I read with great interest the article in the June issue, “Forces and moments generated by removable thermoplastic aligners: Incisor torque, premolar derotation, and molar distalization” (Simon M, Keilig L, Schwarze J, Jung BA, Bourauel C. Am J Orthod Dentofacial Orthop 2014;145:728-36), investigating the forces and moments generated by Invisalign aligners, since this is an area that needs much more research as the technique grows. I applaud the researchers for their excellent work but have a few questions and concerns about the study.
The researchers stated that “the tooth movement to be investigated was performed in isolation at the beginning of the treatment to make it clearly distinguishable from any other tooth movement.” Then in Figure 4, they showed graphs of the forces and simulated tooth movement of the maxillary incisor undergoing what is presumed to be buccal crown torque. The authors highlighted the positive forces in the z-direction (buccal) and the moment about the y-axis (buccal crown torque). However, where did all the other forces and moments come from? According to the graph, there’s a large negative force in the y-direction (distal) on the tooth and moments in all directions. Furthermore, the displacement graph ( B ) shows movement of the tooth in the distal (y) direction. These other data are seemingly not addressed until the end of the discussion when the authors stated “we used patients and not a complete experimental setup. Therefore, only the main force systems corresponding to the tooth movement…can be directly compared…”
This may explain why those data were not compared between groups, but not why the data were seemingly completely ignored. The maxillary incisor was isolated, and buccal crown torque was placed with aligners, but all sorts of forces and moments were measured, and the tooth moved 2.5 mm distally! This seems like a pretty significant finding to me, and similar side effects of aligners have been found in previous studies.
There is also an issue with the way bodily movement was measured in this study. In Table II, distalization was measured only with a force in the x-direction (distal), and then the movement was presumably measured by displacement of the molar crown in that direction. But to distalize a tooth bodily, there must be distal root torque as well, or else all the clinician has done is tipped the crown of the molar. Therefore, the moment about the y-axis should have been investigated further. Figure 7, B shows an average negative moment about the y-axis, presumably showing this distal root torque that produced bodily tooth movement on the distalized molar. But Figure 5, B shows an average positive moment about the y-axis for palatal root torque (or buccal crown torque) on the maxillary incisor. How can 2 moments in opposite directions produce root movement in the same direction? This anomaly needs more research. In the discussion, the researchers stated that “our results suggest that bodily tooth movements…can also be performed using Invisalign aligners…” I do not agree that the data in this study showed this conclusively and feel that this is an irresponsible statement.
The researchers acquired excellent data in this study, clearly demonstrating exponential force and moment decay of the aligners, and useful comparisons of certain forces with and without attachments. However, it seems that the data were “cherry-picked” to make certain statements. I believe that the data in this study could be very useful but should perhaps be analyzed from a different perspective.