High Force Levels Can Be Delivered With Thermoplastic Aligners Initial Forces Generated by Three Types of Thermoplastic Appliances on an Upper Central Incisor During Tipping

Background: Treatment of minor malocclusions with clear thermoplastic aligners is more commonly being used as an aesthetic alternative to fixed appliances. This can be done by manually resetting teeth on a plaster model or by models generated from a 3D computer model (Invisalign®). However, little research has been performed on the complex force systems generated by these types of appliances. Objective: To determine the initial forces placed on a tipped central incisor by 3 types of thermoplastic aligner materials Design: In vitro laboratory study using a simulated dentition. Methods: A sensor capable of measuring all forces and moments generated on a tooth was attached to a manual positioning system and placed into a standardized resin model of a maxillary arch. A plaster model was made of the arch with the incisor in a neutral position, and 5 aligners were manufactured from each of 3 thermoplastic materials: Ideal Clear® 1.0 mm; Erkodur® 1.0 mm; and Biolon® 1.0 mm. The tooth was then tipped in 9 steps from 0 to 0.416 degrees in both a palatal and facial direction (which corresponded to an incisal edge deviation of 0.15 mm). Forces were measured at each step, and statistical analysis was done using the Kruskal-Wallis and Wilcoxon tests. Results: The mean tipping forces ranged from –2.82 N ± 0.62 N to 5.42 N ± 0.56 N, and the mean intrusive forces ranged from –0.41 N ± 0.52 N to –2.3 N ± 0.43 N. The Biolon appliances produced significantly higher forces. Conclusions: These force values are larger than those previously reported and are approximately 5 to 10 times higher than suggested ideal tipping forces (0.35 N to 0.60 N). Intrusive forces were present when tipping movements were simulated. Reviewer's Comments: The authors acknowledge that the lack of a periodontal ligament (PDL) is a major limitation of their model, as the PDL would decrease force levels after initial ligament compression. It is helpful to realize that there are side effects, such as intrusion forces, that are not readily apparent and that not all thermoplastic materials behave the same. It is clear that thermoplastic aligners can generate significant force levels, at least on initial insertion, that may be much higher that force levels thought to be ideal. (ReviewerBrent E. Larson, DDS, MS).