ONLINE ONLY

Photoelastic analysis of forces generated byT-loop springs made with stainless steel ortitanium-molybdenum alloy

Luiz Guilherme Martins Maia,a Mila Leite de Moraes Maia,b Andr�e da Costa Monini,a Alexandre Prot�asio Vianna,a

and Luiz Gonzaga Gandini, Jrc

Araraquara, S~ao Paulo, and Aracaju, Sergipe, Brazil

aPostgEstadbPostcProfelista,DentiThe aucts oReprinmoniacom.bSubm0889-Copyrdoi:10

Introduction: The purpose of this study was to use photoelastic analysis to compare the system of forces gen-erated by retraction T-loop springs made with stainless steel and titanium-molybdenum alloy (TMA) (Ormco,Glendora, Calif) with photoelastic analysis. Methods: Three photoelastic models were used to evaluate retrac-tion T-loop springs with the same preactivations in 2 groups. In group 1, the loop was constructed with a stainlesssteel wire, and 2 helicoids were incorporated on top of the T-loop; in group 2, it was made with TMA and no he-licoids.Results:Upon using the qualitative analysis of the fringe order in the photoelastic model, it was observedthat the magnitude of force generated by the springs in group 1 was significantly higher than that in group 2.However, both had symmetry for the active and reactive units related to the system of force. Conclusions:Both springs had the same mechanical characteristics. TMA springs showed lower force levels. (Am J OrthodDentofacial Orthop 2011;140:e123-e128)

Orthodontists have used many devices to close theremaining spaces after tooth extractions. One ofthem is the T-loop spring suggested by Burstone

in 1982.1 Among its advantages, the low load-deflectionratio (L/D) is significant. Because of the shape of thespring, which incorporates much wire in its form, andwith 0.017 3 0.025-in titanium-molybdenum alloy(TMA) wire, it is possible to work with a greater activa-tion for a longer time and a relatively low force, whencompared with other springs with high L/D.1-3

The moment-to-force ratio (M/F) established by theT-loop spring relating to the center of resistance of thetooth and the force can be changed by the orthodontist,according to the preactivated bends. This results inbetter control of the axial movement and makes itpossible to select different types of movements.4

raduate student, Faculdade de Odontologia de Araraquara, Universidadeual Paulista, Araraquara, S~ao Paulo, Brazil.graduate student, Federal University of Sergipe, Aracaju, Sergipe, Brazil.ssor, Faculdade de Odontologia de Araraquara, Universidade Estadual Pau-Araraquara, S~ao Paulo, Brazil; adjunct clinical professor, Baylor College ofstry, Dallas, Tex, and Saint Louis University, St Louis, Mo.uthors report no commercial, proprietary, or financial interests in the prod-r companies described in this article.t requests to: Luiz Gonzaga Gandini, Jr, Av Casemiro Perez, 560, Vila Har-, Araraquara, S~ao Paulo, Brazil, CEP 14802-600; e-mail, luizgandini@uol.r.itted, November 2010; revised and accepted, March 2011.5406/$36.00ight � 2011 by the American Association of Orthodontists..1016/j.ajodo.2011.03.020

The force system generated by the T-loop spring iscontrolled by the integration of the preactivated bends,the activation amount, and the position of the spring inthe interbracket distance. Hoenigl et al5 evaluateda centralized T-loop force system by first activating itat the maximum level and then deactivating it graduallyuntil it reached the lowest level of deactivation. Theyconcluded that the force system generated by this typeof spring provides movements from a controlled crowntip-back to a radicular correction.

The efficiency of this type of spring was already shownby clinical evidence,6 and its power system was evaluatedby mechanical testing4,5,7-9 and software.10-12 Photo-elastic analysis is a widely used optical technique forexamining and measuring stress distribution instructures exposed to internal or external forces. In thistechnique, polarized light is transmitted througha photoelastic active material, which is doubly refractivewhen stressed. When the emergent light waves areviewed through an analyzer filter, the stress patternsappear as fringes or bands of color.13

In dentistry, photoelasticity was introduced by Zak14

in 1935 during the study in which he assessed the type oftooth movement, strength, and point of application ofthe forces. In orthodontics, this technique has beenused to examine the stresses induced during canine re-traction with other devices that do not use a T-loopspring and with lingual appliances to evaluate stresses

e123

Fig 1. The photoelastic model free of tension.

Fig 2. Preactivated T-loop springs: A, T-loop springmade of SS with helicoids incorporated; B, T-loop springmade of TMA wire without helicoids in its design.

Fig 3. Polariscope machine and it’s components: A, thelight producer; B, polarizers; C, photoelastic model andthe digital camera (D) used to obtain photoelastic fringesimages.

e124 Maia et al

created by occlusal forces in the periodontal tissues andthose produced by orthopedic forces.14

Based on the above-mentioned findings, in this studywe focused on objective evaluation through qualitativeanalysis in a photoelastic model of the force systemsgenerated by the centralized T-loop made with stainlesssteel (SS) and TMA (Ormco, Glendora, Calif) wires, both0.017 3 0.025 in.

MATERIAL AND METHODS

The photoelastic model was prepared by positioningthe 2 teeth separated by a distance of 27 mm (Fig 1).Once the models were constructed, crisscross tubeswere placed on the crowns by using a cylindrical drillat low rotation, and a vertical slot was made where thetubes were placed with acrylic resin.

September 2011 � Vol 140 � Issue 3 American

To obtain high confidence of the results, 2 groupsof T-loops were built, and the tests were repeatedthrice. For the first group, 3 springs were made ofSS wire including helicoids to reduce the L/D ratio(Fig 2, A). For the second group, the springs weremade of 0.017 3 0.025-in TMA wire (Fig 2, B) withno helicoids (Ormco). All T-loop springs were madewith a template of 10-mm length and 7-mm height.Preactivation bends were incorporated on all springs.7

On the apical base, the angle was of 45�, and, on itshorizontal extension, the bends were made symmetri-cally, totaling 187�.

After checking them in a neutral position, the springswere placed in the horizontal slot of the tubes, central-ized at a 27-mm distance. The T-loops were then evalu-ated in 3 activations: 5, 2.5, and 0 mm (neutral position).

The tests were performed by using a circular polari-scope (Fig 3), which comprised a light system (Fig 3, A),2 polarizers (Fig 3, B), a support to handle the photoelas-tic model (Fig 3, C), and a digital camera to record theresults (Fig 3, D).

After activating the T-loop, the fringe order in theradicular surface was read through the interface of 2colors, violet and blue, in the distal, mesial, and apicalareas of each tooth. This fringe order was displayed ingraphs for each surface: mesial, distal, and apical.

RESULTS AND DISCUSSION

In this study, we intended to evaluate comparativelythe force systems generated by centralized T-loopsprings made of SS and TMA wires (0.017 3 0.025 in)in 3 activations (0, 2.5, and 5 mm), shown in Figures

Journal of Orthodontics and Dentofacial Orthopedics

Fig 5. TMAT-loop spring placedwith 2.5mmof activation.

Fig 6. TMA T-loop spring placed with 5 mm of activation.

Fig 7. SS T-loop spring placed with 0 mm of activation.

Fig 4. TMA T-loop spring placed with 0 mm of activation.

Maia et al e125

4, 5, and 6, respectively. To obtain similar L/D ratios ofthe 2 T-loop springs tested, helicoids were incorporatedto the SS T-loop spring design, shown in Figures 7, 8,and 9. The preactivation protocols were equal for bothTMA and SS springs.

The photoelastic model was chosen for this purposeinstead of the finite element method, because the fringesspontaneously generated during the photoelastic testexpress the forces released by each spring configurationin the 3 activations.12-14 These fringes also afforda means of visualizing and analyzing the forcesexerted at the various areas of root surfaces.12 The finiteelement method is a kind of computer-aided test simu-lation that can show the force systems generated by theT-loop springs, based on the records placed on the soft-ware used for this objective.15,16 In this study, althoughthe T-loop springs were made of different alloys,

American Journal of Orthodontics and Dentofacial Orthoped

theoretically, the force system of the 2 T-loop springstested should be the same; thus, similar records wouldbe introduced in the software used with the finiteelement method analysis, and this situation wouldshow similar force releases instead of identifying anypossible differences according to the type of T-loopspring tested.2,4,5,7,9,17

According to the results of this study, this photoelas-tic model showed similar force systems in both sides.

The interpretations were descriptive; the readingswere made through graphics, with each graphic repre-senting a mesial, apical, or distal portion, separatelyanalyzed.

To standardize it and have the data readings moreunderstandable, the following nomenclature wasused: 13, left tooth to the observer; 23, right toothto the observer; A, apical area to the observer; M,

ics September 2011 � Vol 140 � Issue 3

Fig 8. SS T-loop spring placed with 2.5 mm of activation.

Fig 9. SS T-loop spring placed with 5 mm of activation.

Fig 10. Distribution of forces generated from SS andTMA T-loop springs activated at 2.5 mm over the mesialarea of the photoelastic model.

Fig 11. Distribution of forces generated from SS andTMA T-loop springs activated at 0 mm over the apicalarea of the photoelastic model.

e126 Maia et al

mesial area to the observer; and D, distal area to theobserver.

The photoelastic model was observed to be free oftension. In this case, the fringe order of 0.0 in all rootsurfaces for both teeth in the A, M, and D areas wascoincidently at zero point.

The SS T-loop generated a symmetric force system inall root surfaces. The fringe orders in the mesial (Fig 10)and apical (Fig 11) areas were close to 0.5, providinga higher amount of energy when compared with theTMA T-loop. It was observed that the amount of stressgenerated by the SS T-loop with helicoids extended tothe center of the model resulting in a fringe order of0.5, a phenomenon not observed in the same area ofthe model with the TMA T-loop, in which the fringeorder was 0.0. In the distal surface of the SS T-loop,

September 2011 � Vol 140 � Issue 3 American

the fringe order was close to 0.0, which was lowerthan the fringe order observed with the TMA spring(Fig 12).

Some scientific studies in mechanical tryouts demon-strated that the M/F ratio in the force system generatedby the T-loop during the activation of 2.5 mm provideda controlled tipping movement.2,18 In the photoelasticmodel, when evaluating the fringes formed in themesial area observed in Figure 13, both springs showedcharacteristics of controlled tipping movements withoutfringe in the disto-apical area. However, the T-loopmade of SS wire with helicoids in its design provideda higher stress release when compared with the T-loopmade of TMA wire.

The activation of 5 mm of the SS T-loop generateda higher fringe order when compared with the activationof 5 mm of the TMA T-loop. The symmetries were seenin both segments in the 2 groups of springs. In an in-creasing scale of stress in the mesial area, it variedfrom 0.0 in the area closer to the apex to 4.5 in the cer-vical area of the T-loop made of SS and 2.5 to the T-loopspring made of TMA (Fig 14). In the apical (Fig 15) anddistal (Fig 16) areas, the fringe order was greater than forthe SS T-loop.

Journal of Orthodontics and Dentofacial Orthopedics

Fig 12. Distribution of forces generated from SS andTMA T-loop springs activated at 0 mm over the distalarea of the photoelastic model.

Fig 13. Distribution of forces generated from SS andTMA T-loop springs activated at 2.5 mm over the mesialarea of the photoelastic model.

Fig 14. Distribution of forces generated from SS andTMA T-loop springs activated at 5 mm over the mesialarea of the photoelastic model.

Fig 15. Distribution of forces generated from SS andTMA T-loop springs activated at 5 mm over the apicalarea of the photoelastic model.

Maia et al e127

According to our mechanical tryouts, which evalu-ated the same kind of spring, it was observed that theforce system generated by the T-loop was similar toeither the retraction unit or the anchorage unit, knownas symmetric “V,” in which the alpha and beta unitsrelease similar forces and moments.5

In neutral position, 2.5 mm, and 5.0 mm, the resultsshowed that, even by including helicoids in the SS springdesign, it was not adequate to reduce the L/D ratio togenerate a similar amount of energy between thesprings.

When activating the T-loop at 5 mm, the M/F ratio didnot change according to the type of alloy used, and the lit-erature shows that the ratio is 7:1, generating a controlledcrown tipping movement, because its center of rotationwill be located close to the apex.2,5,10 After 1 mm ofdeactivation, the M/F ratio will be 9:1, and the teeth willmove by translation. If this deactivation is maintained,tooth movement will occur by root movement; at thistime, the spring should be reactivated to prevent contact

American Journal of Orthodontics and Dentofacial Orthoped

between the roots adjacent to the extraction site. Withthis study design, we could not evaluate all thesefindings reported in the literature. However, ourphotoelastic model showed energy concentration nearthe cervico-mesial area when the T-loop was in maximumactivation (5 mm). As deactivation occurred (2.5 mm), theenergy concentration moved to the mesial surface of theroot, near the tooth’s center of resistance. After deactiva-tion, the fringe order became greater at the root apex.These findings are supported in the orthodontic literature,which describes the sequence of movements related toT-loop deactivation from controlled crown tipping,passing by translation until the radicular correctionmovement.2,5,7,18

According to Burstone,1 the T-loop’s maximum acti-vation would be of 6 mm, the intermediate activation

ics September 2011 � Vol 140 � Issue 3

Fig 16. Distribution of forces generated from SS andTMA T-loop springs activated at 5 mm over the distalarea of the photoelastic model.

e128 Maia et al

would be 3 mm, and the neutral position would be0 mm. On the other hand, Marcotte17 stated that theT-loop’s maximum activation is 5 mm, the intermediateactivation is 2.5 mm, and the neutral position would be 2mmof activation. This was observed because of a gradualand smooth preactivation of the spring mentioned byBurstone and a more intense preactivation used byMarcotte. Considering these characteristics, we cansuggest that, during the absence of activation (neutralposition), the springs provided a root-movement ten-dency. In the intermediate activation of 2.5 mm, thesprings provided a translation-movement trend, and,in the maximum activation, it provided a controlledcrown-tipping movement.

CONCLUSIONS

The force system released by the springs showed anM/F ratio that was similar in both sides independent ofthe type of the alloy used to construct the springs.Considering the force magnitude, the T-loop made ofTMA showed a lower force magnitude when comparedwith the T-loop spring made of SS.

September 2011 � Vol 140 � Issue 3 American

REFERENCES

1. Burstone CJ. The segmented arch approach to space closure. Am JOrthod 1982;82:361-78.

2. Burstone CJ, Koenig HA. Optimizing anterior and canine retrac-tion. Am J Orthod 1976;70:1-19.

3. Gjessing P. Biomechanical design and clinical evaluation of a newcanine retraction spring. Am J Orthod 1985;87:353-62.

4. Kuhlberg AJ, Burstone CJ. T-loop position and anchorage control.Am J Orthod Dentofacial Orthop 1997;112:12-8.

5. Hoenigl KD, Freudenthaler J, Marcotte MR, Bantleon HP. The cen-tered T-loop—a newway of preactivation. Am J Orthod DentofacialOrthop 1995;108:149-53.

6. Martins RP, Buschang PH, Gandini LG Jr. Group A T-loop fordifferential moment mechanics: an implant study. Am J OrthodDentofacial Orthop 2009;135:182-9.

7. Manhartsberger C, Morton JY, Burstone CJ. Space closure in adultpatients using the segmented arch technique. Angle Orthod 1989;59:205-10.

8. Chen J, Markham DL, Katona TR. Effects of T-looop geometry onits forces and moments. Angle Orthod 2000;70:48-51.

9. RoseD, Quick A, SwainM,Herbison P.Moment-to-force character-istics of preactivated nickel-titanium and titanium-molybdenumalloy symmetrical T-loops. Am J Orthod Dentofacial Orthop2009;135:757-63.

10. Martins RP, Buschang PH, Viecilli R, Santos-Pinto A. Curvatureversus V-bends in a group B titanium T-loop spring. Angle Orthod2008;78:517-23.

11. Martins RP, Buschang PH, Martins LP, Gandini LG Jr. Optimizingthe design of preactivated titanium T-loop springs with loopsoftware. Am J Orthod Dentofacial Orthop 2008;134:161-6.

12. Viecilli R. Self-corrective T-loop design for differential spaceclosure. Am J Orthod Dentofacial Orthop 2006;129:48-53.

13. Glickman I, Roeber FW, Brion M, Pameijer JHN. Photoelastic anal-ysis of internal stresses in the periodontium created by occlusalforces. J Periodontol 1970;41:30-5.

14. Zak B. Photoelastiche analyse in der orthodontischen mechanik. ZStomatol 1935;33:22-37.

15. Sukumar N, Moes N, Moran B, Belytschko T. Extended finiteelement method for three-dimensional crack modeling. IntJ Numer Meth Engng 2000;48:1549-70.

16. Rubin C, Krishnamurthy N, Capilouto E, Yi H. Stress analysis of thehuman tooth using a three-dimensional finite element model.J Dent Res 1983;62:82-6.

17. Marcotte MR. Biomechanics in orthodontics. Philadelphia: BCDecker; 1990.

18. Burstone CJ, Pryputniewicz RJ. Holographic determination ofcenters of rotation produced by orthodontic forces. Am J Orthod1980;77:396-409.

Journal of Orthodontics and Dentofacial Orthopedics