Part 2: Which Aligner Movements are Predictable and which are Not/ What Cases are Easy, Moderate or Difficult to Treat

By Dr. Geoffrey Hall

In 1946 Kesling first introduced the concept of clear orthodontic appliances to move misaligned teeth. The initial cases were minor crowding or spacing. With development of material and computer design of tooth movement, the indication of clear aligners has been greatly enlarged. 

Many researchers reported successful cases to prove that the clear aligners today have been able to treat almost everything from mild to severe malocclusions. Clinicians who provide clear aligner therapy should understand the biomechanical characteristics, know when to treat and be able to predict treatment results and plan distinctively from conventional orthodontic treatment.

Clear aligners are convenient in mild to moderate crowding or diastema, posterior expansion, intrusion of one or two teeth, lower incisor extraction cases, and distal tipping of molars. Movements like extrusion, correction of severe rotations, molar uprighting and closure of extraction spaces are known to be more challenging to accomplish with aligners. Even so, incisor extrusion, molar transition, and closure of extraction spaces are possible with the use of attachments in the clear aligner system.

Indications and Contraindications of Clear Aligners for General Dentist

Effects of Various Tooth Movements According to Review of Literature

1. Djeu et al. (AJO 2005) conducted the first retrospective cohort study on the effectiveness of clear aligners and reported that they are effective in space closure, marginal ridge alignment and, root paralleling; however, aligners are deficient in the correction of anteroposterior discrepancies, providing occlusal contacts, and posterior torque. 

2. Kravitz et al. (AJO 2009)evaluated the accuracy of tooth movement obtained by the aligner system and reported that only 41% of the predicted tooth movement was achieved. The most effective movement was lingual constriction (47.1%), the least accurate was extrusion (29.6%), and only 33% of predicted rotation correction was achieved.

3. Kassas et al. (2013)reported that the clear aligner system is effective in leveling and aligning arches in mild and moderate cases and in correcting buccolingual inclinations effectively, however, it is not sufficient for providing ideal occlusal contacts. The deterioration in occlusal contacts is caused by the thickness of aligners, which interferes with the settling of the occlusal plane

4. Buschang et al. (AO 2014) & Rossini and colleagues (AO 2015)observed that aligners were effective in:

• controlling anterior intrusion 
• posterior buccolingual inclination, 
• in producing about 1.5mm bodily movement of maxillary molars.

Not effective in:

• controlling anterior extrusion, 
• anterior buccolingual inclination, 
• rotations of rounded teeth.

5. Dental arch dimensions

Pavoni et al (2011) found that braces produced significantly more transverse dento-alveolar width of maxillary intercanine and interpremolar and more perimeter of maxillary arch width

than clear aligners did, and increasing intermolar width and maxillary arch depth with aligners.

Grunheid et al (AO 2016) found that clear aligners tended to increase mandibular intercanine

width during alignment in contrast to braces and effects on increasing intemolar width and maxillary arch depth with aligners.

6. Open bite/ Deep Bite

• Open bite is a challenging malocclusion to treat, with a high incidence of relapse.
• Extrusion is the least accurate tooth movement to perform with Clear aligners and it can result in larger deviations compared to other movements.
• This lack of efficiency could be due to the difficulty of the appliance in developing enough force to extrude teeth in a significant way. According to few literature, CAT is not recommended to treat open bite.

Khosravi et al (AJO 2017), indicated that clear aligners corrects mild to moderate open bite mostly through incisor extrusion, rather than posterior intrusion, and relieves deep bites primarily through proclination of the mandibular incisors.

7. Labiolingual movement

Grunheid et al (AO 2016), found that treatment with braces significantly decreased the proclination of mandibular canines in contrast to treatment with clear aligners which tended to increase the intercanine width instead of decreasing inclination. The lower canine is the most difficult tooth to control.

Hennessy et al (AO 2016), found that braces produced more mandibular incisor proclination

during alignment than aligners.

Yildirim et al. (2013) investigated the efficacy of tooth movement and concluded that retrusion was found to be the most accurately obtained tooth movement followed by a rotation, fan-type expansion, and protrusion respectively. 

Retrusion of mandibular central incisors is considered to be the most accurate single-tooth movement, whereas the rotation of mandibular canine is the least accurate movement.

Kravitz et al (AJO 2009) showed that lingual crown tip (53%) was significantly more accurate than labial crown tip (38%), particularly for maxillary incisors.

8. Mesiodistal movement

Clear aligners can produce both uncontrolled and controlled tipping movements. The uncontrolled tipping in closing extraction sites and the least results in tipping canines suggest that teeth with larger roots might have greater difficulty achieving mesiodistal movements.

Baldwin et al (AJO 1996) showed a mean change in radiographic and dental cast interdental

angle of about 17º after Clear aligner therapy.

Kravitz et al (AJO 2009), conducted a study on anterior teeth and showed a mean accuracy of 41% for mesiodistal tipping, in which the highest accuracy was achieved by the maxillary (43%) and mandibular (49%) lateral incisors. The maxillary (35%) and mandibular (27%) canines and the maxillary central incisors (39%) had the lowest accuracy.

9. Rotation

Nguyen and Chen (2006), concluded that incisors achieved 60% of the predicted rotation, while canines and premolars had the lowest accuracy (39%). Thus, teeth with rounded crowns seem to be more difficult to rotate with clear aligners.

Kravitz et al (AO 2008) assessed a mean accuracy for canine rotation of 36%. Canines that received interproximal reduction (IPR) reported the highest mean rotational accuracy (43%).

Kravitz et al (AJO 2009) found the accuracy of rotation for maxillary canines was 32%, lower than that of the maxillary central incisors (55%) and mandibular lateral incisors (52%). The least accuracy was detected for the mandibular canine (29%). For rotations greater than

15º the accuracy of maxillary canine movement was significantly reduced. 

10. Vertical movement

Kravitz et al (AJO 2009), reported that,

Intrusion- Highest accuracy of intrusion was achieved by the maxillary (45%) and mandibular (47%) central incisors. The lowest accuracy of intrusion was achieved by the maxillary lateral incisors (33%). The average amount of true intrusion attempted was 0.72 mm.

Extrusion- It is the least accurate tooth movement achieved with clear aligners (30% of predictability). The maxillary (18%) and the mandibular (25%) central incisors had the lowest accuracy. The average amount of extrusion attempted was 0.56 mm. 

Charalampakis et al (AJO 2018), reported the most accurate movements that could be achieved with were incisor intrusion and canine rotation.

11. Space Closure/ Extraction

Weihong et al. (2015) evaluated the effectiveness of the clear aligner system on mild to moderate cases treated with premolar extractions and compared the treatment results obtained with fixed appliances. Their results revealed that both systems can be used in the treatment of extraction cases, and that root angulation attained with clear aligners are adequate when proper attachments are to be used.

A survey by Best et al (AO2017) and D’Appuzo el al (2019) have concluded that there is a significant difference between general dentists and orthodontists in case perception, both groups treat mainly Class I spacing and crowding, with more orthodontists treating Class I open-bite cases.

General dentists are more willing to treat relatively complex cases with clear aligner therapy only, while orthodontists are more likely to inform patients about the complexity of their cases and of the need to use auxiliaries with clear aligners.

Overall, the higher percentage of both orthodontists and general dentists reported they were more confident treating class I dental relationships and malocclusions with a mild-to-moderate crowding.

Conclusion

Clear aligner could be recommended for the treatment of simple malocclusions with light overbite discrepancies. Efficacy of orthodontic movement revealed that aligners were successful in controlling vertical buccal occlusion. It should be kept in mind that treating extraction cases requires experience and extensive knowledge of the system. They are effective in aligning and levelling the arches in non-growing subjects.

Treatment with aligners is not based on aligners alone. It requires the use of auxiliaries (attachments, interarch elastics, IPR, altered aligner geometries) to improve the predictability of orthodontic movement.

Therefore, in addition to an awareness of clear aligner limitations and case selection criteria, creative treatment planning and an acquisition of experience and competence are critical after the initial learning curve. Appropriate sequencing of movements and utilization of auxiliary techniques can result in more effective and efficient movements.

References

• Ke et al. A comparison of treatment effectiveness between clear aligner and fixed appliance therapies BMC Oral Health (2019) 19:24
• d’Apuzzo et al. Clear aligner treatment: different perspectives between orthodontists and general dentists. Progress in Orthodontics (2019) 20:10
• Park et al, Aligner Corner, JCO 2021 
• Rossini et al, Efficacy of clear aligners in controlling orthodontic tooth movement: A systematic review, Angle Orthodontist, Vol 85, No 5, 2015
• Tamer et al. Orthodontic Treatment with Clear Aligners and The Scientific Reality Behind Their Marketing: A Literature Review, Turk J Orthod 2019; 32(4): 241-6
• T. Wier, Clear aligners in orthodontic treatment, Australian Dental Journal 2017; 62:(1 Suppl): 58–62