Force Decay Evaluation of Thermoplastic and Thermoset Elastomeric Chains - A Mechanical Design Comparison

Introduction: Orthodontic power chains are clinically used for short periods of time, typically being replaced every 3 to 4 weeks. Since the 1970’s, several studies have been published about the force decay of elastomers. The results of these various studies have shown a wide range of force decay. One of the reasons such a wide range was observed is that researchers and clinicians are equally unaware that there is a significant difference in the material behavior between the two types of elastomeric power chains, thermoplastic and thermoset. Therefore, both chain types were included in published results without any differentiation. To date, no study has compared the force decay of both types of power chains whether in vivo or in vitro. Objectives: To compare over a period of 8 weeks :(1) force decay between thermoplastic (TP) and thermoset (TS) power chains, (2) force decay between single and loop chains (3) force decay between using different force levels. Methods: TP and TS orthodontic power chains were obtained from American Orthodontics (AOTP, AOTS) and ORMCO (OrTP, OrTS). Each of the four power chain groups were subdivided into four subgroups with ten specimens per subgroup: (1) single chains light force, (2) single chains heavy force, (3) loop chains light force and (4) loop chains heavy force. The force was recorded with a digital force gauge at 12 time points; zero hours (baseline), 1 hour, 24 hours, 3 days, 1 week, 2 weeks, 3 weeks, 4 weeks, 5 weeks, 6 weeks, 7 weeks and 8 weeks. Specimens were stored in artificial saliva (pH of 6.75) at 37ᵒC degrees. Results: A significant difference (p≤0.001) was found between TP and TS power chains in all four subgroups and all time points with more force decay in the TP power chains. The average mean difference in force decay between the thermoplastic and thermoset power chains was around 20%. No significant difference was found between single and loop chains in the TS power chain groups or in AOTP. However, a significant difference was found between single and loop chains in OrTP power chains with more force decay in single chains. There was no significant difference in force decay between using light and heavy forces. Conclusion: TS power chains decay less than TP power chains, and loop chain retraction design is beneficial when using OrTP power chains. Contrary to the interchangeable use of TP and TS power chains in published literature and in clinical practice, this study demonstrates that they perform differently under stress and that a clear distinction should be made.