Effect of filler level and particle size on dental caries-inhibiting Ca-PO(4) composite

Abstract

Secondary caries and restoration fracture are common problems in restorative dentistry. The aim of this study was to develop Ca-PO(4) nanocomposite having improved stress-bearing properties and Ca and PO(4) ion release to inhibit caries, and to determine the effects of filler level. Nanoparticles of dicalcium phosphate anhydrous (DCPA), two larger DCPA powders, and reinforcing whiskers were incorporated into a resin. A 6 x 3 design was tested with six filler mass fractions (0, 30, 50, 65, 70, and 75%) and three DCPA particle sizes (112 nm, 0.88 mum, 12.0 mum). The DCPA nanocomposite at 75% fillers had a flexural strength (mean +/- SD; n = 6) of 114 +/- 23 MPa, matching the 112 +/- 22 MPa of a commercial non-releasing, hybrid composite (P > 0.1). This was 2-fold of the 60 +/- 6 MPa of a commercial releasing control. Decreasing the particle size increased the ion release. Increasing the filler level increased the ion release at a rate faster than being linear. The amount of ion release from the nanocomposite matched or exceeded those of previous composites that released supersaturating levels of Ca and PO(4) and remineralized tooth lesions. This suggests that the much stronger nanocomposite may also be effective in remineralizing tooth lesion and inhibiting caries. In summary, combining calcium phosphate nanoparticles with reinforcing co-fillers in the composite provided a way to achieving both caries-inhibiting and stress-bearing capabilities. Filler level and particle size can be tailored to achieve optimal composite properties.

Fig. 1

Flexural strengths for the 3 × 6 full factorial design (with three different DCPA particle size and six filler levels in the resin composite). Each value is the mean of six measurements with the error bar showing one standard deviation (mean ± SD; n = 6)

Fig. 2

Elastic modulus of the composites. Increasing the filler level significantly increased the modulus for all three DCPA sizes (P < 0.05). Each value is mean ± SD; n = 6

Fig. 3

PO₄ ion release from the composite with a the nano-DCPA (112 nm); b the ground commercial DCPA (0.88 µm); and c the as-received commercial DCPA (12 µm). Each values is mean ± SD; n = 3. The label at the right side indicates the filler level. The DCPA:whisker mass ratio was 1:1. Note the y-axis scale difference between the plots

Fig. 4

Ca ion release from composite with a the nano-DCPA (112 nm); b the ground commercial DCPA (0.88 µm); and c the as-received commercial DCPA (12 µm). Each values is mean ± SD; n = 3. In each plot, the label at the right side indicates the filler level. The DCPA:whisker mass ratio was 1:1. Increasing the filler level and immersion time significantly increased the amount of Ca release (P < 0.05)

Fig. 5

Effect of calcium phosphate particle size on ion release. a PO₄ release at 56-day with 75% filler level for the three composites having DCPA particle surface area of 18.6 m²/g (particle size = 112 nm), 2.36 m²/g (0.88 µm), and 0.17 m²/g (12 µm), demonstrating the significant effect of particle surface area. b Corresponding Ca ion release

Fig. 6

Effect of calcium phosphate filler level in the composite on release. Ion release after 56 days is plotted versus DCPA filler level for the three composites with DCPA sizes of 112 nm, 0.88 µm, and 12 µm, respectively. The data indicate that the ion release was increased with filler level at a rate faster than being linear