Forces of various nickel titanium closed coil springs

To compare the forces generated by 14 different 9 mm springs supplied by five different companies.

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 degrees 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.

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.

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.