Carel L. Davidson

Effect of adhesive layer properties on stress distribution in composite restorations—a 3D finite element analysis

OBJECTIVESnTeeth, adhesively restored with resin-based materials, were modeled by 3D-finite elements analysis that showed a premature failure during polymerization shrinkage and occlusal loading.nnnMETHODSnSimulation of Class II MOD composite restorations with a resin bonding system revealed a complex biomechanical behavior arising from the simultaneous effects of polymerization shrinkage, composite stiffness and adhesive interface strain. Due to a polymerization contraction, shrinkage stress increases with the rigidity of the composites utilised in the restoration, while the cusp movements under occlusal loading are inversely proportional to the rigidity of the composites. The adhesive layers strain also plays a relevant role in the attenuation of the polymerization and occlusal loading stresses.nnnRESULTSnThe choice of an appropriately compliant adhesive layer, able to partially absorb the composite deformation, limits the intensity of the stress transmitted to the remaining natural tooth tissues. For adhesives and composites of different rigidities, FEM analysis allows the determination of the optimal adhesive layer thickness leading to maximum stress release while preserving the interface integrity. Application of a thin layer of a more flexible adhesive (lower elastic modulus) leads to the same stress relief as thick layers of less flexible adhesive (higher elastic modulus).

Energy dependent polymerization of resin-based composite

OBJECTIVEnThis study explores the relationship between the extent of polymerization and the radiant energy (dose) applied during the photopolymerization of resin-based composites.nnnMETHODnFTIR was used to measure the 5-min and 24-h conversion of four resin-based composites prepared in a thin film and polymerized under conditions of decreasing intensity and a constant exposure time (30s) using a tungsten halogen curing light. The measured conversion was obtained over a wide range of applied radiant energy. Additionally, samples for two of the materials were polymerized at various intensities and exposure times such that the dose remained constant. This process was performed at four dose levels representing approximately 75% of the conversion range.nnnRESULTSnThe curing profiles (percent conversion versus applied radiant energy) depict a gradual decrease in conversion with decreasing energy followed by a rapid descent. Though there are differences in the maximum conversion attained between the materials, when conversion is represented as a fractional conversion relative to the maximum 24-h value, their 5-min and 24-h curing profiles appear quite similar. Additionally, very similar conversion was measured when the films were exposed using equivalent doses providing evidence for a reciprocal relationship between irradiance (power density) and exposure time. For the 24-h measurements, statistical equivalence (Fishers protected LSD at the 0.05 level) was noted for most of the combinations of exposure time and power density within a given dose. Generally, the exceptions occurred with the shortest exposure times.nnnSIGNIFICANCEnA reciprocal relationship between exposure time and power density adds significance to the study of conversion as a function of the total applied dose. This relationship establishes the curing profile as a universal correlation between exposure time and power density.

Clinical significance of polymerization shrinkage of composite resins

The volumetric polymerization shrinkage of some composite resin restorative materials was measured with a modified dilatometer. It is emphasized that only the shrinkage from the start of the gelation, the rigid contraction of the material, is of clinical relevance. The amount of gelation was determined with an instrument that indicates the first moment at which the setting material is rigid enough to exert tensile forces. In addition, the influences of storage, testing conditions, and mixing ratio on the rigid contraction were studied for one light-cured as well as some chemically cured composite resins.

Advances in glass-ionomer cements

This article describes the properties, advances and shortcomings of glass-ionomer cement as a restorative material. The adhesion of glass-ionomer to tooth structure is less technique sensitive than composite resins and its quality increases with time. Therefore glass-ionomer might turn out to the more reliable restorative material in minimal invasive dentistry based on adhesive techniques.

The effect of filler and silane content on conversion of resin based composite

OBJECTIVEnThis study examines the influence of filler loading and silane content on the conversion of photoactivated, resin-based composites as determined using Fourier transform infrared spectroscopy (FTIR).nnnMETHODnZirconia/silica filler was processed with a silane coupling agent (gamma-methacryloxypropyltrimethoxysilane) to achieve a range of silane-to-filler compositions. Treated fillers were compounded with a photoactivated BisGMA/TEGDMA resin to yield a series of pastes all containing 72 wt% total solids. Diffuse reflectance FTIR was used to characterize methacrylate unsaturation of the silane on the filler particles while paste conversion was determined using transmission FTIR. A resin matrix conversion was determined by adjusting the paste conversion for silane unsaturation. Two additional series of pastes were compounded to achieve variable filler-to-resin ratios using non-treated filler and filler processed with 8 wt% silane. Paste conversion was determined using transmission FTIR.nnnRESULTSnPastes compounded from fillers containing variable percent silane demonstrated linearly decreasing conversion (R(2)=0.986) with increasing silane content and ranged from 52.7 to 62.8%. Adjusting paste conversion for silane unsaturation yielded similar resin matrix conversion for all cured pastes (65.1+/-0.8%). Pastes compounded with increasing filler-to-resin ratios had progressively decreasing conversion. Correcting for silane unsaturation suggests this effect was independent of whether the filler was silane-treated or not.nnnSIGNIFICANCEnComposite resin matrix conversion as determined via FTIR can be underestimated by the presence of unreacted methacrylate from silane on the filler. A corrected resin matrix conversion can be estimated by adjusting for silane unsaturation. Additionally, increasing filler-to-resin ratio progressively decreases conversion independent of the presence of silane on the filler.

Polymerization contraction stress in thin resin composite layers as a function of layer thickness

OBJECTIVESnIn the present study, the effect of layer thickness on the curing stress in thin resin composite layers was investigated. Since the value of the contraction stress is dependent on the compliance of the measuring equipment (especially for thin films), a method to determine the compliance of the test apparatus was tested.nnnMETHODSnA chemically initiated resin composite (Clearfil F2, Kuraray) was inserted between two sandblasted and silane-coated stainless steel discs in a tensilometer. The curing contraction of the cylindrical samples was continuously counteracted by feedback displacement of the tensilometer crosshead, and the curing stress development was registered. After 20 min, the samples were loaded in tension until fracture. The curing stress was determined for layer thicknesses of 50, 100, 200, 300, 400, 500, 600, 700 microns, 1.4 mm and 2.7 mm. The compliance of the apparatus was calculated with the aid of a non-linear regression analysis, using an equation derived from Hookes Law as the model.nnnRESULTSnNone of the samples fractured due to contraction stress prior to tensile loading. The contraction stress after 20 min decreased from 23.3 +/- 5.3 MPa for the 50 microns layer to 5.5 +/- 0.6 MPa for the 2.7 mm layer. The compliance on the apparatus was 0.029 mm/MPa.nnnSIGNIFICANCEnA measuring method was developed which was found to be suitable for the determination of axial polymerization contraction stress in this films of chemically initiated resin composites. The method makes it possible to estimate the stress levels that occur in resin composite films in the clinical situation.

3D-finite element analyses of cusp movements in a human upper premolar, restored with adhesive resin-based composites

The combination of diverse materials and complex geometry makes stress distribution analysis in teeth very complicated. Simulation in a computerized model might enable a study of the simultaneous interaction of the many variables. A 3D solid model of a human maxillary premolar was prepared and exported into a 3D-finite element model (FEM). Additionally, a generic class II MOD cavity preparation and restoration was simulated in the FEM model by a proper choice of the mesh volumes. A validation procedure of the FEM model was executed based on a comparison of theoretical calculations and experimental data. Different rigidities were assigned to the adhesive system and restorative materials. Two different stress conditions were simulated: (a) stresses arising from the polymerization shrinkage and (b) stresses resulting from shrinkage stress in combination with vertical occlusal loading. Three different cases were analyzed: a sound tooth, a tooth with a class II MOD cavity, adhesively restored with a high (25 GPa) and one with a low (12.5GPa) elastic modulus composite. The cusp movements induced by polymerization stress and (over)-functional occlusal loading were evaluated. While cusp displacement was higher for the more rigid composites due to the pre-stressing from polymerization shrinkage, cusp movements turned out to be lower for the more flexible composites in case the restored tooth which was stressed by the occlusal loading. This preliminary study by 3D FEA on adhesively restored teeth with a class II MOD cavity indicated that Youngs modulus values of the restorative materials play an essential role in the success of the restoration. Premature failure due to stresses arising from polymerization shrinkage and occlusal loading can be prevented by proper selection and combination of materials.

An SEM evaluation of several adhesive systems used for bonding fiber posts under clinical conditions

OBJECTIVESnThe objective of this investigation was to evaluate the effectiveness of three one-bottle and two three step adhesive systems (as controls) in the formation of resin tags, adhesive lateral branches and a resin dentin interdiffusion zone (RDIZ) when used to bond fiber posts under clinical conditions. This study was performed using standardized SEM observations and scoring resin tag formation and density.nnnMETHODSnFifty root canal treated teeth, already scheduled for extraction for endodontic or periodontal reasons, were selected for this study. The patients were informed and their written consent was obtained. The samples were randomly allocated to five groups of ten samples each. Group 1: All Bond 2 with C & B (Bisco); Group 2: Scotchbond Multipurpose Plus with Opal Luting Composite (3M); Group 3: Scotchbond 1 in combination with Rely X ARC resin cement (3M); Group 4: One-step with C & B resin cement (Bisco); Group 5: All Bond Experimental with Post Cement HI-X (Bisco). The adhesive systems and resin cements were used strictly according to the manufacturers instructions. The priming-adhesive solution of each one-bottle system was light-cured before placing the resin cement and the post. Fifty Aestheti-Plus posts (white quartz fiber posts, RTD, France) were used. A week later, the roots were extracted and processed for SEM observations.nnnRESULTSnAll the adhesive systems showed a RDIZ and resin tag and adhesive lateral branch formation. Microscopic examination of the restored interfaces of Groups 1 and 2 showed a higher percent of RDIZ than those found in samples of Groups 3, 4 and 5. RDIZ morphology was easily detectable and uniform in the first two thirds of the root canals, whilst in the apical third the RDIZ was not uniformly present. Resin tag morphology and formation were significantly more detectable in the cervical and middle areas than in the apical zone. No statistically significant differences were found among the five groups coronally, while in the apical and middle thirds two one-bottle systems (Groups 3 and 4) showed significantly less resin tags than the control Groups 1, 2 and the experimental Group 5. No statistically significant differences were found among the three one-bottle systems in the three evaluated areas. The standardization of SEM observations and scoring resin tag formation allowed statistical evaluation.nnnCONCLUSIONSnThe three step adhesive systems can create a wider micromechanical interlocking between adhesive materials and etched dentin than one-step systems.

The effect of curing light variations on bulk curing and wall-to-wall quality of two types and various shades of resin composites

OBJECTIVESnThis study evaluated the influence of light intensity and irradiation time variations on the curing efficacy of two types and various shades of resin composites and the effect of reduced light intensity on the preservation of wall-to-wall continuity.nnnMATERIALS AND METHODSnThree microfilled composites (in three different shades) and one hybrid composite were used in this study. Polymerization shrinkage, and the hardness and adaptation of adhesive restorations in dentin cavities were determined at light intensities of 175 and 700 mW/cm2 and irradiation times of 10 and 60 s. Data were compared using in a general linear model analysis.nnnRESULTSnShrinkage measurements were the indication of conversion and conversion rate. Reduced intensity slowed down the rate of polymerization but did not reduce the conversion as long as an irradiation time of 60 s was employed. High-energy irradiation caused increased separation of the composite from the tooth structure. On the basis of obtaining optimal conversion and adaption, it was demonstrated that the irradiation time to be more effective than irradiation energy.nnnSIGNIFICANCEnLight-cured composites require an understanding of their structure, pigmentation and irradiation parameters to obtain optimal performance. High intensity light-curing does not necessarily lead to optimal quality.

The role of collagen in the elastic properties of calcified tissues

Abstract Bony tissues, considered as a two phase composite system of HAP and collagen, are described as a family of Reuss solids where the soft matrix becomes stiffer as the hard filler is added until a maximum elastic modulus is reached. Polymeric elastic moduli increase with cross linking density and shortening of maximum cross link length in macromolecules. In some polymers like epoxy these are strongly affected by the solid filler. Close similarity is found between HAP-collagen and FAP-epoxy. The concept is supported from evidence found in chemistry, physical properties, hierarchal structure and mineralization characteristics of collagen. It is postulated that increased cross linking density is associated with increased HAP content and the HAP crystallites provide rigid bases for shortened links to stiffen the composite by reversible enzyme directed processes.