Journal List > Korean J Orthod > v.40(1) > 1043622

Min, Hwang, Yu, Lee, and Cha: The effect of thickness and deflection of orthodontic thermoplastic materials on its mechanical properties



The purposes of this study were to evaluate the force and stress depending on the type, deflection and thickness of the materials and to evaluate the mechanical properties of thermoplastic materials after repeated loading.


Four types of thermoplastic products were tested. Force until the deflections of 2.0 mm and the stress when the materials were restoring to its resting position were evaluated. The mechanical properties of thermoplastic materials evaluated after 5 repeated loading cycles.


The interaction was observed between the thickness and the deflection (p < 0.05) from the regression equation. Thickness and amount of deflection rather than products and materials showed the largest effect on force and stress. In all products, at least 159 gf of force was required for more than 1.0 mm deflection or when materials with 1.0 mm thickness were deflected. The stress recorded was more than 19 gf/mm2. During repeated loading, each group showed significant difference on the force and the stress p < 0.01), 10 - 17% reduction of force and 4 - 7% reduction of stress in average.


Proper thickness of thermoplastic materials and deflection level of tooth movement should be decided for the physiologic tooth movement. Force decay after repeated loading should be considered for the efficient tooth movement.

Figures and Tables

Fig 1
Biostar® (Scheu-Dental, Iserlohn, Germany) and fabricated dental model.
Fig 2
Experimental procedure used in this study. A, Universal test machine, Instron®; B, schematic diagram of 3 point bending test.
Fig 3
Increase in bending force and recovery stress level depending on the thickness and amount of deflection for all four types of products (Lt, Force (gf); Rt, recovery stress (gf/mm2)).
Fig 4
Graph showing bending force and recovery stress changes after repeated loading (Lt, Force (gf); Rt, recovery stress (gf/mm2)).
Table 1
Prescription of materials used in this study

Essix ACE does not provide 0.5 mm thickness sheets.

Table 2
Condition of thermoform used in this study
Table 3
Thickness differentiation after thermoform (Unit : mm)

SD, Standard deviation; ΔThickness, thickness differentiation after thermoform.

Table 4
Factors associated with bending force and recovery stress (top) and multiple regressions for interaction among products, thickness, and deflection rate (bottom)

DF, Degree of freedom; Sig, significant; NS, not significant; SE, standard error. *p < 0.001.

Table 5
Comparative mean force according to three thickness levels and four deflection levels for four types of products

SD, Standard deviation. *p < 0.05.

Table 6
Comparative mean recovery stress according to three thickness levels and four deflection rates for four types of products

SD, Standard deviation.


This study was supported by a faculty research grant of Yonsei University College of Dentistry for 2008 (6-2008-0058).


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