Stress relaxation properties of four orthodontic aligner materials: A 24-hour in vitro study.
OBJECTIVE
To investigate the stress release properties of four thermoplastic materials used to make orthodontic aligners when subjected to 24 consecutive hours of deflection.


METHODS
Four types of aligner materials (two single and two double layered) were selected. After initial yield strength testing to characterize the materials, each sample was subjected to a constant load for 24 hours in a moist, temperature-regulated environment, and the stress release over time was measured. The test was performed three times on each type of material.


RESULTS
All polymers analyzed released a significant amount of stress during the 24-hour period. Stress release was greater during the first 8 hours, reaching a plateau that generally remained constant. The single-layer materials, F22 Aligner polyurethane (Sweden & Martina, Due Carrare, Padova, Italy) and Duran polyethylene terephthalate glycol-modified (SCHEU, Iserlohn, Germany), exhibited the greatest values for both absolute stress and stress decay speed. The double-layer materials, Erkoloc-Pro (Erkodent, Pfalzgrafenweiler, Germany) and Durasoft (SCHEU), exhibited very constant stress release, but at absolute values up to four times lower than the single-layer samples tested.


CONCLUSIONS
Orthodontic aligner performance is strongly influenced by the material of their construction. Stress release, which may exceed 50% of the initial stress value in the early hours of wear, may cause significant changes in the behavior of the polymers at 24 hours from the application of orthodontic loads, which may influence programmed tooth movement.
Luca
L
Lombardo
Elisa
E
Martines
Valentina
V
Mazzanti
Angela
A
Arreghini
Francesco
F
Mollica
Giuseppe
G
Siciliani
D003799:Dental Stress Analysis; D006244:Hardness; D006801:Humans; D008422:Materials Testing; D016382:Orthodontic Appliance Design / Q000379:methods*; D010969:Plastics / Q000737:chemistry; D011092:Polyethylene Glycols; D011093:Polyethylene Terephthalates; D011108:Polymers; D011140:Polyurethanes; D013696:Temperature; D013997:Time Factors; D014087:Tooth Movement Techniques / Q000295:instrumentation*
D010969:Plastics; D011093:Polyethylene Terephthalates; D011108:Polymers; D011140:Polyurethanes; C475920:polyethylene terephthalate glycol; D011092:Polyethylene Glycols
F22 Aligner; Orthodontic aligners; Stress release; Thermoplastic materials