This study was designed to investigate the position of anteroposterior center of resistance for genuine intrusion and the mode of change of the minimum distal force for simultanous intrusion and retraction of the upper and lower incisors according to the increase of labial inclination. For this purpose, we used the three-piece intrusion arch appliance and three-dimensional finite element models of upper and lower incisors.
1. Positions of the center of resistance in upper incisors according to the increase of the labial inclination were as follows; 1) In normal inclination situation, the center of resistance was located in 6mm behind the distal surface of the lateral incisor bracket.
2) In 10° increase of the labial inclination situation, the center of resistance was located in 9mm behind the distal surface of the lateral incisor bracket.
3) In 20° increase of the labial inclination situation, the center of resistance was located in 12m behind the distal surface of the lateral incisor bracket.
4) In 30° increase of the labial inclination situation, the center of resistance was located in 16m behind the distal surface of the lateral incisor bracket.
2. Positions of the center of resistance in lower incisors according to the increase of the labial inclination were as follows; 1) In normal inclination situation, the center of resistance was located in 10mm behind the distal surface of the lateral incisor bracket.
2) In 10° increase of the labial inclination situation, the center of resistance was located in 13mm behind the distal surface of the lateral incisor bracket.
3) In 20° increase of the labial inclination situation, the center of resistance was located in 15m behind the distal surface of the lateral incisor bracket.
4) In 30 increase of the labial inclination situation, the center of resistance was located in 18m behind´ the distal surface of the lateral incisor bracket
3. The patterns of stress distribution were as follows;
1) There were even compressive stresses in and periodontal ligament when intrusion force was applied through determined center of resistance.
2) There were gradual increase of complexity in compressive stress distribution pattern with increase of the labial inclination when intrusion and retraction force were applied simultaneously.
4. With increase of the labial inclination of the upper and lower incisors, the position of the center of resistance moved posteriorly. And the distal force for pure intrusion was increased until 20° increase of the labial inclination.
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