Clear Aligners for maxillary anterior en masse retraction: A 3D finite element study

In earlier times, clear aligners were employed mainly for mild to moderate orthodontic tooth movements. With the development of software technology and the identification of biomechanical properties of aligner plastics, clear aligners have showcased a more promising capability to treat more complex cases, such as those that require tooth extractions.

A common side effect of anterior teeth retraction in extraction cases is the rise of the bowing effect. The bowing effect can cause deep overbite of the anterior teeth, the open bite of premolars, and the mesial tipping of the molars. Experts discovered that appropriate aligner overtreatment with canine attachments can ensure the bodily retraction of incisors in such cases.

That being said, clear aligners are associated with many limitations, especially regarding the failure of complete gap closure in extraction cases. More often than not, these plastic aligners are not rigid enough to retain their original shape in space closure. This may, in turn, result in loss of torque and adverse extrusion of the anterior teeth.

In order to confidently demonstrate the effectiveness of clear aligners in cases that involve maxillary anterior retractions, there are several studies that play a part in uncovering this truth.

This study was conducted by a team from Shanghai, China. It was published in Scientific Reports.

Clear aligners for maxillary anterior en masse retraction: a 3D finite element study.

Jiang T(#)(1), Wu RY(#)(1), Wang JK(1), Wang HH(1)(2), Tang GH(3)(4).

Sci Rep. 2020 Jun 23;10(1):10156. doi: 10.1038/s41598-020-67273-2.

Author information:

• Department of Orthodontics, Shanghai Ninth People’s Hospital, College of Stomatology, Shanghai Jiao Tong University School of Medicine, Shanghai, 200011, China.
• National Clinical Research Center for Oral Diseases; Shanghai Key Laboratory of Stomatology & Shanghai Research Institute of Stomatology, Shanghai, 200011, China.
• Department of Orthodontics, Shanghai Ninth People’s Hospital, College of Stomatology, Shanghai Jiao Tong University School of Medicine, Shanghai, 200011, China. drtanggh@163.com.
• National Clinical Research Center for Oral Diseases; Shanghai Key Laboratory of Stomatology & Shanghai Research Institute of Stomatology, Shanghai, 200011, China. drtanggh@163.com.

(#) Contributed equally

What they asked

The authors aimed:

“To evaluate tooth behaviours under various maxillary incisor retraction protocols for clear aligner therapy via a three-dimensional finite element model of the maxillary dentition.”

What they did

Using GEOMAGIC studio (Raindrop Geomagic, North Carolina, USA), they created three-dimensional finite element analysis models of the maxillary dentition with extracted first premolars, periodontal ligaments (PDLs), and alveolar bone.

They were able to reconstruct teeth and maxillary bone based on cone-beam computed tomography (CBCT) scanning of an adult male subject with well-aligned dentition and a normal upper incisor to a maxillary plane of 110 degrees.

They fabricated the PDLs as a linear elastic film with an average thickness of 0.25 mm around the roots of all teeth. To simulate a sequential pattern of space closure by which canines were distalized first followed by en masse retraction of incisors, a 0.5 mm space was designed in the model between canines and lateral incisors.

The following attachments were designed for the respective teeth:

• Horizontal rectangular attachments (2 mm height, 3 mm width, and 1 mm thickness) for lateral incisor
• Vertical rectangular attachments (3 mm height, 2 mm width, and 1 mm thickness) for teeth other than centre incisors.

The aligners had a thickness of 0.38 mm, in which external offset devices for all crowns and attachments were developed in the simulation.

What they found out

During incisor en masse retraction, different force systems and behaviours of both anterior and posterior teeth were recorded through three protocols. Analyses of the initial tooth displacements and stresses on PDLs were done with ANSYS software.

They found that the central (U1) and lateral (U2) incisors showed uncontrolled lingual tipping and extrusion upon 0.25 mm retraction. While U1 exhibited translation movement, U2 went through less tipping during 0.2 mm retraction and 0.15 mm intrusion. They also observed labial tipping and intrusion of U1 and bodily intrusion of U2 during 0.1 mm of retraction and 0.23 mm of intrusion.

With the additional intrusion on incisors, canines showed extrusion movement, and higher stresses on periodontal ligaments were shifted from U2 to canines.

Incisors also exhibited different mesial-distal angulation in the three simulations, while posterior teeth all suffered mesial inclination. Incorporating intrusion displacement in clear aligners led to a tendency of lingual root movement during incisor retraction. The complexity of tooth movement should be recognized regarding clear aligner therapy.

What we can conclude

The authors explored a unique loading method in this study to mimic the mode of action for clear aligners in anterior en masse retraction cases in a 3D FE model. The results were underwhelming. It showed that incorporating intrusion displacement on aligners led to a tendency of lingual root movement for incisor retraction. The discrepancy between the tooth movement displayed in the FE model and the tooth movement planned for incisors reiterated the complexity of the force system in clear aligner therapy.