What you didn’t learn in dental school: Part 3 — functional anterior cross-bites

July 1, 2010
Previously, we established a priority consideration for all cross-bites: They don’t go away on their own.

Rob Veis, DDS

For more on this topic, go to www.dentaleconomics.com and search using the following key words: orthodontics, space maintenance, appliance therapy, cross-bite, Dr. Rob Veis.

Previously, we established a priority consideration for all cross-bites: They don’t go away on their own. This is especially true of functional anterior (Pseudo Class III) cross-bites, and they are especially critical. You must correct them. Opting out makes you the causative factor behind a host of far-reaching, prohibitively expensive problems — most significantly the development of a permanent Class III dentofacial/mandibular abnormality.

What you need to know

In a growing child, the upper arch can be underdeveloped because of airway issues or abnormal habits like thumb sucking. This can lead to an abnormal contact point, e.g., high cuspids that interfere with the child’s ability to close properly and make contact with the posterior teeth.

In order for the patient to chew, they have to shift their mandible, often bringing it forward so that the posterior teeth can make contact. Left untreated, this intentional shifting creates a functional anterior cross-bite. The patient isn’t truly a skeletal Class III at this point, because they don’t have a long mandible or underdeveloped maxilla, but the automatic — and eventually habitual — sliding of the mandible forward can direct the growth of the bite into a genuine skeletal Class III.

The key to successfully treating a functional Class III is to first distinguish it from a true skeletal Class III. Taking a cephalometric film is essentially useless in making that distinction. Cephs are taken in full occlusion and will only show that the arches are, indeed, in a skeletal Class III position, but they will not always provide the why.

What you need to do

  1. Look in the patient’s mouth and ask — with the teeth not in contact — “What is the molar relationship?” When the teeth come together, does that relationship change? If so, something is causing the mandible to shift, creating a functional Class III.
  2. Observe the primate spacing in the upper and lower anteriors. In a functional cross-bite, there is usually no spacing in the upper arch. Habitually biting forward will prevent the natural development of the premaxilla, eliminating the arch perimeter needed for normal spacing. A skeletal Class III exhibits normal spacing.
  3. Measure the depth of the bite. In a functional Class III, the bite is usually deep. In a skeletal Class III, it’s normal or end-to-end.
  4. Determine the mandibular plane angle. If the angle of the mandible appears parallel to the floor, this is indicative of a functional Class III.
  5. Look for interference points that cause the mandible to shift forward. There are none with a skeletal Class III.
  6. Observe the size of the maxilla. If narrow and high-vaulted, it tends to be functional.
  7. Gauge the position and shape of the premaxillary segment. If the shape of the arch in the maxillary region appears abnormally “caved in” (in the anterior), that’s indicative of a functional Class III. In a skeletal Class III, because the bite is not deep, the pre-maxillary segment tends to grow normally.
  8. Put it all together. Combine the acquired data of your observations, functional to skeletal or otherwise. Even without a cephalometric film, you can make an accurate determination … and might even feel comfortable treating.

To jump or not to jump?

If you recognize a functional Class III early on, then you have the ability to treat it with a removable appliance that will effectively jump the cross-bite by proactively directing the growth of the premaxillary segment and the teeth. You can alleviate the interference points, and allow the maxillary arch to once again encompass the lower arch. That done, the patient has an opportunity to return to normal function.

Whether you decide to treat this yourself or not, it behooves you to at least recognize the problem. Make sure that an orthodontic consultation is arranged, and keep in mind that choosing to wait — or not to treat — is not a good idea.

Jumping the functional Class III cross-bite appropriately, e.g., with an upper anterior cross-bite appliance, will cost somewhere in the neighborhood of $75 to $150. If a patient is functioning forward and you don’t correct, the resultant abnormally directed skeletal Class III will require full-on, years-long orthodontics or surgery with a potential cost of $3,000 to $6,000.

Herein lies the essential purpose of appliance therapy – to provide the best, most economical care possible for the patient and to provide you, the dentist, with the opportunity to address basic orthodontic problems … and, of course, make a good living doing it.

Dr. Rob Veis is CEO of the Appliance Therapy Group® (ATG). He can be reached through www.appliancetherapy.com or by calling (800) 423-3270.

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