Fundamentals of Materials Science

Statement of problem. New luting agents, particularly with adhesive capability, are being introduced in an attempt to improve clinical success. Independent studies of basic comparative data are necessary to characterize these materials in relation to mechanical and physical properties. Purpose. The purpose of this study was to compare the flexural strength, modulus of elasticity, and radiopacity and pH of representatives of 5 types (categories) of luting agents. Material and methods. The luting agents included a zinc phosphate, a conventional and a resin-modified glass ionomer, 2 dual-polymerizing resins ("photopolymerized" after mixing and "unphotopolymerized" conditions), and an auto-polymerizing resin. The specimens were prepared and the testing was conducted by 1 person to maximize standardization. Flexural strength (MPa) and modulus of elasticity (GPa) were determined on bar-shaped specimens (2 × 2 × 20 mm) at 24 hours and 3 months (n = 8). Radiopacity (mm Al) was measured by exposing 1 mm thick specimens along with an aluminum step wedge (n = 4). pH was measured using a pH electrode immediately after mixing; at 1, 5, 15, 30 minutes; and at 1, 2, 4, 6, and 24 hours (n = 4). The data were subjected to statistical analyses with analysis of variance and Duncan's multiple range test ( P Results. The resin luting agents (64 to 97 MPa) showed higher flexural strength than all other materials tested (7 to 27 MPa), with the "photopolymerized" (83 to 97 MPa) conditions higher than "unphotopolymerized" (64 to 81 MPa) ( P P P P P Conclusion. Within the limitations of this study the data showed a wide variation of material properties. The dual-polymerization resin luting agents tested showed the best combination of mechanical and physical properties combined with the highest setting pH. Photopolymerization of these resin-based materials was necessary to maximize strength and rigidity. (J Prosthet Dent 2003;89:127-34.)

Mechanical and physical properties of contemporary dental luting agents

The fracture toughness of a variety of dental composites was evaluated using notched bending specimens, with and without pre-cracks. The pre-crack simulates more accurately a sharp, natural flaw in a material than does a notch, and is standard procedure in fracture-toughness evaluation. The fracture toughness was related to the filler composition and degree of conversion in the composite resins. In general, fracture toughness was highest in the more heavily filled resins, independent of degree of conversion in the matrix. The results also show that producing a pre-crack in certain composites reduced the value of their fracture toughness compared with notched-only specimens.

Variables Affecting the Fracture Toughness of Dental Composites

This article reviews recent studies on: (1) the synthesis of novel calcium phosphate and calcium fluoride nanoparticles and their incorporation into dental resins to develop nanocomposites; (2) the effects of key microstructural parameters on Ca, PO(4), and F ion release from nanocomposites, including the effects of nanofiller volume fraction, particle size, and silanization; and (3) mechanical properties of nanocomposites, including water-aging effects, flexural strength, fracture toughness, and three-body wear. This article demonstrates that a major advantage of using the new nanoparticles is that high levels of Ca, PO(4), and F release can be achieved at low filler levels in the resin, because of the high surface areas of the nanoparticles. This leaves room in the resin for substantial reinforcement fillers. The combination of releasing nanofillers with stable and strong reinforcing fillers is promising to yield a nanocomposite with both stress-bearing and caries-inhibiting capabilities, a combination not yet available in current materials.

Iannetta Rv

Papers

The fracture toughness of dental composites. I. The development of strength and fracture toughness.