Accumulated biomechanical effects of mandibular molar mesialization using clear aligners with auxiliary devices: an iterative finite element analysis

BACKGROUND

The biomechanics generated by the clear aligner (CA) material changes continuously during orthodontic tooth movement, but this factor remains unknown during the computer-aid design process and the predictability of molars movement is not as expected. Therefore, the purpose of this study was to propose an iterative finite element method to simulate the long-term biomechanical effects of mandibular molar mesialization (MM) in CA therapy under dual-mechanical systems.

METHODS

Three groups including CA alone, CA with a button, and CA with a modified lever arm (MLA) were created. Material properties of CA were obtained by in vitro mechanical experiments. MM was conducted by the rebound force exerted by CA material and the mesial elastic force (2N, 30° to the occlusal plane) applied to the auxiliary devices. Stress intensity and distribution on periodontal ligament (PDL), attachment, button and MLA, and displacement of the second molar (M2) during the iterations were recorded.

RESULTS

There was a significant difference between the initial and cumulative long-term displacement. Specifically, compared to the beginning, the maximum stress of PDL decreased by 90% on average in the intermediate and final steps. The aligner was the main mechanical system at first, and then, the additional system exerted by the button and MLA dominated gradually. The stress of attachments and auxiliary devices is mainly concentrated on their interfaces with the tooth. Additionally, MLA provided a distal tipping and extrusive moment, which was the only group that manifested a total mesial displacement of the root.

CONCLUSIONS

The innovatively designed MLA was more effective in reducing undesigned mesial tipping and rotation of M2 than the traditional button and CA alone, which provided a therapeutic method for MM. The proposed iterative method simulated tooth movement by considering the mechanical characteristic of CA and its long-term mechanical force changes, which will facilitate better movement prediction and minimize the failure rate.

[Biomechanics of lower molar mesialization on mini-screws in multi-vestibular technique]

INTRODUCTION

Molar protraction can be used in several clinical situations. It provides a real service to the patient, when it makes it possible to replace an implant, for example, or in case of a very damaged molar compared to the patient’s age.

MATERIAL AND METHOD

In this article, the authors describe the different molar mesialization systems in the vestibular multi-attachment technique, outlining the biomechanics of each.

DISCUSSION

The mini-screws introduction in the practices has made it possible to improve the anchorage management, which is essential in this therapy. However, it is not always offered, depending on the patient’s wishes, but also sometimes of the orthodontist’s.

CONCLUSION

The different biomechanical situations analyzed each have their own characteristics, advantages and disadvantages. A methodical reflection must lead the practitioner to choose the system that suits him, according to his experience and practice, in order to implement this therapy for the well-being of his patients.