An appraisal of growth and reaction to extraoral anchorage. Simulation of orthodontic-orthopedic results.

The purpose of this article was to simulate graphically the probable orthodontic-orthopedic results of different types of extraoral traction applied to the upper dentition. Forces and movements are vectors; apart from individual variation in the amount of reaction to forces, the reactive movement vector at any given site of an influenced hard-tissue unit depends mainly on the position of its center of resistance relative to the force vector and on the amount and time of force application. In growing patients the resulting movement can then be appraised by the addition of reactive movement and average growth-movement vectors. Standardized assumptions have been made to simulate and compare qualitatively the significance and implications of five different types of commonly used extraoral traction on the upper dentition. Biomechanical parameters (such as the system factor describing the reciprocal relation between the lever arm of a force and the corresponding distance between the centers of resistance and rotation) have been arbitrarily assessed. Where cervical gear on the upper dentition is used, the resulting simulations suggest that raising the outer arms of the face-bow means less posterior effect on the upper dentition, less vertical molar control, and an increased tendency to open the bite. Where high-pull traction is applied, graphic simulations using a constant angulation, but different position of the force vector, indicate that moving the force line from a backward to a more forward position would mean a change from posterior to anterior rotation, from pronounced to no vertical molar control, from excessive incisor elongation to complete vertical incisor control, and from considerable distal effects to nearly none on the upper dentition.(ABSTRACT TRUNCATED AT 250 WORDS)