Novel, High Strength Methacrylate Composites Containing Calcium Phosphates and Adhesion Promoting Monomers

Introduction Oral bacteria produce acid that causes tooth decay via de-mineralisation. This remains one of the largest problems to be faced by the dental industry. Aesthetic restoration materials, such as composites and Resin Modified Glass Ionomer Cements (RMGIC’s) are widely used as alternatives to amalgam. Composites have high strength but generally require a bonding agent. Furthermore, polymerisation shrinkage and longer term bond failure allow bacterial microleakage and recurrent caries beneath the restoration. RMGIC bonding is more durable and requires no additional adhesive. Low flexural strength, however, prevents RMGIC’s from being used in high occlusal stress sites. Clinician mixing of the restorative material can also lead to irreproducibility in performance. Our aim is to develop a high strength, dental composite with adhesion promoting and tooth re-mineralising components but also controlled expansion / water sorption to compensate polymerization shrinkage. The material requires suitable rheological and cure characteristics for use with a mixing / delivery system. Our hypothesis was that this could be achieved by combining silica based filler with monocalcium monophosphate (MCPM) and β-Tricalcium Phosphate (βTCP). Incorporation of acidic monomers would provide a bonding mechanism between calcium phosphate in the material (enhancing flexural strength) and the surrounding tooth. Surface MCPM of the hydrated material will dissolve and re-mineralise surrounding collagen. βTCP will react with internal MCPM to produce water binding brushite reducing the plasticising effects of water sorption thereby retaining strength. This mechanism of re-mineralisation has been shown using hydrophilic monomers1 but the excessive water sorption caused significant strength reduction. The following content outlines a novel combination of high molecular weight hydrophobic monomers with acidic monomers and calcium and silica based fillers. Materials and Methods UDMA TEGDMA HEMA PMDM Silica Filler MCPM β-TCP Mix Name 25 0 0 37.5 37.5 C 100 20 0 70 5 5 C 5 60 10 10 C 10 19.2 0.8 70 5 5 P 5 60 10 10 P 10 Table 1 Composition of formulations (wt %) Dimethacrylate UDMA, TEGDMA, monomethacrylate HEMA (4.5: 1.5: 1) and acidic PMDM monomers containing chemical initiators, were combined with a silica based filler, MCPM and βTCP in percentages given in Table 1. FTIR was used to find the polymerisation kinetics and the level of monomer conversion. Polymerisation shrinkage was estimated from initial chemical composition and final polymerisation percentages. Raman mapping was used to identify any changes in calcium phosphate chemistry after 24 hours hydration in deionised water. Volume changes due to water sorption and Biaxial Flexural Strength (BFS) of 10 mm diameter and 1mm thick disc specimens (n=8) were obtained after 24 hours hydration. 95% CI error bars were calculated. Results and Discussion The composition and cure time were such that the formulation could be administered via a syringe and mixing tip (Fig 1). The extent of total polymerisation (71% conversion, with an estimated 3.6% shrinkage) shows that the reaction was 50% and 90% complete in 3.5 and 17 minutes respectively (Fig 2). The 100% MCPM and βTCP formulation had the largest volume increase due to water sorption (Fig 3). The strength of formulations containing 5% and 10% MCPM and βTCP have greatly improved BFS compared to those with 100% and are comparable to that of Fuji IILC. Addition of PMDM results in some increase in BFS (Fig 4). Specimens exhibited regions of brushite formation following hydration. Fig 1 Delivery System Fig 2 Cure Profile at 37°C Fig 3 Volume change after 24hours hydration Fig 4 24hour hydrated Biaxial Flexural Strength Conclusions Handling and cure kinetics enabled a delivery method that fulfills standards and will remove the need for, and irreproducibility of, clinician mixing. A low level of MCPM, βTCP and PMDM can increase BFS at 24hours compared with 100% MCPM & βTCP. The water sorbtion of the formulations was low and partially compensated for polymerisation shrinkage. Brushite was formed when the cured fomulation was hydrated. 1) I. Mehdawi et al Acta Biomaterilia 5(2009)2525