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

Resin shear bond strength to porcelain and a base metal alloy using two polymerization schemes

STATEMENT OF PROBLEM

Fractures in ceramometal restorations can occur and need to be repaired because replacements are not an economic solution.

PURPOSE

This study evaluated the shear bond strengths of 4 porcelain repair systems (Metabond C&B [ME], Silistor [SI], Clearfil Lustre [CL], and Scotchbond Multipurpose Plus [SQ]) to a base metal alloy and porcelain in relation with the polymerization shrinkage of a visible light-cured composite superstructure and compared with the ceramometal bond strength (Vita VMK 68).

MATERIAL AND METHODS

Thirty-two samples were prepared for each bonding system: 16 for resin-metal bond strength test, and 16 for resin-porcelain bond strength test. For each group, bonding agent was applied to 8 substructures and the resin superstructure was polymerized onto the bonding agent; and for the remaining 8 specimens, prepolymerized resin superstructures were bonded with bonding agent. All specimens were subjected to 500 cycles between 5 degrees C and 55 degrees C with 20 seconds dwell time. Tests were performed in a mechanical testing machine with a 0.5 mm/min crosshead speed.

RESULTS

All materials showed an increase in shear bond strength when prepolymerized resin superstructures were used. However, the effect of polymerization shrinkage of resin superstructure was statistically significant only for CL group (P <. 05). The highest metal-resin bond was obtained from ME group with prepolymerized resin superstructures (35.27 +/- 2.40 MPa), and the lowest value was obtained for the SI group in which resin superstructures were polymerized in situ (8.71 +/- 1.03 MPa). The highest porcelain-resin bond was obtained from SC group with prepolymerized resin superstructures (20.71 +/- 1.13 MPa) and the lowest was obtained from SI group (9.99 +/- 1.52 MPa).

CONCLUSION

Higher bond strength values were obtained with prepolymerized resin superstructures compared to in situ polymerized superstructures. Metabond C&B provided the best results for both prepolymerized and in situ polymerized resin superstructure preparation techniques at the failures where metal was exposed. The best results in situations in which the fracture is limited into porcelain were obtained with the use of Scotchbond Multipurpose Plus material. However, a variety of in vivo and in vitro tests are required before a final judgment is made.

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.

Polymers in dentistry

There is a wide choice of materials available for restorative dentistry covering a range of requirements. Fundamental knowledge about the properties of the polymers in use in dentistry is an advantage as it provides information relevant to clinical practice. Dentistry, perhaps, has the unique distinction of using the widest variety of materials, ranging from polymers, metal and metal alloys, ceramics, inorganic salts and composite materials. In the present paper, polymers and polymer composites used directly or indirectly for restorations, prostheses or for production of appliances in dentistry is discussed.

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.

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.

Influence of criteria on the results of in vitro evaluation of microleakage

OBJECTIVES

The aim of this study was to compare and explain the statistical methods employed to evaluate the in vitro sealing efficiency of adhesive restorative systems.

METHODS

Two hundred and sixty sound freshly extracted human premolars were randomly divided into 13 groups. Standardized cavities were prepared, and the teeth were restored with 13 restorative systems. The teeth were thermocycled, immersed in dye, embedded in resin and sectioned. Five evaluation criteria were recorded: mean, median and mode of the data measured on each tooth, maximum dye penetration measurements on each tooth, and percentage of teeth in each group without any dye penetration. For each parameter, one-way ANOVAs and Duncan a posteriori tests were used to compare the 13 systems.

RESULTS

The number of non-statistically different subgroups, pointed out by Duncan tests, was greater when the selected criterion was the maximum dye penetration (6 subgroups) or the percentage of teeth without any penetration (5 subgroups) than when the criterion was the median (3 subgroups), the mode or the mean (4 subgroups). The positioning of the 13 adhesive restorative systems established from the five criteria was different. Equivalent adhesion strategies revealing different experimental results indicate that other factors contribute to the final effectiveness of a particular system: clinical approach with respect to the formation of an elastic bonding layer, and shrinkage, physical and rheological properties of resin composite.

SIGNIFICANCE

The results of these in vitro studies of dye penetration must be considered as comparisons and not as absolute conclusions. The maximum dye penetration measured on each tooth, which complies with the aim of the in vitro evaluation of sealing efficiency defined by Pashley (1990) and allows powerful statistical analysis of results, seems to be the best evaluation criterion.

The influence of comonomer composition on dimethacrylate resin properties for dental composites

During the polymerization of multifunctional monomers for dental restorations, typical final double-bond conversions range from 55 to 75%. The low conversion results in a large amount of extractable monomer, reduced adhesion to the filler, and the potential for increased swelling. In this work, the ability to increase the maximum conversion by optimizing the copolymer composition is explored. A series of multi-ethylene glycol dimethacrylate monomers of various lengths was used as a model system to determine how the copolymer composition affects the final conversion, the mechanical properties, and the predicted shrinkage. It was found that the ultimate conversion can be significantly increased, shrinkage decreased, and mechanical properties maintained. It was found that up to 30 wt% of poly(ethylene glycol) 600 dimethacrylate could be added to diethylene glycol dimethacrylate without reducing the strength and increasing the conversion. Results for other comonomer combinations were similar.

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.

Does an incremental filling technique reduce polymerization shrinkage stresses?

It is widely accepted that volumetric contraction and solidification during the polymerization process of restorative composites in combination with bonding to the hard tissue result in stress transfer and inward deformation of the cavity walls of the restored tooth. Deformation of the walls decreases the size of the cavity during the filling process. This fact has a profound influence on the assumption--raised and discussed in this paper--that an incremental filling technique reduces the stress effect of composite shrinkage on the tooth. Developing stress fields for different incremental filling techniques are simulated in a numerical analysis. The analysis shows that, in a restoration with a well-established bond to the tooth--as is generally desired--incremental filling techniques increase the deformation of the restored tooth. The increase is caused by the incremental deformation of the preparation, which effectively decreases the total amount of composite needed to fill the cavity. This leads to a higher-stressed tooth-composite structure. The study also shows that the assessment of intercuspal distance measurements as well as simplifications based on generalization of the shrinkage stress state cannot be sufficient to characterize the effect of polymerization shrinkage in a tooth-restoration complex. Incremental filling methods may need to be retained for reasons such as densification, adaptation, thoroughness of cure, and bond formation. However, it is very difficult to prove that incrementalization needs to be retained because of the abatement of shrinkage effects.

Curing shrinkage and volumetric changes of resin-modified glass ionomer restorative materials

OBJECTIVES

The objective of the study was to evaluate initial curing shrinkage and volumetric change during water storage of six resin-modified glass ionomer cements (Dyract, DeTrey Dentsply; Fuji II LC, GC Dental Int.; Ionosit Fil, DMG; VariGlass VLC, DeTrey Dentsply; Vitremer, 3M Dental Products; Photac-Fil, ESPE), a hybrid composite (blend-a-lux, Blendax) and a chemical-cured glass ionomer cement (ChemFil Superior, DeTrey Dentsply).

METHODS

The curing shrinkage was determined 5 min and 24 h after polymerization and mixing, respectively. Volumetric changes were examined after 14 d and 28 d water storage. Curing shrinkage and volumetric changes were evaluated using the hydrostatic principle. In addition, the total water content of the materials was measured after 28 d water storage.

RESULTS

Curing shrinkage of most of the resin-modified glass ionomers was greater than the hybrid composite and the chemical-cured glass ionomer cement. After a 28 d water storage, the resin-modified glass ionomers showed volumetric expansion and the chemical-cured glass ionomer showed volumetric loss. All of the examined glass ionomer materials had a higher total water content than the composite. SIGNIFICANCE. The large curing shrinkage of the resin-modified glass ionomer materials measured in this in vitro study could affect the marginal integrity of glass ionomer restorations.

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.

Influence of light intensity on two restorative systems

OBJECTIVES

High intensity curing lights are recommended almost universally on the basis of immediate curing depth measurements. Although this single factor is well documented, the influence of light intensity on other parameters has not been investigated extensively.

METHODS

Two restorative systems were examined with two light intensities in regard to four properties; polymerization shrinkage (density method), flexural modulus and strength (ISO 4049), hardness profiles after post-cure (Vickers), and marginal adaptation in dentine cavities (quantitative margin analysis).

RESULTS

The variation in light intensity did not significantly affect curing contraction or post-cure hardness profiles to a depth of 4.5 mm for either resin composite. Significant differences were found in flexural modulus with both restoratives; only one material demonstrated a light intensity-related influence on flexural strength. Marginal gap formation increased in each bonding agent/resin composite pair with increased light intensity.

CONCLUSION

In clinically relevant layer thicknesses, curing a resin composite with a higher intensity light may demonstrate significant disadvantages due to increased shrinkage stress.