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

This study explores how the resin composition/structure affects the physicochemical properties of copolymers and their amorphous calcium phosphate (ACP)-fflled 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 PO4 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 PO4 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.