Immediate effects of rapid maxillary expansion with haas-type and hyrax-type expander.

Immediate effects of rapid maxillary expansion
with Haas-type and hyrax-type expanders:
A randomized clinical trial
Andre Weissheimer,a
Luciane Macedo de Menezes,b
Mauricio Mezomo,a
Daniela Marchiori Dias,a
Eduardo Martinelli Santayana de Lima,b
and Susana Maria Deon Rizzattoc
Porto Alegre, Rio Grande do Sul, Brazil
Introduction: The purposes of this study were to evaluate and compare the immediate effects of rapid maxillary
expansion (RME) in the transverse plane with Haas-type and hyrax-type expanders by using cone-beam
computed tomography. Methods: A sample of 33 subjects (mean age, 10.7 years; range, 7.2-14.5 years)
with transverse maxillary deficiency were randomly divided into 2 groups: Haas (n 5 18) and hyrax (n 5 15).
All patients had RME with an initial activation of 4 quarter turns followed by 2 quarter turns per day until the
expansion reached 8 mm. Cone-beam computed tomography scans were taken before expansion and at the
end of the RME phase. Maxillary transversal measurements were compared by using the mixed analysis of
variance (ANOVA) model and the Tukey-Kramer method. Results: RME increased all maxillary transverse
dimensions (P0.0001). There was less expansion at skeletal than dental levels. The hyrax group had greater
statistically significant orthopedic effects and less tipping tendency of the maxillary first molars compared with
the Haas group. Conclusions: Both appliances were efficient in correcting a transverse maxillary deficiency.
The pure skeletal expansion was greater than actual dental expansion. The hyrax-type expander produced
greater orthopedic effects than did the Haas-type expander, but this effect was less than 0.5 mm per side and
might not be clinically significant. (Am J Orthod Dentofacial Orthop 2011;140:366-76)
R
apid maxillary expansion (RME) is an important
method used to correct a transverse maxillary de-
ficiency. It was first described in the literature over
a century ago by Angell,1
and it has been disseminated
and made widely popular by Haas since 1961.2
In
RME, rigid and fixed expanders are used to produce
heavy forces to obtain the maximum skeletal response
by opening the midpalatal suture, with minimum
orthodontic movement.2-5
Among the appliances used for RME, the tooth-
tissue–borne (Haas-type) and the tooth-borne (hyrax-
type) expanders are the most recognized in the literature.
The main difference between them is the acrylic pad that
leans on the lateral walls of the palatal vault (Haas-type)
to reinforce the anchorage for greater orthopedic
response and better force distribution during RME.2,4
In the hyrax-type expander, there is no acrylic pad;
therefore, it is more hygienic and prevents soft-tissue
irritation caused by food impaction under the acrylic
plate.6
Although a cephalometric investigation has not
demonstrated any differences between Haas-type and
hyrax-type expanders,7
there is no consensus in the lit-
erature regarding the differences in the immediate
RME effects produced by these appliances.
Several investigations have analyzed the effects of
RME through 2-dimensional cephalometric radiographs,
which do not allow accurate identification of dentoske-
letal structures because of the superimposition of many
bones in the different planes of space.2,7-9
To overcome
these limitations, computed tomography (CT) for the
assessment of the transverse dimensions of the
maxilla was introduced by Timms et al10
in the
1980s. However, the use of conventional CT scans in
orthodontics has been limited because of cost and ra-
diation concerns.11
Cone-beam CT (CBCT) has ushered
in a new era in dental diagnostics. This technology was
designed for imaging hard tissues of the maxillofacial
region with minimum distortion at a lower cost and
with lower radiation emissions compared with
From the Department of Orthodontics, Pontifical Catholic University of Rio
Grande Do Sul, Porto Alegre, Rio Grande do Sul, Brazil.
a
Postgraduate student (Ph.D.).
b
Professor.
c
Assistant professor.
The authors report no commercial, proprietary, or financial interest in the prod-
ucts or companies described in this article.
Reprint requests to: Andre Weissheimer, Pontifıcia Universidade Catolica do Rio
Grande do Sul, Faculdade de Odontologia, Predio 6, Avenida Ipiranga, 6681, sala
209, Porto Alegre, RS, Brazil, CEP 90619-900; e-mail, andre5051@hotmail.com.
Submitted, March 2010; revised and accepted, July 2010.
0889-5406/$36.00
Copyright Ó 2011 by the American Association of Orthodontists.
doi:10.1016/j.ajodo.2010.07.025
366
ORIGINAL ARTICLE

conventional CT. The high resolution of CBCT images
is due to the isotropic voxel (equal in all 3 dimensions),
which produces submillimeter resolutions ranging from
0.4 mm to as low as 0.125 mm.11
Several investigations
have shown the high accuracy of CBCT images for
quantitative and qualitative analyses.12-15
Its use is
recommended in orthodontics for several purposes
such as evaluation of impacted teeth,16,17
evaluation
of bone grafts in cleft regions,18
analysis of alveolar
bone before placement of orthodontic temporary an-
chorage devices,19
and evaluation of RME effects on
nasomaxillary structures.20
The purposes of this study were to evaluate and com-
pare the immediate effects of RME on the transverse
plane with Haas-type and hyrax-type expanders by
using high-resolution CBCT.
Fig 1. A, Haas-type expander and B, hyrax-type expander at the end of the active phase of RME.
Fig 2. Transverse maxillary posterior region evaluation: A and B, preexpansion; C and D, at the end of
the active phase of expansion.
Weissheimer et al 367
American Journal of Orthodontics and Dentofacial Orthopedics September 2011 Vol 140 Issue 3

MATERIAL AND METHODS
This study was approved by the ethical committee of
the Pontifical Catholic University of Rio Grande do Sul in
Brazil. Informed consent was obtained from the parents
of all patients who agreed to participate in this study. The
sample was selected by examining subjects in need of or-
thodontic treatment at the Department of Orthodontics
of the School of Dentistry. The inclusion criteria for
this study were transverse maxillary deficiency, mixed
dentition or early permanent dentition, and no surgical
or other treatment that might affect the RME effects
during the expansion period. Patients with congenital
malformations or periodontal diseases, or above 15 years
of age were excluded from the study sample.
In this prospective study, the sample comprised 33
healthy white children (11 boys, 22 girls) with a mean
chronologic age of 10.7 years (range, 7.2-14.5 years)
and a mean skeletal age of 10.9 years (range, 6.8-15
years). These patients were randomly divided into 2
groups: Haas (n 5 18) and hyrax (n 5 15). In the Haas
group, the Haas-type expander, with 4 bands (first perma-
nent molars and first premolars or first deciduous molars)
and buccal and lingual stainless steel bars of 1.0-mm
diameter was used (Fig 1, A). In the hyrax group, the
hyrax-type expander, with 4 bands, buccal and lingual
stainless steel bars of 1.0-mm diameter and a jackscrew
with 1.4-mm stainless steel extensions soldered to the lin-
gual surfaces of each pair of bands, was used (Fig 1, B).
Both appliances had expansion jackscrews with acti-
vations of a quarter turn equivalent to a 0.2-mm expan-
sion. All patients in the Haas and hyrax groups had RME,
with initial activations of 4 quarter turns (0.8 mm)
followed by 2 quarter turns per day (0.4 mm) until the
expansion screw reached 8 mm.
The i-CAT (Imaging Sciences International, Hatfield,
Pa) was used to obtain CBCT images before RME (T1)
and at the end of the active expansion phase (T2). The
CBCT scans were performed at 120 kV, 8 mA, scan
time of 40 seconds, and 0.3-mm voxel dimension. The
data for each patient were reconstructed with 0.3-mm
slice thickness, and the digital imaging and communica-
tions in medicine (DICOM) images were assessed by us-
ing the EFILM workstation software program (version
2.1.2, Merge Healthcare, Milwaukee, Wis). All linear
and angular measurements were made by a blinded ex-
aminer (M.M.), who had no access to the data or the clin-
ical consultations of the patients in this sample.
For transverse maxillary posterior region evaluation,
the DICOM files with CBCT images at T1 and T2 were im-
ported into EFILM and visualized as axial images
arranged side by side. To obtain standardized axial and
coronal slices and thus allow the comparisons between
T1 and T2, the following references were used. In the
Fig 3. Landmarks used in the evaluation of the maxillary posterior region.
368 Weissheimer et al
September 2011 Vol 140 Issue 3 American Journal of Orthodontics and Dentofacial Orthopedics

axial slices, the images that displayed the root canal in
the most apical region of the palatal root of maxillary
first permanent molars were selected. By using the Mul-
tiPlanar Reformation tool, the MultiPlanar Reformation
line was positioned at the root canal in the most apical
region of the palatal root of the maxillary first perma-
nent molars on the right and left sides. From these ref-
erences, standardized coronal images were produced,
and the measurements were made (Fig 2). The landmarks
used for evaluation of the maxillary posterior region are
shown in Figure 3 and described in Table I.
The analyses of the transversal changes in the maxil-
lary anterior region were performed in a similar way to
those of the posterior region. In the axial slices, images
at T1 and T2 were selected with the root canals in the
most apical region of the roots of the maxillary perma-
nent canines visualized. After that, the MultiPlanar
Reformation line was positioned at the root canal in
the most apical region of the maxillary permanent
canine root on the right and left sides. From theses ref-
erences, standardized coronal images were produced,
and the measurements were made (Fig 4). The landmarks
used to evaluate the RME effects in the anterior region of
maxilla are shown in Figure 5 and described in Table I.
Statistical analysis
Intraexaminer reliability of the measurements was
determined by intraclass correlation coefficients. Double
assessments of each parameter at T1 and T2 (10 days
apart) of 15 randomly selected patients from both
groups were compared (Table II). The data obtained
from all measurements were processed with SAS soft-
ware (version 9.0.2, SAS, Cary, NC). Means and standard
errors for each parameter were calculated, and data at T1
Table I. Landmarks for transverse maxillary evaluation
Skeletal
Line 1-2 Posterior baseline Line formed by the 2 lower points at the inferior inner contour of the
posterior nasal cavity on the right and left sides, respectively.
Line 13-14 Anterior baseline Line formed by the 2 lower points at the inferior inner contour of the anterior
nasal cavity on the right and left sides, respectively.
Distance 5-6 Posterior apical base width Distance between points 5 and 6 (points formed by the intersection of the
line 1-2 with buccal contour of maxilla on the right and left sides,
respectively).
Distance 11-12 Posterior midpalatal suture width Distance between points 11 and 12 (lower points at medial limits of maxillary
palatine processes, on the right and left sides, respectively), representing
the midpalatal suture.
Distance 15-16 Anterior apical base width (inferior) Distance between points 15 and 16 (points formed by the intersection of line
13-14 with buccal contour of maxilla on the right and left sides,
respectively).
Distance 17-18 Anterior apical base width (superior) Distance between points 17 and 18 (intersection of the straight line, which is
parallel and 5 mm superior to line 13-14, with buccal contour of maxilla
on the right and left sides, respectively).
Distance 21-22 Anterior mid-palatal suture width Distance between points 21 and 22 (lower points at medial limits of maxillary
palatine processes, on the right and left sides, respectively), representing
the midpalatal suture in the anterior region.
Alveolar
Distance 3-4 Posterior width at the alveolar crest level Distance between points 3 and 4 (coronal-most points of the maxillary
buccal alveolar processes, on the right and left sides, respectively).
Distance 19-20 Anterior width at midalveolar level Distance between points 19 and 20 (intersection of the straight line, which is
parallel and 5 mm inferior to line 13-14, with buccal contour of maxilla on
the right and left sides, respectively).
Dental
Distance 7-8 Intermolar width at occlusal surface Distance between points 7 and 8 (points formed by the intersection of
a straight line, that superimpose the long axis of the root canal of first
permanent molar palatine root, with the occlusal surface on the right and
left sides, respectively).
Distance 9-10 Intermolar width at palatal root apices Distance between points 9 and 10 (apices of palatine root of permanent first
molars, on the right and left sides, respectively).
Angle 1MD Right first molar angulation Angle formed by the straight line from point 7 and that superimposes the
long axis of the root canal of permanent first molar palatine root, on the
right side, with line 1-2.
Angle 1ME Left first molar angulation Angle formed by the straight line from point 8 and that superimposes the
long axis of the root canal of permanent first molar palatine root, on the
left side, with the line 1-2.

and T2 were compared by using the mixed analysis of
variance (ANOVA) model and the Tukey-Kramer method
at a significance level of 5%.
RESULTS
The overall immediate effects of RME on the trans-
verse plane are shown in Table III. There were signifi-
cant increases in maxillary width at the skeletal,
alveolar, and dental levels for both the Haas (Table
IV) and the hyrax (Table V) groups in all parameters
(P 0.05). There was less expansion at the skeletal
than at the dental level, just as the increase in the max-
illary apical base was smaller in the posterior region
(distances 5-6 and 11-12) compared with the anterior
(distances 15-16, 21-22) (Tables III-V). The hyrax
group had greater statistically significant increases in
the maxillary transverse dimensions at the skeletal
level than did the Haas group in both posterior
(distances 5-6 and 11-12) and anterior (distance 21-
22) regions (Table VI). There was no significant differ-
ence between the groups for the buccal inclination of
the maxillary first permanent molars, except for the
linear measure (distance 9-10), which indicated greater
inclination of these teeth in the Haas group than in the
hyrax group (Table VI).
DISCUSSION
After Broadbent21
introduced the cephalostat in
1931, several investigations have analyzed the effects
of RME through cephalometry in 2-dimensional radio-
graphs.3,8,22
The major problem associated with
cephalometry is projection errors, which have an effect
on linear and angular measurements, caused by
magnification and distortion and are compounded by
incorrect patient positioning.23,24
To overcome these
limitations, we evaluated and compared, using high-
resolution CBCT, the immediate effects of RME on the
transverse planes with Haas-type and hyrax-type ex-
panders. CBCT was used because it is a suitable exami-
nation for imaging craniofacial areas, with minimum
distortion, at a lower cost and with lower radiation dos-
ages than conventional CT.11,25,26
In addition, CBCT is
an accurate and reliable method for assessing changes
associated with RME on nasomaxillary structures.20
Fig 4. Transverse maxillary anterior region evaluation: A and B, preexpansion; C and D, at the end of
the active phase of expansion.

Regarding previous reports that used CT images to
evaluate RME, our study had an adequate sample size
(33 subjects).10,20,27-33
Furthermore, this study design
had some important features: (1) it was a prospective
study; (2) the patients were randomly divided between
the groups; (3) the methodology was highly
standardized in terms of appliance fabrication, and rate
and amount of expansion; and (4) it used high-
resolution CBCT. In this study, since the active expansion
phase lasted only 19 days, there was no need to use
a control group without treatment since normal growth
was not an influencing factor in this short time. In this
study, the overall effects of RME produced a significant
skeletal increase in the transverse maxillary dimension,
confirming previous reports.2-5,20,28,30,34,35
The skeletal
expansion amounts were greater in the anterior
region—2.82 mm (distance 17-18), 3.48 mm (distance
15-16), and 4 mm (distance 21-22)—compared with
the posterior—2.64 mm (distance 5-6) and 2.88 mm
(distance 11-12) (Table III). In agreement with previous
authors, the expansion pattern was triangular with
a wider base at the anterior portion of maxilla.20,29,35
The greater expansion in the anterior region could be
explained by the resistance of the medial and lateral
pterygoid plates of the sphenoid bone to the maxillary
tip movement during the RME.35
Another feasible expla-
nation would be through maxillary expansion biome-
chanics: ie, the direction of the expansion force
produced by the expanders would be located anterior
to the center of resistance of each maxillary half.36
The hyrax-type expander produced greater skeletal
expansion—3.14 mm (distance 11-12) and 4.37 mm
(distance 21-22)—than did the Haas-type expander—
2.62 mm (distance 11-12) and 3.63 mm (distance
21-22) (Table VI). The skeletal gain in the hyrax group
accounted for 38.5% to 39.2% (posterior region) and
37.5% to 54.7% (anterior region) of the total expansion
(8 mm). In the Haas group, the increases were smaller,
ranging from 27.2% to 32.7% in the posterior region
Fig 5. Landmarks used in the evaluation of the maxillary anterior region.
Table II. Intraclass correlation coefficients of the mea-
surements
Measurement ICC
Distance 5-6 0.98
Distance 11-12 0.94
Distance 15-16 0.96
Distance 17-18 0.95
Distance 21-22 0.61
Distance 3-4 0.98
Distance 19-20 0.96
Distance 7-8 0.95
Distance 9-10 0.97
Angle 1MD 0.93
Angle 1ME 0.74

and 32.7% to 45.2% in the anterior region. These
comparison results between the appliances differ from
previous reports.7,28,37
Siqueira et al7
compared the
Haas-type and hyrax-type expanders through frontal
cephalometric radiographs and found no differences
between them. Garib et al28
also found no differences
between these 2 expanders using spiral CT. This phe-
nomenon could be explained by the small study sample
(n 5 8), which reduced the power of the t test to show
statistically significant differences. When significant
differences are demonstrated in such situations, they
clearly exist and most likely have clinical importance.
However, the absence of significant differences does
not necessarily indicate that they do not exist. In addi-
tion, the RME changes were analyzed 3 months after
the active expansion phase, unlike our study, with the
immediate effects of RME on 33 patients evaluated. In
disagreement with the present study, Oliveira et al37
found that the Haas-type expander achieved expansion
with a greater component of orthopedic movement
than the hyrax-type expander. However, the comparison
between the 2 kinds of expanders was performed on
study models and anteroposterior cephalograms.
The main difference between Haas-type and hyrax-
type expanders is the acrylic pad close to the palate in
the Haas-type appliance. According to Haas,4
a purpose
of the acrylic pad is to reinforce the anchorage for
greater orthopedic response during RME. However,
the results of our study did not support this theory,
at least regarding the immediate effects of expansion.
Better results in the immediate skeletal response were
obtained by the hyrax-type expander vs the Haas-
type. This fact can be explained by differences in appli-
ance design: more specifically, in the connection mech-
anism of the jackscrew to the bands of the anchorage
teeth. In the hyrax-type appliance design, the jackscrew
was directly connected to the bands by a rigid stainless
steel framework (1.4 mm), unlike the Haas-type appli-
ance design, where the acrylic was responsible for con-
necting the stainless steel framework (1.0 mm) to the
jackscrew. According to a previous study about the bio-
mechanics of RME, appliance designs that use an
acrylic interface with the teeth are far less stiff than
those constructed solely of soldered stainless steel
wire, as in the case of the hyrax-type expander.36
How-
ever, the acrylic pad against the palate would be impor-
tant, especially during the retention period, when it
would prevent the bone from moving through the
teeth, thus averting an orthopedic relapse of the ex-
panded maxilla.4,5,20
Table III. Immediate changes in the maxillary transverse plane with RME
Variable
T1 T2 Change
PMean SE Mean SE Mean SE
Skeletal
Distance 5-6 (mm)
Posterior apical base width 60.29 0.64 62.93 0.64 2.64 0.11 0.0001*
Distance 11-12 (mm)
Posterior midpalatal suture width 00.00 0.08 02.86 0.08 2.88 0.09 0.0001*
Distance 15-16 (mm)
Anterior apical base width (inferior) 38.37 0.61 41.85 0.61 3.48 0.23 0.0001*
Distance 17-18 (mm)
Anterior apical base width (superior) 38.96 0.83 41.78 0.83 2.82 0.23 0.0001*
Distance 21-22 (mm)
Anterior midpalatal suture width 00.00 0.10 04.00 0.11 4.00 0.13 0.0001*
Alveolar
Distance 3-4 (mm)
Posterior width at alveolar crest level 51.65 0.51 57.28 0.51 5.63 0.16 0.0001*
Distance 19-20 (mm)
Anterior width at midalveolar level 40.06 0.58 44.46 0.58 4.40 0.22 0.0001*
Dental
Distance 7-8 (mm)
Intermolar width at occlusal surface 43.51 0.44 51.31 0.44 7.80 0.15 0.0001*
Distance 9-10 (mm)
Intermolar width at palatal root apices 29.90 0.52 32.55 0.52 2.65 0.14 0.0001*
Angle 1MD (
)
Right first molar angulation 110.6 1.4 118.1 1.4 7.53 0.74 0.0001*
Angle 1ME (
)
Left first molar angulation 117.7 1.2 123.8 1.2 6.17 0.68 0.0001*
*Statistically significant (P 0.05).

In the hyrax group, the transverse expansion at the
suture gradually decreased from the anterior, by 4.37
mm (distance 21-22), to the posterior, by 3.14 mm
(distance 11-12) (Table V). This sutural orthopedic sep-
aration accounted for 54.7% and 39.2% of the total
expansion (8 mm) at distances 21-22 and 11-12,
Table IV. Immediate changes in the maxillary transverse plane with RME in the Haas group
Variable
T1 T2 Change
PMean (mm) SE (mm) Mean (mm) SE (mm) Mean (mm) SE (mm)
Skeletal
Distance 5-6
Distance 11-12
Distance 15-16
Distance 17-18
Distance 21-22
Alveolar
Distance 3-4
Distance 19-20
Dental
Distance 7-8
Distance 9-10
Table V. Immediate changes in the maxillary transverse plane with RME in the hyrax group
Variable
T1 T2 Change
PMean (mm) SE (mm) Mean (mm) SE (mm) Mean (mm) SE (mm)
Skeletal
Distance 5-6
Distance 11-12
Distance 15-16
Distance 17-18
Distance 21-22
Alveolar
Distance 3-4
Distance 19-20
Dental
Distance 7-8
Distance 9-10

respectively. These findings endorse a previous report in
which, of the total expansion achieved, the hyrax-type
expander produced 55% of the suture expansion in the
anterior and 38% in the posterior regions.20
However,
the RME changes were analyzed 3 months after the
active expansion phase, unlike our study, where the
immediate effects of RME were evaluated.
This investigation showed a more significant skele-
tal response compared with other studies.29,30
In
a study by Lione et al,29
the RME was performed
with a modified hyrax-type expander (bands on the
first permanent molars only), and less sutural expan-
sion was obtained in both the anterior (2.17 mm)
and the posterior (1.15 mm) regions. This small ortho-
pedic effect could be explained by (1) the use of a mod-
ified hyrax-type expander, which had less anchorage;
(2) less total expansion (7 mm); and (3) the sutural ex-
pansion evaluated in a more posterior region (posterior
nasal spine) than in our study (in the first molar re-
gion). In our investigation, the amounts of sutural
expansion (2.88 mm in the posterior and 4 mm in
the anterior regions) were greater than the amounts
reported by Podesser et al30
(1.6 mm in the posterior
and 1.5 mm in the anterior regions). This difference
could be explained by less total expansion (7 mm)
and the relapse that might have occurred because of
appliance removal and replacement at the end of the
active phase of RME for CT scan acquisition in their
study. In our investigations, there was no need to re-
move the appliances before the CBCT examination at
T2 because of the lower level of metal artifacts pro-
duced by CBCT compared with conventional CT.11,38
The greater amounts of expansion at the alveolar
level (distances 3-4 and 19-20) than the sutural expan-
sion (distances 11-12 and 21-22) (Table III) show the
bending of the alveolar processes of the maxilla; this re-
sult agrees with previous reports.20,28,30
The expansion
at the alveolar level (distance 3-4) accounted for 70%
of the total expansion, 36% of which represents
sutural expansion and 34% is purely alveolar bending
toward the buccal aspect.
The great changes in maxillary transverse dimensions
occurred at the dental level, where the expansion ac-
counted for 97% (distance 7-8) of the total expansion
Table VI. Comparison between the changes in the maxillary transverse planes in the groups
Variable
Haas group Hyrax group
P
T2-T1 T2-T1
Mean SE Mean SE
Skeletal
Distance 5-6 (mm)
Posterior apical base width 2.19 0.15 3.10 0.17 0.0002*
Distance 11-12 (mm)
Posterior midpalatal suture width 2.62 0.12 3.14 0.14 0.010*
Distance 15-16 (mm)
Anterior apical base width (inferior) 3.29 0.30 3.66 0.34 0.427
Distance 17-18 (mm)
Anterior apical base width (superior) 2.62 0.31 3.00 0.35 0.438
Distance 21-22 (mm)
Anterior midpalatal suture width 3.63 0.17 4.37 0.20 0.007*
Alveolar
Distance 3-4 (mm)
Posterior width at alveolar crest level 5.44 0.25 5.80 0.28 0.342
Distance 19-20 (mm)
Anterior width at midalveolar level 4.03 0.30 4.76 0.34 0.119
Dental
Distance 7-8 (mm)
Intermolar width at occlusal surface 7.70 0.20 7.90 0.23 0.526
Distance 9-10 (mm)
Intermolar width at palatal root apices 2.15 0.18 3.14 0.21 0.0008*
Angle 1MD (
)
Right first molar angulation 8.25 0.98 6.80 1.11 0.334
Angle 1ME (
)
Left first molar angulation 6.14 0.90 6.19 1.02 0.975
*Statistically significant difference (P 0.05).

(8 mm) (Table III). This greater expansion at the dental
level compared with the skeletal level agrees with previ-
ous reports.3,4,20,28,30,34,37
However, the actual dental
expansion can be found by subtracting the total
expansion at the dental level (distance 7-8) from the
suture and alveolar expansions (distance 3-4). Thus,
from 97% (7.8 mm) of the total expansion at the
dental level (distance 7-8), only 27% (2.17 mm)
represents actual dental expansion, which was smaller
compared with 36% (2.88 mm) of pure skeletal
expansion (distance 11-12) and with 34% (2.75 mm)
of pure alveolar bending. RME produced significant
buccal tipping of the first permanent molars,
accounting for 7.53
(angle 1MD) on the right side
and 6.17
(angle 1ME) on the left side (Table III). There
were no statistically significant differences between the
2 groups in angular measurements. The amounts of buc-
cal tipping of the first permanent molars for the Haas
group were 8.25
on the right side (angle 1MD) and
6.14
on the left side (angle 1ME), whereas, in the hyrax
group, the tipping amounts were 6.80
on the right and
6.19
on the left sides. However, there was a statistically
significant difference between the Haas and hyrax
groups in the linear measurement (distance 9-10), which
represents the distance between the apices of the palatal
roots of the first permanent molars. The higher values for
distance 9-10 (nearly 8 mm of expansion) reflected
a small buccal tipping of the first molars. In the hyrax
group, distance 9-10 increased by 3.14 mm, whereas,
in the Haas group, there was an increase of 2.15 mm,
showing greater tipping of the first permanent molars
with that expander (Table VI). Similar results were re-
ported in other investigations.28-37
In the study of
Garib et al,28
the Haas-type expander produced greater
buccal tipping of the first permanent molars (3.5
)
than did the hyrax-type expander (1.6
). Oliveira
et al37
found that the Haas-type expander produced
greater buccal tipping of the first permanent molars
(7.12
right side, 6.64
left side) compared with the
Hyrax-type expander (6.94
right side, 1.21
left side).
However, these differences were not considered statisti-
cally significant in either study.
We assessed the immediate effects of RME; therefore,
long-term evaluation is necessary for a better under-
standing of the differences between Haas-type and
hyrax-type expanders, especially during the retention
and postretention phases of RME.
CONCLUSIONS
Based on this clinical trial with CBCT to assess the
immediate effects of RME on the transverse plane with
2 kinds of palatal expanders, the following conclusions
can be drawn:
1. RME produced significant increases in all maxil-
lary transverse dimensions. The expansion pattern
was triangular, with smaller effects at the skeletal
level than at the dental level. However, the pure
skeletal expansion was greater than actual dental
expansion. The sutural expansion showed a wedge
shape with the wide base in the anterior maxilla.
2. The opening of the midpalatal suture accounted
for 50% of the total expansion (8 mm) in the
anterior region and 36% in the posterior region
(there was a decrease from anterior to posterior).
3. The hyrax-type expander produced greater orthope-
dic effects in 3 of the 5 skeletal points measured
compared with the Haas-type expander. However,
the effects were less than 0.5 mm per side and might
not be clinically significant.
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Immediate effects of rapid maxillary expansion with haas-type and hyrax-type expander.