This study determined the volumetric shrinkage, degree of conversion, sorption, solubility, and flexure properties (before and after aging) of commercial dental composites identified as “low shrinkage”.
Six Bis-GMA-based composites (Point 4, ELS, Filtek Supreme, Aelite LS Posterior, Filtek Z250 and Heliomolar), a silorane-based (Filtek LS) and a dimer dicarbamate dimethacrylate-based composite (N’Durance) were studied. Total shrinkage was measured in a mercury dilatometer ( n = 3). Elastic modulus and flexural strength was determined by the three point bending test before and after 4 months aging in 75% ethanol solution ( n = 10). Sorption and solubility in water was assessed according to ISO 4049. Additionally, composite degree of conversion was determined using near-IR spectroscopy ( n = 3). Data were analysed using one-way/two-way ANOVA or Kruskal–Wallis, and Tukey’s test. Student’s t -test was used to compare storage periods ( α = 0.05).
The volumetric shrinkage values ranged between 1.5% (Filtek LS) and 3.4% (Point 4). The materials presented different behaviors regarding aging in ethanol, as evidenced by the large range in values of percentage reduction for elastic modulus (26–75%) and flexural strength (25–86%). Sorption values ranged from 8.2 μg/mm 3 (Aelite LS Posterior) to 38.2 μg/mm 3 (Point 4). Solubility values ranged from −1.5 μg/mm 3 (Filtek LS) to 5.7 μg/mm 3 (Aelite LS Posterior). The degree of conversion values at 10 min ranged between 28% (Filtek LS) and 73% (Point 4). At 72 h after curing the values ranged between 39% (Filtek LS) and 83% (Point 4).
Among the materials identified by their manufacturers as “low shrinkage”, only Filtek LS presented statistically lower values of shrinkage compared to composites based on conventional dimethacrylates, but lower degree of conversion as well. Overall materials with higher filler content presented higher initial values of flexural properties. However, their resistance to ethanol degradation seems to be influenced by different compositional factors and, therefore, cannot be directly related to water sorption results.
The longevity of posterior composite restorations remains a matter of concern among clinicians. Surveys revealed that composite restorations placed in premolars and molars need to be replaced after 5–6 years, with bulk fracture and secondary caries as the main reasons cited . The need for more durable restorations is one of the driving forces for the development of new materials and, though not always suitable for predicting clinical outcomes, laboratory evaluations are often used to estimate their potential by comparison with already established commercial products .
Dental composites undergo shrinkage during polymerization, which associated with the increase in stiffness of the forming polymer network, generates stresses at the interface between the tooth and the restoration, potentially increasing the risk of premature failure . Recently, some dental composite materials have been developed and marketed as “low shrinkage”. Some of these materials still use Bis-GMA as the base monomer, but resort to greater filler loadings or absence of low molecular weight diluents to achieve lower shrinkage. The introduction of pre-polymerized resin filler is another attempt to reduce the shrinkage. Other approaches include the use of high molecular weight monomers, such as the methacrylate derivatives of dimer acid. In composites containing such components, the reduction in polymerization shrinkage is claimed to be due to not only the higher molecular weight of the monomer, but also to the formation of a heterogeneous network during polymerization, using a mechanism known as polymerization induced phase-separation. Nano-sized domain structures are formed at different rates, which allow viscous flow and shrinkage accommodation within the bulk of the material . This is claimed to reduce polymerization stress, in spite of the higher conversion achieved by this material which is attributed to its more chemically reactive components.
Other alternative monomers have been proposed with epoxide ring-opening polymerization type chemistries , which are known to lead to lower shrinkage than free-radical, vinyl polymerizations. One commercial example is a silorane-based composite, which polymerizes via a cationic mechanism virtually insensitive to oxygen inhibition. Additionally, the presence of the siloxane core (to which the oxirane rings are attached) makes the molecule fairly hydrophobic .
Most in vitro studies focus on immediate mechanical properties, evaluated under dry conditions or after short-term storage in water. However, materials degradation by oral fluids and bacteria have a significant impact on physical , biological , and chemical properties . Mechanical properties of conventional methacrylates used as dental composites have already been extensively explored , but there are still gaps of information where the newest composites are concerned. This study determined the volumetric shrinkage, flexure properties before and after aging, sorption and solubility of commercial dental composites identified as “low shrinkage”. Additionally, degree of conversion of the materials was also obtained. The null hypothesis was that low shrinkage and conventional composites would present similar performance regarding volumetric shrinkage, mechanical properties (before and after aging), and water sorption/solubility.
Methods and materials
Eight commercial composites, all shade A3, were studied ( Table 1 ). Six were Bis-GMA-based: a nanofilled (Filtek Supreme), a highly filled (Aelite LS Posterior), and one that does not contain low molecular weight diluent (ELS). Two composites used alternative monomers: N’Durance contains a dimer dicarbamate dimethacrylate and Filtek LS is silorane-based. Aelite LS Posterior, ELS, N’Durance and Filtek LS are considered low-shrinkage composites by their respective manufacturers.
|Material||Filler content (vol%)||Filler size (μm)||Monomers||Manufacturer|
|Point 4||59%||0.4||Exact composition not informed by manufacturer||SDS Kerr, Orange, CA, EUA|
|N’Durance||65%||0.04–0.5||Dimer Dicarbamate Dimethacrylate (DADMA), Bis-EMA, UDMA||Septodont, Louisville, CO, USA|
|ELS||50%||0.07–2.6||Bis-GMA, Bis-EMA||Saremco, Rohnacker, Switzerland|
|Filtek Supreme||57%||0.08–1.4||Bis-GMA, Bis-EMA, UDMA, TEGDMA||3M ESPE St Paul, EUA|
|Aelite LS Posterior||74%||0.06||Bis-GMA, Bis-EMA, TEGDMA||Bisco, Schamburg, IL, EUA|
|Filtek Z250||60%||0.19–3.3||Bis-GMA, Bis-EMA, UDMA, TEGDMA||3M ESPE St Paul, EUA|
|Heliomolar||46%||0.04–0.2||Bis-GMA, UDMA, D 3 MA||Ivoclar Vivadent, Schaan, Liechtenstein|
|Filtek LS||55%||0.05–5.0||Silorane||3M ESPE St Paul, EUA|
Total volumetric shrinkage measurement
Total shrinkage was measured in a mercury dilatometer (ADA Health Foundation, Gaithersburg, MD, USA). Approximately 0.1 g of composite was placed on a sandblasted and silanized glass slide. A glass column was clamped to the glass slide, filled with mercury and a LVDT probe (linear variable differential transducer) was placed on top of the mercury. The composite was light cured through the glass slide, with a radiant exposure of 18 J/cm 2 (340 mW/cm 2 × 53 s), using a QTH curing unit (QHL 75 – Dentsply, Konstanz, Germany). Volumetric shrinkage ( n = 3) was monitored for 60 min after the photoactivation, and the data recorded by the probe was used to calculate the volumetric shrinkage using previously measured mass and density values.
Degree of conversion
Degree of conversion ( n = 3) was determined using near-IR spectroscopy (Vertex 70, Bruker Optik, Germany). Disc-shaped specimens were made using a silicon mold ( h = 0.8 mm, = 7.0 mm) placed between two glass slides. FTIR spectra were recorded before, at 10 min, and 72 h after photoactivation, and recorded with two scans at a resolution of 6 cm −1 . The composite was light-cured with a radiant exposure of 18 J/cm 2 (570 mW/cm 2 ), using a QTH unit (VIP–Bisco, Schaumburg, IL, USA). Conversion was determined by assessing the variation in peak area of the absorbance intensity at 6165 cm −1 for methacrylate based materials , and at 4155 cm −1 for silorane based material , in relation to the uncured material.
Elastic modulus and flexural strength determination
For each composite, 20 specimens were made using a split steel mold (10 mm × 2 mm × 1 mm). Photoactivation was performed using a QTH unit (VIP–Bisco) and a radiant exposure of 18 J/cm 2 (570 mW/cm 2 × 32 s). The light guide tip has 11 mm in diameter, that cover the entire specimen. Half of the specimens ( n = 10) were stored dry at 37 °C for 24 h. The other half were stored in a 75% ethanol/water solution at 37 °C for 4 months. The solution was changed weekly. The three point bending test was performed using a universal testing machine (Instron 5565, Canton, MA, USA), with 8 mm distance between supports and cross-head speed of 0.5 mm/min. Based on the linear portion of the load vs displacement curve, flexural modulus was calculated according to Eq. (1) :
E = C × L 3 4 × b × h 3 × d × 10 − 3
where E is the flexural modulus (GPa), C is the load recorded (N), L is the span between the supports (mm), b is the width of the specimen (mm), h is the height of the specimen (mm) and d is the deflection (mm) corresponding to C .
Flexural strength was calculated according to Eq. (2) :
σ = 3 × F × L 2 × b × h 2
where σ is the flexural strength (MPa), F is the maximum load recorded before the specimen fractured (N), L is the span between the supports (mm), b is the width of the specimen (mm), and h is the height of the specimen (mm).
Water sorption and solubility
Disk-shaped specimens, 15 mm diameter and 1 mm thick, were made using a steel mold ( n = 5). The photoactivation was performed with a radiant exposure of 18 J/cm 2 (900 mW/cm 2 × 20 s, Flash Lite 1401, Discus Dental, Culver City, CA, USA). Aiming to cover the entire specimen, the light guide was placed 10 mm from the specimen, and the irradiance was measured at this distance to calculate the real radiant exposure. After removal of the specimen from the mold, the opposite surface also received the same radiant exposure to ensure maximum curing of the specimen. The specimens were kept in a vacuum desiccator at 37 °C for 28 days. The specimens were then weighed using an analytical balance (Ohaus-Adventure, AR214N, Shanghai, China) to obtain m 1 . The diameter and height of each specimen were measured to calculate the volume.
Then the specimens were immersed in distilled water at 37 °C, changed weekly to avoid alteration of the pH, for 28 days. The specimens were gently dried with absorbent paper and weighed again to obtain m 2 . The specimens then were returned to the desiccator under vacuum for 90 days, and weighed to obtain m 3 . Sorption and solubility were calculated for each specimen according to Eqs. (3) and (4) (ISO 4049):
W S = m 2 − m 3 V