In-vivo von Mises strains during Invisalign treatment.

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.