An accurate reproducibility of the jaw positions of dental patients on an articulator is prerequisite for the manufacture of various prosthodontic materials and functional analysis of their occlusion. In reproducing the jaw positions on an articulator, various checkbites are used to register necessary interocclusal records for the purpose. Our interest is directed to the fact that, when different materials are employed for checkbites, what kind of premature contacts would occur at centric position of the mandibular casts as well as positional difference from centric relation to centric occlusion, and also a possible effect of the Myo-monitor used to measure the myo-centric position at centric occlusion.
In an effort to study these points, the author was concerned with a series of basic experiments and also with a female dental student at Nihon University School of Dentistry, who was free from obvious systemic complaints and was in possession of normal dental arch and occlusion except for the third molars, having no particular abnormalcy in the gnatho-oral region.
With this subject, occurrence of premature contacts, occlusal contact surface at centric occlusion, amount of cast movement from premature contact at centric relation to centric occlusion and the variation of myo-centric position in association with the use of Myo-monitor.
As a result of the study, the author arrived at the following conclusions:
1. As regards the occlusal contact area, Xanthano in basic experiments and a hard plate wax in a living subject were found to have the least amount of variation.
2. In terms of the cast movement from the occurrence of premature contacts at centric relation to centric occlusion, Uni-fast registered the minimum amount of variation of the materials examined, being true not only of basic experiments but also of a living subject.
3. Uni-fast, which had the least variation in a living subject, was mounted on an articulator and the movement from centric relation to centric occlusion was measured. As a consequence, it was found that, at a hinge point, the movement on the right was 0.27mm on the left.
4. The occlusal contacts by use of the Myo-monitor concerning the myo-centric position gave different values at each checkbite-taking, indicating that this was the jaw position lacking in an accurate reproducibility.
When the movement from myo-centric to centric occlusion on an articulator at a hinge pin point was measured, it was found that myocentric position was located anteriorly to centric occlusion by 0.78mm on the right and 0.18mm on the left.
5. From a clinical point of view, when the findings regarding the prernature contacts, occlusal area at centric relation and the amount of movement from the premature contacts at centric relation to centric occlusion were comparatively examined, it was confirmed that, on taking a checkbite, it was more important to familiarize oneself with the manipulation of a material in use rather than its selection.
But, since the Myo-monitor was found to be lacking in an accurate reproducibility of the jaw position, the author concludes that a myo-centric position is not appropriate to be used as a jaw position for the prosthodontic treatments.

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The aim of this study was to disclose the variation in the surface EMGs recorded from electrodes put on different positions on the masseter and temporal muscles and read on different days.
Seven male subjects with normal tooth alignment and occlusion were studied. For the power spectrum analysis, digital correlator and spectrum analyzer were used.
1) On the relative power spectra, the electrode positioning on the corresponding muscles hardly affected the EMG spectral distribution.
2) Whereas the electrode positioning was found to manifestly affect the root mean square values (rms) of the voltage of the EMG activities.
3) When records were on different days, no great changes in spectral distribution were found. The rms voltage of EMG made a little difference, however, it was much smaller than the variation due to the electrode position.
4) Although significant difference was shown between individual subjects, the masseter and temporal muscles appeared to have respectively charactaristic spectral distributions, which were affected by the functional states of muscle contruction, i. e. chewing and clenching.
5) The EMG power spectra of masticatory muscles might be refrected in their quantitative changes, and could obtain more accurately reproducible result than other parameters.

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A new measuring method for mechanical pressure was recently developed by Fuji Film Company and was quickly introduced into the dental profession. It consists of two sheets, each having a different formation. When these sheets are pressed together, one of them develops a red color pattern measuring the density in proportion to the intensity of pressure exerted. At present three types of sheets are available-low, intermediate and highpressure sheets.
A study was made to determine the effects of this method on measuring pressure. The results indicated that pressure from 8 to 450 kg/cm² can be measured with an accuracy of 2.0 kg/cm² in low-pressure sheet, 8.0 kg/cm² in intermediate and 20.0 kg/cm² in high and that humidity had to be precisely controlled during the measurement. For instance, the same loading of 44 kg/cm² against low-pressure sheet brought various values of color density from 0.94 (22.0°C, 50%) to 1.35 (23.5°C, 83%) in accordance with the grade of humidity.
The sheets didn't keep their original color density, but changed, espcially within the first 5-10 days after loading. During this change there was a specific density area near 1.25 where there was only a slight change in its density. Density thicker than this value became thinner and density thinner than 1.25 became thicker. After this changing period the sheets kept their concentration for 4 months.

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