Optimal specimen geometry in bonded-disk shrinkage-strain measurements on light-cured biomaterials

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 (ε) was given by the curve fit expression: ε=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.