Critical configuration analysis of four methods for measuring polymerization shrinkage strain of composites

Objectives. To compare four methods for measuring polymerization shrinkage strain of composites and to develop a rational basis for comparing data from different methods and laboratories. Methods. Dilatometry, modified bonded disk, strain gage, and a new linear transducer method were used to measure polymerization shrinkage strain of a model composite under similar irradiation conditions. The resin consisted of an untinted resin (50:50 BISGMA/TEGDMA, 0.7% CQ, 0.35% DMAEM, 0.05% BHT) filled with 5% fumed silica and 67wt% untreated hybrid filler. Specimens (n=10) were exposed for 60 s at 600 mW/cm² and then monitored for 300 s. Specimen volumes were 8 mm³ for the strain gage method, 25 mm³ for the linear transducer and dilatometer methods and 43 mm³ for the bonded disk method. The degree of constraint applied to the specimens by each method was calculated and compared. Shrinkage strain values at 60 and 300 s were tested for significance at p=0.05 using one-way ANOVA and Tukey's test. Results. Shrinkage strain magnitudes at 60 and 300 s for the four methods were significantly different (p<0.01). The modified bonded disk method measured the highest shrinkage value and exhibited the highest degree of specimen constraint. There was a 5 s delay after light activation before strain was detected by the strain gage. Significance. Specimen constraint differed in all four methods and was linearly correlated with shrinkage strain magnitude when the degree of constraint was less than 42%.