In the cases of dental orthodontic treatment, in which the jaw must be moved, relapses after treatment become problematical. Recently the question of adaptability of the maxillo-facial muscles has been increasing in importance. Considering that relocating the insertion part of the muscle by surgical operation, as is done as part of an operation, when the muscles do not readily adapt themselves to a newly created oral environment, might achieve a similar state where the muscles adapted themselves, the author undertook the experiment. Twenty-two mature male dogs weighing about 10kg each, which were thought to have ceased growing, were used for the experiment. The insertion part of the masseter muscle on each side together with the periosteum was detached from the mandible. These animals were classified into the following 9 groups : 1) Non Bite-raising Group (30th day, 90th day, 180th day) : 2) Bite-raising Group (30th day, 90th day) : 3) Bite-raising Group, for which the bite-raising plate was used for 3 months and then removed, (30th day, 90th day, 180th day) : 4) Control Group. The subjects in each group were killed at the end of the experiment periods and examined histologically and diametrically. The following conclusions were obtained as the result : 1) When bite-raising was done, vertical dimension was increased and regressive changes were observed in the masseter and temporal muscles. The detached muscle was connected with the mandible by connecting tissue at a position slightly above the former location in one month. On the surface of the mandible at this location, bone apposition was formed. The state of healing of the ablation part was slightly delayed compared with that of the Non Bite-raising Group. 2) When the bite-raising plate was removed after conducting bite-raising, extend of relapses was greater in proportion to the increase in vertical dimention. Increase in vertical dimention in the end, compared with that prior to the experiment, was greater although it was of a slight degree. With regard to the masseter muscle, complete normalization could not be seen even after 6 months. 3) The fact that changes in the masseter muscle is slight, compared with those of bite-raising without performing ablation, appears to be indicate that bite-raising combined with periost-ablation at the insertion part has potentiality of restraining relapses after bite-raising in the cases which have ceased to grow. Because the factors that affect relapses also include the jaw closing muscles and others, it is necessary to expedite researches on these factors in the future.
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