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What do we know about the stress-relaxation or force decay of elastomeric chain?

What are the Elastomeric chains? Elastomeric chains or power chain elastic (PCE) are synthetic elastomers. PCEs are primarily made from polyurethane, which can be either thermoplastic or thermoset. Thermoplastic polyurethanes PCEs are moldable at high temperatures and can be made from plastic. It has been shown that PCEs experience more force decay at 28 days in vitro and require less prestretching than do thermoset PCEs (Kim et al 2005).

What does stress-relaxation or force decay of elastomeric chain mean? As the name suggests this is the decrease in stress that occurs with time when PCE is subjected to constant strain followed by permanent deformation of the elastomer when polymer molecules irreversibly slide past each other (Masoud et al 2014).

What do we know about stress relaxation?

Bishara in 1974 showed that PCE loss half of its force in the first 24 h followed by a more stable phase of 10% to 20% force decay in the first 4 weeks, so, it recommended to over extended the PCE. However, since the PCE probably retain some level of force past 4 weeks, it is unknown whether the force remaining is sufficient to move teeth in a clinical environment. A study by Evan et al in 2017 found that unaltered (old PCE) continued to exhibit space closure throughout the 16 weeks of measurement! A statistically insignificant difference in space closure means between the altered and unaltered paired sites.

Does the manufacturing procedure affect the performance of PCE?

Baty in 1994 found more consistent force if PCE manufactured by stamping rather than injection moulding. He also found that the initial force delivered depend on the company and so the force should be measured by gauge however the degradation is same for all types. Moreover he found that short PCE produce more force which stays longer than long PCE. Finally, the study showed that gray PCE bec of their filler maintain and produces higher force than clear type.

What do you think?

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