Evaluation of open bite closure using clear alignersa retrospective study

Here is the object at the center of this story: a thin, transparent plastic tray, molded precisely to the contours of your teeth. No metal, no wires, and no brackets. For years, orthodontists debated whether that object could close an anterior open bite — a condition where the front teeth simply do not meet, even when the back teeth are together. And the debate was not academic. Millions of adults choose clear aligners specifically because they are invisible, and if they do not work for an open bite, those patients either accept a compromised result or switch to braces. Harris and colleagues decided to find out exactly what happens when you try. An anterior open bite, defined in this study as an overbite of less than 0.5 millimeters, is one of the harder problems in orthodontics. The front teeth fail to overlap vertically, which affects chewing, speech, and facial profile. Conventional fixed appliances, or traditional braces, have a frustrating tendency to extrude posterior teeth as a side effect, which can actually worsen an open bite rather than close it. So, the field has long argued about which mechanism actually does the closing: do you push the front teeth down, pull the back teeth up, or coax the jaw into rotating? The literature offered all three answers, often from small studies with conflicting results. Khosravi and colleagues reported that correction happened mainly through incisor extrusion, roughly 1.5 millimeters. Moshiri and colleagues said it was primarily counterclockwise mandibular rotation from lower molar intrusion. Garnett and colleagues found no significant vertical difference between aligners and fixed appliances, even when the fixed-appliance group included extractions and temporary anchorage devices. The honest summary of the pre-existing evidence: nobody agreed. Harris and colleagues set out to settle at least part of this with a proper measurement. They conducted a single-center retrospective study using records from a private practice in Tokyo, one experienced Invisalign provider, reviewing 250 consecutively treated adult records. Forty-five patients were ultimately included after exclusions for incomplete records, pre-existing positive overbite, or radiographic quality issues. The mean age was 30.73 years. The mean initial open bite was negative 1.21 millimeters — the front teeth were that far from touching. Most patients were female and Japanese, which matters for generalizability. The average treatment lasted just over 14 months, with roughly 74 aligners changed every five days. The measurement tool was cephalometric superimposition. You take a lateral skull X-ray before treatment and another after, overlay them precisely using stable anatomical reference structures, such as the cranial base and the mandibular plane, and you can read off exactly which teeth moved, in which direction, and by how much. There were fourteen landmarks and eighteen measurements. The reliability was high; intra-rater intraclass correlation coefficients ranged from 0.84 to 0.98, while inter-rater ranged from 0.83 to 0.99. The sample of 45 provided the study with 91 percent statistical power to detect a clinically meaningful difference of 0.5 millimeters in molar vertical change. These are not soft numbers. So, what did the incisors do? The upper front teeth extruded — moved downward toward the lower teeth — by a mean of 1.45 millimeters. They also retracted significantly; the upper incisor angle relative to the cranial base changed by negative 10.91 degrees, and the incisor tip moved backward by 2.57 millimeters. The lower incisors showed the same pattern: extrusion of 0.53 millimeters and backward tipping measured as a decrease in the incisor-mandibular plane angle of 3.73 degrees. The front teeth came down and leaned back. The retraction is the unexpected part. Harris and colleagues attribute it to what they call a bite-block effect. The aligner plastic covers the biting surfaces of the back teeth. When the patient bites down, those back teeth press against the plastic, and the forces generated appear to tip the front teeth inward as a byproduct. It is not planned; it is a structural consequence of what an aligner physically is. And it turns out to be one of the two engines driving bite closure. The second engine is molar intrusion, and this is where the findings get mechanistically interesting. The maxillary first molar intruded by 0.47 millimeters. The mandibular first molar intruded by 0.39 millimeters. Both results were statistically significant. And critically, there was no meaningful front-to-back movement of the molars at all; they moved vertically, almost purely. When back teeth move upward, geometry takes over; the jaw acts like a hinge, and if the pivot points rise, the front of the jaw closes. This is mandibular autorotation, a counterclockwise rotation of the lower jaw that closes the anterior gap even without any direct movement of the front teeth. Harris and colleagues found a statistically significant association between the total amount of molar intrusion and the degree of autorotation. The mechanism is real and it is measurable. Here is what makes this finding striking: only 17 of the 45 patients had molar intrusion explicitly programmed into their treatment plan. The other 28 did not. And yet measurable molar intrusion occurred across the sample, with no correlation between planned intrusion and measured intrusion. The bite-block effect appears to operate whether or not the clinician asked for it. The plastic simply does something to posterior tooth position by virtue of being there. These two engines, incisor extrusion and molar intrusion, combined to produce a mean overbite improvement of 3.27 millimeters from a starting point of negative 1.21 millimeters. Anterior facial height decreased by 1.17 millimeters, consistent with the jaw hinging closed. The mandibular plane angle decreased by 0.73 degrees. Harris and colleagues emphasize that these changes were predominantly dental and dentoalveolar rather than skeletal; the bones themselves did not remodel much. The teeth and their supporting structures did the work. A subgroup analysis by initial open bite severity found significant differences in lower molar intrusion between mild and severe cases. Patients with worse initial open bites showed different lower molar behavior, which matters for treatment planning. Severe open bites may respond differently in magnitude even if the mechanism is the same in kind. Now, what the study cannot tell you. There is no post-retention time point. Harris and colleagues measured patients before treatment and after the last refinement aligner. They did not follow anyone for a year or two afterward. This is not a minor gap; anterior open bite is notorious for relapse. The tongue, habits, and jaw growth patterns that created the open bite in the first place do not disappear when the aligners come out. Whether the 3.27 millimeters of correction holds six months or three years later is simply unknown from this data. There is also no comparison group. The study cannot tell you whether clear aligners outperform fixed appliances or whether a different appliance would have achieved more correction with less incisor retraction. The single-center, single-provider design, one experienced clinician in Tokyo, predominantly female and Japanese patients, limits how far the findings travel to other clinical settings. Harris and colleagues call explicitly for randomized controlled trials that follow patients prospectively and compare aligners with matched fixed-appliance groups. That is the study this work sets up, not the study this work is. What this study does deliver is a mechanistic map. When you treat an anterior open bite with clear aligners, the front teeth extrude and retract, the back teeth intrude — whether or not you planned the intrusion — and the jaw hinges closed in response. The correction is real, it is measurable, and it is driven by a combination of forces that the aligner produces partly by intention and partly as a structural byproduct of its own geometry. For a clinician deciding whether to offer a patient clear aligners for an open bite, that map is genuinely useful. It tells you what to expect and what to watch. The durability question is still open. But at least now the mechanism has a name. This lecture was created by ennepō. 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