Polymerization stress, shrinkage and elastic modulus of current low-shrinkage restorative composites

Abstract

Objective

To compare currently available low-shrinkage composites with others regarding polymerization stress, volumetric shrinkage (total and post-gel), shrinkage rate and elastic modulus.

Methods

Seven BisGMA-based composites (Durafill/DU, Filtek Z250/FZ, Heliomolar/HM, Aelite LS Posterior/AP, Point 4/P4, Filtek Supreme/SU, ELS/EL), a silorane-based (Filtek LS, LS), a urethane-based (Venus Diamond, VD) and one based on a dimethacrylate-derivative of dimer acid (N′Durance, ND) were tested. Polymerization stress was determined in 1-mm high specimens inserted between two PMMA rods attached to a universal testing machine. Total volumetric shrinkage was measured using a mercury dilatometer. Maximum shrinkage rate was used as a parameter of the reaction speed. Post-gel shrinkage was measured using strain-gages. Elastic modulus was obtained by three-point bending. Data were submitted to one-way ANOVA/Tukey test ( p = 0.05), except for elastic modulus (Kruskal–Wallis).

Results

Composites ranked differently for total and post-gel shrinkage. Among the materials considered as “low-shrinkage” by the respective manufacturers, LS, EL and VD presented low post-gel shrinkage, while AP and ND presented relatively high values. Polymerization stress showed a strong correlation with post-gel shrinkage except for LS, which presented high stress. Elastic modulus and shrinkage rate showed weak relationships with polymerization stress.

Significance

Not all low-shrinkage composites demonstrated reduced polymerization shrinkage. Also, in order to effectively reduce polymerization stress, a low post-gel shrinkage must be associated to a relatively low elastic modulus.

Introduction

Restorative resin composites have been used in dentistry for nearly 40 years . In spite of the undeniable technological advances introduced during these four decades, the volumetric shrinkage that accompanies the chain-growth polymerization of dimethacrylate monomers remains a major concern for the clinical performance of composite restorations. Composite polymerization can be divided in pre- and post-gel phases. In the pre-gel phase, the reactive species present enough mobility to re-arrange and compensate for the volumetric shrinkage without generating significant amounts of internal and interfacial stresses . After gelation (post-gel phase), the formation of a semi-rigid polymer network hinders plastic deformation . As a consequence, the continued polymerization shrinkage in association with elastic modulus development generates stresses within the material, at the tooth/restoration interface and in the tooth structure . This stress state is likely to facilitate gap formation, jeopardizing the longevity of the restoration . It must be pointed out that even though a direct relationship between polymerization stress and microleakage has been verified in vitro , the deleterious effect of polymerization stress on restoration longevity still lacks clinical evidence .

The organic matrix of dental resin composites is formed by a densely crosslinked network, resultant from the co-polymerization of high molecular weight dimethacrylates, such as BisGMA (bisphenol-A glycidyl methacrylate), UDMA (urethane dimethacrylate) and BisEMA (ethoxylated bisphenol-A glycidyl methacrylate), with diluents, such as TEGDMA (triethyleneglycol dimethacrylate). The volumetric shrinkage resultant from the establishment of covalent bonding among methacrylate groups is determined by the monomeric composition, as the higher the concentration of high molecular weight monomers, the lower the amount of carbon double bonds per unit volume . Also, high molecular weight monomers in general present lower mobility, which reduces the final degree of conversion reached by the composite, also contributing to a lower shrinkage .

Over the years, manufacturers have invested their resources in the development of low-shrinkage restorative composites and, recently, a number of examples of these new materials were made available for clinical use. Some of them are BisGMA-based and use high filler levels or do not contain low-molecular weight dimethacrylates as strategies to reduce polymerization shrinkage. Other materials combine conventional dimethacrylates with new high-molecular weight monomers, for example, tricyclodecane-urethane dimethacrylate (TCD-urethane, Fig. 1 A) or dimer dicarbamate dimethacrylate ( Fig. 1 B).

Fig. 1
Structural formula of base-monomers: TCD-urethane (A), dimer dicarbamate dimethacrylate (B), and silorane (C). Formulas provided by the respective manufacturers.

The TCD-urethane is a low-viscosity monomer that, according to manufacturer’s information, would dispense with the use of diluents responsible for the high polymerization shrinkage of BisGMA-based composites. The molecule has three connected rings in its central portion that increase the flexibility of the monomer backbone and, in theory, would help to accommodate shrinkage. Dimethacrylate-derivatives of dimer acid present higher molecular weight than BisGMA and, when associated with other high-molecular weight dimethacrylates, such as BisEMA and UDMA, polymerization stress development would be reduced due to the occurrence of polymer-induced phase separation, i.e., during the reaction, two physically distinct polymeric phases would be formed with different curing kinetics, allowing for more viscous flow during the pre-gel stage .

In 2007, a silorane-based composite became commercially available . The silorane molecule presents a siloxane core with four oxirane rings attached that open upon polymerization to bond to other monomers ( Fig. 1 C). The oxirane ring opening causes a volumetric expansion that partially compensates the shrinkage resultant from molecular bonding. Literature data confirmed that a silorane-based commercial composite presented less than 1.0% of total volumetric shrinkage, compared to 2.0–3.5% for BisGMA-based composites , causing less tooth deflection and microleakage . Its mechanical properties are comparable to those of dimethacrylate-based materials .

As already mentioned, polymerization stress is not determined by volumetric shrinkage alone, but also by composite elastic modulus . Reaction rate also plays a role, though evidences show it contributes only marginally . Therefore, these new materials should be evaluated in terms of polymerization stress, as well as the other aspects involved in its development, besides shrinkage. There are several reports on the polymerization stress of BisGMA-based composites , and it is important to compare new composites with those showing long laboratory and clinical track records.

The purpose of this in vitro study was to compare commercial low-shrinkage with regular composites in terms of polymerization stress, volumetric shrinkage (total and post-gel), elastic modulus and reaction rate. Shrinkage rate was used as an indirect measurement of polymerization rate. The null hypothesis was that results for the low-shrinkage composites are not different from regular composites.

Materials and methods

Ten composites were tested, all in A3 shade ( Table 1 ). Seven were BisGMA-based, including a nanofilled (SU), a highly-filled (AP) and one that does not contain low-molecular weight diluents (LS), and three used monomers alternative to the conventional dimethacrylates. ND contained dimer dicarbamate dimethacrylate, VD utilized a modified urethane (TCD-urethane), and LS was silorane-based. EL, AP, LS, ND and VD are considered low-shrinkage composites by the respective manufacturers.

Table 1
Composites tested in the study (information provided by the respective manufacturers).
Material Abbreviation Filler content (vol.%) Average particle size Manufacturer Batch number
Durafill DF 40 0.02–0.07 μm Heraus Kulzer GmbH, Hanau, Germany 010213
Heliomolar HM 46 0.04–0.2 μm Ivoclar Vivadent, Schaan, Liechtenstein K30118
ELS EL 50 0.07–2.6 μm Saremco, Rohnacker, Switzerland 04.2012-50
Filtek LS LS 55 0.05–5.0 μm 3M ESPE St Paul, USA 8BF
Filtek Supreme Plus (Body) SU 59.5 20 nm (silica), 0.6–1.4 μm (zirconia/silica clusters) 3M ESPE 8PX
Point 4 P4 59 0.4 μm SDS Kerr, Orange, CA, USA 459692
Filtek Z250 FZ 60 0.19–3.3 μm 3M ESPE 8PF
Venus Diamond VD 64 5–20 μm Heraus Kulzer 010028
ŃDurance ND 65 40–0.5 μm Septodont, Louisville, CO, USA E9810-2
Aelite LS Posterior AP 74 1.1 μm Bisco, Schamburg, IL, USA 0800006817

Polymerization stress measurements

Poly(methyl methacrylate) rods, 5 mm in diameter and 13 or 28 mm in length, had one of their flat surfaces sandblasted with 250 μm alumina. On the shorter rod, in order to allow for the highest possible light transmission during photoactivation, the opposite surface was polished with silicon carbide sandpaper (600, 1200, and 2000 grit) and felt disks with 1 μm alumina paste (Alumina 3, ATM, Altenkirchen, Germany). The sandblasted surfaces received a layer of methyl methacrylate (JET Acrílico Auto Polimerizante, Artigos Odontológicos Clássico, São Paulo, Brazil) followed by two thin layers of unfilled resin (LS System Adhesive for LS or Scotchbond Multi-purpose Plus, bottle 3, for the other tested composites, both from 3 M ESPE). The unfilled resin was light-cured with a radiant exposure of 12 J/cm 2 (400 mW/cm 2 × 30 s).

The rods were attached to the opposing clamps of a universal testing machine (Instron 5565, Canton, MA, USA) with the treated surfaces facing each other with a gap of 1 mm. The composite was inserted into the gap and shaped into a cylinder following the perimeter of the rods. An extensometer (0.1 μm resolution) was attached to the rods (model 2630-101, Instron) for the purpose of monitoring the specimen height and providing feedback to the testing machine in order to keep the height constant. Therefore, the value registered by the load cell corresponded to the force necessary to counteract the polymerization shrinkage force to maintain the specimen’s initial height. The short rod was attached to the testing machine through a hollow stainless steel fixture with a lateral slot that allowed the tip of the light guide of a quartz–tungsten–halogen unit (VIP Junior, Bisco, Schaumburg, IL, USA) to be positioned in contact with the polished surface of the rod. In order to increase the irradiance output, a turbo tip (7 mm in diameter) was used. The irradiance effectively reaching the composite was estimated by interposing a 13-mm rod between the light tip and the sensor of a dental radiometer (model 100, Demetron Res. Corp., Orange, CA, USA). The reading was multiplied by 1.96 to account for the fact that the diameter of the rod was smaller than that of the sensor (7 mm). Radiant exposure was set at approximately 18 J/cm 2 (570 mW/cm 2 × 32 s) for all the composites tested. Force development was monitored for 5 min from the beginning of photoactivation and nominal stress was calculated by dividing the maximum force value by the cross-section of the rod. Five specimens were tested for each of the ten composites.

Post-gel shrinkage measurements

Composite post-gel volumetric shrinkage was determined using strain gages, as previously described in detail . Briefly, a small amount of composite was shaped into a hemisphere, placed on top of a biaxial strain gage and light-cured using a quartz–tungsten–halogen (QTH) unit (XL2500, 3 M ESPE) with the light tip placed 1 mm distant from the surface of the composite. Again, radiant exposure was set at 18 J/cm 2 (600 mW/cm 2 × 30 s). Microstrain resultant from polymerization shrinkage was monitored for 5 min from the beginning of photoactivation in two perpendicular directions. The microstrain values registered in both directions were averaged, given that the materials present homogeneous and isotropic properties on a large scale. This average value was converted to percentage and multiplied by three to represent the volumetric shrinkage. Five specimens were tested for each composite.

Total volumetric shrinkage determination

Composite 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 column. The composite was light-cured from underneath, through the glass slide using a QTH unit (QHL75, Dentsply) with a radiant exposure of 18 J/cm 2 . The irradiance loss through the glass slide was minimal (600 mW/cm 2 × 30 s). Volumetric shrinkage was calculated using the LVDT probe readings and previously recorded mass and density values. Three specimens were tested for each composite. In order to make correlations with polymerization stress values more meaningful, shrinkage data recorded at 5 min were used in the statistical analysis. Maximum rate of shrinkage development was calculated from the first derivative of the shrinkage vs. time curve and was used as a measure of the reaction speed.

Flexural modulus determination

Specimens 12 mm × 2 mm × 1 mm ( n = 10) were made using a split steel mould. Ten minutes after photoactivation (18 J/cm 2 ), the specimen was removed from the mould and subjected to three-point bending in a universal testing machine (Q-Test TM, MTS Systems Corp., NC, USA), with 10 mm distance between the supports and cross-head speed of 0.5 mm/min. Based on the linear portion of the load × displacement curve, flexural modulus was calculated according to the following formula:

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M f = L × D 3 4 × w × h 3 × d × 10 − 3
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Nov 30, 2017 | Posted by in Dental Materials | Comments Off on Polymerization stress, shrinkage and elastic modulus of current low-shrinkage restorative composites
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