Severe Underbite (Class III skeletal discrepancy)

Initial Problem

(the above images are photos of slides - that's why they appear different from the digital images below)

When she presented at age 14, she had a severe discrepancy between her upper and lower jaws. Analysis of her xrays showed that the discrepancy was mostly due to the relative size of her lower jaw (her lower jaw was too large relative to her upper jaw). When there is a discrepancy between the size and/or positions of the upper and lower jaws, the teeth ususally try to compensate for this. If the lower jaw is in front of the upper jaw, usually you see that the upper teeth are flared foward in an attempt to meet the lower teeth, and the lower teeth are leaned in, in an attempt to meet the upper teeth. If the upper teeth are flared, you would expect to see spaces between them, since as they flare out this moves them onto a larger perimeter and there is more space for the teeth. If the lower teeth are leaned inward, you would expect them to be crowded, since moving them inward places them on a smaller perimeter and they therefore have less available space.

Her upper side teeth were also flared out to compensate for the fact that her arch forms were displaced from their normal relationships. The animation below depicts this (you must have animated gifs turned on in your browser's preferences to view the animation). A change in the front to back relationships of the upper and lower arch forms affects the width relationships as well:

In this patient's case, all of the tooth compensations were in her upper arch. Her upper teeth were flared forward leading to the space that you see. Her lower teeth were also flared slightly forward due to her tongue posture (an thus the spaces between them). Her upper side teeth were also leaned out to fit with the lower teeth based upon her jaw positions (see below).

You can also see the the midlines of her upper and lower arches do not line up. Analysis of her xrays also showed that the bone was relatively thin at the inner and outer aspects of her lower front teeth.

In order to correct her bite, her side teeth needed to be positioned as indicated by the red arrows:

We considered the following options for her treatment:

Option 1. Use braces alone without the removal of any teeth. This option would require that I further compensate her teeth for the jaw discrepancy.To accomplish this, I would need to bring all of her top teeth forward to the extent indicated by the red arrows above, move her bottom teeth back that amount, or do some combination of moving her upper teeth forward and her lower teeth back. This option would not address her lower jaw position which is the basic cause of her teeth not fitting together. It is not possible to move her lower teeth back without the removal of any teeth, so this option would necessitate making all of the correction by moving her upper teeth forward. Since they are already leaned forward, I would need to lean them forward to such an extent to meet the bottom teeth that the upper teeth would be moved off the bone (essentially extracting them) -- not a good idea. Option 1 is not a good one.

Option 2. Use braces alone with removal of teeth. this still does not address the basic cause of her problem, but we did consider extracting a tooth on each side to create enough space to move her lower teeth back. Since the bone was thin on the inside surface of her lower front teeth, leaning them inward would have made them more prone to periodontal (gum) problems. In addition, any treatment plan involving extractions and further compensating her teeth would accentuate her chin and leave her upper front teeth leaned out - both of which we decided were undesirable. We therefore decided against option 2.

Option 3. Braces in conjuction with jaw surgery (orthognathic surgery). Since this option addressed the true cause of her problem and met the esthetic objectives that we had, we proceeded with this option.


Since her teeth had compensated for her jaw discrepancy, and since the jaw surgery would correct the jaw discrepancy, her initial treatment would involve putting her teeth in the proper positions in her jaw bones (this is called "decompensating" the teeth). This required constricting the width of her upper arch and leaning in her upper front teeth. The lower spaces were closed by leaning her front teeth in. The space between her upper front teeth was closed before surgery. This pre-surgical stage took 16 visits (scheduled 1 month apart). She then had jaw surgery which involved moving her lower jaw back to fit into proper position with her upper jaw (subcondylar osteotomy). Post-surgical orthodontic treatment involved fine-tuning the coordination of the widths of her upper and lower arches, and trying to correct her midlines 100%. Her midlines were not perfectly aligned when we decided to remove her braces; we decided to stop treatment since she was not responding to attempts to correct the midlines any more and she was satisfied with how she looked at that point. The post-surgical stage took 8 months.

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