Photo-polymerization shrinkage-stress kinetics in resin-composites: methods development.

OBJECTIVES: Studies of free shrinkage-strain kinetics on restoratives have begun to multiply. However, there have been fewer investigations of the more difficult problem of concurrent stress-kinetic measurements. The aim was to outline design parameters for a new methodology for this problem, amenable especially to light-cured materials, and to present illustrative results for a range of restorative composites.
METHODS: Absolute values of stress measurable for a given material and geometry are dependent upon the stiffness of the measurement system. In an infinitely stiff system, the measured stress would also tend towards infinity. Real teeth and their cavities are not infinitely stiff; they have elastic and visco-elastic compliance. Consequently, it is important that some minimal, but essentially constant compliance be allowed, whatever the final or time-dependent modulus of the material may be. This goal has been realised by measurement of the time-development, for a disk-geometry specimen (phi=10, h approximately 1.0 mm) of stress (S(r)), with a calibrated cantilever beam-geometry load cell. A novel specimen-holder design was used for this purpose, held in a rigid base assembly. Specimen thicknesses (or gap-widths) of 0.8 and 1.2 mm were specifically investigated on four representative resin-composites. Concurrent measurements were made of the end-displacement of the cantilever load cell, relative to a lower glass plate retaining the specimen.
RESULTS: Load-calibration of the cantilever load cell gave an end-displacement per unit stress of circa 6 microm/MPa. This compares with literature values for cuspal compliance or displacement of circa 20 microm. Re-normalisation of the stress-data was implemented. This was accomplished assuming Hooke's law behavior at each instant and equivalent to a stiffer system, with a correction (multiplier) factor of 4 on the raw-stress values. For the materials examined, resultant maximum-stress levels determined were circa 5-8 MPa Stress-levels obtained at 1.2mm thickness were slightly higher (11-15%) than the level of stress obtained at 0.8 mm thickness. This is attributable to the greater mass of material undergoing shrinkage at 1.2 mm, offset slightly by the different C-factors. SIGNIFICANCE: The new device is a practical and self-contained system for rapid and accurate measurement of stress-kinetics in photo-polymerising and also self-cure materials.