Comparison of traditional RPE with two types of micro-implant assisted RPE: CBCT study


Recently, various types of the Micro-implant Assisted RPE (MARPE) were introduced to obtain greater skeletal expansion and to minimize dental effects. In the present study, we evaluated skeletal and dental effects immediately after the completion of expansion using three different types of expanders— a traditional tooth-anchored maxillary expander (TAME) and two different types of MARPE, bone-anchored maxillary expander (BAME) and tooth-bone-anchored expander (MSE) using CBCT in adolescents. Overall, the MSE group showed much greater skeletal changes than the TAME and BAME groups, especially, at the nasal floor, maxillary base, and palatal suture. About 72–78% of suture opening was at PNS, which indicates slightly more opening anteriorly than posteriorly; however, it was relatively parallel in nature than anticipated. In all three groups, the greatest transverse changes with expansion occurred at the molar crowns and the 2nd greatest changes at the palatal suture opening at ANS. It is suggested that MSE can be a great alternative method in correcting maxillary skeletal transverse deficiency.


Rapid palatal expansion (RPE) has been widely used in correcting maxillary transverse deficiency by separating the two halves of the maxilla at the midpalatal suture to widen the maxillary basal bone in children and adolescents. RPE effects on the facial skeleton and dentition have been extensively studied in the literature. Using Bjork’s metal implant method, Krebs reported that the two halves of the maxilla separated in a slightly rotary movement, and therefore, the effect of expansion on the facial skeleton diminishes in the cranial direction; there is a greater increase in width in the lower segments than in the upper segments of the maxilla in the frontal plane. Furthermore, a rotation in the transversal plane increased more anteriorly than posteriorly. A recent systematic review evaluating 12 relevant articles concluded that there was no consistent evidence on whether the midpalatal sutural opening was parallel or followed a triangular pattern.

Since the introduction of temporary anchorage devices, a bone-anchored expander has been proposed to obtain greater skeletal expansion and to minimize dental effects. The tooth-anchored expander (traditional hyrax expander) presumably delivers the force to the maxilla through appliance-supporting teeth. On the other hand, a bone-anchored expander that incorporates temporary anchorage devices on the palate delivers the force directly to the maxilla. Various designs of bone-anchored expanders with or without attachment to the teeth have been introduced. Some investigators have shown greater skeletal effects using micro-implant supported RPE (MARPE). One of the more recent developments of MARPE is a maxillary skeletal expander (MSE), which is a tooth-bone anchored RPE. It is designed to produce relatively parallel expansion by anchoring the posterior part of the palatal bone in bi-cortical engagement and has recently gained popularity in orthodontics. Some investigators have reported successful parallel skeletal expansion of the palatal suture using MSE in adult patients that is not limited to the inferior aspect of the nasal cavity, but also extends up to the nasal bone. ,

With recent increased interest in sleep apnea, particular attention has been paid to the skeletal effects of RPE, such as the possibility of increasing upper airway dimensions through widening of the nasal cavity and straightening of the nasal septum, which may contribute to the reduction of nasal resistance and improved nasal breathing.

Recent development of three-dimensional volumetric imaging, cone-beam computerized tomography (CBCT) and 3D reconstruction software programs allow clinicians and researchers to clearly visualize the internal osseous structures and accurately measure changes throughout the basal bone of the maxilla and the adjacent facial structures. Thus far, only a few studies besides case reports have been conducted to evaluate MARPE treatment effects on palatal suture opening and the adjacent facial skeleton. It is a particular interest of the present paper to evaluate the magnitude and pattern of the palatal suture opening and its effects on the adjacent upper facial skeleton in three-dimension.

The purpose of the present study was to quantify and evaluate the changes that occurred immediately after maxillary skeletal expansion using three different types of expanders— a traditional tooth-anchored RME and two different types of MARPE (bone-anchored and tooth-bone-anchored) using CBCT in adolescents.

Materials and methods

This retrospective study was approved (#17–114) by the Institutional Review Board of the University of the Pacific. The study sample consisted of a total of 102 adolescent patients who presented with posterior crossbites and received one of three different types of maxillary expansion treatment. The sample CBCTs were obtained from three sources: (1) a previously conducted randomized clinical trial records from the University of Alberta; (2) consecutively treated cases from a graduate clinic of the orthodontic department at the university of the Pacific; (3) consecutively treated cases from a private orthodontic clinic located in Los Angeles, CA. The main inclusion criteria were patients who received one of the three expansion treatments and had two CBCTs before and after the expansion. The exclusion criteria were poor image quality and being equal to or older than 18years of age. Table1 shows the sample distribution for the three expansion groups.

Comparison of the Age at T1, time interval (T2-T1), and sex for three expansion groups
(n = 37) (n = 41) (n = 24)
Mean SD Mean SD Mean SD
Age at T1 (years) 14.2 1.5 14.01 1.24 13.8 1.9
T2-T1 (years) * 0.34 0.12 0.24 0.14 0.65 0.57
Sex (M:F) 15:22 14:27 9:15

The time interval (T2-T1) was calculated using time at which records were taken rather than the expansion treatment time.

The traditional tooth-anchored maxillary expander (TAME), which is composed of a hyrax screw with bands on the first permanent molars and first premolars, is shown in Fig.1 A. The expansion screw was activated twice a day (0.25 mm per turn, 0.5 mm daily) until posterior dental crossbite overcorrection (20% more of the needed correction) was achieved. After active expansion treatment, a CBCT was obtained (T2) and the screw was fixed with light cured acrylic and kept in place passively for 6 months.

Figure 1.
Three different maxillary expansion appliances. A. Conventional tooth-anchored maxillary expander (TAME), B. Bone-anchored maxillary expander (BAME), C. Tooth-bone anchored expander (MSE). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

For the bone-anchored maxillary expander (BAME) group, two different bone anchor types were used and both types had no direct contact to the teeth. The first type composed of 2 custom-milled stainless steel onplants (diameter, 8 mm; height 3 mm), 2 miniscrews (length, 12 mm; diameter, 1.5 mm; Straumann GBR-System, Andover, Mass) and an expansion screw (Palex II Extra-Mini Expander, Summit Orthodontic Services, Munroe Falls, Ohio). The other type was Dresden-type hyrax expander, which consisted of a temporary anchorage device on one side and a shortened-implant on the other. The appliances were placed on each side between the projection of the permanent first molar and second premolar roots deep into the palatal vault and 6 mm from the suture. A healing period of 1 week was allowed before activation of the expander. With a jackscrew attached to skeletal anchors, semi-rapid expansion was indicated. Activation consisted of 1 turn (0.25 mm) daily until overcorrection was achieved. After active expansion treatment, a CBCT was obtained (T2) and the screw was fixed with light cured acrylic and kept in place passively for 6 months.

The subjects in the third group, tooth-bone anchored MARPE group, received a maxillary skeletal expander (MSE) developed by Moon etal. (MSE, Biomaterials Korea Inc) ( Fig.1 C). Along with bands on the permanent first molars, four micro-implants (two on contralateral sides of the midpalatal suture) were placed on the palate through guide holes in the expander. The micro-implants were each 1.8 mm in diameter and 11or 13 mm in length, allowing for bicortical engagement of the micro-implants at the palatal bone and nasal floor, which prevented unwanted tipping of the micro-implants during the expansion. The MSE was activated 0.5 mm daily (0.13 mm per turn, 4 turns daily) until transverse correction was achieved, and it is kept in place passively for 6 months. A progress CBCT (T2) was obtained before removing the appliance. All CBCT images were taken using the iCAT machine (Imaging Sciences International, Hatfield, Pa) with 8.9 s scan time and 21 × 17 cm or 16 × 13 cm field of view at resolution of 0.3 mm voxels. The MSE group patients were collected from ordinary clinical situations. Therefore, not all of the second CBCT were taken right after the completion of the expansion, but rather at various time intervals ranging from right after the completion of the expansion to 6 months after based upon the clinicians’ needs before removal of the appliances.

For both the TAME and BAME groups, some CBCT scans were taken using a NewTom 3 G (Aperio Services, Verona, Italy) at 110 kV, 6.19 mAs, and 8 mm aluminum filtration, while some scans were taken using the iCAT machine (Imaging Sciences International, Hatfield, Pa) with a collimation height scan of 13 cm, scan time of 20 s, and resolution of 0.3 mm voxel size.

All DICOM files were imported into the InVivo 6.0 software program (Anatomage, San Jose, CA) to display the images. The orientation of the volumetric images was performed prior to landmark location by using three reference planes: (1) Frankfort Horizontal (FH) plane – the primary reference plane that intersects right porion, left porion, and right orbitale; (2) Midsagittal plane – plane passing through nasion and basion and perpendicular to FH plane; and (3) Frontal plane – plane perpendicular to both the FH and midsagittal planes and passing through Nasion. Nasion point was set as the origin.

Landmarks were first located on the 3D volumetric image in which the examiners could freely rotate and crop as necessary for better access and visibility. After the landmark was located on the volumetric image, its location was simultaneously displayed in the “Slice locator” view, where the position of the landmark was adjusted in the sagittal, coronal and axial views. The dual display of the landmark location in the volumetric and cross-sectional views facilitated a more precise landmark identification procedure. Calibration for landmark location was performed by two examiners using randomly selected cases. After satisfactory calibration sessions, each image was traced by 2 calibrated examiners and the estimates were averaged when reporting all measurements.

Table2 shows the 19 landmarks used in the present study. A custom 3D cephalometric analysis was developed through the Invivo software and 17 measurements were calculated using the InVivo 3D Analysis tool ( Fig.2 ). Eleven linear transverse changes were measured between the bilateral structures. Both the right and left sides of the ANS and PNS points were located to measure the magnitude of the midpalatal sutural opening at ANS and PNS at T2. The angular changes of the first molars and central incisors were measured at the internal angle between the long axis of the tooth and the FH 3D plane projected on the frontal plane. The right and left sides were averaged for further data analysis ( Fig.2 ).

Definitions of skeletal and dental landmarks
Landmarks Definition
Landmarks for orientation Nasion (N) Midpoint of the frontonasal suture
Basion (Ba) Most inferior and posterior point at the anterior margin of the foramen magnum
Orbitale (Or) Most inferior point along the inferior margin of the orbital rim
Porion (Po) Most superior and lateral point of the external auditory meatus
Bilateral Skeletal Landmarks NF Nasofrontal suture point
FZ Frontozygomatico suture point
ZA Zygomatic arch point-most lateral point of the zygomatic arch
Key Ridge (KR) Most inferior point of the zygomaticomaxillary ridge along the suture between the zygomatic bone and the maxillary bone
Jugum (J) Intersection of the maxillary tuberosity and the zygomatic buttress in the frontal view
LN Most lateral point of the anterior piriform apparatus of nasal cavity
IN Most inferior point of the inferior border of the anterior piriform apparatus of nasal cavity
ANS * Most anterior point of the premaxilla along the midline of the maxilla
PNS * Most posterior point of the palatine bone
Point A * The deepest point on the contour of the maxilla between the anterior nasal spline and the upper incisor
Bilateral Dental Landmarks U1_M Most mesial point along the upper central incisor incisal edge
U1_D Most distal point along the upper central incisor incisal edge
U1 Apex Upper central incisor root apex
U6 Cr Maxillary first molar mesiobuccal cusp tip
U6 Apex Maxillary first molar mesiobuccal root apex
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Jan 9, 2020 | Posted by in Orthodontics | Comments Off on Comparison of traditional RPE with two types of micro-implant assisted RPE: CBCT study
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