ORIGINAL ARTICLE

Photoelastic stress analysis of mandibularmolars moved distally with the skeletalanchorage systemAtsushi Nakamura,a Tsuyoshi Teratani,b Hidemi Itoh,c Junji Sugawara,d and Hiroyuki Ishikawae

Fukuoka and Sendai, Japan

Introduction: The purpose of this study was to analyze photoelastically the stress distribution around teethin the simulated distal movement of mandibular molars with the skeletal anchorage system. Methods: Twotypes of the photoelastic mandibular dentition models were used, 1 before and 1 after distal movement of thesecond molar. The experiment was performed with 3 forms of traction—first-molar single traction,second-molar single traction, and simultaneous first- and second-molar traction. The direction of tractionwas set parallel to the occlusal plane and at an angle of 30° downward to the occlusal plane. Results: In thefirst-molar single traction model, extremely high stress was generated around the first molar with tractionparallel to the occlusal plane. With the traction 30° downward to the occlusal plane, all models showed thestress around the molars extended distally and downward. Conclusions: Simultaneous traction of the firstand second molars might be preferable to the sequential traction of each molar to prevent the unfavorabledistal tipping of the first molar. Regardless of whether simultaneous or sequential traction is used, thedownward traction to the occlusal plane seems to induce intrusion of the molars as well as their distal

movement. (Am J Orthod Dentofacial Orthop 2007;132:624-9)

The distal movement of mandibular molars isoften required to reduce mandibular anteriorcrowding and to retract protruded anterior teeth

without extracting premolars. Several orthodontic ap-pliances have been used to move the mandibular molarsdistally, including headgear,1,2 the lip bumper,3,4 theJones jig,5 and the Franzulum appliance.6 However,effectiveness of these appliances can be compromisedby limited patient cooperation or reciprocal forces thatcause inappropriate anchor-tooth movement.

Recently, Sugawara7,8 and Daimaruya et al9,10 de-veloped the skeletal anchorage system (SAS) that usespure titanium anchor plates and screws as orthodonticanchorage units. The SAS permits rigid anchorage viaosseointegration of both anchor plates and screws.

aPostgraduate student, Division of Orthodontics, Department of Oral Growthand Development, Fukuoka Dental College, Fukuoka, Japan.bInstructor, Division of Orthodontics, Department of Oral Growth and Devel-opment, Fukuoka Dental College, Fukuoka, Japan.cAssistant professor, Division of Advanced Prosthetic Dentistry, GraduateSchool of Dentistry, Tohoku University, Sendai, Japan.dAssociate professor, Division of Orthodontics and Dentofacial Orthopedics,Graduate School of Dentistry, Tohoku University, Sendai, Japan.eProfessor and chairman, Division of Orthodontics, Department of Oral Growthand Development, Fukuoka Dental College, Fukuoka, Japan.Reprint requests to: Atsushi Nakamura, Division of Orthodontics, Departmentof Oral Growth and Development, Fukuoka Dental College, 2-15-1, Tamura,Sawara-ku, Fukuoka, 814-0193, Japan; e-mail, atussy@college.fdcnet.ac.jp.Submitted, July 2005; revised and accepted, November 2005.0889-5406/$32.00Copyright © 2007 by the American Association of Orthodontists.

doi:10.1016/j.ajodo.2005.11.041

624

Clinical reports described the distal movement of themandibular molars with anchor plates placed at theanterior border of the mandibular ramus or the man-dibular body.11-16 Preferable ages for SAS treatmentwere not indicated. However, the SAS is used mostlyfor adult patients because osseointegrated implants caninhibit surrounding bone growth in children.17,18 Formandibular molar distalization, extraction of the thirdmolars is frequently required to create sufficient spacebehind the second molar.15

Although the SAS is becoming popular in orth-odontic practices, few published studies have focusedon distal movement of the mandibular molars in SAStreatment.15,16 Furthermore, no biomechanical studieshave demonstrated the stresses produced in the peri-odontal tissues around teeth from SAS use. The pur-pose of this study was to photoelastically analyze theinitial stress distribution around the teeth in the simu-lated distal movement of mandibular molars with theSAS to increase our understanding of its likely effectsin the clinical setting.

MATERIAL AND METHODS

Two photoelastic models of a mandibular dentitionwere used in this study, 1 simulating the position ofthe second molar before its distal movement, and theother simulating its position after distal movement. The

photoelastic models was fabricated as follows. First, a

American Journal of Orthodontics and Dentofacial OrthopedicsVolume 132, Number 5

Nakamura et al 625

plaster model of a mandible with dentition (P10-SB.1;Nissin, Kyoto, Japan) was created by using a siliconimpression (Duplicone; Shofu, Kyoto, Japan). The plas-ter models were then set up to simulate the postlevelingstage with no crowding, spacing, or flattening of thecurve of Spee. In 1 model, the second molar waspositioned distally to simulate its completed distalmovement. A n impression was then taken by usingsilicon impression material, and anatomic models ofthe mandibular teeth (B2-306; Nissin) were adaptedto the impression surface. Finally, urethane (Soli-thane C113-300; Thiokol Chemical Corporation, Tren-ton, NJ) was infused into the impression with the modelteeth. The fabricated photoelastic models were dividedon the left side in the sagittal plane to permit stressevaluation. The models were fixed on an acrylic plate,and it was confirmed that no initial stress had beeninduced. The physical properties of each componentmaterial of the model are shown in the Table.

Figure 1 shows the 2 photoelastic mandibular den-tition models, 1 before and 1 after distal movement ofthe second molar. A preformed 0.019 � 0.025-instainless steel archwire was adapted to the mandibulardentition model, and 0.022 � 0.028-in preadjustededgewise brackets were fixed to each archwire withelastic modules. The brackets were then bonded to the

Table. Physical properties of each component materialof the model

Material E (MPa) �

Tooth Epoxy resin 2,930 0.36Surrounding tissue Urethane (solithane) 6.9 0.40

E, Young’s modulus; �, Poisson’s ratio.

Fig 1. Two photoelastic mandibular dentition models:a, before distal movement of second molar, and b, afterdistal movement.

model teeth with light-curing adhesive. The distal ends

of the archwire were adjusted to be less than 1 mmbehind the molar tube.

A bar was fixed behind the model to vary the directionof the distalizing forces. The force was applied to a tooththrough a coil spring, by using an elastic chain extend-ing from the bar (Fig 2). The experiment was per-formed with 3 forms of traction—first-molar singletraction, second-molar single traction, and simulta-neous first-and-second molar traction. First-molar sin-gle traction was performed on the model, simulating thesecond molar position after its distal movement. Theother tractions simulated the second molar positionbefore its distal movement. The direction of tractionwas set parallel to the occlusal plane and at an angle of30° downward to the occlusal plane (Fig 3). A force of250 g was selected because this is similar to that usedin a clinical situation for single molar traction,7 and itprovided a suitable response in the model withoutcausing undue deformation.

In this study, a quasi 3-dimensional technique wasused for the photoelastic observations. Figure 4 showsthe circular polariscope arrangement used to observeisochromatic fringes. In an area of stress, the fringeappears as a repeated pattern of yellow, blue, and red.A cycle from yellow to red is called 1-fringe order, andit is used as an index to judge the level of stress. Theisochromatic fringes occur 3-dimensionally in responseto the intensity of the internal stress generated in themodel. When the overall isochromatic fringe pattern isconsidered, interpretation is based on the followingprinciples: (1) the more fringes, the higher the stressintensity; and (2) the closer the isochromatic fringes areto each other, the higher the stress concentration.17 Toobserve isochromatic fringes easily by preventing lightreflection and refraction, the model was soaked in amineral oil with a refractive index equal to that of themodel material.18

RESULTS

In the photoelastic analysis of the first-and-secondmolar simultaneous traction model (Fig 5), when theforce was applied parallel to the occlusal plane, stresswas generated along the mesial and distal root surfacesof the first and second molars. In the second molar, thestress was distributed along the entire root surface,whereas, in the first molar, the stress was concentratedon the cervical half of the mesial root surface and alongthe apical half of the distal root surface (Fig 5, a).

With the traction 30° downward to the occlusalplane, the stress was generated along the distal rootsurface of the first and second molars, and extended

distally and downward (Fig 5, b).

, mod

American Journal of Orthodontics and Dentofacial OrthopedicsNovember 2007

626 Nakamura et al

In the photoelastic analysis of the second-molarsingle traction model (Fig 6), with traction parallel tothe occlusal plane, the stress was generated along theentire distal root surface of the second molar andextended distally with the intensity of 2-fringe order(Fig 6, a).

With the traction 30° downward to the occlusalplane, the stress was generated around the apex andalong the distal surface of the distal root of the secondmolar and extended distally and downward (Fig 6, b).

In the photoelastic analysis of the first-molar singletraction model (Fig 7), with traction parallel to theocclusal plane, extremely high stress was concentratedon the middle part of the mesial root surface of the firstmolar with the intensity of a 3-fringe order. Stress of a2-fringe order was observed along the apical half of thedistal root surface of the first molar (Fig 7, a).

With the traction 30° downward to the occlusalplane, the stress was less on the mesial root surface of

Fig 2. Mandibular molar distalization on photoligature wire; d, elastic chain.

Fig 3. Direction of traction: a, parallel to oc

Fig 4. Circular polariscope arrangement useddiffuser; P, polarizer; Q, quarter-wave plate; M

the first molar compared with when traction was

applied parallel to the occlusal plane. High stress wasgenerated around the apex of the first molar andextended distally and downward (Fig 7, b).

DISCUSSION

Photoelastic stress analysis allows visualization ofinternal stresses with colored patterns. In this study, weused the quasi 3-dimensional photoelastic techniquedevised by Caputo and Standlee.19 This technique hasthe advantages of using models with good geometricfidelity and of being able to apply multiple complexforce systems to the models. Moreover, the models canbe used repeatedly because they are not destroyed whenobtaining the photoelastic data. With this technique,some studies showed stress distributions in the cranio-facial complex or the periodontal tissues induced byorthodontic forces.20-32

Using the photoelasticity modeling concept, Caputoand Standlee19 demonstrated that it is impossible to

model: a, sliding hook; b, open coil spring; c,

l plane; b, 30°downward to occlusal plane.

erve isochromatic fringes. Ls, Light source; D,el.

elastic

clusa

to obs

model all mechanical properties of a structural element;

us tra

n mo

mode

American Journal of Orthodontics and Dentofacial OrthopedicsVolume 132, Number 5

Nakamura et al 627

they proposed that a decision must be made aboutwhich properties are most pertinent to the clinicalquestion at hand. In the photoelastic model used in thisstudy, the periodontal membrane and the alveolar bonewere not assembled separately, and the entire periodon-tal structure was made from urethane with a low elasticmodulus and a high optical sensitivity, so that theinternal stress induced by several hundred grams oforthodontic force would be easily detectable.

The anchor plate was not assembled directly in themodel to avoid undue deformation of the model anddisturbance of the stress generated around the plate thatcould influence stress distribution around the teeth.

In the first-and-second molar simultaneous tractionand the first-molar single traction models, the stressaround the first molar appeared to induce distal tippingwhen the force was applied parallel to the occlusalplane. The intensity and the concentration of the stressaround the first molar were both higher in the first-molar single traction model than in the 2-molar simul-

Fig 5. First- and second-molar simultaneo

Fig 6. Second-molar single tractio

Fig 7. First-molar single traction

taneous traction model, supposedly because of lack of

contact with the second molar in the single tractionmodel. When the force was applied at an angle of 30°downward to the occlusal plane in both models, thestress appeared to induce intrusion of the molars as wellas their distal movement; the horizontal component ofthe force appeared to move the molars distally, whereasthe vertical component acted to intrude them viaarchwires. The 30° downward traction reduced thedistal tipping effects of the force, and this might be inpart the result of the downward deflection of archwiresoccurring at the mesial side of the first molars; this actsto resist their distal tipping.

In the second-molar single traction model, the stressappeared to induce bodily movement when the forcewas applied parallel to the occlusal plane. A similarstress distribution was found around the second molaras in the first-and-second molar simultaneous tractionmodel. In both models, the distal tipping of the secondmolar appeared to be prevented by the posterior part ofan archwire, which was not easily deflected because of

ction model: a, parallel; b, 30° downward.

del: a, parallel; b, 30° downward.

l: a, parallel; b, 30° downward.

its short length. The 30° downward traction in the

American Journal of Orthodontics and Dentofacial OrthopedicsNovember 2007

628 Nakamura et al

second-molar single traction model appeared to showstress distribution with an intrusive effect resultingfrom the downward component of the traction force.

For the distal movement of the mandibular molarwith the SAS, there are 2 kinds of procedures: first-and-second molar simultaneous traction and sequentialtraction when the first molar is moved distally after thesecond molar has been moved. The stress analysis inthis study suggested that, irrespective of the tractiondirection, there are no differences in tooth movementbetween simultaneous and sequential traction of eachtooth, except for the distal tipping of the first molar.Based on these results, the first molar appears to besusceptible to distal inclination during single traction.To prevent this, the simultaneous traction of the firstand second molars might be preferable.

The force direction from the anchor plate is subjectto some restriction because traction hooks of the platemust be positioned to avoid contact with the maxillaryteeth and the gingivae as well as the archwires, and toprevent injury to the cheek or the gingivae. Thus, thetraction direction for distal movement of the mandibu-lar molar is likely to be downward to the occlusal plane,exerting some intrusive force on the molars. This seemsto be advantageous in the treatment of skeletal Class IIIopen-bite patients requiring distal movement and intru-sion of the mandibular molars. However, in the treat-ment of skeletal Class III deepbite patients, we mustconsider the intrusive forces generated from the down-ward traction to the occlusal plane. In such cases, theparallel traction applied to the occlusal plane must beattempted by adjusting the position and the figure of theanchor plate hooks or the sliding hooks on the archwire.

CONCLUSIONS

Quasi 3-dimensional photoelastic stress analysis wasperformed for the simulated distal movement of themandibular molar with the SAS. First- and second-molarsimultaneous traction might be preferable to the sequentialtraction of each molar to prevent the unfavorable distaltipping of the first molar. Regardless of whether simulta-neous or sequential traction is used, downward traction tothe occlusal plane seems to induce intrusion of the molarsand their distal movement.

REFERENCES

1. Arun T, Erverdi N. A cephalometric comparison of mandibularheadgear and chin-cap appliances in orthodontic and orthopaedicview points. J Marmara Univ Dent Fac 1994;2:392-8.

2. Orton HS, Sullivan PG, Battagel JM, Orton S. The managementof class III and class III tendency occlusions using headgear to

the mandibular dentition. Br J Orthod 1983;10:2-12.

3. Grossen J, Ingervall B. The effect of the lip bumper on lowerdental arch dimensions and tooth positions. Eur J Orthod1995;17:129-34.

4. Davidovitch M, McInnis D, Lindauer SJ. The effects of lipbumper therapy in the mixed dentition. Am J Orthod DentofacialOrthop 1997;111:52-8.

5. Uner O, Haydar S. Mandibular molar distalization with the Jonesjig appliance. Kieferorthop 1995;9:169-74.

6. Byloff F, Darendeliler MA, Stoff F. Mandibular molar distaliza-tion with the Franzulum appliance. J Clin Orthod 2000;34:518-23.

7. Sugawara J. Orthodontic mechanics for the movements ofthe molars with skeletal anchorage system (SAS) utilizingminiplates. Orthodontics Year Book 2000. Tokyo: Quintessence;2000. p. 203-9.

8. Sugawara J. JCO interviews Dr Junji Sugawara on the skeletalanchorage system. J Clin Orthod 2000;33:689-96.

9. Daimaruya T, Nagasaka H, Umemori M, Sugawara J, Mitani H.The influences of molar intrusion on the inferior alveolarneurovascular bundle and root using the skeletal anchoragesystem in dogs. Angle Orthod 2001;71:60-70.

10. Daimaruya T, Takahashi I, Nagasaka H, Umemori M, SugawaraJ, Mitani H. Effects of maxillary molar intrusion on the nasalfloor and tooth root using the skeletal anchorage system in dogs.Angle Orthod 2003;73:158-66.

11. Jenner JD, Fitzpatrick BN. Skeletal anchorage utilising boneplates. Aust Orthod J 1985;9:231-3.

12. Sugawara J, Umemori M, Mitani H, Nagasaka H, Kawamura H.Orthodontic treatment system for Class III malocclusion using atitanium miniplate as an anchorage. J Jpn Orthod Soc 1998;57:25-35.

13. Sugawara J, Baik UB, Umemori M, Nagasaka H, Kawamura H,Mitani H. Treatment and posttreatment dentoalveolar changesfollowing intrusion of mandibular molars with application of askeletal anchorage system (SAS) for open bite correction. Int JAdult Orthod Orthognath Surg 2002;17:243-53.

14. Umemori M, Sugawara J, Mitani H, Nagasaka H, Kawamura H.Skeletal anchorage system for open-bite correction. Am J OrthodDentofacial Orthop 1999;115:166-74.

15. Sugawara J, Daimaruya T, Umemori M, Nagasaka H, TakahashiI, Kawamura H, et al. Distal movement of mandibular molars inadult patients with the skeletal anchorage system. Am J OrthodDentofacial Orthop 2004;125:130-8.

16. Nagasaka H, Sugawara J, Kawamura H, Kasahara H, UmemoriM, Mitani H. A clinical evaluation on the efficacy of titaniumminiplates as orthodontic anchorage. Orthod Waves 1999;58:136-47.

17. Liu WX, Matsui Y, Takaesu K, Shionoya Y, Ohno K, Michi K.Experimental study on influences of implant therapy on mandib-ular growth. J Jpn Stomatol Soc 2002;51:359-65.

18. Thilander B, Odman J, Grondahl K, Lekholm U. Aspects onosseointegrated implants inserted in growing jaws: a biometricand radiographic study in the young pig. Eur J Orthod 1992;14:99-109.

19. Caputo AA, Standlee JP. Biomechanics in clinical dentistry.Chicago: Quintessence; 1987. p. 23.

20. Matsui S, Caputo AA, Hayashi H, Katayama K, Kiyomura H.Effects of L-loops of the multiloop edgewise archwire with classII elastics: a comparison with the ideal archwire. J Jpn OrthodSoc 1997;56:383-90.

21. Caputo AA, Chaconas SJ, Hayashi RK. Photoelastic visualiza-tion of orthodontic forces during canine retraction. Am J Orthod

1974;65:250-9.

American Journal of Orthodontics and Dentofacial OrthopedicsVolume 132, Number 5

Nakamura et al 629

22. Baeten LR. Canine retraction: a photoelastic study. Am J Orthod1975;67:11-23.

23. Chaconas SJ, Caputo AA, Davis JC. The effects of orthopedicforces on the craniofacial complex utilizing cervical and head-gear appliances. Am J Orthod 1976;69:527-39.

24. De Alva JA, Chaconas SJ, Caputo AA, Emison W. Stressdistribution under high-pull extraoral chin cup traction. A pho-toelastic syudy. Angle Orthod 1982;52:69-78.

25. Chaconas SJ, Caputo AA. Observation of orthopedic forcedistribution produced by maxillary orthodontic appliances. Am JOrthod 1982;82:492-501.

26. Kawagoe H, Itoh T, Hirota Y, Kubota A, Hirose T, MatsumotoM, et al. Photoelastic effects of maxillary protraction on thecraniofacial complex. J Jpn Orthod Soc 1984;43:337-45.

27. Hirose T, Kawagoe H, Kubota A, Watanabe T, Itoh T, Matsu-moto M. Photoelastic comparison of various maxillary protrac-

tion force vectors. J Jpn Orthod Soc 1985;44:660-8.

28. Itoh T, Chaconas SJ, Caputo AA, Matyas J. Photoelastic effectsof maxillary protraction on the craniofacial complex. Am JOrthod 1985;88:117-24.

29. Chaconas SJ, Caputo AA, Miyashita K. Force distributioncomparisons of various retraction archwires. Angle Orthod1989;59:25-30.

30. Shetty V, Caridad JM, Caputo AA, Chaconas SJ. Biomechanicalrationale for surgical orthodontic expansion of the adult maxilla.J Oral Maxillofac Surg 1994;52:742-9.

31. Yoshimura O, Matsui S. A mechanical study on multiloopedgewise archwire (MEAW): a comparison with ideal archwireduring combined use of vertical elastics. Orthod Waves 1999;58:214-21.

32. Teratani T, Ohta F, Itoh H, Ishikawa H. Photoelastic stressanalysis of orthodontic force during upper first molar distaliza-tion with the pendulum appliance and distal jet. Orthod Waves

2003;62:1-11.