Load-deformation characteristics of polycarbonate orthodontic brackets R. J. Dobrin,

D.M.D.,*

Philadelphia,

Pa.

I. 1. Kamel,

Ph.D.,*

and D. R. Musich,

D.D.S., M.S.**

F

or many years the best method of attaching orthodontic appliances to teeth has been to fit thin stainless steel band material to the specific tooth structure and to cement this orthodontic band into place. Although this system has many important advantages, such as efficiency and stability of attachments, it also has certain disadvantages, such as poor esthetics, possible loosening of the bands from the teeth, impingement on the gingiva, and time-consuming fitting and placement of the bands. Since the 1960’s, a system of directly bonding plastic orthodontic attachments to teeth has been under investigation by Newman,1-‘F Reteif,‘, 8 Muira,” and MizrahF I* and their co-workers. This type of attachment alleviates many of the aforementioned problems encountered with stainless steel bands. There are inherent problems involved in developing a direct bonding system: (1) An adhesive must be found that is strong, durable, nontoxic, and quick setting, (2) a plastic bracket must be produced that can successfully transmit the desired orthodontic forces to the crown of the tooth and thence t,o the root and surrounding alveolar bone, and (3) the method of placing these attachments should be less complicated and less time consuming than previous techniques using stainless steel bands. The brackets under investigation in this experiment were of the Siamese edgewise design, When the edgewise appliance is used with the mechanics proposed by AngleI’ and Tweed,]” rectangular wire has the abiliby to control tooth position in three planes of space. In edgewise orthodontic treatment which is nearing the fininshing phase, rectangular wire is frequently used to deliver mesiodistal root torque and also to deliver labiolingual root torque in order to This work was supported No. 86.99.8%[226]. *Department of Drexel University, **Department Pennsylvania,

24

by a grant

Metallurgical Philadelphia,

of Pediatric Philadelphia,

from

Engineering Pa. Dentistry, Pa.

the

National

and School

the of

Institute Biomedical Dental

of

Dental

Research,

Engineering

Group,

Medicine,

University

of

Polycnrbmate

Fig. 1. Side view in bracket design.

of

Type

I bracket

(A)

and

Type

II bracket

orthodontic

(B) illustrating

bmckets

the

25

difference

establish the proper axial inclination of the anterior teeth. This report is an attempt to measure quantitatively the torque-deformation characteristics of two edgewise designs of plastic brackets in order to see if they are effective in delivering mesiodistal and labiolingual forces to the tooth crown. An apparatus was designed to stress the brackets in an adjustable manner and to measure their subsequent plastic deformation. Materials

and

methods

Two types of commercially available 0.018 by 0.022 inch edgewise polymeric brackets were tested. The brackets were made by two commercial companies and will be designated Type I and Type 11. Both brackets were matle of clear, unreinforced polycarbonate material but had slight differences in design. In particular, Type II brackets appeared to have a thinner edgewise slot with parallel sides and a squa,re edge. Type I brackets appeared to have a slightly wider slot, more rounded edges, and more surface roughness than Type II brackets (Fig. 1). The material used to bond the bracket to the tooth was a commercially available polymethyl methacrylate adhesive. The adhesive was used in conjunction with a, pit-and-fissure sealant, as per the manufacturer’s recommendations. The edgewise wire used to stress the brackets was stainless steel orthodontic wire. The cross-sectional dimension of this wire was 0.018 by 0.025 inch. Bon&g procedure. The bracket was cemented to an extracted central incisor in accordance with the manufacturer’s suggested procedure as follows : The tooth was etched with phosphoric acid tooth conditioner, rinsed, and blown dry with a chip blower; liquid sealant was then applied to the tooth and polymerized under ultraviolet light. A drop of adhesive liquid was placed on the bracket to be mixed togethrr for apbonded. The two parts of methacrylatc adhesive \xrc proximately 7 seconds in the manufacturer’s recommended ratio. The lingual side of the bracket was then coated with the adhesive and pressed against the tooth for 30 seconds by hand pressure, and the adhcsivc was allowed to set for

26

iln~. J. Orthod. Jnnuary 1976

Dobriq Z

7

/

Y

X

Fig. 2. Free-body diagram of the subjected. The schematic illustration so that the forces illustrated in delivered by the loading apparatus

force and torque to which the of the maxillary central incisor this free-body diagram accurately shown in Fig. 3.

plastic brackets were is purposely inverted, depict the forces

between 5 minutes (the manufacturer’s recommended time) and 15 minutes. After familiarity with the system was gained, no adhesive failures were recorded. Testing appamtus. An apparatus was developed in this study in order to record the response of the orthodontic brackets to a predetermined combinat,ion of torques and forces. The system of torques and force applied to the brackets in this study is shown in the free-body diagram (Fig. 2). The torque component, T,, produces a flexural stress on the bracket which would be similar to that induced by clinical root torque in a labiolingual direction. The torque, T,, produces a rotational shearing stress in the bracket which would be similar to that induced by clinical tipping of the tooth in a mesiodistal direction. The force, F,, produces a vertical shearing stress in the bracket. T, and T, were made equal to each other and their value is 30F, in gram-millimeters. The apparatus was designed to simulate the above-mentioned system of force and torques in their assigned ratios. This apparatus allowed for the variation of the magnitude of these forces, thus enabling quantitation of the plastic deformation of the polymeric brackets. Fig. 3 shows a schematic diagram of the component arrangement used. F, and T, are produced by the weight of the load on pan A. T, is produced by the weight of the load on pan E. F. is equal to the weight on pan A plus the weight of pan A itself and the weight of rod B. The torque T, equals F, x 30 mm., while T, equals 2F, x 15 mm. In order to equalize the magnitudes T, and T,, the weight on pan E was twice the weight on pan A plus 5 Gm. to account for the weight of the rod B. Since no studies have been published which quantitate the deformation of orthodontic plastic brackets, it was of primary interest to study the deformation

Pol.ycnrbonate

orthodo?ltic

27

D.

t

60mm

brackets

UOmm+

&Rod

attached

C.Umversol

to edgawss

wwe

joint

D.~uuey E.Loadmg F.Edgswse

Fig.

3. Schematic

diagram

of

loading

Fig.

4.

diagram

of

measuring

Schematic

~a” wire

(.018x.025)

E.

apparatus. apparatus.

behavior of the selected brackets about the Y axis. This is in the direction necessary to achieve lingual root torquing, a common root movement in orthodontic treatment. Measurement of the bracket deformation was accomplished by simply measuring the angle of rotation of the orthodontic wire after loading and unloading pans A and E. The rotation of the wire was determined by focusing a 1)ea.m of light on a mirror firmly attached to pully II (Figs, 3 and 4). The light beam was then reflected onto a large circular scale which had a mirror at its center. The rotation of the wire and mirror attached to the pulley could be followed by the movement of the reflected beam on this circular scale. The difference between the two angles measured before and after the loading and unloading of the wire indicates twice the angle the wire had moved, thereby allowing a method of determining the degree to which the brackets had deformed. Tcstbg procedure. The displacement-versus-torque data were produced in the following manner: After cementation of the bracket to the tooth, the bracket was ligated to the loading apparatus. Loads in the proper ratios were placed on

Dobrin,

28 Table

I.

Data

Kamel, alid Musich obtained

during

creep

tests

under

a

constant

torque

of

--Initial to

change plastic after

Bra&et

A (Type H (Type C (Type

1) 1) II)

in

B &UC

rlefovmation 1 minute

(‘lrczn~~~

in

lo crc’ep time

B due oz’er (t)

2,000

gm.-mm. .---

_._-

Timi

(21

j

Totctl

(11 formation

clfter

time

(degrees)

(degrees)

(degrees)

7.75 13.50 11.25

9.00 5.00 18.50

16.75 IX.50 29.75

(t)

(ho‘uirs)

(is 17 68

the pans A and E in increments of 5 gm. from 0 to 100 gm. Each load was allowed to remain on the pans for 1 minute and then was removed. The angular position of pulley L) was measured in reference to its initial position. Each test was performed on a new bracket. A total of eight brackets were used to obtain the displacement-versus-torque data. A second test was performed to determine the amount of deformation of the brackets due to prolonged loading, that is, the amount of creep. Three brackets were subjected to a torque of 2,000 gm.-mm. for periods of time ranging from 17 to 69 hours. The weights were unloaded twice during the creep test to measure the angle of rotation. The first measurement, after 1 minute of loading, was designed to account for the initial displacement of the wire. The second reading was taken after loading the bracket for the time intervals indicated in Table I. Results

Figs. 5 and 6 show the graphs of the angular displacement versus torque for the two different brackets at room temperature and normal humidity. When the Type I bracket was loaded with increasing torque (T,), a deformation curve was produced which showed a linear relation between the amount of torque and the amount of deformation, whereas the Type II bracket deforms in P rather exponential manner with increasing torque. The angular displacements at 2,000 gm.-mm., however, are approximately the same for both brackets. (A moment of 2,000 gm.-mm. is considered significant because it is close to the clinical value for torquing central incisor rOOts.lJ) An attempt was then made to reinforce the Type 1 bracket before it was subjected to torque by placing the bonding adhesive completely over the wire and bracket. Although this type of reinforcement is not practical clinically, the findings in Fig, 7 show that there is considerably less deformation with this reinforcement than for the unreinforced bracket. Three sets of data were obtained for creep (Table I). They show that significant creep took place over a short period of time as compared with the time of orthodontic treatment, that is, 18 to 24 months. Discussion

There are two possible reasons for the differences observed in Figs. 4 and 5, which show that there is an apparent difference in the plastic deformation between the Type II bracket. and the Type I bracket : (1) a difference in material

Polycnrbmaate

orthodontic

brackets

29

05

-! i

0

+--i--I 0

315

+.&g-Y

1

675

175

I 1275

1575

1

I

1575

2115

APPLIEDTORQUE, TV(gm-mm)

Fig.

5. Graph

of the

torque

and

deformation

values

for

the Type

fig.

6. Graph

of the

torque

and

deformation

values

for

the Type

2115

bracket. I I bracket.

2115

Fig. 7. reinforced

Graph with

of the acrylic.

torque

and

deformation

values

for

Type

I bracket

after

it

was

or (2) a difference in design. Since both brackets are made of unreinforced polycarbona,te, material differences are ruled out as a possible cause. A closer look at the brackets (Fig. l), however, indicated a definite difference in their design, as was discussed earlier. It was thought that the differences in the shapes of the slots were partially responsible for the differences in the load-deformation behavior of the two brackets. Fig 8 shows brackets with a length of orthodontic wire in their edgewise slots. The contact between the wire and the slot walls covers a large area in the Type II brackets as compared to the Type I brackets. The partial contact in the latter is due to the rounded edges of the bracket and a slightly larger slot dimension. The variation in the load-deformation behavior shown in Figs. 4 and 5 may be partially attributed to the inexact fit of the wire in Type I brackets. This condition allows a smaller area to withstand the applied force, thus producing a greater stress on the bracket wings. Although no data for moist conditions were collected, it has been shown that, even with immersion in water at 212O F., the polycarbonates absorbed little water and had a very slight dimensional change.15 This could lead to the assumption that the temperature and humidity of the oral cavity would have a minimal effect on the mechanical behavior of the brackets; however, cyclic changes in temperature and alterations in pH may affect the bracket strength over a prolonged period of time. The reinforcement of the bracket was an attempt to modify their design. The wire fits into the edgewise Siamese bracket by means of four lugs, through which

Polycnrbonate

Fig. 8. A, Type bracket

with

an

I plastic edgewise

bracket wire

with in the

an slot.

edgewise

wire

orthodontio

in the

slot.

brackets

B, Type

II

31

plastic

it transmits forces. The lugs are situated so that there are two gingival and two occlusal to the wire with a space between each set of lugs. The reinforcement was an attempt to fill this space with the self-curing polymethyl methacrylate. Following this reinforcement, the distortion was markedly reduced (Fig. 7). This treatment eliminates the previously mentioned stress concentration by distributing the force over a larger area. This type of reinforcement is not appropriate for clinical use; therefore, other means of reinforcement should be considered: (1) modification of the bracket material itself, that is, the polycarbonate; (2) the addition of glass fibers (40 per tent by weight), which has been shown to increase the tensile strength of the polgcarbonatc from 12,000 psi to 26,000 psi.‘” (Other suitable fillers may also be found to decrease the rate and amount of creep and increase the modulus of elasticity of polycarbonate.) Also (3) bracket design should be altered so that the bracket would have greater strength than the present Siamese bracket. Even though there are only three sets of data on creep, it is fairly conclusive that the brackets distort with time under a constant stress of 2,000 gm.-mm. This is the most serious of the problems encountered. The amount of angular displacement of the bracket (in degrees) found after certain time intervals under dry conditions equalled or exceeded the amount of torque (in degrees) which an orthodontist would initially place in a rectangular wire during the finishing phase of treatment (10 to 28 degrees) (Table I). Therefore, much of the energy put into the wire would be expended in distorting the brackets, and thus the forces would not be transmitted to the root of the tooth, which is essential to accomplish the desired tooth movement. Although the load deformation resulting from simultaneous application of mesiodistal and labiolingual root-torquing forces to the polycarbonate brackets is quite informative, future studies should be designed to evaluate the load and creep characteristics of these forces when they are applied separately. This information would allow us to augment our understanding of the ability of plastic brackets to deliver forces to the roots during each specific phase of treatment.

Previous work on polycarbonatc~ has shown it to have a relatively low crrcp ratC BS compared to other plastics.‘” The data collcctccl i’or tllc> crc~p c*heI’actcristics of pol,vc*arbonatcs in general also show that when a tonsilc stress is applied at room tcmperaturt thcrc is a scllf-limiting nature to the amount, of deformation which hccom~ ncgligihlc al’tcr approsinl;ltrl,v ZOO ~OUIX" From these studies, it, seems that the creep characteristic oi’ pal>-carbonates poses a minimal problem when the loading is simple; liowcvcr, tlic‘ prcscnt st,utly has shown that the CV2J@XX loadin g used to accomplish orthoclc~ntic tooth movCment~ causes an esccssirc amount of creep in a relatively short time interval. The creep problem ma,v he greatly reduced by a number of mcthotls: (1) slight cross-linking by the USC of high-cncrg,v radiation, which has proved to he a successful trcatmcnt for other plastics. such as poIyclthylenc, and has r(‘suited in a great rcclnction of its c~rerp MioL7 ; (2) ilctditi011 of appropriate fillers can also lead to substantial reduction of creep. Conclusions

There arc numerous advantages that direct bonding techniques can provide for the orthodontist and his patient. The major potential advantage for the orthodontist is reduced chair time, and the major advantage for the patient is improved esthetics. Both of the foregoing advantages are dependent, however, on (1) a durable adhesive system that will not fail over the 18 to 24 months of orthodontic tooth movement and (2) the ability of the plastic, brackets to deliver the necessary forces to the roots of the teeth. Therefore, as new adhesive systems evolve we must be prepared to test their durability under the fluctuating conditions of the oral cavity. Also, as new polycarbonate orthodontic attachments are produced, it is necessary to assess the esthetic advantage they provide in light of their mechanical deficiencies, such as load deformation and creep. Although many of the more recent techniques USClight round wire and auxiliaries to accomplish all tooth movement, many orthodontists continue to use rectangular wire, particularly in the finishing phase of treatment. The results of this study indicate that two of the commercially available polycarbonate brackets have an unacceptable amount of deformation and creep when subjected to forces from rectangular wire that are in the clinical range for incisor root torquing. It is thereforcl recommended that, until the design and/or the components of plastic brackets improve, clinicians should use light wirr torquing auxiliaries or metal hrackcts on patients who require root-torquing movement. Summary

Two types of commercially available plastic brackets were evaluated to determine the tendency of these brackets to deform and creep while a torquing force was applied. A testing device was designed for this purpose. The results indicate that the forces generated by rectangular wire tend to deform the brackets as the load increases. Although the load-deformation curve was not the same for each type of bracket, the amount of deformation was quite

Polycarbmate

orthodontic

brackets

33

similar when the torquing forces reached the clinical range. Both types of brackets also demonstrated a tendency to creep when subjected to a torquing load of 2,000 gm.-mm. over a period of time. The combined effect of plastic deformation and creep indicates that the two types of plastic brackets tested in this study would be of limited use when transmitting a torquing force in the range of 2,000 gm.-mm. from a rectangular wire to the root of the tooth, since much of this force would be dissipated in distortion of the bracket. The authors wish to thank Gordon Moskowitz for his technical assistance Hnack for his helpful comments regarding this manuscript. We also wish personnel of the electrical engineering machine shop at Drexel University.

and to

Donald thank

C. the

REFERENCES

adhesive for bonding attaeh1. Newman, G. V., Snyder, W. H., and Wilson, C. E.: Acrylic ments to tooth surfaces, Angle Orthod. 38: 12-18, 1968. 2. Newman, G. V., Snyder, W. H., Wilson, G. E., and Hanasian, D.: Adhesives and orthodontic attachments (preliminary investigation), J. New Jersey Dent. Sot. 37: 113-120, 1965. 3. Newman, G. V.: Adhesion and orthodontic plastic attachments, AM. J. ORTHOD. 56: 573. 588, 1969. of adhesive systems on tooth surfaces, AX 4. Newman, G. V., and Facq, J. M.: The effects J. ORTHOD. 59: 67-75, 1971. 5. Newman, G. V.: Epoxy adhesives for orthodontic attachments: Progress report, AM. J. ORTHOD. 51: 901-912,1965. 6. Newman, G. V., and Sharpe, L. H.: On the wettability of tooth surfaces (preliminary investigation), J. New Jersey Dent. Sot. 37: 289-293, 1966. bonding of orthodontic attach7. Retief, D. H., Dreyer, C. J., and Gavron, G.: The direct ments to teeth by means of an epoxy resin adhesive, Ah%. J. ORTHOD. 58: 21-40, 1970. 8. Retief, D. H., and Dreyer, C. J.: Epoxy resins for bonding attachments to’teeth, J. Dent. Assoc. S. Afr. 22: 338-346, 1967. 9. Miura, F., Nakagawa, K., and Masuhara, E.: New direct bonding system for plastic brackets, AM. J. ORTHOD. 59: 350-361,197l. 10. Mizrahi, E., and Smith, D. C.: Direct attachment of orthodontic brackets to dental enamel, Dent. J. 130: 392-396, 1971. 11. Mizrahi, E., and Smith, D. C.: Direct cementation of orthodontic brackets to dental enamel: An investigation using zinc polycarboxylate cement, Br. Dent. J. 127: 371.375, 1971. 12. Angle, E. II.: The latest and best in orthodontic mechanisms, Dent. Cosmos 71: 260-270, 1929. 13. Tweed, C. H.: The application of the principles of the edgewise arch in the treatment of malocclusion. II, Angle Orthod. 11: 12-67, 1941. 14. Burstone, C. W.: Unpublished data. 15. Christopher, W. F., and Fox, D. W.: Polycarbonates. New York, 1962, Reinhold Publishing Corp. 16. Raff, R. A. V.: Atomic Energy Commission Technical Report #RLO-2098, 1970, National Technical Information Service. 17. Charlesby, A.: Atomic radiation and polymers, New York, 1960, Pergamon Press. 4001

Spruce

St. (19104)

Load-deformation characteristics of polycarbonate orthodontic brackets.

Load-deformation characteristics of polycarbonate orthodontic brackets R. J. Dobrin, D.M.D.,* Philadelphia, Pa. I. 1. Kamel, Ph.D.,* and D. R. M...
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