A study of the volumetric setting shrinkage of some dental materials

Polymerisation Shrinkage Profiling of Dental Composites using Optical Fibre Sensing and their Correlation with Degree of Conversion and Curing Rate

Traditional polymerisation shrinkage (PS) measurement systems measure average PS of dental composites, but the true local PS varies along the length and breadth of the composite. The PS depends on the curing light intensity distribution, resultant degree of conversion (DOC) and the curing rate. In this paper, optical fibre Bragg grating (FBG) sensing based technology is used to measure the linear post-gel PS at multiple locations within dental composite specimens, and is correlated with DOC and curing rate. A commercial dental composite is used, and its post-gel PS and DOC are mapped using embedded fibre Bragg grating sensors at different curing conditions. The distance between the curing lamp and the composite specimen is varied which resulted in different intensity distribution across the specimen. The effect of curing light intensity distribution on PS, curing rate and DOC are investigated for demonstrating a relationship among them. It is demonstrated that FBG sensing method is an effective method to accurately profiling post-gel PS across the specimen.

Evaluation of the physical properties of dental resin composites using optical fiber sensing technology

OBJECTIVES

The characterization of the physical properties of dental resin composites is fraught with difficulties relating to significant intra and inter test parameter variabilities and is relatively time consuming and expensive. The main aim of this study was to evaluate whether optical fiber Bragg grating (FBG) sensing system may become a viable tool to study dental material characteristics. Of particular focus was the potential for the system to demonstrate a multi parameter all-in-one feature.

METHODS

A miniature FBG was embedded in six different dental resin composites and employed as a sensor to evaluate linear polymerization shrinkage, thermal expansion and water sorption. Six commercially available dental composites with different filler types and volume are evaluated. The tests are repeated with three sets of samples. The curing characteristics and residual strain gradient exhibited by the cured dental composites were also observed and commented.

RESULTS

Among the studied samples, SDR shows lowest polymerization shrinkage, while Beautifil FO3 shows the highest. The results also show clear distinction between particle filler type and fiber reinforcement based composites in their polymerization shrinkage properties. The agreement of the results with existing literatures show that FBG based system provides accurate results. Polymerization shrinkage rate of the samples are also obtained. Thermal expansion of the composites are measured using the FBG sensing method for the first time and is correlated with resin type, volume, filler type and glass transition temperature. The water sorption characteristics of the dental composite are also successfully measured using the FBG sensing method. The high level of repeatability and the low standard deviations shown in the results indicate good reliability with the use of FBG sensors.

SIGNIFICANCE

This study demonstrates how optical fiber technology can provide simple and reliable methods of measuring the critical physical properties of dental composites. In addition due to the embedding and preservation of the sensor within the samples multiple parameters can be tested for with the same sample. These features are expected to greatly assist material science researchers in dentistry as well as other biomedical fields. Of some interest the phenomenon of stress relaxation of dental composite at higher temperature was observed.

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.

Push-Out Bond Strength of Restorations with Bulk-Fill, Flow, and Conventional Resin Composites

The aim of this study was to evaluate the bond strengths of composite restorations made with different filler amounts and resin composites that were photoactivated using a light-emitting diode (LED). Thirty bovine incisors were selected, and a conical cavity was prepared in the facial surface of each tooth. All preparations were etched with Scotchbond Etching Gel, the Adper Scotchbond Multipurpose Plus adhesive system was applied followed by photoactivation, and the cavities were filled with a single increment of Filtek Z350 XT, Filtek Z350 XT Flow, or bulk-fill X-tra fil resin composite (n = 10) followed by photoactivation. A push-out test to determine bond strength was conducted using a universal testing machine. Data (MPa) were submitted to Student's t-test at a 5% significance level. After the test, the fractured specimens were examined using an optical microscope under magnification (10x). Although all three composites demonstrated a high prevalence of adhesive failures, the bond strength values of the different resin composites photoactivated by LED showed that the X-tra fil resin composite had a lower bond strength than the Filtek Z350 XT and Filtek Z350 XT Flow resin composites.

Ability of new obturation materials to improve the seal of the root canal system: a review

New obturation biomaterials have been introduced over the past decade to improve the seal of the root canal system. However, it is not clear whether they have really produced a three-dimensional impervious seal that is important for reducing diseases associated with root canal treatment. A review of the literature was performed to identify models that have been employed for evaluating the seal of the root canal system. In vitro and in vivo models are not totally adept at quantifying the seal of root canals obturated with classic materials. Thus, one has to resort to clinical outcomes to examine whether there are real benefits associated with the use of recently introduced materials for obturating root canals. However, there is no simple answer because endodontic treatment outcomes are influenced by a host of other predictors that are more likely to take precedence over the influence of obturation materials. From the perspective of clinical performance, classic root filling materials have stood the test of time. Because many of the recently introduced materials are so new, there is not enough evidence yet to support their ability to improve clinical performance. This emphasizes the need to translate anecdotal information into clinically relevant research data on new biomaterials.

Comparison of two methods for measuring the polymerization characteristics of flowable resin composites

OBJECTIVES

The purpose of this study was to compare two methods for assessing the polymerization characteristics of flowable resin composites.

METHODS

Two different flowable resin composites and a hybrid resin composite control were investigated. In order to measure the volumetric shrinkage (DeltaV), each material was placed into a mould and extruded into a water-filled dilatometer. The specimens were then light irradiated for 30s using a curing unit with the power density adjusted to either 100 or 600mW/cm(2). For the speckle contrast measurement, each resin composite was condensed into a glass tube and irradiated. The laser-speckle field was recorded in a digital frame. The calculated values were obtained for each pair of adjacent patterns and the changes in speckle contrast as a function of time were obtained.

RESULTS

The average DeltaV values of the resins after 180s ranged from approximately 3.3 to 4.4% for the flowable composites and from approximately 1.8 to 2.3% for the hybrid composite control. The overall magnitude of the speckle contrasts decreased soon after the initial light exposure commenced and gradually increased thereafter. The speckle contrast measurements revealed changes in the pastes due to the polymerization of the flowable resins that were greater than those obtained with the water-filled dilatometer.

SIGNIFICANCE

These data suggest that the polymerization characteristics of flowable resins can be measured successfully using two different methods. Moreover, our findings are of clinical relevance, as the data obtained under laboratory conditions might give an indication of the suitability of flowable resin composites for specific clinical applications.

Volumetric contraction in some tooth-coloured restorative materials

BACKGROUND

Much of the concern about the setting contraction of tooth-coloured restorative materials has been focused on the composite resins. This study investigated setting contraction of a range of glassionomer materials and included, for comparison, products from other groups of restorative materials.

METHODS

A deflecting disk method was used to determine the volumetric contraction of three conventional (non-light cured) glass-ionomer cements (GICs), two restorative, one "lining" consistency and one adhesive/lining consistency resin-modified glass-ionomers (RMGIs), two resin adhesives, three restorative composite resins and two compomers. The influence of powder:liquid ratio on two hand-mixed materials was also examined.

RESULTS

The light-cured materials (including RMGIs) showed substantially greater per cent contraction at 5 minutes than did the three conventional GICs (not light cured) and a substantially greater proportion of the 30-minute contraction had occurred at 5 minutes for these light-cured materials. Their further contraction after 1 hour was generally less than 5 per cent of the 1 hour contraction.

CONCLUSION

Although the conventional GICs contract more slowly in the first 5 minutes, by 30 minutes the current restorative GICs and RMGIs exhibit a volumetric setting contraction that is comparable with the composite resins and compomers and is generally in the range of 2-3 per cent.

A physico-chemical explanation of the post-polymerization shrinkage in dental resins

The main problem of a methacrylated dental resin's photopolymerization is the shrinkage phenomenon. This occurs, as expected, during light irradiation but also, unexpectedly, during about 24h after photopolymerization (i.e. during the so-called 'post-polymerization' stage). During this period, the conversion degree does not change significantly (no more initiation, very limited, if any, propagation reaction) but free radicals concentration decreases.

OBJECTIVES

To better understand what happens during the 24h after the photopolymerization, a thermal study of these resins is investigated at first and an explanation is then discussed.

METHODS

In this paper, the glass transition temperatures (T(g)) are measured at 0 and 24h by DMA. The post-shrinkage phenomenon is observed by TMA. Conversion degree (DC) is followed by Raman and free radical decay by ESR spectroscopy.

RESULTS

T(g) increases significantly during post-polymerization (55-80 degrees C). The same samples were studied by TMA at room temperature and shrinkage is observed. The fact that the degree of conversion (DC) does not increase significantly and that the 'post-shrinkage' occurs at T<T(g) leads to the hypothesis that a physical phenomenon should occur which can, in turn, be responsible for a secondary chemical phenomenon (post-polymerization).

SIGNIFICANCE

The proposed explanation is that, as photopolymerization of dental resins is very fast, a large excess of free volume is trapped in non-equilibrated samples. As they have no time to return to an equilibrium state, free volume should decrease below T(g) and samples do physically shrink during the first 24h. As a consequence, free radicals can come into 'contact' and undergo limited propagation but significant termination justifying the decrease in overall radical concentration.

Polymerization shrinkage and contraction stress of dental resin composites

OBJECTIVE

The aim of this study was to evaluate the shrinkage, contraction stress, tensile modulus, and the flow factor of 17 commercially available dental resin composites.

METHOD

The volumetric shrinkage measurements were performed by mercury dilatometry, and the contraction stress and tensile modulus were determined by means of stress-strain analysis. The statistical analysis was conducted by ANOVA and Tukey's post hoc test, and linear regression.

RESULTS

Strong linear correlation for most resin composites were found for (i) contraction stress and shrinkage (ii) contraction stress and tensile modulus, and (iii) shrinkage and tensile modules. For most of the materials the unpolymerized resin content determines the amount of shrinkage, contraction stress and tensile modules. The pre-polymerized clusters in Heliomolar results in improved shrinkage/contraction stress properties. The shrinkage/contraction stress for Filtek Z100, Aelite Flo, and Flow-it was too high for the amount of resin in the resin composite. This was rationalized by high polymerization rates, a flow factor, and the nature of the resin.

SIGNIFICANCE

High shrinkage and/or high contraction stress may lead to failure of the bond between the resin composites and the tooth structure. This study shows that the unpolymerized resin content determines the amount of shrinkage, contraction stress and tensile modules. Therefore, using pre-polymerized clusters will improve shrinkage/contraction stress properties, as was shown in Heliomolar, while high polymerization rates, and low flow factors have a deteriorative effect on the shrinkage/contraction stress properties.

Real-time dimensional change in light-cured composites at various depths using laser speckle contrast analysis

Laser speckle contrast analysis is an interferometric technique that is used to measure the displacement of the rough surface of a specimen. The purpose of this study was to present a laser speckle correlation method for monitoring real-time dimensional changes of light-cured composites. Uncured composite was condensed into a glass tube and irradiated for 30 s with 600 or 200 mW cm(-2). The speckle patterns obtained from lateral and bottom composite surfaces were monitored using a speckle analyser. The speckle field is recorded in a digital frame and stored by image processing system as the carrier of information on the displacement of the tested surface. The calculated values were obtained for each pair of adjacent patterns and the changes in speckle contrast as a function time were obtained from five repeated measurements. The overall magnitude of the speckle contrasts decreased soon after the initial light exposure and gradually increased thereafter. The speckle contrasts obtained from the bottom surface were smaller than those obtained from the lateral surface. This tendency was more pronounced when the specimen was irradiated with lower power density. It can be concluded that monitoring differential shrinkage at various levels of depth can be achieved with this new technique.

Organosilicon dental composite restoratives based on 1,3-bis[(p-acryloxymethyl) phenethyl] tetramethyldisiloxane

OBJECTIVES

The major concern associated with the use of polysiloxanes as polymer matrices in dental restorative materials, is the generally modest mechanical properties of the polymers. However, it has long been demonstrated that thermal stability, and mechanical properties of polysiloxanes can be substantially modified by incorporation of bulkier substituents such as phenyl groups or more polar groups in the chains. The purpose of this research was to evaluate visible light activated dental composites based on the high molecular weight siloxane monomer 1,3-bis[(p-acryloxymethyl) phenethyl] tetramethyldisiloxane (BAPD).

METHODS

Hardness, diametral tensile strength (DTS), degree of conversion (DC), water sorption (WS) and polymerization shrinkage of BAPD-based composites and bis-GMA-based composites were determined and compared.

RESULTS

Composites based on BAPD exhibited low WS, high DC, low polymerization shrinkage, and had hardness and DTS values that were not significantly lower than those of dental composites based on bis-GMA.

SIGNIFICANCE

BAPD is a high molecular weight monomer (MW = 511) with a low viscosity. It did not require the use of low molecular weight diluent monomers in formulating composite resins. The DC of BAPD was high, ranging from 86 to 94%. Although the DC of BAPD was significantly higher than the conventional difunctional dental monomers, the polymerization shrinkage of the siloxane composites (1.70 - 1.81 vol%) was comparable to several composites based on bis-GMA.

A reappraisal of the incremental packing technique for light cured composite resins

The cuspal flexure caused by a direct placement composite resin was measured using a technique that did not interfere with cuspal movement. Twenty upper premolar teeth with mesial-occlusal-distal (MOD) cavity preparations were restored using a posterior composite resin. Ten teeth were restored using a bulk packing technique and 10 were restored using three 'U' shaped bucco-lingual increments. Total cuspal movement was measured 1 h following the initiation of curing. Bulk placement of the composite resin produced slightly more cuspal movement (12.5 microm) compared with the incremental placement of composite (11.3 microm). This difference was not statistically significant. The technique of incremental packing to reduce cuspal flexure is therefore questioned.

Polymerization shrinkage-strain kinetics of temporary crown and bridge materials

OBJECTIVE

The purpose of this study was to measure the polymerization shrinkage kinetics of four commercially available polymer-based temporary crown and bridge materials, including the effect of ambient temperature.

METHODS

Three dimethacrylate-based materials and one monomethacrylate-based material were investigated. The polymerization shrinkage-strains were measured by using the Bonded-disk method with initial specimen temperature at both 23 and 37 degrees C, with values particularly noted at 5, 10, and 120 min after mixing. Five recordings were taken for each material. The progress of the setting reaction and its temperature-dependence were evaluated by the kinetic curves, and net shrinkage and total shrinkage (inclusive of expansion magnitude) of each material were compared by independent sample t-test and one-way ANOVA.

RESULTS

Most shrinkage occurred in the first 10 min after mixing although there was an early expansion especially with the monomethacylate in the first 5 min. At 120 min, the net shrinkage-strain at 23 and 37 degrees C of the materials used in this test ranged from 3.54 to 4.13%. The fastest setting dimethacrylate-based material and the monomethacrylate-based material showed higher shrinkage-strain than other materials. No significant differences of net shrinkage-strain were found between 23 and 37 degrees C, but higher shrinkage rates were measured at 37 degrees C than at 23 degrees C.

SIGNIFICANCE

The Bonded-disk method is a suitable method for measuring temperature-dependence of shrinkage-strain of polymer-based temporary materials. The dimethacrylate-based materials are preferable to monomethacrylates for temporary restoration as judged by the magnitude of polymerization shrinkage-strain, the majority of which is apparent within 10 min from the start of mixing and may affect the clinical outcome.

Polymerization shrinkage evaluation of three packable composite resins using a gas pycnometer

Modern restorative dentistry has been playing an outstanding role lately since composite resins, allied to adhesive systems, have been widely applied on anterior and posterior teeth restorations. The evolution of composite resins has mostly been verified due to the improvement of their aesthetic behavior and the increase in their compressive and abrasive strengths. In spite of these developments, the polymerization shrinkage inherent to the material has been a major deficiency that, so far, has been impossible to avoid. Using a gas pycnometry, this research investigated the polymerization shrinkage of three packable composite resins: Filtek P60 (3M), Prodigy Condensable (Kerr), and SureFil (Dentsply/Caulk), varying the distance from the light source to the surface of the resins (2 mm or 10 mm). The pycnometer Accupyc 1330 (Micromeritics, USA) precisely records helium displacement, allowing fast and reliable measurements of the volume of composite resin immediately before and after polymerization, without interference of temperature or humidity. Results were not found to be statistically different for the three tested resins, either for 2 mm or 10 mm-distance from the light source to the composite surface.

Comparison of 6 different gutta-percha techniques (part II): Thermafil, JS Quick-Fill, Soft Core, Microseal, System B, and lateral condensation

OBJECTIVES

In this in vitro study, the core-to-sealer ratios were calculated for 6 different gutta-percha techniques: Thermafil, JS Quick-Fill, Soft Core, Microseal, System B, and lateral condensation. The core consisted of gutta-percha or gutta-percha and carrier.

STUDY DESIGN

The sealer-to-core ratios for the Thermafil, JS Quick-Fill, System B, and lateral condensation techniques were published previously. In this study, the sealer-to-core ratio for Microseal and Soft Core techniques was investigated. For this purpose, 20 teeth filled by using the Soft Core or the Microseal technique were embedded in resin and sectioned horizontally at 1, 2, 3, and 4 mm from the anatomic apex. Photographs were taken of each section, and the total area of the canals filled with core material or sealer was calculated.

RESULTS

Once all results were compiled, it became clear that core techniques (Thermafil, JS Quick-Fill, and Soft Core) produced higher gutta-percha content than the Microseal, System B, and lateral condensation techniques (P <.05). The lateral condensation technique produced the least gutta-percha content (P <.05).

CONCLUSION

Thermafil, JS Quick-Fill, and Soft Core were found to be superior to the Microseal, System B, and lateral condensation techniques in terms of the gutta-percha-to-sealer ratio.

Volumetric shrinkage of composites using video-imaging

OBJECTIVE

This study involves investigation of the use of video-imaging for measurement of volumetric shrinkage of composites.

METHODS

Six composites were tested for volumetric shrinkage using video-imaging. The volumetric shrinkage was measured using the single- and multi-view volumetric reconstruction modes. All composites were cured using a VIP(TM) curing light for 40s at 500 mW/cm(2). Dynamic shrinkage was measured using the single-view mode with a red filter placed over the detector opening.

RESULTS

Analysis of the volumetric shrinkage values by a one way ANOVA for each composite showed no difference for the single- and multi-view measurement mode. The shrinkage values determined by video-imaging were compared to those measured for the same composites by mercury dilatometry by one way ANOVA followed by a paired comparison using the Bonferroni method.

CONCLUSION

The video-imaging technique gives reproducible results for volumetric shrinkage of composites comparable to those measured by dilatometry.

Polymerization shrinkage of dental composite resins

Aesthetic materials have always been a priority for the restoration of anterior teeth; increasingly, they have also gained prominence in the restoration of posterior teeth. This has been due to their advantages as an alternative to amalgam. Their drawbacks however, can include poor marginal adaptation, postoperative sensitivity and cuspal movement. These are particularly associated with the polymerization shrinkage accompanying the placement of composite resins. Consequently, a variety of methods have been used for determining the polymerization shrinkage. These range from dilatometer methods, specific gravity methods and deflecting disc systems to optical methods. In this work a unique method for the qualitative analysis of polymerization shrinkage was undertaken. This method utilized a miniature transducer and provided details of the shrinkage from within the material. The results indicated movement of material towards the initiating light, followed by a return movement away from it. The study was expanded to incorporate clinical aspects, whereby the composite resin was in direct contact with dental tissue, as in a restoration. Tests were performed with surface-treated cavity moulds, as in restoration placement, and without surface treatment. Results indicated that the shrinkage was highly dependent upon the region under investigation, as well as on the state of the surface.

Optimal specimen geometry in bonded-disk shrinkage-strain measurements on light-cured biomaterials

OBJECTIVES

The aim was to determine the effect on apparent shrinkage-strain values of varying specimen aspect ratio in the bonded-disk configuration. Thereby one source of possible inter-laboratory variation might be understood and eliminated.

METHODS

The bonded-disk procedure was followed and applied to three representative resin-composites. However, specimen sub-groups were examined, each of different diameters (3.5, 5.0, 7.0 and 9.0 mm), corresponding to C-factors in the range 0.45-7.0.

RESULTS

For each material, no statistically significant difference was apparent between data obtained with 7 and 9 mm diameter specimens. However, statistically significant reductions (p < 0.05, Scheffé and SNK) were apparent at 5.0 and 3.5 mm diameter, in the range 14-18% of the maximum 'true' value. For each material, the diameter (d)-dependence of apparent shrinkage-strain (epsilon) was given by the curve fit expression: epsilon = A - B/d2.

SIGNIFICANCE

For accurate determination of maximum final equilibrium shrinkage-strain values, a high aspect ratio (7-9:1) of the bonded-disk should be utilised. This is important for accurate comparison of data from proposed low-shrinkage amalgam-replacement materials.

Experimental validation of a finite element model of light-activated polymerization shrinkage

An important consideration in improving the longevity of dental resin composite restorations is how to minimize the stresses that develop between resin composites and the wall of the preparation as the resin shrinks during polymerization. If the stress is too great, failure of the bonded interface occurs and microleakage results, with possible margin failure, staining, and secondary caries. This present project was performed to validate a previously developed FEM approach for investigating polymerization shrinkage. Light-cured resin composite was condensed into a cylindrical cavity preparation, which contained a centrally located wire. A profilometer stylus was positioned on the end of the wire to detect the initial surface displacement as the wire moved with the superficial layer of composite resin. The plots of the experimentally derived data were compared to the data plot from the FEM. The initial segments of the experimental plots matched the calculated plot very closely in shape, thus validating the FEM approach.

Diminished leakage along root canals filled with gutta-percha without sealer over time: a laboratory study

AIM

The aim of this study was to investigate the long-term seal of gutta-percha (GP) without sealer.

METHODOLOGY

Extracted human maxillary central incisors were prepared and obturated using heat- or chloroform-softened GP only, or GP in combination with a root canal sealer. Leakage along root fillings was measured at 48 h and after a period of 6 months using a fluid transport model.

RESULTS

At 48 h the GP-filled roots without sealer leaked more than the control roots filled with GP and sealer (P < 0.0001). At 6 months a significant reduction in leakage was found in the GP-only groups (P < 0.05 for both heat- and chloroform-softened GP). After 6 months, roots filled by vertical condensation of GP-only had similar leakage as vertically condensed GP with a sealer (P > 0.05).

CONCLUSIONS

The long-term seal of root fillings is affected by the volume change of both GP and sealer. Leakage reduction due to expansion of GP may compensate to a certain extent for leakage that may occur from sealer dissolution.

A simple method for the measurement of polymerization shrinkage in dental composites

OBJECTIVE

In this study a simple non-contact method was developed to measure the polymerization shrinkage of dental composites.

METHODS

A gas pycnometer was used to determine the volumes of specimens prior to and after photopolymerization and from which the total volumetric shrinkage could be determined.

RESULTS

Four commercial composites were studied and were found to have polymerization shrinkages varying from 1.6 to 2.5%. The method was found to be labour efficient and produced reproducible results with a standard deviation of approximately 10%.

SIGNIFICANCE

This method is appropriate for shrinkage measurements where only the total amount shrinkage is required and in particular for the measurement of shrinkage of photocured materials which are sensitive to water absorption.

Influence of light irradiation on the volumetric change of polyacid-modified resin composites

OBJECTIVES

Recently, a new restorative material called a 'compomer', which is classified as a polyacid-modified resin composite, has become available. The volumetric shrinkage of compomers may create marginal gaps that influence the bonding ability and longevity of a restoration. Since compomers have been introduced recently, their volumetric change during curing is not fully understood. The purpose of this study was to evaluate the volumetric change of compomers.

METHODS

Three compomers, Compoglass (Vivadent), Dyract (Dentsply), and Ionosit Fil (DMG) were employed. The material was placed into a Teflon mould, 4 mm in diameter and 2 mm height, and extruded into the dilatometer. Then the specimens were light activated and the change in the height of the meniscus of water was recorded using a CCD camera and VRC.

RESULTS

The average volumetric shrinkages of the compomers after 160 s were 2.4% for Compoglass, 2.7% for Dyract, and 2.1% for Ionosit-Fil. For all materials tested, there was a tendency of increasing volumetric shrinkage with increased irradiation time.

CONCLUSIONS

The results of this study indicate that the volumetric change of compomer is influenced by the duration of light exposure, light intensity, and environmental conditions to which the materials are exposed.

Intrinsic 'soft-start' polymerisation shrinkage-kinetics in an acrylate-based resin-composite

OBJECTIVES

A resin-composite based on multi-acrylate monomers was to be evaluated for any differences in the setting shrinkage-strain kinetics relative to more conventional formulations based on di-methacrylate monomers.

METHODS

Four resin-composites were examined for shrinkage-strain over time periods up to 60 min, from initial irradiation, using a 'bonded disk' measurement device. One material was evaluated with a standard and also a two-level (low to high) blue light irradiation regime. The test material (Solitaire), and the other two controls were studied under fixed-(normal) level irradiation by blue light. Measurements were made at 23 and 37 degrees C.

RESULTS

The low-high light irradiation applied to one di-methacrylate composite led to a highly significant (p < 0.001) reduction in shrinkage-strain to a minimal level for the initial 0.2 min. The fixed-level irradiation with the acrylate-composite also led to a formally identical, but intrinsic 'soft-start' shrinkage-delay regime; significant at p < 0.001. Di-methacrylate composites under the same fixed-level light regime did not exhibit shrinkage delay. Both di-methacrylate and multi-acrylate composites exhibited increases in the range 18-29% in final equilibrium shrinkage-strain by increase of specimen temperature.

SIGNIFICANCE

Reductions in the rate of initial shrinkage of light-cured restoratives may have clinical benefits for restoration bond-integrity nearly as important as reductions in the final equilibrium shrinkage-strain. This may be achieved either by special light irradiation regimes (low to high, or ramped) or in favourable cases by novel monomer-composite formulations and setting chemistry (e.g. Solitaire). The bonded disk shrinkage-strain measurement technique is suitable for the elucidation of such rapid kinetic and temperature-dependent events during photo-polymerisation setting processes.

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.

Effect of sealer thickness on long-term sealing ability: a 2-year follow-up study

Long-term sealing ability of root-canal sealer is of clinical importance. We compared the performance of five sealers and a dentine bonding agent, at thicknesses of 0.05 mm (thin layer) and 0.3 mm (thick layer), in 212 standard human root specimens, before and after storage in water for 2 years. Thick layers of Roth and Pulp Canal Sealer EWT (PCS-EWT) allowed more leakage than did thin layers (P = 0.0000, 0.0358 for Roth and PCS-EWT, respectively) whereas no significant difference was found between the thin and thick layers for AH26, Ketac-Endo, Sealapex and J&J Bonding Agent. The pooled results of the thin and thick layers, which have clinical relevance, showed that after storing the specimens in water for 2 years, AH26, Sealapex, Ketac-Endo and PCS-EWT leaked more than before storage in water (P = 0.0008, 0.0000, 0.0035, 0.0257 for AH26, Sealapex, Ketac-Endo and PCS-EWT respectively), and that J&J Bonding Agent allowed less leakage and PCS-EWT more leakage than the other four sealers (P = 0.0000).

Polymerization shrinkage of microfilled composites determined by laser beam scanning

An He-Ne scanning laser beam was used to measure for the first time the linear shrinkage of light-cured microfilled composites. A low-power beam, which has a wavelength (632.8 nm) different from the polymerization wavelength (approximately 450 nm), was used. In these conditions no shrinkage is induced by the laser light. This method of measurement makes it possible to analyse small samples with a very low error (1.1 microns). Five different materials were tested using 10-20 mg specimens, and the shrinkage process was examined in detail over 2 days. All these materials showed shrinkage of more than 50% of their original length after 1 min of irradiation and approximately 99% of the total shrinkage occurred 4 h after irradiation.

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.

Effect of thickness on the sealing ability of some root canal sealers

A fluid transport model study was used to evaluate the sealing ability of five root canal sealers, AH 26, Sealapex, Ketac Endo, Roth, and Kerr EWT, and a dental bonding agent at thicknesses of 0.05 mm (thin layer) and 0.3 mm (thick layer) with 270 standard human root sections obturated with sealer combined with standard gutta-percha cylinders. AH26 and Sealapex sealed more tightly in thick layers, whereas Ketac Endo, Johnson and Johnson Bonding Agent, Roth, and Kerr EWT sealed more tightly in thin layers. In thin layers J&J Bonding sealed more tightly than any other sealer tested. In thick layers AH 26 and Sealapex sealed more tightly than Ketac Endo and J&J Bonding Agent, and these sealed more tightly than Roth and Kerr EWT. These findings indicate that the thickness of the sealer layer significantly influences the sealing of a root canal filling and that the influence of thickness varies depending on the type of sealer.

Testing of sealing ability of endodontic filling materials

Incomplete obturation of the root canal system is the major cause of endodontic failure. Endodontic filling materials with ability to seal the root canal are, therefore, essential for successful endodontic therapy. However, assessment of sealing ability is not included in the requirements specified in the current international standard covering root canal sealers. A number of different in vitro methods have been used to evaluate the sealing quality of endodontic filling materials. The tests have usually been leakage tests, i.e. based on assessment of penetration of a tracer along the obturated root canal. Leakage tests have poor reproducibility and are, therefore, not suited for standardized test programs. By selecting the properties necessary for sealing ability and testing them separately, it might be possible to find a number of tests, which individually are suitable for a standardized test program, and which combined will give information on the sealing ability of the test material.

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.

Dimensional changes of endodontic sealers

The purpose of this study was a long-term comparison of the dimensional changes of endodontic sealers of diverse properties. The sealers were injected as thin layers on the internal walls of glass pipettes 1 mm in diameter. The pipettes were filled with deionized water, and the water meniscus levels were recorded periodically up to 180 days. The water was removed from the pipettes, samples were allowed to dry, and the mass and the volume of the sealers were measured. AH26 and Endo-Fill had an initial expansion followed by a volumetric loss. The two zinc oxide eugenol-based sealers studied started to shrink within hours after mixing; the first volumetric loss for AH26 was recorded during the first 30 days and for Endo-Fill after 30 days. The least dimensional change at any time was observed for Endo-Fill. It was concluded that a significant dimensional change and continued volume loss can occur in some endodontic sealers.

Measuring polymerization shrinkage of photo-activated restorative materials by a water-filled dilatometer

A water-filled dilatometer specifically designed for determining the polymerization shrinkage of photo-activated composite restorative materials was used to measure the polymerization shrinkage of three visible light-activated composites. Polymerization shrinkage values ranged from 1.82% for P-50 to 2.15% and 2.19% for Herculite XRV and Prisma APH, respectively. Shrinkage data obtained in this investigation were compared with the published data, and the factors which affect shrinkage measurements were reviewed. It was concluded that maintaining a constant temperature environment (+ or - 0.02 degrees C) for the dilatometer during the shrinkage test was the most critical factor for successful application of the dilatometer.

True linear polymerization shrinkage of unfilled resins and composites determined with a linometer

A device, which was given the name "linometer", has been developed for measuring linear polymerization shrinkage of both unfilled resins and composites. The results were in agreement with those obtained with the mercury dilatometer. The present method is simple, fast, insensitive to temperature fluctuations, is operational at any temperature and, if desired, samples can be maintained in a 100% relative humidity environment. The linometer offers an easy means for routine polymerization shrinkage determinations and therefore may be useful in evaluating new monomer systems for dental composites. The linear polymerization shrinkage for several unfilled resins and composites was measured with this device.

Influence of calcium concentration on the dentin wettability by an adhesive

Plane dentin surfaces were abraded perpendicular to the radicular axis of sound human molars. They were cleaned to reveal the tubules, and the morphological features of each surface were studied microscopically. For a first series of teeth, the Vickers microhardness of the dentin surfaces was measured and the calcium and phosphorus composition was determined by electron microprobe analysis. For a second series, the microhardness and wettability of the surface by the Scotchbond adhesive were compared. Positive correlations were found between the following parameters: degree of minerality, dentin compactness, hardness, and spreading capability of the adhesive.

Method to measure the polymerization shrinkage of light-cured composites

The polymerization shrinkage of light-cured dental composite resins has been reported to cause a marginal gap between the cavity wall and the restoration, leading to the premature and/or tensile stress failure of the composite restoration. This study measured the volumetric shrinkage of six light-cured posterior composites by measuring specific gravity differences between uncured and cured composite test specimens, using a modified version of ASTM method D792 "Specific Gravity and Density of Plastics by Displacement." The measured volumetric shrinkage ranged from 1.35% to 3.22%.

Effect of various incremental techniques on the marginal adaptation of class II composite resin restorations

The effects of various placement techniques on the formation of microgaps were compared at the gingival margins of class II composite resin restorations. Three incremental techniques (occlusogingival layering, oblique layering, and faciolingual layering) and two one-bulk techniques of placing composite resin were studied. In the first one-bulk placement technique the composite resin was photocured occlusally; in the second one-bulk technique the composite resin was irradiated from three directions; facial, lingual, and occlusal. None of the incremental placement techniques improved the adaptation at the gingival margin compared with a one-bulk technique irradiated occlusally. However, the one-bulk placement technique that was irradiated from three directions created a substantial marginal discrepancy.

Determination of polymerization shrinkage kinetics in visible-light-cured materials: methods development

An instrument for the reproducible measurement of polymerization shrinkage kinetics is described, constructed around a disc-shaped specimen sandwiched between two glass plates. Test specimens of light-sensitive dental restorative materials were irradiated through the lower, rigid plate. The upper, non-rigid plate was readily deflected by an increase of the adhesive stress from the polymerizing and shrinking sample. Deflection was measured by an LVDT transducer and computer-recorded. Dimensional changes were confined to the specimen disc-thickness dimension, such that the fractional linear shrinkage approximated the volumetric shrinkage. Shrinkage data are reported for representative materials: unfilled and resin composites, base-lining materials, and an impression material. Equilibrium shrinkage magnitudes ranged from 0.65%, for the impression material, to 7.9% for the unfilled resin. The kinetic behavior was approximately characterized by an overall time constant, ranging from 12.5 to 280 s, associated with an exponential growth curve, although the initial shrinkage was near-linear in time, for many materials, due to non-steady-state concentrations of polymer free-radicals. The test-specimen geometry facilitates rapid and essentially uniform cure and hence the determination of minimum possible time-constants at each ambient temperature and incident light-intensity. Study of hybrid glass-ionomer materials, without spurious dehydration effects, was also achieved.

The polymerization shrinkage of composite resins

The majority of methods used for the determination of volumetric polymerization shrinkage rely on mercury as the immersion medium. With current concern for industrial hygiene, the water-filled dilatometer may have advantages. The purpose of this study was to adapt this method for external-energy-cured materials. Six materials were investigated: Aurafill, Silar, Concise, Heliomolar, P-30, and Occlusin. Measurements were taken over one hour, and results ranged from 0.9 to 2.24 volume percent.