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Introduction

Dental caries involves acid-producing bacteria that enhance dissolution of hydroxyapatite from both enamel and dentin. In dentin, remaining collagen is subsequently degraded by matrix metalloproteinase enzymes (MMPs) . In order to arrest disease progression and restore shape and function, damaged dental structures were previously replaced by amalgam but this is increasingly being replaced by more esthetic composite materials . The use of adhesives with dental composites has reduced the need for over cutting and sound tooth removal procedures previously adopted to ensure amalgam retention . Rapid bond strength deterioration upon thermal or mechanical cycling, however, is a well known problem . Moreover, composites shrink upon polymerization. The resultant stress can enhance tooth adhesion loss, microgap formation and ultimately bacterial microleakage . This, in combination with lack of antibacterial action, can result in current composite resin restorations having a greater risk of secondary caries and being replaced at a higher rate than amalgam .

To overcome bacterial microleakage, chlorhexidine (CHX) has previously been added into dental composites . In addition to being antibacterial, this drug can inhibit MMP action . Composites with early release of chlorhexidine might reduce the need for extensive disease affected tissue removal as advocated in modern tooth restoration procedures . Unfortunately, CHX is not readily released from the bulk of a conventional composite. This problem has been overcome through partial replacement of hydrophobic composite monomers with the hydrophilic monomer, hydroxyethylmethacrylate (HEMA). Hyrophilicity enhances water sorption and expansion which counteracts polymerization shrinkage. It also increases early drug release and antibacterial action . Unfortunately, strength is reduced.

An alternative method employed to reduce bacterial microleakage with composite use has been addition of amorphous calcium phosphate. It has been proposed that release of calcium phosphate may help re-mineralization of surrounding dentin . Poor initial strength, however, was common due to lack of any bonding mechanism between the filler and matrix phase .

Additionally, reactive acidic and basic mono and tri calcium phosphate fillers (MCPM/β-TCP) have been included in dental composites . Use of the hydrophilic and soluble MCPM alone caused rapid water sorption, calcium phosphate release and decline in strength. When β-TCP was added, MCPM in the surface of the material still dissolved. MCPM in the bulk, however, reacted with the β-TCP binding water in brushite (dicalcium phosphate dihydrate) crystals.

MCPM/β-TCP fillers and chlorhexidine were previously incorporated in urethane (UDMA)/triethylene glycol (TEGDMA)/hydroxyethyl (HEMA) di and mono methacrylate resins . The combined presence of high levels of hydrophilic MCPM (25–38 wt% of composite) and HEMA (50 wt% of resin) enabled much higher CHX release than generally possible with dental composites . It also, however, caused excessive water sorption, swelling and early strength decline.

The following new study aim is to address if reduction in HEMA level (10–20 wt%) can control these reactive filler composite problems without overly inhibiting release of chlorhexidine. Furthermore, the reactive calcium phosphate fillers are partially replaced with silica/silicon carbide particles in an attempt to improve early strengths whilst maintaining some calcium phosphate release. Additionally, the calcium binding monomer (Bis[2-(methacryloyloxy)ethyl] phosphate (2MP)) is added to assess if this can provide a bonding mechanism between the calcium phosphate fillers and monomer, thereby raising strength.