The Force-Elongation Characteristics of Orthodontic Elastomeric Chains

Objectives: Elastomeric chains are used extensively to effect orthodontic tooth movement but despite many studies characterizing their properties, there is limited data on the extension needed to achieve a given force. Methods: American Orthodontics long connector open loop, short connector open loop and closed loop elastomeric chains were immersed in water, 0.9% saline and Ringer's lactate for 48 h at RT and 37C. Three loops of chain were tensile tested at a cross-head speed of 5 mm/min to a maximum load of 450 g or 3X initial length. After load release and 30s relaxation, the chains were re-loaded to 450 g. All tests were performed in quadruplicate and the load vs. extension data analyzed by ANOVA and post-hoc Scheffé tests. Results: The tensile data indicated that there are complex relationships between chain extension and delivered force. Overall, all 3 chain configurations require an extension of approximately 50% to deliver a force of 200 g, approximately 75% extension for a force of 300 g, and over 100% for a force of 400 g regardless of the immersion medium. Conclusions: The findings indicate that in clinical use the chain should be pre-stretched to ca. 300% of initial length. When the chain segment spans a gap 50% greater than the segment length, the module will deliver a force of approximately 200 g. If the segment spans a gap that is 75% or 100% greater than the segment length, then the delivered forces will be 300 or 400 g respectively. The data indicate that neither the chain configuration nor immersion medium have a significant effect on the load-extension behavior.