Friday, May 7, 2010

More Questions

How do you decide when to use arch wire curves as opposed to tip-back bends when the bite needs to be opened?

The amount of gingival display often dictates what method of bite opening to use. If the patient has a gummy smile I would rather intrude incisors (tip-backs do this more efficiently than curves) which results in a reduction of gingival display. If the smile is not gummy, erupting molars (curves often do this more efficiently than tip backs) will open the bite without reducing the gingival display.


It is easy to envision that a toe-in bend for maxillary molars helps correct the mesio-lingual rotation of the molars. But how does the toe-in bend applied to the mandibular molars counteract the lingual movement which is often a consequence of eruptive forces produced by tip backs? Also, do you do the toe-in and tip-back bends together or one at a time?

A toe-in results in buccal crown movement. An eruptive force (the tip back) in one plane of space becomes a buccal force (the toe in) in another plane of space. Look at the photo, courtesy of Dr. Tom Mulligan.





















Although some practitioners use both bends simultaneously, I don't. I like to keep my forces as simple as possible. The toe in is used to counter the potential negative side effect of bite opening, which is lingual crown movement. If the side effect isn’t expressed when using the tip back, there is no need for a toe-in. So I wait to see if I need it.
Remember, when using toe-in or tip-back bends, for eruptive(tip-back) or horizontal (toe-in) forces to be produced, the bends must be asymmetric. In other words, the distance between where the wire is bent and where the wire is first engaged must be different on both sides of the wire.If this condition is met, use Mulligan's long and short segment rule (see photos or go to http://www.commonsensemechanics.com/CourseContent.htm )to determine the forces that will be imparted by the wire.
























Now I’m a little confused. The toe-in is used to prevent rolling in of the mandibular molars. Why does this rolling in of the mandibular molars occur?


A side-effect of molar eruption (or any extrusive force for that matter) is lingual crown torque. This force often results in lingual molar movement. Because the wires used for tip backs (usually .016 or.020 stainless steel) are round, in a .022 slot no torque control occurs. Additionally, if rectangular wires are used, the pre-adjusted appliance prescription for the lower molars has lingual crown torque built in. These factors contribute to rolling in, or lingual tipping, of the lower molars. A toe-in bend counteracts these tendencies by providing a buccal force. Ideally, the "bad" forces are cancelled by the "good" forces and the net result is a molar that is upright, not rolled in.

I have another question for you: Why do practitioners who use utility arches expand the upper arch by 5-10 mm by flattening the anterior bridge to correct ClassII Div 1 and 2 patients? Is the expansion necessary?

Upper arch expansion combined with distally rotating the upper molars has been a technique used to correct Class II for over a century (It is often done when using headgear by adjusting the inner bow). The expansion creates the environment where the lower arch can be positioned forward (many ClassII’s are the result of a narrow upper arch which results in the lower arch being positioned, or trapped, distally). This is combined with distal rotation of the upper 1st molar, which places the palatal cusp of the upper 6 in a more anterior position. In ideal occlusion, the upper 6 palatal cusp occludes with the central pit of the lower 6. When the upper 6 palatal cusp is positioned more anteriorly, the lower molar (and hence the whole lower arch) is guided forward. The expanded upper arch allows the mandible to reposition forward, which results in Class II correction.


What’s the easiest way to flare lower incisors forward to gain arch length?

You can do this by manipulation of a utility arch, which has been popularized by Dr. Len Carapezza (http://www.igdpd.com). You could also use a straight wire, leaving the 3's, 4's and 5's unbracketed, packing coil between the 2's and 6's. Make the coil about 2mm longer than the distance between the distal of the bracket on the 2, and the mesial of the bracket on the 6. The force will push the anteriors forward and the molars back. The anterior teeth move forward much more easily than the molars distalize, so the net effect is forward incisor movement. Every month, pack a new piece of coil which is 2mm longer than the coil used in the previous month. Continue until the incisors are where you want them. The same effect can also be achieved by placing stops in the arch wire near the molars so there is a little extra wire length from molar to molar. If you do it this way, you must change the arch wire to gain additional forward movement of the incisors, so this method may be more cumbersome than packing coil.



I have a question about molar uprighting. I have a few adult patients that have lost their lower first molars and I would like to upright their 2nd molars. In order to do this could I simply place a tip back bend just distal to the 2nd premolars rather than just mesial to the molar band? Any suggestions you have on how to upright molars would be greatly appreciated.


Molar uprighting is tough. In theory a center bend (technically an occlusally directed gable bend) will parallel the roots and all vertical forces will cancel. In reality it is very difficult to make the bend a center bend because the bracket position- and hence wire angle of entry- is different on the teeth adjacent to the bend (one bracket is relatively straight, the other is tipped). This contributes to making the bend asymmetric. Unlike a center bend, where vertical forces cancel, the asymmetric bend leads to expression of vertical forces. The big challenge in molar uprighting is to prevent eruption of the molar which often contributes to unwanted bite opening.
Uprighting without eruption occurs with a center bend. You may also get eruption because the bend is usually not precisely a center bend.. Occlusal adjustments must be made so the bite doesn't excessively open. Often, the molar needs to be crowned because so much eruption occurs.
Orthodontists have designed uprighting springs that mitigate the eruptive forces. They are kind of hard to use. Many are now using temporary anchorage (TAD's) to get a more direct force on the molar.
All in all, uprighting is tough. Don't promise your patient much, because you never really know how successful it will be. A combination of a small amount of uprighting, followed by a small amount of mesial movement repeated over and over can yield acceptable results. This is cumbersome and time consuming.



I have a case where, during the initial stage of leveling and aligning, I cannot place the bracket on tooth # 4 in an ideal position. The tooth is blocked out of the arch and also slightly under erupted. Should I continue on to the next stage and wait for this tooth to come in a little more?

Don't go up the wire progression until you get that tooth aligned. Try packing coil for a month; that should free up the tooth to let it erupt. After creating space, get a bracket on it as best you can. If you have to place the bracket too occlusally, thread the initial arch wire (usually this is .012, .014, or .016 nickel titanium) under the gingival tie wings and engage the wire as usual on all the other teeth. This will help erupt the tooth. After a month, reposition the bracket if necessary and tie it in as usual. Once it is aligned, you can move up the wire progression.



I am working on a case where I extracted lower 1st bicuspids to camouflage a Class III occlusion. You suggested to do a tieback right away with .016Niti. Will this move the canine distally into the extraction site? If so, why do I not wait until I am in stronger wire(as suggested in some literature) such as 020ss or rectangular wire?. I have been trying the tieback for about 3 months and still do not see any significant changes. Should I use chain from 3-6 for a month or two instead of the tieback? Can I use the tieback and the power chain from 3-6 at the same time?
Thanks again for all your help.


The tieback (from here on I will refer to it as a laceback) prevents forward movement of incisors during the initial stages of treatment. Without lacebacks in extraction cases, aligning of a crowded arch results in anterior movement of incisors as the crowding is relieved.(For examples of laceback use, see here http://multimedia.3m.com/mws/mediawebserver?mwsId=66666UuZjcFSLXTtM8TamXTyEVuQEcuZgVs6EVs6E666666-- ) This is especially important on the lower arch in ClassIII cases because anterior lower incisor movement is contraindicated. In these cases, you probably won't see much retraction of the canine (it has to do with lack of tip built into preadjusted lower incisor brackets) but the incisors will not move forward. Using a chain instead of a steel tie is not a good idea because the chain is too powerful. It will tip the teeth into the extraction site which makes leveling more difficult.
Once you have completed the initial aligning, remove the laceback and go through a wire progression. When you get into a rigid wire, begin space closure. I prefer en-masse space closure; that is, I retract all 6 anteriors together. In most cases, close space on 019x025 stainless steel; however in many ClassIII's I use .020 stainless steel to do space closure in the lower arch. Space closure mechanics on round will result in de-torquing of the anterior teeth. This is usually beneficial on the lower arch in ClassIII cases because the de-torqued incisors help mask the underlying skeletal ClassIII occlusion.

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