Thio-urethanes Improve Properties of Dual-cured Composite Cements

Abstract

This study aims at modifying dual-cure composite cements by adding thio-urethane oligomers to improve mechanical properties, especially fracture toughness, and reduce polymerization stress. Thiol-functionalized oligomers were synthesized by combining 1,3-bis(1-isocyanato-1-methylethyl)benzene with trimethylol-tris-3-mercaptopropionate, at 1:2 isocyanate:thiol. Oligomer was added at 0, 10 or 20 wt% to BisGMA-UDMA-TEGDMA (5:3:2, with 25 wt% silanated inorganic fillers) or to one commercial composite cement (Relyx Ultimate, 3M Espe). Near-IR was used to measure methacrylate conversion after photoactivation (700 mW/cm(2) × 60s) and after 72 h. Flexural strength and modulus, toughness, and fracture toughness were evaluated in three-point bending. Polymerization stress was measured with the Bioman. The microtensile bond strength of an indirect composite and a glass ceramic to dentin was also evaluated. Results were analyzed with analysis of variance and Tukey's test (α = 0.05). For BisGMA-UDMA-TEGDMA cements, conversion values were not affected by the addition of thio-urethanes. Flexural strength/modulus increased significantly for both oligomer concentrations, with a 3-fold increase in toughness at 20 wt%. Fracture toughness increased over 2-fold for the thio-urethane modified groups. Contraction stress was reduced by 40% to 50% with the addition of thio-urethanes. The addition of thio-urethane to the commercial cement led to similar flexural strength, toughness, and conversion at 72h compared to the control. Flexural modulus decreased for the 20 wt% group, due to the dilution of the overall filler volume, which also led to decreased stress. However, fracture toughness increased by up to 50%. The microtensile bond strength increased for the experimental composite cement with 20 wt% thio-urethane bonding for both an indirect composite and a glass ceramic. Novel dual-cured composite cements containing thio-urethanes showed increased toughness, fracture toughness and bond strength to dentin while demonstrating reduced contraction stress. All of these benefits are derived without compromising the methacrylate conversion of the resin component. The modification does not require changing the operatory technique.

Impact of thio-urethane additive and filler type on light-transmission and depth of polymerization of dental composites

“…Thus, the reduction of light-transmission in this wavelength can be related to moment where the resin matrix achieves high enough conversion to significantly increases it refractive index, resulting in higher discrepancy with that of filler [13]. Considering that the addition of thio-urethane delays the resin vitrification, the modification on light-transmission occurred later when this oligomer was added to composite [16–19]. In opposite, the scattering effect promoted by filler itself is more pronounced when higher wavelengths are incised over the specimens, resulting in early light-attenuation.…”

“…These oligomers have been shown to produce networks with significantly improved fracture toughness and reduced polymerization stress, while at the same time achieving high conversion using the same procedures clinicians use to polymerize composites in practice [16–19]. The additional potential to minimize the mismatch with the inorganic filler phase in dental composites has not been investigated thus far.…”

Linear polymerization shrinkage of Bulk Fill resins

“…The volumetric shrinkage, which is a result of the development of an internal stress, occurs largely in a range of 1.5 to 5%, as product of the rigid nature of the cross-linked polymeric matrix that is formed 3 . The formation of this stress implies in the imperfection of the margins of the restorations with composite resin, which affects significantly its clinical longevity 3 .…”

“…The formation of this stress implies in the imperfection of the margins of the restorations with composite resin, which affects significantly its clinical longevity 3 . The possible partial transmission of this shrinkage stress to the adhesive interface compromises the marginal integrity and results in postoperative sensibility, as well as in a greater susceptibility of micro infiltration of the restorations 4 .…”

“…Thus, it has been previously reported that the stress generated by the polymerization shrinkage directly triggers clinical consequences in the restoration 5 . Therefore, several studies have been carried out to evaluate the measurement, generation and characterization of this stress, as well as the improvement of the physical, chemical and mechanical properties of the composites 3,4 . Increasingly importance has been given to the development of novel composite resins, especially the Bulk Fill composites, which has shown promising results.…”