Document Type: Original Article


1 Orthodontic Research Center, Dept. of Orthodontics, School of Dentistry, Shiraz University of Medical Sciences, Shiraz, Iran

2 Orthodontist


Statement of Problems: The main disadvantage of elastic chains is their rapid force decay. Thus, pre-stretching has been recommended as a method to overcome this ever existing problem. However, the information regarding the magnitude and the method of its performance are still obscure or varied.   
Purpose: This study was designed to evaluate the effect of pre-stretching on force degradation of the synthetic elastomeric chains.
Materials and Method: This experimental study had two categories of synthetic elastomeric chains: 1. Control (unprestretched) 2. Test (prestretched). The test groups were pre-stretched to 33%, 100% and 200% of the original length. We evaluated 4 control and 16 test groups from four manufacturers. In a period of 5 seconds, each specimen was pre-stretched in the rate of 30 mm per minute. Then, all the specimens were stretched to 25 mm and fixed in this distance on the frames and were merged in 37 degree water. Their forces were measured at 0 hour, 1 hour, 24 hours, 1 week, and 3 weeks. Data were analyzed using Friedman, ANOVA and Green house-Geisser test.
Results: All the 20 groups had force degradation in the study period ( p <001). The highest rate of force degradation (about half of the total force degradation) occurred at the first hour. But the rate of force degradation declined over time. In all brands, force degradation was smaller in pre-stretching 200% than the control group ( p <0.01-0.001).
Conclusion: Synthetic elastomeric chains from several companies have different effects from different distances of pre-strectching, so the appropriate pre-stretching length must be defined for each kind of synthetic elastomeric chain.