Residual acrylic adhesive after removal of plastic orthodontic brackets: A scanning electron microscopic study lvar

Caspersen,

Charlottennlund,

D.D.S.*

Denmark

T

he direct application of orthodontic attachments to the tooth surface is a development of the methods that have been evolved since 1955, when Buonocorel published his description of “a simple method of increasing the adhesion of acrylic filling materials to enamel surfaces.” By etching the surface of the enamel with acid, a much stronger adhesion is obtained.ls 5sI2 This technique has been exploited, first, in the sealing of pits and fissures in caries prevention3 and, more recently, in restorative dentistry, where composite plastic filling materials are now used. Among more specific applications are the build-up of fractured anterior teeth71 I1 and intermaxillary plastic fixations in the treatment of maxillofacial fractures.Q Direct cementation of orthodontic plastic brackets to the etched surface of the tooth with self-polymerizing acrylic adhesive has been performed since 1965.l’ The use of the method in orthodontics differs from that in preventive and restorative dentistry in one essential aspect, namely, the application to the tooth surface is a temporary measure. Among the many advantages of the direct application of plastic brackets are cosmetic superiority, less gingival irritation, better oral hygiene, more accurate positioning of the bracket, absence of a separation phase, and lack of spacing of the teeth as a result of the bonding procedure. One disadvantage of the method is the need for an absolutely dry field during cementation47 20,21 Opinions differ widely on the strength of directly applied attachments and hence the risk of their disengagement. Recent reports, however, indicate good results in this respect,21 especially when the method is used in the anterior arch segments. The remineralization of the etched enamel surface has also been discussed.lo~ l5 The Supported mark, *Institute

by

the

Foundation

of

the

Danish

Dental

Association,

Copenhagen,

Den-

1974. of Orthodontics,

Royal

Dental

College,

Copenhagen,

Denmark.

637

Am.

Table

J. Orthod. June 1977

I No. of teeth

30 Experimental teeth bracket removed with: cutter cutter + scalpel cutter + abrasive wheel cutter + abrasive wheel + polishing 4 Control teeth extracted without treatment I2 Reference teeth Etching of surface for Etching of surface for Etching of surface for Etching of surface for Etching of surface for Etching of surface for 8 Teeth withdrawn from to loss of bracket No treatment of Cutter Cutter + scalpel Cutter + abrasive Cutter + abrasive

I 2 3 4 5 6

min. min. min. min. min. min. the experimental

surface

wheel wheel + polishing

15 4 4 7

group

owing

4 7 day before Just before extraction extraction 3 I I 2 I I I I I No. ofteeth 2 2 I 2 1

presence and distribution of any residual acrylic on the enamel surface would seem, however, not to have received attention. The object of the study reported in this article was to examine any acrylic material remaining in the enamel surface after removal of orthodontic plastic brackets affixed to etched tooth surfaces. A scanning electron microscope (SXM) at the Institute of Technology, Tastrup, Denmark, was used for this purpose. Also examined was whether microscopic spaces appear between the adhesive and the enamel structure, with consequent risk of discoloration of the tooth surface. Material

The material for the study consisted of permanent teeth extracted on orthodontic indications from seventeen children (seven boys and ten girls) with ages ranging from 11 to 16 years. Altogether, fifty-four teeth were used, all of them premolars except for one upper lateral incisor (Table I). Thirty-eight teeth were furnished with plastic brackets, but eight teeth lost, their brackets during the week that elapsed hetwcen application of the attachment and extraction of the teeth. These eight teeth were kept apart from the experimental group but were otherwise treated in the same way. Thus, the experimental group comprised thirty teeth. Twelve teeth wcrc used as rcfcrcncc teeth, the tooth surf’aw being etched for between 1 and 6 minutes. The etching of eight of these teeth was performed immediately before extraction, while for the other four it was carried out 1 week before extraction. Four extracted teeth with untreated surfaces served as controls. One of these

Volume 71 Num her 6

Residual acrylic adhesive after

four teeth had not erupted through with the oral environment.

bracket removal

639

the mucosa and thus had not been in contact

Method

Materials in general clinical use were employed. Plastic brackets were cemented with Direction adhesive*l consisting of 35 per cent phosphoric acid in gel form, a primer for drying the tooth surface, a polymer (powder), and a monomer (liquid), Nuclear magnetic resonance analysis showed the polymer to consist of ethylene-glycol-dimethacrylate, and the monomer of methylmethacrylate. The thirty-eight experimental teeth were polished with an aqueous suspension of pure pumice and flushed with a water spray. After the field was dried with cotton rolls the acid gel was applied with a cotton pellet for one minute. The tooth was then flushed again with the spray. The patient was not allowed to rinse and thus contaminate the field with saliva. The tooth was dried with air blasts, and the primer was applied with a cotton pellet, the tooth surface then being matte white. The sole of the plastic bracket was wetted with monomer and then dipped in the polymer, so that the posterior surface of the bracket was covered with a thin film of acrylic. The bracket was pressed on the tooth with tweezers and held there firmly for about one minute until the polymerization had reached the point where the bracket did not drift. After a further 2 minutes’ drying, the patient rinsed and was instructed to perform oral hygiene in the usual way with a toothbrush. After 7 days the polymerization process was assumed to be completed and, as no examination of the long-term effect was required, the bracket was removed with an orthodontic ligature cutter.This was placed under the sole of the bracket, which was then prized from the tooth surface. In fifteen of the teeth this technique sufficed, while in four others residual acrylic was removed with a surgical scalpel. In another four teeth, retained acrylic was removed with a tine rotating abrasive wheel. Finally, in seven teeth the acrylic residue was removed with the abrasive wheel and the surface was polished with pure pumice on a rubber cup (Table I). Teeth in the four quadrants were selected at random for one of the four procedures. Immediately after the bracket had been removed, a local anesthetic was injected and the tooth was extracted gently with elevator and forceps by the investigator. Care was taken to ensure that the surgical instruments did not come into contact with the treated labial surface of the tooth. The teeth were then stored in a moisture-saturated atmosphere with a thymol crystal until the day of the microscopic examination. They were then dried and furnished in vacua with a film of carbon and a film of gold, both about 250 B thick (these are applied to nonconducting substances for SEM examination). All fifty-four teeth were examined by scanning electron microscopy (SEM) and photographs were taken of representative areas. Moreover, in the SEM examination qualitative elemental assays were performed by energy-dispersive

640

Caspermb

Am.

J. Orthod. June 1917

x-ray analysis (ED) to confirm that. there was a fundamental difference in the composition of the substances considered to be acrylic and tooth substance. The principle of the ED analysis is that when the specimen is irradiated with electrons in the SEN examination, secondary x-radiation of specific energy levels for t,hc component elements is produced. The x-ray tletector is connected to a computer module, which analyzes the signals and presents the results in the form of curves on a television screen. Results

The SEM examination of the twelve reference teeth treated only by etching for a varying period usually disclosed a characteristic surface structure reminiscent of a. honeycomb pattern (Fig. 1). There was a large variation in the degree to which the enamel surface was attacked by the acid, this being dependent on both the duration of the etching ant1 the variability of the struct& of the individual tooth surface. For the thirty-eight teeth whose etched enamel surfaces had been furnished with plastic brackets, namelv, the thirty teeth comprising the experimental group and the eight from which the bracket was lost during the week up to extraction, the SEM examination disclosed the presence of greater or lesser amounts of acrylic. The object of the stucly was to obtain a qualit,ative rather than a quantitative estimation of any acrylic retained on the enamel surface. When the bracket was removed with the cutter, it often appeared to the naked eye that the entire ccmcnting acrylic layer came away with the bracket, leaving a clean tooth surface, with the possible exception of a few peripheral “islands” that had split off (Fig. 2). The SE&t examination disclosed, however, t,hat bhe apparently clean surfacc was covered by a microscopically thin, diffuse acrylic film (Fig. 3). These residues appeared under the microscope to vary from well-defined flakes or islands (Fig. 4) to diffuse membranes (Fig. 5). The acrylic in the surface structure was affected but not removed by the mechanical instruments used. Thus, scratch marks from one beak of the cutter could be seen in the acrylic film (Fig. 6). Whcrc an attrmpt hat1 been made to remove the acrylic with a, fine abrasive wheel, thcrc were well-defined scratches on the tooth surface but none on the actual acrylic, (Fig. 7) ; subsequent polishing of the surface with a suspension of pumice smoothed the roughened surface structure (Fig. 8). There was no cvidcncc of space betnccn the acrylic and the tooth substance, the residual acrylic adhering so completely to the enamel structure that the acrylic, filled out even the slightest irregularity in the prism structure (Fig. 9). There is thus no reason to expect that discoloration of the tooth surface will develop as a conscqucncc of the acrylic residue. To check that the structures on the SEM pictures w(lre not misinterpreted, a qualitative assay was performed on characteristic areas by means of ED analysis. From this examination, it was evident t,hat there was a manifest difference in intensity of the calcium signal picked up from a clean area of the tooth (Fig. 10) and from an area covered 1)~ the acrylic film (Fig. 11). This difference in the intensity of the calcium signal and its distribution

Residual acrylic

Fig.

1.

enal

mel

The

b pattern

by t he

characteristic

surface

acid

35

per

appears than

Fig.

2. Corresponding

ual

amounts

pea rs to ture can

scanning

with

of

to

devoid

be

discerned.

the

peripheral

excess of

the

margins

acrylic

acid

center

zone.

acrylic,

microscope

phosphoric

because

the

be

electron

cent

adhesive after

of

the

the

removed

the

area

this

magnification.

(Magnification,

of

x50.)

picture

one

minute.

enamel

prism

(Magnification,

whereas at

(SEM)

for

bracket removal

after

The is

etching

of

the

hor iey-

well-defined

more

641

readily

attac

:ked

~1,000.) plastic covered The

bracket by

the

perichymates

are

large

re sid-

the

bracket

apst ruc-

there sole

of

in the

enamel

A??,. J. Orthod. June 197i

Fig.

3. A tl lin

prism ns.

Fig. partly

4.

residuul

film

(Ml Ignification, Large covered

flakes by

of

acrylic

on

the

tooth

surface

partly

masks

the

etchec 14 enal nel

x3,000.) of quite

residual thin

films

acrylic

and

of acrylic.

a more [Magnification,

open

enamel x300.)

structure

which

is only

Volume Number

71 6

Residual

Fig.

5. Ths e acrylic

[The

fkSUl re is an

Fig.

6.

A

scratch

mov ral

of

the

toot

(M agnification,

h.

retained artifact mark

bracket.

on

the

caused produced The x3,000.]

scratch

tooth

surface

by drying.) by

a is

adhesive after

acrylic

beak impressed

is more

diffuse

[Magnification, of

the in

here

and

varies

643

in thi ckn ess.

x300.)

orthodontic the

bracket removal

acrylic

ligature film

which

cutte !r dur ing rerr lains

on

rethe

Am.

Fig.

7.

left:

Distinct

acry rlic

from

the

furrows

surface

bub ble s. (Magnification,

Fig. strut

8. Polishing

:tur‘es,

whe fel.

such

of as

(Magnification,

of

produced the

tooth.

by

the

Right:

abrasive A

large

wheel; “island”

this

did

of

acrylic

J. Orthod. Jufw 1977

the

not co ntaining

air

x300.) the tooth

enamel

with

substance, x300.)

a suspension acrylic,

and

of the

pumice scratches

has

smoothed produced

all by

the I.he

surf ace abrar

iive

Volume Number

71 6

9. The adhering to the structure of tooth structure and of comparison with

Fig.

Resid~ual acrylic

adhesive after bracket remova.

645

acrylic has flowed out to form a film so thin that it is closely adapted the etched enamel prisms. There is no evidence of space between the the acrylic. lop right: An erythrocyte (6 P in diameter] provides a means the size of the enamel prisms. (Magnification; ~2,000.)

corresponding to the dental tissue and acrylic can also be illustrated in another way, namely, by comparing the general SEM picture (Fig. 12) with the energydispersive x-ray image showing the different distribution of the calcium in the image field (Fig. 13). Discussion

Etching of the enamel surface with phosphoric acid results in a manifold increase in the surface area, a more irregular structure, and hence a greater adhesi0n.l’ 3, 12, I6 Because of the orientation of the crystallites, the acid attacks the enamel prisms centrally.“, 8, LZ It has been shown that the perichymates arc more acid resistant than the imbrications.8 The depth of the etching action centrally in the prisms has been reported as ranging from 10 to 50 microns*~ ‘1 cj,‘3 Z’s ” and is usually about 25 microns ; this has been demonstrated in replicate studies.5 0, 27 It has been shown that enamel with a high fluoride content is more resistant to acid.“, l4 While these factors certainly affect the etching pattern, the chief factor is the duration of etching, which is normally reported as one minute.‘l Wickwire,” however, found that 4 minutes is optimal, while etching for 6 minutes results in disruption of the enamel structure. Another factor that has been the object of a number of studies is the remineralization of the etched enamel surface. Etching with acid results in decalcification. By means of microradiography, .Tohansson”’ found that remineralization occurs in the course of 24 to 48 hours. Lenz and Miihlemannl” state that after as short a time as 2 hours an organic film is deposited from the saliva.

646

Fig. tooth

1 0. The by the

men

t lolder);

Fig.

The

result

of

a

100~second

energy-dispersive

along 4.01.

Am.

Caspersen

axis

of Si,

signal

1 1. For

is shoj uYn. The

x-ray

abscissa 1.74; for

signal

in ascending

P, 2.01; calcium

comparison

with for

qualitative analysis.

calcium

Au,

The order

2.14

assay

performed

emission

lines

of

x-ray

(evaporated

on for

energy

the level:

specimen);

Ca,

a clean elements Al,

1.49

J. Orthod. June 1977

area are (from

of

the

IOU lted SFjeci-

K,-line

3.69;

Kp- line

covered

with

acl rylic

is powerful. Fig.

10,

at

3.69

the and

ED 4.01

analysis is much

for

an

area

weaker.

*al authors have found that the surface stems to be normal again 1 to 2 WC saw after etching. 1. 15,Ii, L’,,xi. 1; It was not possible to caonfirm or refute these fin& on th c basis of the four teeth in this study that were etched 1 week before tracti on ; their surfaces displayed a most \-arid pattern, reflecting the great ra of the: rcsponsc to etching seen in this material.

Volume Number

71 6

Fig. 12. tion

Residual

SEM

shot Nn

Fig. 13. The1 re

is

view in

The

of

Fig.

an

13.

acrylic

nissic )n line

no K,

x-ray

calcium 3.69

“island.”

(Magnification,

energy-dispersive

clearly

acrylic

was

adhesive

This

field

after

is identical

bracket removal

with

the

calciu

647

distl ribu-

x500.) picture

for

signal

from

the

used.)

(Magnification,

the area

calcium covered

distribution by

the

shown acrylic

ir 1 Fig.

“is1 ar Id.”

12. (The

x500.)

Man y investigations have beelt made of the strength of the bond bf?tT , the bra cket, , the acrylic adhesive, and the enamel surface.‘, 11, 12,21,23,w :?i it of the m hzIve relied on tensile tests,‘l, lz, I43=, 2Gbut these do not reflect th es to whi ich t;he bracket is usually exposed in clinical contexts, where the fc3rcc acts laq rely parallel to the tooth surface and thus sets up a shear stress. A nur nber

of autho&* 11’*lri have found that fracturing occurs in the plastic test bodies or iIt t,he acrylic adhesive layer between the enamel al1d the bracket.“‘, ” Tho adhclsion to the etched enamel surface then clearly exceeds the tensile strength of thcL material. Retief2” has recently published some SEM pictures oi’ the site of failure at’ the acrylic-enamel interface which are strong157 suggestive of the acrylic rcsitlucs demonstrated in the present study. Buonocore and associates’ mention t,hc possibility of residual acrylic, while other authorPI “:( state that they hal-c uc*tuall~ observed this phenomenon after using plastic brackets. However, none regard this as a possible shortcoming of the technique of directly bonding plastic brackets. In their in vitro experiments, Miura and (dolleagues’7 found no acrylic but mention that the surface of the toot,11 was cleaned with acetone. 1%~ ww of chemical cleaning of the acrylic residues should bc consider& since in the present study, where mechanical instruments only mere used, acrylic was found to bc rctained on all the teeth. There have been no reports dealing with the problem of the formation of gaps between the acrylic adhesive and the etched enamel structure when plastic brackets are used. In earlier studies carried out ii; connection with the sealing of fissures where the margina,l areas were examined with dyesI’ and radioactive isotopes combined with radioautography,‘, ‘iis y5 it appears that no spaces were found. In the present study there were no signs of gaps between the acrylic a?l(l the etched enamel surface, but the examination was not extended to areas whcrr any excess acrylic, outside the sole of the bracket, might have adhered to weakly etched or unetched enamel. Numerous SE&I studies have been performed of tooth surfaces etched with acid and furnished with acrylic,6-8! lR, 14,1(1,20,22, 24-27but these SE&I studies apparently have not been supplemented with ED analysis to establish whether the described residues differed from the dental tissues in their chemical composition. In the present ED analysis there was a manifest difference in the intensity of thcl signa,ls obtained for calcium from the two areas. Nor has it been shown earlier, either quantitatively or qualitatively, how often acrylic residues occur on the enamel surface after USC of directly bonded plastic brackets. In view of the results of this study, in which acrylic was found on the surfaces of all the teeth examined, it would be of interest to perform long-term observations of the reachtion of this unintended foreign substance on the tooth surface. Summary

After etching for one minute nith 35 per cent phosphoric acid, thirty-eight permanent teeth were furnished with plastic brackets. An additional twelve teeth were usecl as references ant1 were ct~hetl but wcrc not furnished with brackets. Flour teeth served as untreated controls. Eight brackets had fallen off one week la,tcr. Thc~ remaining ha~kcts wcrc rcmo\wl with various instruments. 811 the teeth were extra&cd and examined by scanning clcctron microscopy (SEM). On all of the teeth originally furnishetl with brackets, residual acrylic was found on the tooth surface. The SRM csamiuation was supplemmtcd with energy-

Residzrwl

acrylic

ctdhesiue

after brncket

removal

649

dispersive x-ray analysis (El)) which confirmed that the observed residues differed in their chemical composibion from the dental substance. Appreciation is extended to Dennis Moe of the Institute sen of the Institute of Technology (hot11 of tlr? Royal Ikut:ll analyzing the adhesive.

of Biochemistry Collqy) for

and to J. Asmustheir assistance in

REFERENCES

1. Buonocore, M. G. : A simple method of increasing the adhesion of acrylic filling materials to enamel surfaces, J. Dent. Res. 34: 849-863, 1955. 2. Buonocore, M. G., Matsui, A., and Gminnet, A. J.: Penetration of resin dental materials into enamel surfaces with reference to honding, Arch. Oral Biol. 13: 61-70, 1968. 3. Cueto, E. L., and Buonocore, M. G.: Sealing of pits and fissures with an adhrsiw resin: Its use in caries prevention, J. Am. Dent. Assoc. 75: 121-128, 1967. 4. Dietz, V. S.: A technique for direct bonding of orthodontic attachments, J. Clin. Ortliod. 6: G81-693, 1972. 5. Gminnet, A. J., and Matsui, A.: A study of enamel adhesives, Arch. Oral Hiol. 12: 1615. 1620, 1967. 6. Gwinnet, A. J.: Morphology of the interface between adhesive resins and treated human enamel fissures as seen hy scanning electron microscopy, Arch. Oral Riol. 16: 237-238, 1971. 7. Gminnet, A. J.: Structural changes in enamel and dentin of fractured anterior teeth after acid conditioning in vitro, J. Am. Dent. Assoc. 86: 117-122, 1973. 8. Hoffman, S.: Variations in surface resistance to enamel etching, J. Dent. Res. 51: 795-799, 1972. 9. Iranpour, B. : Application of enamel adhesives in oral surgery, Int. J. Oral Surg. 3: 223-226, 1974. 10. Johansson, B.: Remineralization of slightly etched enamel, J. Dent. Res. 44: Gi-70, 1965. 11. Laswell, H. R., Wclk, D. A., and Reyenos, J. W. : Attachment of resin restorations to arid pretreated enamel, J. Am. Dent. Assoc. 82: 558~563, 1971. of phosphoric acid etching 12. Lee, B. D., Phillips, R. W., and Swartz, M. 1,. : The influence on retention of acrylic resin to bovine enamel, J. Am. Dent. Assoc. 82: 138~1-1386, 1971. 13. Lee, II. L., and Swartz, M. L.: Scanning electron microscope study of composite restorative materials, J. Dent. Res. 49: l-29-158, 1970. 14. Lee, H., Stoffey, D., Orlowski, J., Swartz, M. I,., Ocumpaugh, D., and Nevillc, K. : Sealing of developmental pits and fissures. III. Effects of fluoride on adhesion of rigid and flcsihlr sealers, J. Dent. Res. 51: 191-201, 1972. 15. Lenz, H., and Miihlemann, H. R. : In-vivo and in-vitro effects of saliva on etched 01 mrc.l~anic~:~lly marked enamel aftcar cvrtain priocls of time, Helv. Odontol. Acta 7: 30-33. 1963. 16. Mitchell, 1). L.: Bandlcss orthodontic brackets, J. Am. Dent. Assoc. 74: 103-110, 1967. li. Miura, F., Nakagawa, K., and Masuhara, E.: Nevv direct bonding system for plastic brackets, AM. J. ORTHOD. 59: 350-361, 1974. 18. Newman, G. V.: Epoxy adhesives for orthodontic attachments; progress report, Ahr. J. ORTHOD. 51: 901-912, 1965. 19. Newman, G. V., and Facq, J. M.: The effects of adhesive systems on tooth surfaces, A~I. J. ORTZIO~. 59: 67-75, 1977. 20. Newman, G. V.: Clinical treatment with bonded plastic attachments, AM. J. ORTIIOD. 60: 600-610, 19il. 21. Newman, G. V.: Current status of bonding attachments, J. Clin. Orthod. 7: 425-44’9, 1973. 22. Poole, D. F. G., and Johnson, N. IV.: The effects of different demineralizing agents on human enamel surfaces studied 1)~ scanning electron microscopy, Arch. Oral Biol. 12: 1621. 1634, 1967.

Dental

Orthopedia,

science

which

The

term

dental malpositions mechanically Orthopedia Donnelley

more

has

for

Malocclusion but the

teeth

constructed and & Sons,

object

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Correction p. 3.)

to

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present all

and of

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or

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acceptance,

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appliances

Orthodontia,

correction in that

attached Palate,

not facial

may to Dental

or and

Orthodontics,

dentofacial

only

applies

to

outlines

that

are

be

corrected

the

teeth. Orthopedia,

through (Case,

is

the

malocclusion. every the Calvin

Chicago,

form

caused

of from

medium

of

S.:

Dental

1921,

R.

R.