Orthodontic tooth movement with Invisalign® aligners
Variables affecting orthodontic tooth movement with Invisalign® aligners
Orthodontic tooth movement requires a coordinated reaction of biochemical reactions, cell signaling that allows cellular differentiation and bone remodeling. Bone remodeling involves the coordination of cellular activities for osteoclasts to proceed with bone resorption on the pressure side whereas osteoblasts proceed with bone deposition on the tension side.
Tooth movement is divided in 3 phases:
1- Initial phase
2- Lag phase
3- Secondary or postlag phase
Teeth move the fastest and with a steady rhythm during the secondary phase.
Variability among patients affects the rate at which tooth movement occurs. Factors such as age, gender (male vs female), root length, bone density, medication, and some systemic conditions can have an inhibiting, synergic or additive effect on the rate of orthodontic movement. It is clearly demonstrated that the rate of tooth movement decreases with age and this may be explained by a decrease in biological response. Any medication that interferes or changes bone biology may impact the rate of orthodontic tooth movement.
Percentage of actual movement obtained with aligners
Recent studies (3 studies) from the University of Florida tried to characterize orthodontic tooth movement with Invisalign® aligners. The first two studies analyzed tooth movement actually obtained in 8 weeks with a rate of 0.5 mm every 2 weeks for a total of 2 mm in 8 weeks and a series of 4 aligners.
The third study analyzed the actual movement obtained with a rate of 0.25 mm every two weeks for a total of 1 mm in 8 weeks and a series of 4 aligners.
In the first 2 studies, where the rate of the planned tooth movement was 0.5 mm per aligner multiplied by 4 aligners for a total of 2 mm of movement in 8 weeks, the results show that the percentage of the actual movement obtained compared to the planned movement is 53 ± 16 % and 55 ± 15 %. This means that if a tooth needs to be moved by 2 mm, the actual movement obtained by the series of 4 aligners will only be 1 mm in average.
In the third study, where the rate of the planned movement was 0.25 mm per aligner multiplied by 4 aligners for a total of 1 mm of movement in 8 weeks, the results show that 61,6 ± 20% of the planned movement was obtained. This means that for a planned movement of 1 mm, only 0.6 mm will be obtained.
If the data from these 3 studies are combined, despite the fact that the aligners were programmed to move a central incisor forward (labially) by 1 mm at the rate of 0.25 mm per aligner, in average, only 57% of this movement is obtained.
What does it mean clinically to have an average of only 60% of the movement that actually ends up being done compared to the movement that the aligner is supposed to do?
It is important to understand that a treatment with Invisalign® aligners generally involves more than 10 aligners, often 25, 30, even 45 aligners.
Consequently, the fitting of aligners on teeth decreases with time and with the evolution of the stages of treatment. This leads to a revision of the treatment and a longer duration of treatment.
We can better understand why during or toward the end of treatment, it is essential to revise the ClinCheck® virtual treatment and request a new series of aligners to finish the treatment.
The figure on the opposite side shows the average combined percentage of the obtained tooth movement (dotted lines) compared to the planned movement (continuous line).
The obtained movement is clearly below the planned movement since the lines of studies 1, 2 and 3 are below the solid line of the planned movement.
Tooth movement is verified each week by taking impressions, so it is interesting to see that the whole movement of 0.25 mm is almost done in the first week when a new aligner is worn and that there is a plateau during the second week. Then, a new movement is observed following the insertion of the second aligner, a new plateau appears during the second week of this second aligner and so on.
A good way to understand the concept that aligners do not necessarily provide the whole movement prescribed by the ClinCheck® virtual treatment is to carefully look at the figure above. The black arrows indicate the points of contact between the premolar and its adjacent teeth and the red arrows indicate the points of contact between the lateral incisor and its adjacent teeth. The stage of treatment corresponds to the result obtained after aligner #25 out of a total sequence of 28 aligners. The picture on the right shows what the ClinCheck® planned at aligner #25. The blue line indicates the mesiodistal axis of the premolar and the lateral incisor as obtained by the sequence of treatment from aligner #1 to aligner #25.
The green line on the picture on the right indicates the orientation of the same mesiodistal axis as planned in the ClinCheck®. It is obvious that nearly 10° of derotation are yet to be achieved. Knowing that there are only 3 aligners left to finish the treatment, refinement of the virtual treatment was therefore requested to obtain the ideal correction of the tooth alignment. This treatment was performed with Invisalign® G4 aligners.
Precision of tooth movement
Kravitz et al. measured the precision of tooth movement obtained with aligners compared to the virtual movement planned in the ClinCheck®.
Generally, the mean precision of the movement obtained with Invisalign® aligners is 41% compared to what was planned. Extrusion is the least precise movement (30%).
Concerning rotations, about one-third (⅓) of derotation movement is obtained.
Therefore, it is imperative to either plan overcorrections or use attachments.
Moreover, retraction movement is more precise (47%) than labial tipping movement (40%), which means that it is easier to close excessive spaces by retracting the teeth than to fix crowding by tipping the teeth forward.
This figure on the left shows the presence of an attachment on the buccal side of the canine and the planning of an overcorrection to obtain the desired movement.
It is important to know that the study conducted by Kravitz et al. was performed from 2004 to 2006 and those from the University of Florida (Chisari et al. and Drake et al.) were conducted from 2009 to 2012. The aligner material that was used back then was from the Invisalign® G3 aligners. Conclusions from these studies led to significant improvements with regards to the material used and the planning and realization of tooth movement with aligners.
The aligners being used now are Invisalign® G5, that is 2 generations ahead of the one used in the studies. This does not mean that all the problems are fixed, but we can pretend that Invisalign® G5 aligners have better efficacy than Invisalign® G3 aligners.
Chisari JR, McGorray SP, Nair M, and Wheeler TT. Variables affecting orthodontic tooth movement with clear aligners. Am J Orthod Dentofacial Orthop. 2014, Apr;145(4 Suppl):S82-91.
Drake CT, McGorray SP, Dolce C, Nair M, and Wheeler TT. Orthodontic tooth movement with clear aligners. ISRN Dent. 2012;2012657973.
Kravitz ND, Kusnoto B, BeGole E, Obrez A, and Agran B. How well does Invisalign work? A prospective clinical study evaluating the efficacy of tooth movement with Invisalign. Am J Orthod Dentofacial Orthop. 2009, Jan;135(1):27-35.
Simon M, Keilig L, Schwarze J, Jung BA, and Bourauel C. Forces and moments generated by removable thermoplastic aligners: Incisor torque, premolar derotation, and molar distalization. Am J Orthod Dentofacial Orthop. 2014, Jun;145(6):728-36.