In vitro analysis of the initial tooth mobility in a novel optomechanical set-up.

An optomechanical set-up was developed to accurately measure displacement/force curves of the initial tooth mobility in vitro. The system presented is capable of recording all six components of a tooth movement resulting from an applied orthodontic force system, and consists of a laser diode-based optical part and a six degree of freedom mechanical part. Three laser diodes were mounted in orthogonal arrangement on the tooth of a specimen and their light (lambda = 670 nm) was focused on the surfaces of three position sensing detectors. The laser beams thus defined a cartesian rigid body coordinate system and the movements of the tooth could directly be derived from the movements of the laser spots on the surfaces of the optical detectors. The force system was applied and simultaneously measured via a three-dimensional force-torque transducer mounted on a six-axis positioning table. Measuring accuracy of the tooth displacements was in the range of 9.0 microns and 0.022 deg for translations and rotations, respectively. Resolution and accuracy of the mechanical system was approx. 0.02 N for the measurement of forces and 0.5 Nmm for torques. Displacement/force diagrams of the specimen of a swine's mandible are presented, showing the relationships between applied force system and tooth displacement of the lower first premolar. The accuracy reached proved to be sufficient for the verification of numerical (Finite Element) models of the initial tooth mobility.