Summer 2000 Presentation Summary:

According to Dr. Charles Burstone, symmetry generally works well in orthodontics, but asymmetric occlusions can be a real challenge. Many asymmetric occlusions are functional and should be diagnosed in centric relation, not centric occlusion. These might include crossbites, subdivisions, and midline discrepancies. Functional asymmetry involves a centric shift unlike a structural asymmetry. Dr. Burstone suggested trimming the dental study casts to centric relation rather than centric occlusion. He added that mounting casts is not routinely necessary. There can be dental or skeletal etiology to asymmetric occlusions. The early loss of teeth can contribute to this problem and often manifests itself later as abnormal tipping of teeth. Convergent axial inclinations orient towards the desirable, contrary to divergent inclinations. Crossbites, midline diversions, and posterior occlusions can often be "hidden malocclusions." Asymmetric growth can result in crossbite, although many patients with asymmetric skeletal growth do not have a crossbite. Axial inclination compensations in a lateral direction can sometimes be seen in the upper as well as lower incisor regions and even in the molar areas. Often arch form is constricted at the canine areas or even bowed out. Making asymmetric arch forms in the archwires can sometimes be desirable. We all know that the occlusal plane can be asymmetric. In some cases, a skewed arch with a correct cuspid width could be more unstable with respect to the buccal musculature.

Dr. Burstone described his view of moving teeth in asymmetric situations as analogous to pearls on a chain or string. Nonextraction therapy, may sometimes involve moving molars distally, then moving the incisors across the midline area. Unilateral molar retraction can also involve molar rotation, unilateral expansion, and molar tipback with a transpalatal or lingual arch. Asymmetric headgear, Class II elastics to a cantilever, or unilateral wires using couples are effective. While it is best to create the fewest side-effects, Dr. Burstone observed that there is no "free lunch" in asymmetric mechanics. He prefers symmetrical headgears with asymmetric forces, as opposed to asymmetric headgears. Where there is a deep overbite, unilateral molar tipback is sometimes preferable to translation of molars. Dr. Burstone often uses a rigid anterior wire with a cantilever wire from the first molars to the anterior region. Frequently the canines and premolars will distalize spontaneously if the archwires do not engage the premolars. Class II elastics can be used from a unilateral cantilever extending anteriorly from the upper first molar, allowing a rapid tipback (within four weeks) to occur before the lower arch is affected. In using Class II and Class III elastics, the side-effects are primarily eruptive, rather than introducing an occlusal plane cant. He noted that Herbst appliances have intrusive force side-effects, unlike Class II or Class III elastics, which have extrusive forces. In doing unilateral cuspid retraction, a maxillary transpalatal arch is probably the best support for the first molars.

Dr. Burstone observed how challenging it can be to determine the true facial midline. Most patients have natural asymmetries, which can mislead the operator who may be using dental floss guided by various facial or intraoral landmarks. Determining the facial midline is more of an art than a science because of the variability and nonreliability of those landmarks. However, studies tend to show that relatively minor dental or skeletal asymmetries are less important to the lay public, who tend to focus more on the cupid's bow of the upper lip, the amount of maxillary incisor exposure, and the amount of exposure of the maxillary canines and premolars with lip curvature.

Posterior teeth often determine anterior midline position. Incisors can be tipped without a midline discrepancy or apical base discrepancy. If there is no apical base discrepancy, midline discrepancies are often "self-treating." Anterior crisscross elastics can cause a canting of the occlusal plane from the frontal viewpoint,an undesirable side-effect. For dental midline correction with apical base alignment. Dr. Burstone's uses flexible and segmental wires, lateral forces, lingual arches, and appropriate anchorage. Often teeth are tipped first, and the roots aligned later. Three premolar extractions can sometimes allow treatment of the asymmetric malocclusion where there is a Class II molar relation on one side and a Class I molar relation on the other. Transseptal fibers can be, in Dr. Burstone's words, a "good appliance!" To accomplish incisor tipping in a mesiodistal direction, he suggested using cantilevers, chain elastics, metal figure eights, or transseptal fibers. Incisor mesiodistal translation is best accomplished with sliding mechanics and/or loops. Rigid arch wires, lingual arch, torque control, Class II/III elastics, and low forces provide anchorage for translation and midline correction. Anterior crisscross elastics rotate the occlusal plane, and it matters where the elastics are attached in the horizontal direction. Cuspid torque can also affect the dental midline location. Malcoordination of the arches can be skeletal, dental, or iatrogenic. En mass arch movement can cause occlusal plane cant, unwanted root movement, moving roots through bone, or a pseudo-centric occlusion.

In conclusion, Dr. Burstone cautioned that great care is needed in planning and managing the asymmetric dental arch or arches. And, he quoted Ben Franklin, who said, "A little neglect may breed great mischief."

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