Photoelastic Stress Analysis of Upper Molars IntrusionUsing Temporary Anchorage Device (TAD)

Yuichiro OTSUKA§, Tomoko MOMOI, Michi OI,Yosuke SAKURAI, Shigeyuki MATSUI and Haruhide KANEGAE

Division of Orthodontics, Department of Human Development & Fostering, Meikai University School of Dentistry

Abstract : Recently, the intrusion of maxillary posterior arch segment, which was quite difficult to perform in the past, has also

become possible using Temporary Anchorage Device (TAD), its mechanism is still unclear. In the present study was performed in

order to elucidate the biomechanical influence of upper molars intrusion by TAD.

Material and Methods : A photoelastic model of an adult human maxilla was fabricated using birefringent materials, and the

molar segment was attached with sectional arch. Assumed the implant site to be around zygomatic arch, the direction of traction

was set so as to cross at right angles to the occlusal plane from the center of 1st molar bracket to the stress direction. By setting

the loading condition at 100 gf, 150 gf, 200 gf and 250 gf respectively. Stresses that developed in the supporting structure were

monitored photoelastically and recorded photographically.

Results : At all loading conditions, it was observed that a predominant stress concentrated at the 1st molar’s root apex. How-

ever, stress concentration was not observed in the supporting tissue around the first and second premolar teeth and the second mo-

lar tooth as analysis, except the loading condition at 250 gf.

Conclusion : The traction force to upper molars intrusion was revealed to be limited to the 1st molar at the initial loading de-

spite. Concerning the changes in the stress concentration in response to the increased traction force, the stress at the 1st molar

roots was augmented.

Key words : temporary anchorage device (TAD), upper molars intrusion, biomechanics, photoelastic stress analysis

光弾性実験法による矯正歯科用インプラント（TAD）を応用した上顎臼歯部圧下時における応力解析

大塚雄一郎§ 桃井 知子 大井 迪櫻井 洋介 松井 成幸 鐘ヶ江晴秀明海大学歯学部形態機能成育学講座歯科矯正学分野

要旨：近年，インプラント体を固定源とした歯の移動を行う手法が考案され，多くの施設で臨床応用されている．その結果，過去において困難とされてきた方向および量の歯の移動が可能となった．さらに外科的矯正治療適応症例に対し，非外科的に適切な咬合関係の獲得が得られるなど日常臨床において多大な恩恵を受けている．だが，新しい手法であることから力学的検討はほとんどなされておらず，特に上顎臼歯部の圧下に関する詳細は未だ不明である．そのため歯の移動のメカニクスは術者の経験則にたよるのが現状である．そこで今回我々は，スケレタルアンカッレッジによる上顎臼歯部圧下の力が歯根部周囲支持組織に与える力学的影響を明らかにすることを目的として本研究を行った．【材料と方法】実験モデルは解剖学的形態を等倍大に模した片側上顎歯型モデルを光弾性材料により作製し，臼歯部にセクショナルアーチを装着した．本研究における力学的解析の手法は擬似三次元光弾性法を用いた．矯正力はインプラント埋入部位を頬骨弓下陵付近に想定し，第一大臼歯ブラケット中央から圧下方向へ咬合平面に対し直交するように設定した．荷重条件はそれぞれ 100 gf, 150 gf, 200 gf, 250 gf とし，初期荷重時における歯根部周囲支持組織における内部応力差

明海歯学（J Meikai Dent Med）39（2）, 53−58, 2010 53

Introduction

Open bite is one of the most difficult malocclusionto treat in orthodontic therapy. Many orthodontistshave proposed and applied treatment methods basedon various ideas in past years. When a skeletal im-provement was targeted in an adult severe skeletalopen bite case, on the other hand, SSRO or Two-jawsurgery by means of combined Le Fort I osteotomyand SSRO have been applied following establishmentof treatment strategy for deformed jaw in many insti-tutions with good outcomes1−3). However, a surgical or-thodontic treatment may have some risk not only fromthe medical but also social aspects including relativelylarge surgical invasion or necessity of prolonged hos-pitalization as compared with non-surgical orthodontictreatment. Recently, it has become possible to move atooth toward a direction which had so far beendeemed to be impossible with the aid of a newly de-vised tooth movement approach using skeletal anchor-age system. This tooth-movement method using animplant as an anchorage has now been applied clini-cally in a lot of office since its introduction by Creek-more4) in 1983. By using an implant as an anchorage,such a tooth-movement as those of which directionand distance were considered difficult by conventionalorthodontic treatment has become possible5, 6). In addi-tion, a conventional concept concerning the orthodon-tic treatment has surely changed including non-

surgical acquisition of optimal occlusion in patients sofar deemed to be a candidate for surgical orthodontictreatment. Main implants now used for this methodare roughly divided into two types, screw and platetypes having a known different application mode eachother. The implant of screw type is advantageous inits simplified procedure of use and predominantlyused for reinforced anchorage at molar segment or in-trusion of anterior teeth, but said to be inappropriatefor active tooth-movement at molar segment becauseof potential dropout due to poor implantation. On theother hand, the implant of plate type can be investedinto the jaw tightly although its surgical treatment atinvestment is relatively high. It is therefore consideredto be most suitable as an implant for application intru-sion or distal movement of molar segment7). Thoughthe intrusion of maxillary posterior arch segment,which was quite difficult to perform in the past, hasalso become possible, its mechanism is still unclear.

In the present study was performed in order to elu-cidate the biomechanical influence of upper molars in-trusion by TAD.

Material and Method

1. Experimental modelAn experimental model to study photoelasticity

used in this study was prepared as below according toa procedure described by Matsui et al. 8). The experi-mental model was a one-sided maxillary dentitionmodel including the molar area where was the objectarea of this study and imitated anatomic configurationof the maxillary dentition in the same size. Epoxy arti-ficial teeth were employed as a material for tooth sec-

について解析を行った．【結果】すべての荷重条件において第一大臼歯根尖部に著明な応力集中が認められた．一方，第一小臼歯，第二小臼歯，第二大臼歯において著しい応力集中は認められなかった．【結論】インプラント体から上顎第一大臼歯直下方向への臼歯部圧下の矯正力は，初期荷重時においてセクショナルアーチを装着しているにもかかわらず，第一大臼歯に限局することが明らかとなった．また矯正力の増大に伴う応力集中の変化についても第一大臼歯部の応力は増加するものの，それ以外の歯への応力の変化はごくわずかであった．すなわち，同手法による圧下方向への矯正力は増大するに従い，第一大臼歯のみに過度の矯正力を与える可能性が考えられた．

索引用語：矯正歯科用インプラント（TAD），臼歯部圧下，生体力学，光弾性実験

─────────────────────────────§Correspondence : Yuichiro Otsuka, Division of Orthodontics, De-

partment of Human Development & Fostering, Meikai UniversitySchool of Dentistry, 1-1 Keyakidai, Sakado, Saitama, 350-0283,Japan

54 OTSUKA Y, MOMOI T, OI M, et al J Meikai Dent Med 39, 2010

tion, and polyurethane, which is a photoelastic experi-mental material, was employed as a material support-ing section of the model (Table 1). After preparationof the dentition model, an edgewise appliance and abracket were attached on the first and second premolarteeth, respectively, and a tube was attached on thefirst and second molar teeth with bonding agent, usingthe FA-point9) as the point of reference. Then, sec-tional arch (0.019×0.025 inch, stainless steel wire)was passively applied to the model. The experimentalmodel was secured on near the center of a brass plateof 100×100×5.0 mm using epoxide-based adhesivewith facing the crown side upward. Then, a jig madeof a brass tube with outside diameter 3.0 mm and in-ternal diameter 2.0 mm was set as a traction site as-sumed to be an implanting site on the brass platewhere directly beneath of buccal mesio-distal middleof the first molar tooth. To avoid the loading proce-dure from becoming cumbersome and complicated, themodel was securely bolted down on a laboratory tablewith facing the crown side upward so that the buccalface of the molar tooth and a camera align for follow-ing analysis (Fig 1).2. Analysis method

In this study, a quasi 3-dimensional technique wasused for the photoelastic observations. All loading wasperformed in the field of a circular polariscope, withthe model immersed in a tank of mineral oil to facili-tate isochromatic fringe observation. hese isochromaticfringes are distributed 3-dimensionally in response tothe intensity of internal stress. Stress is high in an areawhen many lines are seen and concentrated when dis-tance between fringes is small8, 10). Fig 2 shows the cir-cular polariscope arrangement used to observe iso-chromatic fringes.3. Loading condition

Load on a segment of the upper molar was set with

assuming that the implanting site locates around thelower border of the zygomatic arch, and so that thedirection of traction crosses at right angle to the oc-clusal plane from a bracket attached to mesio-distalmiddle of the first molar crown to the direction of de-pression. Load of 100, 150, 200, and 250 gf was ap-plied through the jig using weights. Difference in in-ternal stress of the supporting tissue around the rootfor each load was analyzed.

Results

Significant stress concentration was observed in theperiapical supporting tissue of the first premolar toothin all loading conditions. However, stress concentra-tion was not observed in the supporting tissue aroundthe first and second premolar teeth and the secondmolar tooth as analysis results of 100, 150, and 200 gloading. When the load was further increased to 250g, which is the maximum load, a slight stress concen-tration was observed around the periapical tissue ofthe second premolar tooth and the second molar tooth.No stress concentration was observed around the pe-riapical tissue of the first premolar tooth even with themaximum 250 g loading (Fig 3 a b c d).

Discussion

In recent years, a skeletal anchorage system of platetype has been used in a border case between ortho-dontic and surgical orthodontic treatments to have animproved open bite through mandibular counter-clockwise rotation by molar intrusion in order to avoidsuch a risk as reported by Sugawara et al. 5). An appli-cation of orthodontic treatment combined with skeletalanchorage system to such a case as sited above is ex-pected to reduce a patient’s burden from various as-pects in the therapeutic content. In addition, its needseems to have increased as a means for avoiding a or-thognathic surgery because of possible intentional andeffective tooth movement.

As described above, needs for depression techniqueof posterior teeth using TAD are increasing. however,the technique has many unclear points about mechani-cal aspects and there are only few reports about it. Be-cause it is a newly devised manipulation, almost no

Table 1 Elastic modulus of simulant materials

Materials E(MPa)

ToothPeriodontal membrane

EpoxyPolyurethane

29317

E(MPa) : Modulus of elasticity

Stress analysis of upper molars intrusion 55

Fig 1 A experimental model and force system.As for the traction approach, assuming the impacted implant

site in the Keyridge (vicinity of zygomatic arch), it was settledso as to converge at a jig established from tube at the firstmolar to the zygomatic arch immediately below the molar. Fig 2 Circular polariscope arrangement for visualization

of isochromatic fringes.LS : light source D : diffuser P : polarizer M : modelQ : quarter-wave plate

Fig 3a Fig 3b

Fig 3c Fig 3d

Fig 3 Photoelastic visualization of intrusive force.a. Load of 100 gf b. 150 gf c. 200 gf d. 250 gf

Fig 4 Schematic representation of biomecanical influences.

56 OTSUKA Y, MOMOI T, OI M, et al J Meikai Dent Med 39, 2010

mechanical examination has been performed and espe-cially, the details of the upper molar’s intrusion. Forthis reason, it is the status quo that the mechanics oftooth movement is relied on the operator’s empiricalknowledge alone. To the best of our knowledge, thereis only report by Chung et al. 11). Describing retrospec-tive measurement of tooth movement by intrusion ofmaxillary posterior teeth using models before and afterthe treatment, and there is no report on mechanicalstudy using numeric analysis and experimental me-chanical analysis techniques. Therefore, stress analysison mechanics of depression of maxillary posteriorteeth in the periapical supporting tissue at the firstloading was conducted through photoelastic stressanalysis using a maxillary dentition model, and an in-dex to predict movement of the molar segment wasobtained in this study. As a result, it is clarified thatstress to each periapical tissue caused by orthodonticforce of intrusion of the molar area towards directlybeneath of the maxillary first premolar tooth was re-stricted and concentrated on the first premolar tooth,nevertheless, the orthodontic force was loaded ontothe molar segment as a group of molar teeth by apply-ing a sectional arch at the first loading. For changes inthe stress TAD concentration zone associated with in-crease of the load, stress to the supporting tissuearound the first molar tooth was increased ; however,changes in distribution of stress concentration zone atother than the first molar tooth were quite small. Asdescribed above, it is demonstrated that stress at thetime of loading to the first molar tooth was not uni-formly distributed to the supporting tissue around theteeth in the sectional arch. In other words, this resultimplies that the above mechanics cannot always de-press all four teeth uniformly. Therefore, we consid-ered about factors that could affect these results as be-low.

First, flexure of a wire connecting four teeth of themolar segment is suspected as a factor affecting.Theoretically, orthodontic force toward the depressingdirection at the orthodontic force is decreased whenthe wire of the sectional arch is bent to form into anarch with directly loaded first molar tooth as the bot-tom. In addition, orthodontic force towards the overe-

ruption direction is generated onto the first premolartooth around wire end when this flexure of the wirewas further increased. As mentioned above, the impacton this analysis is unignorable when the flexure islarge because of inadequate stiffness of wire. How-ever, the brackets, tubes, and wires used in this studywere brackets and tubes with 0.022-inch slots, andstainless steel wire of 0.019×0.025 inch, which areroutinely used in combination with TAD in clinicalpractice and are one of the thickest wires as rectangu-lar wire used for multi-bracket appliances. The reasonfor selecting this wire size was to obtain the maxi-mum wire stiffness against load of non-reciprocal or-thodontic force, which is applied from absolute andsecured source using implant. Also, we presumed thatimpact on the result of this study would be very smallbecause a maximum loading used in this study was250 g and elastic modulus of the stainless steel wire is197 GPa, therefore flexure of wire would not large.However, it is suggested that there are possibilitiesthat the wire is affected by the effect of the above-mentioned flexure caused by increase of the degree offlexure as a result of use of thinner wire than thatused in this study, or increase of orthodontic force.Accordingly, it may be clinically required to selectwire size with caution based on this result.

The most important factor that should be consideredaffecting the result of this study includes effect ofplay between bracket slots and the wire. According toa study on torque by Meyer et al. 12), when 0.019×0.025 inch stainless steel wire was inserted into abracket with 0.022 inch slots, 10.5°of play was oc-curred between the bracket slots and wire by torquegeneration. Thus, it is suggested that the difference inmoving patter of teeth depending on traction site re-gardless of uniformly loaded single segment may beresulted by that such amount of play interacts with notonly single but every teeth through the wire.

Significant stress concentration was also observedin this study as a result of lack of the effect of playcaused by direct loading to the maxillary first molartooth, where is the traction site. On the other hand, itis considered that direction and strength of orthodonticforce was changed because the orthodontic force is in-

Stress analysis of upper molars intrusion 57

directly loaded to the first and second premolar teethand the second molar tooth with being affected byplay between the bracket slots and the wire, and, as aresult, adequate orthodontic force, which is enough togenerate discernible stress concentration around apicalarea, was not loaded for each teeth. Consequently, or-thodontic force provided stress distribution thatslightly inclines the second premolar tooth and thesecond molar tooth, which are both adjacent teeth ofthe first molar tooth, towards mesio-distal middle andthe first molar teeth direction by effect of the three-dimensional play of the bracket slots even in a maxi-mum loading of 250 g, but expected effect only to-wards intruding direction against the entire segmentwas not obtained as shown in Fig 4. Furthermore, apotential risk of adding an excess orthodontic force onthe 1st molar alone was suggested with the increasedtraction force under upper molars intrusion by TAD.As mentioned above, it is suggested that play betweenthe bracket slots and the wire needs to be dealt tomove the entire molar segment towards intrusion di-rection, for example, by providing a wire curved inopposite direction.

Conclusions

・The traction force to upper molars intrusion was re-vealed to be limited to the first molar at the initialloading despite that the molar region was served asinterconnected firmly by sectional arch.・Concerning the changes in the stress concentration

in response to the increased traction force, thechange of stress in other teeth was all minimal, al-though the stress at the first molar roots was aug-mented. Namely, a potential risk of adding an ex-cess orthodontic force on the first molar alone wassuggested with the increased traction force under

upper molars intrusion by TAD.

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(Received May 13, 2010 ; accepted June 16, 2010)

58 OTSUKA Y, MOMOI T, OI M, et al J Meikai Dent Med 39, 2010