Methods: To determine the force-extension in dry state, elastics were stretched by continuous extension from slack condition to 60 mm at 25°C. Resultant forces versus length changes were recorded. To determine the force-degradation in wet conditions, elastics were stretched to 20 mm and 40 mm and were stored in artificial saliva at temperature of 37°C in a moisture oven. Force measurements were made at 3 intervals: 1, 24, and 72 hours with universal strength testing machine (Hounsfield, UK). Multiple group comparisons were made by ANOVA and pair wise comparisons by t-test.
Results: In dry test, overall, elastics did not perform according to specifications marketed by different companies. In general, the loads at three times the internal diameter were larger than the manufacturer's specified loads. The result of the present study, also, showed a force decay of a 12.38% and 20.5% during the first hour and this decay increased to 20.1% and 28.5% at 24 hours, and to 28.0% and 34.4% up to 72 hours, when extended to 20 and 40 mm respectively. Significant differences between comparable sizes of elastics among all manufacturers were noted.
Conclusions: It is concluded that from mechanical perspectives, though the initial elastic forces were higher than what has been prescribed, they reduce to a clinically acceptable range within hours.