Preadjusted orthodontic brackets have made a significant contribution to orthodontics. They facilitate the esthetic alignment of upper anterior teeth, and they reduce the need for some 1st-, 2nd-, and 3rd-order bends. However, like many other commercial products, they sometimes promise more than they deliver. One early advertisement claimed, "Eliminates need to place torque in the archwire. . . . Eliminates archwire tip bends. . . . Eliminates step-in, step-out first order bends and molar offset bends."
By now, most clinicians seem to know otherwise. I recently conducted a survey of 600 orthodontists, selected at random from the Orthodontic Directory of the World. About 88% of them indicated that they used preadjusted brackets, but only a few said they never made bends in their archwires. The following percentages routinely made bends in these specific areas:
- Eminence bends for lower cuspids 29%
- Step-out bends for upper molars, 36%
- Step-out bends for lower molars, 33%
- Step-down bends for lower anteriors, 24%
- Esthetic bends for upper anteriors, 17%
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Apparently, more than half of orthodontists routinely bend archwires to improve the relationship between the lower anterior teeth and the lower cuspids in preadjusted appliances. Bracket height has a considerable influence on the labiolingual positions of these teeth, because the slope of the labial aspect of the cuspid mandates lingual movement of the tooth as the bracket moves gingivally. To compensate, 29% make an eminence bend for the cuspids, while 24%, who may place the cuspid bracket more incisally, use a step-down bend for the lower anterior teeth.
Another common variation is the molar eminence bend. Preadjusted brackets can achieve molar eminence only by increasing the prominence of the base of the slot to the foil base of the second bicuspid assembly. As a result of the increased projection of the second bicuspid bracket base, a straight wire from cuspid to molar will move the second bicuspid lingually--producing an eminence for the molar. The theory is reasonable, but practical limitations intervene. Before preadjusted appliances, orthodontists commonly placed molar eminence or bayonet bends of 1mm or more. The leading company's preadjusted appliances generate an eminence bend of only .5mm for the upper molar and .15mm for the lower molar. If the molar is banded, the thickness of the band material and cement will likely result in a step-in for the molar, rather than a step-out. Manufacturing the second bicuspid brackets with more prominence would only result in more bond failures of the brackets that are already considered the most likely to fail. Therefore, if one's objective is to place the central grooves of the posterior teeth in a straight line, eminence bends for the molars are a necessity.
Preadjusted appliances produce a lower cuspid eminence by a similar method--increasing the prominence of the lower anterior brackets. Unfortunately, this may be the worst location to place deliberately thicker brackets. Lower anterior bracket projection can cause anterior interferences that lead to bracket shearing, enamel wear of the upper anterior teeth, and even TMJ problems. It would appear more prudent to place brackets with the least possible prominence on the lower anterior teeth and use eminence bends for the lower cuspids.
With current technology, it seems futile to expect a fully programmed appliance that will eliminate all archwire bends. In fact, in some areas of the arch, the risk-benefit ratios suggest that wire bending will always be a wise choice.
RMR