第三大臼歯の発育に関する臨床的観察

Clinical observations on the development of third molars.

抄録

日本人の第三大臼歯の発育状況を調査するために、大阪歯科大学附属病院小児科外来において、7歳0ヵ月から16歳11ヵ月までの小児9,111人（男児4,646人、女児4,465人）の歯科治療中に撮影されたパノラマX線写真と、本学口腔放射線科が保管する本学学生2,769人（男児2,312人、女児457人）のパノラマX線写真を調査した。以下の結果が得られた。1.第3大臼歯の石灰化は男女とも上顎で7歳6ヵ月、下顎で7歳0ヵ月と早く始まっていた。上顎の平均石灰化開始年齢は男子9歳4ヵ月、女子9歳2ヵ月、下顎はそれぞれ9歳1ヵ月、8歳9ヵ月であった。2.2.第3大臼歯の歯冠完成の平均年齢は、上顎では男子が11歳8ヵ月、女子が11歳5ヵ月、下顎ではそれぞれ12歳4ヵ月、12歳3ヵ月であった。3.3.13歳以上で歯胚が確認された割合は、男子が上顎で約70％、下顎で約75％であったのに対し、女子はそれぞれ65％、80％であった。4.4本の第三大臼歯がすべて存在するのは、男子の52.3％、女子の45.5％であったのに対し、第三大臼歯がまったく存在しないのは、男子の9.5％、女子の12.0％であった。5.萌出方向は垂直型、中間型、水平型、遠位型、頬舌型のいずれかに分類された。男性では上顎で70％、下顎で45％、女性ではそれぞれ50％、40％であった。6.第3大臼歯の小歯列形成は上顎にのみみられた。7.第3大臼歯の先天性欠如は男性よりも女性に多く、下顎よりも上顎に多くみられた。本研究の結果は、日本人の第三大臼歯に関する理解を深め、小児臨床における咬合誘導などの計画立案の一助となるものである。

In order to investigate the developmental conditions of third molars in Japanese, we studied the panoramic radiographs taken during dental treatment at the Pediatric Outpatient Section of Osaka Dental University Hospital of 9,111 children (4,646 boys and 4,465 girls) between the ages of 7 years 0 months and 16 years 11 months, in addition to 2,769 panoramic radiographs of students of this university (2,312 men 457 women) kept by the Department of Oral Radiology. The following results were obtained. 1. Calcification of the third molars in both boys and girls began as early as 7 years 6 months in the maxilla and 7 years 0 months in the mandible. The average age for initiation of calcification in the maxilla was 9 years 4 months for boys, and 9 years 2 months for girls, while the ages in the mandible were 9 years 1 month and 8 years 9 months, respectively. 2. The average age for completion of the third molar crowns in the maxilla was 11 years 8 months for boys and 11 years 5 months for girls, while in the mandible it was 12 years 4 months and 12 years 3 months, respectively. 3. At greater than 13 years of age, the tooth germ could be found in boys about 70% of the time in the maxilla and 75% of the time in the mandible, while these figures for girls were 65 and 80%, respectively. 4. All four third molars were present in 52.3% of the males and 45.5% of the females, while 9.5% of the males and 12.0% of the females had no third molars at all. 5. The direction of eruption was classified as either vertical, mesial, horizontal, distal, or buccal/lingual type. The rate for the vertical type in males was 70% in the maxilla and 45% in the mandible, while these figures for females were 50 and 40%, respectively. 6. Microdontia of the third molars appeared only in the maxilla. 7. Congenital absence of the third molars was more common in females than males, and occurred more frequently in the maxilla than in the mandible. The results of this study allow a better understanding of the third molars of Japanese and provide an aid to planning in such areas of clinical pediatric practice as guiding occlusal development.