Combined aging effects of strain and thermocycling on unload deflection modes of nickel-titanium closed-coil springs: An in-vitro comparative study

• Published in: American journal of orthodontics and dentofacial orthopedics Vol. 138; no. 4; pp. 451 - 457
• Main Authors: Vidoni, Gabriele, Perinetti, Giuseppe, Antoniolli, Francesca, Castaldo, Attilio, Contardo, Luca
• Format: Journal Article
• Language: English
• Published: New York, NY Elsevier Inc 01.10.2010; Elsevier
• Subjects: Analysis of Variance; Biological and medical sciences; Dental Alloys; Dental Stress Analysis; Dentistry; Elasticity; Hot Temperature; Linear Models; Materials Testing; Medical sciences; Nickel; Orthodontic Appliances; Otorhinolaryngology. Stomatology; Saliva; Stress, Mechanical; Titanium; Orthodontic procedures; Titanium; Nickel; Dentistry; In vitro; Comparative study; Coil
• ISSN: 0889-5406; 1097-6752
• DOI: 10.1016/j.ajodo.2009.05.022

Introduction There are no reports on the aging effects of thermocycling of nickel-titanium (NiTi) based coil springs, and few studies have investigated their superelasticity phases in full. In this study, we compared the mechanical properties of NiTi-based closed-coil springs after the combined aging effects of prolonged strain and thermocycling, as a reflection of the clinical situation. Methods Ninety NiTi-based closed-coil springs were used, 30 each of the following types: (1) Nitinol (3M Unitek, Monrovia, Calif), (2) Ni-Ti (Ormco, Glendora, Calif), and (3) RMO (Rocky Mountain Orthodontics, Denver, Colo); all had similar dimensions (length, 12 mm). In each sample group, 2 equal subgroups of 15 coil springs were extended by either 50% (to 18 mm) or 150% (to 30 mm), immersed in artificial saliva, and kept at 37°C for 45 days. All springs underwent sessions of 1000 thermocycles (1 minute long) from 5°C to 55°C on days 22 and 45. Unload deflection curves from both the 50% and 150% extensions (according to their strain subgroups) were recorded by using a universal testing machine before the strain (baseline) and at both 22 and 45 days, immediately after thermocycling. Results At baseline, the loads exerted by the NiTi-based coil springs varied from 99.8 to 245.1 gf for the RMO (50% strain) and Ni-Ti (150% strain) groups. Statistically significant, although small, differences were seen at each time point in both the 50% and 150% strain subgroups; generally, the highest and lowest values were recorded in the Ni-Ti and Nitinol groups (all, P  <0.001). Only the Nitinol coil-spring group showed an acceptable superelasticity phase. The strain and thermocycling did not dramatically change the deactivation forces of any coil springs. Conclusions NiTi-based closed-coil springs might not have a superelasticity phase, and prolonged strain and thermocycling do not produce clinically relevant alterations in their deactivation forces.