J.L. Ferracane

Resin composite - State of the art

OBJECTIVESnThe objective is to review the current state of the art of dental composite materials.nnnMETHODSnAn outline of the most important aspects of dental composites was created, and a subsequent literature search for articles related to their formulation, properties and clinical considerations was conducted using PubMed followed by hand searching citations from relevant articles.nnnRESULTSnThe current state of the art of dental composites includes a wide variety of materials with a broad range of mechanical properties, handling characteristics, and esthetic possibilities. This highly competitive market continues to evolve, with the major emphasis in the past being to produce materials with adequate strength, and high wear resistance and polishability retention. The more recent research and development efforts have addressed the issue of polymerization shrinkage and its accompanying stress, which may have a deleterious effect on the composite/tooth interfacial bond. Current efforts are focused on the delivery of materials with potentially therapeutic benefits and self-adhesive properties, the latter leading to truly simplified placement in the mouth.nnnSIGNIFICANCEnThere is no one ideal material available to the clinician, but the commercial materials that comprise the current armamentarium are of high quality and when used appropriately, have proven to deliver excellent clinical outcomes of adequate longevity.

Current trends in dental composites.

The clinical performance of dental composites has been significantly improved over the past decade through modifications in formulation that include: using more stable polymerization promoters for greater color stability; incorporating high concentrations of finely ground fillers to produce adequate strength and excellent wear resistance while retaining translucency; adding radiopacifying agents for improved diagnostics; and utilizing dentin adhesives. However, there are problems which limit the use of composites, especially in posterior teeth. The materials remain very technique-sensitive, due to the extensive contraction which accompanies polymerization and negatively influences marginal sealing. In addition, the materials are generally considered to have inadequate mechanical properties and wear resistance in contact areas to serve as total replacements for amalgams. Current efforts are focusing on several areas, including the development of non- or minimally-shrinking dental composites containing spiro-orthocarbonates as additives to dimethacrylates or epoxy-base resins, and the production of alternative filler materials for ideal wear resistance and esthetics. This paper reviews the composition and characteristics of current dental composites, as well as recent areas of study.

Wear and Marginal Breakdown of Composites with Various Degrees of Cure

Loss of anatomical form due to wear has been cited as one factor limiting the clinical use of posterior composites. The physical properties and possibly the wear resistance of composite are influenced by the extent to which it is cured. The aim of this study was to vary degree of conversion (DC) in composites to test the hypothesis that resistance to wear and marginal breakdown could be improved by enhanced curing. A light-cured hybrid composite containing a 50% Bis-GMA/50% TEGDMA resin and 62 vol% of strontium glass (1 to 2 μm) with microfill silica was formulated (Bisco). Composite was placed into two 2.5-mm-diameter cylindrical holes in Co-Cr teeth replacing first and second molars in the mandibular dentures of 50 edentulous patients. The composites were light-cured for different time periods (9 s, 12 s, 25 s, 40 s, and 40 s + 10 min at 120°C) and then polished. The microfill Heliomolar was also tested. DC (%) was measured by FTIR and ranged between 55% for 9 s of light-curing and 67% for 40 s of light-curing followed by heat application. Impressions were evaluated at baseline, 6 mo, 1 yr, and 2 yrs. Stone casts were evaluated independently by three observers to determine the % of the total margin exhibiting breakdown. Epoxy replicas were measured with a profilometer for wear. Wear of the hybrid composite at 2 yrs ranged from a high of 144 μm with 9 s of light-curing to a low of 36 μm with 40 s of light-curing followed by heat. Heliomolar exhibited from 11 to 16 μm of wear at 2 yrs. There was a strong negative correlation (r2 = 0.91) between the degree of cure and the abrasive wear of the hybrid composites. Marginal breakdown was negligible for the hybrids, and was reduced for the microfill from 40% to 15% of the margin by heat treatment. This study showed that the resistance to abrasive wear of a dental composite could be improved by enhancement of its degree of conversion.

The Effects of Adhesive Thickness on Polymerization Contraction Stress of Composite

A layer of an unfilled adhesive resin placed between the tooth and composite restoration has been shown to absorb some of the stress generated in the composite during polymerization and to reduce interfacial leakage. The objectives of this study were to measure the change in polymerization contraction stress of bonded composite as the thickness of the resin adhesive was systematically varied, and to correlate the effects of the adhesive thickness and reduced stress on marginal leakage in class V cavities. The maximum contraction force of composite (Herculite XRV) was measured in a tensilometer as the thickness of the adhesive bonding agent (Scotchbond MP) was varied from 20 to 300 μm. Composite was placed in Class V cavities prepared on the labial surfaces of bovine teeth to which different thicknesses of adhesive had been applied by layering, and a marginal leakage test was performed by means of staining with silver nitrate. Contraction stress decreased significantly as the adhesive thickness was increased. This result was supported by a theoretical examination of the data. In class V cavities, additional adhesive layering in the marginal area reduced the overall degree of microleakage. The contraction stress generated during the placement of a composite restoration contributes significantly to early marginal leakage, and this stress was significantly absorbed and relieved by the application of an increasing thickness of low-stiffness adhesive.

Reduction of polymerization contraction stress for dental composites by two-step light-activation

OBJECTIVESnThe goal of this study was to assess the reduction of polymerization contraction stress of composites during a two-step light-activation process and to relate this reduction to the process of polymerization shrinkage and specimen thickness.nnnMETHODSnThree test procedures were performed to compare two-step light-activation with delay with one-step continuous irradiation of composites: polymerization contraction stress using a closed-loop servohydraulic testing instrument, polymerization shrinkage by a mercury dilatometer, and degree of conversion by FTIR. For the one-step continuous curing method, the samples were light-activated for 60s at 330 mW/cm(2). For the two-step curing method, a 5s light exposure at 60 mW/cm(2) was followed by 2 min without light exposure, and then a second light exposure for 60s at 330 mW/cm(2). The same light parameters were used for measurements of stress, shrinkage, and degree of conversion. Three composites, Heliomolar, Herculite and Z100 were evaluated. The contraction stress experiments were repeated with varying thickness for Herculite using the one-step and two different two-step techniques.nnnRESULTSnPolymerization contraction stress 10 min after light-activation was significantly reduced (P<0.05) by the two-step method: 29.7% for Heliomolar, 26.5% for Herculite, and 19.0% for Z100. Total volumetric shrinkage and degree of conversion were not significantly different for composites cured by the two different techniques. Increasing the thickness of the composite sample reduced the measured contraction stress, especially for one of the two-step curing methods.nnnSIGNIFICANCEnA combination of low initial energy density followed by a lag period before a final high-intensity light irradiation provides a reduction of polymerization contraction stresses in dental composites. The stress reductions cannot be attributed to reductions in degree of conversion or unrestrained volumetric shrinkage.

Contraction Stress Related to Degree of Conversion and Reaction Kinetics

Polymerization shrinkage of composites bonded to cavity preparations generates stress on the tooth/restoration interface. The purpose of this study was to verify the influence of degree of conversion and speed of polymerization reaction on contraction stress. We prepared experimental composites with different curing rates by varying the concentration of inhibitor (butylated hydroxytoluene). We verified the effect of degree of conversion by submitting one of the composites to different photo-activation times. Contraction stress was monitored for 10 minutes in a tensilometer. Fourier-transformed infrared spectrometry was used for assessment of the degree of conversion. Volumetric shrinkage was determined by means of a mercury dilatometer. Degree of conversion and volumetric shrinkage showed a non-linear relationship with energy density. Degree of conversion showed a pronounced influence on stress. Increased inhibitor concentration reduced curing rate and contraction stress in composites, without compromising the final degree of conversion.

A Comparison of Four Modes of Evaluating Depth of Cure of Light-activated Composites

Four commonly used methods for evaluating depth of cure in light-activated composites were compared. Optical and scraping methods correlate well, but severely overestimate depth of cure as compared with hardness testing or degree of conversion analysis. Degree of conversion appears to be the most sensitive test of depth of cure.

Fourier Transform Infrared Analysis of Degree of Polymerization in Unfilled Resins—Methods Comparison

Two methods of transmission Fourier Transform Infrared analysis were compared to determine the degree of polymerization (DP) in unfilled Bis-GMA-based dental resins. Diluent concentration, curing mode, and activator type were investigated. DP ranged from 55-72% and was highest for the most diluted resins. The effects of polymerization activation mode and type were insignificant. DP was slightly enhanced in the bulk of the resin, as determined by a KBr-pellet technique, in comparison with results from a thin film method, but both techniques seem to provide useful and reproducible results for dental resins.

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.

Alternatives in Polymerization Contraction Stress Management

Polymerization contraction stress of dental composites is often associated with marginal and interfacial failure of bonded restorations. The magnitude of the stress depends on the composites composition (filler content and matrix composition) and its ability to flow before gelation, which is related to the cavity configuration and curing characteristics of the composite. This article reviews the variations found among studies regarding the contraction stress testing method, contraction stress values of current composites, and discusses the validity of contraction stress studies in relation to results from microleakage tests. The effect of lower curing rates and alternative curing routines on contraction stress values is also discussed, as well as the use of low elastic modulus liners. Moreover, studies with experimental Bis-GMA-based composites and recent developments in low-shrinkage monomers are described.