Increase in arch perimeter due to orthodontic expansion.

A mathematical model was developed to compare quantitatively the effects of various types of orthodontic expansion on mandibular arch perimeter. Mandibular arch form was modeled with spline interpolation to fit a smooth curve between assigned molar, canine, and incisor positions. Starting with average arch dimensions, intermolar width, intercanine width, and midline arch length were increased individually and in combination in millimeter increments up to 5 mm, and the consequent changes in arch perimeter were measured. Increasing midline arch length by incisor advancement was nearly four times as effective in increasing arch perimeter as was molar expansion; canine expansion had an intermediate effect. Arch perimeter increments increased slightly with successive amounts of expansion for the molar, canine, and incisor. Combinations of molar-canine and canine-incisor expansion yielded results comparable to the total effects achieved by expansion of those teeth individually. Combined molar-canine expansion created increases in arch perimeter that were only slightly less than those generated by incisor advancement alone.

The morphology of canines in relation to preadjusted appliances.

The canine occupies the transition from anterior to posterior occlusion. Following orthodontic treatment the canine's incisal edge occlusion demonstrates the tip and torque present in the appliance that was used. The effective torque of the bracket, however, is influenced by the tooth morphology at the bracket's base. The morphology of the facial surface can be described by an angle formed between the tangent at the point of bracket placement and the long axis of the crown. In this study, tangent angles at four millimeters and five millimeters from the cusp tip of 100 maxillary and 70 mandibular canines were determined. There was a significant difference between tangent angles at the same location on different canine teeth and also at different locations on the same canine tooth. Proximal collum angles were also measured in this study and there was a significant negative proximal collum angle in maxillary canines and a significant positive proximal collum angle in mandibular canines. The presence of these normal biologic variables will either enhance or minimize the torque supplied by preadjusted appliances, depending on a combination of prescription used and biologic variable present.