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[Three-dimensional change of the occlusal facets in primary dentition before and after the eruption of first permanent molars].

A system to measure the three-dimensional architecture of the occlusal surface was developed to clarify the three-dimensional change of the morphology of occlusal facets that was considered as the trace mark of the chewing function. The three-dimensional measuring and analyzing system of the occlusal surface was composed of two components, the measuring system to obtain the three-dimensional coordinates on the surface contour of the tooth crown and the analyzing system to calculate the minute morphology in the three dimensions. The reproducibility of this system was evaluated with use of the standard block of stainless steel designed for this purpose and was shown to be very accurate. By application of this system, the three-dimensional changes of the occlusal facets on primary canines and primary second molars were investigated on the serial primary dental casts taken before and after the eruption of the first permanent molars from eleven children (seven boys and four girls). The results were as follows: On the primary second molars of the maxilla and mandible, before the eruption of the first permanent molars, the occlusal facets were distributed mainly on the functional cusps. The occlusal facets were found also on the non-functional cusps after the eruption of first permanent molars. As a consequence, the increase in the number of occlusal facets from before to after the eruption of first permanent molars was statistically significant for both maxillary and mandibular primary second molars. In regard to the morphological change in three dimensions, the occlusal facets that were facing mesially or distally before the eruption of first permanent molars changed its direction buccally or lingually and became almost parallel to the datum plane of the mandibular dental arch after the eruption of the first permanent molars. It was also found that the occlusal facets of both jaws in the occlusal relationship were almost parallel to each other. In addition, the amounts of angular change before and after the eruption of first permanent molars were significantly correlated between the maxillary and mandibular occlusal facets. It can be concluded that the coordinating change of the occlusal facets in three dimensions may exist between both jaws during the development of chewing function.