Analysis of stress relaxation in temporization materials in dentistry

Although temporization is intended as an interim step, complexity of individual treatment situations may demand medium to longer term use of temporary appliances in clinical practice. The durability and integrity of these restorations for continued use to meet the treatment demands is therefore an important clinical problem. The goal of this study was to evaluate the short to medium term stability of these materials under controlled loading to study their stress relaxation behavior.

Acrylic resins (poly(methyl) and poly(ethyl) methacrylate) and bis-acryl composite resins were tested in vitro in this study. The stress decay data with time (under an applied constant strain) due to internal strain caused by molecular relaxation were systematically analyzed using important parameters derived from stress changes with time.

The results showed significant differences in the stress relaxation behavior between different materials which may have significant bearing on their durability in medium to longer term interim clinical applications. Poly(ethyl) methacrylate (PEMA) resins subjected to applied constant strain over a period of time showed large time dependent decay of applied stress, indicating very high internal molecular relaxation effects, relative to those of poly(methyl) methacrylate (PMMA) and bis-acryl composites. The results showed that PMMA and composite resins were superior in their ability to maintain constant strain without excessive dissipation of applied stress than PEMA resin. This suggests that internal strain caused by molecular relaxation events may lead to excessive dimensional instability in PEMA.