Forces and moments generated by 3D direct printed clear aligners of varying labial and lingual thicknesses during lingual movement of maxillary central incisor: an in vitro study

Abstract

Objective: The objective of this study was to measure the forces and moments exerted by direct printed aligners (DPAs) with varying facial and lingual aligner surface thicknesses, in all three planes of space, during lingual movement of a maxillary central incisor.

Materials and methods: An in vitro experimental setup was used to quantify forces and moments experienced by a programmed tooth to be moved and by adjacent anchor teeth, during lingual movement of a maxillary central incisor. DPAs were directly 3D-printed with Tera Harz TC-85 (Graphy Inc., Seoul, South Korea) clear photocurable resin in 100-µm layers. Three multi-axis sensors were used to measure the moments and forces generated by 0.50 mm thick DPAs modified with labial and lingual surface thicknesses of 1.00 mm in selective locations. The sensors were connected to three maxillary incisors (the upper left central, the upper right central, and the upper left lateral incisors) during 0.50 mm of programmed lingual bodily movement of the upper left central incisor. Moment-to-force ratios were calculated for all three incisors. Aligners were benchtop tested in a temperature-controlled chamber at intra-oral temperature to simulate intra-oral conditions.

Results: The results showed that increased facial thickness of DPAs slightly reduced force levels on the upper left central incisor compared to DPAs of uniform thickness of 0.50 mm. Additionally, increasing the lingual thickness of adjacent teeth reduced force and moment side effects on the adjacent teeth. DPAs can produce moment-to-force ratios indicative of controlled tipping.

Conclusions: Targeted increases in thickness of direct 3D-printed aligners change the magnitude of forces and moments generated, albeit in complex patterns that are difficult to predict. The ability to vary labiolingual thicknesses of DPAs is promising to optimize the prescribed orthodontic movements while minimizing unwanted tooth movements, thereby increasing the predictability of tooth movements.

Keywords: Digital orthodontics; Direct printed aligners; Forces; Moments.

Fig. 1

Experimental apparatus of multi-axis force/moment sensors. A The experimental apparatus with three multi-axis force/moment transducers connected to the upper central incisors and the upper left lateral incisors. B An individual AFT20-D15 multi-axis force/moment transducer

Fig. 2

Cross-sections demonstrating increased labial thickness of aligner. A Orthogonal view with accompanying thicknesses depicted. B Oblique view

Fig. 3

Diagram of the varying labiolingual thicknesses by the five experimental groups

Fig. 4

Forces (Fx, Fy, Fz) produced by direct printed aligners on the upper left central incisor

Fig. 5

Moments (Mx, My, Mz) generated by direct printed aligners on the upper left central incisor

Fig. 6

Forces (Fx, Fy, Fz) produced by direct printed aligners on the upper right central incisor

Fig. 7

Moments (Mx, My, Mz) generated by direct printed aligners on the upper right central incisor

Fig. 8

Forces (Fx, Fy, Fz) produced by direct printed aligners on the upper left lateral incisor

Fig. 9

Moments (Mx, My, Mz) generated by direct printed aligners on the upper left lateral incisor

Fig. 10

Comparison of forces produced by direct printed aligners on different teeth between the five tested study groups

Fig. 11

Comparison of moments generated by direct printed aligners on different teeth between the five tested study groups