读者砖头

The apple of Sir Isaac Newton

The paper of Xu et al¹ that examined the effectiveness of the popular straight-wire appliance system (SWA) for space closure after tooth extraction is very important. Indeed, surveys suggest that the SWA system is the favored appliance among orthodontist^2,3 and its evaluation in clinical studies are needed. Our own recent surveys (unpublished) confirmed that the SWA is dominating appliance system. In the Republic of Macedonia (37 out of 40 orthodontists, 98%) the SWA was routinely used by 34 orthodontists (95%). The MBT⁴ bracket prescription system, based on SWA, was applied by 7 orthodontists routinely. Similarly in Serbia (57 out of 120 orthodontists, 47.5%), the SWA was routinely applied by 56 orthodontists (99%). Indeed, the excellent study of Xu et al. is complex and could take us astray and some minute explanations may be needed.

There are two basic biomechanical strategies for space closure: frictional and frictionless. The MBT system (this study) applies frictional mechanics. Therefore, it is probably more accurate to state that frictional mechanics was used, whereby retraction en masse versus two stage retraction was compared. Incidentally, the important variables influencing force systems that could in turn influence tooth movement during space closure were not defined (point of force application, direction, and magnitude). When subjected to a given force system, a tooth is set in motion and is a subject of the laws of mechanics and probably much less dependent on some biological traits. Describing the force systems is probably essential.

We may imagine that Sir Isaac Newton would be impressed to hear that gravitational force acting on an apple falling from a tree depends more on the apple's color or on the age of the apple tree then, how simple - on the apple’s mass. The differences which were found in this study between gender and between the age-groups make us believe that the tooth movement would depend on sex hormones and age. However, to prove this, the most important precondition would be that the mechanical factors are precisely controlled. Another argument in favor of the precise measurements would be that if in clinical studies the force systems of the appliances which are used are not defined, the results of the studies would point out more at what we believe then what is the case.

Alexander Spassov

Dragan Pavlovic

Greifswald, Germany

       References

1.      Xu TM, Zhang X, Oh HS, Boyd RL, Korn EL, Baumrind S. Randomized clinical trial comparing control of maxillary anchorage with 2 retraction techniques. Am J Orthod Dentofacial Orthop. 2010;138:544.e1-9.

2.      Banks P, Elton V, Jones Y, Rice P, Derwent S, Odondi L. The use of fixed appliances in the UK: a survey of specialist orthodontists. J Orthod. 2010;37:43-55.

3.      Keim RG, Gottlieb EL, Nelson AH, Vogels DS 3rd. 2008 JCO study of orthodontic diagnosis and treatment procedures, part 1: results and trends. 2008;42:625-40.

4.      Bennett JC, McLaughlin RP. Controlled space closure with a preadjusted appliance system. J Clin Orthod. 1990;24:251-60.