In Vivo Strain Measurement Analyzing Tooth Movement during Invisalign Treatment

Objectives: The Invisalign system is a state-of-the-art appliance for invisible orthodontic treatment. The behavior of force (strain) over time might shed light on the duration of aligner wearing. The objective of this study was to analyze the strain developed in each aligner during a wearing period of 16 days in a mobile and immobile dental unit.

Methods: Two series of aligners (about 40 aligners in each series, per patient) were prepared for each of the three patients who had a maxillary central diastema and an increased over-jet (>6 mm). In the first series, two strain-gauge rosettes were bonded to each aligner, one on the labial aspect of the maxillary central incisor (mobile unit, active tooth) and the other on the buccal aspect of the maxillary first premolar (immobile unit, anchored tooth). These first series of aligners were used for in-vivo strain measurements at days D1, D4, D9, and D16, ie., inserted in the mouth only during strain measurements. The second series was used for alignment, ie., worn by the patient for the 16 days. The Von Mises strains were considered as the parameter characterizing tooth movement during aligner wearing. Variations were statistically determined between aligners per patient using ANOVA with repeated measures.

Results: At D1, the Von Mises strains in the incisor tooth were significantly greater than the remaining days (p<0.05) in all patients. A constant strain level was reached from D9 to D16. Each patient showed a different strain pattern on the premolar.

Conclusions: Tooth movement was nearly completed from D1 to D2, reaching its final position within each aligner. This suggests that from D2 to D16, internal biological processes (bone remodeling, PDL reorganization) occurred, and reduction in aligner wearing can be considered.

Supported by a research grant from Align Technology Inc.