Stability of Rapid Maxillary Expansion

The stability of rapid maxillary expansion (RME) was studied in 20 patients. Study models were evaluated immediately prior to treatment, at the end of treatment, and at least 4 years post-treatment with regards to maxillary and mandibular intermolar widths, intercanine widths, arch perimeters, and irregularity indices. The mean maxillary intermolar width increased by 3.7 mm during treatment, and showed a 0.8 mm (22%) relapse at recall. The mandibular intercanine width increased by an average of 1.2 mm during treatment, and returned to a value nearly the same as that observed prior to treatment. The maxillary and mandibular irregularity indices decreased significantly during treatment, while at post-treatment recall the values had slightly increased (0.6 mm and 1.1 mm respectively). The mean mandibular intermolar width decreased by 1.8 mm during treatment with a 0.5 mm increase at recall. The maxillary and mandibular arch perimeters slightly increased during treatment and at recall had relapsed to nearly the pre-treatment values. The results show that RME was relatively stable, while the mandibular intercanine width relapsed to values similar to those at the start of the treatment. REVIEW OF LITERATURE The objective of rapid maxillary expansion (RME) is to increase the transverse width of the maxillary dental arch at the apical base to correct the skeletal crossbite (due to a narrow maxilla).1,2 The concept of RME was introduced over a century ago. E. H. Angell3 reports the procedure in his 1860 publication describing rapid palatal expansion of the maxillary dental arch to develop space for the maxillary canines. Throughout its history, RME’s popularity has waxed and waned, as there have been conflicting reports regarding the effectiveness and desirability of expanding the maxillary arch by splitting the midpalatal suture.l-18,24 The immediate effects of RME are the splitting of the midpalatal suture, coupled with tipping of the maxillary posterior teeth.4-9 This results in an increased transverse width and arch perimeter of the maxillary arch.lO,11 A Single activation of the appliance produces a large force which decays rapidly with time, with a residual force always present.12-14 The midpalatal suture separation also causes a diastema to form between the maxillary central incisors.I,2 Additionally facial sutures such as the zygomatico-maxillary, zygomatico-temporal and zygomatico-frontal are affected.15 In the frontal plane the maxilla arcs laterally in a triangular pattern, with the center of rotation at the maxillo-frontal suture, causing the mandible to swing downward and backward.l,2,4 This is temporary and is due to lateral tipping of the maxillary teeth, causing the maxillary lingual cusps to contact the mandibular buccal cusps.l,2,4,16 Krebs7-9 used implants to study patients treated with RME, and found that the dental arch width was maintained during fixed retention. Only when fixed retention was discontinued, was there a substantial decrease in the dental arch width, which continued to decrease for 4-5 years. Haas6 reported on the long-term stability of RME. He concluded that 9-12 mm of expansion of the maxillary buccal teeth, and 4-5 mm of expansion of mandibular intercanine width was stable. Sandstrom et. a1.17 looked at mandibular intercanine and intermolar width in patients treated with RME at both the end of the treatment (debanding) and at least 2 years post treatment. They reported an increase in intermolar and intercanine width upon completion of treatment with moderate relapse at the follow-up visit. Davis and Kronman18 looked at patients treated with RME with no treatment in the mandibular arch and found that the increase in mandibular intermolar and intercanine widths not to be significant. Wertz2 also reported on patients who were treated with RME without any mandibular treatment and found no significant increase in the mandibular intermolar width. It has been shown that the arch perimeter increases with RME, however, this increase is variable. Adkins et. al.0 looked at the relationship of arch perimeter and transverse width change in the maxilla upon removal of RME. They found maxillary arch perimeter increased approximately 0.7 times the change in the first premolar width. Little et. al. 19-21 looked at long term stability of orthodontic patients who did not undergo RME and concluded that mandibular arch length continually decreases upon removal of retainers. Numerous studies have been reported on the mandibular irregularity index in orthodontic patients who did not un.dergo treatment with RME. These studies report that irregularity index increases over time after treatment. 19-23 Dental arch changes can be analyzed by studying intermolar and intercanine widths (transverse width), arch perimeter, and irregularity index. Transverse width changes have been studied immediately following RME removal, upon debanding and post-treatment.2,7 There have been varying reports regarding post-treatment stability of intermolar and intercanine widths.6,17,9-23 Variable arch perimeter changes have been observed upon completion of RME in the maxillary arch.0 Long-term stability studies on orthodontic patients not undergoing treatment with RME, have reported that the mandibular arch perimeter continually decreases following treatment.19,23 Long-term irregularity index studies not involving RME have reported an increase in irregularity index over time. 19-23 This study encompasses longterm RME affects with respect to the above parameters. The purpose of this study is to evaluate long-term stability of RME with respect to changes in the maxillary and mandibular intermolar and intercanine widths, arch perimeter, and irregularity index. MATERIAL AND METHODS Maxillary and mandibular casts of 20 patients that had undergone orthodontic treatment with RME were studied. The inclusion criteria for patient selection were as follows; 1) orthodontic treatment with RME, 2) no craniofacial anomalies such as cleft lip or palate, 3) a minimum of 4 years post