Clear Aligner Orthodontic Treatment and Root Resorption
During Dr. Prittinen’s course, we took some time to discuss root resorption using clear aligner therapy. I would like to elaborate on this topic. First, we need to distinguish between generalized and localized resorption. As you are probably aware and have probably seen in your own practices, some patients suffer from generalized resorption. These patients would be excluded from orthodontic treatment during the diagnostic phase. In contrast to severe generalized resorption, severe localized resorption involving a few teeth usually is caused by orthodontic treatment. It has been known for many years that excessive force during orthodontic treatment increases the risk of root resorption, particularly if heavy continuous forces are used. Prolonged duration of orthodontic treatment also increases the amount of resorption.
It is increasingly apparent that some individuals are more susceptible to root resorption. It seems reasonable to presume that the large individual differences relate to genetic factors, although there is not any way to use genetic testing to evaluate resorption risk. However, one way to screen for potential genetic factors relating to root resorption would be to simply ask the patient’s parents if they had braces and if they did, did they experience root “shrinkage”? As a reminder, our US Dental Institute pre-treatment evaluation always includes family history as it relates to growth, orthodontic treatment, extractions etc.
Our policy to detect those who are likely to experience unusually large amounts of resorption is to take a panoramic radiograph 6 to 9 months into treatment and evaluate the amount of resorption during this time. As Dr. Prittinen reminds the attendees at his lectures, this is also the standard of care as determined by the American Association of Orthodontists (AAO).
Patients who show significant resorption in the initial stage of treatment are likely to have greater resorption at the end of treatment. In my own experience, I have stopped treatment in a few cases where I anticipated that the crown root ratio would be adversely affected by continuing treatment. We also had patients in our practice that never completed orthodontic treatment due to resorption. Thankfully this is a rare occurrence.
This gets us to clear aligner orthodontic treatment and root resorption. To date, I know of no report in the literature involving clear aligner therapy and root resorption. This could be just good fortune or it may have something to do with the development and design of CA therapy. First, CA was initially designed for adults. It’s logical to assume that since tooth movement in adults is slower, then the programmed movement in aligners would reflect this. Currently Invisalign and Reveal use the following movements per aligner: 0.25 mm linear, 2 degrees rotation, 1 degree root torque. This was conservative since the original protocol called for a 14 day aligner cycle. Thus, the total movement per month was about half of normal expected orthodontic movement with fixed appliances and was self-limiting since each aligner had a maximum designed tooth movement and force. It seems that the engineering of aligners can be more precise than a fixed system where initial generated forces can be very large. This “division of force” may be the reason that there has not been a report in the literature involving CA therapy and root resorption.
