Rapid palatal expansion in the absence of crossbites: added value?

As noted in a recent article, 1 interest in rapid palatal expansion (RPE), which has traditionally been used to resolve crossbites, has increased markedly in the past 2 decades. Orthodontists using this procedure might be seeking to gain arch perimeter to avoid extractions. This treatment, which is most often started in the mixed dentition, raises some interesting questions. One question is this: In the absence of a crossbite, is RPE necessary to gain arch width to avoid extraction treatment? If so, then is the maxillary arch perimeter the determinant in the extraction-nonextraction decision? These questions are fundamental, because using the maxillary arch to decide whether to extract challenges the concept of mandibular arch-based diagnosis and treatment planning. For this reason, it might be useful to evaluate the rationale for RPE treatment in the correction of crowding. RPE increases the perimeter of the maxillary arch and can provide space to correct moderate (3-4 mm) amounts of crowding. Because no treatment occurs in the mandibular arch, a logical question is this: How is space to be gained to resolve crowding in the mandibular arch? One view is that expansion of the maxillary arch is accompanied by spontaneous transverse expansion of the mandibular arch, with the implication that space would be available to resolve any lower arch-tooth size discrepancy. This supposition is easy to assess because there are adequate data; the conclusion is that any spontaneous expansion provides almost no space to resolve crowding. Brust and McNamara, in a 2-phase treatment protocol, evaluated both mandibular intercanine and intermolar expansion and noted that, immediately after RPE, both dimensions increased approximately 1 mm. However, the intercanine distance decreased before phase 2 treatment, and the net intercanine gain was only 0.3 mm. The 1-mm intermolar increase remained stable. These results are similar to those noted by others who recorded spontaneous mandibular arch change incident to RPE treatment. The increase in intercanine width in these studies was less than 1 mm. As an example, the increase observed by Grayson was only 0.22 mm and led the author to conclude that “the use of rapid palatal expansion as a method of increasing lower arch length cannot be justified.” Interestingly, Brust and McNamara observed that the mandibular arch perimeter decreased 1.3 mm from phase 1 to the beginning of phase 2 treatment. In an investigation of 17 subjects whose comprehensive treatment included RPE, the intercanine dimension increased 2.2 mm during treatment and relapsed 50% postretention, to yield a net long-term gain of only 1.1 mm. In this study, there were no recordings of the intercanine width immediately after RPE to indicate the amount of spontaneous expansion that occurred. Also, variation in response was remarkable, ranging from 0.3 to 3.8 mm. (This large variation is a reason that anecdotal information can be misleading—if one cites only the extreme positive response.) One motive for focusing on the mandibular intercanine dimension in the preceding descriptions is that, in the transverse plane of space, an increase in this area provides the most space to resolve mandibular incisor crowding. Specifically, Germane et al determined that a 1-mm increase in intercanine dimension provides 0.73 mm of space to correct incisor position. In contrast, 1 mm of molar expansion provides only 0.27 mm of space. (Subsequent increases in width provide slightly more space per millimeter. For instance, expansion of 2 mm yields 0.27 mm for the first millimeter and 0.31 mm for the second millimeter, for a total of 0.58 mm.) Clearly, the available data indicate that spontaneous expansion of the mandibular arch usually does not supply adequate space to align crowded incisors. Because spontaneous expansion of the mandibular arch is extremely limited, appliances, such as the Schwartz appliance, have been placed to expand it Professor and chairman, Boston University School of Dental Medicine, Boston, Mass. Reprint requests to: Dr Anthony A. Gianelly, Boston University, Goldman School of Dental Medicine, Department of Orthodontics, 100 E Newton St, Boston, MA 02118-2392; e-mail, gianelly@bu.edu. Submitted and accepted, June 2003. Am J Orthod Dentofacial Orthop 2003;124:362-5 Copyright © 2003 by the American Association of Orthodontists. 0889-5406/2003/$30.00 0 doi:10.1016/S0889-5406(03)00568-7