The lower jaw is a single bone (as is the upper jaw at some point in the teens) so it is impossible to widen the bone structure...all you can do is move the teeth. Now if the lower teeth are leaned in toward the tongue then it is OK to expand the arch to make more room, but if the teeth are level then expansion is unstable. Now there are orthodontists who will say this is not true, but ask them to show you the scientific studies (not anecdotal reports) that support their belief.
Do Retainers That Expand the Lower Jaw Widen the Mandible Too?
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Expanding the mandible?
The upper jaw (maxilla) can be expanded with an expander before growth is complete because there is a cartilage suture that can be widened to actually change the shape of the jaw. In the case of the lower jaw (mandible) there is no cartilage suture that can be used to widen the lower jaw, so any expansion that occurs is generally expansion of only the teeth within the existing jaw bone and not the jaw bone itself. I hope that helps.