Tomographic analysis of midpalatal suture prior to rapid maxillary expansion

ABSTRACT Introduction: In Orthodontics and Facial Orthopedics, the timing of treatment onset may be critical and individual analysis should be applied to promote a favorable treatment planning. In this study, individual analysis of midpalatal suture (MS) and palatal measurements were performed in teenagers and young adult patients treated with rapid maxillary expansion (RME). Description: Twenty-six patients submitted to RME with a tooth-supported appliance (Hyrax) were evaluated. The inclusion criteria were: minimum age of 14 years, presenting all posterior teeth, diagnosed with transverse maxillary discrepancy, and with a clinical indication for maxillary expansion. The pretreatment CBCT scans of these patients were assessed to obtain the stages of MS maturation (MSM); density ratio (MSD); and palatal length, thickness (anterior, intermediate and posterior) and sagittal area. Results: The maturation stages present were C, D or E; the density ranged from 0.6 to 1, and lower density (MSD < 0.75) and higher density (MSD ≥ 0.75) groups were determined. Individuals with higher MSD presented smaller sagittal area, compared to the lower density group. Individuals in D and E MSM stages presented smaller sagittal area and intermediate thickness, compared to stage C. Conclusions: Smaller palatal sagittal area was observed in the high MSD groups and in the stages D and E of MSM.


INTRODUCTION
The ideal moment for orthodontic treatment varies according to each patient's malocclusion. Transverse discrepancies should be treated as soon as possible, 11 since the timing of treatment onset may be critical when treatment is implemented too late. 2 In rapid maxillary expansion (RME), the skeletal effect is expected to be greater than the dental one; therefore, the maxillary arch width increase must be achieved by opening the midpalatal suture (MS), and not by the inclination of posterior teeth. 3,4 However, the resistance of the suture to opening increases as suture fusion advances, which makes the RME controversial in young adults. 4,5 The ossification of the MS occurs from the posterior to the anterior region; 6 and is not directly related to chronological age. 7, 8 There is a consensus that RME in patients up to 14 years of age is predictable, but individual variations in MS fusion process must be analyzed based on the definition of its maturation stage (MSM) 7, 9 and density (MSD). 10 Suture images can be obtained from cone beam computed tomography (CBCT), and that approach has been increasingly used in orthodontics. 11 Christovam IO, Lisboa CO, Vilani GNL, Brandão RCB, Visconti MAPG, Mattos CT, Ruellas ACO Tomographic analysis of midpalatal suture prior to rapid maxillary expansion 5 A recent study 12 suggested that patients older than 15 years of age have a positive prognosis for RME when the MS is at an intermediate stage of maturation, although the efficacy of the MSM analysis is not conclusive to predict the magnitude of expected changes. 13 Therefore, the purpose of the present study was to assess whether palatal baseline measurements differ in teenagers and young adult patients submitted to RME, according to their MS density ratio and maturation stage. The null hypothesis was that there is no difference.

MATERIAL AND METHODS
The protocol of this research was approved by the Research Ethics Committee of the Federal University of Rio de Janeiro (UFRJ, protocol #68388017.5.0000.5257).
In a previous pilot study, the area of the palate was evaluated in the sagittal section of the images of ten patients randomly chosen. The mean and standard deviation of the areas found were calculated. A sample size calculation was performed, considering a test power of 80%, α = 0.05 and a difference to be detected of 45 mm 2 , and a total of thirteen patients were required in each group.
Christovam IO, Lisboa CO, Vilani GNL, Brandão RCB, Visconti MAPG, Mattos CT, Ruellas ACO Tomographic analysis of midpalatal suture prior to rapid maxillary expansion Inclusion criteria were: patients with a minimum age of 14 years, presenting all posterior teeth, diagnosed with transverse maxillary discrepancy, and who had a clinical indication for maxillary expansion. Patients were recruited for RME with a tooth-supported appliance (Hyrax), and obtained MS opening after the active phase. Two orthodontists treated the study patients in a private clinic. The Hyrax was activated twice a day. Patients were followed up weekly until clinical observation of molar transverse relation overcorrection. The clinical favorable accomplishment of RME was confirmed by the presence of the interincisal diastema (Fig 1). 3 The device was maintained for retention, and patients were subsequently treated with fixed orthodontic appliances.      When divided into two groups according to density, a significant difference was observed in relation to MSM stages (p = 0.003) and MSD (p < 0.001) ( Table 1).
Regarding the different stages of MSM (     to individually assess patients and predict response to RME, based on analysis of maturation stages 9 and density ratio of MS. 10 However, information from a recently published systematic review indicates that evidence is still weak. 15 The developmental stages of the MS have been defined histologically and divided into infantile, juvenile, and adolescent periods; in the third stage, MS separation is not possible without fracture occurring in the areas of interdigitation. 17 Angelieri et al. 7 consider the sutures in stages D and E to have fused partially or completely, and surgically-assisted rapid palatal expansion (SARPE) could then be considered. 9,12 Tomographic studies of MSD have provided information about resistance to RME, 18 and changes in MS before and after RME. 19,20 The results indicate that a lower suture density is directly related to a clinical favorable accomplishment of the expansion. 10,18,20 Christovam IO, Lisboa CO, Vilani GNL, Brandão RCB, Visconti MAPG, Mattos CT, Ruellas ACO Tomographic analysis of midpalatal suture prior to rapid maxillary expansion In the present sample, it was verified that participants' mean age was significantly different among MSM stages, similarly to a previous study. 21 Additionally, in a published study 22 with 16 to 20-year-old participants, MSM stages C, D and E were the most often observed, similar to the present sample. Angelieri et al. 23 reported that chronological age could also be considered a viable alternative to predict suture maturation. In the present study, the mean age decreased with progressing stages of MSM, indicating that RME may probably be better accomplished in older patients if they are still in earlier stages of fusion. With respect to the two MSD groups defined in this study (low and high density), no significant difference was found for age, which is similar to previous studies. 10,18 The sample in this study was predominantly female, and no significant difference was found regarding patient sex in different MSM or MSD groups, although the composition of the high-density and the last maturation stage groups were more than 90% and 100% female, respectively. The same was observed in other studies, where 77.2% of the patients in the more advanced MSM stages were female, 12 100% of the patients in stage E were female, 7 or the percentage of female, separated by age (16)(17)(18)(19)(20) years), in stage E was higher than for male. 22 Christovam IO, Lisboa CO, Vilani GNL, Brandão RCB, Visconti MAPG, Mattos CT, Ruellas ACO Tomographic analysis of midpalatal suture prior to rapid maxillary expansion 18 The MSD was significantly different between MSM stages, increasing from stage C to E. This indicates that the more advanced maturation stages present a higher-density suture.
When the suture is not calcified, it is similar to the gray levels at the density of the soft palate. As progression of suture closure advances, some bony spicules begin to appear, and calcified and non-calcified areas are visible. As a consequence, the density increases, which means that the gray levels in the suture are approaching that of the palatine process (cortical bone), until there is fusion of the suture. 10 A significant difference between average MSM was observed when the patients were divided in groups of high and low MSD.
In the low-density group, all individuals were in stages C and D; while in the high-density group, more than 60% of the patients were in stage E. Patients older than 13 years in stages A, B, or C of MSM may have favorable prognosis for RME, 9,12 despite that in stage C the skeletal response is lower than in the previous stages. Nevertheless, other authors 10 reported the correlation between MSM and clinical skeletal measures after RME as negative and not significant, and they considered that density better predicted the response to RME.
Christovam IO, Lisboa CO, Vilani GNL, Brandão RCB, Visconti MAPG, Mattos CT, Ruellas ACO Tomographic analysis of midpalatal suture prior to rapid maxillary expansion 19 The thickness of the palate might interfere in the response to RME, and thinner palates probably have less resistance to the forces of the treatment. 12 In the present study, palate thickness was smaller in individuals who were in the final stages of MSM and in the high density group. These findings suggest that RME years of age. The radiation dose must be as low as reasonably achievable (ALARA principle); therefore, CBCT with reduced field of view should be requested, which can reduce the radiation dose to the patient and present important information to treatment planning. 10,24,25 It is important to mention that, in addition to midpalatal suture, other structures - such as internasal, maxillonasal, frontomaxillary, frontonasal, zygomaticomaxillary, zygomaticotemporal, and zygomaticofrontal sutures and spheno-occipital synchondrosis -can be affected by RME, and may also be used in pre-expansion analysis, in order to determine the best treatment for each patient. 26-28 Christovam IO, Lisboa CO, Vilani GNL, Brandão RCB, Visconti MAPG, Mattos CT, Ruellas ACO Tomographic analysis of midpalatal suture prior to rapid maxillary expansion 21 The clinical relevance of this study is that, although midpalatal suture opening during orthodontic treatment with rapid maxillary expansion is frequent and may reach 12 to 52 percent of the total screw expansion, 29 failures may occur and, although SARPE has proven long-term stability, 30 it is also a rather invasive procedure.
One of the limitations of the present study was its retrospective nature. Ideally, the study should be conducted as a randomized clinical trial with patients treated by the same orthodontist.
The absence of an occlusal radiograph or CBCT after the RME is another limitation of present study, because these exams