Skeletal and dentoalveolar changes after miniscrew-
assisted rapid palatal expansion in young adults:
A cone-beam computed tomography study
Objective:
The aim of this study was to evaluate the skeletal and dentoalveolar
changes after miniscrew-assisted rapid palatal expansion (MARPE) in young
adults by cone-beam computed tomography (CBCT).
Methods:
This retrospective
study included 14 patients (mean age, 20.1 years; range, 16–26 years) with
maxillary transverse deficiency treated with MARPE. Skeletal and dentoalveolar
changes were evaluated using CBCT images acquired before and after expansion.
Statistical analyses were performed using paired
t
-test or Wilcoxon signed-rank
test according to normality of the data.
Results:
The midpalatal suture was
separated, and the maxilla exhibited statistically significant lateral movement (
p
< 0.05) after MARPE. Some of the landmarks had shifted forwards or upwards
by a clinically irrelevant distance of less than 1 mm. The amount of expansion
decreased in the superior direction, with values of 5.5, 3.2, 2.0, and 0.8 mm
at the crown, cementoenamel junction, maxillary basal bone, and zygomatic
arch levels, respectively (
p
< 0.05). The buccal bone thickness and height of the
alveolar crest had decreased by 0.6–1.1 mm and 1.7–2.2 mm, respectively, with
the premolars and molars exhibiting buccal tipping of 1.1
o
–2.9
o
.
Conclusions:
Our results indicate that MARPE is an effective method for the correction of
maxillary transverse deficiency without surgery in young adults.
[Korean J Orthod 2017;47(2):77-86]
Key words:
Cone-beam computed tomography, Miniscrew-assisted rapid palatal
expansion, Adults, Expansion
Jung Jin Park
a
Young-Chel Park
a
Kee-Joon Lee
a
Jung-Yul Cha
a
Ji Hyun Tahk
b
Yoon Jeong Choi
a
a
Department of Orthodontics, Institute
of Craniofacial Deformity, College of
Dentistry, Yonsei University, Seoul,
Korea
b
Graduate of Harvard School of Dental
Medicine, Harvard University, Boston,
MA, USA
Received May 11, 2016; Revised July 6, 2016; Accepted July 13, 2016.
Corresponding author:
Yoon Jeong Choi.
Assistant Professor, Department of Orthodontics, College of Dentistry, Yonsei University,
50-1 Yonsei-ro, Seodaemun-gu, Seoul 03722, Korea.
Tel
+82-2-2228-3101
e-mail
yoonjchoi@yuhs.ac
*This study was supported by a faculty research grant of Yonsei University College of
Dentistry for 6-2015-0123.
©
2017 The Korean Association of Orthodontists.
The authors report no commercial, proprietary, or financial interest in the products or companies
described in this article.
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License
(http://creativecommons.org/licenses/by-nc/4.0) which permits unrestricted non-commercial use, distribution, and
reproduction in any medium, provided the original work is properly cited.
THE KOREAN JOURNAL of
ORTHODONTICS
Original Article
pISSN 2234-7518 • eISSN 2005-372X
https://doi.org/10.4041/kjod.2017.47.2.77
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https://doi.org/10.4041/kjod.2017.47.2.77
INTRODUCTION
Rapid palatal expansion (RPE) has been widely used
in the field of orthodontics since the mid-1960s for
increasing the transverse dimensions of the maxilla in
growing patients.
1
Rapid palatal expansion enables the
separation of the midpalatal suture, which is followed by
skeletal orthopedic expansion.
2
Surgically assisted RPE
(SARPE) is a treatment modality that helps overcome
increased resistance from the bony palate and zygomatic
buttress in adults.
3,4
However, SARPE has several limita-
tions, including high cost, a complex treatment process,
and surgical morbidity,
5
and most patients are reluctant
to undergo this surgical procedure. Therefore, several
efforts have been made to minimize the surgical risks
and limitations of RPE.
Previous histological studies have shown that the
midpalatal suture begins to obliterate during the juvenile
stage, with a marked degree of closure observed in the
third decade of life.
6,7
Therefore, conventional RPE can
produce unwanted effects in adults, such as expansion
failure, alveolar bone dehiscence, buccal crown tipping,
root resorption, reduction in buccal bone thickness, and
marginal bone loss.
8
To minimize these side effects,
orthopedic expansion of the basal bone is essential in
non-growing patients.
9,10
Nonsurgical maxillary expansion can be achieved
through conventional, bone-anchored, or combination-
type RPE. Bone-anchored devices have been reported
to successfully expand the maxilla after lateral osteo-
tomy.
4,11
To ensure expansion of the basal bone without
surgical intervention and maintain the separated bone in
consolidation, Lee et al.
12
introduced miniscrew-assisted
RPE (MARPE) and reported successful expansion of the
maxilla through opening of the midpalatal suture.
Periapical or occlusal radiographs are adequate for
assessing the opening of the midpalatal suture. How-
ever, the movements of each tooth and its alveolus are
barely identifiable on conventional two-dimensional
(2D) radiographs such as lateral or posteroanterior (PA)
cephalograms. Cone-beam computed tomography (CBCT)
allows imaging at relatively low radiation dosages
and presents a clear view of bony structures, with
minimal image distortion.
13
Skeletal and dentoalveolar
changes after conventional tooth-borne and tooth–
bone-borne RPE have been investigated using CBCT in
growing patients.
14-16
In contrast, there is limited infor-
mation regarding nonsurgical expansion using bone-
borne techniques such as MARPE in young adults.
Therefore, CBCT can be used to accurately assess not
only the changes in each tooth and its alveolus, but
also quantitative three-dimensional (3D) changes in the
maxillofacial complex after MARPE.
17
The aim of the present study was to evaluate the
following null hypothesis: short-term skeletal and
dentoalveolar measurements obtained before and after
MARPE in young adults do not differ significantly. To
test the hypothesis, CBCT data acquired before and after
MARPE were compared.
MATERIALS AND METHODS
Subjects
The CBCT records of 19 patients with maxillary
constriction who had undergone maxillary expansion by
MARPE between January 2012 and October 2013 and
had a complete set of CBCT images acquired before (T1)
and after (T2) expansion were retrieved from the archives
of the Department of Orthodontics, Yonsei University
Dental Hospital. Maxillary constriction was diagnosed at
maxillomandibular transverse differential index values
> 19.6 mm.
18
Of the 19 patients, 5 were excluded on
the basis of the following exclusion criteria: failure
of opening of the midpalatal suture (n = 3), systemic
diseases, craniofacial anomalies (n = 1), and history
of orthodontic treatment (n = 1). Finally, 14 patients
(male, 9; female, 5) with a mean age of 20.1 ± 2.4
years (range, 16–26 years) were retrospectively enrolled
in the study. None of the subjects exhibited functional
displacement. The mean duration of expansion was
27 days (range, 18–35 days), and the mean amount of
expansion was 6.7 mm (range, 4.5–8.8 mm). The second
set of CBCT images were acquired within 5 weeks (mean
duration, 10.7 days; range, 1–35 days) of completion of
expansion. The mean duration between T1 and T2 was
38 days (range, 24–66 days). This study was approved
by the institutional review board of Yonsei University
Dental Hospital (No. 2-2015-0017). Because of the
retrospective nature of the study, the institutional review
board waived the requirement for written informed
consent from patients.
The MARPE device was fabricated by modifying
the conventional hyrax-type RPE device.
12
Four rigid
connectors, composed of 0.8-mm stainless steel wire
with helical hooks, were soldered onto the base of
the conventional hyrax screw body (Hyrax
®
Click;
Dentaurum, Ispringen, Germany). Two anterior hooks
were positioned in the rugae area, and two posterior
hooks were positioned in the para-midsagittal area. The
MARPE device made passive contact with the underlying
tissue. Following cementation of the appliance to the
maxillary first premolars and molars, four orthodontic
miniscrews (Orlus; Ortholution, Seoul, Korea), with a
collar diameter of 1.8 mm and length of 7 mm, were
placed at the center of each helical hook (Figure 1).
12
Maxillary expansion was initiated on the day after
MARPE device placement. The appliance was activated
at a rate of one turn per day (0.2 mm per turn) until the
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required expansion was achieved.
Measurements
The CBCT device (Alphard VEGA; ASAHI Roentgen
IND, Kyoto, Japan) was set at 5.0 mA and 80 kV, and
images were acquired for 17 seconds, with a voxel size
of 0.3 mm. During image acquisition, the patients were
seated upright, with the Frankfort horizontal plane
parallel to the floor and the patient’s head stabilized
by an ear rod. Patients were asked to open their mouth
slightly during image acquisition to prevent overlapping
of the maxillary and mandibular teeth. The images
were imported as digital imaging and communications
in medicine (DICOM) files using InVivo5
®
software
(Anatomage, San Jose, CA, USA) and reoriented with
the palatal plane parallel to the floor in the sagittal
and coronal planes. Subsequent measurements were
performed using the same software (InVivo5
®
).
Interpremolar (IPMW) and intermolar (IMW) widths,
Figure 1. Clinical application of miniscrew-assisted rapid
palatal expansion.
Z
N
J
MA
C6
Ag
Z
N
J
MA
C6
Ag
Figure 3. Two-dimensional posteroanterior cephalogram
reconstructed from a three-dimensional skull model.
Refer to Table 1 for the definitions of abbreviations.
Figure 2. Three-dimensional tooth models used for the cone-beam computed tomography assessment of interpremolar
and intermolar widths after miniscrew-assisted rapid palatal expansion.
Solid arrow, interpremolar width; dashed arrow, intermolar width.
Park et al • Changes after MARPE in young adults
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https://doi.org/10.4041/kjod.2017.47.2.77
defined as the distances between the right and left
buccal/mesiobuccal cusp tips of the first premolars
and first molars, respectively, were measured on 3D
tooth images (Figure 2). For measurement of transverse
dimensions, 2D PA cephalograms were reconstructed
perpendicular to the midsagittal plane. Bilateral
landmarks (Z, N, J, MA, C6, and Ag) were identified in
these images, and the distances between right and left
corresponding landmarks were measured (Figure 3). The
landmarks evaluated in this study and their definitions
are summarized in Table 1.
Three-dimensional skull images acquired at T1
Table 1. Definition of landmarks used in this study
Landmark Description
Z The most lateral point of the zygomatic arch
N The most lateral wall of the nasal cavity
J The junction between the maxillary tuberosity outline and the zygomatic process
MA Midpoint of the J and C6 points on the lateral contour of the maxillary alveolus
C6 The most lateral point of cemento-enamel junction of the maxillary first molar
Ag Antegonial notch
Alare The most infero-lateral point of the nasal aperture in a transverse plane
Ectocanine The most infero-lateral point on the alveolar ridge opposite the center of the maxillary canine
A-point* The most posterior and deepest point on the anterior contour of the maxillary alveolar process in
the mid-sagittal plane
Prosthion* The most antero-inferior point on the maxillary alveolar margin in the mid-sagittal plane
Ectomolare The most infero-lateral point on the alveolar ridge opposite the center of the maxillary first molar
Processus zygomaticus The most infero-lateral point of the processus zygomaticus
Z, N, J, MA, C6, and Ag were identified on the reconstructed two-dimensional posteroanterior cephalogram of a three-
dimensional skull model.
Alare, ectocanine, A-point, prosthion, ectomolare, and processus zygomaticus were defined according to the study by
Magnusson et al.
19
*Although A-point and prosthion were one-point landmarks before expansion, they were separated into right and left
landmarks after expansion.
Figure 4. Superimposition of three-dimensional cone-beam computed tomography images acquired before (white) and
after (blue) miniscrew-assisted rapid palatal expansion.
1 and 2, alare, right and left; 3 and 4, A-point, right and left; 5 and 6, prosthion, right and left; 7 and 8, ectocanine,
right and left; 9 and 10, ectomolare, right and left; 11 and 12, processus zygomaticus, right and left.
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and T2 were superimposed using a point-to-point
and volumetric registration method with specifically
normalized mutual data based on the anterior cranial
base (Figure 4).
19
Upon volumetric registration, the T1
and T2 images shared the same coordinate system,
which compensated for discrepancies and minimized
the risk of measurement errors. On each image, six
pairs of bony landmarks were identified on the basis
of a previous report,
19
following which, each landmark
was coordinated. Displacements in the maxilla were
analyzed along the x, y, and z axes by calculating
the deviation of each landmark between T1 and T2.
The distances between right and left corresponding
landmarks were measured, and the differences between
the measurements in T1 and T2 were calculated.
Two coronal scans were obtained perpendicular to
the midsagittal plane, passing through the buccal/
mesiobuccal cusp tips and furcations of the maxillary
first premolars and molars. For the maxillary first pre-
molars with a single root, coronal scans were obtained
perpendicular to the midsagittal plane, passing through
the buccal and palatal cusp tips. For measure ment of
nasal cavity and basal bone widths, the anterior-most
slice showing the entire palatal roots of the maxillary
right first premolars and molars was selected (Figure
5). On each image, the following parameters were
measured: nasal cavity width, defined as the transverse
width between the lateral-most points of each nasal
cavity; basal bone width, defined as the transverse
width between the right and left intersection points of
the maxillary lateral border and a line passing through
the nasal floor; interdental angle, defined as an angle
between the right and left tooth axes determined by
connecting the central fossa and palatal root apex;
buccal bone thickness, defined as the distance from the
buccal root surface to the outer border of the alveolar
bone, along a horizontal line passing through the
furcation; and buccal alveolar height, defined as the
distance from the buccal/mesiobuccal cusp tip to the
buccal alveolar crest. Buccal bone thickness and alveolar
height were measured on the right and left sides, and
the mean values of the two measurements were used for
statistical analyses.
Statistical analysis
The normality of data was determined using the
Shapiro-Wilk test. Comparison of skeletal and dento-
alveolar measurements before and after MARPE was
performed using paired
t
-tests or Wilcoxon signed-rank
tests, according to the normality of data distribution.
Values of
p
< 0.05 were considered statistically significant.
All statistical analyses were performed using the SPSS
version 15.0 software (SPSS Inc., Chicago, IL, USA).
Table 2. Comparison of skeletal and dentoalveolar
measurements before (T1) and after (T2) expansion (n =
14; mm)
T1 T2 ΔT2–T1
p
-value
IPMW 39.2 ± 3.1 44.7 ± 3.1 5.5 ± 1.4 0.000
‡
IMW 50.2 ± 3.6 55.7 ± 4.1 5.4 ± 1.7 0.000
‡
Z–Z 124.9 ± 3.5 125.7 ± 3.5 0.8 ± 0.5 0.048*
N–N 23.8 ± 1.8 25.2 ± 1.4 1.4 ± 1.0 0.000
‡
J–J 65.0 ± 4.4 67.0 ± 4.8 2.0 ± 1.4 0.000
‡
MA–MA 62.3 ± 4.8 64.7 ± 4.6 2.4 ± 1.3 0.000
‡
C6–C6 59.7 ± 4.5 62.9 ± 4.4 3.2 ± 1.5 0.000
‡
Ag–Ag 89.1 ± 5.3 89.0 ± 4.9 0.0 ± 1.3 0.947
Data are presented as mean ± standard deviation.
IPMW, Interpremolar width; IMW, intermolar width.
Please refer to Table 1 for the definition of each landmark.
Paired
t
-tests were performed according to the normality of
the data; *
p
< 0.05,
‡
p
< 0.001.
Figure 5. Coronal cone-beam
computed tomography images
acquired before expan sion
at furcations of the first
premolar (left) and first molar
(right).
a, buccal bone thickness; b,
buccal alveolar height.
Buccal bone thickness and
alveolar height were measured
on the right and left sides, and
the mean value of the two
measurements was calculated.