Water sorption of flowable composites

Abstract

ABSTRACT Objectives: Flowable composites are characterized by lower filler loading and a greater proportion of diluent monomers in their formulation. These composites were traditionally created by retaining the same small particle size of the conventional hybrid composites, but reducing the filler content and allowing the increased resin to reduce the viscosity of the mixture However, their various mechanical properties such as flexural strength and wear resistance have been reported to be generally inferior compared to those of the conventional composites. Dental restorative materials are in continuous contact with fluids and saliva in the patient’s mouth. Consequently, the water sorption and solubility of these materials are of considerable importance. Resin based materials demonstrate water sorption in the oral cavity, which is the amount of water absorbed by the material on the surface and into the body while the restoration is in service. The water intrusion in the dental material can lead in a deterioration of the physical/mechanical properties, decreasing the life of resin composites. Water uptake can promote breakdown causing a filler-matrix debonding. Water sorption affects the physical and mechanical properties of resin composite such as dimensional change, decrease in surface hardness and wear resistance, filler leaching, change in color stability, reduction in elastic modulus, and an increase in creep and a reduction in ultimate strength, fracture strength, fracture toughness, and flexural strength. In addition, penetration of water into the composite may cause release of unreacted monomers (solubility) which may stimulate the growth of bacteria and promote allergic reactions. The effect of water sorption on conventional composites has been extensively studied and reviewed in the dental literature. However , there are no published studies on the water sorption of flowable composites. Water sorption increases as the amount of resin matrix increases and filler content decreases, since the filler particles do not absorb water. Thus, it is of utmost importance to study the water sorption of flowable composite. Hence the aim of this study was to evaluate and compare water sorption and solubility values of different light-activated flowable composite materials in solutions with varying pH values. And, since water filled porosities in the flowable composites may form small incubation chambers, a second related objective was to compare and correlate water sorption values of the various flowables to their ability to form Streptococcus mutans and Streptococcus sanguis single species biofilms in/on their surfaces. Methods: In this study, water sorption and solubility tests were performed according to the ISO standards (International Organization for Standardization specification 4049:07-2009- Dentistry- Polymer Based Restorative Materials [available at http://www.iso.org/iso/home/store.htm]). Three disc-shaped specimens of each flowable composite were made in a jig consisting of a Teflon mold (15 mm in diameter by 1 mm in thickness) compressed between 2 glass slabs with mylar strips used as separating sheets. The flowable resin was inserted in the Teflon mold in a single increment. All specimens were cured with a light-emitting diode curing unit. According to the ISO standard, discs were weighted every day for 35 days using the same balance, with a repeatability of 0.1 mg, until a constant mass (M1) was obtained. Once a constant M1 was obtained, the volume (V) was then calculated in cubic millimeters as follow: V =π(d/2)2h, where π=3.14; d is the mean diameter of the specimen; and h is the mean thickness of the specimen. After M1 was achieved, each flowable composite resin group of 3 discs was placed into buffers of pH = 4.0,5.5 and 7.0. After 24 hrs, specimens were wiped free of excess buffer with absorbent paper and weighed. This cycle was repeated at one week , one month, and six months. When a constant mass was achieved it was designated M2. Mass gain (Mg) was defined as follows: (M2 –M1). Per cent mass gain (%Mg) was defined as follows: (M2-M1/M1). Finally, the specimens were reconditioned to constant mass, once again following the above-mentioned procedure. This constant mass was recorded as M3. Water sorption (Wsp) was calculated in micrograms per cubic millimeter for each of the specimens by using the following equation provided by ISO 4049 standard: Wsp=(M2-M3)/V, where M2 is the mass of the specimens in micrograms after immersion in buffer for 30 days; M3 is the reconditioned mass of the specimen, in micrograms; and V is the volume of the specimen in cubic millimeters. Water solubility (Wsl) was calculated in micrograms per cubic millimeter for each of the specimens, using the following equation, provided by ISO 4049 standard: Wsl=(M1-M3)/V, where M1 is the conditioned mass of the specimen in micrograms before immersion in buffer; M3 is the reconditioned mass of each specimen in micrograms, and V is the volume of the specimen in cubic millimeters. For biofilm experiments, flowable discs were prepared as described above. Each disc was then sectioned into three equal portions using high speed and low speed handpieces , a diamond bur, and sandpaper discs, such that the three samples of each flowable had the same mass to within 0.3 mg. The samples were sterilized by dipping in 1.2% sodium hypochlorite (Chlorox), followed by rinsing with sterile distilled water, and then conditioning to a constant mass as described above, inside a desiccator that was wiped with 1.2 % Chlorox. Biofilm experiments were conducted as follows: three equal mass specimens of each flowable composite were placed in a series of wells of a sterile culture disc. Then sterile BHI broth (2 ml) was added to each well. One well served as control and no growing bacteria were added to it. To the other specimens was added 40 μl log phase S. mutans or S. sanguis cells. The culture dishes were then placed on a rotator at 37C for six hrs. Biofilm formation was measured by staining attached cells with crystal violet, destaining with 30% acetic acid, and measuring the satin spectrophotometically. Results: The pH of the solution influenced the % mass gain, as all samples gained more mass at pH 4.0 as compared to pH 5.5 and 7.0. The flowable resin SureFill showed the least % mass gain at each pH. However, there was no statistical difference in % mass gain based on pH of storage buffer for any of the flowable composites (P=.05) . Time had a significant influence on the % mass gain for the first week for all samples, with minor gains thereafter, and became steady after 1 month. Surefill showed the least water sorption when stored in buffer for 30 days, however it was not significant compared to the other flowables (P= 0.05). Filtek showed the least water solubility, but is not significant compared to the other flowables (P=0.05). The highest significant values (P< 0.05) for water sorption and solubility were observed for Virtuoso. Two trials indicated that strains of S. mutans and S. sanguis form biofilm readily on the surface of the composites, with S. sanguis having a higher predilection to form biofilm on all composites (Figure 6). However, no correlation was found between water sorption and solubility values of the flowable composites and biofilm formation. Conclusions: Within the limitations of this study the following is concluded: Time and storage conditions are important to the % mass gain due to water, with all flowable composites showing more mass gain at low PH. Due to its hydrophilic nature, as well as to the filler characteristics, the flowable composite Virtuoso exhibited significantly higher values of water sorption and water solubility than the other flowable composites that were tested. All flowable composites formed S. sanguis and S. mutans single species biofilm on their surfaces, with S. sanguis forming higher concentrations of biofilm on all samples. There was no clear correlation to water sorption and biofilm formation characteristics of the composites.