An In Vitro Comparison of the Forces Released by Closed Nickel-Titanium Springs

The aim of this study was to evaluate the force-release behavior of closed coil nickel-titanium springs of three different commercial brands available on the market, as well those from different batches, tested over multiple distances, according to the number of deactivations.

For this in vitro analysis, closed coil nickel-titanium springs of the following numbers and brands: eight, 3 mm GAC® springs; eight, 3 mm springs from TP Orthodontics® and eight, 5 mm springs from MORELLI® were standardized at 200 gf force. Two batches of all three brands were tested and compared. All springs were evaluated on a test machine and extended to a distance of five times beyond their original length. The springs were extended, and the distances of force released were measured and marked at every 20% of the total spring length, during their deactivation, and the process was repeated five times for each spring.

The intraclass correlation coefficients showed that the value of 200 gf indicated by the manufacturers was poorly reproduced. The springs did not change their force release after successive extensions. All springs presented higher deactivation force values as their stretching increased.

All springs presented the greater release of force as their stretch was increased, but the GAC® brand springs presented a more uniform pattern. TP Orthodontics® springs exhibited an acceptable force release pattern, but MORELLI® springs proved to be far below expected strength.

As a clinical consideration of the study, the results obtained in the present research are relevant to the use of nickel-titanium closed springs with regard to the quantity of force released during the mechanics of space closure after extraction (main application of nickel-titanium closed springs), and to the pattern of forces released by springs in view of the real context of gradual closure of spaces after tooth extractions.