What are unreliable tooth movements in Aligners?

The field of orthodontics is no more how it used to be. Out of all the major sectors of dentistry, orthodontics has seen the most advancements and tactical adaptations to the needs of its customers. Today, we can proudly commend orthodontic marvels like clear aligner therapy (CAT) for changing the game in ortho-planning and treatment.

Conventional orthodontic treatment with braces and similar metal frameworks has now been pushed to the backseat, owing to the aesthetic, biological and psychological advantages CAT seem to provide over traditional pre-adjusted appliances. This is not to say, however, that aligners are a one-fit-all glove for orthodontic malalignment. Instead, it is recognized widely that clear aligners may not be complimentary to some types of orthodontic movements. Even the modern computer-aided design/computer-aided manufacturing (CAD/CAM) system has its own set of limitations.

How does orthodontic tooth movement with aligners work?

The mechanism of tooth movement with aligners can be explained through two perspectives: the displacement-driven system and the force-driven system. The displacement-driven system is mainly responsible for controlling simple movements such as tipping or minor rotations and may not be useful for controlling root movements.

The force-driven system, on the other hand, utilizes biomechanical principles to facilitate tooth movement. By using pressure points and power ridges, they are able to produce more difficult movements such as intrusions, axial root movements, and torque.

Predictability of movements with aligners

Depending on the types of teeth and the severity of malalignment, some teeth are more amenable to certain movements than others. Typically, aligners work best with minor movements such as facial or lingual movements because there’s a broader surface to apply force.

Aligners are also fairly conducive to mesiodistal tipping movements. A study suggests that the most predictable movement with aligners was mesiodistal tipping.

On the contrary, aligners perform poorer when orthodontic cases involve the following movements:

• Rotation: Unless there is adequate space to rotate the tooth, it may be difficult to reposition the tooth in a normally inclined fashion. In fact, a study concluded that the rotation of the lower canines was the most unpredictable movement achieved by aligners (54.2%).
• Torques: Crowns that are particularly shorter in length or those that lack defined crown morphology like bulbous-shaped crowns may not be easily torqued, at least not without the help of engagers.
• Extrusions: Considering the crown morphology and available interproximal areas, extrusions can be tricky to perform with aligners. For example, triangular-shaped crowns or teeth with interproximal areas that allow the aligner to engage more of the crown may be successfully extruded without engagers. Short clinical crowns or multi-rooted teeth may require engagers.
• Intrusions: If the adjacent anchor teeth do not provide sufficient retentiveness, intrusions may not be possible with aligners alone. The intrusion of multi-rooted teeth is extremely rare with aligners as well.

Conclusions

Clear aligners are good for providing an aesthetic experience to the patients as compared to conventional fixed mechanics. We must accept, however, that they are not well-suited for all treatment plans and orthodontic tooth movements. As one study published in the Turkish Journal of Orthodontics concluded, “clear aligners can be used in mild to moderate crowding cases but caution must be exercised in complex cases.”