A number of methods have been so far developed for the evaluation of human masticatory efficiency. Of these the most acceptable method seems to be the one based on the principle of screen analysis of solid particles of any material as crushed by a definite number of masticatory strokes. Such analysis, however, of particle size distribution of crushed material on screens of successive meshes naturally requires to be expressed in terms of more than one variable and makes it most difficult to evaluate the masticatory efficiency by means of any single numerical value. The use of a single mesh screen for sifting out the crushed particles, as proposed by Manly and others, for an evaluation of the masticatory efficiency also seems to have only very limited application. The use of several successive meshes of screen in an experiment, on the other hand, as proposed by Ishihara and by Ueda, has also disadvantage that it fails to elicit a common numerical value to express the efficiency of an individual mastication, and hence is inconvenient for the comparative study. In the present study, an attempt was made to obtain some significant information for estimation of the masticatory efficiency by calculating the net energy input required for the crushing of a material both in a specially designed mechanical apparatus and by the mouths of human subjects. The test material used for the crushing experiments was the same with the synthetic tablet as devised by Sato in this laboratory. The tablet was of a cylindrical form, 5mm in diametr and 3mm long, consisting of the mixture of equal volume of barium sulfate and Carnauba wax. For the purpose of model experiments, ten grams of the test tablet were placed in a steel mortar, and an iron ball weighing 2.7 kilograms was allowed to drop directly on the tablets one to four times to reduce them into respectively smaller sized particles. The mortar was held upon the base of hardened steel, and between the bottom of the mortar and the steel base were placed 3 pieces of aluminum cushion wire 1 cm long, being arranged radially with an equal angle of 120 degree between each other. These metal wires served to prevent the ball from bouncing at the drop by absorbing the impact, and furthermore served to provide a means to calculate an energy loss by measuring the amount of deformation produced on the wires by the impact. The net energy input required for the crushing of the tablets could be obtained in the following manner. The crushed material was packed in the cell of an air permeability apparatus, and compressed air was allowed to pass through the cell. Increments of the specific surface area of the material could be calculated from the pressure drop according to the equation as developed by Kozeny-Carman. The experiments conducted by the above procedures confirmed the usefulness of the test tablets in that there existed a first order proportionality between increment of the specific surface area and net energy input as evidenced in many previous experiments on other brittle substances. Similar experiments conducted on the test tablets after they were allowed to be mast-icated by human subjects also confirmed the applicability of this procedure to obtain a quantitative measurement of the masticatory efficiency in terms of net energy input. Since the above experiments using the air permeability apparatus were too time-con-suming to be applied clinically, an attempt was made to correlate the energy input data thus obtained to data obtainable by the conventional sieving method. For this purpose, a total of 22 subjects ranging in age from 10 to 55 with varying dental conditions from normal to denture wearer were selected as experimental subject. The test tablets were allowed to chew for these subjects, and the resulting particles of the tablets were analyzed by the two methods. Results showed that a significant correlation existed between 8-mesh oversize
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