Biomechanics of self-ligating brackets

One of the hottest subjects in town is the difference between self-ligating and conventional brackets, especially from the biomechanical point of view.

We applaud the group from Edmonton, Alberta, Canada, for their impressive study in the October issue (Badawi HM, Toogood RW, Carey JPR, Heo G, Major PW. Three-dimensional orthodontic force measurements. Am J Orthod Dentofacial Orthop 2009;136:518-28).

However, we have several comments.

The loading procedure described in the study cannot be performed in vivo. Therefore, we believe that the meaning of the loading graph for the clinician is questionable. In a clinical situation, there are 2 main ways to place the wire in the malaligned canine. 1. Engage the wire in the neighboring teeth and later pull the engaged wire toward the canine; sometimes, using conventional brackets, binding and friction effects on both sides decrease the extrusive force applied to the tooth to zero. 2. Engage the teeth one after the other: first, the lateral incisor or the first premolar and then the canine, continuing to the next tooth. We believe that this would produce a different loading graph.

The article ignores that, immediately after about 0.5 to 1 mm of canine extrusion, the extrusive forces on the canine in the self-ligating bracket, as can be seen in the graph, are higher than in the conventional bracket. This means that, during most of the treatment time with self-ligating brackets, the canine is exposed to higher forces in this direction than with conventional brackets.

The time factor affects not only the elastomeric qualities of the ties, but also the whole force system. Unfortunately, the time factor in the study was not mentioned. The scale of the forces and moments in Figures 4 and 5 are different and might lead to mistakes because of some optical fixations.

The unloading forces of the conventional brackets in the Fx and Fy directions clear more room for the canine to move vertically and buccally. This might be an advantage in most patients when gaining space is required.

We believe that the measured moments and their depiction in the graphs need more in-depth analysis and explanation. The moments developed on each tooth have 2 sources. The first is the couple produced in the bracket, and the second is the force applied on the bracket from the wire multiplied by the distance to the center of resistance. For that matter, using a round wire, Mx (moments in the buccolingul direction) consists only from the moment generated by the force in that direction, multiplied by the distance to the center of resistance. My and Mz are the sum or the difference (depending on the direction) of the 2 above-mentioned moments. Unfortunately, we did not find in the article the distance from the bracket to the center of resistance in any of the 3 axes.

Most forces and moments on the lateral incisor and premolar are the result of binding and not friction. Using the same bracket, even with different methods of ligation, should not influence binding, and we found no notes related to binding in the article.

The sentence, “we would expect to see more vertical canine movement and less tipping of the adjacent teeth with passive ligation compared with conventional ligation,” fits only this specific laboratorial mechanical system. Moreover, that conclusion involves the prediction of the biologic response. Since no theory regarding the response to force was proven correct, this assumption might be inappropriate.

We find it pretentious to describe the self-ligating force system as “more accurate,” as mentioned in the conclusion section. The meaning of accuracy in the context of the article can easily be misunderstood and misused. From the article, we believe that a further conclusion might be that the results are an initial argument point to an ongoing discussion with a new and innovative tool.

Again, we praise the authors for their interesting and outstanding article.

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Apr 14, 2017 | Posted by in Orthodontics | Comments Off on Biomechanics of self-ligating brackets
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