Segmented arch or continuous arch technique? A rational approach
Identifieur interne : 002C10 ( Pmc/Corpus ); précédent : 002C09; suivant : 002C11Segmented arch or continuous arch technique? A rational approach
Auteurs : Sergei Godeiro Fernandes Rabelo Caldas ; Alexandre Antonio Ribeiro ; Hallissa Simplício ; André Wilson MachadoSource :
- Dental Press Journal of Orthodontics [ 2176-9451 ] ; 2014.
Abstract
This study aims at revising the biomechanical principles of the segmented archwire technique as well as describing the clinical conditions in which the rational use of scientific biomechanics is essential to optimize orthodontic treatment and reduce the side effects produced by the straight wire technique.
Url:
DOI: 10.1590/2176-9451.19.2.126-141.sar
PubMed: 24945523
PubMed Central: 4296600
Links to Exploration step
PMC:4296600Le document en format XML
<record><TEI><teiHeader><fileDesc><titleStmt><title xml:lang="en">Segmented arch or continuous arch technique? A rational
approach</title>
<author><name sortKey="Caldas, Sergei Godeiro Fernandes Rabelo" sort="Caldas, Sergei Godeiro Fernandes Rabelo" uniqKey="Caldas S" first="Sergei Godeiro Fernandes Rabelo" last="Caldas">Sergei Godeiro Fernandes Rabelo Caldas</name>
<affiliation><nlm:aff id="aff01"> Adjunct professor, Department of Pediatric Dentistry, Federal University of Rio Grande do Norte (UFRN), professor at the Specialization course of Orthodontics, UnP.</nlm:aff>
</affiliation>
</author>
<author><name sortKey="Ribeiro, Alexandre Antonio" sort="Ribeiro, Alexandre Antonio" uniqKey="Ribeiro A" first="Alexandre Antonio" last="Ribeiro">Alexandre Antonio Ribeiro</name>
<affiliation><nlm:aff id="aff02"> PhD resident in Orthodontics, State University of São Paulo (UNESP)/ Araraquara. Professor.</nlm:aff>
</affiliation>
</author>
<author><name sortKey="Simplicio, Hallissa" sort="Simplicio, Hallissa" uniqKey="Simplicio H" first="Hallissa" last="Simplício">Hallissa Simplício</name>
<affiliation><nlm:aff id="aff03"> Adjunct professor, Department of Pediatric Dentistry and Orthodontics, UFRN.</nlm:aff>
</affiliation>
</author>
<author><name sortKey="Machado, Andre Wilson" sort="Machado, Andre Wilson" uniqKey="Machado A" first="André Wilson" last="Machado">André Wilson Machado</name>
<affiliation><nlm:aff id="aff04"> Adjunct professor, Department of Orthodontics, Federal University of Bahia (UFBA). Visiting professor, Master's program in Orthodontics, UCLA.</nlm:aff>
</affiliation>
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<sourceDesc><biblStruct><analytic><title xml:lang="en" level="a" type="main">Segmented arch or continuous arch technique? A rational
approach</title>
<author><name sortKey="Caldas, Sergei Godeiro Fernandes Rabelo" sort="Caldas, Sergei Godeiro Fernandes Rabelo" uniqKey="Caldas S" first="Sergei Godeiro Fernandes Rabelo" last="Caldas">Sergei Godeiro Fernandes Rabelo Caldas</name>
<affiliation><nlm:aff id="aff01"> Adjunct professor, Department of Pediatric Dentistry, Federal University of Rio Grande do Norte (UFRN), professor at the Specialization course of Orthodontics, UnP.</nlm:aff>
</affiliation>
</author>
<author><name sortKey="Ribeiro, Alexandre Antonio" sort="Ribeiro, Alexandre Antonio" uniqKey="Ribeiro A" first="Alexandre Antonio" last="Ribeiro">Alexandre Antonio Ribeiro</name>
<affiliation><nlm:aff id="aff02"> PhD resident in Orthodontics, State University of São Paulo (UNESP)/ Araraquara. Professor.</nlm:aff>
</affiliation>
</author>
<author><name sortKey="Simplicio, Hallissa" sort="Simplicio, Hallissa" uniqKey="Simplicio H" first="Hallissa" last="Simplício">Hallissa Simplício</name>
<affiliation><nlm:aff id="aff03"> Adjunct professor, Department of Pediatric Dentistry and Orthodontics, UFRN.</nlm:aff>
</affiliation>
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<author><name sortKey="Machado, Andre Wilson" sort="Machado, Andre Wilson" uniqKey="Machado A" first="André Wilson" last="Machado">André Wilson Machado</name>
<affiliation><nlm:aff id="aff04"> Adjunct professor, Department of Orthodontics, Federal University of Bahia (UFBA). Visiting professor, Master's program in Orthodontics, UCLA.</nlm:aff>
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<front><div type="abstract" xml:lang="en"><p>This study aims at revising the biomechanical principles of the segmented archwire
technique as well as describing the clinical conditions in which the rational use of
scientific biomechanics is essential to optimize orthodontic treatment and reduce the
side effects produced by the straight wire technique.</p>
</div>
</front>
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<pmc article-type="research-article"><pmc-dir>properties open_access</pmc-dir>
<front><journal-meta><journal-id journal-id-type="nlm-ta">Dental Press J Orthod</journal-id>
<journal-id journal-id-type="iso-abbrev">Dental Press J Orthod</journal-id>
<journal-title-group><journal-title>Dental Press Journal of Orthodontics</journal-title>
</journal-title-group>
<issn pub-type="ppub">2176-9451</issn>
<issn pub-type="epub">2177-6709</issn>
<publisher><publisher-name>Dental Press International</publisher-name>
</publisher>
</journal-meta>
<article-meta><article-id pub-id-type="pmid">24945523</article-id>
<article-id pub-id-type="pmc">4296600</article-id>
<article-id pub-id-type="doi">10.1590/2176-9451.19.2.126-141.sar</article-id>
<article-categories><subj-group subj-group-type="heading"><subject>Special Article</subject>
</subj-group>
</article-categories>
<title-group><article-title>Segmented arch or continuous arch technique? A rational
approach</article-title>
</title-group>
<contrib-group><contrib contrib-type="author"><name><surname>Caldas</surname>
<given-names>Sergei Godeiro Fernandes Rabelo</given-names>
</name>
<xref ref-type="aff" rid="aff01">1</xref>
<xref ref-type="corresp" rid="c01"></xref>
</contrib>
<contrib contrib-type="author"><name><surname>Ribeiro</surname>
<given-names>Alexandre Antonio</given-names>
</name>
<xref ref-type="aff" rid="aff02">2</xref>
</contrib>
<contrib contrib-type="author"><name><surname>Simplício</surname>
<given-names>Hallissa</given-names>
</name>
<xref ref-type="aff" rid="aff03">3</xref>
</contrib>
<contrib contrib-type="author"><name><surname>Machado</surname>
<given-names>André Wilson</given-names>
</name>
<xref ref-type="aff" rid="aff04">4</xref>
</contrib>
</contrib-group>
<aff id="aff01"><label>1</label>
Adjunct professor, Department of Pediatric Dentistry, Federal University of Rio Grande do Norte (UFRN), professor at the Specialization course of Orthodontics, UnP.</aff>
<aff id="aff02"><label>2</label>
PhD resident in Orthodontics, State University of São Paulo (UNESP)/ Araraquara. Professor.</aff>
<aff id="aff03"><label>3</label>
Adjunct professor, Department of Pediatric Dentistry and Orthodontics, UFRN.</aff>
<aff id="aff04"><label>4</label>
Adjunct professor, Department of Orthodontics, Federal University of Bahia (UFBA). Visiting professor, Master's program in Orthodontics, UCLA.</aff>
<author-notes><corresp id="c01"><bold>Contact address:</bold>
Sergei Godeiro Fernandes Rabelo Caldas, Disciplina de
Clínica Infantil, Departamento de Odontologia, UFRN, Avenida Senador Salgado Filho,
1787 - Lagoa Nova - Natal/RN - Brazil. CEP: 59056-000 - E-mail:
<email>sergeirabelo@uol.com.br</email>
</corresp>
</author-notes>
<pub-date pub-type="epub-ppub"><season>Mar-Apr</season>
<year>2014</year>
</pub-date>
<pmc-comment>Fake ppub date generated by PMC from publisher
pub-date/@pub-type='epub-ppub' </pmc-comment>
<pub-date pub-type="ppub"><season>Mar-Apr</season>
<year>2014</year>
</pub-date>
<volume>19</volume>
<issue>2</issue>
<fpage>126</fpage>
<lpage>141</lpage>
<history><date date-type="received"><day>20</day>
<month>1</month>
<year>2014</year>
</date>
<date date-type="accepted"><day>24</day>
<month>1</month>
<year>2014</year>
</date>
</history>
<permissions><copyright-statement>© 2014 Dental Press Journal of Orthodontics</copyright-statement>
<license license-type="open-access" xlink:href="http://creativecommons.org/licenses/by/4.0/"><license-p>This is an Open Access article distributed under the terms of the Creative
Commons Attribution Non-Commercial License, which permits unrestricted
non-commercial use, distribution, and reproduction in any medium, provided the
original work is properly cited.</license-p>
</license>
</permissions>
<abstract><p>This study aims at revising the biomechanical principles of the segmented archwire
technique as well as describing the clinical conditions in which the rational use of
scientific biomechanics is essential to optimize orthodontic treatment and reduce the
side effects produced by the straight wire technique.</p>
</abstract>
<trans-abstract xml:lang="pt"><p>O objetivo desse trabalho é revisar os princípios biomecânicos da técnica do arco
segmentado, bem como descrever situações clínicas onde o uso racional da biomecânica
científica é fundamental na otimização do tratamento ortodôntico e eliminação dos
efeitos colaterais da abordagem com arco contínuo.</p>
</trans-abstract>
<kwd-group><kwd>Orthodontics</kwd>
<kwd>Corrective Orthodontics</kwd>
<kwd>Biomechanics</kwd>
</kwd-group>
</article-meta>
</front>
<body><sec sec-type="intro"><title>INTRODUCTION</title>
<p>Specialists in Orthodontics, particularly beginners, often have many questions in mind:
Which technique should be used? What is the best bracket prescription? Which is the most
recommendable type of bracket: conventional or self-ligating? When are the straight
archwire and segmented mechanics techniques best recommended?</p>
<p>Making an accurate diagnosis of the changes observed in patients is necessary. Likewise,
an effective treatment plan performed within a shorter period of time, with as little
injuries to protective and supporting tissues as possible, is also essential. Thus,
regardless of the type of bracket, prescription, technique or slot, recommending the
most appropriate treatment is what really matters. For this reason, knowing the
principles on which each technique is based on, as well as its limitations, is
essential. For instance, treating a clinical case of a maxillary canine in
infralabioversion by means of the straight archwire technique used to level the tooth is
a harmful procedure for adjacent teeth. Canine extrusion would occur regardless of the
type of bracket, whether conventional or self-ligating, however, it would be followed by
undesired intrusion and failure of the lateral incisor and first premolar (<xref ref-type="fig" rid="f01">Fig 1</xref>
). Many authors claim that such undesired effect
would not result in any further issues, given that it would be solved with
intermaxillary rubber bands, arch bends or wire progression. However, it results in a
side effect of which solution demands extra treatment time, results in increased
biological damage and potential root resorption. Thus, in this case, it is incongruous
to use straight wires to level a canine, regardless of the type of bracket.</p>
<fig id="f01" orientation="portrait" position="float"><label>Figure 1</label>
<caption><p><bold>A</bold>
) Straight wire mechanics used for canine extrusion. <bold>B</bold>
)
Note the side effects on the lateral incisor and first premolar, which made the
conditions of the case worse.</p>
</caption>
<graphic xlink:href="dpjo-19-02-0126-g01"></graphic>
</fig>
<p>Conversely, a complementary technique can be used to employ such procedure without
producing further side effects on adjacent teeth. With the aid of the segmented arch
technique (SAT) and after healing of the anchorage unit, only the canine is extracted by
a cantilever or a rectangular loop (<xref ref-type="fig" rid="f02">Fig 2</xref>
).
Differently from the conventional techniques, which normally use an arch made of one
single alloy, connecting all brackets and adjacent tubes; the SAT uses arch segments
connected to each other, but not necessarily connected to brackets and adjacent tubes.
This allows a combination of wires made of different alloys, dimensions and hardness to
be used. Rigid and thick archwires can connect groups of teeth into anchorage units,
whereas flexible archwires are used to exert forces between these units.<sup><xref rid="r01" ref-type="bibr">1</xref>
</sup>
</p>
<fig id="f02" orientation="portrait" position="float"><label>Figure 2</label>
<caption><p>Cantilever (<bold>A</bold>
) and rectangular loop (<bold>B</bold>
) for canine
extrusion.</p>
</caption>
<graphic xlink:href="dpjo-19-02-0126-g02"></graphic>
</fig>
<p>After employing the segmented mechanics, treatment can be conventionally performed with
straight wires. Thus, this article aims at discussing how SAT can aid orthodontists to
solve specific cases by means of a rational approach.</p>
</sec>
<sec><title>PRINCIPLES BEHIND THE SEGMENTED ARCH TECHNIQUE</title>
<p>SAT was developed by Dr. Charles Burstone in 1962. It consists of a sequence of
orthodontic procedures based on the mechanical principles of Mechanics. Thus, in order
to fully understand the principles behind it, it is essential to comprehend the
following concepts: horizontal force, torque, moment of force, moment of torque,
equivalent system of forces, moment-to-force ratio and load/deflection ratio.<sup><xref rid="r02" ref-type="bibr">2</xref>
</sup>
</p>
<p>Horizontal force is a vector able to overcome inertia, thus changing the position of the
moving body. Torque refers to two noncollinear, coplanar forces of same intensity, but
opposite direction. A body subjected to torque tends to rotate about an axis, in which
the center of resistance (CRes) coincides with the center of rotation (CRot), causing a
tooth to go through pure rotation.<sup><xref rid="r01" ref-type="bibr">1</xref>
,<xref rid="r02" ref-type="bibr">2</xref>
,<xref rid="r03" ref-type="bibr">3</xref>
</sup>
</p>
<p>Nevertheless, whenever a horizontal force is applied to a tooth and this force bypasses
the CRes, the latter produces a rotational movement known as "moment of force". In this
case, the crown moves in one direction, while the root moves in the opposite direction,
with the CRot more apical towards the center of resistance. This movement is known as
uncontrolled tipping (<xref ref-type="fig" rid="f03">Fig 3</xref>
).<sup><xref rid="r01" ref-type="bibr">1</xref>
,<xref rid="r02" ref-type="bibr">2</xref>
,<xref rid="r03" ref-type="bibr">3</xref>
</sup>
</p>
<fig id="f03" orientation="portrait" position="float"><label>Figure 3</label>
<caption><p>Uncontrolled tipping with force bypassing the CRes. Note that the CRot is apically
displaced. (Source: adapted from Almeida<sup><xref rid="r31" ref-type="bibr">31</xref>
</sup>
).</p>
</caption>
<graphic xlink:href="dpjo-19-02-0126-g03"></graphic>
</fig>
<p>On the other hand, should the force go exactly through the CRes, this tooth will move by
translation (<xref ref-type="fig" rid="f04">Fig 4</xref>
), in which case the CRot is in
the infinite. However, it is worth mentioning that it is extremely difficult to have
force going exactly through the CRes. Therefore, in order to achieve a translatory
movement, it is important to establish an equivalent system of forces that is applicable
to a bracket. In other words, a different system of forces that produces the same effect
on the CRes of a tooth, even if applied to the crown.<sup><xref rid="r01" ref-type="bibr">1</xref>
,<xref rid="r02" ref-type="bibr">2</xref>
,<xref rid="r03" ref-type="bibr">3</xref>
</sup>
</p>
<fig id="f04" orientation="portrait" position="float"><label>Figure 4</label>
<caption><p>Translatory movement with force going exactly through the CRes. Note that the CRot
is in the infinite. (Source: adapted from Almeida<sup><xref rid="r31" ref-type="bibr">31</xref>
</sup>
).</p>
</caption>
<graphic xlink:href="dpjo-19-02-0126-g04"></graphic>
</fig>
<p>Didactically speaking, to increase our understanding on the subject, it has been
established that the distance from a canine bracket to its CRes (approximately ⅓ of the
distance from the alveolar crest to the apex, more cervically) is of 10 mm (in normal
clinical conditions). In these cases, if a horizontal force of 100 gf is applied to the
bracket, the moment of force (M = F x d) equals 1000 gf.mm. Therefore, the tooth moves
by uncontrolled tipping. However, biomechanically speaking, by means of an equivalent
system of forces, it is possible to move a tooth by translation, even if force is
applied to the brackets (<xref ref-type="fig" rid="f05">Fig 5</xref>
).<sup><xref rid="r01" ref-type="bibr">1</xref>
,<xref rid="r02" ref-type="bibr">2</xref>
,<xref rid="r03" ref-type="bibr">3</xref>
</sup>
</p>
<fig id="f05" orientation="portrait" position="float"><label>Figure 5</label>
<caption><p>Example of an equivalent system. Canine subjected to a 100-g force (blue arrow) 10
mm away from its CRes, with a 1000-g.mm torque moving by translation. In red, the
moment of force. In yellow, the moment of torque; and in green, the moment
resulting from the CRes (Source: adapted from Almeida<sup><xref rid="r31" ref-type="bibr">31</xref>
</sup>
).</p>
</caption>
<graphic xlink:href="dpjo-19-02-0126-g05"></graphic>
</fig>
<p>In order to make such a movement, it is necessary to expel the moment of force, letting
the horizontal force act. To this end, it is necessary to make a gable bend in order to
create a torque in the bracket slot, producing a moment of -1000 gf.mm (in the opposite
direction of the moment of force). That moment is known as moment of torque. Thus, the
moment of force and the moment of torque are neutralized, and the horizontal force will
be the only one acting in the tooth, promoting tooth translation (<xref ref-type="fig" rid="f05">Fig 5</xref>
). The ratio between the moment of torque (-1000 gf.mm) and the
horizontal force (100 gf) is known as moment-to-force ratio (MF), and it determines the
position of the CRot of the tooth, which, in this case, is 10/1 (absolute value). The MF
ratio is the scientific basis to establish differential movements by means of an
equivalent system of forces<sup><xref rid="r01" ref-type="bibr">1</xref>
,<xref rid="r02" ref-type="bibr">2</xref>
,<xref rid="r03" ref-type="bibr">3</xref>
</sup>
(<xref ref-type="fig" rid="f05">Fig 5</xref>
). Retraction of anterior teeth is a
classic clinical example. Whenever orthodontic archwires exert forces onto brackets,
anterior teeth undergo movement of retraction (horizontal force) and uncontrolled
tipping (moment of force). In order to maintain correct axial tipping, it is necessary
to apply a moment so as to oppose such tendency towards tipping (moment of torque) by
means of incorporating vestibular torque of the crown (to brackets and/or archwires).
Ideally, should there be a balance between the moment of torque and the moment of force
(tendency towards uncontrolled tipping), a moment of retraction is established by
translation.</p>
<p>The concept of load/deflection ratio (LF) is also key to understand SAT. Orthodontic
wire tension-deformation charts show that such ratio is characterized by the slope of
the line during the elastic phase of the wire, which corresponds to Hooke's law (<xref ref-type="fig" rid="f06">Fig 6</xref>
). Thus, clinically speaking, it is
characterized by the amount of load (force) that is lost when the appliance is unloaded
(deactivated).</p>
<fig id="f06" orientation="portrait" position="float"><label>Figure 6</label>
<caption><p>LF ratio of a wire is calculated by the slope of the line during its elastic
phase.</p>
</caption>
<graphic xlink:href="dpjo-19-02-0126-g06"></graphic>
</fig>
<p>In other words, the more bended a line is (for instance, a steal wire), the more force
is lost for each millimeter of deactivation. On the other hand, beta-titanium wires
(β-Ti) present a lower slope of the line, which results in a minor loss of force during
deactivation, i.e., a lower LF ratio.</p>
<p>Low LF ratio wires are ideal for segmented mechanics, given that they tend to keep
satisfactory levels of force during system deactivation, proving frequent follow-ups or
continuous adjustments to be unecessary.<sup><xref rid="r04" ref-type="bibr">4</xref>
,<xref rid="r05" ref-type="bibr">5</xref>
</sup>
</p>
<p>Additionally, an increase in the distance between brackets, as a result of segmentation
and the use of auxiliary tubes, increases the points of force application and allows
more predictable mechanics to be carried out. Furthermore, increased distances allow
springs to have their LF ratio decreased, which results in greater activations and a
lower number of adjustments. Such increase in the distance between brackets allows
accessories to be safely positioned and activations to be more safely performed in
comparison to conventional mechanics.<sup><xref rid="r01" ref-type="bibr">1</xref>
</sup>
</p>
<p>The advantage of not connecting adjacent brackets (with greater distance between them)
can be used to selectively dissipate forces and moments. Differently from straight wire
mechanics, in which reactive forces are dissipated around points of force application as
a result of adjacently connecting the brackets, in SAT, the site where the reaction will
be dissipated can be previously selected. Canine extrusion reactive forces and moments
can, for instance, be dissipated in molars instead of incisors and bicuspids.<sup><xref rid="r01" ref-type="bibr">1</xref>
</sup>
</p>
<p>In addition to that, the new metal alloys allowed excellent forces to be produced
without changing the transverse section, in which case the concept of variable module
was introduced to Orthodontics.</p>
<p>The advantages of the variable module include a better control of tooth movement as a
result of opting for rectangular wires since the beginning of treatment, as well as
reduction in the number of archwires used during mechanics and fabrication of appliances
that produce horizontal force of less magnitude and low LF ratio. In this context, β-Ti
wires have become indispensable for producing biological tooth movement through ideal
forces, in which case these wires are chosen for SAT.<sup><xref rid="r02" ref-type="bibr">2</xref>
</sup>
</p>
<p>β-Ti alloy, commercially known as TMA or titanium molybdenum alloy (Ormco Corporation,
Glendora, USA), presents the properties of a "higher quality" wire, for instance: high
springback properties, lower hardness values in comparison to steel, high formability
and weldability with no reduction in resilience, and resistance to corrosion. It has
higher springback properties in comparison to stainless steel and it is able to be
deflected twice as much without permanent deformation. Additionally, it releases forces
that correspond approximately to half of the forces released by steel alloys under
similar activation, which causes its LF ratio to be approximately half of the stainless
steel ratio.<sup><xref rid="r06" ref-type="bibr">6</xref>
</sup>
</p>
<p>However, the majority of clinicians do not remember that, in most types of material,
plastic deformation occurs if tension exceeds its limit of elasticity,<sup><xref rid="r05" ref-type="bibr">5</xref>
</sup>
in other words, that plastic deformation
is also time-dependent.<sup><xref rid="r05" ref-type="bibr">5</xref>
,<xref rid="r07" ref-type="bibr">7</xref>
</sup>
Whenever a given material is constantly
subjected to load or stress within its elastic limit, it may undergo progressive
deformation known as creep.<sup><xref rid="r05" ref-type="bibr">5</xref>
,<xref rid="r07" ref-type="bibr">7</xref>
</sup>
From a microscopic prospect, creep in
metals results from displacements in the crystal structure of matter. Such microscopic
phenomenon can be experimentally considered as an increase in deformation associated
with constant stress (<xref ref-type="fig" rid="f07">Fig 7</xref>
) or an increase in
stress associated with constant deformation (stress relaxation) (<xref ref-type="fig" rid="f08">Fig 8</xref>
). Creep depends on the intensity of stress and temperature,
given that high tension and temperature favor displacement. In Engineering, creep in
metals is only considered when the temperature accounts for at least 30% of the material
melting point, given that structural components often undergo high tension.<sup><xref rid="r05" ref-type="bibr">5</xref>
,<xref rid="r07" ref-type="bibr">7</xref>
</sup>
</p>
<fig id="f07" orientation="portrait" position="float"><label>Figure 7</label>
<caption><p>Example of creep (Source: adapted from Caldas et al<sup><xref rid="r04" ref-type="bibr">4</xref>
</sup>
).</p>
</caption>
<graphic xlink:href="dpjo-19-02-0126-g07"></graphic>
</fig>
<fig id="f08" orientation="portrait" position="float"><label>Figure 8</label>
<caption><p>Example of stress relaxation.</p>
</caption>
<graphic xlink:href="dpjo-19-02-0126-g08"></graphic>
</fig>
<p>As for orthodontic springs, acute bends are generally employed to shape the wire. Such
bends centralize stress, producing unstable spacing and displacement in the crystal
structure of high tension sites.<sup><xref rid="r05" ref-type="bibr">5</xref>
</sup>
Orthodontists have tried to overcome such problem by thermally treating orthodontic
stainless steel appliances so as to rearrange the crystal structure and, as a result,
relieve residual tension.<sup><xref rid="r08" ref-type="bibr">8</xref>
</sup>
The
Bauschinger<sup><xref rid="r02" ref-type="bibr">2</xref>
</sup>
effect is also
another strategy commonly employed. It consists in overactivating the wire and
simulating activations until the wire is properly shaped for applying the system of
forces.</p>
<p>Such progressive deformation, as a result of structural stress relief, has been recently
proved by mechanical trials conducted with β-Ti T-loop springs previously activated by
bends, in which case reduction in force and moments of 15.5 and 17.15%, respectively,
were observed within the first 24 hours. In clinical practice, applying an acute bend to
shape an orthodontic appliance may cause the latter to loose and, as a consequence,
change its system of forces under constant deformation. In these cases, the appliances
must be overactivated or the acute bend must be replaced by curvature activation, i.e.,
gradual activation without significant sites of stress.<sup><xref rid="r08" ref-type="bibr">8</xref>
</sup>
</p>
<p>Another fact that should be carefully assessed is the interchangeable use of β-Ti wires
that are available on the market. Once the patent for the first commercial brand of β-Ti
expired (TMA, OrmcoCo., Glendora, USA), the use of β-Ti alloys drastically expanded with
a wide range of price and quality.<sup><xref rid="r09" ref-type="bibr">9</xref>
</sup>
In
spite of the large number of brands available, only a few researches have been conducted
to investigate the properties of this type of wire.<sup><xref rid="r10" ref-type="bibr">10</xref>
,<xref rid="r11" ref-type="bibr">11</xref>
,<xref rid="r12" ref-type="bibr">12</xref>
</sup>
Those researches compared the mechanical properties of
β-Ti alloys by means of traction tests<sup><xref rid="r11" ref-type="bibr">11</xref>
,<xref rid="r12" ref-type="bibr">12</xref>
</sup>
or 3-point flexural
tests<sup><xref rid="r10" ref-type="bibr">10</xref>
</sup>
in straight wire
segments. This may not demonstrate how different β-Ti alloys behave when bends are
placed in the wires or when more elaborate designs are chosen (springs, loops,
cantilevers, etc.). A recent research conducted with four different β-Ti wires (TMA
[Ormco Co.], Beta Flexy [Orthometric Imp. Exp. Ltda, Marília/SP, Brazil], Beta III Wire
[Morelli Ortodontia, Sorocaba/SP, Brazil] and Beta CNA [Ortho Organizers, INC., San
Marcos, USA]) revealed that the wires produced different systems of force when used in
the form of a more elaborate design (T-loop spring) due to the fact that each one of the
wires responded differently when a bend was created. TMA and Beta CNA revealed more
consistent MT ratios during trials. Such results do not mean that the other types of
wire must not be clinically used; although they need to be employed with a different
approach.<sup><xref rid="r09" ref-type="bibr">9</xref>
</sup>
</p>
<p>Thus, SAT, as it has been previously stated, involves several mechanical concepts and
particularities that must be well understood by those employing it. Its application is
rather useful for performing a rational orthodontic treatment. However, it is
recommended for specific cases, i.e., to treat specific conditions such as canine
extrusion and retraction, deep bite correction, molars uprighting and occlusal plane
correction.</p>
</sec>
<sec><title>CANINES EXTRUSION</title>
<p>Maxillary canines are often impacted due to being the last teeth to irrupt in the oral
cavity, having a complex eruption path, or due to the lack of space for appropriate
positioning in the dental arch.<sup><xref rid="r13" ref-type="bibr">13</xref>
</sup>
</p>
<p>In these cases, orthodontic traction may be carried out by means of SAT with the aid of
a cantilever or a rectangular loop. As it has been previously mentioned, the use of
mechanics with continuous wire, regardless of the type of bracket, produces several side
effects on teeth adjacent to the canine. For this reason, it must be avoided. Therefore,
by means of SAT, the desired movement is achieved when force is directly applied to the
mal-positioned/impacted tooth, while reactive force is dissipated or controlled in the
posterior anchorage unit as a result of an increase in the number of teeth joined to the
segment and/or the use of an anchorage appliance.</p>
<p>Clinically speaking, both appliances are feasible and effective. However,
biomechanically speaking, they present peculiar characteristics that need to be well
understood by the clinician. A cantilever can be described as a segment of a stainless
steel 0.017 x 0.025-in wire or a β-Ti wire inserted inside a bracket or tube in the
reactive member, tied to the opposite end by means of contact point in the active member
(impacted or mal-positioned canine).Therefore, the wire is not inserted into the bracket
slot where movement is desired. Should the force produced by an activated cantilever
(ranging from 40 to 60 gf) be directed along the CRes of the tooth, it will produce a
translatory movement. Should it bypass the CRes of the tooth (in most cases, it is
applied to the crown), it will result in a rotational tendency of force due to the
moment of force established (<xref ref-type="fig" rid="f09">Fig 9</xref>
).</p>
<fig id="f09" orientation="portrait" position="float"><label>Figure 9</label>
<caption><p><bold>A</bold>
) System of forces produced by a cantilever (statically determined
system).This system also reveals a tendency towards palatalization of #13 with
force going in the vestibular direction of CRes. <bold>B</bold>
) Palatal bar used
to control any undesired effects on the anchorage unit.</p>
</caption>
<graphic xlink:href="dpjo-19-02-0126-g09"></graphic>
</fig>
<p>Conversely, given that the wire has been inserted inside the bracket slot or tube, a
force of equal intensity and opposite direction is observed in the reactive member.
Similarly, a torque is produced inside the appliance when the moment arm of the
activated cantilever is tied to the tooth to be moved. Such torque produces movement
that, in association with force, must be restricted or reduced to nothing by reinforcing
the anchorage of the reactive member (for instance, by using a palatal bar), so as to
avoid side effects on the mechanics. Therefore, a cantilever is a statically determined
system, characterized by force and moment of force in the active member, whereas it is
characterized by force of similar intensity, but opposite direction, and moment of
torque in the reactive member. Such system provides a system of forces that can be
easily visualized and predicted during the process of deactivation (<xref ref-type="fig" rid="f09">Fig 9</xref>
).</p>
<p>A rectangular loop is recommended for tridimensional control of a tooth with more severe
positioning anomalies. It may be fabricated with stainless steel 0.017 x 0.025-in wire
or β-Ti wire between 6 and 7 mm in the cerviccal-occlusal direction, and between 8 and
10 mm in the mesiodistal direction.<sup><xref rid="r13" ref-type="bibr">13</xref>
</sup>
Unlike a cantilever, a rectangular loop is inserted inside the bracket slot of an active
member. Therefore, in this case, in addition to force (moment of force), a moment of
torque is also established as a result of inserting the wire inside the slot. In the
reactive member, the system is similar to a cantilever. This system is statically
undetermined, given that during deactivation, the system of forces may be little
predictable due to changes in tooth positioning (<xref ref-type="fig" rid="f10">Fig
10</xref>
).</p>
<fig id="f10" orientation="portrait" position="float"><label>Figure 10</label>
<caption><p>System of forces produced by a rectangular loop (statically undetermined system).
The activated rectangular loop (light blue) must be adjusted so as to determine
the final position of the tooth. When inserted into the slot (dark blue), its
deactivation will move the tooth in the previously established position.</p>
</caption>
<graphic xlink:href="dpjo-19-02-0126-g10"></graphic>
</fig>
<p>Thus, it is recommended that a cantilever be used in buccally or lingually placed
impacted canines that need to be lingually or labially repositioned, respectively. A
cantilever allows a system of forces to be applied in the desired direction, which may
be determined by the method of preactivation. A cantilever may be replaced by a
rectangular loop, provided that the crown of the canine is apparent and its movement may
be three-dimensionally controlled until correct positioning is achieved in the dental
arch.<sup><xref rid="r13" ref-type="bibr">13</xref>
</sup>
</p>
</sec>
<sec><title>CANINE RETRACTION</title>
<p>Canine retraction for resolution of previous crowding is a clinical condition that
largely favors the use of SAT before straight wires are used. Should the orthodontist
choose to perform mechanics with straight wires avoiding buccal tipping of incisors,
straight wires will be progressively used without including the incisors in the arch.
Canine retraction begins with an E-arch (rectangular steel wire). Another option would
be passive E-arch in the posterior region and canine, which would allow retraction since
the first month of treatment.</p>
<p>Another relevant point is mesiodistal tipping of canines, in which case three situations
should be considered: canine mesially tipped with the apex more distally placed than the
cuspid tip; upright canine with root apex and cuspid tip virtually occupying the same
position in the mesiodistal direction; distally tipped canine with the apex more
mesially placed in relation to its cuspid (<xref ref-type="fig" rid="f11">Fig
11</xref>
). Using straight wires for cases of mesial tipping and upright teeth favor the
resolution of crowding and canine retraction (<xref ref-type="fig" rid="f11">Figs
11A</xref>
and <xref ref-type="fig" rid="f11">11B</xref>
). Placing the straight wire
with the canine mesially tipped favors distal movement of the crown, which results in
diastema between the canine and the lateral incisor, thus favoring the resolution of
crowding (<xref ref-type="fig" rid="f11">Fig 11A</xref>
).</p>
<fig id="f11" orientation="portrait" position="float"><label>Figure 11</label>
<caption><p><bold>A</bold>
) Mesially tipped canine and the initial need for horizontal force,
only. (<bold>B</bold>
) Upright canine must undergo translation movement, i.e., a
combination of horizontal force and moment. (<bold>C</bold>
) Distal tipping of
canine initially suggests the application of moment for root movement.</p>
</caption>
<graphic xlink:href="dpjo-19-02-0126-g11"></graphic>
</fig>
<p>Conversely, upright canines do not favor the opening of diastema, but it is probable
that, in this case, the onset of retraction will happen more quickly (<xref ref-type="fig" rid="f11">Fig 11B</xref>
).</p>
<p>On the other hand, in case of distal tipping of canines (<xref ref-type="fig" rid="f11">Fig 11C</xref>
), alignment and leveling with straight wires may worsen the condition
of crowding, in which case there is a tendency towards mesial movement of the crown. The
use of elastomeric chain or springs for canine bracket in the posterior region is also
contraindicated to avoid mesialization of the crown, given that these devices favor
distal tipping of the crown. Additionally, extrusive and undesired forces will be
incorporated in the region of incisors during alignment.</p>
<p>Thus, it is recommended that segmented mechanics be used with reactive members and
pre-activated T-loop springs, and that it be horizontally activated in accordance with
the mesiodistal tipping of canines. For instance, in case of mesial tipping, the T-loop
spring may be horizontally activated until canine uprighting is achieved. As for upright
canines, a standard T-loop spring is used, i.e., with pre-activations (moment) and
horizontal activation, producing translatory movements. In case of distal tipping,
pre-activations (moment) may be initially used. After distal movement of canine roots is
produced and uprighting is achieved, standard T-loop spring activation begins.
Anti-rotation bends must be considered to overcome the tendency towards rotation of
canines, also treated according to the position of canines in the dental arch. A
complete description of T-loop spring biomechanics is found in publications by Martins
et al.<sup><xref rid="r01" ref-type="bibr">1</xref>
,<xref rid="r04" ref-type="bibr">4</xref>
,<xref rid="r08" ref-type="bibr">8</xref>
,<xref rid="r09" ref-type="bibr">9</xref>
,<xref rid="r14" ref-type="bibr">14</xref>
-<xref rid="r17" ref-type="bibr">17</xref>
</sup>
</p>
</sec>
<sec><title>DEEP BITE</title>
<p>Deep bite is a multifactorial vertical malocclusion that needs a differential diagnosis
comprising facial, dental and cephalometric data as well as data on the vertical
positioning of incisors at rest, smile and speech.<sup><xref rid="r18" ref-type="bibr">18</xref>
</sup>
</p>
<p>It may be treated by extrusion of posterior teeth, intrusion of maxillary and/or
mandibular incisors or a combination of procedures.<sup><xref rid="r18" ref-type="bibr">18</xref>
</sup>
</p>
<p>Intrusion of incisors was considered for many years as a complex orthodontic movement.
In fact, it is a technical movement that must be carefully planned. The use of straight
wires hardly produce pure intrusion of incisors, instead, it leads to extrusion of
posterior teeth. In other conditions, such as in appropriate anterior overbite,
alignment and leveling achieved with straight wires may lead to deep bite, given that
canines with severe root mesial tipping (<xref ref-type="fig" rid="f11">Fig 11C</xref>
)
during correction cause anterior overbite to deepen. Following this line of reasoning,
using archwires to level the curve of Spee (excessive in the maxillary arch and reverse
in the mandibular arch), regardless of the type of alloy, will promote extrusion of
posterior teeth, especially of premolars, followed by pseudo-intrusion of incisors (more
buccal tipping than pure intrusion) (<xref ref-type="fig" rid="f12">Fig
12</xref>
).<sup><xref rid="r18" ref-type="bibr">18</xref>
</sup>
</p>
<fig id="f12" orientation="portrait" position="float"><label>Figure 12</label>
<caption><p>Summary of the mechanical effects of arches used to manipulate the curve of
Spee.</p>
</caption>
<graphic xlink:href="dpjo-19-02-0126-g12"></graphic>
</fig>
<p>A systematic review conducted in 2005 concluded that the movement of intrusion is
feasible and more easily achieved in the mandibular arch than in the maxillary arch. The
study revealed that the segmented arch technique is most commonly employed for intrusion
of incisors, with 1.5 mm of intrusion of maxillary incisors and 1.9 mm of mandibular
incisors.<sup><xref rid="r19" ref-type="bibr">19</xref>
</sup>
</p>
<p>Intrusion of anterior teeth can be basically achieved by means of two types of
mechanics: straight wire appliance and three-piece arch appliance. The former consists
of a segment of wire placed along the dental arch and, in the anterior region, placed on
the teeth by directly fitting into the bracket slots (Ricketts arch) (<xref ref-type="fig" rid="f13">Figs 13A</xref>
, <xref ref-type="fig" rid="f13">13B</xref>
and <xref ref-type="fig" rid="f13">13C</xref>
) or by being tied to another anterior
segment (Burstone arch) (<xref ref-type="fig" rid="f13">Fig 13D</xref>
).</p>
<fig id="f13" orientation="portrait" position="float"><label>Figure 13</label>
<caption><p>Illustration of the straight wire intrusion system. (<bold>A</bold>
) maxillary
intrusion arch; (<bold>B</bold>
) mandibular arch; (<bold>C</bold>
) bracket
fitting; and (<bold>D</bold>
) fitting in another segment of wire.</p>
</caption>
<graphic xlink:href="dpjo-19-02-0126-g13"></graphic>
</fig>
<p>As for the three-piece arch appliance, two intrusion cantilevers are fabricated (one for
each side) and inserted into another segment in the anterior region (<xref ref-type="fig" rid="f14">Fig 14</xref>
). Using a segment of wire in the anterior
region, isolating the area, is a much more predictable and understandable alternative.
Should the intrusion arch be directly inserted into the anterior bracket slots (<xref ref-type="fig" rid="f13">Fig 13C</xref>
), undesired torques may be incorporated, and
a more complex and undetermined mechanical system may be produced.</p>
<fig id="f14" orientation="portrait" position="float"><label>Figure 14</label>
<caption><p>Illustration of the three-piece arch appliance intrusion system. (<bold>A</bold>
)
maxillary arch; (<bold>B</bold>
) mandibular arch.</p>
</caption>
<graphic xlink:href="dpjo-19-02-0126-g14"></graphic>
</fig>
<p>Regardless of the type of fitting, good anchorage control must be achieved due to the
forces and, especially, the moments created by the system of intrusion (<xref ref-type="fig" rid="f15">Fig 15</xref>
). The largest number of posterior teeth must
be incorporated into posterior segments, with lingual or palatal arches used to gather
posterior segments into one single anchorage segment.<sup><xref rid="r02" ref-type="bibr">2</xref>
</sup>
Additionally, the use of a high-pull headgear and short
power arm angulated upwards is also recommended to balance the adverse effects of this
mechanics. Likewise, the use of mini-implants or miniplates is also recommended.</p>
<fig id="f15" orientation="portrait" position="float"><label>Figure 15</label>
<caption><p>Intrusion system with the forces and moments produced.</p>
</caption>
<graphic xlink:href="dpjo-19-02-0126-g15"></graphic>
</fig>
<p>According to Burstone, the key to success in intrusion is controlling the system of
forces. Light, constant forces must be employed, with the point of application and
direction of force carefully assessed. The magnitude of force must range from 10 to 15
gf per incisor. To calculate the total force, the teeth that will potentially undergo
intrusion must be added up and the corresponding load must be applied. The magnitude of
force is of paramount importance to yield satisfactory clinical results. For this
reason, it is recommended that precision dynamometers be used to calculate the
appropriate load.<sup><xref rid="r02" ref-type="bibr">2</xref>
</sup>
</p>
<p>Researches have been conducted on the point of force application in order to provide
appropriate intrusion of maxillary incisors. The results yielded by these studies led to
clinical assertions that oftentimes result in errors. When all four incisors need to be
intruded with maintenance of axial inclination (without moment production), intrusive
force must be applied near the distal surface of lateral incisors. In order to produce
intrusion and buccal tipping of incisors, force must be mesially applied to lateral
incisors, whereas for intrusion with retroclination, load must be distally applied to
canines.<sup><xref rid="r20" ref-type="bibr">20</xref>
</sup>
</p>
<p>Santos-Pinto<sup><xref rid="r21" ref-type="bibr">21</xref>
</sup>
also suggests other
points of force application, which vary between pure intrusion and intrusion associated
with buccal tipping or retroclination, according to the objectives of treatment (<xref ref-type="fig" rid="f16">Fig 16</xref>
).</p>
<fig id="f16" orientation="portrait" position="float"><label>Figure 16</label>
<caption><p>Different points of force application and mechanical effects on incisors: 1, 2 and
3 = intrusion + buccal inclination; 4 = intrusion; and 5 = intrusion +
retroclination.</p>
</caption>
<graphic xlink:href="dpjo-19-02-0126-g16"></graphic>
</fig>
<p>It is worth mentioning that these points of force application must be adapted to the
conditions of each clinical case. Depending on the degree of axial inclination of
incisors, such points may be more anteriolly or posteriolly displaced.</p>
<p>Some clinical conditions require not only intrusion of incisors, but also intrusion of
canines, in which case it is recommended to begin deep bite correction by canines
followed by incisors. <xref ref-type="fig" rid="f17">Figure 17</xref>
, for instance,
shows the use of a three-piece arch appliance, with two segments posteriorly placed and
one segment placed on the incisors without including the canines. Two cantilevers were
fabricated with β-Ti wire for intrusion of canines on both sides. Subsequently, when
canines are overcorrected or on the same occlusal plane of posterior teeth, they are
incorporated to the poster anchorage unit, after which intrusion of incisors is
performed.</p>
<fig id="f17" orientation="portrait" position="float"><label>Figure 17</label>
<caption><p>Clinical illustration of a cantilever used for canine intrusion.</p>
</caption>
<graphic xlink:href="dpjo-19-02-0126-g17"></graphic>
</fig>
<p>Today, intrusion of maxillary incisors has been rarely used as the only resource to
treat deep bite, given that exposure of maxillary teeth and a narrow section of gingival
tissue yield more esthetic results, giving the patient a younger appearance.<sup><xref rid="r18" ref-type="bibr">18</xref>
</sup>
Slightly extruded maxillary incisors are
generally more esthetic than slightly intruded teeth.<sup><xref rid="r22" ref-type="bibr">22</xref>
</sup>
On the other hand, in very specific cases, intrusion of
maxillary incisors is well recommended.</p>
<p>In addition to the aforementioned procedures, intrusion and simultaneous retraction of
maxillary incisors are highly recommended not only for antero-superior intrusion, but
also for retraction, effectively controlling axial inclination of those teeth.<sup><xref rid="r23" ref-type="bibr">23</xref>
</sup>
In these cases, in addition to the
three-piece arch appliance, retraction appliances, such as elastomeric chain or springs,
are also used, thus incorporating new forces and moments into the system (<xref ref-type="fig" rid="f18">Fig 18</xref>
). Similarly to the mechanics described in
<xref ref-type="fig" rid="f15">Fig 15</xref>
, good anchorage control must be
implemented so as to overcome the side effects of the mechanics.</p>
<fig id="f18" orientation="portrait" position="float"><label>Figure 18</label>
<caption><p>Mechanical illustration of forces and moments in the segmented arch technique
employed for simultaneous retraction and intrusion of incisors.</p>
</caption>
<graphic xlink:href="dpjo-19-02-0126-g18"></graphic>
</fig>
<p>As it has been previously mentioned, in many clinical situations, alignment and leveling
achieved by means of straight wires may cause deep bite. Furthermore, in case of
extraction of first bicuspids, partial retraction of canines with straight wires also
increases deep bite. <xref ref-type="fig" rid="f19">Figure 19</xref>
shows how deep bite
was achieved after alignment, leveling and partial retraction of maxillary canines. In
these cases, retraction of incisors becomes a difficult task to be achieved due to the
lack of overbite and overjet necessary for retraction. In this example (<xref ref-type="fig" rid="f19">Fig 19C</xref>
), the arch of intrusion and simultaneous
retraction was used for retraction of incisors with greater control of axial inclination
without necessarily promoting real tipping. At treatment completion (<xref ref-type="fig" rid="f19">Fig 19D</xref>
), we observed that retraction was achieved
with correct inclination of incisors, whereas anterior overbite remained unchanged,
preventing potential deleterious impact on the degree of teeth exposure at smile, rest
and speech.</p>
<fig id="f19" orientation="portrait" position="float"><label>Figure 19</label>
<caption><p>Clinical illustration of SAT used for simultaneous retraction and intrusion of
incisors:(<bold>A</bold>
) initial case; (<bold>B</bold>
) during maxillary
alignment and leveling; (<bold>C</bold>
) deep bite after alignment and leveling
completion, and partial retraction of canines; (<bold>D</bold>
) final result after
placement carried out by means of the segmented arch technique.</p>
</caption>
<graphic xlink:href="dpjo-19-02-0126-g19"></graphic>
</fig>
</sec>
<sec><title>MOLAR UPRIGHTING</title>
<p>Molar uprighting is widely employed in cases of early loss of deciduous teeth followed
by deleterious dental migration in young patients and in orthodontic treatment of adult
patients with tooth loss. In these cases, molar uprighting is generally associated with
extrusion of antagonist teeth, reduction in edentulous space, bone dehiscence in the
mesial surface of tipped molars, gingival recession of tipped molars, early contact in
centric relation and occlusal interference on excursion of the mandible.<sup><xref rid="r24" ref-type="bibr">24</xref>
</sup>
With regard to integrated planning,
clinicians must decide whether the tooth subject to uprighting will undergo movement for
space closure, opening of space for prosthetic rehabilitation or implant placement.</p>
<p>Mesial movement of molars may be rendered difficult due to the following: alveolar bone
resorption resulting from tooth loss, which causes the molar mesial bone to become too
thin; unfavorable root morphology for movement of lower molars; greater mandibular bone
density in relation to the maxilla; and thin buccolingual bone thickness from distal to
mesial in the mandibular arch.</p>
<p>Using straight wires to upright tipped molars is considered unfeasible, given that, in
these cases, there is a strong tendency towards extrusion of molars, especially due to
the short distance between brackets. Additionally, incorporating a T-loop spring into
the arch will lead to extrusion of premolars.<sup><xref rid="r24" ref-type="bibr">24</xref>
</sup>
A cantilever, extended up to the anterior region, may be used to
reduce the effects of extrusion on molars. Researches have proved a moment of 1200 gf.mm
to be appropriate for molar uprighting.<sup><xref rid="r25" ref-type="bibr">25</xref>
,<xref rid="r26" ref-type="bibr">26</xref>
</sup>
Should a 30-mm
cantilever be used, an activation of 40 gf is enough for molar uprighting (<xref ref-type="fig" rid="f20">Fig 20</xref>
), in which case 40 gf corresponds to intrusive
forces in the anterior region and extrusive forces in the region of molar teeth.
Mesocephalic or brachycephalic patients are able to eliminate or reduce this effect of
extrusion by their own muscular pattern; however, additional studies are warranted to
further investigate this topic. Another aspect that should be highlighted is whether or
not the tooth subject to uprighting allows attrition, such as extensive restoration, on
its occlusal surface. In other words, whether it is a tooth that will become a
metal-ceramic crown that allows extrusion as it will undergo attrition.</p>
<fig id="f20" orientation="portrait" position="float"><label>Figure 20</label>
<caption><p>Illustration showing the moment produced by the cantilever used for molar
uprighting.</p>
</caption>
<graphic xlink:href="dpjo-19-02-0126-g20"></graphic>
</fig>
<p><xref ref-type="fig" rid="f21">Figure 21</xref>
shows a clinical case of side effects on
occlusion resulting from loss of first molar and lower second premolar. The second molar
was not included in the straight archwire during alignment and leveling. Subsequently, a
β-Ti cantilever was used for two purposes: 1) molar uprighting and space opening for
implant rehabilitation; and 2) correction of the occlusal plane in the anterior segment
as a result of installing the cantilever in the midline, thus producing intrusive forces
in this area (<xref ref-type="fig" rid="f21">Figs 21B</xref>
, <xref ref-type="fig" rid="f21">21C</xref>
). With the molar near the occlusal plane, straight wires were
used to go on with treatment and reach completion (<xref ref-type="fig" rid="f21">Figs
21D</xref>
, <xref ref-type="fig" rid="f21">21E</xref>
, <xref ref-type="fig" rid="f21">21F</xref>
).</p>
<fig id="f21" orientation="portrait" position="float"><label>Figure 21</label>
<caption><p>Clinical illustration of a cantilever used for uprighting tooth #37.</p>
</caption>
<graphic xlink:href="dpjo-19-02-0126-g21"></graphic>
</fig>
<p>In cases in which molar extrusion is impossible, springs or cantilevers supported by
mini-implants can be used with the vector line of force passing under the molar CRes, so
as to produce a resultant force that intrudes the molar. Another option to eliminate
extrusion force is to use a double cantilever,<sup><xref rid="r27" ref-type="bibr">27</xref>
</sup>
in which case one cantilever is used as in conventional molar
uprighting, while the other is activated to produce intrusive forces on the molar. Thus,
the effect of extrusion produced by the conventional cantilever is neutralized by the
second cantilever. One disadvantage of this option is patient's discomfort caused by the
concurrent use of two devices (<xref ref-type="fig" rid="f22">Figs 22</xref>
, <xref ref-type="fig" rid="f23">23</xref>
).</p>
<fig id="f22" orientation="portrait" position="float"><label>Figure 22</label>
<caption><p>Illustration showing the effects of intrusion caused by activation of a second
cantilever inserted into the cross tube, neutralizing the extrusive effects
produced by the first cantilever.</p>
</caption>
<graphic xlink:href="dpjo-19-02-0126-g22"></graphic>
</fig>
<fig id="f23" orientation="portrait" position="float"><label>Figure 23</label>
<caption><p>Illustration showing the neutralization of vertical effects on molar subject to
uprighting.</p>
</caption>
<graphic xlink:href="dpjo-19-02-0126-g23"></graphic>
</fig>
</sec>
<sec><title>OCCLUSAL PLANE CORRECTION</title>
<p>Occlusal plane inclination is a daunting challenge faced by orthodontic treatment. It is
mainly caused by dental issues such as loss and/or ankylosis of antagonist teeth,
deleterious oral habits, inappropriate orthodontic bonding leading to asymmetric dental
alignment, skeletal disharmony or a combination of factors.<sup><xref rid="r28" ref-type="bibr">28</xref>
</sup>
</p>
<p>Most cases of skeletal facial asymmetry present significant changes in maxillary and/or
mandibular occlusal plane. TJD-related pathologies (rheumatoid arthritis, condylar
hyperplasia, osteochondroma, condylar resorption, and others) and craniofacial
syndromes/deformities are the most common primary etiological factors of facial
asymmetry that impairs the occlusal plane. In fact, abnormal mandibular impairment is
initially observed in growing and non-growing patients.</p>
<p>However, to compensate such changes, the maxillary occlusal plane is also impaired so as
to establish a balance between the mandibular changes.<sup><xref rid="r28" ref-type="bibr">28</xref>
,<xref rid="r29" ref-type="bibr">29</xref>
</sup>
</p>
<p>Treatment of those pathologies depends on the cause and degree of severity of the
change. In cases of significant facial alterations, dental correction may be achieved by
mini-implants or miniplates; however, no improvements in facial esthetics will be
observed. Thus, an approach combining orthognathic surgery and orthodontic treatment is
recommended. Nevertheless, in borderline cases in which patient's chief complaint is not
facial, orthodontic approach alone is employed with considerable forseability.<sup><xref rid="r29" ref-type="bibr">29</xref>
</sup>
</p>
<p>Mini-implants became popular and allowed those issues to be addressed in a more
predictable manner, yielding excellent results.<sup><xref rid="r30" ref-type="bibr">30</xref>
</sup>
Nevertheless, other biomechanical procedures that do not require
skeletal anchorage can be employed to treat occlusal plane inclination. The rational use
of biomechanics by means of SAT and asymmetric cantilevers proves to be a feasible
option. As shown in <xref ref-type="fig" rid="f24">Figure 24A</xref>
, the patient, whose
chief complaint was having an asymmetric smile, presents occlusal plane inclination. Due
to the absence of facial complaints, she was advised to undergo dental treatment, only,
which would be performed with asymmetric cantilevers.</p>
<fig id="f24" orientation="portrait" position="float"><label>Figure 24</label>
<caption><p>Clinical illustration showing the use of two asymmetric cantilevers. A, B, C)
Initial case; D, E, F) Intrusion on the right side and extrusion on the left side
for correction of the occlusion plane; G, H, I) After mechanics was applied.</p>
</caption>
<graphic xlink:href="dpjo-19-02-0126-g24"></graphic>
</fig>
<p>Initially, alignment and leveling were carried out in association with anchorage
preparation of the reactive member by means of a palatal bar.</p>
<p>The 0.019 x 0.025-in wire was achieved and the arch was segmented into three pieces
(from #16 to #14, from #13 to #21 and from #22 to #25). Initial alignment and leveling
did not include tooth #27 which was coupled with #16 by a palatal bar.</p>
<p>The segment going from #13 to #21 underwent a clockwise movement, with the point of
force application between #13 and #12, where greater intrusion was observed. A 70-g
force was applied by a cantilever made of 0.017 x 0.025-in β-Ti wire (<xref ref-type="fig" rid="f24">Fig 24B</xref>
).</p>
<p>The segment going from #22 to #25 underwent an anti-clockwise movement, with the point
of force application between #22 and #23. Extrusive force was applied by a cantilever
made of 0.017 x 0.025-in steel wire. In order to achieve flexibility and decrease the LF
ratio, a helix was placed in the wire with a force of (<xref ref-type="fig" rid="f24">Figs 24D</xref>
, <xref ref-type="fig" rid="f24">24E</xref>
, <xref ref-type="fig" rid="f24">24F</xref>
).</p>
<p>The side effects produced by the mechanics were balanced by the palatal bar reinforced
with the use of a steel stabilization 0.019 x 0.025-in archwire placed from #16 to 14 on
the right side. Treatment lasted for three months, followed by rebonding of brackets and
new procedures of alignment and leveling (<xref ref-type="fig" rid="f24">Figs
24G</xref>
, <xref ref-type="fig" rid="f24">24H</xref>
, <xref ref-type="fig" rid="f24">24I</xref>
).</p>
</sec>
<sec><title>FINAL CONSIDERATIONS</title>
<p>Today, orthodontists must keep distance from conceptual dogma and be open to new
information that contributes to perform orthodontic treatment as effectively as
possible. Knowing how to deal with potential side effects is essential for orthodontic
therapy, given that such effects may hinder treatment and, in many cases, cause lost of
control.</p>
<p>In addition to the aforementioned situations, the segmented mechanics may be employed in
many other branches of Orthodontics, namely: anterior retraction, use of palatal bar and
removable lingual arch (correction of rotation, distalization, arch expansion or
contraction), extrusion of incisors, etc. Therefore, understanding the principles behind
scientific biomechanics is crucial to yield great clinical results with as little side
effects as possible. In this context, SAT proves to be essential as an auxiliary
resource that aids the straight wire technique in several clinical situations.</p>
</sec>
</body>
<back><fn-group><fn fn-type="other"><p>» The patient displayed in this article previously approved the use of her facial and
intraoral photographs.</p>
</fn>
<fn fn-type="other"><p>» The authors report no commercial, proprietary or financial interest in the products
or companies described in this article.</p>
</fn>
<fn fn-type="other"><p><bold>How to cite this article:</bold>
Caldas SGFR, Ribeiro AA, Simplício H, Machado
AW. Segmented arch or continuous arch technique? A rational approach. Dental Press J
Orthod. 2014 Mar-Apr;19(2):126-41. doi: <ext-link ext-link-type="uri" xlink:href="http://dx.doi.org/10.1590/2176-9451.19.2.126-141.sar">http://dx.doi.org/10.1590/2176-9451.19.2.126-141.sar</ext-link>
</p>
</fn>
</fn-group>
<ack><title>Acknowledgments</title>
<p>This article was written in honor to the members of the Department of Orthodontics of
the State University of São Paulo (UNESP) / Araraquara. Thank you very much for teaching
us about the fascinating world of scientific biomechanics!</p>
</ack>
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