

Self-adhesive resin cements - chemistry, properties and clinical considerations.

Self-adhesive resin cements were introduced to dentistry within the past decade but have gained rapidly in popularity with more than a dozen commercial brands now available. This review article explores their chemical composition and its effect on the setting reaction and adhesion to various substrates, their physical and biological properties that may help to predict their ultimate performance and their clinical performance to date and handling characteristics. The result of this review of self-adhesive resin cements would suggest that these materials may be expected to show similar clinical performance as other resin-based and non-resin based dental cements.

Nanogel-Based Filler-Matrix Interphase for Polymerization Stress Reduction.

A novel filler-resin matrix interphase structure was developed and evaluated for dental composite restoratives. Nanogel additives were chemically attached to the filler surface to use this created interphase as a potential source of compliance to minimize stress development during polymerization. In addition, we evaluated the effects of free nanogel dispersion into the resin matrix, combined or not with nanogel-modified fillers. Nanogels with varied characteristics were synthesized (i.e., size, 5 and 11 nm; glass transition temperature, 28 °C to 65 °C). Glass fillers were treated with trimethoxyvinylsilane and further reacted with thiol-functionalized nanogels via a free radical thiol-ene reaction. γ-Methacryloxypropyltrimethoxysilane-surface treated fillers were used as a control. Composites were formulated with BisGMA/TEGDMA resin blend with 60 wt% fillers with nanogel-modified fillers and/or free nanogel additives at 15 wt% in the resin phase. Polymerization kinetics, polymerization stress, volumetric shrinkage, and rheological and mechanical properties were evaluated to provide comprehensive characterization. Nanogel-modified fillers significantly reduced the polymerization stress from 2.2 MPa to 1.7 to 1.4 MPa, resulting in 20% stress reduction. A significantly greater nanogel content was required to generate the same magnitude stress reduction when the nanogels were dispersed only in the resin phase. When the nanogel-modified filler surface treatment and resin-dispersed nanogel strategies were combined, there was a stress reduction of 50% (values of 1.2 to 1.1 MPa). Polymerization rate and volumetric shrinkage were significantly reduced for systems with nanogel additives into the resin. Notably, the flexural modulus of the materials was not compromised, although a slight reduction in flexural strength associated with the nanogel-modified interphase was observed. Overall, modest amounts of free nanogel additives in the resin phase can be effectively combined with a limited nanogel content filler-resin interphase to lower volumetric shrinkage and dramatically reduce overall polymerization stress of composites.

Dental Restorative Materials Based on Thiol-Michael Photopolymerization.

Step-growth thiol-Michael photopolymerizable resins, constituting an alternative chemistry to the current methacrylate-based chain-growth polymerizations, were developed and evaluated for use as dental restorative materials. The beneficial features inherent to anion-mediated thiol-Michael polymerizations were explored, such as rapid photocuring, low stress generation, ester content tunability, and improved mechanical performance in a moist environment. An ester-free tetrafunctional thiol and a ultraviolet-sensitive photobase generator were implemented to facilitate thiol-Michael photopolymerization. Thiol-Michael resins of varied ester content were fabricated under suitable light activation. Polymerization kinetics and shrinkage stress were determined with Fourier-transform infrared spectroscopy coupled with tensometery measurements. Thermomechanical properties of new materials were evaluated by dynamic mechanical analysis and in 3-point bending stress-strain experiments. Photopolymerization kinetics, polymerization shrinkage stress, glass transition temperature, flexural modulus, flexural toughness, and water sorption/solubility were compared between different thiol-Michael systems and the BisGMA/TEGDMA control. Furthermore, the mechanical performance of 2 thiol-Michael composites and a control composite were compared before and after extensive conditioning in water. All photobase-catalyzed thiol-Michael polymerization matrices achieved >90% conversion with a dramatic reduction in shrinkage stress as compared with the unfilled dimethacrylate control. One prototype of ester-free thiol-Michael formulations had significantly better water uptake properties than the BisGMA/TEGDMA control system. Although exhibiting relatively lower Young's modulus and glass transition temperatures, highly uniform thiol-Michael materials achieved much higher toughness than the BisGMA/TEGDMA control. Moreover, low-ester thiol-Michael composite systems show stable mechanical performance even after extensive water treatment. Although further resin/curing methodology optimization is required, the photopolymerized thiol-Michael prototype resins can now be recognized as promising candidates for implementation in composite dental restorative materials.

Academy of Dental Materials guidance-Resin composites: Part II-Technique sensitivity (handling, polymerization, dimensional changes).

OBJECTIVE: The objective of this work, commissioned by the Academy of Dental Materials, was to review and critically appraise test methods to characterize properties related to critical issues for dental resin composites, including technique sensitivity and handling, polymerization, and dimensional stability, in order to provide specific guidance to investigators planning studies of these properties. METHODS: The properties that relate to each of the main clinical issues identified were ranked in terms of their priority for testing, and the specific test methods within each property were ranked. An attempt was made to focus on the tests and methods likely to be the most useful, applicable, and supported by the literature, and where possible, those showing a correlation with clinical outcomes. Certain methods are only briefly mentioned to be all-inclusive. When a standard test method exists, whether from dentistry or another field, this test has been identified. Specific examples from the literature are included for each test method. RESULTS: The properties for evaluating resin composites were ranked in the priority of measurement as follows: (1) porosity, radiopacity, sensitivity to ambient light, degree of conversion, polymerization kinetics, depth of cure, polymerization shrinkage and rate, polymerization stress, and hygroscopic expansion; (2) stickiness, slump resistance, and viscosity; and (3) thermal expansion. SIGNIFICANCE: The following guidance is meant to aid the researcher in choosing the most appropriate test methods when planning studies designed to assess certain key properties and characteristics of dental resin composites, specifically technique sensitivity and handling during placement, polymerization, and dimensional stability.

Academy of Dental Materials guidance-Resin composites: Part I-Mechanical properties.

OBJECTIVE: The objective of this project, which was initiated from the Academy of Dental Materials, was to review and critically appraise methods to determine fracture, deformation and wear resistance of dental resin composites, in an attempt to provide guidance for investigators endeavoring to study these properties for these materials. METHODS: Test methods have been ranked in the priority of the specific property being tested, as well as of the specific test methods for evaluating that property. Focus was placed on the tests that are considered to be of the highest priority in terms of being the most useful, applicable, supported by the literature, and which show a correlation with clinical findings. Others are mentioned briefly for the purpose of being inclusive. When a standard test method exists, including those used in other fields, these have been identified in the beginning of each section. Also, some examples from the resin composite literature are included for each test method. RESULTS: The properties for evaluating resin composites were ranked in the priority of measurement as following: (1) Strength, Elastic Modulus, Fracture toughness, Fatigue, Indentation Hardness, Wear-abrasion (third body) and Wear-attrition (contact/two body), (2) Toughness, Edge strength (chipping) and (3) Wear determined by toothbrush. SIGNIFICANCE: The following guidance is meant to aid the researcher in choosing the proper method to assess key properties of dental resin composites with regard to their fracture, deformation and wear resistance.

Simultaneous Measurement of Fluorescence, Conversion and Physical/mechanical Properties for Monitoring Bulk and Localized Photopolymerization Reactions in Heterogeneous Systems.

An FT-NIR spectrometer, rheometer and fluorescence spectrophotometer were coupled for the real-time monitoring of polymerization reactions, allowing the simultaneous tracking of polymerization kinetics, storage modulus as well as fluorescence. In this study, a methacrylate functionalized dansyl chromophore (DANSMA) was synthesized and two different nanogels were made from urethane dimethacrylate and isobornyl methacrylate. Two series of resin formulations were prepared using the DANSMA probe, ethoxylated bisphenol A dimethacrylate as the matrix monomer, Irgacure® 651 as the initiator and the dispersed, monomer-swollen nanogels to give clear UV-curable resins. Placement of the fluorescent probe either throughout the resin or linked into the nanogel before its dispersion in the matrix provides a tool to study how the nanogel structure affects local network development by means of fluorescence from the DANSMA probe. We demonstrate the potential of this new technique using a composite as the two phase system (resin and polymerizable nanogel) including a dansyl derivative as a polymerizable probe to follow the reactions that are taking places in both phases.

Impact of curing protocol on conversion and shrinkage stress.

Since considerable shrinkage stress develops during the curing of dental composites, various soft-start photocuring protocols, aiming to lower stress but not compromise conversion, have been proposed. We hypothesized that utilizing soft-start photocuring will result in not only reduced stress, but also decreased conversion. We evaluated the impact of 3 protocols (soft-start, pulse, and standard) on the stress development and polymerization extent of an experimental composite. A novel set-up capable of simultaneous shrinkage stress, conversion, and temperature measurements on the same specimen was utilized. Analysis of the data shows that stress rises dramatically as a function of conversion in the vitrified state, and the utilization of soft-start or pulse curing results in specimens with reduced final conversion and shrinkage stress, compared with specimens cured according to the standard full-intensity protocol. Finally, this study demonstrates that the predominant reason for the reduced shrinkage stress attained with soft-start or pulse curing is a modest decrease in final conversion.

Volumetric contraction and methacrylate conversion in photo-polymerized amorphous calcium phosphate/methacrylate composites.

Because of its relatively high solubility in aqueous media and its rapid transformation to hydroxyapatite, amorphous calcium phosphate (ACP) has been utilized as the filler phase of resin-based bioactive composites that have remineralization potential. The objectives of this study were to determine how various methacrylate resins and various types of ACP fillers affect acrylic vinyl conversion and polymerization shrinkage (PS). Several types of photo-crosslinkable resin systems were prepared and admixed with a mass fraction of 40% of either unhybridized, silica- or zirconia-hybridized ACP. After visible light-activated photo-polymerization ACP composites were assessed by near infrared spectroscopy for degree of vinyl conversion and by mercury dilatometry for PS. It was found for these composites that vinyl conversion was independent of filler type but strongly dependent on the type and composition of the resin phase. PS, on the other hand, showed more complex dependence both on the resin type and composition and, in some cases, on the type of ACP. In order to obtain ACP/methacrylate-based composites with maximal vinyl conversion, resin type and composition are of primary importance. However, in order to minimize volume contraction on polymerization it appears necessary to consider both the resin and filler type of these bioactive composites.

Cyclopolymerizable monomers for use in dental resin composites.

The simple reaction of conventional monofunctional acrylate monomers with paraformaldehyde has yielded a new class of difunctional monomers. The unique proximity of the double bonds within these compounds allows a facile cyclopolymerization to incorporate a cyclic ether structure into the polymer backbone. The external position of the acrylate esters means that these pendant groups can be varied for alteration of the physical properties of the monomers and the corresponding polymers. A series of the new monomers has been prepared and polymerized under dilute solution and bulk conditions to yield non-crosslinked and crosslinked polymers, respectively. The polymers exhibited high degrees of conversion and significantly reduced polymerization shrinkage, compared with polymers obtained from conventional diacrylate or dimethacrylate monomers. These properties appear to be a direct result of an efficient cyclopolymerization process. The combination of improved conversion to polymer with less contraction makes these monomers ideal candidates for use in dental resin composites. A preliminary evaluation of their potential in this application has shown that with suitable ester functionality, they can be formulated to provide composites with excellent mechanical strength properties.

Synthesis and evaluation of new oxaspiro monomers for double ring-opening polymerization.

Polymerization with expansion in volume can be achieved with spiro orthocarbonate monomers through a double ring-opening process wherein two bonds are cleaved for each new bond formed. The resulting expansion can be applied to counter the polymerization shrinkage associated with the conventional methacrylate monomers used in dental composites and thereby provide formulations with drastically reduced degrees of shrinkage. New monomers have been prepared that exhibit enhanced reactivities and ring-opening efficiencies compared with earlier free-radical-polymerizable oxaspiro compounds. In dental composite formulations, the monofunctional oxaspiro monomers provided DTS values equivalent to those of the controls under certain curing conditions; however, only modest reductions in polymerization shrinkage were observed. 2,3-Bis(methylene) spiro orthocarbonate monomers with a conjugated diene structure were also synthesized and evaluated. These novel monomers appear to offer significant potential for future development of free-radical ring-opening polymerization. While visible-light-cured formulations of the bis(methylene) compounds with methacrylate comonomers did not yield acceptable composite materials in this initial attempt, the high reactivity and the ability to form rigid, cross-linked polymers make this type of monomer worthy of continued investigation. These properties may allow the bis(methylene) oxaspiro monomers to be used alone or in concert with other ring-opening monomers for special applications.

Synthesis and evaluation of novel multifunctional oligomers for dentistry.

A new type of multifunctional oligomer was synthesized, and its potential as a base monomer in dental composite formulations was evaluated. The oligomer of ethoxylated bis-phenol A diacrylate (OEBPA) was prepared in good yield by a formaldehyde insertion/condensation reaction. Although double bonds along the oligomer backbone are arranged in pairs such that cyclopolymerization is possible, it is not presently known whether this process plays a significant role in the polymerization. Indirect evidence supporting efficient cyclopolymerization involves the reduced polymerization shrinkage observed for polymerized OEBPA relative to polymers of other monomers used as base resins. Photo-cured composites containing either OEBPA, BIS-GMA, or ethoxylated bis-phenol A dimethacrylate (EBPADM) as base resin and TEGDMA as diluent were compared. While the resulting diametral tensile and transverse strengths did not differ significantly, the values for the energy absorbed to failure indicated that the OEBPA- and EBPADM-based formulations yielded composites with somewhat greater toughness than that of the BIS-GMA material. This multifunctional oligomer offers mechanical strength and conversion values that are comparable with those of existing base resin monomers while providing an approximate 30% reduction in polymerization shrinkage.

Cements containing syringic acid esters -- o-ethoxybenzoic acid and zinc oxide.

Fissure caries is reduced when syringic acid is incorporated into a cariogenic diet of rats. It was therefore of interest to synthesize n-hexyl and 2-ethylhexyl syringate and to evaluate the properties of cements with these compounds as ingredients. Liquids containing the esters dissolved in o-ethoxybenzoic acid (EBA) - when mixed with powders made up from zinc oxide, aluminum oxide, and hydrogenated rosin - hardened in from four to nine min. Properties of the cements were determined, when possible, according to ANSI/ADA specification tests. Depending on the powder-liquid ratio employed, we obtained compositions with varying physical properties desirable for different dental applications. The syringate cements, compared with the commonly used ZOE materials, have improved compressive and tensile strength, lower water solubility, do not inhibit polymerization, and are compatible with acrylic monomers. These cements pass, and mostly greatly exceed, the requirements for ZOE-type restorative materials. They also bond significantly to resins, composites, and non-precious metals. The bond strength is somewhat less than that of n-hexyl vanillate-EBA cement, but greatly exceeds the adhesion to various substrates of ZOE luting agents. Cements containing n-hexyl syringate were somewhat brittle. Best results were obtained with liquid compositions containing 5% 2-ethylhexyl syringate, 7% n-hexyl vanillate, and 88% EBA, which yielded non-brittle materials. These cements, because of the syringate ingredient, may possess caries-reducing properties. Thus, perhaps in conjunction with fluoride additives, they would be useful as insulating bases, pulp capping agents, root canal sealers, soft tissue packs, or intermediate restoratives.

Intermediate restoratives from n-hexyl vanillate-EBA-ZnO-glass composites.

Vanillate esters such as n-hexyl vanillate (HV) dissolved in a suitable chelating agent - e.g., o-ethoxybenzoic acid (EBA) - react with zinc oxide, aluminum oxide, and hydrogenated rosin powder to yield non-eugenol-containing cements that do not inhibit polymerization and are compatible with acrylic monomers. These cements can be modified by adding methyl methacrylate, or the less-volatile, higher-molecular-weight dicyclopentenyloxyethyl, or cyclohexyl methacrylate to the HV-EBA liquid, and silanized glass to the powder. On incorporating a suitable initiator-accelerator system, one can prepare powder-liquid mixes that have good working properties and harden in five to 10 min. The cured materials containing monomethacrylate ingredients have compressive and tensile strength one and one-half to three times that of eugenol-based intermediate restoratives. Cements with even better mechanical properties are obtained using dimethacrylates as monomeric components. Storage stability of the liquids comprising vanillates-EBA and monomethacrylates is excellent. The vanillate-EBA-dimethacrylate liquid containing amine accelerators polymerizes within days when left standing at 45 degrees C. The cement composites adhere strongly to composites, non-precious metals, or porcelains. Rupture of the bond occurs cohesively within the cement. Because of their high strength, low solubility, and excellent adhesion, these cements, subject to their biocompatibility with dental tissues, show great promise as intermediate restorative resins and in the repair of fractured porcelain or porcelain-to-metal crowns and bridges.

Preparation and characterization of cyclopolymerizable resin formulations.

An amine-catalyzed reaction between acrylates and formaldehyde has been used to convert mono-acrylates to difunctional monomers and di-acrylates to multifunctional oligomers by linking the acrylic double bonds together in 1,6-diene pairs. The resulting monomers and oligomers undergo efficient cyclopolymerization to high conversion with significantly less shrinkage than normally found for acrylates. In this study, a convenient single-step process was used with mixtures of mono- and di-acrylate starting materials to produce a series of resins with potential for effective cyclopolymerization. Incremental changes in the ethyl acrylate (EA) to ethoxylated bisphenol A diacrylate (EBPAD) ratio directly supplied cyclopolymerizable resins with a broad range of viscosities and product distributions. Those resins produced from reaction mixtures rich in EA have low viscosities because of high diluent monomer contents and limited oligomerization of EBPAD due to end-cap formation. Resin viscosity and average molecular weight of the oligomeric component of the resin were inversely related to the amount of EA used in the reaction. Through the choice of reactants and their ratio, this simple technique has the potential to provide cyclopolymerizable resins for use in a variety of dental polymer applications.

Semiempirical and ab initio conformational analysis of 2-methylene-8,8-dimethyl-1,4,6,10-tetraoxaspiro[4.5] decane with application of GIAO-SCF methods to NMR spectrum interpretation.

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.

Evaluation of methylene lactone monomers in dental resins.

alpha-Methylene-gamma-butyrolactone (MBL), which can be described as the cyclic analog of methyl methacrylate, exhibits greater reactivity in free radical polymerizations than conventional methacrylate monomers. Unfilled resin formulations composed of Bis-GMA/MBL or Bis-GMA/TEGDMA/MBL were light-cured. The effect of the more reactive methylene lactone monomer on mechanical properties and the degree of conversion of the polymers was examined. The infrared absorption bands for the carbon-carbon double bonds of MBL and the methacrylate monomers are well resolved and allow the conversion of each component to be calculated individually. The incorporation of a small amount of MBL (5 w/o) to Bis-GMA significantly increased the conversion; however, additional MBL (10 to 30 w/o) did not further increase the Bis-GMA conversion level. This appears to indicate an incompatibility between MBL and the bulky Bis-GMA monomer. Addition of 10 w/o MBL to Bis-GMA/TEGDMA (7:3) resulted in a cured resin with 71% methacrylate and 75% overall conversion efficiencies compared with the 57% conversion of the control formulation. The diametral tensile and the transverse strengths were approximately 10% greater for the MBL resin compared with the Bis-GMA/TEGDMA control; however, these differences were not statistically significant. The synthesis and polymerization of several substituted methylene lactones was also studied.

Determination of double bond conversion in dental resins by near infrared spectroscopy.

OBJECTIVES: This study determined the validity and practicality of near infrared (NIR) spectroscopic techniques for measurement of conversion in dental resins. METHODS: Conversion measurements by NIR and mid-IR were compared using two techniques: (1) The conversion of 3mm thick photopolymerized Bis-GMA/TEGDMA resin specimens was determined by transmission NIR. Specimens were then ground and reanalyzed in KBr pellet form by mid-IR. (2) As further verification, thin resin films were photocured and analyzed by mid-IR. Multiple thin films were then compressed into a thick pellet for examination by NIR. RESULTS: Conversion values obtained by NIR and mid-IR techniques did not differ significantly. A correction for changing specimen thickness due to polymerization shrinkage was applied to NIR conversion measurements since an internal standard reference peak was not employed. Sensitivity of the NIR technique was superior to those based on the mid-IR. SIGNIFICANCE: The nondestructive analysis of conversion in dental resins by NIR offers advantages of convenience, practical specimen dimensions and precision compared with standard mid-IR analytical procedures. Because glass is virtually transparent in the NIR spectrum, this technique has excellent potential for use with filled dental resins as well.

Dimethacrylate monomers with varied fluorine contents and distributions.

OBJECTIVES: There are many unique properties associated with fluorinated polymers that make these materials attractive for use in the challenging oral environment. This study was devised to better define the influence of fluorine content and its structural distribution on properties of fluorinated resins and composites, especially with regard to their water-related and mechanical properties. METHODS: A series of fluorinated dimethacrylate monomers was prepared by reaction of aromatic diepoxides with fluoroalcohols and subsequent conversion of the resulting diols to the methacrylates. Composites based on monomer systems comprised of the fluorinated monomers with 1,10-decamethylene dimethacrylate and reinforced with silanized quartz filler were evaluated for conversion, water contact angle, water sorption and diametral tensile strength. RESULTS: By selection of reactants, fluorine was introduced as trifluoromethyl groups, extended fluoroalkyl pendant chains, or combinations of the two. Photopolymerization conversion among the experimental composites was generally equal to or greater than that of a conventional Bis-GMA/TEGDMA composite. While the water contact angles generally increased with fluorine content, no correlation was obtained between fluorine content and water sorption of the composites. The mechanical strength of the fluorinated composites showed a general decline with increasing fluorine content and consistent variations due to specific structural features. SIGNIFICANCE: A versatile route to fluorinated dimethacrylates with diverse structural and fluorine distribution patterns is presented. Composites from these monomers are very hydrophobic but have relatively low mechanical strength. The monomers described can be considered as useful additives to moderate the water sorption of conventional resins. However, the results of this study point to specific fluorinated resin structures that are expected to provide a more optimal balance between hydrophobicity and mechanical strength that will improve the long-term performance of dental composites.

The effect of cure rate on the mechanical properties of dental resins.

OBJECTIVE: This study investigates the effect of cure rate on the mechanical properties of a common dimethacrylate dental resin formulation (75/25 wt% bis-GMA/TEGDMA). METHODS: The polymerization rate and final conversion of the exact specimens subsequently used for mechanical testing were monitored by near-infrared (near-IR) spectroscopy. The glass transition temperature (T(g)) and modulus, as a function of temperature, were determined by dynamic mechanical analysis (DMA). Iniferter initiating systems were used to create partially cured networks that did not contain any trapped radicals. By the elimination of trapped radicals from the system, the formed networks can be characterized as a function of both temperature and double bond conversion without inducing additional thermal cure during testing. RESULTS: Copolymer specimens were cured with UV and visible light initiating systems, UV light intensities that varied by over four orders of magnitude, and cure temperatures that differed by 60 degrees C. Even though the polymerization rates for these resins were vastly different, similar T(g) and modulus were measured for specimens cured to the same final double bond conversion. SIGNIFICANCE: This study shows that highly cross-linked dimethacrylate systems, such as bis-GMA/TEGDMA, exhibit similar network structure and properties as a function of double bond conversion, regardless of the method or rate of cure.