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Mechanical Characterization of Thermoplastic Aligner Materials: Recommendations for Test Parameter Standardization
F Elkholy1, S Schmidt1, M Amirkhani2
1Department of Orthodontics, University of Ulm, Ulm, Germany.
Journal of Healthcare Engineering
|July 6, 2019
Summary
Standardizing three-point-bending tests for thermoplastic aligner materials is crucial. This study establishes thickness-dependent deflection guidelines to prevent cracks and ensure comparable results in aligner material testing.
Area of Science:
- Biomaterials Science
- Mechanical Engineering
- Dental Materials
Background:
- Mechanical properties of thermoplastic aligners are vital for treatment success.
- Existing material testing data is difficult to compare due to inconsistent methodologies.
- Standardized testing protocols are needed for reliable aligner material evaluation.
Purpose of the Study:
- To evaluate test parameters for standardized three-point-bending aligner material tests.
- To establish guidelines for future, comparable aligner material testing.
- To determine optimal testing conditions for polyethylene terephthalate glycol (PET-G) aligner foils.
Main Methods:
- Varied specimen preparation and three-point-bending test parameters.
- Tested polyethylene terephthalate glycol (PET-G) foils of different thicknesses, including thermoformed samples.
- Investigated span lengths (8 and 16 mm) and deflection ranges (0.1–0.2 mm).
- Assessed the effect of water storage on bending forces and validated results with a mathematical model.
Main Results:
- Thickness-dependent deflection ranges prevent macroscopic cracks by maintaining stresses between 14–18 MPa.
- Water immersion significantly decreased bending force by 50% in loaded specimens, versus 14% in unloaded ones.
- Mathematical modeling confirmed the relationship between deflection, stress, and crack prevention.
Conclusions:
- In vitro three-point bending tests should mimic clinical aligner therapy's geometrical and stress conditions (14–18 MPa).
- Established thickness-dependent deflections for PET-G specimens at an 8 mm span length are recommended.
- Adopting these parameters will yield valid and comparable results for future aligner material studies.
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