Abstract
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.
