Prevalence and severity of apical root resorption during orthodontic treatment with clear aligners and fixed appliancesa cone beam computed tomography study

There's a clear plastic tray sitting on millions of bathroom counters right now — nearly invisible and custom molded, the modern alternative to a mouth full of metal. Invisalign and its competitors have reshaped orthodontics partly on the promise of being gentler. Gentler on your social life, sure. But gentler on your actual teeth? That question turns out to have a specific, measurable answer, and until recently, nobody had looked at it with the right tool. Li and colleagues looked. What they found should matter to anyone who's ever sat in an orthodontist's chair and had to choose. The side effect in question is called apical root resorption, or ARR for short. This is the permanent shortening of tooth roots that happens as a consequence of orthodontic force. When your teeth move through bone, the tissue at the root tips can get damaged and resorbed by the body. A little is common and often inconsequential. A lot can compromise the long-term stability of your teeth, alter the crown-to-root ratio, and in serious cases, contribute to tooth loss. The reported prevalence across orthodontic patients ranges widely — anywhere from 20 percent to nearly 100 percent depending on the study — but severe apical root resorption, meaning more than five millimeters of root loss or more than a quarter of the root length gone, occurs in roughly one to five percent of cases. It concentrates in the anterior teeth, particularly the upper incisors, which makes it both functionally and aesthetically significant. The problem with studying apical root resorption accurately is that it's a three-dimensional change, and most of the historical literature measured it with two-dimensional X-rays. Panoramic radiographs can overestimate the prevalence of apical root resorption by about 20 percent compared with periapical films, and two-dimensional imaging in general can both over- and underestimate root loss. What you want is cone beam computed tomography, or CBCT, which gives you a true three-dimensional image of each root and has demonstrated higher accuracy for this kind of measurement. Li and colleagues built their study around CBCT precisely because they wanted to know the real number, not the X-ray approximation. Here's how they set it up. Seventy patients, with a mean age just under 24, were split evenly into two groups: thirty-five treated with Invisalign clear aligners and thirty-five treated with Victory Series fixed appliances from 3M Unitek. All anterior teeth — upper and lower — were measured, giving a total of 373 roots. The crucial design decision was matching the two groups on the American Board of Orthodontics discrepancy index, a standardized scoring system for case difficulty. The mean discrepancy index score was 18.80 in the aligner group and 17.14 in the fixed appliance group — statistically identical, with a p-value of 0.445. That matters because the obvious confound in any aligner versus braces comparison is that clinicians often use aligners for simpler cases. By matching on case difficulty, Li and colleagues tried to close that door. Treatment time was also comparable: about 21.5 months for aligners and 23.3 months for fixed appliances. Root length was measured on cone beam computed tomography images taken before and after treatment, all on the same machine with standardized settings. Two investigators, blinded to which treatment each patient had received, measured each root independently. Their agreement was excellent: an inter-rater intraclass correlation coefficient of 0.98, and intra-rater coefficients of 0.98 and 0.97. Apical root resorption for each tooth was simply root length before treatment minus root length after treatment, so a positive number means root lost. Now the results. The headline numbers are stark. In the clear aligner group, 56.3 percent of anterior roots showed some degree of apical root resorption. In the fixed appliance group, 82.1 percent did. That difference is statistically significant with a p-value below 0.001. But prevalence only tells you how many teeth were affected — severity tells you how badly. Mean root loss in the aligner group was 0.13 millimeters. In the fixed appliance group, it was 1.12 millimeters. That is nearly a tenfold difference in mean loss. The per-tooth breakdown makes the pattern more vivid. In the fixed appliance group, the hardest-hit teeth were the maxillary canine — the upper eye tooth — with a mean loss of 1.53 millimeters, and the maxillary lateral incisor at 1.31 millimeters. The maxillary central incisor lost 1.23 millimeters on average. Every single tooth in the fixed appliance group showed a statistically significant decrease in root length from before to after treatment. In the aligner group, the story is almost reversed. The mandibular canine showed a mean value of negative 0.06 millimeters — meaning no net resorption at all, just slight measurement variation in the direction of apparent lengthening. The mandibular lateral incisor averaged 0.04 millimeters of loss. Statistically significant root length change in the aligner group appeared only in the maxillary incisors and the mandibular central incisor. Li and colleagues also classified severity using Sharpe's grading system, where first degree is slight apex blunting of one to two millimeters, second degree is moderate blunting up to one quarter of root length, and third degree is excessive blunting beyond that. In the fixed appliance group, about 62 percent of teeth fell into the first degree category, nearly 20 percent into the second degree, and less than one percent into the third. In the aligner group, 43.7 percent of teeth showed no resorption at all — classified as zero degree — and the remaining 56.3 percent fell into the first degree only. Not a single tooth in the aligner group reached second or third degree severity. So why does the appliance type make this much difference? Li and colleagues discuss two mechanical arguments, and both are framed as working hypotheses rather than settled causal proof. The first is force continuity. Fixed appliances apply continuous force to teeth around the clock. Clear aligners, by contrast, are removed to eat and function differently when worn — they apply force more intermittently. The literature they cite suggests intermittent force gives the cementum, the hard tissue covering the root surface, time to heal between loading cycles. The second argument is force magnitude and distribution. Heavier forces are associated with higher prevalence and greater severity of apical root resorption, and according to their analysis, aligners deliver relatively lighter, more computer-controlled forces compared with the concentrated forces that archwires can transmit to specific teeth. The upper canines and lateral incisors are particularly vulnerable under fixed mechanics, likely because they travel through larger arcs of movement during treatment. Taken together, these two factors — intermittency and lighter load — may reduce the inflammatory cascade that drives root resorption. The authors are careful to present this as the field's current thinking, not an established mechanism. What can't this study tell us? The design is retrospective — patients weren't randomized to treatment; they were assigned by their clinicians. That opens the door to selection bias beyond what the discrepancy index controls for. A clinician might choose aligners for cases that happen to involve less demanding tooth movement even within a matched difficulty score. The sample, while adequately powered by the study's own calculations, consists of 70 patients — reasonable for a cone beam computed tomography study but limited for broad generalization. The analysis covers only anterior teeth; molars were not assessed. Furthermore, cone beam computed tomography, while more accurate than two-dimensional radiography, still carries measurement uncertainty and a radiation dose reported elsewhere as one point five to thirty-three times higher than panoramic X-ray — a tradeoff the authors acknowledge. Within those constraints, the cone beam computed tomography data from Li and colleagues offer the most rigorous imaging-based evidence to date on this question. The prevalence difference — 56 percent versus 82 percent — and the severity difference — 0.13 millimeters versus 1.12 millimeters — are both clinically meaningful and statistically solid. For any patient facing a choice between treatment modalities, and for any clinician advising them, this study gives a specific, three-dimensional answer to a question that the field had been circling with less precise tools for years. The plastic tray on the bathroom counter, it turns out, really does appear to be easier on your roots. The evidence, measured in fractions of a millimeter on 373 individual roots, now says so. This lecture was created by ennepō. Go to https://ennepo.ai to Discover, Create and Follow the latest research in your field. Read when you can. Listen when you want to.