“I have had excellent results with the Schwarz appliance on mixed dentition cases requiring arch development. What modifications would you recommend for improving unilateral posterior crossbite? This case also has a deviation between the upper and lower midlines.”
The Schwarz Expansion Appliance is frequently prescribed for the type of case. Most doctors indicate that unilateral posterior crossbites are usually the result of a lateral shift of the mandible due to maxillary bilateral construction. A developed mandible with an underdeveloped maxillary arch results in a cusp-to-cusp posterior bite relationship at the rest position. On closure, the mandible shifts to one side into a bite of convenience or functional crossbite in order to attain interdigitation. This presents the deceiving appearance of unilateral construction and is usually evidenced by a deviation of the upper and lower frenums when fully closed. The upper arch is developed bilaterally so the posteriors can occlude in the proper lingual-buccal relationship. With the lateral shift averted, the labial frenums should align vertically on closure.
The Schwarz Expansion Appliance is constructed with one or two large expansion gears and bilateral posterior bite blocks. The bite blocks allow use of the forces of mastication to provide positive pressure for development. The occlusal acrylic is ground flat to remove interferences and to allow the mandible to attain its proper relationship. The gears are adjusted once each four to seven days depending on the patient’s initial response to the pressure. Auxiliary springs can be added for anterior alignment.
Once the crossbite has been improved, the occlusal acrylic can be removed to reestablish good posterior coupling. Although some doctors feel that the occlusion will sustain the development, most will use the appliance as a retainer at night only for approximately six months after active treatment.