Novel bonding systems for resin composites

Abstract

Resin composites are commonly used in the restorative management of diseased or damaged teeth via adhesive bonding, which has evolved significantly over the last few decades. Current bonding systems allow self-etching of tooth tissues via adhesive functional monomers which reduces the number of clinical steps and technique sensitivity. However, the resin-dentine interface is prone to degradation because of disruption of the hybrid layer mainly due to hydrolytic or enzymatic activity, while the resin-enamel interface is still considered sub-optimal when compared to the use of phosphoric acid etching. The aim of this study was to develop a bonding system that would be suited to both enamel and dentine by combining therapeutic monomers to preserve the resin-tooth interface. A two-step self-etching system containing bis[2-(methacryloyloxy) ethyl] phosphate (BMEP) as the acidic functional monomer was developed with it present either in the primers only or additionally at a low concentration in the adhesive and results of surface roughness, interaction of the primer with neat hydroxyapatite using FTIR spectroscopy, microshear bond strength and mode of failure compared to a commercial reference containing the functional monomer 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP), Clearfil SE Bond 2 (CFSE). BMEP-based primers were found to be an effective etchant on both enamel and dentine especially with a higher concentration of BMEP compared to 10-MDP-based CFSE that also exhibited spontaneous polymerisation without photoinitiation upon interaction with hydroxyapatite; ensuring polymerisation of any residual acidic monomer that may remain in deeper areas of the tooth during clinical application. Two types of experimental adhesives, one comprising of commonly used monomers in dental adhesives whilst the other contained a low concentration of BMEP, were evaluated for wettability, curing and thermal properties. The inclusion of BMEP in the adhesive resulted in curing parameters inferior to CFSE but remaining within the clinically acceptable range whilst significantly higher surface free energy, lower water sorption and solubility were observed with or without BMEP in the experimental adhesives. The BMEP bonding systems revealed that BMEP primers penetrated deeper into dentine with formation of a well infiltrated hybrid layer and longer resin tags compared to the CFSE primer using confocal imaging and microRaman spectroscopy. BMEP also demonstrated potent inhibition of endogenous matrix metalloproteinases, which is expected to provide the dentinal collagenous extracellular matrix with resistance to gelatinolytic and collagenolytic degradation. The experimental bonding systems further demonstrated that the combination of 15 wt% of BMEP in the primer with a hydrophobic BMEP-free adhesive resulted in high bond strengths to both enamel and dentine, excellent durability and lowest nanoleakage after up to 6 months storage in an aqueous environment. Since microbial contamination is a frequent reason for failure of the tooth-resin interface, imparting antimicrobial properties using eugenyl methacrylate (EgMA), a hydrophobic monomer with intrinsic antimicrobial properties due to the presence of an eugenol moiety which can participate in polymerisation as a result of its methacrylation, was explored. EgMA incorporated into the adhesive at two different concentrations resulted in a significant decrease in water sorption and solubility with a significant increase in the glass transition temperature owing to presence of the aromatic ring in EgMA and the resultant branched polymer structure. The bond strengths and nanoleakage values of the EgMA-based bonding systems to dentine were maintained for the test period of 6 months. EgMA, both as a monomer and on polymerisation, exhibits antimicrobial properties, and this action was evident from the sur