Expansion patterns in surgically assisted rapid maxillary expansion : Transpalatal distractor versus hyrax appliance

The purpose of this work was to compare the patterns of expansion resulting from a tooth-borne hyrax appliance and a bone-borne transpalatal distractor in patients treated by surgically assisted rapid maxillary expansion (SARME).

A total of 28 patients with transverse maxillary hypoplasia were treated by SARME, using a hyrax appliance in 12 and a transpalatal distractor in 16 cases. Before and after palatal expansion, an impression of each maxilla was taken. Casts fabricated on this basis were digitized with a 3D scanner. Based on the resultant virtual models, both the amounts of expansion and the angles of crown tipping from the canines through the second molars were determined and the results of the two groups were compared.

Almost all measurement sites revealed significant expansion of the dental arches over the course of SARME. The sites of maximum expansion were more anterior in the distractor group, while larger total amounts of expansion were seen in the hyrax group. Both appliances involved buccal tipping of almost all canines, premolars, and molars; these findings were more pronounced in the hyrax group.

Either a transpalatal distractor or a hyrax appliance can be successfully used to expand a narrow maxilla by SARME. The two appliances, however, cause different amounts of dental arch expansion and buccal crown tipping. A hyrax appliance should be expected to result in a parallel expansion pattern with the largest increase in the premolar area. A transpalatal distractor is likely to cause more of a V-shaped pattern of expansion. Thus, in clinical practice, specific patterns of distraction can be selectively achieved by taking advantage of specific appliances and various options of positioning.