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Optimal specimen geometry in bonded-disk shrinkage-strain measurements on light-cured biomaterials
1University of Manchester Dental School, Higher Cambridge Street, Manchester M15 6FH, UK. david.watts@man.ac.uk
Objectives:
The aim was to determine the effect on apparent shrinkage-strain values of varying specimen aspect ratio in the bonded-disk configuration. Thereby one source of possible inter-laboratory variation might be understood and eliminated.
Methods:
The bonded-disk procedure was followed and applied to three representative resin-composites. However, specimen sub-groups were examined, each of different diameters (3.5, 5.0, 7.0 and 9.0 mm), corresponding to C-factors in the range 0.45-7.0.
Results:
For each material, no statistically significant difference was apparent between data obtained with 7 and 9 mm diameter specimens. However, statistically significant reductions (p < 0.05, Scheffé and SNK) were apparent at 5.0 and 3.5 mm diameter, in the range 14-18% of the maximum 'true' value. For each material, the diameter (d)-dependence of apparent shrinkage-strain (epsilon) was given by the curve fit expression: epsilon = A - B/d2.
Significance:
For accurate determination of maximum final equilibrium shrinkage-strain values, a high aspect ratio (7-9:1) of the bonded-disk should be utilised. This is important for accurate comparison of data from proposed low-shrinkage amalgam-replacement materials.