Load system of segmental T-loops for canine retraction.
BACKGROUND
The orthodontic load system, especially ideal moment-to-force ratios, is the commonly used design parameter of segmental T-loops for canine retraction. However, the load system, including moment-to-force ratios, can be affected by the changes in canine angulations and interbracket distances. We hypothesized that clinical changes in canine position and angulation during canine retraction will significantly affect the load system delivered to the tooth.


METHODS
The load systems of 2 T-loop groups, one for translation and the other for controlled tipping, from 9 bilateral canine retraction patients were made to the targeted values obtained from finite element analyses and validated. Each loop was tested on the corresponding maxillary dental cast obtained in the clinic. The casts were made before and after each treatment interval so that both initial and residual load systems could be obtained. The pretreatment and posttreatment interbracket distances were recorded for calculating interbracket distance changes.


RESULTS
As the interbracket distances decreased, the average retraction-force drop per interbracket distance reduction was 36 cN/mm, a 30% drop per 1 mm of interbracket distance decrease. The average antitipping-moment drops per interbracket distance reductions were 0.02 N-mm per millimeter for controlled tipping and 1.4 N-mm per millimeter for translation, about 0.6% and 17% drops per 1 mm of interbracket decrease, respectively. Consequently, the average moment-to-force ratio increases per 1 mm of interbracket distance reduction were 1.24 mm per millimeter for controlled tipping and 6.34 mm per millimeter for translation. There was a significant residual load, which could continue to move the tooth if the patient missed the next-scheduled appointment.


CONCLUSIONS
Clinical changes in canine position and angulation during canine retraction significantly affect the load system. The initial planned moment-to-force ratio needs to be lower to reach the expected average ideal value. Patients should be required to follow the office visit schedule closely to prevent negative effects because of significant moment-to-force ratios increases with time.
Associate professor, Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences and Chinese University of Hong Kong, Hong Kong, China.
Zeyang
Z
Xia
Jie
J
Chen
Feifei
F
Jiangc
Shuning
S
Li
Rodrigo F
RF
Viecilli
Sean Y
SY
Liu
D003481:Cuspid / Q000502:physiology*; D003799:Dental Stress Analysis / Q000379:methods; D020342:Finite Element Analysis; D006801:Humans; D021621:Imaging, Three-Dimensional; D016014:Linear Models; D016382:Orthodontic Appliance Design; D009969:Orthodontic Wires; D013314:Stress, Mechanical; D014087:Tooth Movement Techniques / Q000295:instrumentation*