Saturday, 27 November 2010

In-Vitro studies of resistance to sliding and its clinical relevance.

The issue of Seminars in Orthodontics of December 2010 is dedicated to evaluation of different in-vitro study models and its clinical relevance. This is of major importance to the clinician who likes to question and not only digest what he is confronted with in the literature.

What is the reason for spending our precious and limited time reading articles, if the results are of little to no use to what actually happens clinically. As such a small presentation is attempted of resistance to sliding in-vitro models.

Starting with tooth movement while sliding on a main wire:
  1. The tooth initially starts moving tipping back towards the force applied to it. This happens until a critical angle is achieved and the bracket slot edges come in contact with the main wire.
  2. Binding happens and the tooth movement is reduced drastically.
  3. Notching hinders the tooth movement completely and uprighting of the root will have to take place for the circle to commence again.
Tooth movement in other words is not a dynamic effect, the teeth do not move "like beads on a string" as some like to present for their commercial and marketing needs. Tooth movement is more of a contact and release process. 

This is an attempt to visualise the model of tooth movement as it happens clinically when a tooth is sliding with its bracket on a main wire.                   This is a static model.

This is what is measured by the in-vitro studies that have as a model a wire pulled through a bracket or a series of brackets with none or different tip, torque angulation set ups. 
               This is a dynamic model.

  • As such kinetic friction is of little to none relevance to the biology of tooth movement.
  • Static friction is of relevance only during the first phase of tooth movement as described above.
  • Binding and notching are the forces that commence root uprighting and make the sliding mechanics approach of space closure possible.
  • All in-vitro studies with a wire or bracket pulled models, measure static and kinetic friction, and are of little clinical relevance. 
  • They are however of scientific relevance to build the knowledge baseline for further studies.
Those few points should be bared in mind by the reader of the vast numbers of in-vitro studies on resistance to sliding, that especially in the self ligating field have been flooding the orthodontic literature for over a decade now.

The trend is changing and more studies and reviews are been published that confront this issue. We are hopefully moving to a more critical era.

Suggested literature:
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