[A three-dimensional 3-D finite element study on direction of maxillary protraction].

PURPOSE: To offer a scientific basis on the direction of maxillary protraction by establishing and using three-dimensional finite element model (FEM) of cranio-maxillary complex.

METHODS

Using thin-slice high resolution CT scanning and digital image transfer and transcription, in combination with Mimics 10.0 software, a three-dimensional FEM of cranio-maxillary complex of Class III malocclusion(maxillary retrusion), whose modeling sample was a 9 years old female, was established. Then 8 directions between -20° and 50° against horizontal plane of anteriorly-directed extraoral forces (5N) were added in the area of maxillary canine. The displacement and stress distribution in the cranio-maxillary complex were analysed by using ANSYS 10.0 software.

RESULTS

Vertically, when the protraction angle was less than 30°, the cranio-maxillary complex showed anticlockwise rotation. And the displacement showed decrease with increasing of angle. When 30° to 40°, the perpendicular displacement of any marker point was zero. When the protraction angle was greater than 40°, it showed clockwise rotation. Horizontally: When the protraction angle was between -20° and 50°, the maxillary moved forwards. And the displacement got its top when the angle was zero. Medially, the cranio-maxillary complex and maxillary dental arch had the trend of medial movement. With the angle increasing, the displacement was decreased, and the major principal stress was increased. Von Mises stress was decreased and then increased. When the protraction angle was greater than 10°, these stresses showed increase.

CONCLUSIONS

When the protraction angle is between 30° and 40°, the cranio-maxillary complex shows movement without any rotation. The risk of deep -bite may be present while the angle diminished. On the other hand, there is the risk of open-bite. The greater the protraction angle, the less of the craniomaxillary complex advances, but the stress increases. With increase of the protraction angle, the antedisplacement of cranio-maxillary complex decreases but the stress increases.