Eugenol has been used in dentistry due to its ability to inhibit the growth of a range of microorganisms, including facultative anaerobes commonly isolated from infected root canals. The aim of this study was to evaluate the antibacterial activity of the experimental composites containing eugenyl methacrylate monomer (EgMA), a polymeric derivative of eugenol, against a range of oral bacteria, commonly associated with failure of coronal and endodontic restorations. In vitro composite behavior and wettability were also studied in conjunction with their antibacterial activity.
EgMA monomer (5 and 10% by weight) was added into BisGMA/TEGDMA resin based formulations with filler mixtures of hydroxyapatite (HA) and zirconium oxide ZrO 2 . The antibacterial activity of the experimental composites against Enterococcus faecalis , Streptococcus mutans and Propionibacterium acnes were evaluated by direct contact test and compared with composite formulation without inclusion of EgMA. To clarify the antibacterial mode of action, agar diffusion test (ADT) was also performed. Water sorption, solubility, diffusion coefficient, contact angle and surface free energy as complementary clinically relevant properties were determined.
Water sorption and wettability studies showed reduction of water uptake and surface free energy values with increasing content of EgMA monomer, resulting in significant increase in the hydrophobicity of the composites. No inhibition zones were detected in any of the composites tested against the three bacteria employed as expected, due to the absence of any leachable antibacterial agent. The covalently anchored EgMA monomer with the composite surface exhibited an effective bacteriostatic activity by reducing the number of CFUs of the three species of bacteria tested with no significant dependence on the concentration of EgMA at 5 and 10% by weight. The surface antibacterial activity R of the experimental composites were different against the three tested species with values in the range 2.7–6.1 following the order E . faecalis < S. mutans < P. acnes .
The incorporation of EgMA monomer within polymerizable formulations provides a novel approach to yield intrinsically antibacterial resin composites for different dental applications.
Endodontically treated teeth (ETT) are more susceptible to fracture than vital teeth due to the significant reduction of tooth tissue as a result of the endodontic and restorative treatment accompanied with changes in chemical composition of dentin due to loss of water and collagen . Although it is well established that coronal coverage significantly improves the clinical success rate of endodontically treated posterior teeth, the choice of restoration depends on the amount of remaining tooth structure and functional requirements.
Composite resins have the advantage of bonding to residual coronal and root canal dentin, which may assist in strengthening the tooth and offer an alternative technique for restoration of ETT . Dual cure resin composites core materials with different viscosity are currently used within the canal for fiber posts cementation, to restore the structurally compromised ETT. They have superior mechanical properties than those of resin cements and result in lower stress, reducing the load transfer on the root dentin and surface of the post .
However, resin composites lack antibacterial properties and result in more plaque accumulation than other restorative materials . In addition, any microleakage allows for new bacterial invasion, compounded by the fact that it is difficult to completely remove bacteria from the root canal system even after careful cleaning and shaping and the minimally invasive approach during restoration of teeth will possibly maintain more residual bacteria within the dentinal tubules .
There is a rising interest to endow dental restorative materials with sustained antibacterial activity to enhance long term performance , which is expected to lower the risk of reinfection and secondary caries . Different antibacterial agents such as chlorhexidine, fluoride, quaternary ammonium salts and metallic agents (silver, gold and zinc) have been incorporated in acrylic based composite formulation in order to achieve this goal . However, most of these additives cause an adverse effect in terms of mechanical properties, discoloration of the material , toxicity and short-term antibacterial effectiveness .
Most antibacterial studies reported in literature evaluate the activity of different incorporated antibacterial agents against Streptococcus mutans , the main microbial etiological agent of dental caries and the leading cause of resin based composite failure , however other oral microorganisms such as, Enterococcus faecalis , Candida albicans and Propionibacterium acnes are also frequently associated with endodontic infections . E. faecalis , in particular, is difficult to remove owing to its considerable virulence factors constituting a source of recurrent infection after conservative as well as surgical treatments . P. acnes is an anaerobic Gram-positive bacterium responsible for a wide range of infections and inflammatory conditions . Therefore, development of antibacterial restorative filling materials to be reliable for a variety of dental applications need a potent antimicrobial agent which acts against a wide range of oral microorganisms.
Eugenol (4-allyl-2-methoxyphenol) is a natural phenolic anti-oxidant essential oil that possesses antifungal activity and inhibits the growth of several microorganisms including Escherichia coli and facultative anaerobes commonly isolated from infected root canals . This compound has been used in combination with zinc oxide in different dental applications such as temporary filling materials and root canal sealers. However, eugenol is not compatible with other methacrylate based restorative materials because of the presence of free eugenol, which interferes with the polymerization reaction of dental composite resins.
In contrast, eugenyl methacrylate (EgMA) an eugenol derivative possess in its chemical structure a polymerizable methacrylic group ( Fig. 1 ) that allows the monomer to participate in free radical polymerization reactions whilst maintaining the antibacterial activity of its natural precursor against different Gram-negative and Gram-positive bacterial species .
In our previous study, the experimental composites from Bis-GMA/TEGDMA, a commonly used dental resin system and EgMA were formulated with 65% by weight filler phase comprising of HA/ZrO 2 . These composites were tailored to function as an antibacterial restorative material for intracanal posts cementation and core build-up in the restoration of ETT. The influence of EgMA monomer incorporation on curing, physical and mechanical properties of these new formulations showed that these composites were suited for the application.
However, properties such as water sorption and wettability have detrimental effects on the composite material and bacterial adhesion which are important parameters toward clinical relevance. Hence in this study, the in vitro behavior and antibacterial activity of these EgMA containing resin composites against a range of oral bacteria commonly associated with the failure of coronal and endodontic restorations are reported.
Materials and methods
Materials and composites formulations
Three batches of dual cure resin composites were prepared by combining 2,2-bis [4-(2-hydroxy-3-methacryloyloxypropyl)-phenyl] propane (Bis-GMA) (Esschem Europe, Durham, UK) and tri-ethyleneglycol dimethacrylate (TEGDMA) (Esschem Europe, Durham, UK) in a fixed ratio of 1:1 by weight, representing a total resin phase of 35 wt% in the formulation. EgMA monomer (MW = 232.23 g/mol.) was synthesized as reported previously and added at a level of 0 (reference), 5 and 10 wt% of the resin phase ( Table 1 ). All composites were formulated with 65 wt% filler phase, which contained hydroxyapatite (HA) with a mean particle size diameter of 3–5 μm (Plasma Biotal Ltd., Tideswell, Derbyshire, UK) and ZrO 2 with a mean particle size diameter 18 μm (Fisher Scientific Ltd., Loughborough, UK) in a ratio of 4:3 by weight. The filler particles were silanated with 10% of A-174 (Merck-Frankfurt, Germany) by a wet silanation treatment in 70/30 mix of acetone and distilled water following a method described previously . The resin phase was first prepared and divided in two separate portions where initiator system (0.5% benzoyl peroxide (Merck-Frankfurt, Germany) +0.5% camphorquinone (Sigma-Aldrich, Dorset, UK)) and activator (N,N dimethyl p-toluidine (Sigma-Aldrich, Dorset, UK) 1:1 molar ratio) were added respectively to avoid self-polymerization. Then the corresponding amount of silanised filler was added to each portion and mixed on a magnetic stirrer for 24 h.
|Composites||Monomers (in weight percent)|
Equal masses of the two pastes were hand-mixed using a stainless steel spatula for 30 s and carefully placed into Teflon molds to produce discs of 10 mm diameter and 1 mm thickness avoiding bubble entrapment. The upper and lower surface of the mold was covered with glass slides and then cured by visible light for 40 s each side by overlapping, using Optilux 501 (Demetron, Danbury, USA) dental curing unit performing an irradiance of 400 ± 50 mW cm −2 .
Water sorption and solubility
Water sorption and solubility were measured according to ISO 4049 . Three disk specimens were prepared for each material. The thickness and diameter of each specimen were measured at 4 and 2 points respectively, using a digital electronic caliper (DURATOOL, UK). Mean values were used to calculate the volume of each specimen in mm 3 . The specimens were then placed in a desiccator with anhydrous calcium chloride and maintained at 37 °C. After 22 h, they were removed, stored in another desiccator at 23 °C for 2 h and then weighted to an accuracy of ±0.0001 g using a Mettler-Toledo AG64 balance to obtain the constant initial weight ( M i ) and to ensure completion of polymerization and dehydration. Specimens of each material were immersed in 10 ml distilled water in individual glass containers and then incubated at 37 °C for a total immersion time of 28 days. At noted intervals, the specimens were gently dried on filter paper until free from visible moisture, waved in air for 15 s and weighed 1 min later and returned to the glass containers filled with distilled water. The recorded weight was denoted as the mass of saturated specimen M s ( t , time). Each specimen was then desorbed in a drying oven maintained at 37 °C and weighed again until a constant dry mass ( M d ) was found. A second absorption–desorption cycle followed to obtain 2nd M s ( t , time) and 2nd M d in the same way as the first cycle.
Mass change percentage was calculated by the following equations:
Sorption mass change percentages = M s ( t ) − M i M i × 100
The water sorption ( W SP ) and solubility ( W SL ) in μg/mm 3 were calculated using the following equations:
where V is the volume of the sample.
The early stages of diffusion-controlled uptake of water in composites are given by