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Rapid maxillary expansion (RME) is a common technique to improve the dental and skeletal transverse width in cases of constricted maxillary arches. Although retention after RME has been widely examined, there is still no clear statement about the minimal retention time in postpubertal patients and many practitioners have retention concepts varying between three and six months.    Methods  This retrospective study consisted of 14 patients who were either treated with a Haas-type RME (6 patients) or a Hybrid-RME (8 patients). The average age was 15.8 years (min. 13.5 years, max. 23.0 years).  Low-dose CT scans were taken initially before placement of the RME (T0), directly after maximal activation (T1) and (in six cases) also in retention after 6 months (T2). Using a 3D-software (“OnDemand3D”/Cybermed Inc.) in analogy to the method published by Franchi et al. ( AJODO Volume 137 /  Number 4 ) all values were measured twice at an interval of 1 month to assess the method error and the intraoperator reliability.  Statistical analysis was performed using SPSS 21 for Mac. Possible influences of the RME-type were assessed using the univariate ANOVA. Changes in the sutural density between the different points of time were examined using paired t-tests.    Results  The density of the suture decreased significantly after expansion (T0-T1) with both types of RME (p = 0.000). In the retention period there was a significant increase of the sutural density (p = 0.007) although it did not achieve the initial level (p = 0.002).    Conclusions  1. The midpalatal suture was opened in all analysed patients.  2. In postpubertal patients a retention time of six months does not allow sufficient reorganization of the suture.  3. Therefore, a retention period longer than six months seems to be beneficial to prevent relapses in postpubertal patients.

Density of the midpalatal suture after RME treatment – a retrospective comparative low-dose CT-study