Intrabracket space and interbracket distance: critical factors in clinical orthodontics.

The engineers who designed the Houston Astrodome, Walter Moore and Associates, were engaged to study the effect that different edgewise appliances have on the function of orthodontic beams or wires. They were supplied with tooth width, bracket width, wire size, slot size information, and stainless steel wire specifications. With these data their computer was programmed to model each appliance as a simple beam reflecting its different support conditions. In the study the 0.018, 0.022, and 0.016-inch traditionally slotted appliances were tested in single and twin brackets. In addition the 0.016-inch bimetric appliance (0.016 inch on anterior teeth, 0.022 inch on posterior teeth) was tested. The following wires were used for testing: 0.016 x 0.022 inch, 0.017 x 0.022 inch, 0.018 x 0.025 inch (0.018 inch); 0.018 x 0.025 inch, 0.019 x 0.025 inch, 0.022 x 0.028 inch (0.022 inch); 0.014 x 0.018 inch, 0.015 x 0.019 inch, 0.016 x 0.022 inch (0.016 inch and bimetric). The results as stated in the conclusion statement by Rick Horn, PhD, of Walter Moore and Associates, are (1) for a given appliance and wire size, the amount of deflection allowable at permanent set decreases with decreasing size of teeth; (2) for a given appliance and wire size, the force imparted to the teeth at permanent set increases with decreasing size of teeth; (3) for a given appliance, the amount of deflection at permanent set decreases with increasing wire size; (4) for a given appliance, the force imparted to the teeth at permanent set increases with increasing wire size; (5) the amount of deflection allowable at permanent set is larger for single brackets than double brackets and larger for bimetric brackets than single brackets; (6) the force imparted to the teeth at permanent set is smaller for single brackets than double brackets and smaller for bimetric brackets than single brackets; and (7) of the six types of appliances examined, the bimetric appliance is the most flexible, allowing the most deflection at permanent set with the smallest force imparted to the teeth. This study supports the following thesis: the only way to take advantage of smaller wires and thereby have an appliance deliver maximum resiliency with lighter forces and not loose control is through differential slot sizing.