Skeletal response to maxillary protraction

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ORIGINAL ARTICLE
Skeletal response to maxillary protraction
with and without maxillary expansion: A finite
element study
Pawan Gautam,a Ashima Valiathan,b and Raviraj Adhikaric
Manipal, Karnataka, India
Introduction: The purpose of this finite element study was to evaluate biomechanically 2 treatment modali-
ties—maxillary protraction alone and in combination with maxillary expansion—by comparing the displace-
ment of various craniofacial structures. Methods: Two 3-dimensional analytical models were developed
from sequential computed tomography scan images taken at 2.5-mm intervals of a dry young skull. AutoCAD
software (2004 version, Autodesk, San Rafael, Calif) and ANSYS software (version 10, Belcan Engineering
Group, Cincinnati, Ohio) were used. The model consisted of 108,799 solid 10 node 92 elements, 193,633 no-
des, and 580,899 degrees of freedom. In the first model, maxillary protraction forces were simulated by apply-
ing 1 kg of anterior force 30� downward to the palatal plane. In the second model, a 4-mm midpalatal suture
opening and maxillary protraction were simulated. Results: Forward displacement of the nasomaxillary com-
plex with upward and forward rotation was observed with maxillary protraction alone. No rotational tendency
was noted when protraction was carried out with 4 mm of transverse expansion. A tendency for anterior max-
illary constriction after maxillary protraction was evident. The amounts of displacement in the frontal, vertical,
and lateral directions with midpalatal suture opening were greater compared with no opening of the midpalatal
suture. The forward and downward displacements of the nasomaxillary complex with maxillary protraction and
maxillary expansion more closely approximated the natural growth direction of the maxilla. Conclusions: Dis-
placements of craniofacial structures were more favorable for the treatment of skeletal Class III maxillary ret-
rognathia when maxillary protraction was used with maxillary expansion. Hence, biomechanically, maxillary
protraction combined with maxillary expansion appears to be a superior treatment modality for the treatment
of maxillary retrognathia than maxillary protraction alone. (Am J Orthod Dentofacial Orthop 2009;135:723-8)
M
axillary protraction is recommended for skel-
etal Class III patients with maxillary defi-
ciency.1,2 The principle of maxillary
protraction is to apply tensile force on the circummaxil-
lary sutures and thereby stimulate bone apposition in the
suture areas. Biomechanical studies on dry skulls have
demonstrated that application of an anteriorly directed
force results in forward movement of the maxilla.3,4
These investigations also showed that the direction of
a Assistant professor, Department of Orthodontics and Dentofacial Orthopedics,
Manipal College of Dental Sciences, Manipal, Karnataka, India.
b Professor and head, Director of Post Graduate Studies, Department of Ortho-
dontics and Dentofacial Orthopedics, Manipal College of Dental Sciences,
Manipal, Karnataka, India.
c Reader, Department of Mechanical Engineering, Manipal Institute of Technol-
ogy, Manipal, Karnataka, India.
The authors report no commercial, proprietary, or financial interest in the prod-
ucts or companies described in this article.
Reprint requests to: Ashima Valiathan, Department of Orthodontics and Dento-
facial Orthopedics, Manipal College of Dental Sciences, Manipal, Karnataka,
India, 576104; e-mail, avaliathan@yahoo.com.
Submitted, March 2007; revised and accepted, June 2007.
0889-5406/$36.00
Copyright � 2009 by the American Association of Orthodontists.
doi:10.1016/j.ajodo.2007.06.016
the force is critical in controlling rotation of the maxilla.
A force generated parallel to the maxilla or above the
palatal plane produces counterclockwise rotation of
the palatal plane.
Maxillary protraction therapy often is supplemented
with maxillary expansion. Maxillary expansion is com-
monly needed in the treatment of patients with Class III
malocclusion, because of insufficient maxillary arch
width. Rapid maxillary expansion is typically used in
young patients and has been shown to help in Class III
correction. Midfacial orthopedic expansion has been
recommended with protraction forces on the maxilla be-
cause it supposedly disrupts the circummaxillary sutural
system and presumably facilitates the orthopedic effect
of the facemask.5-7 There is some evidence in the liter-
ature that maxillary expansion alone can be beneficial in
the treatment of certain types of Class III malocclusion,
particularly borderline malocclusions.7
The purposes of this finite element study were to
evaluate the displacements of various craniofacial struc-
tures with maxillary protraction therapy and compare
them with the pattern of displacement with combined
rapid palatal expansion and maxillary protraction.
723
mailto:avaliathan@yahoo.com
724 Gautam, Valiathan, and Adhikari American Journal of Orthodontics and Dentofacial Orthopedics
June 2009
MATERIAL AND METHODS
The details of the modeling procedure were de-
scribed in a previous study.8 The analytical model was
developed from sequential computed tomography scan
images taken at 2.5-mm intervals of a dry young skull
with dental age of 7 years. These computed tomography
scan sections were traced by using AutoCAD software
(2004 version, Autodesk, San Rafail, Calif). The next
step was to generate geometric surfaces by joining lines
together. These tracings were sequentially transferred to
ANSYS software (version 10, Belcan Engineering
Group, Cincinnati, Ohio). The complete geometry in-
cluded an assemblage of discrete pieces called ele-
ments, connected at a finite number of points, called
nodes; 10 node solid 92 elements (tetrahedron) were
used for meshing. The solid 92 element has a quadratic
displacement behavior, is well suited to model irregular
solid geometry, and gives a better representation of the
variable bone thickness that is characteristic of the cra-
niofacial skeleton. Also, the use of solid 92 element
gives better stress transmissibility and bending defor-
mations. This tetrahedron element is defined by 10 no-
des having 3 degrees of freedom at each unstrained
node: 3 translations (x, y, and z). Our model consisted
of 108,799 elements, 193,633 nodes, and 580,899 de-
grees of freedom. Hence, this model (Fig 1) is a better
representation of the skull than previous models in
which the sections were made at 10-mm sections.9-12
To avoid any inconsistencies associated with inaccuracy
of modeling the teeth and the periodontal ligament
through computed tomography, this study was restricted
to the analysis of skeletal displacements.13,14 The teeth
and the periodontal ligament were not modeled, and the
model comprised the compact and cancellous bones of
Fig 1. Three-dimensional finite element method skull
model of a 7-year-old child.
the craniofacial skeleton. Nine craniofacial sutural sys-
tems were integrated into the model as described in the
previous study.8 The mechanical properties of the com-
pact and cancellous bones in the model were defined ac-
cording to the experimental data in previous studies as
shown in Table I.10 Restraints were established at all
other nodes of the cranium lying on the symmetrical
plane, and appropriate boundary conditions were im-
posed. In addition, zero-displacement and zero-rotation
boundary conditions were imposed on the nodes along
the foramen magnum.
In the first model, 1 kg of force was directed anteri-
orly and 30� downward relative to the occlusal plane
near the canine to simulate orthopedic maxillary pro-
traction forces.
In the second model, a known transversal (x) dis-
placement with a magnitude of 2 mm was applied on
the surface nodes of the intermaxillary suture in the first
molar region to simulate the initial phase of maxillary
expansion before maxillary protraction therapy. It was
assumed that the 2 plates of the transversal orthopedic
appliance moved apart by a total distance of 4 mm. Af-
ter this, maxillary protractionforces were simulated by
applying a 1-kg force directed anteriorly and 30� down-
ward relative to the occlusal plane near the canine.
RESULTS
With maxillary protraction alone, Point A moved
anteriorly and inferiorly by 0.33 and 0.056 mm, respec-
tively. The inferior movement of the posterior nasal
spine (PNS) was 0.20 mm, which was more than that
of anterior nasal spine (ANS), indicating a slight ten-
dency for upward rotation even with 30� of downward
pull (Fig 2). Prosthion (or supradentale) moved anteri-
orly and inferiorly by 0.34 and 0.045 mm, respectively.
With maxillary protraction alone, the total vector
displacements of the medial and lateral pterygoid plates
were 0.3 to 0.45 mm; the displacements of other areas of
nasomaxillary region were 0.25 to 0.4 mm. Lateral bend-
ing of the medial and lateral pterygoid plates were evi-
dent with maxillary protraction (Table II).
In response to maxillary protraction with maxillary
expansion, Point A moved anteriorly and inferiorly by
Table I. Young’s modulus and Poisson’s ratio for various
materials used in this study
Material Young’s modulus kg/mm2 Poisson’s ratio
Compact bone 1.37 3 103 0.3
Cancellous bone 7.9 3 102 0.3
Data from Tanne et al.9
American Journal of Orthodontics and Dentofacial Orthopedics Gautam, Valiathan, and Adhikari 725
Volume 135, Number 6
0.15 and 0.68 mm, respectively. ANS moved anteriorly
and inferiorly by 0.12 and 0.69 mm, respectively. The
inferior movement of PNS was 0.57 mm (less than
that of ANS), indicating downward and forward transla-
tion of the maxilla in response to maxillary protraction
with maxillary expansion (Fig 3). Prosthion (or supra-
dentale) moved anteriorly and inferiorly by 0.14 and
0.53 mm, respectively (Table III).
Maxillary expansion resulted in a wedge-shaped
opening in both the anteroposterior and superoinferior
planes (Fig 4). Maximum displacement in response to
maxillary protraction with maxillary expansion was ob-
served in the nasomaxillary region followed by the body
of the zygomatic bone. The overall displacements of the
pterygoid plates were considerably less than the dis-
placement of the zygomatic bone in maxillary protrac-
tion with maxillary expansion. This is different from
the pattern of displacement with maxillary protraction
alone in which the overall displacement of the pterygoid
region was more than that of the zygomatic bone.
The nasal cavity wall, the nasal bone, and the zygo-
matic bone were displaced medially, anteriorly, and in-
feriorly with maxillary protraction alone. Similarly, the
zygomatic arch also was displaced in the medial, ante-
rior, and inferior directions. With maxillary protraction
alone, the orbital part of the greater wing of the sphe-
noid was displaced in the lateral direction, whereas
the orbital part of the lesser wing of the sphenoid was
displaced in the medial direction. The sphenoid bone
as a whole was displaced in the anterior and inferior
directions.
With maxillary protraction with expansion, the zy-
gomatic bone and the zygomatic process of the temporal
bone were displaced laterally, posteriorly, and superi-
orly. The nasal cavity wall was displaced laterally,
Fig 2. Maxillary protraction alone, causing counter-
clockwise rotation of the nasomaxillary complex.
whereas the nasal bone moved medially with maxillary
protraction with expansion. The orbital part of the
greater wing of the sphenoid was displaced laterally,
posteriorly, and superiorly, whereas the orbital part of
the lesser wing of the sphenoid was displaced laterally,
anteriorly, and inferiorly.
DISCUSSION
Several studies noted counterclockwise rotation of
the maxilla with protraction headgear treatment.3,15,16
To minimize the counterclockwise rotation produced
by the protraction forces, investigators changed the
point of force application and the direction of the pro-
traction forces. Some investigators applied the force
from the canine region.17 Spolyar18 applied the force
Table II. Displacement of the craniofacial structures
with maxillary protraction alone (mm)
x y z
Maxilla
Point A 0.45 3 10�7 0.33 3 10�3 0.56 3 10�4
ANS 0.43 3 10�7 0.30 3 10�3 0.68 3 10�4
Supradentale 0.23 3 10�5 0.34 3 10�3 0.45 3 10�4
Tuberosity �0.78 3 10�5 0.32 3 10�3 0.22 3 10�3
Zygomatic buttress 0.11 3 10�4 0.36 3 10�3 0.17 3 10�3
Inferior orbital rim 0.86 3 10�6 0.25 3 10�3 0.18 3 10�3
Frontal process �0.38 3 10�6 0.20 3 10�3 0.21 3 10�3
PNS 0.27 3 10�7 0.33 3 10�3 0.20 3 10�3
Sphenoid bone
medial pterygoid
Inferior �0.14 3 10�4 0.32 3 10�3 0.25 3 10�3
Superior �0.10 3 10�4 0.32 3 10�3 0.23 3 10�3
Lateral pterygoid
Inferior �0.80 3 10�5 0.30 3 10�3 0.38 3 10�3
Superior �0.12 3 10�4 0.32 3 10�4 0.26 3 10�3
Nasal cavity wall
Lateral 0.77 3 10�5 0.28 3 10�3 0.86 3 10�4
Inferior 0.43 3 10�7 0.30 3 10�3 0.67 3 10�4
Superior 0.45 3 10�7 0.23 3 10�3 0.48 3 10�4
Nasal bone 0.37 3 10�5 0.20 3 10�3 0.65 3 10�4
Zygomatic bone
Frontal 0.14 3 10�5 0.20 3 10�3 0.13 3 10�3
Temporal 0.12 3 10�3 0.19 3 10�3 0.20 3 10�3
Maxillary 0.26 3 10�5 0.29 3 10�3 0.13 3 10�3
Body 0.26 3 10�5 0.28 3 10�3 0.15 3 10�3
Frontal bone
Zygomatic process 0.24 3 10�7 0.19 3 10�3 0.21 3 10�3
Superior orbital
ridge
�0.40 3 10�5 0.15 3 10�3 0.55 3 10�4
Temporal bone
Zygomatic process 0.64 3 10�4 0.19 3 10�3 0.16 3 10�3
Sphenoid bone
Greater wing
(orbital)
�0.59 3 10�5 0.19 3 10�3 0.20 3 10�3
Lesser wing
(orbital)
0.53 3 10�6 0.19 3 10�3 0.22 3 10�3
726 Gautam, Valiathan, and Adhikari American Journal of Orthodontics and Dentofacial Orthopedics
June 2009
at the premolar or the deciduous molar region. Other in-
vestigators moved the point of force application distal to
the lateral incisors, and some changed the direction of
force at an angle of 15� to 30� from the occlusal
plane.19,20
A 1-kg force applied 30� downward to the palatal
plane induced forward displacement of the nasomaxil-
lary complex with upward and forward rotation. This
is in contrast to the study by Tanne et al,10 who showed
that the nasomaxillary complex repositioned in an al-
most translatory manner with a slight rotation in loading
with a 30� downward force. Keles et al21 observed that
the maxilla advanced forward with a counterclockwise
rotation with 30� downward maxillary protraction.
The counterclockwise rotational tendency of the
maxilla was why maxillary protraction therapy in Class
III patients with maxillary deficiency and open bite was
contraindicated.22,23
A 1-kg force applied 30� downward to the palatal
plane along with 4 mm of transverse expansion induced
forward displacement of the nasomaxillary complex
with no rotational tendency. This agrees with the find-
ings of Yu et al,24 who demonstrated that the amounts
of displacement and deformation when the midpalatal
suture was opened showed decreases in upward-forward
rotation of the maxilla and the zygomatic arch. This is
a more favorable displacement pattern in patients with
open bite or vertical growers. Hence, a combination of
maxillary protraction with rapid maxillary expansion
can be used in Class III maxillary retrognathic patients
with open bite without the adverse effect of increasing
lower anterior facial height.25 Contrary to this, Williams
Fig 3. Maxillary protraction with maxillary expansion
(lateral view), causing the nasomaxillary complex
to translate anteroinferiorly, approximating its growth
direction.
et al26 and Baccetti et al27 observed backward and
downward rotation of the mandible in patients undergo-
ing maxillary protraction with palatal expansion. These
findings suggest that the vertical dimension is difficult
to control with maxillary protraction.
Keles et al21 observed that the force applied extraor-
ally 20 mm above the maxillary occlusal plane resulted
in anterior translation of the maxilla without rotation;
they recommended this method for Class III patients
with anterior open bite.
The displacement of various craniofacial structures
was considerably more after maxillary protraction
with maxillary expansion. This agrees with Yuet al,24
who showed greater displacements in the frontal,
Table III. Displacement of craniofacial structures with
maxillary protraction with maxillary expansion (mm)
x y z
Maxilla
Point A �0.97 0.15 0.68
ANS �0.75 0.12 0.69
Supradentale �1.09 0.14 0.53
Tuberosity �0.61 �0.29 3 10�1 �0.38 3 10�1
Zygomatic buttress �0.79 �0.10 �0.43
Inferior orbital rim �0.20 0.54 3 10�2�0.93 3 10�1
Frontal process 0.12 3 10�1 0.11 0.21
PNS �0.67 0.16 0.57
Sphenoid bone
medial pterygoid
Inferior �0.55 0.36 3 10�1 0.14
Superior �0.67 0.37 3 10�1 0.19
Inferior �0.46 �0.27 3 10�1 �0.60 3 10�1
Superior �0.48 �0.37 3 10�1 �0.71 3 10�1
Nasal cavity wall
Lateral �0.54 0.66 3 10�1 0.29
Inferior �0.75 0.12 0.69
Superior �0.89 3 10�5 0.13 3 10�1 0.37
Nasal bone 0.52 3 10�2 0.87 3 10�1 0.28
Zygomatic bone
Frontal �0.19 �0.17 �0.40
Temporal �0.44 �0.11 �0.50
Maxillary �0.51 �0.53 3 10�1 �0.28
Body �0.49 �0.10 �0.46
Frontal bone
Zygomatic
process
�0.55 3 10�5 0.12 0.24
Superior orbital
ridge
�0.51 3 10�1 �0.18 3 10�1 0.37 3 10�1
Temporal bone
Zygomatic
process
�0.18 �0.15 �0.28
Sphenoid bone
Greater wing
(orbital)
0.40 3 10�2�0.60 3 10�1 �0.11
Lesser wing
(orbital)
0.29 3 10�1 0.12 0.22
American Journal of Orthodontics and Dentofacial Orthopedics Gautam, Valiathan, and Adhikari 727
Volume 135, Number 6
vertical, and lateral directions with midpalatal sut-
ure opening when compared with no midpalatal suture
opening.
The nodes in the anterior region of the maxilla were
displaced medially with maxillary protraction alone, in-
dicating the tendency for anterior maxillary constriction
after maxillary protraction. A tendency for constriction
at the anterior region of the maxilla was also noted pre-
viously.9 A wedge-shaped opening, as seen clinically,
was evident both in the anteroposterior and the superoin-
ferior directions.6 This suggests that maxillary expan-
sion in conjunction with maxillary protraction tends to
counteract the side effect of anterior constriction.
Kapust et al28 showed that the treatment of Class III
malocclusion with a bonded maxillary expander and
a facemask in the early mixed dentition results in signif-
icant advancements of ANS, PNS, Point A, and the
maxillary dentition.
The anterior structures of the maxilla—Point A,
ANS, and prosthion—were displaced more anteriorly
with maxillary protraction and expansion than with
maxillary protraction alone, with downward and for-
ward translation of the maxilla, indicating that maxil-
lary protraction with expansion yields more favorable
results than maxillary protraction alone. This forward
and downward displacement of the nasomaxillary com-
plex with maxillary protraction with expansion more
closely approximates the natural growth direction of
the maxilla.29
Rapid palatal expansion has been recommended
with protraction forces on the maxilla because it sup-
posedly disrupts the circummaxillary sutural system
Fig 4. Maxillary protraction with maxillary expansion (in-
ferior view), with wedge-shaped opening after maxillary
expansion.
and presumably facilitates the orthopedic effect of the
facemask.6 This is supported by high stresses generated
in various craniofacial sutures after rapid maxillary
expansion.8
These results showed that maxillary protraction pro-
duced similar changes to normal downward and forward
growth of the maxilla and was achieved with accompa-
nying opening of the midpalatal suture.
CONCLUSIONS
The amounts of displacement in the frontal, vertical,
and lateral directions with midpalatal suture opening
were more than those with no midpalatal suture open-
ing. The amounts of displacement and deformation
when the midpalatal suture was opened showed de-
creases in upward-forward rotation of the maxilla and
the zygomatic arch. The forward and downward dis-
placement of the nasomaxillary complex with maxillary
protraction and maxillary expansion more closely ap-
proximates the natural growth direction of the maxilla.
Hence, biomechanically, maxillary protraction com-
bined with maxillary expansion appears to be a superior
treatment modality for maxillary retrognathia than max-
illary protraction alone.
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	Skeletal response to maxillary protraction with and without maxillary expansion: A finite element study
	Material and methods
	Results
	Discussion
	Conclusions
	References