First off I just have to say that I appreciated Dr Khan's frequent questions. It's a great stimulus for us to review what we know, and to evaluate why we believe it to be true. Thanks!

With respect to the question:

Leaving only 2 mil of clearance will limit the angle between the bracket slot at the wire, while larger clearance will allow increased angulation. Increased bracket-wire angulation leads to wire binding on the bracket, and has been demonstrated to account for the majority of "friction" in our orthodontic systems (as opposed to classic "sliding" friction which is sometimes still proposed as the major contributor in orthodontics). For a review of this topic, I would recommend:

Friction and resistance to sliding in orthodontics: a critical review                        AJODO. Burrow (2009) Vol 135, p 442-7

Also worth considering is that it is not only M-D angulation, but also torque between the wire and bracket that will contribute to this binding:

Influence on binding of third-order torques to second-order angulation                    AJODO. Kusy (2004) Vol 125, p726-732

With this in mind, I would say using > 2 mil of clearance is probably not justified in most situations, and may have adverse results of increased frictional resistance to space closure, and potential anchorage loss.

One situation I can think of where initiating space closure with a smaller wire might be appropriate is when it is desirable to upright very proclined anterior teeth. Clinicians may begin closure on round steel wires, smaller rectangular steel wires, or rectangular NiTi wires. I am not aware of any research regarding which of these clinical preferences is most effective.

If anyone has other reasons for space closure on smaller wires, I would be intrigued to hear them. Thanks!

Dear Gibson thanks for again answering  my question .

No 1 i make my question more clear why we use 3 mil clearance when the recommended clearance is 2 mil (Ref contemporary orthodontics by William proffit 5 th e).

i personally believe we are a part of a bandwagon and we do orthodontics which is imposed upon us from some businessmen owning manufacturing companies.

We don't use 2 mil clearance because we don't have a option to do it . at least i don't got it . 2 mil clearance on 0.22 mil slot mean a 20x 26 mil wire .i have never gone through a study that uses such a dimension .so they finding in articles are 

21x25 wire (1 mil clearance in one dimension )offer too much friction so use 19x25 (3 mil clearance ) .So we miss the 2 mil theory . 

Also  when you say you use 3 mil clearance it not 3 mil clearance you are getting . it more because of manufacturer tolerance.An example of manufacturer tolerance of a 16 x 22 and 21 x 25 wire is given fig from one of my unpublished study . 

about contact angle its not the dimension of the wire that matters . its dimension of the bracket that matters . wider the bracket less would be the contact angle . ideal the length of the power arm less would be the contact angle . Bracket and power arm lever activation decides the contact angle during sliding mechanics if at least 2 mil of clearance is present.

There are two aspects of space closure . Anterior and posterior dentition including the canine  . A lighter wire do tipping movement . So in buccal segment we will lose tip and in labial segment we will lose torque keep the arch form constant. So we can easily do retraction of buccal segment teeth on round wires without loosing much torque.While retracting anterior segment teeth greater the clearance greater the torque loss . 

Very interesting points. Admittedly my preference is an 018 slot appliance with space closure on 16x22 (ie: 2 mils difference). I suppose there is no common 20x25 wire for 022 slot appliances. Perhaps it is the manufacturers as you suggest. Perhaps a 20x25 is simply too stiff to be practical? I'm not sure.

Also an excellent point regarding M-D bracket width, which will have a huge influence on binding.

There are 3 variation of 0.018" slot . The standard slot is 18x25 and manufacturer or unknown prescription variation is 18x28 and 18x 30 " for piggy mechanics . Most of us dont know but dentaurum universally sell sell 22x 30 and 18x30 slots . 

So your clearance in horizontal dimension is 2 mil but in vertical dimension is 3 mil on 25 slot, 6 mil in 28 and 8 mil in 30 . Now its the horizontal dimension that is effected more with manufacturer tolerance. Beveling of the wire = horizontal clearance.Bump and hum in slot base = horizontal clearance   rounding of slot base = horizontal clearance.  Rounding of base walls connection = horizontal clearance ..But all these horizontal clearance don't make up equal or greater than your vertical clearance . 

unfortunately for most of us we use less clearance for horizontal dimension and its the horizontal dimension that is effected more by friction . On the top by ligatures and on down from base of bracket and plaque . see the pics taken for my personal understanding of friction . 

Well the above discussion is much related to mechanics . My question still remain . Do you ever thought why you use this wire and why not any other wire which manufacturer dont manufacture ? Are we part of a bandwagon ? just doing what every one else is doing 

Hello Haris,

you are indeed part of a bandwagon. In order to avoid this the only chance you have is to use segmented arch mechanics.

Kind regards

Ulrich Kritzler

I know this is late and that @harris probably got his answer after all those years. But I would like to agree with your thoughts. Bill Profitt has an online video recording with Jim McNamara which lean to this point. In summary he highlights that an 022x028 slot stems from the original Angel's edgewise appliance. In that era, the only archwire material available was gold. And we know that gold is a soft material that requires decent dimensions to be stiff enough to do its job etc... But the habit of using 022x028 slot brackets continued after the introduction of stainless steel wires in orthodontics in the ninteen 30s 40s etc...