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Elastic ligatures, binding forces, and anchorage taxation
LETTERS TO THE EDITOR
Elastic ligatures, binding forces, and anchorage taxation To the Editor: ligatures, Binding Forces and The abstract of a study by P. Michael Echols on “Elastic Anchorage Taxation” (AMERICAN JOURNAL OF ORTHODONTICS, February, 1975, p. 219) concludes with a clinical suggestion that requires further examination. As the experiment is described, it was used to measure the breakaway force required to slide an arch wire through a bracket when ligated with an elastic ligature. Two factors tend to reduce this force in clinical application. Saliva serves as an excellent lubricant with elastic ligatures, and the small movements of the teeth in function provide a “walking” effect that allows the arch to move much more easily over a period of time than it might at the time of initial loading. Suitable lubrication and three-axis vibration would have to be used in any such experiment to provide clinically relevant measurements, and the resulting movement should be considered on an orthodontic rather than an instantaneous time scale. Any force above the breakaway force will produce the desired slippage, and even his artificially elevated force levels are not unusually high for many movements. Adding these values to those normally used, however, could certainly result in excessive force application and anchorage “taxation.” Force levels should be based on biologic considerations and not increased arbitrarily to overcome either real or imagined mechanical drag which is ultimately transferred to the tissues. The important point in sliding movements with elastic ligatures is their relatively low maximum force. Binding forces are limited along with all other forces. Wire ligatures have much greater strength, giving them the capacity to apply much higher binding forces as teeth slide along the arch. Minor irregularities in the arch can be “locked up” and stop movement much more readily than with elastic ligatures. The elasticity and lower strength of elastic ligatures require extra care in collateral rotation control in sliding movements, but when used properly they are exceptionally well suited for these applications. Above all, force levels should not be increased just because elastomers replace metal for ligation.
Kaywond C. Thwow, D.D.8. R West Mifliu St. Madisott, Wisconsi~s 53703
694
Elastic ligatures, binding forces, and anchorage taxation To the Editor: ligatures, Binding Forces and The abstract of a study by P. Michael Echols on “Elastic Anchorage Taxation” (AMERICAN JOURNAL OF ORTHODONTICS, February, 1975, p. 219) concludes with a clinical suggestion that requires further examination. As the experiment is described, it was used to measure the breakaway force required to slide an arch wire through a bracket when ligated with an elastic ligature. Two factors tend to reduce this force in clinical application. Saliva serves as an excellent lubricant with elastic ligatures, and the small movements of the teeth in function provide a “walking” effect that allows the arch to move much more easily over a period of time than it might at the time of initial loading. Suitable lubrication and three-axis vibration would have to be used in any such experiment to provide clinically relevant measurements, and the resulting movement should be considered on an orthodontic rather than an instantaneous time scale. Any force above the breakaway force will produce the desired slippage, and even his artificially elevated force levels are not unusually high for many movements. Adding these values to those normally used, however, could certainly result in excessive force application and anchorage “taxation.” Force levels should be based on biologic considerations and not increased arbitrarily to overcome either real or imagined mechanical drag which is ultimately transferred to the tissues. The important point in sliding movements with elastic ligatures is their relatively low maximum force. Binding forces are limited along with all other forces. Wire ligatures have much greater strength, giving them the capacity to apply much higher binding forces as teeth slide along the arch. Minor irregularities in the arch can be “locked up” and stop movement much more readily than with elastic ligatures. The elasticity and lower strength of elastic ligatures require extra care in collateral rotation control in sliding movements, but when used properly they are exceptionally well suited for these applications. Above all, force levels should not be increased just because elastomers replace metal for ligation.
Kaywond C. Thwow, D.D.8. R West Mifliu St. Madisott, Wisconsi~s 53703
694