Calcium and phosphate ion releasing composite: effect of pH on release and mechanical properties

Abstract

Objectives: Secondary caries and restoration fracture are the two main challenges facing tooth cavity restorations. The objective of this study was to develop a composite using tetracalcium phosphate (TTCP) fillers and whiskers to be stress-bearing, and to be "smart" to increase the calcium (Ca) and phosphate (PO(4)) ion release at cariogenic pH.

Methods: TTCP was ball-milled to obtain four different particle sizes: 16.2, 2.4, 1.3, and 0.97 microm. Whiskers fused with nano-sized silica were combined with TTCP as fillers in a resin. Filler level mass fractions varied from 0 to 75%. Ca and PO(4) ion releases were measured vs. time at pH of 7.4, 6, and 4. Composite mechanical properties were measured via three-point flexure before and after immersion in solutions at the three pH.

Results: TTCP composite without whiskers had flexural strength similar to a resin-modified glass ionomer (Vitremer) and previous Ca-PO(4) composites. With whiskers, the TTCP composite had a flexural strength (mean+/-S.D.; n=5) of (116+/-9)MPa, similar to (112+/-14)MPa of a stress-bearing, non-releasing hybrid composite (TPH) (p>0.1). The Ca release was (1.22+/-0.16)mmol/L at pH of 4, higher than (0.54+/-0.09) at pH of 6, and (0.22+/-0.06) at pH of 7.4 (p<0.05). PO(4) release was also dramatically increased at acidic pH. After immersion, the TTCP-whisker composite matched the strength of TPH at all three pH (p>0.1); both TTCP-whisker composite and TPH had strengths about threefold that of a releasing control.

Significance: The new TTCP-whisker composite was "smart" and increased the Ca and PO(4) release dramatically when the pH was reduced from neutral to a cariogenic pH of 4, when these ions are most needed to inhibit caries. Its strength was two- to threefold higher than previously known Ca-PO(4) composites and resin-modified glass ionomer. This composite may have the potential to provide the necessary combination of load-bearing and caries-inhibiting capabilities.

Figure 1

Tetracalcium phosphate (TTCP), Ca₄(PO₄)₂O, particle diameter vs. ball milling time. The median (50th percentile) particle sizes were 16.2 μm, 2.4 μm, 1.3 μm, and 0.97 μm, respectively. These particles were used as fillers in dental resin to release Ca and PO₄ ions to inhibit tooth caries.

Figure 2

Results of group 1 on the effect of TTCP filler level on composite mechanical properties. Each value is the mean of five measurements with the error bar showing one standard deviation (mean ± sd; n = 5). The TTCP particle size was 0.97 μm, and no whiskers were used in the composite.

Figure 3

Results of group 2 on effects of TTCP particle size and whiskers. Each value is mean ± sd; n = 5. The total filler level was fixed at 75% for each experimental composite. The TTCP particle sizes were 16.2 μm, 2.4 μm, 1.3 μm, and 0.97 μm, respectively. Two commercial controls were also included, which were measured in the same manner. TTCP:whisker ratio of 1:1 means 37.5% of TTCP and 37.5% of whiskers. TTCP:whisker ratio of 1:0 means 75% of TTCP and no whiskers.

Figure 4

Ca and PO₄ release for group 3: Composite filled with 0.97-μm TTCP to 75% mass fraction, without whiskers. Each value is mean ± sd; n = 3. Decreasing the solution pH significantly increased the ion release from the composite (p < 0.05).

Figure 5

Ca and PO₄ release for group 3: TTCP-whisker composite with 37.5% of TTCP (0.97μm TTCP), and 37.5% of whiskers. Each value is mean ± sd; n = 3.

Figure 6

Results of group 4, showing (A) flexural strength, and (B) elastic modulus, after immersion for 28 days in solutions of pH of 7.4, 6, and 4, respectively. Solution pH had only minor effects on composite strength and modulus. TTCP-whisker composite matched the strengths of TPH at all three pH. Both TTCP-whisker composite and TPH had strengths about 3-fold the strength of Vitremer. Each value is mean ± sd; n = 5. The TTCP-whisker composite had 75% fillers at a TTCP:whisker ratio of 1:1.