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INTRODUCTION
In every orthodontic force system when the
line of force does not pass through the center
of resistance (Cr), a moment is created [2].The magnitude and the direction of that
moment are dependent on the direction of
the applied force, its magnitude, and its
perpendicular distance from the Cr [3]
(Fig. 1).
Many analytic and experimental studies
have been carried out in order to determinethe location of the Cr. According to most of
these studies, the Cr in a single root tooth
i s located at 0.24–0.55 of the root length
measured from the alveolar crest, or slightly
apically to it in teeth with more than a single
root [3–10]. The location of the Cr depends
o n many factors, such as the root shape
and length [9], the alveolar bone height [11],and the direction and the magnitude of the
applied force. The materials and methods
were different in all the above-mentioned
studies, and therefore we found it difficult to
compare their findings. The location of the Cr
in a tooth is not related to the technique used
to straighten it. Nevertheless, the relationships
between the Cr location and the bracket place-ment (buccal or lingual) directly influence the
magnitude and the direction of the moments
created by the applied forces (Fig. 2).
BONDING
As in buccal orthodontics (BO), accurateb r a c ket placement in lingual orthodontics
( LO) is of the utmost importance. The bonding
process in LO is much more complicated, and
is usually done using the indirect technique,
for the following reasons:
1. The lingual aspects of the teeth show great
variability in tooth size and morphology. Inaddition, the cingulum, the marginal ridge
1
Lingual orthodontic appliances represent an excellent alternative to labial appliances for
esthetically conscious patients. The mechanotherapy involved is altered in some respects,because of the difference in the position of the brackets [1]. The purpose of this article is to
describe the biomechanical aspects that one must consider when the traditional labial
appliances are replaced by lingual appliances.
Lingual orthodontics (LO) versus buccal
orthodontics (BO): biomechnical and
clinical aspects
S. Goren, R. Zoizner, S. Geron, R. Romano
Orthodontic Department, Israel Defense Force, Tel-Hashomer, Israel
Figure 1. The magnitude and the direction of the
moment are dependent on the direction of the applied
force, its magnitude, and its perpendicular distance
from the Cr.
anatomy and the steep curvature of the
lingual surface make direct assessment
o f the correct position of the bracke t simpossible [12] (Fig. 3a,b).
2. Since the correct and esthetic alignment
of the buccal surface of the teeth is our
treatment goal, brackets should be placed
at variable distances from the incisal edge,
due to the different angulations on the
lingual aspects [13].
3. A smaller interbracket distance (IBD) inLO, especially in the anterior region, makes
the use of compensatory bends very
d i f ficult [1, 13–15]. Since indirect bonding
i s more accurate, less wire bending is
required (Fig. 3c).
THE WIRE STIFFNESS
When the brackets are placed on the lingual
s u rface instead of on the labial surf a c e s ,
t h e IBD in the anterior region of the arch is
decreased signific a n t l y. The overall ratio of
anterior lingual distance to labial interbracket
distance was calculated to be 1:1.47. Mandi-
bular ratios are significantly smaller thanm a x i l l a ry ratios [1]. The load/deflection ratio
2 Journal of Lingual Orthodontics, Volume 3 Number 1
is determined by the modulus of elasticity of
the wire, the wire cross-sectional area, and
the wire length. The smaller IBD makes the
same wire stiffer in LO than in BO. According
to Moran [1], the decreased IBD associatedwith LO makes a wire approximately three
times stiffer for first- and second-order bends
than when used with BO, and approximately
1.5 times stiffer for third-order bends.
MOVEMENTS IN THE VERTICAL
DIRECTION: INTRUSION–EXTRUSION
OF A SINGLE ROOT TOOTH
When the intrusive–extrusive line of force
passes buccally or lingually to the Cr, a
moment is created. This moment direction
can be clockwise (CW) or counter- c l o c k w i s e
(CCW), depending on the direction of the
force vector relative to the Cr. The tooth’scrown can therefore move lingually or buc-
cally. The magnitude of the moment in LO is
much smaller than that in BO, due to the
smaller distance between the lingual bracket
and the Cr (Fig. 4). One must estimate, for
each treated case, if the line of force is
passing labially, lingually or right through the
Figure 2. (a) The sagittal distance between the buccal bracket (D1) or the lingual bracket (D2) and the Cr determine
the magnitude of the moments created when force is applied. (b) The vertical distance between the buccal bracket
(D3) or the lingual bracket (D4) and the Cr determine the magnitude of the moments created when force is applied.
(a) (b)
D2
D1
D15Distance of a buccalbracket from the Cr in thesagittal plane
D25Distance of a lingualbracket from the Cr in thesagittal plane
D3=Distance of a buccal bracketfrom the Crin the vertical plane
D4=Distance of a lingual bracketfrom the Crin the vertical plane
D4 D3
Cr
Cr
S. Goren et al Lingual orthodontics (LO) versus buccal orthodontics (BO) 3
C r, and evaluate whether the moments that
will be created by the intrusion–extrusion
mechanics are desired or should be compen-
sated by changing the bracket torque, angu-
lation and inclination at the laboratory stage,
or by clinical modifications. According to
Geron et al [16], applying an intrusive forcei n BO on a tooth that is initially positioned
between retroclination of 20° and proclination
of 45° will create a lingual root movement
(proclination). In LO, labial root moment
(retroclination) will occur when the tooth is
retroclined more than 220°. The composite
pad on the lingual bracket might change thedistance between the point of force applica-
tion and the Cr and can therefore minimize
the moment magnitude and even reverse the
moment direction (Fig. 5).
MOVEMENTS IN THE SAGITTAL
DIRECTION
In both systems, LO and BO, the direction
o f force applied passes relatively far from
t h e C r, and therefore a moment is created.
The moment tends to move the crown in the
force direction and the apex in the opposite
direction. In this sense, there is no difference
between LO and BO. The sagittal force alsocreates a moment in the buccal–lingual
Figure 3. (a) The lingual aspects of the teeth show great variability in tooth size and morphology. In addition, the
cingulum, the marginal ridge anatomy and the steep curvature of the lingual surface make direct assessment of
the correct position of the brackets impossible. (b) In LO, brackets should be placed at variable distances to avoid
the necessity for first- and second-order bends. (c) A smaller interbracket distance (IBD) in LO, especially in the
anterior region, makes the use of compensatory bends very difficult.
(a) (b)
(c)
4 Journal of Lingual Orthodontics, Volume 3 Number 1
d i r e c t i o n, which tends to rotate the tooth.
I n B O, the vector of force passes buccal to
t h e C r, and in the LO, it passes lingual to it.
Therefore, the directions of these rotationsare opposite. For example, retracting a pre-
molar in BO will tend to rotate the tooth in
a distal–lingual direction. In LO, the same
retraction will create a mesial–lingual rotation
of the tooth (Fig. 6). The clinical implication
i s that the over-corrections applied to the
teeth in the laboratory or clinical stages are
opposite in LO and BO.
MOVEMENTS IN THE TRANSVERSAL
DIRECTION
In both systems, the vectors of force are
passing similarly relative to the Cr, and hence
produce similar moments: the moment tends
to create a movement of the crown in theforce direction and a root movement in the
opposite direction. Clinically, it seems that
expansion is easier in LO than in BO, due to
the elimination of posterior occlusion caused
by the anterior bite plane and the equilibrium
change in LO between the tongue and the
lips.
FRICTION
Many studies have been conducted to evaluate
the principal factors that may influence fric-
tional resistance. The most important factorsare bracket and wire materials [17–19], type
and force of ligation [20–22], the relative
bracket–wire clearance [23], and the archwire
size as related to stiffness [24]. The wire and
b r a c ket materials in LO are the same as in
BO.
The type of ligation is different between
LO and BO. In LO, the double overtie iscommonly used with metal or elastic ligatures
in order to hold the wire in the bracket slot.
There is a lack of information concerning the
normal force created when using this ligation
technique.
As mentioned before, the smaller IBD
makes the same wire stiffer in LO than in BO
[1]. According to some researchers [25,26],stiffer wires reduce the binding and thus
reduce the resistance to friction. On the other
hand, Creekmoore [27] and Articolo and Kusy
[28] suggest that stiffer wires will increase
resistance to sliding in tipped brackets. One
must remember, however, that most of the
Figure 4. The magnitude of the moment in LO is much
smaller than that in BO, due to the smaller distance
between the lingual bracket and the Cr (DL) compared
to the distance between the buccal bracket and the Cr
(DB).
Figure 5. The composite pad on the lingual bracket
might change the distance between the point of force
application and the Cr, and therefore minimize the
moment magnitude and even reverse the moment
direction.
DL
DB
Intrusionforce
MomentDirection
Force Direction
MomentDirection
CompositePad
Force Direction-No Moment
sliding procedure is done clinically at the
posterior regions, where the difference in IBD
between LO and BO is less .
CLINICAL IMPLICATIONS
CL II/1 malocclusion
The classical manifestations of CL II/1 mal-
occlusion are class II molar relationships,
increased overjet, upper incisors normally in-
clined or proclined, and a constricted maxilla
in relation to the mandible [29].
When CL II/1 malocclusion is treated with
LO, attention must be paid to the contacts
between the lower incisors and the bite-planes of the upper incisor brackets. When
the upper incisors are proclined, the vector of
force could pass labial to the Cr, which would
create a moment that will worsen the pro-
c l i n a t i o n. The anterior-bite plane, created
by the LO brackets, can make the expansion
o f the upper dentition easier, due to the
p o s t e r i o r disarticulation. The vertical openingand the CCW rotation of the mandible (down
a n d back) caused by LO induces a Class II
tendency [28].
CL II/2 malocclusion
The classical manifestations of CL II/2 mal-
occlusion are Class II molar relationships,upper incisor retroclination, and an anterior
deep bite [29].
The upper anterior bite-planes in LO create
intrusive forces on the upper and lower
incisors and simultaneously encourage erup-
tion of the posterior teeth. Because the upper
incisors are retroclined, the vector of theanterior intrusive force can pass palatinally to
the Cr and hence worsen the retroclination.
SUMMARY
LO has some advantages and disadvantagesrelative to BO. Among the advantages is the
improved esthetic appearance, and among
the disadvantages is the difficulty in direct
viewing and access. However, most of the
unique features of LO can be considered either
as advantages or disadvantages, depending
on the malocclusion and according to the
specific goals of treatment. Teeth move underthe force we apply according to biomechanical
principles and the biological environment.
B r a c ket position does not change the need
to understand basic biomechanical rules. As
clinicians, we must analyze in each case the
forces we apply and try to predict the tooth
movement.
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S. Goren et al Lingual orthodontics (LO) versus buccal orthodontics (BO) 5
Figure 6. (a) Retracting a premolar in BO will tend to rotate the tooth in a distal–lingual direction. (b) In LO, the
same retraction will create a mesial–lingual rotation of the tooth.
(a) (b)
6 Journal of Lingual Orthodontics, Volume 3 Number 1
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Reprint requests: Dr Rafi Romano, Orthodontic Depart-
ment, Israel Defense Force, 34 Habarzel, Tel-Aviv 69710,
Israel.
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