Topics of the day:
1. Bracket design, friction and self-ligating mechanisms
I appreciated reading some of the opinions of our colleagues about self-ligating brackets and their experiences. However, I think science, especially biomechanics, deserves a place on this discussion.
I think the main questions to be answered are:
1) How much a self-ligating bracket is physically different from a regular bracket?
2) How much a self-ligating system can increase efficiency and quality of treatment, if it does?
To answer these questions using science rather than anecdotal or “magical” explanations, we would need to go reviewh some ideas, particularly on friction.
Kusy and others have been published extensively on this. It surprises me how this evidence has been ignored when people talk about self-ligating mechanisms. We do have some studies with questionable methodology that give confusing results, but I believe physics and some of the best studies published give us a clear explanation about what happens in self-ligating brackets. These findings are not new in the light of what is physically a fact about friction. A few comments on this may add to this discussion:
1) Friction is dependent on normal forces and coefficient of friction, only. It doesn't depend on the area of surfaces.
2) Friction is not always bad. For example, when you try to align a crowded lower incisor with a NiTi wire without opening space for it with a coilspring, friction is what moves the adjacent teeth apart in order to get you the space you need. A zero friction bracket would make this type of strategy impossible.
3) Friction can help you a lot: if you have a high cuspid, it actually helps you to avoid tipping of the adjacent teeth towards the cuspid space. When you use a base arch, friction between the molar tube and the arch helps to delay incisor flaring and molar tipping. Understand friction allows an orthodontist to be able to judge when it is useful or not.
4) If there is play between wire and bracket during space closure sliding mechanics, friction is only dependent on the binding normal force of the ligature to the archwire and of the archwire to the bracket. A self ligating bracket avoids these, but these are just very small components of friction (see Kusy's studies and comments below). In fact, if you tie the wire not tight with a metallic ligature, you avoid the binding force as much as you avoid it using a self-ligating bracket.
5) Play between bracket and archwire depends on bracket design. For example, brackets with larger widths have less play than brackets with smaller widths. In general, the more you use undersized wires, the less friction you will have, but also less control.
6) Everytime you apply a single force at the level of the bracket, you tend to have tipping of this tooth. The wire reacts against that tipping tendency and this generates normal forces between bracket and wire. These normal forces are the largest components of friction. Self-ligating brackets do not avoid these components in any manner. Actually, Damon brackets have a triangular shape, which decreases the distance between normal forces. This means that the normal forces that the wire will exert on the bracket will be larger to keep the tooth on bodily movement, increasing friction.
7) Brackets of larger width and larger slot depth will in general generate less friction. Again, this is because when the wire reacts to the tipping tendency, the normal forces will be less to have equilibrium.
8) Friction can be decreased if you try to move the tooth with a resultant force near to the Cres of the tooth.
9) An example: let's estimate the friction of a canine attached to a rigid wire with a Damon bracket
*with a vertical distance of 12mm from the bracket to the center of resistance
*horizontal distance of 4mm from bracket to center of resistance
*4mm of bracket width
*pulled with a force of 100cN (~ the weight of 100g).
* No torque or play are present.
*coefficient of friction between steel and steel is ~0.11:
2 nd order moment generated on cuspid by force: 12x100= 1200cNmm
2 nd order Reaction moment generated by wire on bracket: 1200cNmm
2 nd order normal force: 1200/4= 300cN (one on each bracket end)
Total 2 nd order friction= 2 x 300 x 0.11= 66cN
1 st order moment generated on cuspid by force: 4x100= 400cNmm (equilibrium)
1 st order reaction moment generated by wire on bracket: 400cNmm (equilibrium)
1 st order normal force: 400/4= 100cN (one on each bracket end)
Total first order friction= 2x100x0.11= 22cN
Total translation force acting on cuspid: 12cN (only 12% of the original force!!)
The meaning of this is that in the clinic you'd get only 12cN of translation force of the cuspid if you start with 100cN (or grams) of force, * even with a Damon bracket *. If you add more normal forces (with ligatures) it obviously gets worse and the teeth won't move at all. If you tie a bracket with a binding force of 50cN (what would generate friction of about 2x5.5cN, 5.5 for wire-ligature interface and 5.5 for wire-bracket), you can't even move the tooth. Imagine now if you have more normal forces out of torsion of the archwire!
Now , 11cN is much less than the 88cN we get from other sources of friction, which a self-ligating bracket doesn't avoid at all. Why people don't realize that? It's just basic high school physics.
This leaves you three possibilities:
1) Use frictionless systems with controlled force systems (loops, my personal preference)
2) Use undersized wires and get tipping, correcting the roots later (round tripping of roots and side effects possible later)
3) Use loops and sliding mechanics at the same time to decrease the wire reaction moment, decreasing normal forces and friction.
I believe a lot of orthodontists that deal with the Damon system may treat with the second option.
In my opinion, one possible advantage of using a self-ligating bracket is if in your practice management calculations, the time you spend tying brackets adds more to your costs than paying about 100% more for brackets. But still, even considering that option, sometimes it is good to be able control the amount of friction we have in treatment. I could imagine a parallel world where self-ligating brackets are the standard and someone would be lecturing on cases where friction can help getting better results in treatment, trying to sell a regular edgewise bracket.
One final comment I'd like to make is that some studies that claim to have less friction with self-ligating brackets do not separate the variables involved the way they should. If you compare a self-ligating bracket to a bracket that had ligatures bound with pressure, the results will be that self-ligating brackets have less friction. A different result would be a physical absurdity. Kusy has good literature on this- he actually goes in detail about the variables involved, and his conclusion is that when it concerns friction, taking out of the equation small variations due to play, bracket shape and the binding force of the ligature, a self-ligating bracket behaves just like a regular bracket. There's no mystery and no magic. We can't forget our basic science and surrender to what companies want orthodontists to believe: that buying their next bracket and believing on their new guru is always the solution to all the bad results of your practice. Maybe switching to a new bracket and spending lots of time on that is a really bad investment if compared to some dedicated study of biomechanics.
I hope I contributed somehow to this discussion.
Rodrigo F. Viecilli, DDS, Orthodontics Specialist
Indiana University School of Dentistry
“Seek simplicity, and distrust it. We think in generalities, but we live in detail.”
Alfred N. Whitehead
I would like to lend Scott Smoron some support in asking how Damon brackets can possibly carry out transverse arch expansion as with an RME or Quad. I used to rely mainly on a Quad Helix (the 3M removable system made that process so much easier) for arch expansion and RME for the really narrow arches which simply need a good orthopaedic correction.
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