To study hygroscopic dimensional changes in new resin-matrix composites during water sorption/desorption cycles.
Five materials were examined: a self-adhering flowable composite: Vertise ® Flow (VF), a universal composite: GC Kalore (GCK), two micro-fine hybrid composites: GC Gradia Direct Anterior (GDA) and GC Gradia Direct Posterior (GDP), and a posterior restorative composite: Filtek ® Silorane (FS). Five disk-shaped specimens of each material were prepared (15 mm diameter × 2 mm thickness) according to ISO 4049. The mean diameter of each specimen was measured by a custom-built laser micrometer (to a resolution of 200 nm) periodically over 150 d water immersion and 40 d recondition periods at (37 ± 1) °C. Perspex controls were used. Data analysis was performed by repeated measures ANOVA, one-way ANOVA and Tukey’s post hoc test ( p < 0.05).
Differences in hygroscopic expansion were found for all test materials during sorption, ranging from 0.74% (±0.05) for FS to 4.82% (±0.13) for VF. The differences were significant for all materials ( p < 0.001), except between GCK and GDA. The mathematical relationship between diametral expansion and square root of time was non-linear. VF exhibited significant dehydration shrinkage.
The silorane composite FS had the lowest hygroscopic expansion. The extent of compensation of polymerization shrinkage by hygroscopic expansion depends on materials, specimen dimensions and time-scale. So the clinical situation must be taken into consideration in the application of these findings.
A dramatic growth in applications of esthetic resin-matrix composites has arisen in restorative dentistry. One principal limitation can be dimensional instability during polymerization and after restoration. The initial volumetric reduction, which is generally known as polymerization shrinkage, may develop detrimental stress on the cavity walls . Consequently, de-bonding, micro-leakage through marginal gaps, post-operative sensitivity, secondary caries, enamel fracture and clinical failure of restorations could occur . Clinically, stress might be relieved in various ways: by cavity design—taking configuration factor into consideration, applying liners, modulating curing initiation or by means of water sorption . However, most of this information has been gathered in vitro or on the basis of well-educated assumptions. There is still the need for more direct clinical evidence to support these approaches.
Resin-matrix composites are constantly exposed to an aqueous environment after intra-oral application. Water diffused into the matrix may contribute to the relaxation of the polymerization shrinkage stress to some extent . In addition, absorbed water may expand the polymer matrix and induce crazing and cause hygroscopic expansion after occupying the microvoids and free volume between chains . This might increase the bulk volume of resin-composites and possibly decrease marginal gaps generated by polymerization shrinkage . On the other hand, water could elute some residual monomers and other components from certain resin composites, resulting in further shrinkage, reduced bulk , weakened mechanical properties in restorations and potential allergic reactions with some patients. Therefore, the dimensional change of a resin composite immersed in water is somewhat complicated and is material dependent .
Although hygroscopic expansion (strain) occurs slowly in resin-composites, eventually it is still expected to partially compensate polymerization shrinkage . A greater expansion that exceeds the shrinkage is undesirable because the potential hygroscopic stress may cause micro-cracks or even cracked cusps in restored teeth , as well as adverse clinical consequences to any overlying restorations .
Dimensional changes of conventional dimethacrylate-based resin composites , core materials , a compomer , a calcium aluminate cement and glass-ionomer cements have been studied by several researchers. However, only prior work on resin-composites is of principal relevance to the present study. Hygroscopic expansion has not been previously reported on silorane, DX-511 monomer (with a long rigid core and high molecular mass, Fig. 1 ) or self-adhering resin-composites. Diametral and volumetric changes of newly introduced resin composites based on those monomers may significantly affect their clinical performance after either a short or relatively long-term restoration period. The aim of this study was therefore to evaluate the hygroscopic dimensional changes of five new resin-matrix composites when water stored at 37 °C and subsequently reconditioned. The working null-hypothesis was that the hygroscopic expansion behavior of investigated resin composites does not vary between materials.
Materials and methods
Five commercially available dental restorative resin composites were investigated ( Table 1 ). 25 disk-shaped specimens ( n = 5) were produced according to ISO FDIS 4049: 1999 and their manufacturer’s instructions. Care was taken to minimize entrapped air while uncured material was placed into brass ring molds (15 mm diameter × 2 mm thickness) in a laboratory environment at 23 (±1) °C and relative humidity of 50 (±2)%. The molds were sandwiched between two pieces of transparent polyester film with two glass microscope slides pressed on each side and then clamped. Five overlapping sections on each side of the specimen were irradiated respectively using a halogen curing light (Optilux 501, Kerr Corporation, USA; 11 mm exit window) under the standard curing mode (output wavelength range: 400–505 nm; output irradiance: 580–700 mW/cm 2 ). Atmospheric oxygen was minimized by the polyester film during light curing. A radiometer was used to check the stability of the curing light periodically. Each specimen was carefully removed from its mold and irregularities were finished against 1000 grit silicon carbide abrasive paper. Any specimen with visual voids was discarded. Then specimens were stored separately in glass vials in a lightproof desiccator with anhydrous self-indicating silica gel at (37 ± 1) °C, until the mass change of each specimen was less than 0.1 mg in a 24 h period, which indicated sufficiently completion of polymerization and dehydration. Each specimen was weighed using a calibrated electronic analytical balance with a precision of 0.01 mg (Ohaus Analytical Plus, Ohaus Corporation, USA). Two Perspex specimens (Polymethylmethacrylate, Imperial Chemical Industries, UK) were used as control. One was stored dry in a lightproof desiccator and the other was kept in de-ionized water at (37 ± 1) °C.
|Code||Material||Manufacturer||Batch no.||Elastic modulus (GPa)||Filler loading (wt%)||Fillers||Organic matrix|
|VF||Vertise ® Flow||Kerr Corporation, Orange, CA, USA||3172311||5.2||70||Prepolymerized filler, barium glass, nano-sized colloidal silica, nano-sized ytterbium fluoride||GPDM and methacrylate co-monomers|
|GCK||GC Kalore||GC America Inc. USA||0903171||8.6||82||Prepolymerized filler (with lanthanoid fluoride), fluoro-alumino-silicate glass, strontium/barium glass, silicon dioxide, lanthanoid fluoride||DX-511, UDMA and dimethacrylate co-monomers|
|GDA||GC Gradia ® Direct Anterior||GC Dental Products Corp., Tokyo, Japan||0901134||5.2||73||Silica, prepolymerized filler||UDMA and dimethacrylate co-monomers|
|GDP||GC Gradia ® Direct Posterior||GC Dental Products Corp., Tokyo, Japan||0905201||6.6||77||Silica, prepolymerized filler, fluoro-alumino-silicate glass||UDMA and dimethacrylate co-monomers|
|FS||Filtek ® Silorane||3M ESPE, St. Paul, MN, USA||20080514||7.9||76||Quarz (silane layer), radiopaque yttrium fluoride||Silorane|
Hygroscopic dimensional changes
Mean diametral measurements, and thus diametral changes, of the specimens were monitored by a custom-built non-contact laser micrometer. This was similar in some respects to a previously described instrument though developed independently. The device incorporated a laser-scan micrometer (LSM) system (Measuring Unit LSM-503s and Display Unit LSM-6200, Mitutoyo Corporation, Japan), mounted on a heavy stainless steel base, 2.5 cm thick, with rubber feet. A disk specimen holder was rotated in a horizontal plane about a vertical axis by an electronic stepper-control unit. The LSM was interfaced, via the Display Unit, to a PC, with USB input, for further recording and data processing.
The measuring LSM system ( Fig. 2 ) obtained the specimen dimensional data rapidly and accurately using a highly directional parallel-scanning laser beam. The laser beam, generated by a laser oscillator, was directed at a polygon mirror rotating at high speed and synchronized by clock pulses. Then the reflected laser beam passed through a collimator lens and maintained its constant direction through the beam window toward the disk specimen. The measurement light rays traveled as ‘parallel beams’ toward a photo-electric detector unit, but they were partially obstructed by the disk specimen. The extent of the beam-obstruction was directly proportional to the disk diameter. The resulting electrical output signal changed according to the duration over which the beam was obstructed. This was processed by the Display Unit CPU and the disk dimension was displayed digitally. The laser beam system was able to measure each disk specimen diameter to a resolution of 200 nm.
The LSM system was calibrated before each measurement using two reference gages, according to the manufacturer’s instructions.
The stepper-control unit maintained stepwise rotation of the disk specimen (mounted on its holder) with a total of 800 steps per rotation. The speed of rotation was 28 steps per second. The scanning speed of the laser beam was 3200 scans per second and so 91,428 diametral measurements were taken per revolution and these were averaged, in sets of 1024, to give 89 recorded readings/revolution. At each sorption time-period, specimens were measured overfive complete rotations. Therefore, the diametral values presented for each specimen at each time point were obtained as overall averages of 445data values, which were transferred to an Excel file. The grand mean for the 5 specimens per group was then obtained for each sorption time-period.
The initial mean diameter of each specimen was measured and denoted as d 1 . Then specimens were immersed in 10 mL de-ionized water at (37 ± 1) °C for periods of time. Diametral measurements were taken at different time intervals until equilibrium was reached. The mean diameters measured at time t and at equilibrium were respectively denoted as d 2 ( t ) and d ∞ . For measurement, each specimen was carefully taken out, dried on filter paper until there was no visible moisture and then mounted on the specimen holder. The diameter was measured after which the specimen was returned to water storage. After 150 d water immersion, the specimens were reconditioned in the desiccator using the cycle described above until the values of the diameter were constant. The percentage diametral change from water-immersion was calculated by:
d ( % ) = d 2 ( t ) − d 1 d 1 × 100
The percentage hygroscopic dimensional change (volumetric change) can be calculated by the following equation , which assumes isotropic expansion behavior :
V ( % ) = 1 + d ( % ) 100 3 − 1 × 100