BITE FORCES AND HARDNESS PERCEPTION IN PRODUCTS PRESENTING PLASTIC-DEFORMATION

The oral perception of the hardness of plastic deformable products was investigated. Differential sensitivity was studied over a large range of hardness during psychophysical tests in which bite forces were rec orded with small intra-oral load cells. The mechanical properties of 2 1 products obtained from dental waxes and modelling clays were measure d. They covered a stress range from 19N/cm(2) to 265N/cm(2) when the p lastic deformation plateau was reached. For psychophysical experiments , these products were divided into four series according to their hard ness. For each series, a differential threshold was obtained for 10 in dividuals who were asked to make hardness comparisons by biting the sa mples with the incisors. During each bite, forces were recorded from a n intra-oral load cell placed beneath the sample. Differential thresho lds varied between 6 and 13% depending on the series. For the 21 produ cts taken together, a significant correlation (R = 0.98) was found bet ween bite forces and stresses. Furthermore, significant correlations w ere also found within each series of comparisons except for the series of the softest products. Bite forces appeared therefore to play the m ain role in hardness perception of products exhibiting plastic deforma tion, except for the softest products for which developed forces might not correspond to the normal field of forces activated during mastica tion.