Original article
In-vivo von Mises strains during Invisalign treatment

https://doi.org/10.1016/j.ajodo.2008.11.027Get rights and content

Introduction

The Invisalign (Align Technology, Santa Clara, Calif) force delivery system has not been investigated. Since forces are related to the strains developed on the aligner surface, the behavior of the maxillary incisor and premolar von Mises strains (IVM, PVM) were studied during biweekly wear of an aligner.

Methods

Maxillary aligners (n = 61) were examined from 3 patients requiring maxillary incisor retraction and stationary anchored premolars. Two series of maxillary aligners were manufactured. Series 1 was worn by the patient, and series 2 served for in-vivo measurements with 2 strain gauge rosettes bonded to each aligner on the vestibular sides of the incisor and the premolar. Measurements were taken at days 1, 2, 9, and 15.

Results

All aligners demonstrated a peak IVM strain at day 1 (P <0.001); it then decreased at day 2 and plateaued through day 15. No anchorage loss was found in 2 patients (IVM ≥PVM), and a minute loss was evident in 1 patient (PVM >IVM).

Conclusions

Each aligner should be worn close to 24 hours in the first 2 days, with the time subsequently reduced (remaining 12 days). Final aligners should be thicker or worn for longer period (eg, 3 weeks). In spite of the inherent anchorage property of the aligner, attachment reinforcement should be considered in demanding anchorage requirements.

Section snippets

Material and methods

In this prospective cohort study, we examined 61 aligners. Inclusion criteria were pure retraction of the measured maxillary central incisor (active dental unit) with no contact with the mandibular incisors and no movement of the measured anchored tooth (passive dental unit), interdental spaces around the retracted tooth, and increased overjet. These 61 aligners were selected from 117 maxillary aligners; 51 aligners were excluded from the study because other tooth movements were incorporated

Results

The treatment plan for patient 1 included closure of the diastema between the maxillary central incisors (9 aligners) and retraction of all 6 maxillary anterior teeth to reduce the overjet (29 aligners) (Fig 2).

Figure 3, A, and the Table give the mean values of IVM and PVM strains measured during maxillary incisor retraction treatment. Both variables (time and location) significantly affected the developed strains (P <0.001). Additionally, an interaction was found between time and location (P

Discussion

This study reports a novel in-vivo methodology to assess the force behavior of the Invisalign technique during retraction of the anterior segment, based on the assumption that the force exerted by the aligner on a tooth can be interpreted by the von Mises strains developed on the aligner surface. An additional set of aligners (series 2), used only for strain measurements, avoided moisture that could interfere with SG measurements.24 It can be suggested that changes that could occur in the

Conclusions

  • 1.

    A novel methodology was introduced to measure in-vivo strains during Invisalign treatment.

  • 2.

    Peak IVM strains developed at day 1, decreased at day 2, and maintained a plateau from days 2 to 15; this means that, in each aligner, most tooth movement (incisor) occurred within the first 24 hours.

  • 3.

    Because the von Mises strain level did not decrease to zero from days 2 to 15, this is probably related to undersized aligner manufacturing; even in a passive form, the aligner will exert some pressure on all

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    Supported by a grant from Align Technology, Santa Clara, Calif.

    In partial fulfillment of the second author's requirements for the degree of master in orthodontics, Maurice and Gabriela School of Dental Medicine, Faculty of Medicine, Tel Aviv University.

    The authors report no commercial, proprietary, or financial interest in the products or companies described in this article.

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