INTRODUCTION

Rapid maxillary expansion (RME) treatment has been used widely since the mid 1960s.1 It is frequently used to correct maxillary width deficiency or posterior crossbite or to expand arch perimeters to alleviate dental crowding.2 Although the major effect of this treatment is noticed clinically in the dentition and maxilla area, RME therapy appears to involve an ample portion of the craniofacial complex,3 as the maxilla is associated with 10 bones in the face and head.4–6 This involvement has been hypothesized following investigations based on histologic methods,7,8 radiologic imaging, photoelastic models,7,9 bone scintigraphy,10 and finite element analysis.11–18

Histologic studies on animals demonstrated a sign of increased cellular activity at suture level and immature bony tissue depositing along the suture borders.7,8 Scintigraphic investigation showed a statistically significant increase in metabolic activity around the maxillary, zygomatic, sphenoid, and nasal bones and midpalatal sutures.10 Finite element analysis studies on the effect of RME associated, or not, with maxillary protraction13–19 found signs of high stress around the circumaxillary sutures.

However, even though the above studies have been well designed, some issues may have affected their conclusions. In fact histologic findings are limited to experimental studies in animals. Moreover, accuracy of the results of the finite element model (FEM) depends on the detailed geometry, material properties, and boundary conditions of the FEM.

Therefore, until now, there have been only speculations about whether RME can or cannot disarticulate these structures in humans. With advanced technology and the introduction of three-dimensional computed tomography (3D CT) imaging, this speculation can be verified. Therefore, to study the osseous effects of RME therapy on the midfacial complex, the use of 3D CT was considered in this investigation.

The aim of this investigation was to test the null hypothesis that circumaxillary sutures do not show bony displacement in response to RME therapy. This was done using high-resolution multislice multidetector computed tomography to evaluate quantitatively the early extent of these suture disarticulations in young orthodontic patients following treatment with RME.

MATERIALS AND METHODS

The materials for this prospective study consisted of good quality cephalometric radiographs and CT records taken from eight growing patients (two male and six female), with Angle Class I malocclusion, treated with RME at the Department of Orthodontics, Faculty of Dentistry, Catania University, Catania, Italy.

To be included in this study all patients had to have a bilateral posterior crossbite, transverse maxillary deficiency, deep palatal vault, and dental crowding at the start of the treatment. The mean chronologic age of the patients was 9.8 ± 1.8 years (range 8 to 11.4 years). Patients less than 8 and more than 11.4 years of age, and patients with missing maxillary posterior permanent teeth, presence of metal restorations on maxillary teeth, periodontal disease, previous orthodontic treatment, or craniofacial syndromes were not included in the study. All procedures were explained to the patients and their parents, and informed consent forms were signed by parents. This was the same patient population used in a previous investigation.20 The study was approved by the ethical committee of Catania University, Italy.

A Hyrax palatal expander was used for each patient. The activation protocol required the screw to be turned three times per day (0.25 mm per turn) for an average of 18 days for all subjects. Expansion was considered adequate when the occlusal aspect of the maxillary lingual cusp of the permanent first molar came into contact with the occlusal aspect of the mandibular facial cusp of the permanent first molar. The average amount of screw expansion was 8.00 mm, the average total expansion at the molar crown (measured at the cusp tips of the mesiobuccal cusp on the maxillary first molars, left and right, respectively) was 3.20 mm, and the average sutural opening at the same level was 1.72 mm.

Multislice CT scans were performed before rapid palatal expansion (time T0) and again at the end of the active expansion phase (time T1) without removing the expander. The CT scans were carried out by a trained radiographer, using the same scanner console with the primary indication of evaluating buccal bone of maxillary posterior teeth.21

A low-dose CT scan protocol was used. This was similar to protocols already described in the literature,22 but lower voltage and current were used. Briefly, patients were examined with a multidetector helical CT scanner (Lightspeed Ultra, GE Medical Systems, Giles, UK). The scanning parameters were 80 kV, 10 mA (low dose), 0.625 mm collimation, pitch 1, and gantry tilt 0°. Multiplanar reformation and 3D post processing were performed on a workstation (Advantage Windows 4.1, GE Medical Systems). The patients were scanned in the supine position, with chin and shoulder rests, having the head positioned with Camper's plane perpendicular to the ground.21

The data of each patient were reconstructed with 0.5-mm slice thickness and saved as DICOM (digital imaging and communications in medicine) files. The data were then transferred to a workstation (Mac Pro Quad, 2.66 GHz; Apple, Cupertino, Calif) and visualized by using the OsiriX Imaging software program which allowed us to take measurements (open source, OsiriX Imaging software, www.osirix-viewer.com). Prior to carrying out measurements, in order to minimize eventual measurement errors due to the absence of a cephalostat, the 3D images were reoriented according to two reference planes: NFZ (a frontal plane passing through the two frontozygomatic sutures at the inner rim of the orbit and nasion) and the Frankfort horizontal (FH) plane as described recently by Cho.23 Six circumaxillary sutures were examined (five were bilateral sutures and one a single suture). They were classified into four groups according to the kind of articulation and their running pattern.24 Thus, we had: (1) sutures running sagittally and articulating directly to maxilla (nasomaxillary sutures); (2) sutures running coronally and articulating directly to maxilla (frontomaxillary suture and zygomaticomaxillary suture); (3) sutures running sagittally and articulating indirectly to maxilla (internasal suture, zygomaticotemporal suture); and (4) sutures running coronally and articulating directly to maxilla (frontonasal suture) (Figure 1).

View Figure

• Download as Powerpoint
• Download Figure

Measurements were carried out directly at the CT image slices either on axial section parallel to the FH plane or on sagittal section perpendicular to the NFZ plane. In order to ensure that the same point was used on the subsequent CT scans, ie, between T0 and T1, and because opening varied along the length and thickness of each suture,24 the middle site of sutures was chosen as the recording point for each suture. Briefly, the first the middle site of each suture was landmarked on the 3D images. Thereafter, the OsiriX Imaging software allowed to find the same point on CT slice, and at this site the suture width was recorded.

The suture width at T0 and T1 was assessed on the right and left side of the skull for bilateral sutures; the single sutures were measured alone. Linear measurements were taken to the nearest 0.01 mm. Figure 2 illustrates the linear variables obtained from the magnified image (4×) before and after expansion.

View Figure

• Download as Powerpoint
• Download Figure

RESULTS

Evaluating the changes between T0 and T1 (Table 1), all linear measurements showed an increase and RME determined a widening of the suture of about 30% ± 12%. The quantity of suture opening at each right and left suture was not significantly different (P > .05); therefore, right and left sutures were combined in order to compare differences between T0 and T1 (Figure 2). Although all values showed clear differences between T0 and T1, indicating that in all patients the appliance had a widening effect on circumaxillary suture, there were individual variations. Descriptive statistics at T0 and T1 are reported in Table 1. Differences between T1 and T0 and P values are shown in the same table.

Table 1 Changes in Circumaxillary Sutures Before Rapid Palatal Expansion (T0) and at the End of the Active Expansion Phase (T1)

View Fullscreen

The average amount of opening was generally higher in the sutures articulating directly to the maxilla (zygomaticomaxillary and frontomaxillary suture), and differences between T0 and T1 were highly significant. Among the sutures indirectly articulating to the maxilla, the internasal suture showed the highest degree of disarticulation, ie, it showed a mean widening of 0.386 mm, and differences between T0 and T1 were statistically significant. Sutures far from the maxilla showed also a certain degree of disarticulation.

DISCUSSION

Tomographic studies on RME have provided useful findings, even though they have been limited in precise evaluation of the biomechanical effect of orthopedic forces on midpalatal suture. In recent years, some experimental data concerning the effect of RME on circumaxillary sutures have been accumulated. In fact, although RME force is concentrated on splitting the maxillary suture, there are concomitant changes to the surrounding circumaxillary sutures.26,27 Therefore, the purpose of this study was to demonstrate, through 3D CT imaging, the osseous effects on the orofacial skeleton induced by RME on circumaxillary sutures. seeing that it has still not been well explored quantitatively.

In fact, most investigations have been carried out on animals, or through FEM studies, few of which were validated with the use of posteroanterior x-rays, whose usefulness due to the low reliability of cephalometric points can be questioned.28 Therefore, no clinical evidence is available on human beings. It has been stated that palatal expansion “disarticulates” the maxilla and initiates cellular responses at the circumaxillary sutures.10,29 This leads to an increased anabolic rate of sutural cells and a modified activity of many genes and transcriptional factors and therefore enhances transverse growth of the circumaxillary anatomical regions.30–32

Findings from our investigation showed that circumaxillary sutures articulating directly to the maxilla were opened more extensively than those indirectly articulated. In fact, the distant structures of the craniofacial skeleton (zygomatic bone and temporal bone) were also affected by transverse orthopedic forces, although to a lesser extent. All in all, an increased width in circumaxillary sutures, with the highest amount of opening at the internasal suture (0.386 mm) and the lower at the zygomaticotemporal suture (0.213 mm) was observed. Our findings support the results from previous FEM studies that demonstrated higher stress levels in the zygomatic process, external walls of the orbit, the frontozygomatic suture, and the frontal process of the maxilla.12,14–18

For the sake of clarity, it should be emphasized that results from our CT study may have underestimated suture disarticulation as the evaluations were carried out only on a plane of the CT scan and at the middle portion of the suture, ie, in a point. As a matter of fact, due to the complexity of the facial skeleton and the topography of sutures, it is reasonable to assume that compression, shear, and tension forces may coexist in the same suture at different sites. Thus, experimental models like FEM would, apparently, seem to depict better what really happens on circumaxillary sutures following RME.

However, it should be pointed out that FEM studies have their own limitations. In fact, it is important to realize that results from FEM studies depend on the selections of nodes and elements valid for a specific human skull as the analytical model is generally developed from just one skull. Accordingly, one should be aware that the structural and spatial relationship of various craniofacial components varies among individuals.17 Acquaintance with these initial mechanical reactions helps the orthodontist to understand better the final therapeutic effects and the way the orthodontic appliance actually acts on sutures of the craniofacial system.

An increased success rate for orthodontic treatment of Class III malocclusion using RME followed by facial mask therapy33–35 is also rational and supported by our findings. From a biomechanical point of view, knowing the displacement and stress patterns in the craniofacial skeleton will help us to gain a better understanding of how the internal bony structures of the craniofacial skeleton respond to RME therapy.

CONCLUSIONS

• The null hypothesis that circumaxillary sutures do not show bony displacement in response to RME is rejected.

• The amount of widening after RME therapy is different among sutures and highly variable among subjects.

• Sutures articulating directly to the maxilla are more affected by the RME therapy.