Forces of various nickel titanium closed coil springs

Objective: To compare the forces generated by 14 different 9 mm springs supplied by five different companies. Materials and Methods: Five replicates of 14 different 9 mm springs were evaluated, resulting in 70 total specimens. Each was extended once from its resting length to 12 mm and then was deactivated. All tests were performed in a 37°C water bath. Forces were recorded at the 12 mm extension and deactivation distances of 9 mm, 6 mm, 3 mm, and 1 mm using an MTS force gauge. Data were collected with Testworks software, version 4.0, and were analyzed by analysis of variance (ANOVA) with one factor alternated. Results: Mean peak load forces at 12 mm were significantly different between springs, and these forces varied from 147 to 474 grams. Mean unload forces measured at 9 mm, 6 mm, and 3 mm of deactivation values were highly variable, and only 6 of the 14 springs exhibited a ''physiologic'' mean unload force of 50 grams or less over the total deactivation range. Conclusions: Few springs tested exhibited physiologic peak load forces and constant deactivation forces. This study suggests that labeling of nickel titanium closed coil springs is confusing and misleading.