Do the NiTi low and constant force levels remain stable in vivo? 
The aim was to evaluate the alteration of the deactivation forces of the most commonly used nickel-titanium wires under long-lasting oral cavity environmental influence.  Randomized in vitro and in vivo trials of 540 pieces of orthodontic archwires, NeoSentalloy®, Copper NiTi® 35°C and Titanol Superelastic(®), round (0.016 inch), and rectangular (0.016 × 0.022 inch), were carried out.  Randomization and blinding was achieved with 12-colour system that ensured encoding of key information on the tested specimens.  Each of 270 patients (females, 18-20 years old, in the finishing stage of orthodontic treatment) received the piece of NiTi wire ligated piggyback, for a period of 4-6 weeks.  Eventually, all samples were subjected to a three-point bending test.  Data were statistically analysed at a 5 per cent significance level.  In the group of the round used wires, when compared with the new ones, the deactivation force (F dav) values increased significantly for Titanol Superelastic®; the NeoSentalloy® and Copper NiTi® 35°C wires did not change their values of F dav.  In the group of rectangular wires, F dav decreased for Titanol Superelastic®, but increased for NeoSentalloy® wires.  The F dav values of the Copper NiTi® 35°C used wires practically did not change, but a remarkable increase of the standard deviation was noted.  No calculations concerning effectiveness of in vivo aligning of analysed wires were made.  For the purpose of a 4-6-week aligning stage, round NeoSentalloy® with a diameter of 0.016 inches seems to be the wire of choice because of the low level of F dav.