Stiffness of incisor segments of edgewise arches in torsion and bending

Abstract

A mechanical model dentition is used in a laboratory study to relate incisor segment third-order activation to actual torque induced upon engagement of maxillary Edgewise arches. A portion of the sample of stainless-steel aches was subjected to stress-relief heat treatment. Torsional stiffness values are calculated, and the accompanying vertical displacement force on the incisor segment is evaluated. The results seem to warrant the following conclusions. Broad ranges of incisor segment torque magnitudes may be obtained from rectangular orthodontic wires and arch designs presently in common clinical use. Torsional behavior is associated with the elastic shear modulus or modulus of rigidity, which is essentially the same for all stainless and chrome-cobalt alloys. Vertical extrusive force is generated as a secondary effect directly related to the torsional stiffness and torque activation. Compensation is possible through archwire adjustments to cause the wire to lie above the bracket slots of the incisor segment before activation of lingual root torque. Stress relief of rectangular stainless steel arches following placement of V-bends, twists, or loops did not have a significant effect on force values. Further investigation, modeling other configurations such as labiolingual movements of the six maxillary anterior teeth to determine the torsional stiffnesses for commonly-used arches could be worthwhile. Similarly, the quantification of torsional stiffnesses of arches fabricated in rectangular nickel-titanium, titanium-molybdenum, and braided stainless-steel wires may be of value.