Academy of Dental Materials guidance—Resin composites: Part II—Technique sensitivity (handling, polymerization, dimensional changes)

Abstract

Objective

The objective of this work, commissioned by the Academy of Dental Materials, was to review and critically appraise test methods to characterize properties related to critical issues for dental resin composites, including technique sensitivity and handling, polymerization, and dimensional stability, in order to provide specific guidance to investigators planning studies of these properties.

Methods

The properties that relate to each of the main clinical issues identified were ranked in terms of their priority for testing, and the specific test methods within each property were ranked. An attempt was made to focus on the tests and methods likely to be the most useful, applicable, and supported by the literature, and where possible, those showing a correlation with clinical outcomes. Certain methods are only briefly mentioned to be all-inclusive. When a standard test method exists, whether from dentistry or another field, this test has been identified. Specific examples from the literature are included for each test method.

Results

The properties for evaluating resin composites were ranked in the priority of measurement as follows: (1) porosity, radiopacity, sensitivity to ambient light, degree of conversion, polymerization kinetics, depth of cure, polymerization shrinkage and rate, polymerization stress, and hygroscopic expansion; (2) stickiness, slump resistance, and viscosity; and (3) thermal expansion.

Significance

The following guidance is meant to aid the researcher in choosing the most appropriate test methods when planning studies designed to assess certain key properties and characteristics of dental resin composites, specifically technique sensitivity and handling during placement, polymerization, and dimensional stability.

Introduction

Much of the testing of dental resin composites is designed to ascertain various universal or standardized properties such as strength, hardness, and resistance to wear or deformation. Appropriate test methods for these important properties have recently been reviewed . However, it is well recognized that obtaining the maximum level of these properties is dependent on the clinician and how well they manipulate the materials . Therefore, characteristics that may affect the manipulation of the material, or the so called “technique sensitivity”, may have a profound impact on the ultimate properties obtained, and the clinical success of a resin composite restoration. Characteristics such as the stickiness and slump resistance, are very important for clinical handling, but are less amenable to being analyzed by typical standardized tests. Other properties, such as porosity and viscosity, are more likely to have standard tests, but can only be considered as imperfect surrogate measures for assessing the handling characteristic in question. Congruently, some handling characteristics, such as stickiness, can have an impact on other, more well-defined properties, such as porosity.

Extent of polymerization can be well characterized using methods such as Infrared or Raman spectroscopy. But the property itself is affected by a myriad of factors, some of which are inherent in the material (photoinitiator type and amount, resin monomer type), others of which are under the influence of the curing light (irradiance, beam profile, spectral output), and others yet that are under the control of the clinician (exposure time, exposure distance, light guide position) and therefore subject to technique sensitivity. Polymerization likewise results in dimensional changes that are readily measured with tests such as dilatometry, Archimedes principle, or the bonded disk, but the clinical results of such dimensional changes, which may include marginal leakage, interfacial gap formation and tooth fractures, are more difficult to accurately assess. While there might not be true “standard test methods” for properties related to technique sensitivity, there are specific recommended test methods for many of the pertinent properties. The purpose of this paper is to review various tests available for assessing properties associated with the placement technique sensitivity of resin composite, including handling, for characterizing the polymerization reaction of resin composites, and for assessing dimensional change during and after the curing process and its associated outcomes. These properties are summarized in Table 1 , and are accompanied by a value representing their relative importance for measurement and study.

Table 1
Summary of available methods to assess technique sensitivity characteristics, polymerization and dimensional change properties of dental resin composites, ranked in the priority of the specific property/characteristics being tested, as well as of the specific test methods for evaluating that property.
Clinical issue/requirement Properties Property rank Method Test rank
Technique sensitivity: handling — placement Stickiness 2 Unplugging force, unplugging work 1
Measure length/area of withdrawn composite 2
Slump resistance 2 Extrude and cure 1
Imprint and cure (slump resistance index-SRI) 1
Viscosity 2 Rheology (with Viscometer) 1
Pressing under standard load and measure size 2
Porosity 1 Section — assess under magnification 1
3D micro-tomography 2
Archimedes method, ASTM D3171-11 2
X-ray 2
Radiopacity 1 ISO 4049 (vs. aluminum step wedge) and ISO13116 1
Sensitivity to ambient light 1 ISO 4049 (Xenon light box) 1
Polymerization Degree of conversion 1 FTIR (Fourier transform infrared) spectroscopy 1
Near IR 1
FT-Raman 2
DSC (differential scanning calorimetry) 2
NMR 4
Polymerization kinetics — rate 1 FT-IR, NIR, Raman 1
DSC 1
Shrinkage/dilatometry 2
Optical — interferometry, fluorescent probes 2
Elastic modulus — DMA, rheometer 3
Acoustic 2
DEA 2
Depth of cure 1 ISO 4049 — scraping 2
Microhardness vs. depth 1
Chemical vs. depth (FTIR, NIR, Raman) 1
Penetrometer 2
Solvent dissolution 3
Transition in translucency 3
Dimensional stability Polymerization shrinkage 1 Bonded disk 1
SSA = stress–strain analyser 1
Linometer 2
Dilatometer (e.g. Hg) 2
Pyknometer 2
Archimedes method — ISO (17304) 1
Accuvol camera imaging 2
Strain gage 2
Digital image correlation 2
Polymerization shrinkage rate/kinetics & gel point 1 e.g. from bonded disk or SSA 1
Polymerization stress 1 via UTM = universal testing machine 1
Bioman method 1
Tensilometer (cantilever beam) 1
SSA = stress–strain analyser 1
Photoelastic method 2
Thin ring 2
Indentation crack analysis 3
Hygro (swelling) expansion 1 Laser scanning of disk — stored for 3 months or more in water or other solvent. 1
Calipers or dial gage 2
Measuring microscope 2
Thermal expansion 3 Displacement measured over a temperature range 2

Technique sensitivity: handling, placement

Stickiness

Stickiness refers to the propensity of a resin composite to be retained on an instrument while the material is being placed into the cavity preparation. There is an ideal, yet poorly defined level of stickiness whereby the resin composite will be retained in the cavity and not pulled out or deformed as the placement instrument is removed. A number of tests have been devised to assess stickiness, most of which follow a similar scheme. A set volume of composite is placed in a mold, and then a steel rod or instrument is inserted into the unset material at a constant rate or until a predetermined force or depth is reached; then the motion is reversed until the composite separates from the instrument ( Fig. 1 ). Immediately upon separation, the composite is irradiated with a curing light to harden the material, leaving the surface in the shape of a peak. This peak of composite, sometimes called a “composite flag” can be measured for height and/or area and used as a measure of stickiness . Depending on the instrumentation and measurement capability, the unplugging force and work (the force and work required for the composite to detach from the instrument in withdrawal direction) can also be measured and calculated ( Fig. 1 ). All of these methods have been found to be reliable measures of composite stickiness that allow for good differentiation among current materials. In addition, one study correlated the unplugging work and force of various resin composites to the subjective handling characteristics as assessed by dentists and found a good association between the two, indicating that these tests are a good proxy by which to evaluate resin composite stickiness . Resin composite temperature, speed of the instrument/rod insertion and removal, and the surface area and roughness that the composite is in contact with have all been shown to be important factors influencing the results of these tests, and therefore should be well described whenever publishing results in this area.

Fig. 1
Schematic diagram and photo showing experimental setup to measure forces during application and removal of plugger tip from unset composite to assess stickiness (courtesy of Dr. Martin Rosentritt). Graph shows force–time relationship during application and removal, with work being calculated as the area under the curve (force × distance) (adapted from Rosentritt et al. ).

Slump resistance

Slump resistance is the ability of a resin composite to maintain its shape after placement and prior to curing. This is important in a clinical situation when a clinician desires to sculpt the anatomy of a restoration in the unset paste prior to light curing, in part to reduce the amount of finishing required. This is particularly the case in class III or class V restorations, in large anterior restorations, e.g. a direct composite veneer or a class IV restoration, and when reconstructing the cuspal or crestal anatomy in posterior restorations (class I and II), especially larger ones. Conversely, since slump resistance runs counter to adaptability, there may be situations in which the dentist might prefer that the composite readily flow and adapt, such as into pits and fissures. Because the viscosity of resin composites can vary widely, different tests are needed to assess differences in slump resistance for different classes of resin composites.

Two tests have been used to test slump resistance of more heavily filled resin composites. In one, a 3D laser scan is used to assess the dimensional change in pre-shaped resin composite at one-minute intervals . Image registration and image matching are used to compare differences in the vertical axis from baseline to subsequent scans and is defined as slumping. The evaluation should be limited to clinically relevant periods, e.g. no more than 3–4 min. An advantage to this test is that tooth-shaped samples can be used. Limitations of this technique are a vertical resolution of approximately 10 μm, an inability of the optical setup to measure surface inclinations greater than 60°, thereby making the specimen shape an important factor, and the necessity that the laser being used must emit light in a wavelength that will not initiate polymerization of the resin composite.

The other test ( Fig. 2 ) for more viscous resin composites uses disc-shaped specimens in which a mold with a specific shape is pressed into the surface of the uncured composite . One set of specimens is immediately light cured, while another set is allowed to slump for two minutes prior to light curing. White stone replicas are fabricated and specimen geometries are assessed with 3-D laser profilometry. Changes in the dimensions of the specimens before and after slumping are used to calculate a Slump Resistance Index, which has shown a strong correlation with shear modulus and shear viscosity. This test method has very good resolution (approx. 1 μm with a high quality scanner). The results are independent of mold shape (triangular, circular, square), but are dependent on time and temperature.

Fig. 2
Schematic showing the aluminum mold used to imprint on composite disks and the cutting profiles of the square, round and triangular shapes (a), the procedure to make an imprint on a composite disc with the aluminum mold (b), the shape of the imprint immediately after removal of the mold (c), and the shape of the imprint after two minutes (d). The slumping resistance index (SRI) was defined as Hs–Ls/Hi–Li (Hi, Li: before slumping heights of the highest and lowest point from the base line, respectively; Hs, Ls: after-slumping heights of the highest and lowest point from the base line, respectively) (adapted from Lee et al. ).

Neither of the above two tests can be applied to flowable resin composites due to their lower viscosity, and therefore an alternative methodology has been developed to measure slump resistance of such materials. This method utilizes a custom-made loading device to extrude flowable resin composite from a syringe and needle of a specific size onto a glass slide . The speed of extrusion is controlled by a stepper motor that moves the piston of the syringe down at a specific and precise rate. Once extrusion of the specified amount of material volume is complete, the resin composite is allowed to slump for 10 s before being light-cured. The outcome measure for this test is the aspect ratio (height/diameter) of the cured, slumped specimen. This test was found to reliably differentiate among flowable resin composites of varying slump resistance, and demonstrated a close correlation to complex viscosity as measured by dynamic oscillatory shear testing .

Viscosity

Closely related to slump resistance is viscosity, where viscosity is essentially defined as the resistance to flow for a material or fluid. The above citations note strong correlations between various rheologic (i.e. flow) properties of resin composite and slump resistance. Viscosity is a complex property and is most thoroughly assessed using a parallel plate or cone and plate rheometer to perform a rotational shear test at a specific rotation speed or over a range of frequencies ( Fig. 3 ). The parallel plate arrangement is probably most ideal for filled and very viscous resin composites, because it is difficult to thin the viscous composite sufficiently to ensure that the tip of the cone is in contact with the plate for the latter method. A liquid holding cup with a cylinder rotating within it can also be used for assessing viscosity of very fluid materials, but is not suitable for viscous composites. This basic design of the parallel plate system can also be employed using a dynamic oscillatory shear force to provide an assessment of complete rheologic properties, including storage shear modulus (G′), loss shear modulus (G″), loss tangent (tan δ ), and complex viscosity ( η *) . Frequency ( ω ) is an important variable in the measurement of viscosity using dynamic oscillatory shear tests. Due to the complex interactions of the particles during flow, the viscosity of filled systems is more difficult to measure than that of neat resins or lesser filled resin composites, although even unfilled resins can yield complex shear thinning or thickening behavior.

Fig. 3
Schematic of the rotational cone and plate rheometer for measuring the viscosity of fluids.

A much simpler and less expensive assessment of composite viscosity has been described . It employs a methodology used in determining the consistency of elastomeric impression materials . Samples of resin composite are prepared in a mold of predetermined dimensions, removed from the mold and weighed, placed between sheets of plastic and subjected to a constant predefined load for 1 min. At that time, the specimen is imaged, the circumference measured and the surface area computed to define the composite consistency. This test does not determine the actual viscosity of the resin composite, but rather provides a relative comparison of viscosity among various resin composites. A similar method has been used with composite specimens subjected to different heat treatments to reduce viscosity . In this case, the thickness to volume ratio of flattened resin composites was measured after a specific volume of material was subjected to a specified load (4 kg) for a specific period of time (180 s), followed by light-curing. These methods may be more practical and easy to perform than experiments with more expensive rheological equipment, but they are typically less discriminating and provide less overall information.

Porosity

Porosity can exist in the form of voids at the surface or at interfaces between the resin composite and the tooth or another material, or they can be internal to the supplied material. Internal voids can be introduced during the manufacturing of the resin composite, but can also be formed during its clinical manipulation. When present at interfaces, porosity relates, at least indirectly, to the stickiness of the resin composite toward the cavity preparation. The stickier a composite is, the more likely it is to adhere to the placement instrument, poorly adapt to the preparation or previously placed resin composite increment, and form a void. Voids are significant in that they can degrade mechanical and esthetic properties of resin composite restorations . Several assessments have been suggested for evaluating porosity. A simple, non-destructive and inexpensive test is to simply take a radiograph of a composite specimen and evaluate it for the presence of voids . However, this is merely a 2-D representation of a 3-D specimen, therefore it is not possible to locate the voids within the bulk of the specimen, or to differentiate if there is more than one void superimposed on another. In addition, the void size that can be detected is limited by the resolution of the radiograph, making this test more valuable as a screening tool than a true measure of porosity.

Another relatively simple and inexpensive test for porosity is to fabricate resin composite specimens, and then section the specimens with a saw. The sections can be observed directly under magnification, or imaged and analyzed digitally, for example with image J software (free of charge from the National Institutes of Health, Bethesda, MD, and available at imagej.nih.gov ). These sections are then amenable to a number of different assessments, including the number, diameter, area, and/or the location of voids . The disadvantages to this method are that it is once again a 2-D representation of a 3-D phenomenon, and the outcomes will depend on the location and number of sections obtained from each specimen. Also, the assessment can be quite laborious and time consuming, depending on the number of specimens, sections and particular analysis used.

The use of a pycnometer can provide a determination of the overall density of a resin composite. When combined with a high-pressure mercury intrusion porosimeter, it is possible to estimate the pore size distribution as well . This method requires more sophisticated and expensive testing equipment. Another relatively simple and easy method for measuring true density for dental composite is a helium pycnometer , though again this method does require special equipment.

A highly accurate, yet sophisticated assessment of porosity within resin composite can be obtained using 3-D X-ray micro-tomography . Standardized samples are fabricated and subjected to high-resolution X-ray tomography (basically imaging a specimen with X-ray energy at a sequence of depths and then compiling the individual 2-D images), resulting in a 3-D rendering of the sample. The images must then undergo digital processing, such as low-pass filtering and edge contrast enhancement, followed by analysis with software to quantitate specimen porosity. This technique not only allows computation of the percentage of porosity, but also permits visualization of the location of porosities within the sample.

Radiopacity

Radiographic examination is often the best, and sometimes the only way to adequately assess various aspects of the clinical quality of a restoration, including its integrity, the presence of secondary caries, voids, overhangs or open margins, as well as its contour and adaptation to the cavity wall and its contact with other teeth or restorations. To meet these goals, resin composite must have a certain level of radiopacity, the optimal level being debatable. The current ISO 4049 standard calls for resin composite to be at least as radiopaque as an equal thickness of highly pure aluminum (Al) ( Fig. 4 ). This has been shown to correspond to the same level of radiopacity as dentin . However, the literature is equivocal as to whether this is the level of radiopacity that is most effective at distinguishing secondary caries from normal adjacent tooth structure or the restoration. One study indicated that a resin composite radiopacity close to dentin was most effective at diagnosing a simulated recurrent caries lesion . Others have determined that the resin composite should have radiopacity equal to, or slightly greater than, that of enamel to accurately discern the tooth-restoration margin and to accurately detect recurrent caries . In addition, it has been suggested that specimens of enamel and dentin be included in the assessment along with the resin composite samples (see Fig. 4 ) to enhance clinical relevance . One must also be aware that many resin dentin adhesives do not contain radiopacifying agents, and therefore may show up as a radiolucency between the resin composite and the tooth on a radiograph, depending upon its thickness.

Fig. 4
Digital dental X-ray showing two different dental composites (top-left and top-center) and a tooth slice (top right) in comparison to an aluminum step wedge, with thickness of 6, 5, 4, 3, 2, and 1 mm when going from left to right (courtesy of Dr. Wen Lien, United States Air Force Dental Evaluation & Consultation Service).

The ISO 4049 test for radiopacity calls for the fabrication of 1 mm thick cylindrical resin composite specimens that are exposed to a standardized radiograph of specified voltage (kV) and target film distance along with an aluminum step wedge of at least 98% purity with a thickness range of 0.5–5.0 mm in 0.5 mm increments. The optical density (standard film radiograph) or grey scale (digital radiograph) of the step wedge is plotted for each thickness, which is used to convert the radiopacity of the specimen into a corresponding radiopacity of aluminum.

Sensitivity to ambient light

Most current resin composites are visible light cure (VLC), that is, the setting reaction is initiated when the resin composite is irradiated with energy in the visible wavelength region that is absorbed by a photoinitiator contained in the resin. While alternative photoinitiators are being used to some extent, camphorquinone (CQ) is most commonly incorporated into resin composites solely or in conjunction with another photoinitiator. Camphorquinone has a peak absorption around 470 nm, which is in the blue range of the visible light spectrum. Sources of light other than VLC units emit light in this range, such as the operatory room lights, dental operating lights, and sunlight, which can all initiate premature cure of VLC resin composite . If the resin composite begins to harden prior to the dentist completing insertion and manipulation of the material, it can affect the handling characteristics, void formation and adaptation into the cavity preparation. ISO 4049 specifies a value that a material must meet to be considered adequately resistant to ambient light. The current ISO dental standard involves exposure of resin composite to an 8000 lux light source, which is less than other ISO standard testing methods with a 24,000 lux light source. Both methods may differ from the ambient light conditions that may be experienced in a dental operatory. Therefore, as with radiopacity, there is some question as to whether the ISO standard adequately assesses the effect of ambient light on initiating the cure of resin composite. The results of one study showed that the working time as determined when mimicking normal dental conditions was statistically the same as those obtained with the ISO test using 24,000 lux, while the current ISO standard of 8000 lux, being relatively low, significantly over estimated the “real world” working time of the resin composites .

The ISO 4049 test for ambient light sensitivity currently uses a xenon lamp, or alternate source with equivalent performance, with appropriate filters to simulate the light spectrum from a dental operating light. The light is placed in a dark room and an illuminance measuring device is used to determine the distance from the light source that provides 8000 lux illuminance. The measuring device is shielded with a black matt cover upon which a glass slide is placed. Approximately 30 mg of spherical-shaped resin composite is placed on the glass slide for 60 s, after which a second glass slide is placed on top of the resin composite and rotated. If no voids or inhomogeneities are discerned, the material is deemed to have passed the test.

Technique sensitivity: handling, placement

Stickiness

Stickiness refers to the propensity of a resin composite to be retained on an instrument while the material is being placed into the cavity preparation. There is an ideal, yet poorly defined level of stickiness whereby the resin composite will be retained in the cavity and not pulled out or deformed as the placement instrument is removed. A number of tests have been devised to assess stickiness, most of which follow a similar scheme. A set volume of composite is placed in a mold, and then a steel rod or instrument is inserted into the unset material at a constant rate or until a predetermined force or depth is reached; then the motion is reversed until the composite separates from the instrument ( Fig. 1 ). Immediately upon separation, the composite is irradiated with a curing light to harden the material, leaving the surface in the shape of a peak. This peak of composite, sometimes called a “composite flag” can be measured for height and/or area and used as a measure of stickiness . Depending on the instrumentation and measurement capability, the unplugging force and work (the force and work required for the composite to detach from the instrument in withdrawal direction) can also be measured and calculated ( Fig. 1 ). All of these methods have been found to be reliable measures of composite stickiness that allow for good differentiation among current materials. In addition, one study correlated the unplugging work and force of various resin composites to the subjective handling characteristics as assessed by dentists and found a good association between the two, indicating that these tests are a good proxy by which to evaluate resin composite stickiness . Resin composite temperature, speed of the instrument/rod insertion and removal, and the surface area and roughness that the composite is in contact with have all been shown to be important factors influencing the results of these tests, and therefore should be well described whenever publishing results in this area.

Fig. 1
Schematic diagram and photo showing experimental setup to measure forces during application and removal of plugger tip from unset composite to assess stickiness (courtesy of Dr. Martin Rosentritt). Graph shows force–time relationship during application and removal, with work being calculated as the area under the curve (force × distance) (adapted from Rosentritt et al. ).

Slump resistance

Slump resistance is the ability of a resin composite to maintain its shape after placement and prior to curing. This is important in a clinical situation when a clinician desires to sculpt the anatomy of a restoration in the unset paste prior to light curing, in part to reduce the amount of finishing required. This is particularly the case in class III or class V restorations, in large anterior restorations, e.g. a direct composite veneer or a class IV restoration, and when reconstructing the cuspal or crestal anatomy in posterior restorations (class I and II), especially larger ones. Conversely, since slump resistance runs counter to adaptability, there may be situations in which the dentist might prefer that the composite readily flow and adapt, such as into pits and fissures. Because the viscosity of resin composites can vary widely, different tests are needed to assess differences in slump resistance for different classes of resin composites.

Two tests have been used to test slump resistance of more heavily filled resin composites. In one, a 3D laser scan is used to assess the dimensional change in pre-shaped resin composite at one-minute intervals . Image registration and image matching are used to compare differences in the vertical axis from baseline to subsequent scans and is defined as slumping. The evaluation should be limited to clinically relevant periods, e.g. no more than 3–4 min. An advantage to this test is that tooth-shaped samples can be used. Limitations of this technique are a vertical resolution of approximately 10 μm, an inability of the optical setup to measure surface inclinations greater than 60°, thereby making the specimen shape an important factor, and the necessity that the laser being used must emit light in a wavelength that will not initiate polymerization of the resin composite.

The other test ( Fig. 2 ) for more viscous resin composites uses disc-shaped specimens in which a mold with a specific shape is pressed into the surface of the uncured composite . One set of specimens is immediately light cured, while another set is allowed to slump for two minutes prior to light curing. White stone replicas are fabricated and specimen geometries are assessed with 3-D laser profilometry. Changes in the dimensions of the specimens before and after slumping are used to calculate a Slump Resistance Index, which has shown a strong correlation with shear modulus and shear viscosity. This test method has very good resolution (approx. 1 μm with a high quality scanner). The results are independent of mold shape (triangular, circular, square), but are dependent on time and temperature.

Fig. 2
Schematic showing the aluminum mold used to imprint on composite disks and the cutting profiles of the square, round and triangular shapes (a), the procedure to make an imprint on a composite disc with the aluminum mold (b), the shape of the imprint immediately after removal of the mold (c), and the shape of the imprint after two minutes (d). The slumping resistance index (SRI) was defined as Hs–Ls/Hi–Li (Hi, Li: before slumping heights of the highest and lowest point from the base line, respectively; Hs, Ls: after-slumping heights of the highest and lowest point from the base line, respectively) (adapted from Lee et al. ).

Neither of the above two tests can be applied to flowable resin composites due to their lower viscosity, and therefore an alternative methodology has been developed to measure slump resistance of such materials. This method utilizes a custom-made loading device to extrude flowable resin composite from a syringe and needle of a specific size onto a glass slide . The speed of extrusion is controlled by a stepper motor that moves the piston of the syringe down at a specific and precise rate. Once extrusion of the specified amount of material volume is complete, the resin composite is allowed to slump for 10 s before being light-cured. The outcome measure for this test is the aspect ratio (height/diameter) of the cured, slumped specimen. This test was found to reliably differentiate among flowable resin composites of varying slump resistance, and demonstrated a close correlation to complex viscosity as measured by dynamic oscillatory shear testing .

Viscosity

Closely related to slump resistance is viscosity, where viscosity is essentially defined as the resistance to flow for a material or fluid. The above citations note strong correlations between various rheologic (i.e. flow) properties of resin composite and slump resistance. Viscosity is a complex property and is most thoroughly assessed using a parallel plate or cone and plate rheometer to perform a rotational shear test at a specific rotation speed or over a range of frequencies ( Fig. 3 ). The parallel plate arrangement is probably most ideal for filled and very viscous resin composites, because it is difficult to thin the viscous composite sufficiently to ensure that the tip of the cone is in contact with the plate for the latter method. A liquid holding cup with a cylinder rotating within it can also be used for assessing viscosity of very fluid materials, but is not suitable for viscous composites. This basic design of the parallel plate system can also be employed using a dynamic oscillatory shear force to provide an assessment of complete rheologic properties, including storage shear modulus (G′), loss shear modulus (G″), loss tangent (tan δ ), and complex viscosity ( η *) . Frequency ( ω ) is an important variable in the measurement of viscosity using dynamic oscillatory shear tests. Due to the complex interactions of the particles during flow, the viscosity of filled systems is more difficult to measure than that of neat resins or lesser filled resin composites, although even unfilled resins can yield complex shear thinning or thickening behavior.

Fig. 3
Schematic of the rotational cone and plate rheometer for measuring the viscosity of fluids.

A much simpler and less expensive assessment of composite viscosity has been described . It employs a methodology used in determining the consistency of elastomeric impression materials . Samples of resin composite are prepared in a mold of predetermined dimensions, removed from the mold and weighed, placed between sheets of plastic and subjected to a constant predefined load for 1 min. At that time, the specimen is imaged, the circumference measured and the surface area computed to define the composite consistency. This test does not determine the actual viscosity of the resin composite, but rather provides a relative comparison of viscosity among various resin composites. A similar method has been used with composite specimens subjected to different heat treatments to reduce viscosity . In this case, the thickness to volume ratio of flattened resin composites was measured after a specific volume of material was subjected to a specified load (4 kg) for a specific period of time (180 s), followed by light-curing. These methods may be more practical and easy to perform than experiments with more expensive rheological equipment, but they are typically less discriminating and provide less overall information.

Porosity

Porosity can exist in the form of voids at the surface or at interfaces between the resin composite and the tooth or another material, or they can be internal to the supplied material. Internal voids can be introduced during the manufacturing of the resin composite, but can also be formed during its clinical manipulation. When present at interfaces, porosity relates, at least indirectly, to the stickiness of the resin composite toward the cavity preparation. The stickier a composite is, the more likely it is to adhere to the placement instrument, poorly adapt to the preparation or previously placed resin composite increment, and form a void. Voids are significant in that they can degrade mechanical and esthetic properties of resin composite restorations . Several assessments have been suggested for evaluating porosity. A simple, non-destructive and inexpensive test is to simply take a radiograph of a composite specimen and evaluate it for the presence of voids . However, this is merely a 2-D representation of a 3-D specimen, therefore it is not possible to locate the voids within the bulk of the specimen, or to differentiate if there is more than one void superimposed on another. In addition, the void size that can be detected is limited by the resolution of the radiograph, making this test more valuable as a screening tool than a true measure of porosity.

Another relatively simple and inexpensive test for porosity is to fabricate resin composite specimens, and then section the specimens with a saw. The sections can be observed directly under magnification, or imaged and analyzed digitally, for example with image J software (free of charge from the National Institutes of Health, Bethesda, MD, and available at imagej.nih.gov ). These sections are then amenable to a number of different assessments, including the number, diameter, area, and/or the location of voids . The disadvantages to this method are that it is once again a 2-D representation of a 3-D phenomenon, and the outcomes will depend on the location and number of sections obtained from each specimen. Also, the assessment can be quite laborious and time consuming, depending on the number of specimens, sections and particular analysis used.

The use of a pycnometer can provide a determination of the overall density of a resin composite. When combined with a high-pressure mercury intrusion porosimeter, it is possible to estimate the pore size distribution as well . This method requires more sophisticated and expensive testing equipment. Another relatively simple and easy method for measuring true density for dental composite is a helium pycnometer , though again this method does require special equipment.

A highly accurate, yet sophisticated assessment of porosity within resin composite can be obtained using 3-D X-ray micro-tomography . Standardized samples are fabricated and subjected to high-resolution X-ray tomography (basically imaging a specimen with X-ray energy at a sequence of depths and then compiling the individual 2-D images), resulting in a 3-D rendering of the sample. The images must then undergo digital processing, such as low-pass filtering and edge contrast enhancement, followed by analysis with software to quantitate specimen porosity. This technique not only allows computation of the percentage of porosity, but also permits visualization of the location of porosities within the sample.

Radiopacity

Radiographic examination is often the best, and sometimes the only way to adequately assess various aspects of the clinical quality of a restoration, including its integrity, the presence of secondary caries, voids, overhangs or open margins, as well as its contour and adaptation to the cavity wall and its contact with other teeth or restorations. To meet these goals, resin composite must have a certain level of radiopacity, the optimal level being debatable. The current ISO 4049 standard calls for resin composite to be at least as radiopaque as an equal thickness of highly pure aluminum (Al) ( Fig. 4 ). This has been shown to correspond to the same level of radiopacity as dentin . However, the literature is equivocal as to whether this is the level of radiopacity that is most effective at distinguishing secondary caries from normal adjacent tooth structure or the restoration. One study indicated that a resin composite radiopacity close to dentin was most effective at diagnosing a simulated recurrent caries lesion . Others have determined that the resin composite should have radiopacity equal to, or slightly greater than, that of enamel to accurately discern the tooth-restoration margin and to accurately detect recurrent caries . In addition, it has been suggested that specimens of enamel and dentin be included in the assessment along with the resin composite samples (see Fig. 4 ) to enhance clinical relevance . One must also be aware that many resin dentin adhesives do not contain radiopacifying agents, and therefore may show up as a radiolucency between the resin composite and the tooth on a radiograph, depending upon its thickness.

Fig. 4
Digital dental X-ray showing two different dental composites (top-left and top-center) and a tooth slice (top right) in comparison to an aluminum step wedge, with thickness of 6, 5, 4, 3, 2, and 1 mm when going from left to right (courtesy of Dr. Wen Lien, United States Air Force Dental Evaluation & Consultation Service).

The ISO 4049 test for radiopacity calls for the fabrication of 1 mm thick cylindrical resin composite specimens that are exposed to a standardized radiograph of specified voltage (kV) and target film distance along with an aluminum step wedge of at least 98% purity with a thickness range of 0.5–5.0 mm in 0.5 mm increments. The optical density (standard film radiograph) or grey scale (digital radiograph) of the step wedge is plotted for each thickness, which is used to convert the radiopacity of the specimen into a corresponding radiopacity of aluminum.

Sensitivity to ambient light

Most current resin composites are visible light cure (VLC), that is, the setting reaction is initiated when the resin composite is irradiated with energy in the visible wavelength region that is absorbed by a photoinitiator contained in the resin. While alternative photoinitiators are being used to some extent, camphorquinone (CQ) is most commonly incorporated into resin composites solely or in conjunction with another photoinitiator. Camphorquinone has a peak absorption around 470 nm, which is in the blue range of the visible light spectrum. Sources of light other than VLC units emit light in this range, such as the operatory room lights, dental operating lights, and sunlight, which can all initiate premature cure of VLC resin composite . If the resin composite begins to harden prior to the dentist completing insertion and manipulation of the material, it can affect the handling characteristics, void formation and adaptation into the cavity preparation. ISO 4049 specifies a value that a material must meet to be considered adequately resistant to ambient light. The current ISO dental standard involves exposure of resin composite to an 8000 lux light source, which is less than other ISO standard testing methods with a 24,000 lux light source. Both methods may differ from the ambient light conditions that may be experienced in a dental operatory. Therefore, as with radiopacity, there is some question as to whether the ISO standard adequately assesses the effect of ambient light on initiating the cure of resin composite. The results of one study showed that the working time as determined when mimicking normal dental conditions was statistically the same as those obtained with the ISO test using 24,000 lux, while the current ISO standard of 8000 lux, being relatively low, significantly over estimated the “real world” working time of the resin composites .

The ISO 4049 test for ambient light sensitivity currently uses a xenon lamp, or alternate source with equivalent performance, with appropriate filters to simulate the light spectrum from a dental operating light. The light is placed in a dark room and an illuminance measuring device is used to determine the distance from the light source that provides 8000 lux illuminance. The measuring device is shielded with a black matt cover upon which a glass slide is placed. Approximately 30 mg of spherical-shaped resin composite is placed on the glass slide for 60 s, after which a second glass slide is placed on top of the resin composite and rotated. If no voids or inhomogeneities are discerned, the material is deemed to have passed the test.

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Nov 22, 2017 | Posted by in Dental Materials | Comments Off on Academy of Dental Materials guidance—Resin composites: Part II—Technique sensitivity (handling, polymerization, dimensional changes)
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