Development and characterisation of dental composites containing anisotropic fluorapatite bundles and rods

To develop dental composites incorporating fluorapatite (FA) crystals as a secondary filler and to characterise degree of conversion, key mechanical properties and fluoride release.

FA rod-like crystals and bundles were hydrothermally synthesised and characterised by scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS), X-ray diffraction (XRD) and ¹⁹F MAS-NMR. Composites were formulated containing BisGMA/TEGDMA/BisEMA and barium-aluminium-silicate glass (0FA). FA crystals were incorporated at 10 (10FA), 20 (20FA), 30 (30FA) and 40wt% (40FA) maintaining a filler content of 80wt% (63-67vol%). Degree of conversion (DC), flexural strength (FS), flexural modulus (FM), fracture toughness (K_1C), Vickers hardness (HV) and 2-body wear were measured. Fluoride release was measured in neutral and acidic buffers.

XRD and ¹⁹F MAS-NMR confirmed that only FA was formed, whilst SEM revealed the presence of single rods and bundles of nano-rods. DC ranged between 56-60% (p>0.05). FA composites showed lower FM and lower FS (p<0.05), but comparable wear resistance and HV (p>0.05) to 0FA. 30FA and 40FA showed similar K_1C to 0FA (p>0.05), with SEM showing evidence of toughening mechanisms, whereas 10FA and 20FA showed lower K_1C (p<0.05). FA containing composites released fluoride that was proportional to the amount of FA incorporated (p<0.05) but only under acidic conditions.

The addition of FA to the experimental composites reduced strength and stiffness but not the DC, hardness or wear rate. 30FA and 40FA had a higher K_1C compared to other FA groups. Fluoride release occurred under an accelerated acidic regime, suggesting potential as a bioactive 'smart' composite.