An application of magnet and magnetic sensor: measurement system for tooth movement.

A measurement system for tooth displacement and rotation in the sagittal and frontal planes was designed and tested. Four small magnetic sensors were arranged at each corner of a rectangle on a plastic sheet. They were then placed in the mouth and fixed to the front teeth with a plastic splint. A powerful magnet made of a rare earth metal was fixed to the target tooth and placed at the center of the sensors. Movement of the magnet was detected by the four sensors as the tooth trajectory. This system was tested by measuring first molar movement in human subjects with a load generated by an orthodontic aid. This system was small enough to fit in a human oral cavity and did not interfere with the orthodontic aid. Although the oral environment with high humidity and high temperature was not agreeable for the sensors, resolution was estimated at better than 3 microns including the effects of system drift. System calibration was done extra orally and distortion was calculated at less than 6% in a square of 200 microns. The accuracy of this system was not affected by temperature due to the application in the oral cavity at a relatively constant temperature. The advantage of this system was not only the two-dimensional measurement but also rotational measurement in which a 0.05 degree resolution was calculated.