Open Journal of Stomatology, 2013, 3, 425-432 OJST
http://dx.doi.org/10.4236/ojst.2013.38071 Published Online November 2013 (http://www.scirp.org/journal/ojst/)
Dentoalveolar changes following maxillary distraction
osteogenesis
Lili Yang1, Eduardo Yugo Suzuki2*, Boonsiva Suzuki2
1Faculty of Dentistry, Chiang Mai University, Chiang Mai, Thailand
2Department of Orthodontics and Pediatric Dentistry, Faculty of Dentistry, Chiang Mai University, Chiang Mai, Thailand
Received 20 August 2012; revised 23 September 2013; accepted 15 October 2013
Copyright © 2013 Lili Yang et al. This is an open access article distributed under the Creative Commons Attribution License, which
permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ABSTRACT
The purpose of this study was to compare the den-
toalveolar changes produced when using two differ-
ent intraoperative surgical procedures for maxillary
distraction osteogenesis. Eight patients were assigned
into two groups according to the surgical procedure:
down-fracture (DF, n = 6) vs non-down-fracture
(NDF, n = 2). Lateral cephalograms and 3-D models
before and after maxillary distraction were analyzed.
The Mann-Whitney U test was used to compare the
differences in the amounts of advancement and
dento-alveolar changes between the DF and NDF
groups. The significance level was established at 0.05.
Although a significantly greater amount of maxillary
movement was observed in the DF group (10.0 mm ±
2.2) than in the NDF group (5.9 mm ± 2.3), signifi-
cantly greater arch length (8.7 mm ± 5.2) and arch
width changes (6.0 mm ± 1.0) were observed in the
NDF group than in the DF group, (arch lengths 3.0
mm ± 1.1 and arch width changes 3.2 mm ± 2.0). A
significantly greater amount of dental anchorage loss
was observed in the NDF group. The use of the NDF
procedure resulted in greater amounts of dental an-
chorage loss than resulted from the DF procedures
when tooth-borne devices were used during maxillary
distraction osteogenesis. The type of surgical proce-
dure might play an important role in the amount and
direction of the dental changes.
Keywords: Maxillary Distraction; Non-Down-Fracture;
Down-Fracture; 3-D Model Analysis
1. INTRODUCTION
Distraction osteogenesis is a biomechanical process
where the application of incremental traction forces leads
to new bone formation between the surfaces of osteoto-
mized bone segments that are gradually separated [1,2].
This technique not only allows the development of in-
crements of new bone, but also allows the stretching of
the surrounding soft tissue [3-5]. Therefore, distraction
osteogenesis has become a very important alternative in
the treatment of patients with severe maxillary hypopla-
sia in craniofacial syndromes and cleft-related deformi-
ties [6,7].
Maxillary distraction osteogenesis has been applied
successfully for the management of patients with clefts
and has several advantages over conventional orthog-
nathic procedures. These advantages, such as allowing
large amounts of maxillary advancement [6,7], thus
eliminating the need for bone grafting, have reduced
rates of relapse [8].
Conventional surgical procedures for maxillary dis-
traction osteogenesis often involve a Le Fort I complete
osteotomy with pterygomaxillary disjunction, septal dis-
junction and careful medial sinus wall separation fol-
lowed by an intraoperative DF to achieve the complete
mobilization of the maxilla [9,10]. However, the DF is
considered as a high-risk and aggressive procedure, since
it may induce undesirable fractures extended to the
pterygoid plate, sphenoid bone and cranial base, edema
and bleeding [11]. In order to minimize the risk of the
surgical procedure and to shorten the operation time, the
use of maxillary osteotomy without the complete intra-
operative DF, also known as the NDF technique, has
been proposed by several authors [3,12,13]. In the NDF
technique, the maxilla is mobilized just enough to ensure
that the skeletal osteotomy has been completed [12].
Therefore, the traditional and aggressive DF procedure is
not fully performed [12,13].
Some reports have shown that cases treated without
the DF technique allow for sufficient mobilization of the
maxillary bone, consequently providing similar surgical
outcomes to those of cases treated with the conventional
*Corresponding author.

L. L. Yang et al. / Open Journal of Stomatology 3 (2013) 425-432 426
DF technique [13,14]. However, our hypothesis is that
using two different surgical procedures may provide dif-
ferent levels of maxillary mobility at the time of the
maxillary distraction.
As a consequence, maxillary mobility might play an
important role in the total amount of maxillary move-
ment, but also in the amount of dental movement.
Cephalometric measurement is the traditional tech-
nique for analysis of dental movement after treatment
[9,10]. However, a cephalometric radiograph is a two-
dimensional projection of a three-dimensional structure
and thus cannot be used to evaluate tooth movement in
three dimensions [15]. Recently, digital dental models
have been used to accurately document malocclusions
and to evaluate tooth movement in three dimensions [15].
The measurement parameters for dental movement were
the results of three-dimensional digitizing and not a read-
ing of a two-dimensional radiograph, which can in theory
be exposed with a change in orientation, which may in-
fluence the result [16]. Therefore, the three-dimensional
model analysis can be used as an efficient approach to
compare the different dental changes in the osteotomized
maxilla following different surgical procedures.
The purpose of this study was to compare the dental
changes following maxillary distraction when using two
different intraoperative surgical procedures (DF and
NDF) for maxillary distraction osteogenesis.
2. MATERIALS AND METHODS
2.1. Patient Selection
Between November 2009 and November 2011, eight
patients (six male and two female, aged 15 to 26 years)
who underwent maxillary distraction osteogenesis at the
Faculty of Dentistry, Chiang Mai University were in-
cluded in this retrospective study. Six patients had uni-
lateral cleft lip and palate (UCLP), one had bilateral cleft
lip and palate (BCLP) and one had cleft palate (CP). All
patients volunteered and signed an informed consent
based on the Helsinki declaration of 1975, as revised in
2000.
The patients were divided into two main groups ac-
cording to the distraction device; six patients who re-
ceived the DF procedure and two patients who did not
receive the NDF procedure.
2.2. Measurements
2.2.1. Cephalometric Measurement
Dento-skeletal changes were analyzed using serial sets of
lateral cephalograms obtained in centric occlusion before
and after distraction. All lateral cephalograms obtained at
each interval were traced on acetate paper. The anterior
cranial base was used for overall superimposition. Four-
teen skeletal and dental landmarks and two reference
planes were identified (Figures 1(a) and (b)). Custom-
made digitizer software (Smart Ceph v 9.0 XP, Y & B
Products, Chiang Mai, Thailand) was used to perform all
linear and angular cephalometric measurements.
An XY coordinate system was constructed on the sella
turcica (S). A line parallel to the Frankfort horizontal
(FH) plane passing through S was used as the X axis; a
line drawn perpendicular to this plane through S was
used as the vertical or Y axis [17]. The SN line and the X
and Y axes were transferred from pre-distraction to post-
distraction as accurately as possible by using the anterior
cranial base for overall superimposition. The subtraction
of the X and Y values for each landmark at each interval
was calculated to estimate the horizontal and vertical
displacement of the landmarks. The magnification of the
cephalograms was 10%. No correction was made be-
cause all radiographs were made in the same cephalostat
with the same object-film distance. The radiographs were
obtained with the lips in the relaxed position.
1) Skeletal change measurement
The linear and angular skeletal changes after maxillary
distraction osteogenesis were measured according to
Figure 2.
 Linear changes (Figure 2( a) )
The amount of skeletal movement was measured
(a) (b)
Figure 1. (a) Skeletal cephalometric landmarks and reference
planes. The following skeletal points were assessed: Sella (S):
the midpoint of the cavity of the sella turcica, Nasion (N), Or-
bitale (Or), Porion (Po), Anterior nasal spine (ANS), Posterior
nasal spine (PNS), Point A (a), Point B (b), Menton (Me),
Gonion (Go). Frankfort horizontal plane: extending from the
porion to the orbitale. This plane is used as a reference for an-
gular measurement of palatal plane and mandibular plane
angulations. (b) Dental cephalometric landmarks and reference
planes. U1: the tip of the crown of the most anterior maxillary
central incisor. U1r: the apex point of the most anterior maxil-
lary central incisor. U6: the midpoint of the maxillary first mo-
lar crown. U6r: the apex of the mesial root of the maxillary first
molar. Palatal plane: extending from the anterior nasal spine to
the posterior nasal spine. This plane is used as a reference for
angular measurement of maxillary central incisor and molar
tooth angulations.

L. L. Yang et al. / Open Journal of Stomatology 3 (2013) 425-432 427
(a) (b)
Figure 2. Linear and angular skeletal changes after maxillary
distraction osteogenesis (a) Linear changes, (b) Angular
changes.
through the horizontal (A-x) direction movement of point
A before and after distraction.
 Angular changes (Figure 2( b ))
The angular changes, including SNA, SNB, ANB, and
the inclination of the palatal plane (PP-FH) and man-
dibular plane (MP-FH) relative to the Frankfort horizon-
tal plane, which represented the maxillary rotation and
resulting mandibular rotation, were measured before and
after distraction. The angular changes after the activation
period were calculated by comparing the measurements
before and after distraction.
2) Dental change measurement (Figure 3)
The movement of the maxillary central incisors and
first molars were measured by comparing the linear
changes and angular changes before and after distraction.
 Linear changes (Figure 3( a) )
U1-PP (mm): vertical distance from the maxillary in-
cisor edge to the palatal plane.
U6-PP (mm): vertical distance from the medial buccal
crown top of the upper first molar to the palatal plane.
 Angular measurement (Figure 3(b))
U1-PP (degree): inclination of the maxillary central
incisor (U1 to U1r) relative to the palatal plane.
U6-PP (degree): inclination of the maxillary first mo-
lar (U6 to U6r-f) relative to the palatal plane.
The linear and angular changes after the activation pe-
riod were calculated by comparing the measurements
before and after distraction.
2.2.2. 3-D Model Measurement
The orthodontic models were digitized by using a 3-D
scanner (Maestro, Age, Italy). Then the data were trans-
ferred from the 3-D scanner to the Maestro 3-D Ortho
Studio software, in order to perform the measurements.
All measurements were performed on 3-D scanned cast
models before and after maxillary distraction osteogene-
sis by the same observer.
(a)
(b)
Figure 3. Dental change measurements after maxillary distrac-
tion osteogenesis. (a) Linear changes, (b) Angular changes.
1) Transverse plane measurement
Reference points (Figure 4)
 U3: the cusp tips of the right/left maxillary canines.
 U4: the midpoints of the central groove of the right/
left maxillary first premolars.
 U5: the midpoints of the central groove of the right/
left maxillary second premolars.
 U6: the midpoints of the transverse fissure on maxil-
lary first molars.
 The arch length was determined by measuring the
length of a perpendicular line constructed from the
mesial contact point between the central incisors to
the line connecting the reference point on the right
and left first molars.
 The arch width was determined by measuring the
length of the line connecting the midpoints of trans-
verse fissure on the right and left first molars.
Measurement list:
 U3-U3: the distance between the right and left labial
cusp tips of maxillary canines.
 U4-U4: the distance between the midpoints of trans-
verse fissure on the right and left first premolars.
 U5-U5: the distance between the midpoints of trans-
verse fissure on the right and left second premolars.
 U6-U6: the distance between the midpoints of trans-
verse fissure on the right and left first molars.

L. L. Yang et al. / Open Journal of Stomatology 3 (2013) 425-432 428
Figure 4. Transverse plane.
 Arch length: the length of perpendicular line con-
structed from mesial contact point between central
incisors to the line connecting the reference points on
the right and left first molars.
 Arch width: the length of the line connecting the
midpoints of transverse fissure on the right and left
first molars.
2) Sagittal plane measurement
Reference points (Figure 5)
 U3c/U4c/U5c: the buccal cusp tips of canine, first
and second premolar.
 U6c: the midpoints between two buccal cusp tips of
first molars.
 A line was drawn connecting the contact points of the
canine, premolars and first molar. The midpoints of
each tooth along this line were identified, and con-
nected to the points U3c, U4c, U5c and U6c by lines
which were extended at each end.
Measurement list:
The angles between the LU3/LU4/LU5/LU6 and the
Y-axis.
3) Coronal plane measurement
Reference points (Figure 6)
 U3: the cusp tips of the right/left maxillary canines.
 U4 the midpoints of the central groove of the right/
left maxillary first premolars.
 U5 the midpoints of the central groove of the right/
left maxillary second premolars.
 U6: the midpoints of the transverse fissure on maxil-
lary first molars.
 By connecting the buccal and palatal junction points
of canine, premolar and first molar to get the mid-
point, and draw the lines (LU3/LU4/LU5/LU6) through
the midpoints and the U3/U4/U5/U6.
Figure 5. Sagittal plane.
Figure 6. Coronal plane.
 The palatal heights were determined by measuring
the height of a perpendicular line constructed from
the surface of palate to the lines connecting the ref-
erence points U3/U4/U5/U6.
Measurement list:
 The angles between the LU3/LU4/LU5/LU6 and the
Y- ax is.
 The palatal height: the height of a perpendicular line
constructed from the surface of palate to the lines
connecting the reference points U3/U4/U5/U6.
2.3. Error Analysis
Dahlberg’s formula [18] was used to determine the meas-
urement error. Each radiograph was retraced, superim-
posed, and re-digitized for the error determination. The
errors of 3-D model measurement were calculated based
on the measurements of sixteen casts of all eight patients.
Each cast was scanned and digitized twice with a one-
week interval by the same observer. The reliability of the
measurements was evaluated by paired t test with a 5%
level of significance (SPSS Inc., Chicago, IL).
2.4. Statistical Analysis
The Mann-Whitney U test was used to compare the dif-
ferences in the amounts of advancement and dentoalveo-
lar changes between the DF and NDF groups. The sig-
nificance level was established at 0.05.
3. RESULTS
3.1. Cephalometric Analysis
Cephalometric analysis demonstrated a significantly
greater change in the value of SNA in the DF group than
in the NDF group. Opposite vectors of displacement

L. L. Yang et al. / Open Journal of Stomatology 3 (2013) 425-432 429
were observed in the inclination of the palatal plane be-
tween the DF and NDF groups. The DF group experi-
enced a clockwise rotation of the palatal plane (2.2˚ ±
2.3˚), while the NDF group experienced a counterclock-
wise rotation (−8.3˚ ± 8.5˚). No significant differences in
the mandibular plane were observed between groups.
Significant differences in the amounts and patterns of
dental changes throughout the distraction period between
DF and NDF were observed. In the DF group, U1 (−5.3˚
± 6.2˚) and U6 (−6.3˚ ± 4.1˚) were palatally inclined with
a minimal amount of dental extrusion of U1 (0.9 mm ±
1.3) and U6 (−0.3 mm ± 2.1). In contrast, in the NDF
group, U1 (12.6˚ ± 16.1˚) was buccally inclined, whereas
U6 (5.8˚ ± 2.4˚) was mesially inclined. A large amount
of dental extrusion was observed in U1 (3.5 mm ± 3.1)
and U6 (1.3 mm ± 1.1). (Table 1)
3.2. 3-D Model Analysis (Table 2)
In the transverse plane, significantly greater arch lengths
(8.7 mm ± 5.2) and arch width changes (6.0 mm ± 1.0)
were observed in the NDF group than in the DF group,
(arch lengths 3.0 mm ± 1.1 and arch width changes 3.2
mm ± 2.0). A large amount of extrusion was observed in
U5 and U6 in NDF group. The widths between U3/U4/
U5 on right and left side teeth were significantly wider in
the NDF group than in the DF group.
In the sagittal plane, U3 (−6.4˚ ± 4.3˚), U4 (−5.5˚ ±
3.9˚), U5 (−6.4˚ ± 3.7˚) and U6 (−12.1˚ ± 6.1˚) were
palatally inclined in the DF group. In contrast, in the
NDF group, U3 (8.3˚ ± 3.5˚), U4 (6.8˚ ± 5.2˚), U5 (9.0˚ ±
7.1˚) and U6 (18.4˚ ± 6.6˚) were buccally inclined.
In the coronal plane, there was no significant differ-
ence in bucco-lingual tipping of U3/U4/U5/U6. In the
NDF group, the arch heights measurements at the posi-
tion of U5 (2.6 mm ± 3.6) and U6 (3.6 mm ± 7.2) indi-
cated that these teeth were extruded; in contrast, in the
Table 1. Dental changes after maxillary distraction following
two surgical procedures (DF vs NDF).
DF NDF
Variable Mean SD Mean SD Sig
SNA (˚) 7.5 2.7 4.4 1.3 0.039*
SNB (˚) −1.3 1.8 −1.7 3.4 0.474NS
ANB (˚) 8.8 3.2 4.6 1.1 0.029*
Palatal Plane (˚) 2.2 3.5 −8.3 8.5 0.015*
Mand-plane (˚) 1.2 3.2 3.3 4.3 0.297NS
U1-PP (˚) −5.3 6.2 12.6 16.1 0.019*
U1-PP (mm) 0.9 1.3 3.5 3.1 0.064**
U6-PP (˚) −6.3 4.1 5.8 2.4 0.003**
U6-PP (mm) −0.3 2.1 1.3 1.1 0.179**
*P < 0.05; **P < 0.01; NS = No significant difference.
Table 2. 3-D Dental Model Analysis. Comparison of dental
changes between DF and NDF groups.
DF NDF
Variable Mean SD Mean SD Sig
Arch Length3.01 1.13 8.73 3.42***
Arch Width3.18 1.98 5.97 1.00***
U3 2.04 3.58 4.22 2.51*
U4 2.15 2.43 4.15 3.55*
U5 2.10 2.51 5.12 3.43*
Transverse
(mm)
U6 3.71 1.84 5.86 4.51
U3 −6.35 4.30 8.63 3.52***
U4 −5.50 3.93 6.82 5.25***
U5 −6.41 3.73 8.95 7.15***
Sagittal (˚)
U6 −12.14 6.10 18.39 6.58***
U3 6.15 2.98 9.60 4.66
U4 8.43 4.01 11.75 4.22
U5 10.87 5.40 13.88 5.03
Coronal (˚)
U6 15.68 6.40 19.17 5.94
U3 1.52 4.81 3.20 3.19
U4 1.93 3.14 2.70 2.13
U5 −2.16 4.38 2.58 3.64*
Palatal
Height
(mm)
U6 −2.77 4.42 3.60 7.20**
*P < 0.05; **P < 0.01; ***p < 0.001.
DF group, U5 (−2.2 mm ± 4.4) and U6 (−2.8 mm ± 4.4)
were intruded.
4. DISCUSSION
Cephalometric measurement has been widely used for
measuring tooth movements after orthodontic treatment
[19]. However, there are some shortcomings of conven-
tional cephalometric measurement, included the difficul-
ties in evaluating three-dimensional dental movement
and identifying inherent landmarks. Further disadvan-
tages are tracing errors, frequent radiation exposure, and
high cost [20]. The 3-D model analysis can offer more
information, not only in the sagittal plane, but also in the
transverse and coronal planes, which are impossible to
evaluate by lateral cephalometric analysis.
Different intraoperative surgical protocols involving
the use of DF and NDF procedures have been applied to
perform maxillary distraction osteogenesis [9,10,12,13].
The main advantage of the NDF over the DF procedure
is the reduction of risks and complications, thus reducing
the duration of surgery [12,13]. However, little is known
about the biomechanical changes and stability promoted
by the application of such different surgical protocols. In
this study, comparisons of dental changes between these
two different intraoperative surgical procedures have
been performed.

L. L. Yang et al. / Open Journal of Stomatology 3 (2013) 425-432 430
Analysis of dento-skeletal changes demonstrated sig-
nificant differences in the amount and direction of the
rotation of the osteotomized maxillary bone. In the DF
group, a clockwise rotation pattern was observed, where-
as a counter-clock wise rotation of the maxillary bone
was clearly observed in the NDF group. Although it was
not possible to confirm the amount or type of bone at-
tachment through radiographic examination, the main
explanation for such differences might be attributed to
the differences in the bone attachment at the posterior
maxilla. In the DF group, the maxillary bone was com-
pletely mobilized, consequently allowing the unrestricted
down-forward movement of the maxilla at the planned
position. In contrast, in the NDF group, the presence of
bone contacts at the posterior maxilla or incomplete os-
teotomies, limited the movement of the maxilla. And as a
consequence, when the distraction force was applied, the
partially ostotomized maxilla did not moved to the
planned down-forward position; instead it moved up-
forward [21]. Such undesirable and unplanned move-
ments would lead to unsatisfactory results.
The presence of incomplete osteotomies has also been
reported by several authors [3,14,22]. Dolanmaz et al.
[23] also have observed different types of unpredictable
fractures after the DF procedures in a group of cadavers.
In their study, the incomplete osteotomies were evaluated
using CT to identify the areas with incomplete fractures.
Cephalometric analysis and the 3-D model analysis in
the sagittal plane demonstrated significant differences in
the amount and direction of dental movement between
the DF and NDF groups.
In the DF group, palatal inclination of U1 and distal
tipping of U6 were observed. In contrast, in the NDF
group, buccal inclination of U1 and mesial tipping of U6
were observed. Such contrasting dental movements can
be explained by the different amounts of maxillary bone
resistance to the movement during the maxillary ad-
vancement between DF and NDF groups. As a result,
palatal tipping of U1 combined with distal tipping of U6
was observed. In the NDF group, since a relatively great
amount of force was necessary to advance the maxillary
bone, both U1 and U6 were moved mesially, indicating a
large amount of dental movement (Figure 7). It is im-
portant to note that although a large amount of force was
used in the NDF group, the maxillary bone did not move
forward to the planned position.
The results of this study are in accordance with those
of Block et al. [24] who investigated the amount of den-
tal anchorage loss associated with the use of tooth-borne
distractors. Block et al. [24] have demonstrated that
some amount of dental anchorage loss is expected when
tooth-borne devices are used. However, the pattern of
dental changes between the DF and NDF groups ob-
served in this study can be attributed to the different lev-
(a)
(b)
Figure 7. Dental changes resulting from different surgical pro-
cedures. (a) The dental movement in DF group (complete mo-
bilization of the maxilla); (b) The dental movement in NDF
group (Incomplete mobilization of the maxilla).
els of resistance to movement offered by the osteoto-
mized maxillary bone. Such differences in the dental
changes indicate that the type of surgical procedure
might play an important role in the amount and direction
of the dental changes. This can be critically important
considering the use of tooth-borne devices. The distribu-
tion of force coming from the distraction pull may affect
not only the skeletal structures but the maxillary teeth as
well. Because the maxillary teeth serve as anchor units
for the distraction device, the dental changes are very
likely that dental changes may occur in addition to the
skeletal changes [25]. The use of bone-borne distractors,
or the use of distractors connected to miniscrew implants
can reduce or avoid the undesirable dental effects during
distraction osteogenesis [26,27].
There is no report in the literature to date of any study
on 3-D model analysis after maxillary distraction. In this
study, significantly greater arch length and width
changes were observed in the NDF group than in the DF
group. The higher levels of distraction force applied to
the first molar in the NDF group, may have led to more

L. L. Yang et al. / Open Journal of Stomatology 3 (2013) 425-432 431
dental movement than in the DF group. Greater buccal
inclination of the maxillary first molar produced a
greater arch width increase, and greater anterior move-
ment, and labial inclination of the maxillary incisors
produced a greater arch length increase. The mostly force
applied to the maxillary first molar might cause this ex-
pansion effect because the distraction force was deliv-
ered by a tooth-borne device that was attached to the
band on the first molar. The inclination effect on the first
molar was greater than on the incisor, canine and premo-
lar due to the attended mode.
In the NDF group, the second premolar and first molar
extrusion might be due to the counter rotation of the
tooth-borne device. The posterior maxilla remained con-
nected to the skull base following the NDF procedure,
which might have led to the extrusion of the second
premolar and first molar. In contrast, the osteotomized
maxilla was completely mobilized in the DF group. In
this study, the mobilized maxilla was not just the result
of forward movement, but also of downward movement.
This downward movement may have led to clockwise
rotation of the tooth-borne device, which may have pro-
duced the second premolar and first molar intrusion.
5. CONCLUSIONS
The use of the NDF procedure resulted in greater
amounts of dental anchorage loss than resulted from the
DF procedures when tooth-borne devices were used dur-
ing maxillary distraction osteogenesis. The type of sur-
gical procedure might play an important role in the
amount and direction of the dental changes.
Further studies, with increased numbers of subjects,
are necessary to evaluate the effects of different cleft
types on the dentoalveolar changes during maxillary dis-
traction osteogenesis.
6. ACKNOWLEDGMENTS
The authors gratefully acknowledge a grant from Thailand research
Fund (RSA 5480029), and grant from the Faculty of Dentistry, Chiang
Mai University. The authors acknowledge the assistance of Dr. M. K.
Kevin O. Carroll, Professor Emeritus of the University of Mississippi
School of Dentistry, USA, and Faculty Consultant at the Chiang Mai
University Faculty of Dentistry, Thailand, in the preparation of the
manuscript.
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