Continuing with extraction treatments, todays blog will explore other variables beyond the aligners that can influence the predictability of the planned movements. We know that en masse movement is not possible, that torque control of the anterior teeth during their retrusion is not easy... but will both arches show the same behaviors?
The type of bone present in the maxilla has different characteristics to the mandibular bone, which has thicker cortices and, in general, a denser bone structure than the upper arch. If the movement of the teeth is conditioned by the tissues and structures that surround them, the upper and lower teeth will not respond in the same way to the forces we apply to them, whether with braces, aligners or any other orthodontic device.
These differences between tooth movements in both arches have been studied by Dai et al. who, after performing a series of CBCT overlays on 33 patients who underwent premolar extractions, obtained the following findings:
- Anchorage: molar anchorage was not achieved exactly as planned. Anchorage control of first molars with the G6 protocol did not provide significant advantages over the use of conventional horizontal rectangular attachments for anchorage control of upper molars. However, the lower molars showed better anchorage control than the upper molars. This finding is consistent with what occurs in fixed orthodontic treatment, as the upper molars move more easily to mesial than the lower molars. This may be due to two factors: the higher bone density of the lower jaw and the smaller size of the anterior teeth in the lower arch compared to those in the upper arch.
- En masse movement: both upper and lower canines showed a markedly greater distal inclination than planned, and both upper and lower central incisors lingualized more than planned. These results suggest that tipping rather than en masse movement is more likely to occur in the canines and central incisors during extraction space closure.
- Overbite: after closing of extraction spaces, the overbite increased by an average of 4 mm despite ending at 1.2 mm in digital planning. This unexpected increase in the overbite is due to the relative extrusion caused by the lingual inclination of the incisors during their retraction and to the greater flexibility of the aligner due to the absence of the premolars, which causes a decrease in the intrusion force of the aligner on the anterior teeth.
Considering these results, we shall summarize its practical application in our extractions treatments:
- Manage the loss of anchorage of the upper arch with intermaxillary elastics or micro-screws. Knowing that upper molars mesialize more easily than lower molars, we will decide in which cases this phenomenon is desirable and we will adapt the anchorage (elastics, micro-screws) accordingly.
- Attachments for en-masse movements. Attachments allow the creation of more complex force systems to achieve better root control. Even so, if the root displacement is significant, we will not exclude the use of auxiliary techniques to clear aligners.
- Avoid the undesired increase of the overbiteusing anterior ramps and overcorrecting the intrusion of the upper and lower incisors. Although with these modifications the final result of the ClinCheck may not seem very accurate, the result we will achieve in the patient will be closer to our objective.
Dai FF, Xu TM, Shu G. Comparison of achieved and predicted crown movement in adults after 4 first premolar extraction treatment with Invisalign. Am J Orthod Dentofacial Orthop 2021;160:805-13