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 Our own recent surveys (unpublished) confirmed that the SWA is the dominating appliance system.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

There are 2 basic biomechanical strategies for space closure: frictional and frictionless. The MBT system applies frictional mechanics. Therefore, it is probably more accurate to state that frictional mechanics was used, whereby retraction en masse vs 2-stage retraction were 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 subject to the laws of mechanics and is probably much less dependent on some biologic traits. Describing the force systems is probably essential.

We might imagine that Sir Isaac Newton would be impressed to hear that the gravitational force acting on an apple falling from a tree depends more on the apple’s color or age of the apple tree than, how simple, the apple’s mass. The differences that were found in this study between sexes and age groups make us believe that tooth movements 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 that are used are not defined, the results of the studies would point out more what we believe than what is the case.