The effect of resin composite pre-heating on monomer conversion and polymerization shrinkage

Objectives. To determine monomer conversion and polymerization shrinkage of a resin composite after different pre-heating procedures and storage intervals. Methods. For a commercial resin-based composite the immediate (5 min) and final (24 h) degree of conversion was measured on top and bottom surfaces utilizing FTIR spectroscopy Composite pre-heating temperatures were selected between 10 and 68 degrees C. Polymerization shrinkage was measured according to Archimedes’ principles of buoyancy after 5 min at respective pre-heating temperatures and after 24 h dark and wet storage at 37 degrees C. Intra-cavity temperature development was monitored using a K-type thermocouple. Results. No significant increase in immediate as well as in final degree of conversion were measured from composite pre-heating at 68 degrees C compared to 54 and 39 degrees C. Linear correlations were detected immediately after photo-polymerization and on the top surface after 24 h storage. Polymerization shrinkage as a function of pre-heating temperatures exhibited a linear correlation after 5 min, but no statistically different behavior after 24 h. Significance. Pre-heating of resin composites does not increase degree of conversion over time. It can be clinically beneficial, due to a superior marginal adaptation. This advantageous effect of reduced material paste viscosity has to be clinically addressed, since temperature rapidly drops to the physiological level upon removal from the pre-heating device. (C) 2008 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved

In vivo shell-like fractures of veneered-ZrO2 fixed dental prostheses

Fractographic analyses are performed in two fixed dental prosthetic reconstructions made of ZrO2 frameworks covered by a veneering ceramic that fractured during function in the mouth. Processing histories, material properties, recovered broken parts and replicas of the fracture surface were used, along with fractographic markings to determine fracture origins and cause of failure. A shell-like fracture pattern was found common for both cases, although different factors were identified to be involved in each fracture event. Internal thermal residual stresses and occlusal surface defects from localized contact overloading were found to precipitate the fracture in Case 1, whereas extreme occlusal surface damage from sliding chewing contact was determinant in the fracture of Case 2. The interface between the veneering ceramic and the ZrO2 framework was unaffected by the fractures

Fracture toughness of dental incremental composite-composite interfaces at elevated temperatures

The aim of the present laboratory study was to mechanically characterize the interface between two dental resin-based composite (RBC) increments, and to investigate if elevated temperatures have an influence on the quality of the interface mimicking clinical filling procedure. Four RBCs (CLEARFIL MAJESTY (TM) Posterior, Kuraray (CMP)/Filtek (TM) Supreme XTE, 3M (FSX)/Grandio (R) SO, VOCO (GSO)/VisCalor (R) bulk, VOCO (VCB)) were tested with a fracture toughness test using Chevron notched beams (K-I,K-CNB) at 23, 37 and 54 degrees C. K-I,K-CNB specimens (3 x 4x25mm) with a V-shaped notch at the incremental interface were loaded until failure in a 4-point bending set-up. Failure modes were characterized using light microscopy, microstructural interface was analyzed using SEM. Statistical analysis was performed using Kolmogorov-Smirnoff test, two-way ANOVA and Tukey Post-Hoc test (p = 0.05). Mean K-I,K-CNB ranged between 0.73 +/- 0.14 MPam(0.5) (VCB, 23 degrees C) and 1.11 +/- 0.11 MPam(0.5) (FSX, 23 degrees C). The tested conventional highly filled RBCs presented fracture toughness at the incremental interface comparable to the cohesive strength of the bulk materials. VCB showed reduced interfacial fracture toughness at 23 and 37 degrees C, but performed well at elevated temperature of 54 degrees C

Crack growth behavior of a biomedical polymer-ceramic interpenetrating scaffolds composite in the subcritical regimen

We subjected a commercial dental composite formed by interconnecting polymer-ceramic scaffolds to extensive quasi-static, static and cyclic experiments under biaxial flexure. By this we meant to obtain static and cyclic subcritical crack growth exponents that, based on stablished relationships describing the degradation of frictional bridging mechanisms, challenging the notion of a suggested R-curve behavior. By exploring the fracture statistics of specimens with increasing effective volumes and effective areas, we demonstrated the presence of a bi-modal defect size distribution in disaccord with the Weibull behavior across length scales. Lifetime distributions seemed to follow the strength distributions, and were used to derive crack growth velocity diagrams for combined levels of applied stress. Ultimately, the claim of an R-curve behavior could not be supported based on the absence of any significant cyclic fatigue effect, i.e. bridging degradation

Repair Bond Strength of Aged Resin Composite after Different Surface and Bonding Treatments

The aim of this study was to compare the effect of different mechanical surface treatments and chemical bonding protocols on the tensile bond strength (TBS) of aged composite. Bar specimens were produced using a nanohybrid resin composite and aged in distilled water for 30 days. Different surface treatments (diamond bur, phosphoric acid, silane, and sandblasting with Al2O3 or CoJet Sand), as well as bonding protocols (Primer/Adhesive) were used prior to application of the repair composite. TBS of the specimens was measured and the results were analyzed using analysis of variance (ANOVA) and the Student–Newman–Keuls test (α = 0.05). Mechanically treated surfaces were characterized under SEM and by profilometry. The effect of water aging on the degree of conversion was measured by means of FTIR-ATR spectroscopy. An important increase in the degree of conversion was observed after aging. No significant differences in TBS were observed among the mechanical surface treatments, despite variations in surface roughness profiles. Phosphoric acid etching significantly improved repair bond strength values. The cohesive TBS of the material was only reached using resin bonding agents. Application of an intermediate bonding system plays a key role in achieving reliable repair bond strengths, whereas the kind of mechanical surface treatment appears to play a secondary role

A Photoelastic Assessment of Residual Stresses in Zirconia-Veneer Crowns

Residual stresses within the veneer are linked to the high prevalence of veneer chipping observed in clinical trials of zirconia prostheses. We hypothesized that the thermal mismatch between the zirconia infrastructure and the veneer porcelain, as well as the rate used for cooling zirconia-veneer crowns, would be directly proportional to the magnitude of residual stresses built within the veneer layer. Two porcelains with different coefficients of thermal expansion were used to veneer zirconia copings, to create high or low thermal mismatches. The crowns were cooled according to a fast- or a slow-cooling protocol. The retardation of polarized light waves was used to calculate the residual stress magnitude and distribution across the veneer, according to the photoelasticity principle, in 1.0-mm-thick crown sections. While thermal mismatch was an important factor influencing the maximum stress development in the veneer, cooling rate had a minor role. Curved surfaces were preferential sites for stress concentration regardless of thermal mismatch or cooling rate

The potential of novel primers and universal adhesives to bond to zirconia

Objectives: To investigate the adhesive potential of novel zirconia primers and universal adhesives to surface-treated zirconia substrates.Methods: Zirconia bars were manufactured (3.0 mm x 3.0 mm x 9.0 mm) and treated as follows: no treatment (C); air abrasion with 35 mu m alumina particles (S); air abrasion with 30 mu m silica particles using one of two systems (Rocatec or SilJet) and; glazing (G). Groups C and S were subsequentially treated with one of the following primers or adhesives: ZP (Z-Prime Plus), AZ (AZ Primer); MP (Monobond Plus); SU (ScotchBond Universal) and; EA (an Experimental Adhesive). Groups Rocatec and SilJet were silanized prior to cementation. Samples form group G were further etched and silanized. Bars were cemented (Multilink) onto bars of a silicate-based ceramic (3.0 mm x 3.0 mm x 9.0 mm) at 90 degrees angle, thermocycled (2.500 cycles, 5-55 degrees C, 30 s dwell time), and tested in tensile strength test. Failure analysis was performed on fractured specimens to measure the bonding area and crack origin.Results: Specimens from group C did not survive thermocycling, while CMP, CSU and CEA groups survived thermocycling but rendered low values of bond strength. All primers presented a better bond performance after air abrasion with Al2O3 particles. SilJet was similar to Rocatec, both presenting the best bond strength results, along with SMP, SSU and CEA. G promoted intermediate bond strength values. Failure mode was predominately adhesive on zirconia surface combined to cohesive of the luting agent.Conclusions: Universal adhesives (MP, SU, EA) may be a considerable alternative for bonding to zirconia, but air abrasion is still previously required. Air abrasion with silica particles followed by silane application also presented high bond strength values. (C) 2013 Elsevier Ltd. All rights reserved