The development of fracture toughness and fracture strength in posterior restorative materials

Abstract

The changes which take place in the fracture toughness (K IC), the fracture stress and the inherent flaw size with storage time (in water at 37°C) have been determined for three commercial products, all which have been suggested as alternatives to amalgam. Their performances have been compared with each other, and with other relevant restorative materials. The fracture toughness of the composite Occlusin rises rapidly to a high value which is maintained. Though the fracture toughness of the composite P30 peaks at a value greater than that for the selected reference amalgam, it falls considerably and progressively after this peak. The fracture toughness of the silvercontaining glass ionomer (Ketac Silver) is no greater than a conventional glass ionomer and inferior to amalgam. The inherent flaw sizes are similar for all composites, rising to become constant in time. By contrast the sizes of the inherent flaws in glass ionomers and amalgam fall in time towards constancy. Relative values and time dependencies for fracture stresses and fracture toughnesses show apparent similarities but differences do exist, which may be important to clinical performance. The main conclusion is that the fracture toughness of posterior composite varies with time and product, in one case comparable only to the lower end of the range for amalgams but in the other case substantially greater than any amalgam. Thus, a posterior composite can possess a fracture toughness in excess of that for amalgam and, on this criterion, is an acceptable substitute.