Comparative Properties of Low-Shrinkage Composite Resins
John Burgess, DDS, MS; and Deniz Cakir, DDS, MS
Abstract: OBJECTIVE: To measure the polymerization shrinkage, wear, surface roughness, gloss, color stability, and stain resistance of N’Durance® , SureFil ® SDR® , and Filtek ® LS, and compare those values to currently used clinically successful composites. METHODS: Polymerization shrinkage: Specimens were placed into an AcuVol™, and percent volume shrinkage calculated after light curing. Wear: Specimens were loaded in the University of Alabama at Burlington wear machine. Volumetric loss was determined using a 3D noncontact profilometer (Proscan 2000). Gloss and Ra: Gloss was measured with gloss meter using 60º geometry in a gloss unit. Surface roughness was measured with a 3D noncontact profilometer. Color stability: Initial and after storage (distilled water, ultraviolet light, and staining solution) L*a*b* values were measured using a spectrophotometer against white and black backgrounds. RESULTS: Filtek LS had the lowest polymerization shrinkage values. Gloss and roughness average values were similar for all materials measured. Color stability measurements for all composite resins in water and UV light were less than 3. All composite resins had similar values when placed in the staining solution.CONCLUSION: Low-shrinkage composite resin restorative materials have mechanical and physical properties similar to clinically successful composite resins.
Composite resins have been used in restorative dentistry with good clinical success for almost 45 years. However, these esthetic materials have limitations that restrict their use as universal restorative materials. Specifically, composite resins shrink when polymerized, wear when used as a restorative material in high occlusal stress areas, and differ in gloss retention and color stability.
Visible light-cured composite resin is placed in 2-mm increments into a cavity preparation, contoured to the desired shape, and light-cured. This time-consuming process is necessary because light attenuates as it passes through composite resin. Photoinitiators1 in the composite resin are light-activated and initiate polymerization of the dimethacrylate resin monomers. Composite resins may contain a combination of photoinitiators, each requiring its own specific wavelength for maximum reactivity. The spectral emission of the light-curing unit should be compatible with the spectral absorption of the photoinitiators to produce maximum conversion in the composite resin. Poorly cured composite resin shrinks less, but has poor mechanical properties, color stability, and wear resistance. Therefore, as a general guide, composite resin should be cured in 2-mm increments with a curing light that emits the proper wavelengths to cure the composite resin being used. Although curing mechanisms vary, polymerization of composite resin diminishes molecular vibration, producing polymerization shrinkage as the resin solidifies. Most currently marketed composite resins have polymerization shrinkage values from 0.9 vol% to 3.7 vol%.2-7 Shrinkage produces stress at the marginal area, which compromises marginal seal thereby increasing staining and postoperative cold sensitivity. This may eventually allow marginal caries. Often, polymerization shrinkage tears the composite resin or the tooth in the margin. When the restoration is finished, debris falls into the crevice and the “white line”8 becomes visible (Figure 1).
Stress reduction during polymerization is an area of concentrated research in composite resin technology. One approach to reducing marginal stress and improving the marginal seal is to use a more viscous bonding agent with increased filler loading to provide a thicker adhesive layer to accommodate contraction stress and provide a more durable bond. In theory, as the resin composite is polymerized, the elastic bonding agent is stretched, not torn, and an improved marginal seal is produced. However, clinical studies evaluating the use of flowable composite resin under highly filled composite restorations do not show improved clinical performance. Another method increases filler content of the composite, which decreases shrinkage by reducing the monomer content and has the additional benefit of improving wear resistance. However, increased loading increases composite stiffness, reduces flow, and increases the stress developed during polymerization.9 Attempts to control shrinkage stress by using various light-curing techniques have not been clinically successful; no reduction in marginal staining or improvement in marginal adaptation was observed in several clinical studies.10-15
The most recent attempt to reduce shrinkage uses new resin monomers with novel chemistries to reduce polymerization shrinkage stress. Some currently marketed composite resins have low polymerization shrinkage (N’Durance® , Septodont, https://www.septodontusa.com; and Filtek™ LS, 3M ESPE, https://www.3mespe.com) or have significant flow (SureFil® SDR® , DENTSPLY, https://www.dentsply.com) to compensate for polymerization shrinkage. The first low-shrinkage, commercially available composite resin was Filtek LS, which reduced shrinkage with ring-opening silorane chemistry. In the authors’ laboratory, shrinkage for this composite ranged from 0.7 vol% to 0.9 vol%. Another newly developed composite, N’Durance, uses a blend of methacrylate and dimer chemistry to reduce shrinkage to 1.2 vol%. Shrinkage is only one factor contributing to polymerization stress in composite resins. As composite resin polymerizes, the material flows from the unbonded free margin to compensate for shrinkage. As polymerization continues, the composite stiffens and flow decreases. At the gel point, the composite is too rigid to flow, and the forces continuing to be generated by polymerization shrinkage are transferred to the cavity wall, creating stress at the marginal interfaces. It is important to note that only a portion of the total shrinkage is translated into stress; the remainder is reduced by the composite flow. SureFil SDR is designed to reduce shrinkage stress by increasing flow with a unique chemistry that slows the rate of polymerization to reduce shrinkage stress, even though its polymerization shrinkage remains similar to other flowable composite resins (3.1 vol%). This composite resin is used as a dentin replacement material and is polymerized in 4-mm increments.
To date, little information has been presented measuring comparative properties of low-shrinkage composites and conventional clinically successful composite resins. The purpose of this study was to measure and compare the polymerization shrinkage, wear, surface roughness, gloss, color stability, and stain resistance of N’Durance, SureFil SDR, and Filtek LS, with currently used, clinically successful composite resins.
POLYMERIZATION SHRINKAGE
Polymerization shrinkage was measured using an AcuVol™ (BISCO, https://www.bisco.com), which uses a camera to measure the size and shape of the composite placed on the device’s rotating pedestal. After the composite is imaged, the volume is calculated and the difference is gauged after the composite is light-cured. Polymerization shrinkage is expressed as vol% for the five measured materials.
WEAR
In vitro wear resistance was measured using the University of Alabama at Birmingham (UAB) wear machine for seven composite resins: Filtek LS, Filtek Supreme Plus, Z250 (3M ESPE), Heliomolar (Ivoclar Vivadent, https://www.ivoclarvivadent.us), Tetric Ceram HB (Ivoclar Vivadent), N’Durance, and Grandio (VOCO, https://www.voco.com). N’Durance and Filtek LS were the highly filled low-shrinkage materials, and SureFil SDR was the flowable-type low-shrinkage material used in this study. Eight replications were made for each material. The machine parameters were wear stylus tips of stainless steel (d = 4.7 mm) and third body media of 50-µ polymethylmethacrylate beads (15 g beads for 9 g water). A cylindrical silicone mold (diameter = 10 mm, height = 6 mm) was used to prepare the composite samples, eight for each material. The composite resin specimen was formed in three increments—2 mm each—and cured with a Coltolux LED curing light (Coltène/Whaledent, https://www.coltene.com) (600 mW/cm2 ) using the manufacturer’s recommended curing time. After fabrication, the specimens were stored in distilled water at 37º C for 24 hours. Then the specimens were positioned in brass holders (d = 15 mm) using a self-cured acrylic. Each specimen was polished with 600-grit and 1200-grit SiC abrasive paper under water spray using a rotational polishing device (No: 233-0-1997, Buehler, https://www.buehler.com) and wet finished with 0.05 µ alumina slurry and a polishing cloth. To remove any adherent finishing debris, each specimen was sonicated in an ultrasonic bath in distilled water for 5 mins. After polishing, the specimens mounted in the brass cylinders were placed into the UAB wear machine. A cyclic load of 75 N was applied for 400,000 cycles. Specimens were scanned using a noncontact 3D surface profilometer (Proscan 2000, Scantron Industrial Products. https://www.scantronltd.co.uk) to determine the wear volume (Pro-Form software, Scantron Industrial Products) by superimposing the two scans (Figure 2).
Results
Wear data were statistically analyzed using analysis of variance (ANOVA) followed by Tukey/Kramer post-hoc tests (P = .05) and are displayed in Figure 3 (Mean±SD). Z250 produced significantly lower wear volume (P = .0014) compared with Filtek LS. No other differences were significant.
GLOSS AND SURFACE ROUGHNESS
The ability of a composite resin to be finished to a smooth, reflective, toothlike surface is an important property, allowing a composite resin restoration to mimic tooth structure. The roughness average (Ra) measures the smoothness of the finished composite resin: low values indicate a smooth surface. The ability of a composite to obtain a highly reflective surface is measured in gloss units (GU); these values were measured for Filtek Supreme, Heliomolar, Filtek LS, and N’Durance (Table 1). For the surface gloss and roughness tests, five specimens were made for each composite resin tested. Five composite resin discs 12 mm in diameter and 4 mm thick were made for each of the four composite resins by placing the composite in an elastomeric mold. The notched mold provided a method to orient the polymerized discs during the measurements. The composite was placed and cured in two increments of 2 mm, and the last increment was covered with a clear strip and compressed with a glass microscope slide. The composite was polymerized through the slide to produce a flat composite surface with a Coltolux LED curing light (output: 600mW/cm2, verified with a radiometer [FieldMate, Coherent, https://www.coherent.com]). After the specimens were separated from the mold, they were immersed in distilled water, stored in an incubator at 37º C for 24 hours, and polished using a rotational polishing device (No: 233-0-1997, Buehler) with a series of 320-grit, 400-grit, 600-grit, 1200-grit, and 2000-grit SiC abrasive papers concluding with 0.05 µ alumina slurry on a polishing cloth. Afterwards, the specimens were ultrasonically cleaned in distilled water for 5 mins, followed by rinsing, before measuring gloss and surface roughness to remove any polishing material from the composite surface. Gloss was measured with a small area gloss meter using a scale of 1 GU to 100 GU with a square measurement area of 2 mm x 2 mm and 60º geometry (Novo-Curve™, Rhopoint, https://www.rhopointinstruments.com) (Figure 4 ).
One reading was made and the specimen rotated 90º, then a second reading was taken. Specimens were rotated to record any residual finishing lines. The mean of two readings was recorded as the GU for each specimen. To eliminate the influence of the overhead light, the aperture of the gloss meter and specimen was covered with a dark box during the gloss reading.
Surface roughness (µm) produced on each polished composite was measured with a 3D noncontact profilometer (Proscan 2000) following ISO 4288 for cut-off and evaluation length (0.8/4 mm), with 3 µm step size. The area (4 mm x 2 mm) was analyzed for each specimen to determine the surface roughness, and the Ra recorded. Data were statistically analyzed using ANOVA and Tukey/Kramer tests (P = .05).
Results
Gloss: Filtek LS produced the lowest gloss value (P < .0001). No other significant differences were found among the materials (Figure 5). Surface roughness: For both X and Y values, N’Durance had significantly lower Ra values than Heliomolar and Filtek Silorane (P = .0025). No other differences were significant (Figure 6).
STAIN RESISTANCE AND COLOR STABILITY
For each material and shade tested, nine cylindrical samples (12.2 mm diameter, 3.6 mm height) were prepared in the same silicone mold used for the surface roughness specimens; three for stain resistance, three for UV light, and three for deionized water. After light curing, the specimens were stored in the dark at 37º C for 24 hours. The notched specimens allowed them to be polished and measured at repeatable orientations. Each specimen was cured and polished using a sequence of 320-grit, 400-grit, 600-grit, and 1200-grit silicon carbide grinding papers followed by 0.05 µm slurry of alumina powder and water on a Buehler polishing cloth. Each specimen was polished for 80 secs, rotating 90º every 20 secs for each paper. The polishing was extended to 30 secs for each orientation during the final slurry polish. Initial L*a*b* values were taken using a CM-700d spectrophotometer (Konica Minolta, https://www.konicaminolta.com) against white or black backgrounds made from poster board. For each background, every sample was measured at two orientations 90º from each other, relative to the orientation notches on the specimen (Figure 7 , ). Silicone fixtures for the spectrophotometer positioned the composite resin specimens for repeatability, and 1-second sampling delay minimized instrument vibration during measurement. CIELab 1976 formulas were used, SCE (specular component excluded), SAV (small area view), 10º, with each orientation measured twice and averaged by the spectrophotometer.
Specimens to be stained were placed into a solution made from a mixture of coffee, tea, and cranberry juice for 1 week. After ultrasonic cleaning, L*a*b* values were recorded with the spectrophotometer against white and black backgrounds before and after storage.
Plastic wells were used to apply storage treatments to the samples. Three were stored in distilled water at room temperature in darkness for 1 week, three in a staining solution for 1 week at 37º C, and three immersed in distilled water and placed in a foil-lined chamber under UV light with a spectral range of 310 nm to 410 nm, a peak wavelength of 350 nm, and an output of 17 mW for 48 hours at 2.16 KJ/m2 x 105 KJ/m2. After storage, L*a*b* and gloss measurements were taken in the same orientations as the initial values.
Results
The spectrophotometer readings obtained for color stability and stain resistance were statistically analyzed using paired T-tests within the same sample groups and unpaired T-tests across different groups. The initial L*a*b* spectrophotometer measurements and posttreatment readings were entered into a formula (Delta E = {(Delta L*)2 + (Delta a*)2 + (Delta b*)2}1/2) to obtain ∆E values, which were statistically analyzed. Although the analyses were complex, no composite resin produced visually perceptible color changes (Delta E greater than 3) in water or UV light. All composites had perceptible color changes (Delta E greater than 3) after being stored in the staining solution for 1 week (Figure 8).
CONCLUSIONS
While no perfect composite resin has been produced to date, this article details some advantages and disadvantages of low-shrinkage composite resins. Although SureFil SDR (flowable) has 3.1 vol% polymerization shrinkage, it may reduce marginal stress by increased flow during its 4-mm posterior bulk fill. Due to its significant wear, SureFil SDR should be covered with a wear-resistant composite resin in occlusal areas. Filtek LS has greater wear than all other highly filled, low-shrinkage composite resins, as well as the least polymerization shrinkage (less than 1%). Because of its silorane chemistry, Filtek LS requires a dedicated self-etching bonding agent to bond to tooth structure. N’Durance has low shrinkage (1.2 vol%), high wear resistance, high surface gloss (good polishability), along with good smoothness, color stability, and stain resistance. In addition, N’Durance can be used with any bonding agent.
DISCLOSURE
The authors have received grant/research support from Septodont, Caulk, 3M ESPE, and VOCO.
ESSENTIAL CHARACTERISTICS OF N’DURANCE HIGH CONVERSION LOW VOLUME SHRINKAGE LOW WATER ABSORPTION BONDING COMPATIBILITY |
CLINICAL RESEARCHWear Resistance in Posterior Restorations These characteristics—wear resistance, combined with higher conversion than most other posterior composites, volumetric shrinkage of about 1.2%, and shrinkage stress of 1.4 MPa to 1.5 MPa—are proving very beneficial for durability and biocompatibility of restorations. Until now, this combination has seldom been found in posterior composites. |
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 Figure 1 White line formation produced during polymerization but seen when finishing debris fills the gap created by the shrinkage stress. |  Figure 2 Wear pattern obtained from the Proscan 2000. | ||||||
 Figure 3 Volume loss of tested materials after wear test. |  Figure 4 Orientation of the polished composite specimen on the Novo-Curve gloss meter. | ||||||
 Figure 5 Gloss values of four composites (in gloss units). |  Figure 6 Surface roughness (Ra) values of four composites (in microns). | ||||||
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| Figure 7 Specimen orientation on spectrophotometer for color measurement against white or black backgrounds. | |||||||
 Figure 8 Delta E values after color stability (distilled water and UV light) and stain resistance (staining solution) tests. |  | ||||||
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