The straight wire approach, even with small NiTi archwires, generates very high forces and binding if the wire is deflected more than 2mm. There is evidence that root resorption is often seen in straight wire cases / prescription appliances.
Kind regards
Katja Kritzler
There is a broad range of scientific documentation of the force levels in complex tooth alignment and of friction properties according to vertical and horizontal tooth alignment. Most of the found outcome analyses of the in vitro studies primary focus on alignment efficiancy. As the measured high forces in levelling with varius NiTi archwires bracket combinations can lead to detrimental effects on the periodontium a word of caution should be mandatory. Systematic reviews suggest that comprehensive orthodontic treatment causes increased incidence and severity of root resorption, and heavy forces might be particularly harmful. The evidence to support the use of light forces should not be disregarded. In order to avoid adverse side effects particularly on the adjacent teeth in cases with more than 2 mm of arch wire deflection a bypass arch technique should always be employed. Equally efficient archwires of smaller diameter are recommended during leveling and aligning to prevent the heavy forces that may cause unwanted side effects. Displacement of teeth more than 3 mm should not be treated with a straight wire approach alone.
Dear Katja,
Would you believe it , I saw your question only now. I guess it did not come straight to me but was embedded in this project. So here are my thoughts in brief:"
Your question (slightly rephrased): Will advanced biomechanics flourish /reignite the use of segmented arch mechanics ?
Answer: No I believe it will not. Because 'segmental mechanics' has to be accepted by the clinicians out there in general. It cannot flourish if it just remains an academic subject of interest among the select few. But it will do a whole lot of other things:
1. Make orthodontics precise.
2. Increase predictability of tooth movement.
3. Make it more specialized, and knowledge based i.e. bring in more science into it.
There have been three major drawback of segmental mechanics in the past : 1) Wire bending, sometimes quiet extensive bending is required and has to be repeated for consecutive appointments. With ' chair time' being a huge factor very few want to invest in it. In this day and age 'efficiency' is the key. 2) Although the F/M system can be precisely determined on paper, that determination only works for instantaneous force systems . Clinically once the appliance has been placed the F/M system changes quiet rapidly and keeps on undergoing changes till the next intervention. There is no reliable method right now to determine this change in each individual or for a particular clinical situation.
3) Side effects: And that is the biggest , I guess. if not executed precisely and with full knowledge, this technique leads to a lot of side effects. Any technique which is so demanding for the clincian will not easily flourish. sliding mechanics , here works beautifully , because the side effects are automatically shared by multiple teeth. The side effects only show up when they exceed a certain value/level ( primarily through arch wire deflection). Till that point they act as a 'safety valve' or buffer for the side effects.
I think the segmental approach has lots of application, but it has to be integrated with sliding mechanics in general. This is the basis of our extensive Biomechanics in Orthodontics (BIO) courses ( divided into I & II) at University of Connecticut. We go from the fundamental level to the really advanced all the time making it clinically very applicable and easy to execute.
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