Adhesive bonding of various materials to hard tooth tissues: forces developing in composite materials during hardening

Optical comparison between micro-CT and OCT in imaging of marginal composite adaptation: Observational study

Dental composite is the most used aesthetic restorative biomaterial worldwide. However, it undergoes polymerisation shrinkage that could lead to loss of the interfacial seal between tooth and resin in some circumstances. This demands high-resolution imaging technologies to detect these defects. This study carried out a comparison between microcomputed tomography (micro-CT; Shimadzu, Japan) and swept-source optical coherence tomography (SS-OCT; Santec, Japan) in the detection of marginal adaptation defects at the tooth-resin interface. Unlike in micro-CT, it was possible to outline interfacial gaps along with tooth-resin interfaces with SS-OCT, which was attributed to the Fresnel diffraction of light. This in vitro comparison demonstrates SS-OCT has great potential in dental imaging to effectively assess dental composite adaptation and marginal defects when high resolution is desired in real time. LAY DESCRIPTION: Detection of tooth-colored restoration defects had been assessed by different radiographic methods. However, most of these methods are either invasive or suffer from low-resolution. In this study, a comparison has been carried out between two different high-resolution imaging systems; microcomputed tomography and optical coherence tomography, to explore their potentials in detecting restorations defects. The results showed optical coherence tomography has a great accuracy in locating the underlying defects when the obtained images were validated against confocal laser scanning microscopy images.

Simulated Cuspal Deflection and Flexural Properties of Bulk-Fill and Conventional Flowable Resin Composites

CLINICAL RELEVANCE

Some bulk-fill flowable resin composites produce less cuspal deflection than a conventional incrementally filled flowable resin composites.

SUMMARY

Objective: This study investigated simulated cuspal deflection and flexural properties of bulk-fill and conventional flowable resin composites.Methods and Materials: Five bulk-fill and six conventional flowable resin composites were evaluated. Aluminium blocks with a mesio-occlusal-distal cavity were prepared and randomly divided into groups for each of the different measurement techniques and were further subdivided according to the type of flowable resin composite. The simulated cuspal deflection caused by the polymerization of resin composite within an aluminium block was measured using a highly accurate submicron digimatic micrometer or a confocal laser scanning microscope (CLSM). In addition, the flexural properties of tested resin composites were measured to investigate the relation between cuspal deflection and flexural properties, and the resin composites were observed using scanning electron microscopy.Results: Simulated cuspal deflection of some bulk-fill flowable resin composites was found to be significantly lower than or similar to those for conventional counterparts, regardless of the measurement method. There were statistically significant differences in flexural properties depending on the material, regardless of the type of flowable resin composite. Pearson correlation analysis did not reveal a statistically significant relationship between flexural properties and cuspal deflection.Conclusion: These results indicate that some bulk-fill flowable resin composites exhibit lower cuspal deflection with the bulk-filling technique than is shown by conventional flowable resin composites using the incremental filling technique. Simulated cuspal deflection can be measured using either a micrometer or CLSM, but this experiment failed to show a significant relationship between cuspal deflection and flexural properties of flowable resin composites.

Microcomputed tomography evaluation of cement shrinkage under zirconia versus lithium disilicate veneers

STATEMENT OF PROBLEM

Computer-aided design and computer-aided manufacturing (CAD-CAM) technology and the improved translucency of recently developed high-strength monolithic zirconia could make them clinically acceptable for veneers if bonding to zirconia was as predictable as to glass-ceramics. Few studies have compared how resin cements behave between glass-ceramic and zirconia veneers before and after polymerization.

PURPOSE

The purpose of this in vitro study was to evaluate the volumetric polymerization shrinkage of resin cement, marginal discrepancy, and cement thickness before and after polymerization for glass-ceramic and zirconia veneers with light-polymerizing resin cement.

MATERIAL AND METHODS

Ten lithium disilicate veneers and 10 zirconia veneers were fabricated with a CAD-CAM workflow on extracted human maxillary anterior teeth with intact enamel surfaces. Zirconia veneers were treated with airborne-particle abrasion, and lithium disilicate veneers were etched with 5% hydrofluoric acid. All specimens were treated with ceramic primer and cemented with a light-polymerized resin cement. All specimens were scanned before and after resin cement polymerization by microcomputed tomography. The data were processed by the Amira software program to compare polymerization volumetric shrinkage, cement thickness, and marginal discrepancy. The data were compared by using a t test and analysis of variance (α=.05). Two bonded veneers were loaded in a mastication simulator for 400 000 cycles to investigate the effect of cyclic fatigue loading.

RESULTS

Mean volumetric polymerization shrinkage was 4.2 ±0.8% for the lithium disilicate group and 6.4 ±3.5% for the zirconia group. No significant difference was found for volumetric shrinkage between materials (P=.132). The mean ±standard deviations of the marginal discrepancies before and after polymerization were 178 ±41 μm and 158 ±37 μm for lithium disilicate and 115 ±33 μm and 107 ±32 μm for zirconia. A smaller marginal discrepancy was found for both materials after polymerization (P=.011) and for zirconia compared with lithium disilicate (P=.004). The mean ±standard deviation cement thickness values before and after polymerization were 157 ±27 μm and 147 ±27 μm for lithium disilicate and 162 ±53 μm and 147 ±52 μm for zirconia. Smaller cement thickness was found after polymerization (P<.001), whereas no significant difference was found in cement thickness between materials (P=.144). No changes were noted in marginal discrepancy and cement thickness as a result of the fatigue loading.

CONCLUSIONS

The difference in the volumetric polymerization shrinkage of cement between lithium disilicate and zirconia veneers was not statistically significant. Polymerization shrinkage resulted in smaller marginal discrepancy and cement thickness for both veneer materials.

Simulated cuspal deflection and flexural properties of high viscosity bulk-fill and conventional resin composites

OBJECTIVES

The purpose of this study was to investigate the simulated cuspal deflection and flexural properties of high viscosity bulk-fill and conventional resin composites.

METHODS

Seven high viscosity bulk-fill resin composites and eight conventional resin composites were used. Aluminum blocks (10 mm x 8 mm x 15 mm) with a mesio-occlusal-distal (MOD) cavity [4 (W) mm x 8 (L) mm x 4 (D) mm] were prepared and randomly divided into groups for different measurement techniques [micrometer vs CSLM] and further subdivided according to type of resin composite (high viscosity bulk-fill vs conventional resin composite). The simulated cuspal deflection resulting from the polymerization of resin composite bonded to a precisely machined MOD cavity within an aluminum block was measured with either a novel highly accurate submicron digimatic micrometer (MDH-25 M, Mitsutoyo, Tokyo, Japan) or a confocal laser scanning microscope (CLSM, VK-9710, Keyence, Tokyo, Japan) cuspal measurement method. In addition, flexural properties of tested resin composites were measured to investigate the relationship between simulated cuspal deflection and flexural properties. Scanning electron microscopy observation of tested resin composites was also conducted.

RESULTS

The simulated cuspal deflection of high viscosity bulk-fill resin composites was similar to that of conventional resin composites, regardless of measurement method. There were no statistically significant differences (p > 0.05) between the micrometer and CLSM cuspal measurement methods. There were statistically significant differences (p < 0.05) in flexural strength and elastic modulus depending on the material, regardless of the type of resin composite. Pearson correlation analysis did not show any statistically significant (p < 0.05) relationship between flexural properties and cuspal deflection.

CONCLUSIONS

The results of this study indicate that high viscosity bulk-fill resin composites show similar cuspal deflection with bulk-filling techniques, to those shown by conventional resin composites with incremental filling techniques. Simulated cuspal deflection can be measured using either a micrometer or CLSM, but this experiment failed to show any relationship between the flexural properties and simulated cuspal deflection of resin composites.

SIGNIFICANCE

High viscosity bulk-fill resin composites produce the same level of cuspal deflection as a conventional incrementally filled resin composite.

The Influence of Water Sorption of Dental Light-Cured Composites on Shrinkage Stress

The contraction stress generated during the photopolymerization of resin dental composites is the major disadvantage. The water sorption in the oral environment should counteract the contraction stress. The purpose was to evaluate the influence of the water sorption of composite materials on polymerization shrinkage stress generated at the restoration-tooth interface. The following materials were tested: Filtek Ultimate, Gradia Direct LoFlo, Heliomolar Flow, Tetric EvoCeram, Tetric EvoCeram Bulk Fill, Tetric EvoFlow, Tetric EvoFlow Bulk Fill, X-tra Base, Venus BulkFil, and Ceram.X One. The shrinkage stress was measured immediately after curing and after: 0.5 h, 24 h, 72 h, 96 h, 168 h, 240 h, 336 h, 504 h, 672 h, and 1344 h by means of photoelastic study. Moreover, water sorption and solubility were evaluated. Material samples were weighted on scale in time intervals to measure the water absorbency and the dynamic of this process. The tested materials during polymerization generated shrinkage stresses ranging from 6.3 MPa to 12.5 MPa. Upon water conditioning (56 days), the decrease in shrinkage strain (not less than 48%) was observed. The decrease in value stress in time is material-dependent.

Polymerization shrinkage kinetics and shrinkage-stress in dental resin-composites

OBJECTIVES

To investigate a set of resin-composites and the effect of their composition on polymerization shrinkage strain and strain kinetics, shrinkage stress and the apparent elastic modulus.

METHODS

Eighteen commercially available resin-composites were investigated. Three specimens (n=3) were made per material and light-cured with an LED unit (1200mW/cm(2)) for 20s. The bonded-disk method was used to measure the shrinkage strain and Bioman shrinkage stress instrument was used to measure shrinkage stress. The shrinkage strain kinetics at 23°C was monitored for 60min. Maximum strain and stress was evaluated at 60min. The shrinkage strain rate was calculated using numerical differentiation.

RESULTS

The shrinkage strain values ranged from 1.83 (0.09) % for Tetric Evoceram (TEC) to 4.68 (0.04) % for Beautifil flow plus (BFP). The shrinkage strain rate ranged from 0.11 (0.01%s(-1)) for Gaenial posterior (GA-P) to 0.59 (0.07) %s(-1) for BFP. Shrinkage stress values ranged from 3.94 (0.40)MPa for TET to 10.45 (0.41)MPa for BFP. The apparent elastic modulus ranged from 153.56 (18.7)MPa for Ever X posterior (EVX) to 277.34 (25.5) MPa for Grandio SO heavy flow (GSO).

SIGNIFICANCE

The nature of the monomer system determines the amount of the bulk contraction that occurs during polymerization and the resultant stress. Higher values of shrinkage strain and stress were demonstrated by the investigated flowable materials. The bulk-fill materials showed comparable result when compared to the traditional resin-composites.

Visualization study on distortion of a metal frame by polymerization shrinkage and thermal contraction of resin

Three types of metal specimens (ring-shaped, plate-shaped, and a simulated anterior arch) for distortion observations were made from Au-Ag-Pd-Cu alloy. Distortion due to polymerization shrinkage and thermal contraction of a heat-curing acrylic resin containing 4-META (4-methacryloyloxyethyl trimellitate anhydride, 4-META resin) could be visualized for the ring-shaped specimen, which showed increasing distortion of the metal frame upon adhesion of the resin to the outer metal surface. Distortion of the plateshaped specimen adhering to 4-META resin decreased with increasing thickness of the cured resin. The distortion of the metal frame simulating an anterior arch of a six-unit bridge with a facing composite resin showed that the curvature of the metal frame was larger after curing of the facing composite resin. However, it recovered most of its original curvature with an associated increase in the number of cracks between the crowns after trimming the resin to a tooth profile.

Analysis of residual stress in the resin of metal-resin adhesion structures by scanning acoustic microscopy

The residual stress caused by polymerization shrinkage and thermal contraction of a heat-curing resin containing 4-META on a metal-resin structure was measured by a scanning acoustic microscope. The tensile residual stress in the resin occurred within 70 µm of the adhesion interface with a flat plate specimen. The maximum tensile stress was about 58 MPa at the interface. On a metal plate specimen with retention holes, ring-like cracks in the resin occurred around the retention holes with the adhesive specimen and many linear cracks occurred in the resin vertical to the longitudinal direction of the metal frame with the non-adhesive specimens. There was tensile residual stress on the resin surface at the center of the retention holes of the adhesion specimen, indicating that the stress in the specimen with surface treatment for adhesion was higher than in that without surface treatment.

Shear bond strength and SEM morphology evaluation of different dental adhesives to enamel prepared with ER:YAG laser

CONTEXT

Early observations of enamel surfaces prepared by erbium lasers motivated clinicians to use laser as an alternative to chemical etching.

AIMS

Evaluate shear bond strength (SBS) values of different dental adhesives on Erbium:Yttrium Aluminum Garnet (Er:YAG) laser prepared enamel and to evaluate possible etching patterns correlations between dental adhesives and SBS values.

SUBJECTS AND METHODS

One hundred bovine incisors were randomly assigned to SBS tests on enamel (n = 15) and to enamel morphology analysis (n = 5) after Er:YAG laser preparation as follows: Group I - 37% phosphoric acid (PA)+ ExciTE(®); Group II - ExciTE(®); Group III - AdheSE(®) self-etching; Group IV - FuturaBond(®) no-rinse. NR; Group V - Xeno(®) V. Teeth were treated with the adhesive systems and subjected to thermal cycling. SBS were performed in a universal testing machine at 5 mm/min.

STATISTICAL ANALYSIS USED

One-way ANOVA and post-hoc tests (P < 0.05). For the morphology evaluation, specimens were immersed in Ethylenediamine tetraacetic acid (EDTA) and the etching pattern analyzed under Scanning Electron Microscope (SEM).

RESULTS

Mean bond strengths were Group I - 47.17 ± 1.61 MPa (type I etching pattern); Group II - 32.56 ± 1.64 MPa, Group III - 29.10 ± 1.34 MPa, Group IV - 23.32 ± 1.53 MPa (type III etching pattern); Group V - 24.43 MPa ± 1.55 (type II etching pattern).

CONCLUSIONS

Different adhesive systems yielded significantly different SBSs. Acid etching significantly increased the adhesion in laser treated enamel. No differences in SBS values were obtained between AdheSE(®) and ExciTE(®) without condition with PA. FuturaBond(®) NR and Xeno(®) V showed similar SBS, which was lower in comparison to the others adhesives. No correlation between enamel surface morphology and SBS values was observed, except when PA was used.

Adhesion of amorphous calcium phosphate composites bonded to dentin: a study in failure modality

AIMS

As a bioactive filler capable of remineralizing tooth structures, the main disadvantage of as-made amorphous calcium phosphate (am-ACP) are its large agglomerates. The objective of this study was to mill ACP, and compare the adhesive strength with dentin, work to fracture, and failure modes of both groups to glass-filled composites and one commercial compomer after 24 h, 1 week, 1, 3, and 6 months of exposure to simulated saliva solution (SLS). Flat dentin surfaces were acid-etched, primed, and photopolymerized. Composites were applied, photo-cured, and debonded in shear. The resin used in each composite was identical: ethoxylated bisphenol A dimethacrylate, triethylene glycol dimethacrylate, 2-hydroxyethyl methacrylate, and methacryloxyethyl phthalate. Fillers consisted of am-ACP and milled ACP (m-ACP), and a strontium-containing glass (Sr-glass) at respective mass fractions of (40, 60, and 75%).

FINDINGS

Ninety percent of the fracture surfaces in this study showed adhesive failure, with most of these occurring at the dentin/primer interface. Fifty-two percent of failures after 24-h immersion occurred at the primer/composite interface. After 6 months of SLS exposure, 80% of specimens failed at the dentin/primer interface, with a 42% overall reduction in bond strength.

CONCLUSIONS

Milled ACP composites showed initial mechanical advantages over am-ACP composites and the compomer, and produced a higher incidence of a failure mode consistent with stronger adhesion. Evidence is provided which suggests that milled ACP composites may offer enhanced potential in clinical bonding applications.

Polymerization stress development in dental composites: Effect of cavity design factor

The objective of the study was to assess the effect of the cavity design factor (C-factor) on polymerization stress development (PSD) in resin composites. An experimental resin (BT resin) was prepared, which contained 2,2-bis[p-(2’-hydroxy-3’-methacryloxypropoxy)phenylene]propane (B) and triethylene glycol dimethacrylate (T) in 1:1 mass ratio, and an activator for visible light polymerization. An experimental composite with demonstrated remineralizing potential was also formulated by inclusion into the BT resin of zirconia-hybridized amorphous calcium phosphate (ACP) filler at a mass fraction of 40 % (BT/ACP composite). A commercial glass-filled composite (TPH) was used as a control. To assess the effect of the test geometry on PSD, C-factor was systematically varied between 0.8 and 6.0 by varying the height of the cylindrical composite specimens. The measured PSD values obtained by cantilever beam tensometry for specimens with variable C-factors were normalized for mass to specimens with a C-factor of 1.33 (h=2.25 mm) as controls to give calculated PSD values. Degrees of vinyl conversions (DC) attained in the TPH control and in the experimental BT/ACP composites were measured by near-infrared spectroscopy. In both the TPH and BT/ACP composite series, PSD_calc increased with the increasing C-factor, confirming the hypothesis that the C-factor value influences PSD values. The higher PSD_meas and PSD_calc values for the experimental BT/ACP composite compared to the commercial TPH composite probably reflect differences in the type and mass of the resin and filler phases in the two types of composite. These differences also account for the observed variation (21 %) in DC attained in a BT/ACP composite 2 h after cure (69.5 %) and in the DC of the TPH composite (57.5 %) having the same C-factor. The cavity design factor seems to play a key role in influencing the PSD of bonded composites, but other factors such as composite mass and composition also must be considered for their effects on PSD.

Comparative analysis of the shrinkage stress of composite resins

The aim of this study was to compare the shrinkage stress of composite resins by three methods. In the first method, composites were inserted between two stainless steel plates. One of the plates was connected to a 20 kgf load cell of a universal testing machine (EMIC-DL-500). In the second method, disk-shaped cavities were prepared in 2-mm-thick Teflon molds and filled with the different composites. Gaps between the composites and molds formed after polymerization were evaluated microscopically. In the third method, the wall-to-wall shrinkage stress of the resins that were placed in bovine dentin cavities was evaluated. The gaps were measured microscopically. Data were analyzed by one-way ANOVA and Tukey's test (alpha=0.05). The obtained contraction forces were: Grandio = 12.18 +/- 0.428N; Filtek Z 250 = 11.80 +/- 0.760N; Filtek Supreme = 11.80 +/- 0.707 N; and Admira = 11.89 +/- 0.647 N. The gaps obtained between composites and Teflon molds were: Filtek Z 250 = 0.51 +/- 0.0357%; Filtek Supreme = 0.36 +/- 0.0438%; Admira = 0.25 +/- 0.0346% and Grandio = 0.16 +/- 0.008%. The gaps obtained in wall-to-wall contraction were: Filtek Z 250 = 11.33 +/- 2.160 microm; Filtek Supreme = 10.66 +/- 1.211 microm; Admira = 11.16 +/- 2.041 microm and Grandio = 10.50 +/- 1.224 microm. There were no significant differences among the composite resins obtained with the first (shrinkage stress generated during polymerization) and third method (wall-to-wall shrinkage). The composite resins obtained with the second method (Teflon method) differed significantly regarding gap formation.

Influence of light intensity on contraction stress of flowable resins

The purpose of this study was to evaluate the influence of power density on contraction stress of resin composite restorative materials during photo-polymerization. Six flowable resin composites, and a hybrid resin composite for comparison, were used. The composites were polymerized with the power density adjusted to either 100 or 600 mW/cm(2). Stress development was determined with a custom-made tensilometer. The adhesive was placed in a thin layer on a steel rod and resin paste was packed into the mold. The contraction force (N) generated during polymerization was continuously recorded and the maximum contraction stress (MPa) was calculated. Data were analyzed statistically. When the power density was adjusted to 100 mW/cm(2), the average contraction stress ranged from 0.30 to 0.50 MPa for the flowable composites, compared with 0.35 MPa for the hybrid composite. When the power density was adjusted to 600 mW/cm(2), the average contraction stress ranged from 0.34 to 1.00 MPa for the flowable composites and 0.69 MPa for the hybrid composite comparison. For all materials tested except Estelite Flow Quick, contraction stress increased with higher power density. The present results indicate that contraction stress during polymerization is influenced by power density and resin composite type.

Influence of Tg, viscosity and chemical structure of monomers on shrinkage stress in light-cured dimethacrylate-based dental resins

OBJECTIVES

The aim of this study was to investigate the influence of the molecular mobility and the chemical structure of dimethacrylates most commonly used in dental composites on shrinkage stress from experimental matrices.

METHODS

Three established neat monomers BisGMA (B), UEDMA (U) and TEGDMA (T), two experimental comonomers BisGMA-based (B-T(70/30) and B-T(50/50)) and two comonomers UEDMA-based (U-T(88/11) and U-T(66/33)) in weight%, were elaborated. Camphorquinone (CQ) and N,N-cyanoethylmethylaniline (CEMA), as photoinitiator and reducing agent, were added. Then the matrices were mixed by centrifugal force at room temperature. The viscosity (eta), the glass transition temperature of the monomers and comonomers systems (T(g(monomer)) and the maximum shrinkage stress (MSS) of each material (five replications) were statistically analysed by one-way ANOVA/Tuckey's test and Pearson's correlation procedure (p = 0.05).

RESULTS

All formulations exhibited a newtonian rheological behavior. The viscosity of the comonomers systems can be divided in two groups: the pair B-T(70/30)/U-T(88/11) with the viscosity 3.5+/-3.10(-3)Pa.s and the pair B-T(50/50)/U-T(66/33) with the viscosity 0.28+/-3.10(-3)Pa.s. This pairs constituted samples allowing to compare the shrinkage stress of the BisGMA and UEDMA-based matrices with each other. The T(g(monomer)) of each group showed equivalent statistically values: -37.1 +/- 0.02 degrees C (U-T(88/11)) with -39.3 +/- 0.02 degrees C (B-T(70/30)) for the 3.5 Pa.s pair, and -53.1 +/- 0.03 degrees C (U-T(66/33)) with -58.5 +/- 0.01 degrees C (B-T(50/50)) for the 0.28 Pa.s one. There was a correlation between eta and T(g(monomer)) (r < 0.45 and p < 0.01). In decreasing order, the shrinkage stress was 14.11 +/- 0.3 MPa (T), 10.64 +/- 0.6 MPa (U-T(66/33)), 8.16 +/- 0.25 MPa (B-T(50/50)) without a significant difference compared to 8.04 +/- 0.5 MPa (U-T(88/11)), 6.83 +/- 0.52 MPa (U), 4.44 +/- 0.25 MPa (B-T(70/30)) and 0.33 +/- 0.3 MPa (B). There was a negative correlation between eta (r < -0.42 and p < 0.01), T(g(monomer)) (r < -0.41 and p < 0.01) and MSS. Whatever the viscosity, the UEDMA-based matrices developed higher shrinkage stresses than the BisGMA homologues.

SIGNIFICANCE

The shrinkage stress development increase with the molecular mobility of the reacting medium. For the same molecular mobility, the large differences in stress values of the matrices studied are correlated to the structure and particularly the functionality of the monomers used.

Microleakage in the proximal walls of direct and indirect posterior resin slot restorations

This study compared the degree of microleakage in the proximal walls of direct and indirect resin slot restorations in relation to the types of dentin bonding systems and the location of gingival margins. Two Class II slot preparations were prepared and restored in each of 60 extracted human molars using direct (Filtek Supreme) and indirect (Tescera ATL) restorative resin materials. Various types of dentin bonding systems, including self-etching (OneStep Plus/Tyrian SPE, iBond, Xeno III) and etch and rinse systems (All-Bond 2, Prime & Bond NT) were used to restore the prepared teeth. The gingival proximal wall was placed apical to the cementoenamel junction (CEJ) in 1 proximal box and coronal to the CEJ in the other. The specimens were stained and evaluated for microleakage using a digital imaging and analysis system. Significant differences were found in the degree of microleakage observed in the various restorative groups. In general, the group restored with indirect resin had less microleakage than the direct resin groups. Factors, such as type of dentin bonding system and location of gingival margins, exert a substantial influence on the degree of microleakage that occurred along the walls of proximal resin restorations.

Shrinkage stress in light-cured composite resins: influence of material and photoactivation mode

OBJECTIVES

The aim of this study was to record the effect of composite type and photoactivation mode on the stresses resulting from polymerization of five established composite resins: packable (Solitaire, Solitaire 2), micro-hybrid (Aelitefil, Z100) and hybrid (Clearfil AP-X).

METHODS

A mechanical testing machine was used to record the polymerization contraction stress (MPa) of cylindrical composite specimens (d=5mm; h=2mm; C-factor=1.25) at 0.1s intervals over a period of 400s. The samples were photopolymerized using a halogen light curing device under two types of light exposure: group 1, Standard (800mW.cm(-2)x60s); group 2, Exponential (logarithmic increase from 150 to 800mW.cm(-2) over 15s+800mW.cm(-2)x45s). The stress rate (SR: slope(MPa>0-60s)) and the maximum shrinkage stress (MSS: MPa(400s)) of each material (five replications) were statistically analysed by one-way ANOVA/Scheffe's test and Pearson's correlation procedure (alpha=0.05). Finally, Student's t-test (two matched series) enabled the assessment of the effect of the irradiation method on the results.

RESULTS

For group 1, in decreasing order, the MSS was 1.51+/-0.07MPa (Solitaire) statistically equivalent to 1.45+/-0.06MPa (Aelitefil), 1.29+/-0.08MPa (Solitaire 2), and 1.04+/-0.03MPa (Z100) statistically equivalent to 0.92+/-0.05MPa (Clearfil AP-X). Z100 showed the highest SR (0.045+/-6x10(-3)) and Solitaire, the lowest (0.017+/-2x10(-3)). There was no correlation between SR and MSS (r<-0.33, p<0.05). For group 2, the MSS and SR values were distributed in a similar way to those from group 1. There was a negative correlation between SR and MSS (r<-0.43 and p<0.01). The exponential ramp successfully reduced the MSS (-3.9%) and SR (-11%) values.

SIGNIFICANCE

There is no relationship between composite resin type, stress rate and shrinkage stress levels. The slower stress rate development, resulting from ramped light intensity, helped slightly to reduce the maximum polymerization stress.

Volume contraction in photocured dental resins: The shrinkage-conversion relationship revisited

Polymerization shrinkage and degree of conversion (DC) of resin composites are closely related manifestations of the same process. Ideal dental composite would show an optimal degree of conversion and minimal polymerization shrinkage. These seem to be antagonistic goals, as increased monomer conversion invariably leads to high polymerization shrinkage values.

OBJECTIVES

This paper aims at accurately determining the polymerization volume contraction of experimental neat resins and to link it to the number of actual vinyl double bonds converted in single ones instead of, as generally done, to the degree of conversion.

METHODS

Different mixtures of Bis-GMA/TEGDMA (traditionally used monomers) were analyzed. Contraction of the polymers was determined by pycnometry and the use of a density column. DC was determined by the use of Raman spectrometry.

RESULTS

An univocal relationship has been found between the volume contraction and the actual number of vinyl double bonds converted into single ones. A contraction value of 20.39 cm3/mole (of converted C=C) was deduced from 27 measurements.

SIGNIFICANCE

This relationship helps in finding solutions to the polymerization shrinkage problem. A reduction of the polymerization shrinkage due to the chemical reaction may obviously be expected from the addition of molecules allowing a decrease in the number of double bonds converted per unit volume of resin matrix, while maintaining the degree of conversion (of Bis-GMA and TEGDMA) and thus the mechanical properties. Further research will be directed at this objective.

Clinical performance and marginal adaptation of class II direct and semidirect composite restorations over 3.5 years in vivo

OBJECTIVE

The study evaluated the clinical performance and marginal adaptation of direct and semi-direct class II composite restorations in a split-mouth design over 3.5 years.

DESIGN

44 upper posterior teeth in 11 adults with primary carious lesions were treated with 22 direct and 22 semi-direct restorations. Conventional cavities were prepared for both types of restorations. A fine fine hybrid composite (APH) and a multifunctional adhesive system (Prisma universal bond 3) were used for all restorations. The incremental "3-sited light curing" technique was applied to direct restorations. Semi-direct inlays were prefabricated on silicone casts and post-cured using light and heat. Clinical performance was evaluated using modified USPHS parameters, while marginal adaptation was judged on replicas, using SEM and a standardized evaluation technique.

RESULTS

Clinical results after 3.5 years revealed a 100% retention rate with no fractures, sensitivity or recurrent caries for both types of restorations. SEM-evaluation of the occlusal margins showed at the tooth-restoration interface relatively low rates of marginal openings over the observation period (4-8%). Marginal restoration fractures ranged between 1 and 2%, marginal tooth fractures between 3 and 9%. Differences between the restorative techniques and after the different time observation periods were not statistically significant. Proportions of marginal fractures and openings at the restoration-luting composite interface were less than 10% after 3.5 years.

CONCLUSION

The results indicated no significant differences for direct and semi-direct fine hybrid composite restorations in medium size cavities in posterior teeth with respect to clinical performance and marginal adaptation over 3.5 years.

In vitro evaluation of marginal and internal adaptation after occlusal stressing of indirect class II composite restorations with different resinous bases

Composite inlays are indicated for large cavities, which frequently extend cervically into dentin. The purpose of this study was to compare in vitro the marginal and internal adaptation of class II fine hybrid composite inlays (Herculite, Kerr) made with or without composite bases, having different physical properties. Freshly extracted human molars were used for this study. The base extended up to the cervical margins on both sides and was made from Revolution (Kerr), Tetric flow (Vivadent), Dyract (Detrey-Dentsply) or Prodigy (Kerr), respectively. Before, during and after mechanical loading (1 million cycles, with a force varying from 50 to 100 N), the proximal margins of the inlay were assessed by scanning electron microscopy. Experimental data were analysed using non-parametric tests. The final percentages of marginal tooth fracture varied from 30.7% (no base) to 37.6% (Dyract). In dentin, percentages of marginal opening varied from 9.2% (Tetric Flow) to 30.1% (Prodigy), however, without significant difference between base products. Mean values of opened internal interface with dentin varied from 11.06% (Tetric Flow) to 28.15% (Prodigy). The present results regarding dentin adaptation confirmed that the physical properties of a base can influence composite inlay adaptation and that the medium-rigid flowable composite Tetric Flow is a potential material to displace, in a coronal position, proximal margins underneath composite inlays.

Marginal and internal adaptation of class II restorations after immediate or delayed composite placement

Direct class II composite restorations still represent a challenge, particularly when proximal limits extend below the CEJ. The aim of this in vitro study was to evaluate the influence of the type of adhesive and the delay between adhesive placement and composite insertion on restoration adaptation. Direct class II MOD box-shaped composite restorations (n=8 per group) were placed on intact human third molars, with proximal margins 1mm above or under CEJ. All cavities were filled with a horizontal layering technique, immediately after adhesive placement (IP) or after a 24h delay (DP). A filled three-component adhesive (OptiBond FL: OB) and a single-bottle, unfilled one (Prime & Bond 2.1: PB) were tested. Marginal adaptation was assessed before and after each phase of mechanical loading (250000 cycles at 50 N, 250000 cycles at 75 N and 500000 cycles at 100 N); internal adaptation was evaluated after test completion. Gold-plated resin replicas were observed in the SEM and restoration quality evaluated in percentages of continuity (C) at the margins and within the internal interface, after sample section. Adaptation to beveled enamel proved satisfactory in all groups. After loading, adaptation to gingival dentin degraded more in PB-IP (C=55.1%) than PB-DP (C=86.9%) or OB-DP (C=89%). More internal defects were observed in PB samples (IP: C=79.2% and DP: C=86.3%) compared to OB samples (IP: C=97.4% and DP: C=98.3%). The filled adhesive (OB) produced a better adaptation than the 'one-bottle' brand (PB), hypothetically by forming a stress-absorbing layer, limiting the development of adhesive failures. Postponing occlusal loading (such as the indirect approach) improved also restoration adaptation.

The influence of the restoration-tooth interface in light cured composite restorations: a finite element analysis

This study examines the early shrinkage behaviour of dental composite resins, and in particular the interfacial stresses around the margins of a composite restoration. The development of stresses at the restoration-tooth interface can have a detrimental effect on the longevity of a restoration. The influence of this interface on the stress system generated in the tooth was examined using finite element analysis. The restoration-tooth interface was simulated using spring elements of varying spring constants (k = 1, 10(2), 10(4), 10(10) N/mm). Interfacial stresses varied from -0.15 to 0.42 MPa for a spring constant of 1 N/mm, and from -19 to 68 MPa for a spring constant of 10(10) N/mm. Correlations between stiffness at the restoration-tooth interface and higher shrinkage stresses due to restricted shrinkage were found. Interfacial failure at the upper and lower regions of the interface, as well as cuspal movements of the order of 2 microm were predicated for the model of the highest spring constant, 10(10) N/mm. The restoration-tooth interface modelled by the spring elements was seen to have a conclusive effect on the ensuing stress system, as well as the longevity of the restoration.

Polymerization Contraction Stress of Resin Composite Restorations in a Model Class I Cavity Configuration Using Photoelastic Analysis

PURPOSE

An important factor that contributes to deterioration of resin composite restorations is contraction stress that occurs during polymerization. The purpose of this article is to familiarize the clinician with the characteristics of contraction stress by visualizing the stresses associated with this invisible and complex phenomenon.

MATERIALS AND METHODS

Internal residual stresses generated during polymerization of resin composite restorations were determined using micro-photoelastic analysis. Butt-joint preparations simulating Class I restorations (2.0 mm x 5.0 mm, 2.0 mm in depth) were prepared in three types of substrates (bovine teeth, posterior composite resin, and transparent composite resin) and were used to examine contraction stress in and around the preparations. Three types of composite materials (a posterior composite, a self-cured transparent composite, and a light-cured transparent composite) were used as the restorative materials. The self-cured composite is an experimental material, and the others are commercial products. After treatment of the preparation walls with a bonding system, the preparations were bulk-filled with composite. Specimens for photoelastic analysis were prepared by cutting sections perpendicular to the long axis of the preparation. Fringe patterns for directions and magnitudes of stresses were obtained using transmitted and reflected polarized light with polarizing microscopes. Then, the photoelastic analysis was performed to examine stresses in and around the preparations.

RESULTS

When cavity preparations in bovine teeth were filled with light-cured composite, a gap was formed between the dentinal wall and the composite restorative material, resulting in very low stress within the restoration. When cavity preparations in the posterior composite models were filled with either self-cured or light-cured composite, the stress distribution in the two composites was similar, but the magnitude of the stress was greater in the light-cured material. When preparations in the transparent composite models were filled with posterior composite and light-cured transparent composite material, significant stress was generated in the preparation models simulating tooth structure, owing to the contraction of both restorative materials.

CLINICAL SIGNIFICANCE

Polymerization contraction stress is an undesirable and inevitable characteristic of adhesive restorations encountered in clinical dentistry that may compromise restoration success. Clinicians must understand the concept of polymerization contraction stress and realize that the quality of composite resin restorations depends on successful management of these stresses.

Visco-elastic parameters of dental restorative materials during setting

Contraction stresses generated in restoratives during setting are among the major problems in adhesive dentistry, since they often result in loss of adhesion from the cavity walls or in post-operative pain. The rate of stress development and the ultimate magnitude of the stress, which determine the seriousness of these problems, depend on the relatively unknown visco-elastic behavior of the restoratives during setting. The aim of this study was to determine the visco-elastic parameters during setting, to aid our understanding of the process of contraction stress development. A dynamic mechanical method was used in which the materials were subjected to periodic strain cycles in a universal testing machine during the first 60 min of setting. The visco-elastic parameters (viscosity eta and Young's modulus E) were calculated by analysis of the experimental stress-strain data with a simple mechanical model according to Maxwell. Two restorative materials from different classes were investigated: a two-paste resin composite and a conventional glass-ionomer cement. A comparison of the results showed significant differences in the development of viscosity and stiffness in the early stage of setting. The resultant relaxation time (eta/E) of the glass ionomer remained at a low level during the first 15 min, whereas that of the resin composite increased markedly. This is of clinical importance, since it implies that, during the early setting stage, glass ionomers are better capable of reducing the contraction stresses than resin composites, thus increasing the likelihood that the bond with the cavity walls will form and survive during setting.

Effect of luting composite shrinkage and thermal loads on the stress distribution in porcelain laminate veneers

STATEMENT OF PROBLEM

Cyclic thermal fatigue has demonstrated a significant influence of the thicknesses of luting composite and ceramic in crack propensity of porcelain laminates.

PURPOSE

This study was conducted to define potentially involved parameters for crack development in porcelain laminates bonded to teeth. Finite element modeling was used to evaluate the respective effects of luting composite shrinkage and significant thermal changes.

MATERIAL AND METHODS

A buccolingual cross-section of a maxillary incisor was digitized and used as a template to generate a single 2-dimensional mesh, including all the different restorative designs. Luting composite shrinkage was simulated at a baseline temperature of 20 degrees C. The effect of thermal loads from 20 degrees C to 5 degrees C and from 20 degrees C to 50 degrees C was assessed with and without preexisting composite shrinkage.

RESULTS

Shrinkage of the luting composite alone generated important compressive forces on the ceramic, either at the restoration surface or interface. Compression intensity was related to geometry and ratio of thicknesses between the ceramic and luting composite (CER/CPR). Lower ratios produced higher compression forces in the ceramic. When thermal loads were combined to the composite shrinking forces, the stress pattern was significantly changed only for the experimental conditions with the lowest CER/CPR ratio. Temperature increase reduced compressive stresses and exacerbated tensile stresses. Thermal loads were simulated alone (situation of an "ideal nonshrinking" luting composite) and generated mainly tensile stresses in the ceramic, which intensity was again modulated by the CER/CPR ratio and the local geometry of composite and ceramic. Because of ceramic brittleness, these tensile forces were more detrimental than the high compression created by composite shrinkage alone. The stress pattern was not influenced by the incisal length of the veneer but rather by the facial thickness of ceramic. The worst record made with a shrinking luting agent (500 microm of luting composite, lowest CER/CPR ratio, 5 degrees C) was much less harmful than the worst record made with a hypothetical "nonshrinking" luting material.

CONCLUSIONS

The ratio of the thickness of cement and luting composite appears to have a relevant influence on the stress distribution in porcelain laminates. Restorations that are too thin, combined with poor internal fit, resulted in higher stresses at both the surface and interface of the restoration. Because of its precompressed state given by composite shrinkage, ceramics performed better with regard to temperature-induced tensile forces.

Photoelastic analysis of polymerization contraction stresses in resin composite restorations

OBJECTIVES

The objective of this study was to determine the distribution and the magnitude of the internal stresses in a resin composite restoration resulting from polymerization shrinkage by using photoelastic analysis.

METHODS

Butt-joint box-shaped cavities (5.0 x 2.0 mm, 2.0 mm in depth) prepared in bovine teeth and in composite moulds were filled with the light-activated transparent composite. The restoration was cross-sectioned perpendicularly to the longitudinal side of the cavity and observed with polarized microscopes. The principal stresses of the restoration, normal and shear stresses at the cavity wall were evaluated by the graphical integration method. The integrity of the bond along the cavity wall was also examined by staining method.

RESULTS

The internal stresses of the restorations in bovine teeth were not large enough to observe, apparently because the gaps along the dentinal wall acted as a stress relief. On the other hand, there were no gaps along the cavity walls at the restorations in the composite moulds. As flow of the resin composite was severely limited, the maximum normal tensile stress at the cavity wall, which occurred near the internal line angle of the cavity, was calculated to reach as high as 23 MPa. The stress level near the internal line angle was higher than that near the cavo-surface margin.

CONCLUSION

The distribution of the internal stresses in a composite restoration in a box-shaped cavity is considered to be unfavorable for the deep dentin bond. A good understanding of these phenomena may improve the clinical effectiveness of resin composite restoration.

Polymerization shrinkage and polymerization shrinkage stress in polymer-based restoratives

OBJECTIVES

This paper is intended to contribute to the recognition and understanding of problems related to polymerization shrinkage.

DATA SOURCES

Scientific publications of relevance with regard to this subject were critically reviewed.

STUDY SELECTION

The dimensional changes which develop during the curing of resin composites and glass polyalkenoate cements are studied, with special reference to methods of determining shrinkage, shrinkage stress and stress relief.

CONCLUSIONS

As no method for handling the adhesive restorative materials has yet been described which guarantees a leakproof restoration, the practitioner has to accept the problem of polymerization shrinkage and destructive shrinkage stress. Only a proper understanding of the mechanisms that cause these problems and the techniques that may reduce their effects will enable the practitioner to derive maximum benefit from the application of resin composites and glass polyalkenoate cements in restorative dentistry.

Analysis of strain gage method for measurement of post-gel shrinkage in resin composites

OBJECTIVE

The objective of this study was to refine a strain gage method for measuring polymerization contraction of resin composites and to isolate the net post-gel contraction by identifying factors contributing to the measured strains. The hypothesis to be tested was that carefully controlled strain gage measurements of composite polymerization could isolate post-gel contraction events.

METHODS

Composite was placed on a biaxial strain gage and light-cured. This method enabled real-time registration of the progress of shrinkage strain, corresponding to elastic modulus development. Strain from the two axes of the strain gage were averaged and plotted as a function of time. A representative curve was calculated from the mean of ten measurements. The following factors influencing the total contraction measurement were evaluated: thermal expansion of the gage, thermal expansion of the composite due to the exothermic reaction and exposure to the curing light, and adhesion of the composite to the gage. These parameters were measured so that the net deformation of the composite during polymerization could be calculated.

RESULTS

Parametric studies of pre-cured and photointiator-free materials confirmed the hypothesis that strain gages measure post-gel contraction. Thermal artifacts were measured and subtracted from the total strain output.

SIGNIFICANCE

Strain gages are suitable for measuring the clinically significant phase of composite polymerization contraction.

In vitro measurement of cuspal strain and displacement in composite restored teeth

OBJECTIVES

This study was carried out to measure changes in cuspal strain and displacement occurring during placement and polymerization of bonded composite restorations in extracted human teeth in vitro.

METHODS

Strains were measured using electrical resistance strain gauges bonded to the buccal and lingual cusps of each specimen and cuspal displacement was recorded with a linear variable differential transformer. Mesio-occluso-distal cavities of two types were prepared in lower molar teeth. Following enamel acid etching and application of a dentine adhesion promoter, specimens were restored incrementally with a light curing posterior composite material.

RESULTS

It was shown that the shrinkage of a composite material during polymerization generated stresses which resulted in tensile strains on the tooth surface. Strains of up to 882 microns/m were recorded and a maximum cusp displacement of 14 microns was also measured. These strains were reduced but not eliminated by preparation and restoration a mesio-distal slot running the full length and depth of the restoration. Statistical analysis revealed significantly higher strains and displacement produced on the buccal cusps of teeth that had a reduced cusp width (P < 0.001).

CONCLUSIONS

The in vitro restoration of posterior teeth with a bonded composite material generates polymerization stresses which can be recorded as tensile strains and displacements on the tooth surface. Strains measured during composite placement were greater when the remaining cusp width was less. A stress relief procedure resulted in a decrease in cuspal strain and displacement of approximately 30-40%.

The effect of filler content and processing variables on dimensional accuracy of experimental composite inlay material

STATEMENT OF PROBLEM

Dimensional accuracy of a composite inlay restoration is important to ensure an accurate fit and to minimize cementation stresses.

PURPOSE OF STUDY

A method was developed to measure dimensional accuracy and stability of a composite inlay.

MATERIAL AND METHODS

A standard Class II (MOD) inlay cavity stainless steel mold was made with six circular indentations placed on the occlusal floor of the cavity and four indentations on each gingival floor to act as datum points in the measurement of linear polymerization shrinkage. The inlay restorations were prepared from an inlay-onlay composite material of different filler contents (50%, 65%, and 79% by weight). For each filler content group, three curing methods were used: light curing only, light curing and heat curing at 100 degrees C for 5 minutes, and light curing and heat curing at 100 degrees C for 5 minutes and then storage in distilled water for 7 days. The accuracy of the MOD inlays was determined by measuring the shrinkage of the restoration on the occlusal floor areas and the gingival seats.

RESULTS

The results demonstrated an inverse linear relationship between filler content and polymerization shrinkage. There was a tendency for the light-curing and heat-curing method to show an increase in polymerization shrinkage. An expansion was recorded between the mesial and distal boxes when the specimens were soaked in water for 7 days.

CONCLUSIONS

This study suggested that the inlay mold limits the physical shrinkage that can occur between the mesial and distal axial walls of the inlay restoration because the inlay cannot shrink to a smaller dimension than the mold. Water sorption then causes hygroscopic expansion, which enlarges the distance between the mesial and distal walls.

Stress analysis of a bulk-filled Class-V chemical-cured dental composite restoration

Clinical problems associated with the polymerization shrinkage of dental composite restorations include tooth sensitivity, crown fracture, discoloration, recurrent decay, and loss of restoration. Our goal was to determine whether these complications could be attributed to the transient stresses developed during contraction. Thus, a finite element model was used to calculate the transitory deformations and composite-tooth interface stresses produced during the shrinkage of a chemical-cured ideal Class-V composite restoration. It was found that the interface stress peaks moved with the polymerization front, and that in some instances, their intermediate magnitudes were higher than the final, fully cured, values. Therefore, the results indicate that clinical failure may be related to these transitory changes during the polymerization process.

Finite element stress analysis of three filling techniques for class V light-cured composite restorations

An important disadvantage of current dental resin composites is polymerization shrinkage. This shrinkage has clinical repercussions such as sensitivity, marginal discoloration, and secondary caries. The objective of this study was to compare three filling techniques in terms of the transient stresses induced at the resin composite/tooth interface during polymerization. The techniques were: bulk filling (B), three horizontal increments (HI), and three wedge increments (WI). A simple Class V cavity preparation was modeled in finite element analysis. Polymerization shrinkage was simulated by a thermal stress analogy, thereby causing 1% shrinkage due to an arbitrary coefficient of thermal expansion. Interface normal and shear stresses were calculated at nine steps during polymerization, proceeding from 0% to 100% volume of cured resin. The importance of the interface transient stresses was revealed by the finding that, in most cases, their peak values exceeded the final or residual stress. Also, the WI and B techniques consistently exhibited the highest and lowest maximum transient stresses, respectively. These results from the simple model of a Class V restoration suggest that bulk filling of light-cured resin composites should be used in restorations which are sufficiently shallow to be cured to their full depth.

Influence of humidity on dimensional stability of a range of ion-leachable cements

The dimensional changes of a variety of dental restorative materials, occurring during and after setting, were investigated. The materials were tested under four different environmental conditions: 25 degrees C at laboratory humidity, 25 degrees C at 100% humidity, 37 degrees C at laboratory humidity and 37 degrees C at 100% humidity. Two materials setting by an acid-base reaction were also examined when protected with both a conventional varnish and a low-viscosity light-curable resin. The dimensional changes were recorded continuously using linear variable displacement transducers (LVDTs) over periods of up to 2 h. The materials investigated showed varying magnitudes of dimensional change. The shrinkage of conventional glass-ionomer cements (Fuji II and Opusfil) were the highest at 37 degrees C in air. This was attributed to the highest rate of water loss in the most desiccating environment. The shrinkage observed for the materials which set, even only in part, by a polymerization reaction will probably be due to the water loss and/or polymerization shrinkage. Exposure of these materials to a high-humidity environment reduced the shrinkage because of the swelling associated with water absorption. Application of the varnish and the protective resin over the cement surfaces also reduced shrinkage in Fuji II due to prevention of water exchange. The apparatus used in this study provided a simple and reliable method for measuring linear dimensional change. Data obtained in this study were comparable, where appropriate, to the values found in the literature.

Stress analysis of a bulk-filled Class V light-cured composite restoration

Clinical failures are often associated with the polymerization shrinkage of resin composite restorative materials. These problems include tooth sensitivity and fracture, marginal leakage, loss of the restoration, and recurrent decay. Our goal was to examine transient composite distortions and interface stresses as a bulk-filled light-cured composite polymerized in a Class V restoration. The analysis was based on a finite element model. The curing of the restoration was divided into 4 steps: approximately 1/30, 1/4, 1/2, and full depth (1/1) of cure. Since the actual curing pattern is not known, calculations were performed for three hypothetical (flat, convex, and concave) polymerization front shapes. The calculations showed that the assumed shape was a critical factor in determining cured surface deformations. For example, the initial cure depth (1/30) resulted in a surface bulge if the polymerization front was presumed convex, while the concave front resulted in a large intrusion. By the time that about 1/2 the depth of the restoration was cured, the differences were essentially gone. The final surface outline was intruded. Interface stresses in the curing restoration were qualitatively similar regardless of the assumed polymerization shape. As with surface distortions, the stresses changed with curing depth. It was concluded that (1) transient events during polymerization are possible contributors to clinical complications, and (2) more must be known about the polymerization pattern.

Light-cured lining materials: a laboratory study

OBJECTIVES

The purpose of this study was to compare seven light-cured lining materials (four glass ionomer and three non-glass ionomer), with a chemical-cured glass ionomer lining material.

METHODS

Specimens were prepared and tested according to the methods prescribed in Australian/International Standards for compressive strength, diametral tensile strength, depth of cure, radiopacity, and acid erosion. Other tests were adhesion to dentin and adhesion to resin composite.

RESULTS

Compressive strengths ranged from 48.6 MPa to 307.1 MPa, and diametral tensile strengths from 5.8 MPa to 34.8 MPa. Most of the light-cured materials were stronger than the chemical-cured glass ionomer. Depth of cure increased with increased irradiation times, with values of 0.63 mm to 6.43 mm for 60 s irradiation. Radiopacity was in the range of 0.91 mm Al to 1.07 mm Al. All materials were unaffected or only slightly affected by acid erosion. Adhesion to dentin was effectively zero for the non-glass-ionomer products. Adhesion to composite was from 6.23 MPa to 25.1 MPa, and thermocycling affected only three light-cured and the one chemical-cured glass ionomer.

SIGNIFICANCE

All products complied with the requirements of standards, where available. Incremental placement and curing is necessary for some products. Adhesion to dentin is better with glass ionomers, whereas their adhesion to composite may be unreliable.

Polymerization shrinkage of composite resins: comparison with tooth deformation

Polymerization shrinkage of two posterior composite resin restorative materials was measured by dilatometry. The results were compared with a decrease in cavity width of MOD preparations in extracted premolars restored with the composite resins. A highly filled hybrid composite exhibited greater free shrinkage and cuspal deformation than a hybrid composite with a lower filler content. Deformation of the cusps was less than the unrestricted shrinkage of the composite resins. Hydrated teeth exhibited less deformation than dehydrated teeth because of polymerization shrinkage. Greater cuspal deformations were measured with the microscopic technique than with interferometry because of differences in experimental design.

Interferometric measurements of cusp deformation of teeth restored with composites

A Michelson interferometry apparatus was used for measurement of the displacement of the buccal cusps of premolars after restoration of MOD preparations with composites. The effects of composite type, cavity size, and hydration conditions were examined. Interferometry permitted real-time measurement of cusp movement as it occurred. Contraction occurred very rapidly, about 1/3 of the 60-minute amount within the two-minute period of exposure to the curing light. Cusp movement was smooth rather than interrupted, indicating lack of microfracturing at deformations of 11-46 microns. Contraction, 0.94% for Heliomolar and 1.2% for P-50, was similar to the linear polymerization shrinkage of the resins. Less cusp movement occurred in small cavities than in large cavities. Hydrated teeth had less cusp movement than dehydrated teeth.

Polymerization contraction and conversion of light-curing BisGMA-based methacrylate resins

The aim of this study was to investigate the polymerization contraction and the conversion of light-curing methacrylate resins based on bisphenol-A bis(2-hydroxypropyl)methacrylate (BisGMA) diluted with triethylene glycol dimethyacrylate (TEGDMA), methyl methacrylate (MMA), hydroxypropyl methacrylate (HPMA) or (+/-)-2-ethylhexyl methacrylate (EHMA). The contraction measurements were carried out with a linometer, a simple device to determine true linear polymerization contraction of liquid monomers at ambient temperature. The contraction increased with the amount of diluting monomer. The estimated conversion of the BisGMA-TEGDMA, calculated using the contraction, is consistent with literature values. The BisGMA-HPMA mixtures showed high conversions at moderate contraction.

A comparison of the shear bond strengths to a resin composite of two conventional and two resin-modified glass polyalkenoate (ionomer) cements

The inability of resin composite to bond directly to dentine has compromised the clinical success of this restorative material. This problem may be overcome by the so-called laminate technique in which glass polyalkenoate cement is placed upon dentine and then covered with a resin composite. The cement both adheres to dentine and affords micromechanical attachment to the composite. The introduction of the resin-modified glass polyalkenoate cements (RMCs) offers the potential for chemical union between cement and composite. This investigation examined the shear bond strengths of P-50 resin composite to four glass polyalkenoate lining materials, with and without the application of an intermediate bonding agent (Scotchbond 2). Two of the cements were RMCs (Vitrebond, XR-Ionomer) and the others were conventional base materials (Baseline, Ketac-Bond). The bond between P-50 and Vitrebond with or without Scotchbond 2 was significantly (P < 0.01) stronger and more consistent (P < 0.05) than that observed for all other materials. The treatment of the conventional materials and XR-Ionomer with Scotchbond 2 significantly (P < 0.01) improved the bond strengths to P-50. It is concluded that Vitrebond formed the most favourable cement-resin composite bond and that the other materials studied should be used in conjunction with an effective intermediate bonding agent, such as Scotchbond 2.

Cusp movement in premolars resulting from composite polymerization shrinkage

This study examined the effect of cavity size, hydration conditions and type of composite on cusp movement following placement of MOD composite restorations in vitro. A microscope with a micrometer stage was used to measure deflection of cusps for 14 days after initiation of curing. The intercuspal width decreased for the first few minutes after the composites were placed, then increased toward the original dimensions. Teeth with small cavities contracted less than those with large cavities, and recovery was complete or nearly complete only in teeth with small cavities. Hydrated teeth and restorations recovered more rapidly and more completely than those that were dry. Differences in deflection of the cusps were observed with two different restorative materials.

Dentin bond shear strength and microleakage for Syntac/Heliomolar: a comparison between the manufacturer's and total etch technique

Recent improvements in formulation have led to significant improvements in the bond strength and clinical performance of dentin bonding agents. The growing interest in the concept of total etch prompted a comparison between manufacturer-recommended procedure and that of total etch for the product Syntac. An in vitro study to examine shear bond strength to dentin and microleakage and an in vivo component to investigate the micromorphologic relationship between restoration and tissue was conducted. No statistical significance was observed in bond strength values for the nonetched versus the etched group. Significantly, however, a larger number of specimens in the nonetched group exhibited microleakage. In vivo, only the etched group exhibited a zone of hybridization although both nonetched and etched restorations showed no interfacial gaps. It was concluded that total etch does not compromise the shear bond strength of Syntac but it appears to significantly reduce microleakage possibly due to the formation of a hybrid zone and may have merit clinically.

Effects of polymerization contraction in composite restorations

Post-gel polymerization contraction of resin composite induces contraction stresses at the composite-tooth bond and in surrounding tooth structure. Strain gauges have been shown to be an effective method for measuring linear post-gel polymerization contraction of composites. A new model was developed in which the composite sample was bonded to and circumscribed by an acrylic ring. The model simulates a composite restoration surrounded by dentine. A strain gauge measured the deformation of the ring while a second strain gauge simultaneously recorded the dimensional change of the sample. Stresses placed on the acrylic ring as a result of polymerization contraction of the composite were calculated, based on the strains on the ring and the ring's material properties. Four composites (Heliomolar, Vivadent, Tonawanda, NY, USA; Herculite XR, Kerr Manufacturing Co., Romulus, MI, USA; P-50, 3M Co., St Paul, MN, USA; Silux Plus 3M Co.) were evaluated for polymerization contraction strain and stress on the surrounding acrylic ring during polymerization. At the end of the 60 s light application, Heliomolar demonstrated significantly lower post-gel contraction (0.12 per cent, P less than 0.05) when compared to the other materials. When the strain reached an equilibrium at the end of 14 min Heliomolar continued to demonstrate lower post-gel contraction, however this was not statistically significant at P less than 0.05. When the contraction stress on the surrounding acrylic ring was considered, P-50 rapidly developed and produced the largest stress values (1.7 MPa) at the end of the light application while Heliomolar produced the lowest stress values (0.3 MPa). These values, however, were not significantly different when evaluated statistically.(ABSTRACT TRUNCATED AT 250 WORDS)