By Christopher Cetta, DMD
While incisors and molars rotate fairly easily and consistently with plastic aligners, the cylindrical morphologic features of premolars and canines often necessitate the use of composite attachments. Rotation attachments can vary from vertical rectangular attachments to optimized rotation attachments with the Invisalign system. Some optimized and conventional configurations—such as Dr Jonathan Nicozisis’ famous “sash” or oblique attachment1—can provide simultaneous rotational and extrusive forces.
While rotation of premolars and canines can certainly be accomplished with composite attachments and aligners, it is often a slow and less predictable process compared to traditional brackets and wires. With most aligner systems such as Invisalign, the rotational movement is limited to 2° of rotation per aligner.2 In fact, a recent finite element analysis model even suggested ideal forces with an attachment might be closer to 1.2° of rotation per aligner for proper tracking.3
As an alternative, bondable buttons and chain elastomerics (ie, power chain) are a tried-and-true method for generating a biomechanical force couple to efficiently rotate teeth. Auxiliaries have been employed in this fashion as a pre-treatment phase prior to scanning for aligner therapy.
A recent method I have utilized is to incorporate buttons and an elastic chain simultaneously with the aligners. The benefit of this approach is twofold. First, the patient is often highly motivated to “begin treatment” with their aligners; and second, the plastic can be effectively used to stabilize the anchor tooth by resisting the rotational force moment. My selection criteria are premolars that are rotated more than 45° (this can be measured in the aligner software). For mild to moderate rotations, I will just use a composite rotation attachment. This approach could also be used on rotated canines.
Precision cut windows are placed on the rotated tooth, the LL4 in Case #1 (see Figures 1 through 3), and an anchor tooth, ideally a molar due to its rectangular anatomy. I bonded a clear Precision Aligner Button from DynaFlex to the mesial surface of the premolar and a metal Precision Button to the LL6 molar. A clear elastic power chain was connected to the buttons, leaving an extra link between as needed. The cylindrical anatomy of the premolar will allow the tooth to spin within the confines of the aligner, while the plastic will either prevent or at least deter the rotation of the rectangular-shaped molar. The patient is seen every 3 to 4 weeks to change out the elastic power chain while continuing their weekly switches of the aligners. Once the tooth is fully de-rotated, my current protocol is to remove the buttons and scan for refinement aligners.
In Case #1, the 76° rotation of the LL4 was corrected in the first 3 months (12 weeks) of aligner treatment. With 2° of rotation per aligner, this same movement would have taken 38 weeks or 9.5 months with a composite attachment and aligners. Therefore, it was three times faster and more efficient to incorporate the button auxiliaries and arguably more predictable than with aligners alone.
In Case #2 (see Figures 4 through 6), the LL4 had an 89° rotation that was corrected in 23 weeks as opposed to the typical 45 aligners (this patient was seen at 8-week appointment intervals prior to our latest protocol). So incorporating buttons proved roughly two times faster and more efficient.
While the majority of aligner platforms do not currently allow the doctor to manipulate the speed of tooth movement (a notable exception being uLab Systems), I would propose the tooth could rotate 6° per stage with these auxiliaries. For Invisalign and other outside manufacturers, the bondable buttons would ideally be removed and attachments added to those particular teeth with a new attachment template corresponding to that intermediate stage. This would avoid the mid-course correction scan and waiting 3 to 4 weeks for the new aligners to arrive, even though this approach cuts down on the total number of aligners when correcting the rotation is the rate-limiting step. With in-house aligners, if the rotational speed cannot be adjusted beyond 2°, I would recommend only manufacturing the first half of the aligners to minimize waste.
Keep an eye out for these severe rotations during your aligner setups and be sure to consider auxiliaries to do the twist! OP
- Nicozisis, J. (2013) Tripping the Plastic Fantastic. Orthodontic Products. Nov. 2013, 28-34.
- Simon, M., Keilig, L., Schwarze, J. et al. Treatment outcome and efficacy of an aligner technique – regarding incisor torque, premolar derotation and molar distalization. BMC Oral Health 14, 68 (2014).
- Cortona, A.; Rossini, G.; Parrini, S. et al. Clear aligner orthodontic therapy of rotated mandibular round-shaped teeth: A finite element study. Angle Orthod 1 March 2020; 90 (2): 247–254.
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