Rapid maxillary expansion: treatment stability and changes in mandibular interdental distance.

Aim: Rapid maxillary expansion (RME) represents one of the most widely used treatment for posterior crossbites. It has been used not only to correct maxillary constriction but also to create additional space in dental arches to relieve crowding, as additional tool in the treatment of III classes and oral breathing. Comparisons between the RME treatment outcome and stability reports usually represent a challenge because studies vary widely regarding sample size, age range, expansion and retention methods used. Contradictory information of long-term stability (0-45%) have been published and this factor is most important for borderline extraction cases. Moreover, we analyze the concurrent behavior of mandible during and after treatment of expansion. Materials and Methods: Thirteen patients (6 males and 7 females; mean age ± DS: 11,69 ± 2,17 years) have been enrolled for this study. All of them presented with a variable degree of crowding sometimes associated with posterior crossbite. They underwent a complete clinical and radiological examination before treatment and, successively, a standardized protocol of RME with two turns a day for about 15 days. The expander was kept on the teeth for 6 months. Dental casts were obtained at three observation times: pretreatment (T1), after removal of expander (T2), 3 (T3) and 12 (T4) months later. The following measurements were made for both maxilla and mandible: inter-canine, inter-premolar and inter-molar width. Results: Distance between upper canine, first premolar and first molar meanly increased 4,2 mm, 5,4 mm and 5,4 mm, respectively from T1 to T2. In the mandible, the increase was 2,4 mm, 1,6 mm and 1,6 mm for inter-canine, inter-premolar and inter-molar width respectively. At T3, upper inter arch width decreased of 17% (0,7 mm), 9% (0,5 mm) and 20% (1,1 mm) while, al T4, the decrease was of 24% (1 mm), 26% (1,4 mm) and 26% (1,4 mm) respectively for canine, premolar and molar width. Discussion: Treatment with RME presented good stability for upper inter-canine, inter-premolar and inter-molar widths, showing a recurrence rate of only 24-26% in 12 months. Recurrences was mostly present in the first 3 months for canines and molars. Apparently, the mandible showed a concurrent expansion without treatment which was larger for canines (2,4 mm) than premolars and molars (1,6 mm). Conclusions: Although RME was unanimously considered to have predictable results, it usually showed a variable rate of recurrences. They should be always considered in the orthodontic plan. Iper-correction and contention seems to be important to prevent recurrence