ANALYSIS OF STRAIN-GAUGE METHOD FOR MEASUREMENT OF POST-GEL SHRINKAGEIN RESIN COMPOSITES

Objective. The objective of this study was to refine a strain gage met hod for measuring polymerization contraction of resin composites and t o isolate the net post-gel contraction by identifying factors contribu ting to the measured strains. The hypothesis to be tested was that car efully controlled strain gage measurements of composite polymerization could isolate post-gel contraction events. Methods. Composite was pla ced on a biaxial strain gage and light-cured. This method enabled real -time registration of the progress of shrinkage strain, corresponding to elastic modulus development. Strain from the two axes of the strain gage were averaged and plotted as a function of time. A representativ e curve was calculated from the mean of ten measurements. The followin g factors influencing the total contraction measurement were evaluated : thermal expansion of the gage, thermal expansion of the composite du e to the exothermic reaction and exposure to the curing light, and adh esion of the composite to the gage. These parameters were measured so that the net deformation of the composite during polymerization could be calculated. Results. Parametric studies of pre-cured and photointia tor-free materials confirmed the hypothesis that strain gages measure post-gel contraction. Thermal artifacts were measured and subtracted f rom the total strain output. Significance. Strain gages are suitable f or measuring the clinically significant phase of composite polymerizat ion contraction.