Will similar force magnitude cause differential tooth movement?


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We read with great interest the article on force degradation from elastomeric chains in the April 2017 issue (Evans KS, Wood CM, Moffitt AH, Colgan JA, Holman JK, Marshall SD, et al. Sixteen-week analysis of unaltered elastomeric chain relating in-vitro force degradation with in-vivo extraction space tooth movement. Am J Orthod Dentofacial Orthop 2017;151:727-34).We want to acknowledge and appreciate the efforts taken in this multicenter study. The authors concluded that in the unaltered site, elastomeric chain continued to exhibit space closure throughout the 16 weeks of measurement, and there was a statistically insignificant difference in space closure between the altered and unaltered sites. However, many studies in the past have concluded that the force levels from elastomeric chains reduce by 50% to 70% in the first 24 hours.Although the results were significant, we would like to put forth our queries regarding the methodology. Proffit et al1 stated that the anchorage value of each tooth differs, which affects the amount of orthodontic tooth movement to the applied force. The authors measured the amount of space closure when canine retractions, reciprocal molar to premolar, and en-masse retraction were being carried out. Thus, the amount of force required would vary, and the space closure measured would be substantially different. The authors did not discuss any difference in the amount of space closure with regard to the type of tooth movement. Any difference in the rates of retraction in the maxillary and mandibular arches would validate the study further.2With regard to the results in Table I, would it be prudent to compare mean space closure accomplished with a reference value of zero? Regarding results mentioned in Table II, “When 2 sites from the same patient are positively correlated,” which were the 2 sites correlated here?The details of the study with respect to these queries would help us to understand better.

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