Idealized force decay of orthodontic elastomeric chains follows Nutting Equation

Abstract

The force decay of orthodontic elastomeric modules has been of considerable interest since these biomechanical force-generating appliances were introduced five decades ago. Two mathematical approaches, a power-law equation and a Maxwell–Weichert model, have been used by previous investigators to predict the in vitro force degradation under idealized conditions. In this study, the principal sensor approach had the test specimen being strained continuously using a digital force gauge. Test specimens of 13 products from 8 manufacturers were stretched to 25 mm and tested in an artificial saliva at 37°C in both the as-received condition and after 24 h conditioning. Measurements for specimens strained continuously on the digital force gauge were compared with intermittent measurements for other specimens transferred to this gauge after 19.2 h ageing on a stretching rack, the approach in most past studies. Force data at equally spaced logarithmic time points, normalized to initial force and induced strain, were used to present degradation curves for individual products. Nonlinear regression analyses were used to fit both mathematical models to each product and conditioning state. Statistical analyses show that the power-law equation (historically the Nutting equation) provides a more accurate prediction of idealized in vitro force decay for the elastomeric products.