下顎歯槽部基底上に萌出した歯が安定できる後方限界の研究

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タイトル別名
  • A DETERMINATIVE STUDY OF POSTERIOR LIMIT IN LOWER JAW THAT PERMITS A STABLE ACCOMODATION OF ERUPTED DENTITION WITHIN THE APICAL BASE
  • 下顎歯槽部基底上に萠出した歯が安定できる後方限界の研究
  • シタアゴ シソウブ キテイ ジョウ ニ ボウダシタ ハ ガ アンテイ デキル コウホウ ゲンカイ ノ ケンキュウ

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The term "discrepancy" in the field of orthodotics denotes such case of malocclusion in which the magnitude of apical base fails to provide a stable accomodation for the full dentition. Although routine treatment of such malocclusion consists of surgical extraction of one or more teeth, there has been no definite criterion as to what is the true extent of the discrepancy in each individual case. In evaluating the extent of discrepancy in any patient, one must first of all know relative sizes of both the apical base and the existing dentition in the jaw. The size or area occupied by the dentition may well be determined by measuring the distance between mesio-distal ends of the dentition. With regard to the size of apical base, however, measurement should be done on the extent of area to be occupied by every member of the dentition. The limit of area occupied by teeth in the anterior and lateral parts of jaw is physiologically determined in accordance with state of balance among muscle forces surrounding the teeth. However, in the posterior part of jaw, especially in the lower jaw, there apparently lacks any physiological landmark or muscle force to demarcate the posterior limit of the hindermost tooth and thus makes it difficult to get accurate diagnosis of discrepancy. The present study was undertaken to establish the posterior limit of lower jaw within which could be placed all the erupting teeth with stability. In this connection, we had previously noticed on the soft tissue behind the hindermost molar the presence of a boundary line which served to demarcate the mucous membrane into the immovable (gingival part) and movable (muscle part), and we designated the line as linea terminalis posterior gingivae. The present study was directed to know how this line was constituted and whether or not it had any significance in determining the posterior limit for accomodating the correct dentition. Results obtained are summarized as follows. 1. Histological studies also showed that the linea terminalis posterior gingivae served an approximate junction between the immovable and movable mucous membrane lining the soft tissue behind the hindermost tooth of lower jaw. 2. Possible change of this line with increasing age of subjects was investigated on the serial gnathostatic models taken from the patients from 6 to 15 years of age, and distances from Point b to the line were measured. The result showed that the yearly increase of the distance was approximately 1 mm in both sexes of the subjects. 3. Hardly any shift of position of the line was observed to occur even by physiological change of surrounding tissues such as eruption of hindermost molar and its mesial migration. 4. It was confirmed that when the line was located 4 to 5 mm far behind the distal side of the hindermost molar to erupt, the latter tooth could be expected to make normal eruption. These evidences pointed to the conclusion that the linea terminalis posterior gingivae could serve as a criterion to determine the posterior limit in the measurement of the magnitude of apical base of lower jaw and that any stable eruption of the hindermost molar should come 4 to 5 mm far mesially from this limit.

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