Wednesday, May 16, 2012

Unilateral Crossbite With a Functional Shift

Unilateral crossbite with a functional shift is a very common problem that is best corrected early because it affects how the jaws develop.  The cause of the problem is a narrow upper jaw.  Because it is narrow relative to the lower jaw, the upper teeth are not wide enough to form a perimeter around the lower teeth.  Essentially, the upper and lower dental arches do not match when the teeth come together.  This mismatch causes the child to compensate by shifting the jaw to one side or the other in order to chew.  This shifted position eventually becomes a habitual rest position for the jaw as well.  Clinically, this habitual position is evident when the back teeth on one side of the mouth fit normally with the upper teeth forming a perimiter around the lower teeth and there is a crossbite on opposite side with the upper teeth fitting inside the perimiter of the lower teeth.

Another indicator that there is a "functional shift" of the lower jaw is the child's facial appearance.  Examination of facial symmetry shows the lower jaw to be sitting off to one side.

Correction of the problem is achieved by using a palate expander to widen the upper jaw.  In children and adolescents, the upper jaw consist of two bones that are connected by ligaments.  A palate expander is a metal appliance that anchors to upper back teeth on either side of the upper jaw and traverses the roof of the mouth next to the palate.  In the center of the appliance is a screw mechanism that can be turned at regular intervals (usually once or twice per day) that causes the bones of the upper jaw to gradually separate.  As the two bones move apart, a space opens between the top front teeth (which is very helpful when there is crowding of the front teeth).  After the desired amount of expansion has been achieved, the palate expander is left in place to hold the expansion for at least 3 months during which time new bone fills in where the expansion has occurred and the space between the front teeth gradually closes.  While the child does feel pressure when the expansion mechanism is turned, the whole process is typically painless.

 When the upper jaw has been widened to the correct size, the lower jaw will naturally find the proper position within the perimiter of the upper jaw and facial symmetry returns to normal.  The longer a unilateral crossbite with a functional shift is allowed to go uncorrected, however, the more ingrained the "habitual assymetry" becomes.  If such a bite is left untreated into adulthood, what was once a habitual position becomes a true skeletal position to which the body has adapted and a return to normalcy with upper jaw expansion will no longer occur. 

Several years after the expander treatment, once all of the permanent teeth had grown in, a second phase of orthodontic work was performed using braces and elastics to finalize bite correction.

Without early intervention to correct her crossbite, it would have been difficult to achieve such good bite and facial symmetry.