Force degradation of closed coil springs: an in vitro evaluation.

This in vitro study was designed to determine the force degradation of closed coil springs made of stainless steel (SS), cobalt-chromium-nickel (Co-Cr-Ni) and nickel-titanium (Niti) alloys, when they were extended to generate an initial force value in the range of 150 to 160 gm. The specimens were divided into two groups. Group I included SS, Co-Cr-Ni, and two nickel-titanium spring types (Niti 1 and Niti 2), 0.010 x 0.030 inch with an initial length of 12 mm. Group II was comprised of SS, Co-Cr-Ni, and Niti 3 0.010 x 0.036-inch springs, with an initial length of 6 mm. A universal testing machine was used to measure force. A pilot study determined the extension required for each spring type, so that the initial force was in the range of 150 to 160 gm. Initial force was recorded, and then the springs were extended to the respective distances at 4 hours, 24 hours, 3 days, 7 days, 14 days, 21 days, and 28 days resulting in a total of eight time periods. Between the time intervals, all springs were extended to the same initial extension on specially designed racks and stored in a salivary substitute at 37 degrees C. Means and standard deviations of force values, percent force loss, and mean extension were statistically analyzed. All springs showed a force loss over time. Of the total, the major force loss for most springs was found to occur in the first 24 hours.(ABSTRACT TRUNCATED AT 250 WORDS)