Compa ison in shear bond strength of orthodontic brackets between five bonding systems related to different etching times: An in vitro study Paul Surmont, DDS," Luc Dermaut, DDS, PhD, b Luc Martens, DDS, PhD, = and Michel Moors a

Gent, Belgium New adhesives are .continuously being marketed. The adhesive-bracket bond is still the weakest point in orthodontic bonding. A new device for the measurement of shear bond strength values (SBS) of orthodontic brackets is presented in this article. With this device, five commercial products were evaluated for their SBS: Lee Bond (LB), Concise (C), Super C (SC), Achieve-no-mix (ANM), and Panavia Ex (PE). After applying Begg brackets of laminated minimesh on lower human incisors, the specimens were thermocycled. The products investigated produced high bond strengths. The products tested showed shear bond strengths that were significantly lower than those stained with PE. The application of PE yielded the highest shear values. Nevertheless, some characteristics of this product might be clinically somewhat inconvenient. There was no significant difference in shear bond strength between 15 and 60 seconds enamel etching before bond application. (AM J OFITHOD DENTOFAC ORTHOP 1992;101:414-9.)

S i n c e the introduction o f the acid-etch technique in 1955 by Buonocore, bonding to enamel has found a lot of applications in all disciplines of dentistry. 1'2 One of them is the bonding of orthodontic brackets, either direct or indirect. This technique has enormously reduced the need for orthodontic banding. From the literature, it is known that the adhesivebracket interface is the "bottleneck" o f bonding orthodontic brackets to etched enamel. 3 Recently, however, a new product, Panavia Ex (PE), has been generated that is known to bond to metal chemically. In addition to the development o f new products, research has indicated that shorter etching times might yield equally high bond strengths to enamel. 4 However, this influence has not yet been fully determined for the bonding capacity of currently employed orthodontic brackets.

PURPOSE OF THE STUDY The aims o f this study were the following: 1. Evaluation of a new device for measuring the shear bond strength (SBS) of orthodontic brackets bonded to human tooth enamel.

From the State University of Gent, Belgium. "Assistant Professor, Department of Restorative Dentistry, Dental School. T'Professorand Chairman, Department of Orthodontics. Dental School. 'Associate Professor, Department or Restorative Dentistry. Dental Scho~f.eEngineer. Department of Nonferrous Metallurgy. 811/29807

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2. Comparison o f the SBS obtained with five different bonding cements. 3. Measurement o f SBS of orthodontic brackets bonded to enamel as a function of a reduced etching time.

MATERIAL AND METHODS A. The testing device

The sample holder consists of six parts (Figs. l and 2). An upper rod (a) is fixed to the upper part of an lnstron testing machine (Instron Ltd., Buckinghamshire, England). The upper part of a plate-like hook (b), that can be connected to the upper rod ground thin at its lower end to fit precisely under the flanges of the orthodontic brackets used in this study. In this way, there are 2 degrees of freedom between the bracket and the hook that are a vertical translation and a rotation around a shaft, perpendicular to the hook. The thiid part, the tooth holder itself (c) has two parallel perforations that allow the placement of a shaft (d). Plain cut to these perforations, a screw hole is drilled through in which a large screw (e) with a flattened end can be inserted. This screw will counteract the reaction forces caused by the shear forces transferred from the bracket to the tooth root. The tooth holder itself is attached to the lower fixation rod (f). Between the two latter parts, there are 2 degrees of freedom: one rotation around a vertical shaft and one translation in the horizontal plane. After the application of the bracket tothe tooth surface, the tooth is positioned in the tooth holder by means of the shaft (d). This shank is inserted in the tooth through a hole that is drilled through the root in a mesiodistal direction. The

S/tear bond strength of brackets 415

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Fig. 1. Photograph of dismantled sample holder, displaying different parts (for the legend see text).

upper rod is fixed in the testing machine, and the hook i s linked to this rod. The bracket is held by the hook and by means of the large screw (e). The tooth is rotated until the bracket and the hook are in a vertical plane. Finally, the lower rod is attached to the tooth holder and fixed in the testing machine. Because the lower rod can rotate in relation to the tooth holder, there is no possibility of any force being transferred to the bracket-tooth interface by the closure of the clamps of the testing machine.

B. Sample preparation A flowchart of the sample preparation is shown in Fig. 3. Eighty human lower incisors were chosen at random. Incisors were chosen since it is believed that they provide more reliable SBS values than premolars and molar teeth2 The teeth were macroscopically inspected for cracks, defects and the presence of calculus. They were stored in plastic boxes at 4 ~ C on a grid above water containing 1% thymol from the time of extraction until they were required for the study. The enamel surfaces were cleaned with a fluoride-free prophylaxis paste. Then a hole was drilled in a mesiodistal direction through the middle of the root. All teeth were divided at random into five groups of 16 specimens each. In every group, the enamel was etched with the etching medium available in the package (Table I). In each group, eight teeth were etched during a 15-second period, whereas for the remaining eight teeth, the itching time was extended to 60 seconds. The etching time of 60 seconds was chosen because it is universally recommended in most manuals, whereas the 15-second period was considered to be the shortest possible time span for the adequate etching of tooth enamel. ~.-"After the application of the etching medium, the etehant was thoroughly rinsed off for 15 or 60 seconds, respectively. Thereafter, the facial tooth surfaces were dried

C Fig. 2. Prepared sample positioned in apparatus.

with oil-free, compressed air. In all cases the frosty white appearance of etched enamel was noticed. For the following steps, all products were used according to the manufacturer's instructions. Every group of 16 teeth was bonded with one of the bonding systems mentioned in Table I. Begg brackets on laminated minimesh (TP Orthodontics, Inc., part no. 256550 LaPorte, Ind.) were used. After bonding, the samples were thermocycled (100 cycles from 4 ~ to 56 ~ C) in an attempt to simulate the oral circumstances. Thereafter, the samples were stored in water at 37 ~ C for 1 week. To test the shear bond strength, the samples were mounted in an universal testing machine (lnstron 1195), as described previously. The angulation was defined, and the machine was activated with a crosshead speed of 0.5 ram/minute until a fracture was noticed. The forces were automatically recorded in kilograms and convened into MegaNewtons per meters squared. During the sample preparation, only a minimal amount of bonding material was applied on the tooth surface. Before any sign of setting was noticed, excess cement was carefully removed. The real contact surface of the brackets of 9 mm-" was thus taken into account. Finally, after breakage, the bracket base, as well as the enamel surface of each tooth, was-visually inspected in an attempt to identify the weakest point in orthodontic bonding (adhesive bracket bond versus bracket tooth bond).

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80 human incisors cleaned with pumice

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and 60 seconds, respectively, are compiled, no statistical significant difference in overall SBS between the two etching times could be detected. Except for PE, which showed many cohesive failures in the tooth structure itself, over 90% of bond failures with the other products were a combination of a bond-bracket interface failure and a cohesive bond failure in the product itself. Two tooth-adhesive failures, however, were recorded after the use of Super C (SC) as bond medium.

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DISCUSSION (n=8) (n=8)

(n=8) (n=8) (n=8)

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shear bond strength measurements with an Instron i195 c r o s s h e a d s p e e d 0.5 mm/sec

I I

data statistical

processing

I Fig. 3. Flowchart of sample preparation used in this study.

RESULTS

All data are presented in Table II and Fig. 4. With the F test and the t test, no statistical significant difference, in SBS could be detected between the teeth etched for 15 or 60 seconds within one bonding material. Achieve-no-mix (ANM) was found to resist shear strength bettei" than Lee bond (LB), whereas a Significant differen6e in SBS between PI~ arid all other products was noticed. Moreover, the PE yielded crown fractures in most samples. The t w o samples that did not dem0nsira~e iooth fracture showed a S B S of 22.6 and 2.2-MN~rri2. Consequently, the mean SBS values listed in t~e.table for pE can be considered as being the mean [rac!ure resistance of h u m ~ lower incisors that have been thermocycled rather than th~ SBS of orthodoiitic brackets bonded to etched enamel. Whenall SBS values for teeth etched 15 seconds

The newly developed device produced data comparable to other studies. 69 It is very easy to manipulate. Moreover, with this apparatus, the angulation can be installed very precisely. Because there is only 1 degree of freedom between the hook and the bracket, it is a requisite that the angulation of the hook is identical to the angulation of the bracket. According to the literature '~ a biologic variation exists between teeth originating from different persons. In this study, however, the data of the same group derived after the loading experiment were. found to have small standard deviations. Every investigated product yielded relatively high bond strengths. Recently, similar results were reported in another study, with no significant difference between commercial products. 6 Consequently, at present it seems more important to improve and simplify the clinical operating procedures rather than to increase the adhesive strength of the currently available adhesives." Moreover, an increased bond strength to enamel could provoke more damage of the enamel because of the difficulties in debonding. 'z''3 However, higher bond strengths could reduce the surface area needed for a strong bond, which ultimately result in the use of smaller brackets. The morse recently developed ceramic brackets, when used with an appropriate bonding, provide very high bond Strengths, even higher than the stainless steel brackets used previously.'4 Although the in vivo conditions differ from in vitro circumstances, a ranking between several products according to their bond strengths obtained under ideal conditions may be established. 7 However, torque and tensile bond strength measurements would give, joinily w i t h t h e SBS, a more complete evaluation of these materials.'S The use of PE clearly demonstrated the highest SBS. The SBS itself, however, could not be measured in this group, ~ince in 11 of 16 cases, a fracture within the tooth crown was perceived. If the teeth had withstood the shear forces, the measured values would have been evefi highei'.

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Table I. Listing of the five bonding products used in the study, with some of their delivery properties

ProductlCodelManufacturer Lee Bond

LB

Concise

C

Super C

SC

Achieve no mix

ANM

Panavia EX

PE

Primerl unfilled resin

PresentationlMi.ringlEtchant

Lee Pharmaceutical, Lee Insta-Bond, South El Monte, Calif. Dental Products, 3M Co., Munrovia, Calif. AMC Manufacturing0 Philadelphia, Pa. "A"-Co., Johnson & Johnson, San Diego, Calif. Kuraray Co., Kyoto, Japan

Compules

No

Paste

Primer on tooth; primer on bracket

Two pastes

Yes

Fluid

Unfilled resin on tooth

Powder/liquid

Powder absorbed by liquid

Gel

Compules

No

Gel

Powder/liquid

Yes

Gel or liquid

Primer on tooth; primer on bracket

Table II. Shear bond strength values (MN/m 2) classified according to etching time and product

[LeeBondlConciselSuperClachieve-no-mi-r

I

Panaviaex

15-secondetching SBS X s

15,2 2.6

16.7 3.7

17.5 2.1

18.0 1.6

>'20.1 3.4

15.7 3.9

15.7 3.8

17.9 1.5

18.1 1.5

>'20.1 3.3

60-secondetching SBS X s

Although PE demonstrated a rigid bond, the brackets could easily be removed with specially designed forceps and without macroscopic damage to the enamel surface. If the bond between orthodontic brackets and etched enamel mediated by PE is that high, perhaps brackets could be applied on molar teeth instead of orthodontic bands. According to'the litrrature,~'!6't7 the SBS values in the posterior region, however, are lower compared with those obtained in 'the 'anterior region. The use of PE, which cures in an aerobic atmosphere, requires an isolating gel. The setting reaction only starts after a relatively long fixation period during which the bracket might be displaced from its original position. Therefore PE will be clinically more useful in combination with the indirect bonding technique. Although some investigators '~'~" indicate that after an etching time of 60 seconds or more the enamel etching pattern would be more uniform compared with a 15-second etch, this uniformity, however, is not reflected in the values of the bond strengths: '~3The results of this study are in agreement with other researchers

who, although they used other products, did not discover any variation in bond strength between enamel etched for 15 or 60 seconds. 2~ From a recent in vitro study, it was revealed that differences in surface roughness because of different etching procedures did not reflect differences in shear bond strength of an orthodontic bonding resin. 22Several studies even indicate that increased bond strengths can be achieved with a reduced acid-etch time. 22"23 Moreover, etching the enamel for 15 seconds is a more conservative procedure since less enamel is dissolved. From a clinical point of view, the shorter etching time should be preferred. 24 Recent in situ studies indicate that reducing the duration of etching has no adverse effect on the clinical retention of bonded attachments. 2~26 There is still a lot of controversy about the use of bonding agents (unfilled resin) before the application of the composite material. '''27'2s In this study, the two powder/liquid systems, SC and PE, in which no bonding agent is to be applied on the etched enamel, produced equally high SBS values. Every step in the bonding procedure that can be omitted, without affecting the quality of the bond, should be considered as an

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SHEAR BOND STRENGTH Influence

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Fig. 4. Bar diagram representing the shear bond values obtained with different products and different etching times.

Table III. Comparison of bond strength values between the different products used by means of the t test

(level of significance p -< 0.05) ILeeBond[Concise[SuperCltchieve-no-mixlPanaviaex Lee b o n d Concise Super C Achieve-no-mix P a n a v i a ex

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improvement in the application technique. According to several investigators, 9,2s3~there is still some controversy whether the Use of unfilled resin would have any impact on the adhesive strength to etched enamel. Another interesting point emerging from the data is the difference in standard deviation between the groups. A considerablevariance (s 2) of the.Concise (C) group found in this study was previously reported by other investigators as well. 3' This variabilitY Could be explained by the fact that perhaps in some instances brackets were applied after the initial polymerization reaction has already initiated. ~2The variations in layer thickness might be an additional and reasonable explanation for a higher variance. Every product has its own critical

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