Dentoalveolar changes following maxillary distraction osteogenesis

doi:10.4236/ojst.2013.38071

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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

Email: *[email protected]

Received 20 August 2012; revised 23 September 2013; accepted 15 October 2013

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.

OPEN ACCESS

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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