YOU ARE DOWNLOADING DOCUMENT

Please tick the box to continue:

Transcript
Page 1: Effective en-masse retraction design with orthodontic mini ... · during space closure is important for maximum retraction of the anterior teeth after premolar extractions.6,7 OMIs

ORIGINAL ARTICLE

Effective en-masse retraction design withorthodontic mini-implant anchorage: A finiteelement analysis

Sang-Jin Sung,a Gang-Won Jang,b Youn-Sic Chun,c and Yoon-Shik Moond

Seoul and Kunsan, Korea

Introduction: The strategic design of an appliance for correcting a bialveolar protrusion by using orthodonticmini-implant anchorage and sliding mechanics must take into account the position and height of the mini-implant, the height of the anterior retraction hook and compensating curve, and midline vertical traction. Inthis study, we used finite element analysis to examine effective en-masse retraction with orthodontic mini-implant anchorage and sought to identify a better combination of the above factors. Methods: Base modelswere constructed from a dental study model. Models with labially and lingually inclined incisors were also con-structed. The center of resistance for the 6 anterior teeth in the base model was 9 mm superiorly and 13.5 mmposteriorly from the midpoint of the labial splinting wire. The working archwires were assumed to be 0.019 3

0.025-in or 0.016 3 0.022-in stainless steel. The amount of tooth displacement after finite element analysiswas magnified 400 times and compared with central and lateral incisor and canine axis graphs. Resultsand Conclusions: The tooth displacement tendencies were similar in all 3 models. The height of the anteriorretraction hook and the placement of the compensating curve had limited effects on the labial crown torque ofthe central incisors for en-masse retraction. The 0.016 3 0.022-in stainless steel archwire showed moretipping of teeth compared with the 0.019 3 0.025-in archwire. For high mini-implant traction and 8-mmanterior retraction hook condition, the retraction force vector was applied above the center of resistance forthe 6 anterior teeth, but no bodily retraction of the 6 anterior teeth occurred. For high mini-implant traction,2-mm anterior retraction hook, and 100-g midline vertical traction condition, the 6 anterior teeth wereintruded and tipped slightly labially. (Am J Orthod Dentofacial Orthop 2010;137:648-57)

Orthodontic mini-implants (OMIs) are used forvarious anchorages.1-5 For treatment ofbialveolar protrusion, anchorage preservation

during space closure is important for maximumretraction of the anterior teeth after premolarextractions.6,7 OMIs have been reported to be effectiveanchorage for en-masse space closure (Fig 1).3 In somepatients, however, retroclined anterior teeth after maxi-mum retraction can cause a poor esthetic outcome(Fig 1, B).

aAssociate professor, Division of Orthodontics, University of Ulsan College of

Medicine, Asan Medical Center, Seoul, Korea.bAssistant professor, School of Mechanical Engineering, Kunsan National

University, Kunsan, Korea.cProfessor, Division of Orthodontics, Department of Dentistry, School of Med-

icine, Ewha Womans University, Seoul, Korea.dProfessor, Division of Orthodontics, University of Ulsan College of Medicine,

Asan Medical Center, Seoul, Korea.

The authors report no commercial, proprietary, or financial interest in the

products or companies described in this article.

Reprint requests to: Sang-Jin Sung, 388-1 Pungnap-2dong, Songpa-gu, Seoul

138-736, Korea; e-mail, [email protected].

Submitted, April 2008; revised and accepted, June 2008.

0889-5406/$36.00

Copyright � 2010 by the American Association of Orthodontists.

doi:10.1016/j.ajodo.2008.06.036

648

The working rectangular wires for en-masse ante-rior retraction with an OMI and sliding mechanics canbe made of 0.019 3 0.025-in (0.022-in slot) or 0.0163 0.022-in (0.018-in slot) stainless steel (SS).8-10 Theanterior retraction hook (ARH) is placed bilaterallybetween the lateral incisor and the canine at a heightof 2 to 5 mm. The position of the OMI is about10 mm above the posterior archwire.3,10

Although we designed the retraction force vectorthrough the center of resistance (CR) for the 6 maxillaryanterior teeth, bodily tooth movement might not occur.Because these teeth were not splinted rigidly to forma multi-rooted tooth, a relatively flexible continuousfull archwire was used.6,11 Furthermore, application ofan experimental11,12 or estimated CR7,13,14 to a patientmight be limiting.15

The reaction of teeth to sliding mechanics can beanalyzed by using finite element analysis (FEA).16

With 3-dimensional (3D) computer models, variousconditions can be simulated by varying the simulationparameters. The initial reactions of the teeth, periodon-tal ligament (PDL), and alveolar bone can be evaluatedqualitatively and quantitatively.

Page 2: Effective en-masse retraction design with orthodontic mini ... · during space closure is important for maximum retraction of the anterior teeth after premolar extractions.6,7 OMIs

Fig 1. Various OMI placements: A, low OMI traction; B, high OMI traction. Arrows indicate momentsgenerated by force components. C, Combination of anterior and vertical traction.

American Journal of Orthodontics and Dentofacial Orthopedics Sung et al 649Volume 137, Number 5

Major factors affect the accuracy of FEA includinganatomic accuracy of the geometry of the finite element(FE) model and uncertainties about material propertiesand boundary conditions. In most previous studies, FEmodels were obtained by constructing coarse mappedmeshes for manually modeled computer aided design(CAD) geometries, so that the resulting analyses cannotavoid inaccuracies.16,17 Recently, with the help of 3Dlaser scans and corresponding CAD-computer aidedmanufacturing technologies, anatomically correct 3Dsurface modeling has become popular.9,15,18

Innovatively increased performance of the centralprocessor unit also enables fast FE modeling with finetetrahedron solid elements with even better accuracythan time-consuming mapped meshes. In addition, the ac-curacy of FE models has been improved by the publica-tion of newly calibrated material properties of the PDLsobtained from animal19 and cadaver20 studies.

In this study, we used FEA for an effective en-masseretraction design with orthodontic mini-implant anchor-age. We examined the effect of anterior torque controlas a function of the original anterior tooth axis, position,and height of the OMI, height of the ARH, compensat-ing curve (CC), and midline vertical traction, and soughtto identify a better combination of these factors.

MATERIAL AND METHODS

The central incisor (tooth 11), lateral incisor (12),and canine (13) from a dental study model (i21D-400G,Nissin Dental Products, Kyoto, Japan) were scannedand aligned with a True-arch form (‘‘A’’ Company, SanDiego, Calif). The axes of these teeth were reproducedfrom the dental study model (base model). The thicknessof the PDL was considered to be uniform (0.25 mm).16

The alveolar bone crest was constructed to follow thecurve of the cementoenamel junction (Fig 2, A). Lingualand labial models were also constructed with the anteriorteeth tipped lingually or labially by 10� compared withthe base model (Fig 2, B and C).

Using Ansys version 10 (Canonsburg, Pa), the arch-wire was modeled by beam 4 elements with a cross sec-tion of 0.019 3 0.025-in or 0.016 3 0.022-in SS wire.The ARH (0.05-in SS wire) was set between the lateralincisor bracket and the canine bracket bilaterally. Thecenter hook was set at the midpoint of the archwire(Fig 2, A).

At the connected nodes between the archwire and thebrackets, translational degrees of freedom in the 2 flexuraldirections of the archwire were coupled to deformtogether, and translational degrees of freedom in the axialdirection of the archwire were unconstrained.

Therefore, free axial rotation movement of the arch-wire in the brackets was allowed, while friction betweenthe archwire and brackets along the axial direction wasignored.

In the system studied, the y-axis was the midsagittalline of the dental arch on the occlusal view, and thez-axis was perpendicular to the y-axis. The x, y, andz coordinates of the archwire midpoint were 0, 49.6,5 in the base model (Fig 2, A).

The total numbers of nodes (or total numbers of el-ements) comprising the model were 49039 (249796) forthe base model, 48429 (246646) for the labial model,and 49039 (249796) for the lingual model. For the dis-cretization of the teeth, the PDL, and the alveolar bone,4-node tetrahedron element (ANSYS solid45) wereused, and material properties15,19,20 in the modelswere assumed to be isotropic and homogeneous9,16

(Table I).The position of the OMI was assumed to be 10 mm

(low OMI traction) or 12 mm (high OMI traction)(Fig 2, A). Retraction force vectors of 200 g from theARH (0, 2, 5, and 8 mm) to low or high OMI tractionwere resolved into components along the x, y, and zaxes and applied to the ARH (Fig 2, A; Table II).

For the 2-mm ARH and high OMI traction condi-tion, the OMI between the central incisors was assumedto apply midline vertical traction. The additional intru-sion force (50 or 100 g) from the center hook was

Page 3: Effective en-masse retraction design with orthodontic mini ... · during space closure is important for maximum retraction of the anterior teeth after premolar extractions.6,7 OMIs

Fig 2. Three FEM models: A, base model with 4�, 2�, and 2� V-bends placed in the archwireto simulate 3-mm CC. B, Lingual model, tipped 10� lingually compared with the base model. C, Labialmodel, tipped 10� labially compared with the base model. ARH, anterior retraction hook; CH,center hook; CC, compensating curve; LOT, low OMI traction; HOT, high OMI traction; CR, centerof resistance.

Table I. Material properties

Young’s modulus (g/mm2) Poisson’s ratio

Teeth 2E6 0.3

Periodontal ligament 5.0 0.3

Alveolar bone 2E5 0.3

Stainless steel 2E7 0.3

E, times ten raised to the power, as (scientific) E notation.

650 Sung et al American Journal of Orthodontics and Dentofacial Orthopedics

May 2010

combined to simulate the effect on labial crown torque(Fig 2, A).

To simulate a 3-mm CC, 4�, 2�, and 2� V-bends wereplaced in the archwire between the distal side of the ca-nine bracket and the mesial side of the second molartube. The vertical distance from the second molar tubeto the end of the curved archwire was 3 mm (Fig 2, A).

The 6 anterior teeth of the base model were splintedwith 0.3 3 0.3-in SS wire. The wire post was set at themidpoint of the labial splinting wire, and the wire framewas set at the lingual sides of the 6 anterior teeth. To findthe CR of the 6 anterior teeth, a 200-g retraction orintrusion force was applied in a superior or posteriordirection from the midpoint of the labial splintingwire at 0.5-mm intervals (Fig 3, A). After simulation,the initial tooth displacement was magnified 400times.16 The CR was estimated from the point of forceapplication that resulted in bodily movement of the 6anterior teeth in the base model. The CR was 9 mmsuperiorly and 13.5 mm posteriorly from the midpointof the labial splinting wire (Fig 3, B).

For the analysis, ANSYS software was used on thepersonal-computer platform with a Pentium 4 centralprocessor unit (3.0 GHz, Intel, Santa Clara, Calif).The deformed shapes of the teeth were studied withthe tooth axis graph in the y-z plane. The axes of teeth11, 12, and 13 were constructed by connecting the yand z coordinates of the nodes at the root apex andcrown (middle of the incisor edge or canine tip; Fig 3,C and D). The y and z coordinates of each node after dis-placement of the teeth were calculated by adding theinitial displacement of the node that was magnified 20times to the y and z coordinates of each node beforedisplacement.

When a force is applied to a tooth, initial movementis produced, and then orthodontic movement starts.Therefore, it is important to clarify the forces appliedto the teeth and the stresses produced in the PDL at ini-tial movement. If the magnitudes of these forces andstresses are not suitable to produce bone remodeling,orthodontic movement might not start.

In this study, hydrostatic stresses were calculated toinvestigate the tensile or compressive status of the PDL:sH 5 (s1 1 s2 1 s3)/3, where s1, s2, and s3 denotethe principal stresses.21,22

RESULTS

Low OMI traction with 0.019 3 0.025-in SS wire inthe labial, base, and lingual models was measured. Forthe system with 0-mm CC and 0-mm ARH, teeth 11,12, and 13 were tipped lingually by en-masse retractionin all 3 models. As the height of the ARH increased (2or 5 mm), lingual tipping of teeth 11 and 12 reducedmore than with the 0-mm ARH (Fig 4, Table III). Theincisor axes, however, did not reach the original axesin the 3 models. For the 3-mm CC, tooth 13 reachedits original axis, but there was no effect on tooth 11 inany model.

Page 4: Effective en-masse retraction design with orthodontic mini ... · during space closure is important for maximum retraction of the anterior teeth after premolar extractions.6,7 OMIs

Table II. The components of 200-g retraction force from ARH to OMI

Traction type Low OMI traction High OMI traction

ARH height 0 mm 2 mm 5 mm 0 mm 2 mm 5 mm 8 mm

Fx (g) 87.85 89.24 90.83 97.01 101.05 106.33 110.27

Fy (g) 169.85 172.50 175.60 142.91 148.86 156.64 162.44

Fz (g) 58.47 47.60 30.28 100.00 87.56 64.50 38.22

Fz/Fy (%) 34.40 27.60 17.20 70.00 58.80 41.20 23.50

Fx, Amount of force in the direction of X axis of the FE model; Fy, amount of force in the direction of Yaxis of the FE model; Fz, amount of force in

the direction of Z axis of the FE model; Fz/Fy, ratio of Fz and Fy.

Fig 3. CR for the 6 anterior teeth (base model) and tooth axes. A, Wire splinting of the 6 anterior teeth.B, Deformed model (grey contour) shown with original model (white mesh). The initial displacement(magnified 400 times) showed bodily movement by a given intrusion (white arrow) or retraction (blackarrow) force. C and D, The axes of teeth 11-13 were constructed from the y and z coordinates of thenodes at the root apex and crown and compared with the graph in the y-z plane (D).

American Journal of Orthodontics and Dentofacial Orthopedics Sung et al 651Volume 137, Number 5

With low OMI traction with 0.016 3 0.022-in SSwire, the lingual tipping tendencies in the labial,base, and lingual models were similar to that observedfor the 0.019 3 0.025-in SS archwire. For the 2-mmARH and 0-mm CC condition, the tooth axes after dis-placement in the 3 models were compared (Fig 5,Table IV). Because 0.016 3 0.022-in SS wire ismore flexible than 0.019 3 0.025-in wire, there wasmore labial tipping of tooth 12 and more distal tippingof tooth 13. A 3-mm CC in the 0.016 3 0.022-in SSwire did not reduce distal tipping of tooth 13 morethan with the 0.019 3 .025-in SS wire (Fig 6, TablesIII and IV).

The effects of high OMI traction in the base modelwere reduced distal tipping of tooth 13 and lingual tip-ping of tooth 12 compared with low OMI traction con-

ditions. The axis of tooth 13 was almost independent ofARH. The axes of teeth 11 and 12 did not reach theiroriginal axes (Fig 7, Table IV).

The force vector can be applied just above the CRfor the 6 anterior teeth for the high OMI traction and8-mm ARH condition. Instead of bodily retraction,tooth 11 intruded with controlled tipping, and tooth 12intruded, and the root apex moved more lingually. Tooth13 had uncontrolled tipped, and the crown extruded (Fig8, A; Table IV).

For the 2-mm ARH and 100-g midline vertical trac-tion condition, the force vectors were above the CR forthe 6 anterior teeth; all teeth were intruded and slightlytipped labially (Fig 8, A; Table IV).

The PDL hydrostatic stress contour plot canshow tensile and compressive stress distributions.21

Page 5: Effective en-masse retraction design with orthodontic mini ... · during space closure is important for maximum retraction of the anterior teeth after premolar extractions.6,7 OMIs

Fig 4. Comparison of the effects of ARH and CC for 0.019 3 0.025-in SS archwire in the labial, base,and lingual models. Labial, labial model; Base, base model; Lingual, lingual model; ARH, anteriorretraction hook; CC, compensating curve; 019, 0.019 3 0.025-in SS wire; LOT, low orthodonticmini-implant traction.

Table III. Comparison of the initial tooth displacements in the y and z axes with 0.019 3 0.025-in SS wire in the basemodel

Tooth Axis

Original coordinates

Low OMI traction

0-mm ARH 5-mm ARH 5-mm ARH 1 3-mm CC

Crown Apex Crown Apex Crown Apex Crown Apex

11 y 45.8279574 33.5601520 �1.309E-01 3.096E-02 �8.872E-02 7.485E-04 �7.672E-02 �6.129E-03

z 0.5000000 21.5338045 �4.497E-02 4.803E-02 �8.569E-03 4.453E-02 �3.039E-04 4.272E-02

12 y 43.2892962 29.5725181 �1.280E-01 6.809E-03 �8.598E-02 �2.873E-02 �6.331E-02 �3.879E-02

z 0.2564025 20.0000000 �2.977E-02 6.440E-02 7.463E-03 5.023E-02 2.680E-02 4.172E-02

13 y 38.5166876 28.6921322 �5.911E-02 1.279E-02 �5.760E-02 2.184E-02 �1.405E-02 �8.599E-03

z �0.9141889 24.5369266 �1.295E-02 2.367E-02 �4.322E-02 1.257E-03 �1.623E-02 �3.915E-03

ARH, Anterior retraction hook; CC, compensating curve; E, times ten raised to the power, as (scientific) E notation.

652 Sung et al American Journal of Orthodontics and Dentofacial Orthopedics

May 2010

For the high OMI traction and 8-mm condition, thelingual apices of teeth 11 and 12, and the labioapicaland linguocervical thirds of tooth 13 were com-pressed areas. The highest tensile stress was distrib-

uted at the labiocervical third of tooth 13 (Fig 8,B-D). As the ARH increased from 2 to 8 mm, theARH and main archwire joint deformation increased(Fig 8, E and F).

Page 6: Effective en-masse retraction design with orthodontic mini ... · during space closure is important for maximum retraction of the anterior teeth after premolar extractions.6,7 OMIs

Fig 5. Comparison of the effect of 0.016 3 0.022-in SSarchwire for 2-mm ARH, 0-mm CC, and low OMI tractionconditions. Base, base model; Labial, labial model;Lingual, lingual model; 016, 0.016 3 0.022-in SS.

American Journal of Orthodontics and Dentofacial Orthopedics Sung et al 653Volume 137, Number 5

DISCUSSION

Premolar extraction and maximum en-masse retrac-tion are preferred for patients with lip protrusion toachieve a harmonious lower profile.3,6 Without torquecontrol, however, the inclination of the incisor will bereduced, and the amount of retraction will beincreased (Fig 1, B). To maintain or control the inclina-tion of the incisor, we can build additional torque in thearchwire or use high-torque brackets. Reinforcement ofincisor torque, however, might cause incisor extrusionor posterior anchorage loss.23 The OMI can be an effi-cient tool, not only in solving the anchorage problembut also in controlling anterior torque.

Because OMIs are positioned apically to the crownat the proximal root space, they can generate intrusiveforce vectors. For en-masse retraction with the lowOMI traction and the 2-mm ARH condition, the ARHbetween the lateral incisor and the canine is the pointof force application. The intrusive force component(30-60 g, Table II), therefore, was applied anteriorlyto the CR for the 6 anterior teeth and was expected toinduce a counterclockwise moment that would counter-balance the distal tipping of the incisors (Fig 1, B).

The optimum combination of the position of OMIthat can generate more intrusive force and the lengthof ARH that makes the application of the force throughCR is necessary to control incisor inclination.

Initially, we needed to investigate the CR for the 6anterior teeth in the base model to apply the force closeto the CR. The CR varies among patients, depending onroot length, alveolar bone support, and number ofteeth.11-13,15,24 The teeth of our FE model werereproduced from a dental study model and arrangedaccording to the Roth arch form.

The initial tooth displacement in this study was be-tween 9.815E-4 mm and –1.309E-1 mm (Tables III andIV). We magnified the amount of tooth displacement400 times to judge the bodily displacement of thecentral incisor axis superiorly and posteriorly (Fig 3,B). The location of the CR in the base model varieddepending on the material properties of the PDL andalveolar bone. This was approximately 1 to 2 mm inthe y-z plane. We used the newly calibrated Young’smodulus of the PDL (5 g/mm2).15,20

Pedersen et al12 reported that the CR for 6 anteriorteeth was located on a line 3 mm behind the distal sur-face of the canines in a study of human autopsy mate-rial. Melsen et al13 deduced the CR for 6 anteriorteeth from other studies. Their estimated CR was half-way between the midpoint of the 4 incisors’ CR andthe canines’ CR. The CR for our base model was located13.5 mm posteriorly and 9 mm superiorly from the cen-ter of the archwire, similar to the estimate of Melsenet al13 (Figs 3, B, and 8, A).

When the ARH increased, the line of action of the forceshould be closer to the CR for the 6 anterior teeth.14,25 Thevertical force component generated by the verticaldifference between the ARH and the OMI, however, willbe decreased. Clinically, deflection of the ARH bya retraction force might cause gingival impingement;hence, the ARH (cantilever beam) of a 0.05-in SS wasmodeled to minimize the deflection.

In sliding mechanics, the dimensions of the mainarchwire can vary according to the bracket slot size toreduce friction of the posterior wire. Generally, 0.0193 0.025-in SS wire is recommended for a 0.022-inslot and 0.016 3 0.022-in SS wire for a 0.018-in slot.In a combination of a main wire and a 0.05-in SSARH, the bending moments are highest at the joint (sup-ported end) of the relatively flexible main archwire, andthe deformation of the main archwire will induce labialtipping of the lateral incisor and distal tipping of thecanine (Fig 8, E and F).26

The high bending moments developed depend moreon the length of the ARH and the flexibility of the mainarchwire (Figs 4, A-C, 6, and 8, A). Even though theline of action of the force was applied close to the CRfor the 6 anterior teeth with an 8-mm ARH, the centraland lateral incisors were not bodily retracted. Figure 8,B-D, shows that the maximum PDL compressive stress

Page 7: Effective en-masse retraction design with orthodontic mini ... · during space closure is important for maximum retraction of the anterior teeth after premolar extractions.6,7 OMIs

Table IV. Comparison of the initial tooth displacements in the y and z axes with 0.016 3 0.022-in SS wire, high OMItraction, and MVT in the base model

Tooth Axis

Low OMI traction High OMI traction

5 mm ARH (016*022) 5 mm 8-mm ARH 2-mm ARH 1 100-g MVT

Crown Apex Crown Apex Crown Apex Crown Apex

11 y �8.452E-02 9.815E-04 �6.876E-02 �1.365E-02 �4.938E-02 �2.623E-02 �1.326E-02 �5.070E-02

z �8.112E-03 4.320E-02 8.920E-03 4.358E-02 2.443E-02 4.105E-02 5.748E-02 3.183E-02

12 y �8.239E-02 �3.942E-02 �5.087E-02 �4.636E-02 �3.685E-02 �5.839E-02 �2.262E-02 �4.611E-02

z 2.378E-02 5.287E-02 4.101E-02 4.126E-02 4.860E-02 3.607E-02 4.813E-02 3.094E-02

13 y �6.814E-02 3.208E-02 �1.193E-02 �5.582E-03 �3.267E-02 1.561E-02 1.049E-02 �2.525E-02

z �5.576E-02 �2.676E-03 �1.200E-02 1.570E-03 �4.700E-02 �1.298E-02 2.015E-02 1.324E-02

The y and z coordinates of each node after the displacement of the teeth. ARH, anterior retraction hook; CC, compensating curve; MVT, midline

vertical traction; 016, 0.016 3 0.022-in SS wire; E, times ten raised to the power, as (scientific) E notation.

Fig 6. Differences between 0.016 3 0.022-in SS and0.019 3 0.025-in SS wires for the 5-mm ARH conditionin the base model. 016, 0.016 3 0.022-in SS; 019,0.019 3 0.025-in SS; CC, compensating curve.

Fig 7. Comparison of the effects of high OMI tractionand ARH for the 0.019 3 0.025-in SS wire in the basemodel.

654 Sung et al American Journal of Orthodontics and Dentofacial Orthopedics

May 2010

(–0.62 g/mm2) was distributed on the lingual root apicesof teeth 11 and 12 from intrusion and retraction. The highPDL tensile stress (0.67 g/mm2) was on the labiocervicalthird of tooth 13 because of extrusion and lingual tipping.

The forces generated by the reverse curve of Speeplaced in the mandibular achwire transmitted both in-trusive and torquing forces to the roots of the incisorsand molars, encouraging lingual movement of the inci-sor roots and mesial movement of the molar roots.16,27

To overcome the limitations of the ARH, CCs can beplaced in the maxillary archwire to encourage labialtipping and intrusion of the incisors. However, the

binding between the bracket slot and curved archwiremight restrict sliding in the posterior slot. Hence, weassumed a clinically applicable and minimum CC(depth of the curve from the distal aspect of thecanine to the distal aspect of the second molar wasabout 0.5 mm) to reduce friction in the 0.016 3

0.022-in and 0.019 3 0.025-in SS archwires.In a previous FEA study, the deactivation force from

the curved archwire was analyzed first.16 The reactionforces that corresponded to the mesial and distal nodesof each bracket were then substituted for CC simulation.However, this approach might cause a large error if the

Page 8: Effective en-masse retraction design with orthodontic mini ... · during space closure is important for maximum retraction of the anterior teeth after premolar extractions.6,7 OMIs

Fig 8. Comparison of the effects of various force applications above the CR with the 0.019 3 0.025-inSS wire: A, comparison of tooth displacements of 8 mm for the ARH or midline vertical traction con-dition; B-D, contour plots of hydrostatic stress (g/mm2) in condition of 8-mm ARH and high OMI trac-tion; comparison of archwire deformation (magnified 20 times) and von Mises stress (g/mm2)distribution between E, 2-mm ARH, and F, 8-mm ARH and high OMI traction. ARH, anterior retractionhook; CC, compensating curve; MVT, midline vertical traction; 019, 0.019 3 0.025-in SS archwire; ,CR for the 6 anterior teeth in the base model; red arrow, 200-g force vector from 8-mm ARH to highOMI traction; blue arrow, resultant force vector from 100-g force of MVT and 200-g force of 2-mmARH to high OMI traction.

American Journal of Orthodontics and Dentofacial Orthopedics Sung et al 655Volume 137, Number 5

archwire is curved much to the out-of-plane directionbecause the accurate nodal positions of the archwireafter connecting brackets cannot be known in advance.

In this study, to reduce such errors, we placed smallV-bends in the archwire between the brackets to simu-late the CC effect. Compared with the previous study,the CC effect on the dentition was similar.9 The 3-mmCC encouraged distal tipping of the molar and mesialtipping of the canine, and counteracted the distal tippingof the canine caused by side effects of the ARH. Noadditional labial tipping of the incisors occurred.

To produce a more vertical intrusion force compo-nent than low OMI traction, the OMI can be placed atthe mesial and apical sides of the second premolar(high OMI traction, Figs 1 and 7). In high OMItraction, as ARH increased, the force and the z-y ratioswere decreased which was also observed in low OMItraction. For the same ARH height, however, theaverage Fz/Fy ratio was increased by 218%, and a 5-mm ARH produced the highest ratio (239.5%) (Table II).

Although the line of action of the force from 5-mmARH to high OMI traction passed under the CR for the 6anterior teeth, lingual tipping of the incisors and distaltipping of the canine (caused by increased ARH) werereduced (Fig 7). Labial tipping of the central incisors

beyond their original axes, however, did not occur inthe high OMI traction condition.

In the 8-mm ARH and high OMI traction condition,the line of action of the force passed above the CR forthe 6 anterior teeth without decreasing the Fz and theFz/Fy ratios compared with the 5-mm ARH and lowOMI traction condition (Table II). The central incisor,however, remained in a lingual position with respectto the original axis, and there was less tipping of the lat-eral incisor and the canine (Figs 8).

The design of en-masse retraction appliances forthe bodily retraction of 6 anterior teeth so that theline of action of the force passes through the CR is,therefore, desirable. The design should include rigidsplinting of the 6 anterior teeth to prevent deforma-tion between the ARH and the main archwire11,15

and a bilaterally connected rigid palatal lever armthat enables force application around the CRwithout deflection of the ARH and gingivalimpingement.14,25

Placing an OMI between the incisors helps to applyadditional vertical intrusion forces, and we can expecta more direct effect on the central incisor (Fig 1,C).1,5 In the 2-mm ARH, high OMI traction, andmidline vertical traction conditions (100 g force

Page 9: Effective en-masse retraction design with orthodontic mini ... · during space closure is important for maximum retraction of the anterior teeth after premolar extractions.6,7 OMIs

656 Sung et al American Journal of Orthodontics and Dentofacial Orthopedics

May 2010

applied at the center hook) in the base model, theresultant retraction force vectors passed above the CRfor the 6 anterior teeth and were more verticallydirected than under the 8-mm ARH and high OMItraction condition (Fig 8, A). With 100 g of midline ver-tical traction, the 6 anterior teeth tended to tip labiallyand intrude. Labial tipping of the central incisor oc-curred; this was not observed under the ARH and CCconditions.

An additional OMI to apply midline vertical tractionmight place a burden on the patient with regard to moresurgical procedures, but it is effective in inducing labialtipping of the incisors and intrusion of the 6 anteriorteeth. It could, therefore, be the treatment of choicefor patients with deep bite and those who need maxi-mum en-masse retraction.

These results suggest that en-masse bodily move-ment of anterior teeth seems to be difficult with conven-tional sliding mechanics by using OMI. However, theseresults are right only for the initial movement, which isproduced by elastic deformation of the PDL. This isa limitation of this study. Long-term orthodontic move-ment might not be the same as the initial movement.Especially when many teeth are connected with an arch-wire, the force system varies with tooth movement.

CONCLUSIONS

1. For the 0-mm CC and ARH condition, the centraland lateral incisors and the canine were tippedlingually in all 3 models. As the height of theARH increased, lingual tipping of the central andlateral incisors was reduced. With the 3-mm CC,the canine axis reached its original axis, but therewas no effect on the central incisor axis in anymodel.

2. In the system with 0.016 3 0.022-in SS wire and5-mm ARH, labial tipping of the lateral incisorand distal tipping of the canine were worse com-pared with the system with the 0.019 3 0.025-inSS wire.

3. For the high OMI traction and 8-mm ARH condi-tion, the force vector was applied just above theCR for the 6 anterior teeth, but no bodily retractionoccurred.

4. For the 2-mm ARH and 100-g midline verticaltraction condition, the central and lateral incisorsand the canine were intruded and slightly tippedlabially.

REFERENCES

1. Creekmore TD, Eklund MK. The possibility of skeletal anchor-

age. J Clin Orthod 1983;17:266-9.

2. Umemori M, Sugawara J, Mitani H, Nagasaka H, Kawamura H.

Skeletal anchorage system for open-bite correction. Am J Orthod

Dentofacial Orthop 1999;115:166-74.

3. Park HS, Bae SM, Kyung HM, Sung JH. Micro-implant anchor-

age for treatment of skeletal Class I bialveolar protrusion. J Clin

Orthod 2001;35:417-22.

4. Park YC, Lee SY, Kim DH, Jee SH. Intrusion of posterior teeth

using mini-screw implants. Am J Orthod Dentofacial Orthop

2003;123:690-4.

5. Kim TW, Kim H, Lee SJ. Correction of deep overbite and gummy

smile by using a mini-implant with a segmented wire in a growing

Class II Division 2 patient. Am J Orthod Dentofacial Orthop 2006;

130:676-85.

6. Burstone CJ. The segmented arch approach to space closure. Am J

Orthod 1982;82:361-78.

7. Guray E, Orhan M. En masse retraction of maxillary anterior teeth

with anterior headgear. Am J Orthod Dentofacial Orthop 1997;

112:473-9.

8. Bennett JP, McLaughlin RP. Orthodontic treatment mechanics

and the preadjusted appliance. Alyesbury, United Kingdom:

Wolfe Publishing; 1993. p. 13-26.

9. Jeong HS, Sung SJ, Moon YS, Cho YS, Lim SM. Factors influenc-

ing the axes of anterior teeth during SWA en masse sliding retrac-

tion with orthodontic mini-implant anchorage: a finite element

study. Korean J Orthod 2006;36:339-48.

10. Chung KR, Nelson G, Kim SH, Kook YA. Severe bidentoalveolar

protrusion treated with orthodontic microimplant-dependent en-

masse retraction. Am J Orthod Dentofacial Orthop 2007;132:

105-15.

11. Dermaut LR, Vanden Bulcke MM. Evaluation of intrusive me-

chanics of the type "segmented arch" on a macerated human skull

using the laser reflection technique and holographic interferome-

try. Am J Orthod 1986;89:251-63.

12. Pedersen E, Isidor F, Gjessing P, Andersen K. Location of centres

of resistance for maxillary anterior teeth measured on human

autopsy material. Eur J Orthod 1991;13:452-8.

13. Melsen B, Fotis V, Burstone CJ. Vertical force consider-

ations in differential space closure. J Clin Orthod 1990;

24:678-83.

14. Hong RK, Heo JM, Ha YK. Lever-arm and mini-implant system

for anterior torque control during retraction in lingual orthodontic

treatment. Angle Orthod 2005;75:129-41.

15. Reimann S, Keilig L, Jager A, Bourauel C. Biomechanical finite-

element investigation of the position of the centre of resistance of

the upper incisors. Eur J Orthod 2007;29:219-24.

16. Sung SJ, Baik HS, Moon YS, Yu HS, Cho YS. A comparative

evaluation of different compensating curves in the lingual and la-

bial techniques using 3D FEM. Am J Orthod Dentofacial Orthop

2003;123:441-50.

17. Yu HS, Baik HS, Sung SJ, Kim KD, Cho YS. Three-dimen-

sional finite-element analysis of maxillary protraction with

and without rapid palatal expansion. Eur J Orthod 2007;29:

118-25.

18. Jeong SJ, Kim WS, Sung SJ. Numerical investigation on the flow

characteristics and aerodynamic force of the upper airway of

patient with obstructive sleep apnea using computational fluid

dynamics. Med Eng Phys 2007;29:637-51.

19. Ziegler A, Keilig L, Kawarizadeh A, Jager A, Bourauel C. Numer-

ical simulation of the biomechanical behaviour of multi-rooted

teeth. Eur J Orthod 2005;27:333-9.

20. Poppe M, Bourauel C, Jager A. Determination of the elasticity pa-

rameters of the human periodontal ligament and the location of

the center of resistance of single-rooted teeth a study of autopsy

Page 10: Effective en-masse retraction design with orthodontic mini ... · during space closure is important for maximum retraction of the anterior teeth after premolar extractions.6,7 OMIs

American Journal of Orthodontics and Dentofacial Orthopedics Sung et al 657Volume 137, Number 5

specimens and their conversion into finite element models.

J Orofac Orthop 2002;63:358-70.

21. Dorow C, Sander FG. Development of a model for the simulation

of orthodontic load on lower first premolars using the finite ele-

ment method. J Orofac Orthop 2005;66:208-18.

22. Hohmann A, Wolfram U, Geiger M, Boryor A, Sander C, Faltin R,

et al. Periodontal ligament hydrostatic stress with areas of root re-

sorption after application of a continuous torque moment. Angle

Orthod 2007;77:653-9.

23. Mulligan TF. Common sense mechanics. J Clin Orthod 1980;14:

180-9 contd.

24. Shroff B, Lindauer SJ, Burstone CJ, Leiss JB. Segmented ap-

proach to simultaneous intrusion and space closure: biomechanics

of the three-piece base arch appliance. Am J Orthod Dentofacial

Orthop 1995;107:136-43.

25. Park YC, Choy K, Lee JS, Kim TK. Lever-arm mechanics in lin-

gual orthodontics. J Clin Orthod 2000;34:601-5.

26. Thurow RC. Edgewise orthodontics. St Louis: C.V. Mosby; 1982.

p. 19-25.

27. Clifford PM, Orr JF, Burden DJ. The effects of increasing the re-

verse curve of Spee in a lower archwire examined using a dynamic

photo-elastic gelatine model. Eur J Orthod 1999;21:213-22.