天然歯の中心線と重心線との関係

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When occlusal force applies to natural teeth, and if the force applies in the direction of the longitudinal axis of the tooth root, the stress born by the tooth root is maximal. However, if the occlusal force deviates from the direction of the longitudinal axis and lateral force adds, the stress bearing ability considerably decreases and the tooth life is said to be shortened. Teeth are stabilized when the relation of contact with the neighboring teeth which form the dentition is maintained. However, when a tooth becomes isolated, its ability to withstand functional stress is said to decrease considerably. It was considered important to examine the relation between the tooth crown longitudinal axis and tooth root longitudinal axis which helps to clarify the foregoing matter. Of longitudinal axis, attention was focused on the gravity line of tooth, and mesial and distal edge line angle and root apex on the labial (buccal) side were designated on about the same plane and this plane was designated as the base line. The section perpendicular to this base line was recorded at three points each at the crown and the root portions, and the gravity point and center point were measured on this section. From these, the gravity line and center line were determined. Relations between the gravity point and center point and between the gravity line and center line from the upper and lower central incisors to second premolars were examined. The results were as follows : 1. The mesial and distal angles of the clinical crown axis to the middle line for central incisors, lateral incisors, canines, first premolars, and second premolars in the upper jaw were 1.0±2.2°, 2.8±4.2°, 4.6±3.7°, -0.9±4.0°, and 0.9±4.0°, respectively. In the lower jaw, the figures were -0.7±3.8°, -0.6±4.0°, 4.3±2.5°, -0.5±4.9°, and 1.2±3.7°, respectively. 2. Gravity point versus center point showed about the same tendency buccolingually. At the cervical portion, the gravity point and center point about coincided and were always positioned buccally from the center point at other portions. 3. The gravity point for the upper anterior teeth was positioned mesially from the center point, and for other teeth distally. In every case, the gravity point and the center point about coincided near the cervical portion. 4. The directions of the center line and gravity line at the tooth crown portion and those of the center line and gravity line at the tooth root portion did not completely coincided for the anteior teeth through the premolars. 5. The angle of the tooth root gravity line to the tooth crown gravity line mesiodistally was about 180° for the upper central incisors, lateral incisors, and lower central incisors, but was over 180° for other teeth. The tooth root gravity line was mesially inclined to the tooth crown gravity line. 6. Functional stress exerted on the tooth crown was dispersed vertically and mesially to the tooth root and was presumed to have resulted in a force causing the tooth to incline mesially.
1993-02-25

著者

伊藤文隆
九州歯科大学歯科補綴学第1講座

天然歯の中心線と重心線との関係

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