Thank you for this opportunity to reply to Dr Katz’s letter, published in the January issue (Katz MI. Appearances count when industry underwrites research. Am J Orthod Dentofacial Orthop 2010;137:3-4), and to address the letter of Drs Brezniak, Proter, Herman, Turgman, and Zoizner.
“Leafing through” the October 2009 issue of the AJO-DO would definitely result in a distorted image of any article in that issue. We would have preferred to receive Dr Katz’s concerns about potential bias in a study (due to research funds provided by industry), along with a discussion of the content in the article. We encourage him to critically read our article and give us his feedback about our method, data, and interpretation of the data.
We welcome academic scrutiny of our research efforts, which is a sign of scientific rigor. Because of the title of Dr Katz’s letter, we feel obligated to explain that, until recently, our research was entirely funded by the McIntyre Memorial Research Fund, a trust fund available to the orthodontic department at the University of Alberta. It was established by a generous orthodontist who cared about the future of his specialty. The research conducted and published in the October issue was not funded by a private entity. We are glad that Dr Katz understands the need to go after industry funding for orthodontic research, something we had to do to continue the development of our research laboratory. We want to remind Dr Katz that passive self-ligation is not a bracket system belonging to 1 company; almost every orthodontic supplier has a passive self-ligation bracket system. Dr Katz incorrectly assumed that our experiments cannot be repeated; we welcome Dr Katz, his residents, and curious academics and clinicians to visit our research facility and spend time with us and our research assistant. The experiments can be easily repeated, and the method is not operator dependent. Finally, to answer Dr Katz’s question: no, the financial support probably would not have been forthcoming from the company if the results had shown its product to be inferior. Dr Katz must decide for himself whether passive self-ligation (in vitro) produces a distinctly different force system than conventional ligation, or, on the other hand, he can believe that an orthodontic department program director (who decided not to use self-ligation in his private practice) with 2 mechanical engineering professors and a statistics professor conspired with a graduate PhD resident to produce evidence supporting self-ligation to acquire research funding in the form of student scholarships. We did not hide our relationship with industry and are proud that we attracted orthodontic research funds from corporations to continue developing our research. The alternative was to stop our research after the university funds ran out. We hope that Dr Katz and many others will agree that we made the right decision.
We thank Drs Brezniak et al for their critical appraisal of our article; they show a deep understanding of orthodontic biomechanical concepts. We will respond to their comments in order.
Their first point, regarding the loading procedure, is well made, and we recognize that in a clinical setting there are many ways to ligate a high canine, and each might produce a different loading curve. This is why we standardized our procedure and ligated the teeth in the passive position and then moved the canine into the displaced position before bringing it back to perfect alignment. When we designed our first experiment, we had to start with a well-controlled test that would allow similar conditions for both ligation methods; as Brezniak et al stated correctly, the sequence of ligation might affect the loading curve of conventional ligation more than self-ligation. We are replicating the same test by starting the canine in the default position and tying it first. Another reason that we wanted to test the loading and unloading curves was to investigate the effect of the ligation method on the superelastic behavior (difference between the loading and unloading curves in Fz) of nickel-titanium wires.
Their next comment, that extrusive forces on the canine are higher in the self-ligating bracket system, requires a correction. At about 0.5 mm of extrusion during unloading, the Fz with self-ligation is around 2.2 N, and, at about 1 mm, the Fz is around 1.9 N; with conventional ligation, the Fz values at the same extrusions (0.5 and 1 mm) are about 3.7 and 1.7 N, respectively. This pattern shows that with self-ligation the extrusive force drops from 4 to 2 N and is maintained at 2 N throughout unloading. Conventional ligation did not show the same consistent pattern as self-ligation.
Brezniak et al are correct in their statement that duration might be a factor affecting not just the elastomeric ties but the whole system, especially when we consider that degradation of elastics takes place in the oral environment. We mentioned that our in-vitro experiment does not replicate the oral environment; we intend to study the phenomenon of elastic degradation and its effect on the load-deflection curves of our experiment in the future.
Our intention for Figures 4 and 5 was to show the software display and user interface. With the software’s ability to display the force system from more than 1 view, the scaling of the elastic ligation view and the passive self-ligation view are identical in each graph. However, we had to change the scaling and zoom out in Figure 4 when compared with Figure 5 to include the force vector in the screen capture frame.
Regarding the unloading forces of conventional brackets clearing more room for canine movement, we mentioned in our conclusion that “Based on those findings, we might not be able to make definite predictions on the effect of these differences on the actual tooth movements.” We simply believe that, for vertically displaced teeth, a vertical alignment force is desirable, and mesiodistal and buccolingual forces are undesirable. This led us to conclude that the force system produced by passive ligation is more accurate in terms of delivering a more consistent desired force (Fz in this case) and lower undesired forces (Fx and Fy in this case).
We agree with Brezniak et al that the moments require more in-depth analyses; a challenge in this study was trying to explain every aspect of our data and produce a logical explanation for it, especially moments. The sources of the moments are not limited to those mentioned in their letter; in reality, the bracket-wire interface can produce moments that are extremely complicated and cannot be currently explained by using the center of resistance as a reference point. We hope that further tests can help us to an understanding of the bracket-wire interface.
In our study, there was simply no way to quantify friction and binding, and distinguish between them. Our Fx forces constituted resistance to sliding, which includes friction and binding. We are in the final stage of building a device dedicated to studying resistance to sliding in detail and in 3 dimensions, and we look forward to studying this phenomenon in more detail. The concept of binding is poorly understood, and this has produced many misconceptions in orthodontics; that is why we need to study this phenomenon.
Near the end of their letter, Brezniak et al quoted a sentence from our conclusion section and suggested that the conclusion is inappropriate. However, immediately preceding the quoted sentence, we wrote “Based on those findings, we might not be able to make definite predictions on the effect of these differences on the actual tooth movements. However, it is safe to conclude that different force systems produce different types of tooth movement; therefore, we would expect to see more vertical canine movement and less tipping of the adjacent teeth with passive ligation compared with conventional ligation.”
Finally, we stated in our conclusion that the force system produced in this in-vitro study by passive self-ligation was more accurate because more consistent vertical extrusion forces (the desired force) and less mesiodistal or buccolingual forces (undesired forces) were generated. We assumed that AJO-DO readers would recognize that this is the first of many tests with the orthodontic simulator, and our data provided an example of its capabilities. It has taken 6 years to prepare and validate a 3-dimensional orthodontic force measurement tool that will be used for many tests to better understand a vaguely understood area of orthodontics. We continue to investigate the effects of the ligation method, and we are now gathering data, using a much larger sample size to be able to perform statistical tests. These data will be collected by starting the canine in the displaced position and ligating the teeth from the anterior to the posterior sequentially.
We have been approached by many academics and clinicians with numerous interesting research proposals to use this device, which is giving us a unique view into the world of 3-dimensional orthodontic mechanics. We plan to start an internship program for those interested in using the University of Alberta orthodontic research laboratory to investigate specific orthodontic biomechanic applications as part of a degree program or simply out of curiosity. We thank the doctors for their input and look forward to more discussions as more evidence is published and made available to AJO-DO readers.