Investigating the Formation of Apatite and The Role of Bioactive Glass and Hydroxyethyl Methacrylate in Resin-Modified Glass Ionomer Cement

Abstract

Background: Secondary caries is one of the most common causes of dental restoration replacement. A compromised marginal seal caused by polymerisation shrinkage might result in secondary caries. Therefore, the capacity of a restorative material to form apatite at the tooth-restoration interface could lower the incidence of secondary caries.

Objective: This project aimed to evaluate the capability of resin-modified glass ionomer cements (RMGICs) to form apatite on immersion in a medium simulating the oral environment. The role of bioactive glass (BAG) incorporated in experimental RMGICs in forming apatite was evaluated. Factors that enhance apatite formation, including hydroxyethyl methacrylate (HEMA), were also explored.

Methods: Novel RMGICs with varying HEMA concentrations, 30% (referred to as HEMA-RMGIC) or 15% (referred to as THFM-RMGIC), and different polyacrylic acid (PAA) concentrations, (20%, 10%, or 0%), with and without 10 wt% experimental BAG, were developed. These RMGICs were evaluated alongside an experimental glass ionomer cement (GIC) and a commercial restorative material (ACTIVA™ BioACTIVE-RESTORATIVE™). The ability of these materials to form apatite in set cement discs immersed in artificial saliva (AS) for up to 24 months was evaluated using 31P magic-angle spinning nuclear magnetic resonance (31P-MAS-NMR), 19F-MAS-NMR, X-ray Diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR). The post-experiment AS solutions for each material were analysed using pH metre, ISE, and ICP-OES. A water absorption experiment was conducted for up to 6 months in AS, to evaluate the impact of the materials’ hydrophilicity on apatite formation. Three-point flexural strength (TFS) and modulus (TFM) were analysed after 24 hours and 1 month in AS.

Results: 31P MAS-NMR, 19F MAS-NMR, XRD and FTIR of the experimental compositions demonstrated evidence of increasing apatite formation over time. No significant difference in apatite formation was found between the experimental RMGICs and the different HEMA and PAA concentrations, while the hydrophilicity of experimental compositions had a limited effect. The substitution of 10 wt% of the experimental glass with BAG accelerated apatite formation. Conversely, the commercial material showed no signs of apatite formation. High fluoride release was observed from HEMA-RMGIC and GICs. As the immersion time of experimental samples increased, substantial consumption of Ca and P ions from their immersion media was identified. THFM-RMGIC exhibited the lowest % weight changes among experimental compositions. Incorporating BAG into THFM-RMGIC increased the % weight change, however, it was comparable to HEMA-RMGIC. THFM-RMGICs displayed the highest TFS amongst the other experimental compositions. BAG incorporation did not significantly affect the TFS except for HEMA-RMGIC. Following immersion for 1 month in AS, a significant reduction in TFS was observed. TFM was also significantly reduced, only in RMGICs with BAG, after one month in AS. TFM of THFM was significantly higher than all experimental and commercial materials.

Conclusions: Experimental RMGICs and GICs formed apatite, following immersion in AS, with time, however, the apatite formation within RMGICs was higher. In addition, substituting 10 wt% of ionomer glass powder with BAG significantly enhanced apatite formation by experimental RMGICs. The presence of HEMA played a role in enhancing apatite formation within RMGICs. Substituting 50% of HEMA for biocompatible monomer (THFM) in RMGIC significantly decreased % weight change, enhanced flexural strength and modulus, and did not compromise apatite formation. These materials could be beneficial in reducing microleakage at the tooth restoration interface and in preventing the development and progression of secondary caries.