Extractions with aligners. Part II

Continuing with the treatment of extractions, in today's blog we will explain other variables beyond the aligners themselves that may influence the predictability of planned movements. We know that mass movement is not possible, that controlling the torque of the anterior teeth during their retrusion is not easy either... but will the same phenomena occur in both arches?

The type of bone present in the maxilla has different characteristics from 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 surrounding tissues and structures, the upper and lower teeth are more dense.
The lower teeth will not respond equally to the forces we apply to them, whether with braces, aligners or any other orthodontic device.

You are at 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 undergoing premolar extractions had the following findings:

• AnchoringMolar anchorage was not achieved exactly as planned. The anchorage control of the first molars with the G6 protocol did not provide significant advantages over the use of conventional horizontal rectangular attachments for anchorage control of the 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 mesially than the lower molars. This may be due to two factors: the greater bone density of the mandible and the smaller size of the anterior teeth in the lower arch compared to those in the upper arch.
• Mass movementThe results of this study show that both upper and lower canines showed a significantly higher distal inclination than planned, and both upper and lower central incisors were more lingualised than planned. These results suggest that a tilting movement is more likely to occur than a mass movement of the canines and central incisors during extraction space closure.
• OverbiteAfter closure of the extraction spaces, the overbite increased by an average of 4 mm despite the fact that the digital planning ended with 1.2 mm. This undesired increase in the overbite is due to the relative extrusion caused by the lingual inclination of the incisors during their retraction and to the higher
  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.

In the light of the results, we will now summarise their practical application in our extraction treatments:

• Controlling the loss of anchorage of the upper arch with intermaxillary elastics or micro-screws. Knowing that upper molars mesialise more easily than lower molars, we will decide in which cases we are interested in this phenomenon occurring and we will adapt the anchorage (elastics, micro-screws) accordingly.
• Place of ataches for mass movement control. The attachments make it possible to create more complex force systems with which to achieve better root control. Even so, if the root displacement is significant, the use of ancillary techniques to aligners should not be ruled out.
• 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.