Correlation of bottom-to-top surface microhardness and conversion ratios for a variety of resin composite compositions

Knoop microhardness (KHN) and degree of conversion (DC) values were obtained from the bottom and top surfaces of 1-, 2- and 3-mm thick samples of three types of resin composite: an anterior microfill, an anterior hybrid and a posterior hybrid, all having differing filler size and loading but similar shade (A2) and basic monomeric content. Sample infrared spectra were obtained using attenuated total reflectance (ATR) in a Fourier transform infrared (FTIR) spectrometer. The samples were exposed using a 40-second exposure to a quartz-tungsten-halogen light source with an irradiance of ≈ 560 mW/cm ². They were stored for 24 hours in complete darkness at 37°C and 100% humidity prior to obtaining cured spectra and KHN readings. KHN and DC values were obtained from the same sample specimen made at similar surface depths, but separate groups were made for obtaining top and bottom values. Cure and hardness data were analyzed with one- and two-way ANOVAs followed by the Tukey-Kramer post-hoc test. Linear regression was also applied. Statistical testing was performed at a pre-set 0.05 level of significance. KHN and DC were significantly different according to composite type and depth (p=0.0001), with an interaction effect (p=0.0022). KHN, DC and corresponding bottom/top surface (B/T) ratios decreased with depth. Regression revealed a linear relationship between DC and KHN for each composite type, with no r² less than 0.96. When B/T ratios were correlated, a B/T-KHN ratio of 0.80 corresponded to a narrow range of B/T-DC ratios (between 88 to 91%) for the three composites tested. When combining results from composite types, linear regression of B/T-DC and B/T-KHN produced a very predictable relationship (r²=0.959), for which a B/T-KHN ratio of 0.80 corresponded to a B/T-DC ratio of 0.90. As a measure of completeness of conversion, B/T-KHN was approximately 2.5 times more sensitive than the B/T-DC ratio. In summary, while KHN cannot be used to directly compare conversion of the different composites tested, the use of B/T ratios for both hardness and conversion resulted in a linear relationship independent of filler size or loading.