This skeletal change reduces the amount of dental compensation needed in order to achieve positive overbite.
Clear aligner treatment resulted in no significant skeletal change and molar vertical position was not significantly changed throughout treatment. Increase in overbite was successfully achieved by incisor extrusion.
Though not statistically significant, the average incisor extrusion was less for the skeletal anchorage group (0.78 and 0.60mm for U1 and L1, respectively) even though the initial severity of open bite was more severe (-1.83mm). Clear aligner treatment resulted in more average incisor extrusion (1.14mm and 0.77mm for U1 and L1, respectively) with less severe initial overbite (-0.94mm).
These results indicate that incisors extrude more during treatment of AOB in patients treated with clear aligners than in patients with skeletal anchorage. Reduction in overbite during clear aligner treatment is mainly due to maintenance of the vertical position of molars and extrusion of incisors, whereas during skeletal anchorage AOB treatment reduction in overbite occurs due to molar intrusion, autorotation of the mandible and incisor extrusion.
Comparison of growing patients treated with skeletal anchorage and clear aligners shows that more significant treatment effect can be observed in non-growing patients as the vertical position of the molars exhibited larger effects of treatment in the desired direction: maintenance of molar position in clear aligner patients and more molar intrusion in skeletal anchorage intrusion patients.
Results from this investigation support previous findings of studies of AOB treatment with both clear aligners and skeletal anchorage, however no studies have directly compared the
two treatment modalities. Clear aligner AOB treatment has been observed to result in
maintenance of molar position and incisor extrusion. Garnett et al reported minimal change of molar position and extrusion of 0.97mm and 0.82mm for maxillary and mandibular incisors (Garnett et al. 2019). Khosravi et al demonstrated similar treatment effects with clear aligners. No significant molar vertical change was noted and incisors extruded 0.9mm and 0.8mm, maxillary and mandibular (Khosravi et al. 2017). Moshiri et al demonstrated molar intrusion of molars: 0.4mm and 0.6mm for maxillary and mandibular, respectively. Incisor extrusion was also noted but to a lesser degree (0.5mm and 0.8mm for maxillary and mandibular incisors, respectively) (Moshiri et al. 2017). The results in this investigation support previous findings with the exception of molar intrusion. On average, molars extruded 0.10mm and 0.17mm for maxillary and mandibular molars, respectively. Incisors extruded 1.14mm and 0.77mm for maxillary and mandibular.
The results of the skeletal anchorage group are also comparable to previous studies. In a study of 10 patients, Kuroda et al reported molar intrusion of 2.3mm in the maxilla and 1.3mm in the mandible as well as 0.5mm maxillary incisor extrusion and maintenance of the mandibular incisor position (Kuroda et al. 2007). In a smaller study of 4 adults, Sherwood et al reported mean molar intrusion of 1.99mm (Sherwood et al. 2002). Results from these studies support our findings in the current sample.
Results indicate that in in patients with AOB, the treatment modality can have important effects in incisor position and smile esthetics. In patients where minimal incisor extrusion and skeletal change is desired, skeletal anchorage may be a superior treatment option. If incisor extrusion is desired, clear aligners may be more effective.
Limitations of this study include a mix of growing and non-growing patients. As 38% of the subjects were growing, it could be argued that treatment may have a more effective role in intruding molars as the position of molars was maintained to the relative planes as a patient grows. Additionally, as this was a cephalometric study, there are known limitations in the accuracy of tracings with errors up to 2mm (Baumrind and Frantz 1971). ICC reliability tests demonstrate excellent reliability for all variables except for measurement of OJ which was only good. The good ICC value for overbite could be attributed to the small distances measured between incisors relative to other measures in this study and the difficulty in accurately tracing crowded incisors.
It is also of note that the stability of AOB treatment is not well known. To date, there are no studies assessing the stability of incisor extrusion as a result of AOB treatment with clear aligners. Similar tooth movements directed by fixed appliances have been studied and observed to range from 61.9%-75% in combined studies of extraction and non-extraction treatment (Al- Thomali et al. 2017; Greenlee et al. 2011). Scheffler et al. reported on the stability of 30 patients with AOB treated with skeletal anchorage molar intrusion and found that all patients maintained positive overbite after 1 year, but observed 0.2mm molar extrusion along with 0.5mm increase in LFH and slight elongation (<2mm) of the incisors to maintain overbite (Scheffler et al. 2014). Similar ranges could be expected but future studies should investigate the specific stability of AOB treatment and perhaps the effects of different methods of retention.
Conclusions
These results demonstrate that open bite closure is effective primarily by way of anterior extrusion unless active molar intrusion is achieved with skeletal anchorage. This also
demonstrates that open bite closure with skeletal anchorage aids in preserving the vertical position of the incisors while intruding maxillary molars, decreasing LFH and increasing the occlusal plane more predictably than open bite closure with clear aligners.
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