The aim of this in vitro study was to evaluate some sorption characteristics [sorption ( S ), solubility ( SL ) and the percentages of mass change (M g %), solubility (SL%) as well as sorbed liquid (S%)] of self-adhesive resin cements when immersed in distilled water and lactic acid.
A disc-shaped specimen of each self-adhesive resin cements [G-Cem (GC), SmartCem™ 2 (SC2), RelyX™ U100 (R1), RelyX™ Unicem 2 (RU2)] were prepared in a split-Teflon mold and irradiated by an Optilux 501 light cure at 580 mW/cm 2 for 40 s in eight overlapping sections each side. The volume of each specimen was calculated and placed inside a desiccator containing anhydrous calcium chloride, then weighed on an analytical electronic balance. Two independent groups were established according to the immersion media or liquids (distilled water and 0.01 M lactic acid) maintained at 37 °C for the time intervals: 1, 6, 12, 24, 48 and 168 h, where the sorption ( S ) property (μg/mm 3 ) was calculated. However, the SL , M g %, SL% and S% were measured after 168 h of immersion. The data were statistically analyzed by repeated measures ANOVA, one-way ANOVA and post hoc Tukey’s test ( p < 0.05).
Analysis of variance revealed highly significant differences between the materials for the sorption and solubility values examined with some exceptions ( p < 0.05). However, independent samples T -test expressed significant differences of all the sorption values between both water and lactic acid media for the resin cements with some border significances ( p > 0.05). The highest liquid’s sorption was exhibited by GC material after immersion in lactic acid for 168 h period followed by SC2 (37.83 and 34.15 μg/mm 3 , respectively), while the lowest sorption was presented by RU2 cement after 1 h immersion period in water (3.89 μg/mm 3 ). Stereomicroscope showed homogenous surface topography in RU2 and R1 samples, while some striated cracks and microvoids were observed in GC and SC2 materials, respectively. The SL values followed this order: RU2 < R1 < SC2 < GC.
Knowing the best self-adhesive cement that can provide less sorption and solubility values will help the dentist to choose the most suitable luting material for indirect restorations.
Resin-based cements are widely used luting material because of their enhanced ability to bond tooth surface to several indirect restorations. Nowadays, there has been a marked increase in the development of dual-cured resin cements because of their ability to be self cured under indirect esthetic polymeric or ceramic restorations. Compared to conventional luting agents, these resin cements can achieve better marginal seal, show retentive capability and possess adequate physical and mechanical properties, such as increased fracture resistance of overlying restorations, along with an optimal esthetic result . The properties of resin luting agents have been improved by modifying their composition. Resin cements can be classified either by their mechanism of interaction to the tooth substance or by their mode of cure. Conventional resin cements require the application of a total-etch system or a self-etching primer. More recently, new materials were introduced called self-adhesive resin cements that were applied immediately to enamel and dentin without previous use of an adhesive system .
These self-adhesive dual-cured resin cements become increasingly common and mostly used for bonding the indirect restorations. They have been invented to simplify the resin bonding process and minimize the steps as well as the time consumed during the bonding procedures. They are being attractive for this option, as well as for being dual-cured and easy to use with no need for dental tissues pretreatment . These cements were manufactured on a new technology of monomers, fillers and initiators. According to manufacturers, these products include acidic and hydrophilic monomers in their composition, which simultaneously demineralize and infiltrate enamel and dentin, resulting in a strong bonding . The organic matrix of some of them is based on a newly developed multifunctional phosphoric-acid methacrylate system (phosphates and phosphonates groups), while others are based on carboxylic acid group . These acidic monomer groups are provided in specific concentration that can demineralize and condition enamel and dentin as well as allow adhesion to the tooth surface through micromechanical retention . Therefore, they require no conditioning or priming pretreatment step for the tooth surface. One of these cements is RelyX™ Unicem possessing functionalized monomers of phosphate groups that are claimed to react with calcium ions of hydroxyapatite of the tooth substance, which results in a chemical bonding, thus adding more retention . The remaining phosphoric acid groups of the methacrylate monomers are neutralized by other ions released from the fillers during the setting reactions, while the released fluoride ions are absorbed by the tooth structure. It is claimed by manufacturer that immediately after mixing, the formed paste is very acidic, and then within few minutes the pH value starts to increase reaching a neutral level after 24 h . Therefore, by these reactions, the phosphoric acid is neutralized and the resin cement becomes hydrophobic. Its initial hydrophilicity is important to achieve wetting of the tooth, while hyrdophobicity developed later is endeavor to facilitate bonding. This mechanism of action of the new self-adhesive cement is crucial for its application on the tooth surface and essential prerequisite for its long-term stability.
The sorption properties of the resin cement materials have an important value in terms of durability of indirect restorations. The ideal luting material should be impenetrable to oral fluids and resists dissolution over the life-time of the restoration. In the oral environment, there is more sensitivity of the restoration to moisture that may increase the risk of bond degradation and cement dissolution at the marginal gap. In consequence, this can result in weakening and fracture of the indirect restoration . Other properties such as restoration’s retention, tooth sensitivity, microleakage and secondary caries are also affected by the sorption ability of the cement . Moreover, sorption and solubility can influence strength, biocompatibility, dimensional and color stability of polymeric-based cements . For example, slight water sorption may have an essential effect in compensating polymerization shrinkage of the resin, thus relieving internal stresses created during shrinkage, and possibly improving marginal seal by decreasing gaps .
It is important to understand the sorption and solubility characteristics of the resin cements, which have been exhaustively studied by many researchers . Most of the previous studies examined the sorption and solubility of polymeric materials as composite, soft lining and acrylic denture base resins in different immersion media such as water, artificial saliva and ethanol . However, few researches have evaluated the effect of acids produced by human dental plaque such as lactic acid on these properties. Moreover, several earlier studies have shown that lactic and other acids produced by dental plaque had detrimental effects on softening and surface degradation of polymeric resin materials . Studies about the action of this acid on the newly developed self-adhesive resin cements may increase the knowledge regarding their durability in the oral environment. Thus, it is essential to analyze the sorption and solubility characteristics of these cements when they are immersed, not only in water and artificial saliva, but also in lactic acid. Therefore, the aim of this in vitro study was to evaluate the sorption and solubility characteristics [sorption ( S ), solubility ( SL ) and percentage of mass change (M g %), resin solubility (SL%) and sorbed liquid (S%)] of some self-adhesive resin cements when immersed in distilled water and lactic acid. The tested null hypotheses were: (1) there are no differences in sorption ( S ) of these cements after being immersed in the two liquids at different time intervals, (2) there are no differences between the cements for the values of each of the followings: SL , M g %, SL% and S%, after 168 h of immersion in the two liquids, (3) there is no difference between the two liquids with respect to all sorption and solubility characteristics examined.
Materials and methods
Four self-adhesive resin cements were investigated in this study: G-Cem (GC Corporation), SmartCem™ 2 (Dentsply DeTrey), RelyX™ U100 (3M-ESPE), RelyX™ Unicem 2 automix (3M-ESPE). Their composition specifications are listed in Table 1 .
|Material||Code||Mode of paste–paste mixing||Shade||Composition||Batch no.||Manufacturer a|
|G-Cem||GC||Mechanical mixing of a capsule in amalgamator for 10 s||Light||Powder: Fluoro-alumino-silicate glass (65–70 wt%, average size 4 μm), initiator, pigment||0906051||GC Corporation, Tokyo, Japan|
|Liquid: UDMA, dimethacrylate, 4-META, distilled water, phosphoric acid ester monomer, silicon dioxide, initiator, inhibitor|
|SmartCem™ 2||SC2||Paste/paste dual syringe, Automix through a dispensing tip||Light||Base paste: UDMA, di- and tri-methacrylate resin, phosphoric acid acrylate resin, polymerizable dimethacrylate resin, barium boron fluoroaluminosilicate glass, 95.8% glass fillers (3.8 μm) and 4.2% aerosil (16 nm), 69% filler weight (46 vol.%), titanium dioxide, iron oxide, hydrophobic amorphous silicon dioxide||1005241||DENTSPLY DeTrey, GmbH, Konstanz, Germany|
|Catalyst paste: Barium boron fluoroaluminosilicate glass, UDMA, dipentaerythritol pentaacrylate phosphate, polymerizable dimethacrylate resin, organic peroxide initiator, camphorquinone, phosphene oxide photoinitiator, BHT|
|RelyX™ U100||R1||Clicker dispenser of 2 paste hand-spatula-mixing||A2||Base paste (white): methacrylate monomers containing phosphoric acid groups, methacrylate monomers, 70% wt inorganic fillers of <12.5 μm grain size, silanated calcium ions, alumina, strontium, fluoride fillers, initiator components, stabilizers||421936||3M ESPE Dental Products, Seefeld, Germany|
|Catalyst paste (yellow): methacrylate monomers, alkaline (basic) fillers, initiator components, stabilizers, pigments|
|RelyX™ Unicem 2 Automix||RU2||Paste/paste dual syringe, Automix through a dispensing tip||A2||Base paste: phosphoric acid modified methacrylate monomers, bi-functional methacrylate, 70% wt inorganic fillers of <12.5 μm grain size, silanated calcium ions, alumina, strontium, fluoride fillers, alkaline amines initiator||414088||3M ESPE Dental Products, Seefeld, Germany|
|Catalyst paste: methacrylate monomers, alkaline and silanated fillers, stabilizers, pigments|
The sorption and solubility tests were performed according to the method described in ADA standard specification No. 27 (ISO FDIS 4049:1999) of polymer-based filling, restorative and luting materials . A disc-shaped specimen (1 ± 0.1 mm in thickness and 15 ± 0.1 mm in diameter) of the resin cements were prepared at room temperature (23 ± 1 °C) and relative humidity of 50 ± 2%. The specimens were fabricated by injecting the material into a split-Teflon mold that was rested on a glass slide of 1 mm thickness (76 × 26 × 1 mm Surgipath glass). Careful handling of the material during the injection was taken to minimize the inclusion of entrapped air bubbles. The material was confined between two opposing transparent polyethylene films to minimize exposure to oxygen from atmosphere. Another 1 mm thick glass slide was overlaid on top of the mold and pressed by slight finger pressure to allow extrusion of excess material and avoid porosities. The specimen was irradiated using a visible light curing unit (Optilux 501; Demetron/Kerr Corp., West Collins Orange, CA, USA) utilizing a standard curing mode of an output wavelength range 400–505 nm; and output irradiance of 580 mW/cm 2 . The light curing was performed initially on the middle of the specimen through the glass slide, which was then removed and the curing process was continued through the polyethylene film. The glass slide was removed after the first shot of irradiation in order to avoid the light attenuation. Then, the irradiation was carried out from both top and bottom surfaces in eight overlapping sections on each side; for 40 s each section. This was performed to ensure optimum polymerization of the material resulting in a total curing time of 320 s on each side. The output irradiance was checked periodically with a build-in radiometer associated with the Optilux 501 unit.
The polymerized specimens were carefully removed by dismantling the mold; and each one with an inspected visual void was excluded. The excesses at the edge of the specimen were eliminated with a scalpel blade and lightly rotated and finished against 1000–1500 grit silicon carbide (SiC) abrasive paper to remove irregularities yielding a visually smooth edge. The finished specimens were then dried with tissue and blown with a dust off blower to ensure removal of all debris. The specimens were divided randomly into two subgroups according to the storage media and then subdivided into another six subgroups according to the storage time of each material for the sorption property. Six specimens for each subgroup ( n = 6).
Sorption and solubility measurements
The specimens were placed in a desiccator by spreading them out on a metallic mesh rested inside the desiccator containing anhydrous calcium chloride (CaCl 2 ) underneath the mesh. A suction vacuum (Cole-Parmer, Mexico) was applied through a connecting hose attached to the top-cover of the desiccator at 23 °C to allow removal of moisture during the first 2 h. Then, the suction vacuum was disconnected and the desiccator contained the specimens were transferred in a pre-conditioned oven at 37 °C and stored for 21 h. Thereafter, the discs were stored in another desiccator at 23 °C for 1 h and weighed on an analytical calibrated electronic balance (XT 220A Precisa instrument AG, CH-Dletlkon, Switzerland) to an accuracy of ±0.0001 g (0.1 mg). This procedure was repeated until a constant mass ( m 1 ) was attained, i.e. until the mass loss of each specimen was not more than 0.1 mg within a period of 24 h to ensure complete dehydration of the specimen. The diameter of each specimen was measured from 3 perpendicular planes using a digital electronic caliper (0–20 cm; Mitutoyo Corporation, Tokyo, Japan). Also, the thickness was measured at 5 points of the specimen, one at the center and four at equally spaced points on the specimen’s circumference using a micrometer gauge (0–25 mm; Moore & Wright, Sheffield, UK) with an accuracy of 0.02 mm (20 μm). These measurements were taken in order to calculate the volume of each specimen (V) in mm 3 by the following equation:
V = π r 2 h
where r is the mean specimen radius, h is the mean specimen thickness.
Then, the specimens of each material were randomly divided into two subgroups according to the liquids used for specimen’s storage. They were immersed separately in 20 mL lightproof glass vial of each corresponding immersion media (distilled water of pH 7 and 0.01 M buffered lactic acid of pH 4) maintained at 37 °C for the following time intervals of immersion: 1, 6, 12, 24, 48, and 168 h. After completing each storage period, the specimens were washed with distilled water and carefully handled by tweezers, wiped and blotted dry with an absorbent filter paper to remove excess of visible surface liquid. Additionally, they were waived in air for 10 s and then weighed again until sorption equilibrium was reached a constant weight donated as m 2 ( t ). A fresh distilled water and lactic acid solutions were replaced daily in order to avoid variation in their pH level due to ions leaching process. Thereafter, the specimens were reconditioned in the desiccator until they reached a constant weight ( m 3 ) using the same procedure described for m 1 . The values for the sorption ( S ) at specific times of measurements ( t ) and the solubility ( SL ) after 168 h, in micrograms per cubic millimeter (μg/mm 3 ), were calculated using the following equations, respectively:
S ( t ) = m 2 ( t ) − m 3 ( t ) V
S L ( 168 h ) = m 1 − m 3 V
where m 1 is the conditioned-initial-dry mass prior to immersion in the corresponding liquid media; m 2 ( t ) is the saturated mass of specimen after immersion at a specific time ( t ); m 3 ( t ) is the reconditioned-final-dry mass at a specific time ( t ); V is the volume of the specimen.
The percentage of mass change or increase of the specimen (M g %) is the apparent value for sorbed liquid by the specimen. This measure excluded the leached species because unreacted monomers are simultaneously extracted resulting in a loss of specimen’s weight. It was determined after 168 h of sorption according to the following equation:
M g % ( 168 h ) = m 2 − m 1 m 1 × 100
where m 2 represents the weight of saturated specimen after one week (168 h) of sorption.
The percentage of resin cement solubility (SL%), which represents the amount of unreacted monomers that may have been extracted by water or lactic acid after 168 h immersion period. This was given by the formula:
S L % ( 168 h ) = m 1 − m 3 m 1 × 100