To evaluate the extent and rate of hygroscopic expansion of resin composites at 37 °C.
Eight resin composites were examined: 1 micro-hybrid (Bright Light ® ), 5 nano-hybrids (Experimental Vertise™; Nanoceram-Bright ® ; Tetric EvoCeram ® ; Grandio ® SO; Ceram X™ duo) and 2 flowables (X-tra base; Venus ® Diamond Flow). Five disks (15 mm × 2 mm) of each material were prepared. The mean change in specimen diameter was recorded by a custom-built non-contact laser micrometer. Specimens were initially measured dry and then at fixed time intervals, over 150 days, after storage in distilled water at 37 ± 1 °C. Data were re-expressed in volumetric terms and analysed by repeated measures ANOVA, one-way ANOVA and Tukey’s post hoc test ( α = 0.05).
The volumetric hygroscopic expansion ranged from 0.58 to 2.26 and can be considered in three bands. First, Experimental Vertise had the highest expansion ( p < 0.001). Venus Diamond Flow, Tetric EvoCeram and Ceram X duo were the second band. The third band, with still lower expansion, consisted of Bright light, Grandio So, Nanoceram-Bright and X-tra base , with no significant difference between them.
For the size (2 mm thickness) and shape of specimen measured, equilibrium was attained in all cases by 60 days. Within this set of resin-composites the equilibrium expansion varied by almost 400% of the lowest material.
There is increasing concern amongst clinicians about dimensional changes of dental resin composites during and after curing . Recently, several in vitro laboratory studies have investigated long-term dimensional instability and suggested that this might lead to clinical problems such as: post-operative pain, secondary caries, marginal staining breakdown of the restorations, gap formation, micro-leakage and enamel crack propagation . Dimensional stability of resin composites can be caused by polymerization shrinkage, thermal contraction/expansion and interactions with an aqueous environment .
An appropriate cavity design, filling technique and bonding agent may reduce gap formation caused by polymerization shrinkage . Also, water sorption by a resin composite may cause expansion of the composite filling and reduce any gap formed due to polymerization shrinkage . Hygroscopic expansion could also relax any internal stresses of the resin matrix and compensate for resin composite shrinkage . However, shrinkage occurs within minutes but water sorption takes from days to months. So to get full compensation, the absorption may need to occur for an extended period of several weeks . Consequently, the positive effect of water sorption on marginal gap compensation still requires further direct evidence .
There are two contrasting processes during water sorption by composite restoratives in the oral aqueous environment. Firstly, water can leach out unreacted monomers, if they are present, which can lead to loss in mass, shrinkage and changes in mechanical properties . Secondly, water diffusing into the material leads to mass increase and usually can cause a progressive bulk expansion until equilibrium .
The aim of this study was to evaluate the extent and rate (kinetics) of hygroscopic expansion of resin composites stored in distilled water at 37 °C. The null hypotheses were that there would be no difference in either (i) the final magnitude or (ii) the rates of hygroscopic expansion between the examined materials.
Materials and methods
Eight representative resin composite materials were investigated ( Table 1 ). Five disk-shaped specimens ( n = 5) were prepared of each material according to ISO 4049: 1999 and their manufacturers’ instructions. Care was taken to minimize entrapped air while the uncured material was placed into brass ring molds (15 mm × 2 mm) in a laboratory environment at 23 (±1) °C. The mold was sandwiched between two transparent Mylar™ strips with two glass slides on either side and then pressed together. Five overlapping sections on each side of the specimen were irradiated in turn using a halogen curing unit with a tip diameter of 10 mm approximately 1 mm away from the specimens for both sides (Optilux ® 501, SDS, Kerr, Danbury, CT, USA) under the standard curing mode (output wavelength range: 400–505 nm; output irradiance: 580–700 mW/cm 2 , applied for 40 s). A calibrated radiometer was used to verify the irradiance for each use of the light cure unit. Each specimen was carefully removed from its mold and any edge ‘flash’ was removed with a 1000 grit silicon carbide abrasive paper. Then the specimens were stored separately in glass vials in a lightproof desiccator with silica gel at 37 ± 1 °C, until the mass change of each specimen was less than 0.2 mg in a 24 h period, which indicated sufficient completion of dehydration. Each specimen was weighed using a calibrated electronic analytical balance with a precision of 0.01 mg (Ohaus Analytical Plus, Ohaus Corporation, USA).
|Code||Product||Manufacturer||Matrix||Filler type||Filler %w/w||Lot Number|
|Exp.VT||Expermintal Vertise™||Kerr Corp, Orange, USA||GPDM and methacrylate co-monomers||Prepolymerized filler, Ba glass; nanoscale SiO 2 & YbF 3||–||3379131|
|VDF||Venus ® Diamond Flow||Heraeus Kulzer GmbH Hanau, Germany||UDMA, EBADMA||Ba-Al-F silicate glass,YbF 3 and SiO 2||65||010100|
|TEC||Tetric EvoCeram ®||Ivoclar Vivadent AG, Schaan, Liechtenstein||Bis-GMA, UDMA||Ba glass, silicate, SiO 2 , mixed oxide 40 nm-3000 nm||76||L56579|
|CXD||Ceram-X™ duo||Dentsply GmbH Konstanz, Germany||Methacrylate modified polysiloxane, Dimethacrylate||Barium–aluminum–borosilicate glass, SiO 2||76||0811000572|
|NCB||Nanoceram-Bright ®||DMP Ltd, Greece||Bis-GMA, TEGDMA||Barium glass, mixed oxide 0.05–0.7 μm||80||630212|
|GSO||Grandio SO||Voco GmbH Cuxhaven, Germany||Bis-GMA, Bis-EMA, TEGDMA||Ba glass 1 μm SiO 2 20–40 nm||89||1048014|
|XB||X-tra base||Voco GmbH Cuxhaven, Germany||Aliphatic di-methacrylate, Bis-EMA||Ba glass, YbF 3 , fumed silica||75||V45252|
|BL||Bright Light ®||DMP Ltd., Greece||Bis-GMA, TEGDMA||Ba glass, mixed oxide 0.04–0.25 μm||81||610230|
The mean change in diameter of the specimens was recorded by a custom-built non-contact laser micrometer to a resolution of 200 nm. The device consisted of a laser-scan micrometer (LSM) system (Measuring Unit LSM-503 s and a Display Unit LSM-6200, Mitutoyo Corporation, Japan), mounted on a heavy stainless steel base. A disk specimen holder was rotated in a horizontal plane around a vertical axis by an electronic stepper-control unit. The LSM was connected via the Display Unit to a computer with USB input for further recording and data processing ( Fig. 1 ). The LSM system working mechanism was previously described .
The initial mean diameter d 1 of each specimen was measured. Then the specimens were immersed in 10 ml of distilled water at 37 ± 1 °C for periods of time. Measurements of the diameter were taken after 1 and 3 h, and 1, 2, 3, 4, 5, 6,7,14, 21, 28, 60, 90, 120 and 150 days, to allow sufficient time for equilibrium to be achieved. The mean diameter d 2 was measured at each time t and d ∞ at 150 days. For each measurement, each specimen was carefully taken out of storage medium, dried on filter paper until there was no visible moisture and then mounted onto the specimen holder. The multiple diametral measurements were obtained, after which the specimen was returned to water storage. After 150 days of water immersion each specimen diameter had reached a constant value. The percentage diameter change Δ d (%) was calculated by:
The percentage volumetric hygroscopic expansion V (%) can be calculated using the following equation, which assumes isotropic expansion behavior :