Third Molars

Our first topic: “post hoc ergo propter hoc” or why third molars don’t cause orthodontic relapse.

The orthodontic literature is pretty clear that, in general, third molars do not cause orthodontic relapse or crowding. The reason why this is such a strongly held belief is that it is almost “normal” for the anterior teeth (especially the lower incisors) to crowd up in the late teens or early twenties. This happens whether or not the patient had braces (or even wisdom teeth) and is probably related to late mandibular growth and a flattening of the anterior facial profile. Since this is the same time that the wisdom teeth “try” to erupt it is very easy to confuse cause and coincidence. The Latin phrase that speaks to this logical fallacy is: “post hoc ergo propter hoc” which translates “after this therefore because of this”.
This is not to say that there are not good reasons to extract the third molars, but that these reasons have more to do with whether the third molars can erupt into a self cleaning area or whether they will stay impacted and resorb roots or cause cysts. These are questions best left to the long-term evaluation of the patient’s general dentist.
The dental arches do not expand much as a child ages but they do get longer with the first molars erupting distal to the primary teeth, the second molars farther distal and finally the third molars. Unfortunately most of the time growth is complete before sufficient room is created for the third molars.
It is often the case that even with additional room provided by premolar extractions and some mesial movement of the molars during space closure there is still insufficient room for the thirds. For this reason we cannot even “promise” a patient that extracting premolars is really a trade where we extract four easily removed teeth (premolars) to save four bigger and more difficult to remove teeth (third molars).
In some borderline cases surgical removal of the distal gingival tissue can allow the patient to keep the third molars clean.
Bottom line is that the dentist must make a decision when growth is complete if there sufficient room for the third molars to come in to a self-cleaning area. If there is room, but the third molars are not coming in correctly, then a decision has to be made whether it is worth it to place partial orthodontic appliances to get the third molar aligned or just extract them as “unnecessary” teeth. Of course it is always hard to get a 20 year old to consent to further orthodontic treatment but sometimes they would rather do some ortho than some oral surgery!
As the above items point out there is nothing extra that the orthodontist can add to the decision as to whether to extract the third molars except in those rare cases when there is sufficient room but the third molars are erupting incorrectly and the patient would consent to additional orthodontic treatment.



Hope this helps!

Sincerely,




Robert M. Waxler, D.M.D., M.S.
Jeffrey T. Cavanaugh, D.D.S.
For Century Orthodontics

P.S.

Topics for our next letters may include:
Retention
Class III growth
Class II correction techniques (pros and cons)
Clear Appliance Therapy (CAT) appliances (Invisalign and OrthoClear)
TMD
Extractions
Wires and brackets

If you have any request please let us know (century@centuryorthodontics.com)

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