In-vivo force decay of nickel-titanium closed-coil springs.
BACKGROUND
Nickel-titanium closed-coil springs are purported to deliver constant forces over extended ranges of activation and working times. In-vivo studies supporting this claim are limited. The objective of this study was to evaluate changes in force-decay properties of nickel-titanium closed-coil springs after clinical use.


METHODS
Pseudoelastic force-deflection curves for 30 nickel-titanium coil springs (used intraorally) and 15 matched laboratory control springs (simulated intraoral conditions: artificial saliva, 37°C) were tested before and after retrieval via dynamic mechanical analysis and a testing machine, respectively, to evaluate the amounts of force-loss and hysteresis change after 4, 8, or 12 weeks of working time (n = 10 per group). The effects of the oral environment and clinical use on force properties were evaluated by comparing in-vivo and in-vitro data.


RESULTS
The springs studied showed a statistically significant decrease in force (approximately 12%) after 4 weeks of clinical use (P <0.01), with a further significant decrease (approximately 7%) from 4 to 8 weeks (P = 0.03), and force levels appearing to remain steady thereafter. Clinical space closure at an average rate of 0.91 mm per month was still observed despite this decrease in force. In-vivo and in-vitro force-loss data were not statistically different.


CONCLUSIONS
Nickel-titanium closed-coil springs do not deliver constant forces when used intraorally, but they still allow for space-closure rates of approximately 1 mm per month.
Private practice, Wilson, NC.
Crystal
C
Cox
Assistant professor, Department of Orthodontics, University of North Carolina, Chapel Hill, NC.
Tung
T
Nguyen
Associate professor, Department of Orthodontics, University of North Carolina, Chapel Hill, NC.
Lorne
L
Koroluk
Professor, Department of Orthodontics, University of North Carolina, Chapel Hill, NC. Electronic address: Ching-Chang_Ko@unc.edu.
Ching-Chang
CC
Ko
D000293:Adolescent; D000328:Adult; D001831:Body Temperature / Q000502:physiology; D002152:Calorimetry, Differential Scanning; D003722:Dental Alloys / Q000737:chemistry*; D003799:Dental Stress Analysis / Q000295:instrumentation; D004548:Elasticity; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D008297:Male; D008422:Materials Testing; D055595:Mechanical Phenomena; D009055:Mouth / Q000502:physiology; D009532:Nickel / Q000737:chemistry*; D016382:Orthodontic Appliance Design; D016910:Orthodontic Brackets; D019516:Orthodontic Space Closure / Q000295:instrumentation; D009969:Orthodontic Wires; D011446:Prospective Studies; D012464:Saliva, Artificial / Q000737:chemistry; D013314:Stress, Mechanical; D013696:Temperature; D013997:Time Factors; D014025:Titanium / Q000737:chemistry*; D055815:Young Adult
D003722:Dental Alloys; D012464:Saliva, Artificial; C040654:titanium nickelide; D009532:Nickel; D014025:Titanium