Dental composites based on amorphous calcium phosphate - resin composition/physicochemical properties study

Abstract

This study explores how the resin composition/structure affects the physicochemical properties of copolymers and their amorphous calcium phosphate (ACP)-filled composites. A series of photo-polymerizable binary and ternary matrices are formulated utilizing 2,2-bis[ p-(2(')-hydroxy-3(')methacryloxypropoxy)phenyl]propane, 2,2-bis[ p-(2(')-methacryloxypropoxy)phenyl]-propane (EBPADMA), or a urethane dimethacrylate as base monomers, and triethylene glycol dimethacrylate or hexamethylene dimethacrylate (HmDMA) with or without 2-hydroxyethyl methacrylate (HEMA) as diluent monomer. Unfilled copolymers and composites filled with 40% by mass zirconia-hybridized ACP are evaluated for biaxial flexure strength (BFS), degree of conversion (DC), mineral ion release, polymerization shrinkage (PS), and water sorption (WS). The average DC values are 82-94% and 74-91% for copolymers and composites, respectively. Unrelated to the resin composition, the PS values of composites are up to 8.4 vol. % and the BFS values of wet composite specimens are on average 51 +/- 8 MPa. The maximum WS values attained in copolymers and composites reach 4.8 mass%. Inclusion of hydrophobic HmDMA monomer in the matrices significantly reduces the WS. The levels of Ca and PO(4) released from all types of composites are significantly above the minimum necessary for the re-deposition of apatite to occur. Elevated Ca, and to a lesser extent PO(4) release, is observed in HEMA-containing, ternary EBPADMA formulations. Further resin reformulations may be needed to improve the PS of composite specimens. Poor dispersion of ;as-synthesized' ACP within the composite contributes to their inferior mechanical performance.

Fig. 1

Chemical structure of the monomers and photo-curing agents utilized in the study.

Fig. 2

Physicochemical characteristics of Zr-ACP filler: Structural features (XRD and FTIR spectra; parts a and b, respectively), particle size distribution (PSD, part c) and the surface morphology (SEM image; part d).

Fig. 2

Physicochemical characteristics of Zr-ACP filler: Structural features (XRD and FTIR spectra; parts a and b, respectively), particle size distribution (PSD, part c) and the surface morphology (SEM image; part d).

Fig. 2

Physicochemical characteristics of Zr-ACP filler: Structural features (XRD and FTIR spectra; parts a and b, respectively), particle size distribution (PSD, part c) and the surface morphology (SEM image; part d).

Fig. 2

Physicochemical characteristics of Zr-ACP filler: Structural features (XRD and FTIR spectra; parts a and b, respectively), particle size distribution (PSD, part c) and the surface morphology (SEM image; part d).

Fig. 3

Degree of conversion (DVC; mean + standard deviation (SD; indicated by bars)) attained in copolymer and composite specimens at 24 h post-curing. The number of samples in each group n ≥ 7.

Fig. 4

Polymerization shrinkage (PS; mean + SD) of Zr-ACP composites formulated with various binary and ternary matrices. The number of samples in each group n = 9.

Fig. 5

Biaxial flexure strength (BFS; mean + SD) of dry and wet Bis-GMA, EBPADMA and UDMA copolymers and composites. The number of samples in each group n ≥ 14.

Fig. 6

Maximum water sorption, W_max (mean + SD; in mass %), of binary and ternary copolymers and their corresponding composites after 30 d of exposure to 75 % relative humidity at room temperature. Number of specimens in each group n ≥ 4.

Fig. 7

Concentration of the mineral ions (mean + SD) attained after 216 h of immersion of composite specimens in buffered saline. The number of samples in each group n = 3.