Korean J Orthod.  2018 Nov;48(6):384-394. 10.4041/kjod.2018.48.6.384.

Effects of prestretch on stress relaxation and permanent deformation of orthodontic synthetic elastomeric chains

• Chang JH
• Hwang CJ
• Kim KH
• Cha JY
• Kim KM
• Yu HS

• KMID: 2427776
• DOI: http://doi.org/10.4041/kjod.2018.48.6.384

Abstract

OBJECTIVE
This study was performed to investigate an appropriate degree of prestretch for orthodontic synthetic elastomeric chains focusing on time-dependent viscoelastic properties.
METHODS
Orthodontic synthetic elastomeric chains of two brands were prestretched to 50, 100, 150, and 200% of the original length in one and three cycles, and the hysteresis areas of the obtained stress-strain curves were determined. Acrylic plates were employed to maintain constant strain during the experiment. A total of 180 samples were classified into nine groups according to brand, and their stresses and permanent deformations were measured immediately after prestretch (0 hour), after 1 hour and 24 hours, and after 1, 2, 3, 4, 5, 6, 7, and 8 weeks. The relationship between stress relaxation and permanent deformation was investigated for various degrees of prestretch, and the estimated stress resulting from tooth movement was calculated.
RESULTS
The degree of prestretch and the stress relaxation ratio exhibited a strong negative correlation, whereas no correlation was found between the degree of prestretch and the average normalized permanent strain. The maximal estimated stress was observed when prestretch was performed in three cycles to 200% of the original length.
CONCLUSIONS
Although prestretch benefited residual stress, it did not exhibit negative effects such as permanent deformation. The maximal estimated stress was observed at the maximal prestretch, but the difference between prestretch and control groups decreased with time. In general, higher residual stresses were observed for product B than for product A, but this difference was not clinically significant.

Keyword

Orthodontic synthetic elastomeric chain; Prestretch; Stress relaxation; Permanent deformation

MeSH Terms

Elastomers*
Relaxation*
Tooth Movement
Elastomers

Figure

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  Figure 1 A, Orthodontic synthetic elastomeric chains used in the present study: product A (above) and product B (below). B, Photograph of control vs. permanent deformation (above), with the measured length indicated by an arrow (below). C, Fabricated acrylic plate. D, Classification table (product A: A1–A9; product B: B1–B9). E, Distance measurement for constant strain. F, Storage in artificial saliva. G, Storage in an incubator at 37℃. H, Measurement of stress using a universal testing machine. I, Distance measurement utilizing a digital Vernier caliper. J, Observation using the Micro Hi Scope system (Hirox, Tokyo, Japan).

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  Figure 2 A, Plot of the hysteresis area enclosed by the stress-strain curve vs. the degree of prestretch. B, Stress relaxation curves measured for 8 weeks and enlarged curves recorded at times of up to 24 hours. C, Temporal dependence of normalized permanent strain (gx(t), Eq. 10). D, Temporal dependence of estimated stress (δx(t), Eq. 15) resulting from tooth movement (1 mm/4 weeks). E, Three-dimensional plot for group B9. Hysteresis representing the degree of prestretch is shown as the area of the stress-strain curve (a). A decrease in stress (b) and an increase in permanent deformation (c) were observed with increasing time. The reduction of estimated stress resulting from tooth movement (arrow) is shown in (d).

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  Figure 3 A, Scatter plot of the degree of prestretch (HX) vs. the stress relaxation ratio (RX). B, Scatter plot of HX vs. average normalized permanent strain (gX(c)). C, Scatter plot of HX vs. permanent deformation strain (εper) immediately after prestretch. D, Scatter plot of sustained total strain (εtotalx) vs. average permanent strain (εperx(c)).

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  Figure 4 Average of recorded stress (σx(c)) and elastic restoring stress (δx(c)) of products A (A) and B (B) obtained after 8 weeks.

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  Figure 5 Images captured immediately after prestretch by the Micro Hi Scope system (Hirox, Tokyo, Japan; original magnification, ×50).

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