To investigate the variation in water sorption and solubility across a range of different core build-up materials.
Five materials were tested, four of which are resin-based materials (Grandio Core, Core.X Flow, Bright Flow Core, Speedee) and one resin-modified glass ionomer (Fuji II LC). All specimens ( n = 10) were immersed in 10 ml distilled water in individual glass containers and weighed at one week, 14 and 28 days. After a total immersion time of 28 days, 7 specimens were dried to a constant mass, in a desiccator for 28 days. Three samples of each material were not dried, but were left in distilled water for 1 year, to determine the long-term water sorption properties. Specimens were weighed at monthly intervals until 6 months and then at the 9th and 12th months. Each specimen was measured using a digital electronic caliper (Mitutoyo Corporation, Japan).
After 28 days immersion, the change in water sorption and solubility of the materials ranged from 12.9 to 67.1 μg/mm 3 ( P < 0.001) and 0.9–6.4 μg/mm 3 respectively ( P < 0.001). Except for Fuji II LC, an independent T -test showed significantly higher water sorption and solubility for the other materials after 1-year total immersion in water compared to 1 month ( P < 0.05). Using repeated measures ANOVA, all materials showed mass changes over time (1 month) ( P < 0.001).
Grandio Core had the lowest water sorption and solubility among the tested materials. According to the ISO 4049 standards, all the tested materials showed acceptable water sorption and solubility, apart from the water sorption behavior of Fuji II LC.
Water sorption and solubility can affect the mechanical strength, color stability, and abrasion resistance of resin-composites . Moreover, resin-composites have shown reduced strength and longevity, as a result of extensive water sorption and solubility behavior .
The resin composite’s polymer network will absorb a certain amount of water and release monomers and ions to the surrounding environment . Water most likely permeates through the network due to its porosity and intermolecular spaces, so the water gained is highly dependent on the density of the polymer network.
Dental composite restorations in a patient’s mouth are continuously exposed to a wet environment. The water absorbed and consequent hygroscopic expansion of a composite may compensate for the polymerization shrinkage-stress. Endodontically treated posterior teeth as well as anterior teeth with extensive coronal loss, may need to receive crowns after being built-up with core materials . If a final restoration is a resin based direct restoration, it will be exposed to a wet environment and will absorb water. Water sorption for a core material may not be avoided even when the final restoration is a crown, as after the cementation of a crown, water may still reach the core material from diffusion through the cement . Thus, evaluating the water sorption of core materials is critical for an understanding of their long term success.
In situations where there has been extensive coronal destruction, marginal leakage under crowns may result in the core build-up margins coming in contact with moisture. Marginal leakage will occur with poor adaptation of a casting to the margins, or lack of integrity of the chosen luting cement. Also, a core material may come in contact with moisture if it is used as an interim restoration. Moisture in the oral environment may result in dimensional changes, swelling and hygroscopic expansion. This can lead to micro cracks of the resin based core materials and subsequently a failure of the restoration . There are no long term studies that have investigated water sorption of core build-up materials.
Using resin based core build-up materials may contribute to the failure of the overlying restorations. The fracture resistance of zirconia copings has been shown to be affected by the water absorption of the luting agent . It is well known that the hygroscopic expansion of resin modified glass ionomers and compomers can lead to the failure of all ceramic crowns, when these materials are used for either core build up or adhesive bonding .
Significant differences between dental resin-based materials, regarding their water sorption and solubility have been shown . Accordingly, the present study aimed to measure the water sorption and solubility of different core build-up materials.
The null hypotheses tested were:
There is no difference in the water sorption and solubility of the tested core materials after 1 month.
There is no difference in the water sorption and solubility of the tested materials between 1 month total immersion and 1 year total immersion.
Materials and methods
The tested materials and relevant information are shown in Table 1 . The specimens were prepared according to ISO 4949:2009 and the manufacturer’s instructions. Specimens were prepared in cylindrical molds (16.0 mm diameter × 1.0 mm thickness) at (23 ± 1) °C and the material packed was slightly overfilled into a brass ring mold set on a piece of transparent polyester film on a glass microscopic slide. It was then covered with another piece of polyester film while being pressed by another glass slide. The specimens were then light cured by an Optilux curing unit (Optilux 501, USA) with an irradiance of 620 mw/cm 2 . Five overlapping sections (20 s) on each side of the specimen were irradiated. A 1000 grit silicon carbide paper was used to remove any excess flash, to finish the specimens and to obtain uniform thickness. The thickness and diameter of each specimen were measured at 4 and 2 points respectively, using a digital electronic caliper (Mitutoyo Corporation, Japan). Mean values were used to calculate the volume of each specimen in mm 3 . The specimens were then incubated in a lightproof desiccator with anhydrous self-indicating silica gel at (37 ± 1) °C. After 22 h, the specimens were transferred to another desiccator at (23 ± 1) °C for 2 h and then weighed to an accuracy of ±0.1 mg using a calibrated electronic analytical balance with precision of 0.01 mg (Ohaus Analytical Plus, Ohaus Corporation, USA). This cycle was repeated until the mass change of each specimen was not more than ±0.1 mg in any 24 h period to ensure completion of polymerization and dehydration. This constant mass ( m 1 ) was the initial mass of the specimen.
|Grandio Core||Dual cure composite: UDMA, BIS-GMA. Fillers (77% wt)||Voco, Germany|
|Core.X Flow||Dual cure composite: UDMA, Di&tri-functional methacrylates. Fillers (69% wt)||DENTSPLY, Caulk, USA|
|Bright Flow Core||Dual cure composite: Methacrylate polymers (48%). Fillers (52% wt)||DMPLTD, Greece|
|Speedee||Dual cure composite: Self-adhesive resin hydrophilic based monomer||DENTSPLY, Caulk, USA|
|Fuji II LC||Light cured resin reinforced glass-ionomer cement:
Powder: fluoroaluminosilicate glass, polycyclic acid
Liquid: water, polyacrylic acid, HEMA
|GC, UK Ltd.|
Specimens of each material ( n = 10) were immersed in 10 ml distilled water in individual glass containers for a total immersion time of 28 days. They were weighed every day for the first week, then after 14 days and finally after 28 days. Specimens were gently dried on filter paper until free from visible moisture, waved in air for 15 s and weighed 1 min later to ±0.01 mg and returned to the glass containers filled with distilled water. The recorded mass was denoted as m 2 ( t , time).
After a total immersion time of 28 days, not all 10 specimens were dried to a constant mass. Seven specimens of each material were dried to constant mass ( m 3 ) in the desiccators for 28 days using the cycle described above. Three samples of each material were not dried and were left in distilled water for a total immersion time of 1 year just to determine whether the water sorption and solubility will be different from the one 1 month group after long term water immersion in water of 1 year. These groups (3 samples for each material) were measured monthly up to 6 months and then at 9 and 12 months. After a total of 1 year immersion, these groups were, dried to constant mass ( m 3 ) for 120 days as previously described. Water sorption and solubility were calculated for tested materials after 1 month and after 1 year. Sorption and desorption cycle mass change percentage was calculated for both groups (1 month group and 1 year group). Water sorption, solubility and mass change percentages were calculated by the following equations:
1. Mass change percentages
Sorption mass change percentages
Mg % = m 2 ( t ) − m 1 m 1 × 100
Desorption mass change percentages
Mg % = m 3 ( t ) − m 1 m 1 × 100