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Angle Orthodontist, Vol 76, No 5, 2006851DOI:10.2319/061205-201

Original Article

Shear Bond Strength of Metal Brackets on Enamel

Paola Cozzaa; Leonardo Martuccib; Laura De Toffolb; Santiago Isaza Pencoc

ABSTRACT

Objective:To compare the shear bond strength of different metal orthodontic brackets.Materials and Methods: Five types of orthodontic metal brackets were selected (S1, Victory

Series, 3M Unitek, Monrovia, Calif; S2, Mini Dyna-Lock, 3M Unitek; S3, Mini Sprint, Forestadent,Pforzheim, Germany; S4, Topic, Dentaurum, Inspringen, Germany; and S5, equilibrium 2, Den-taurum). Brackets were bonded on enamel surfaces of bovine incisors (Transbond XT, 3M Unitek)

and were tested for shear bond strength with an Instron universal testing machine (Instron Corp,Canton, Mass). Data obtained in newtons and megapascals were analyzed with descriptive sta-

tistics and with analysis of variance and Tukey honestly significant difference (HSD) tests. Theadhesive fracture site was classified with the adhesive remnant index (ARI).

Results:All the specimens tested had shear bond strength adequate to resist orthodontic forces.

S5 showed significantly greater bond strength when compared with the other samples, except forS1. S1, S3, and S5 showed a significantly greater bonding force. The ARI index demonstrated a

large variability. Retentive structure of S1, S3, and S5 had equal validity. The enlargement of theretentive surface enhances adhesion but affects the adaptability to surface irregularity of the

enamel, increasing the risk of fracture at the interface with the bracket.Conclusions: The results of this study suggest that probably the retentive base extension can

be lower than 7 mm2 proposed in previous studies as the minimal area.

KEY WORDS: Bracket; Adhesion; Shear bond strength; ARI index

INTRODUCTION

The introduction of the acid etching technique byBuonocore1 allowed the substitution of metal bands

with directly cemented brackets.2 Mitchell,3 in 1967,was the first author to report on the use of metal brack-

et with a retentive base.The adhesion of metal brackets is obtained with me-

chanic interlock between base-adhesive resin-enamel.

Many types of metal bracket bases exist, and they canbe classified into two principal groups: brackets with

soldered bases and brackets with integral bases.In the first group, the metal bases are soldered to

the bracket bodies. The bases used in this class are

a

Professor and Head, Department of Orthodontics, Universityof Rome Tor Vergata, Rome, Italy.

b Fellow, Department of Orthodontics, University of Rome,Tor Vergata, Rome, Italy.

c Graduate, Department of Orthodontics, University of Mode-na and Reggio Emilia, Modena and Reggio Emilia, Italy.

Corresponding author: Dr. Paola Cozza, via Veio 53, Rome00183, Italy(e-mail: p.cozza@flashnet.it)

Accepted: September 2005. Submitted: June 2005.

2006 by The EH Angle Education and Research Foundation,Inc.

perforated bases, mesh foils, and photoetched bases.In the second group, the base and the remaining parts

of the bracket are a unique piece. Four types of basesbelong to this group: retention groove bases, meshbases, waffle bases, and laser-structured bases.

The base retentive system is only one of the factors

that influences the shear bond strength of metal brack-

ets. Furthermore, cleaning and conditioning proce-

dures of the enamel, adhesive systems, polymeriza-

tion type, and time and moisture contamination of con-

ditioned enamel can modify the retention of metal

brackets.4,5

Therefore, the authors recommend a retention of

metal brackets greater than 68 MPa.6,7

Many studies have been performed to compare

bond strength of metal brackets with different retentivebases.4,6,811

The aims of this study were to (1) compare the

shear bond strength of different generations of re-

tentive bases, mesh foil, a new type of waffle base,

and laser-structured base and (2) evaluate the influ-

ence of the base extension on the bond strength.

Furthermore, the specimens were evaluated with

the adhesive remnant index (ARI) index to localize the

sites of adhesive fracture. Before testing, the base sur-

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852 COZZA, MARTUCCI, DE TOFFOL, PENCO

Angle Orthodontist, Vol 76, No 5, 2006

Table 1. Descriptive Statistic of Shear Bond Strength (N) and Nominal Area (mm2)

Bracket Area (mm2) Mean (N) SD SE

95% CI for Mean

Lower

Bound

Upper

Bound Minimum Maxi mum

Victory Series S1 8.97 273.40 71.67 22.66 222.13 324.67 184 391

Mini Dyna-Lock S2 9.25 145.20 52.46 16.59 107.67 182.73 87 248

Mini Sprint S3 5.9 200.60 30.57 9.67 178.73 222.47 143 236Topic S4 12 215.10 80.80 25.55 157.30 272.90 34 300

Equilibrium 2 S5 10.4 302.30 90.14 28.50 237.82 366.78 144 417

faces were observed using a scanning electron micro-

scope (SEM) at 20.

MATERIALS AND METHODS

Five types of maxillary incisor metal brackets wereselected for this study: Victory Series (S1) and Mini

Dyna-Lock (S2) (3M Unitek, Monrovia, Calif), MiniSprint (S3) (Forestadent, Pforzheim, Germany), and

Topic (S4) and equilibrium 2 (S5) (Dentaurum, Insprin-gen, Germany). Ten brackets for each type were used.

In the first part of this study, the base surface of the

brackets selected was observed using a SEM (ZeissDSM 950) at 20. For the morphologic analysis with

SEM, the slot surfaces of the bracket were bonded onappropriate stubs leaving the retentive surface ex-

posed. The mean base surface area of the bracketswas calculated by measuring length and width andcomputing the area.

A bovine tooth model was used in this study be-cause the enamel of bovine incisor has been shown

to be histochemically similar to human enamel.12 Fifty

bovine incisors were extracted, washed, and deprivedof pulps and of the greater portion of the roots. Thecriteria of tooth selection were crown grossly perfectwith absence of cracks caused by extraction forceps.

The teeth were then embedded in self-curing acrylicresin (Ortocryl, Dentaurum), leaving the labial enamel

exposed. The specimens were stored in normal salinesolution for 1 week until testing.

A prophylactic treatment was performed with pum-ice-powder paste-water containing no fluoride, thenrinsed with an air-water syringe for 10 seconds, and

dried with an air-water syringe. Before bonding, theenamel was etched for 15 seconds with a 35% ortho-

phosphoric acid gel solution, sprayed for 20 seconds,and dried with an air-water syringe. The primer (Trans-

bond XT Primer, 3M Unitek) was applied on the enam-el and sprayed with air to promote the complete pen-etration of the resin. After photopolymerization for 10

seconds, brackets with a small layer of adhesive(Transbond XT, 3M Unitek) were positioned and

pressed on the labial surfaces of the teeth. The excessof adhesive extruding from the periphery of the base

was removed, and the adhesive was cured at 380

mW/cm2 (Curing Light XL 3000, 3M Unitek) applying

the light from both interproximal sides for 10 secondseach. The intensity of the lamp has been measured

with a radiometer (Optilux Radiometer Model 1000,SDS Kerr, Danbury, Conn), and the error reported bythe producer was 5%.

The specimens were tested on an Instron universaltesting machine (Instron Corp, Canton, Mass): a blade

was placed at the bracket base-enamel interface at theocclusal side using a crosshead speed of 6 mm/min

and a 50-kg load cell; in this way the brackets wereshear tested to failure. The force producing failure wasrecorded in newtons and converted into megapascals

by dividing the measured force values by the meansurface area of the brackets.

The ARI was used to evaluate the amount of ad-hesive left on the enamel surface after debonding and

to establish the sites of adhesive fracture. Bracketswere observed with a stereomicroscope at 10 mag-nification, and the remaining adhesive was scored with

respect to the amount of resin material remaining on

the enamel: ARI 0, less than 10% of the adhesive re-mained on the enamel; ARI 1, more than 10% but lessthan 50% of the adhesive remained on the enamel;

ARI 2, more than 50% but less than 90% of the ad-hesive remained on the enamel; ARI 3, more than90% of the adhesive remained on the enamel with a

clear impression of the bracket base on the adhesive-enamel surface.13

Statistical method

Descriptive statistics included mean and standarddeviation (SD) for each group, in megapascals and in

newtons. A one-way analysis of variance (ANOVA)and Tukeys HSD post hoc tests were carried out toanalyze the effect of bracket base design on mean

shear bond strength.

RESULTS

The overall mean bond strengths are shown in Ta-

ble 1. The one-way ANOVA test showed that therewere statistically significant differences among the five

groups with respect to shear bond strength (P .001).

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Angle Orthodontist, Vol 76, No 5, 2006

Table 2. Statistical Comparison (Tukeys HSD Post hoc Test)a

Bracket Base Comparison

Newtons

Mean

Difference P

Megapascals

Mean

Difference P

Victory Series S1 S2 Mini Dyna-Lock 128.20 .001*** 14.78 .000***

S3 Mini Sprint 72.80 .141 2.52 .931

S4 Topic 58.30 .331 12.55 .002**S5 Equilibrium 2 28.90 .878 1.41 .992

Mini Dyna-Lock S2 S1 Victory Series 128.20 .001*** 14.78 .000***

S3 Mini Sprint 55.40 .382 17.30 .000***

S4 Topic 69.90 .170 2.23 .955

S5 Equilibrium 2 157.10 .000*** 13.37 .001***

Mini Sprint S3 S1 Victory Series 72.80 .141 2.52 .931

S2 Mini Dyna-Lock 55.40 .382 17.30 .000***

S4 Topic 14.50 .989 15.08 .000***

S5 Equilibrium 2 101.70 .015* 3.93 .730

Topic S4 S1 Victory Series 58.30 .331 12.55 .002**

S2 Mini Dyna-Lock 69.90 .170 2.23 .955

S3 Mini Sprint 14.50 .989 15.08 .000***

S5 Equilibrium 2 87.20 .050* 11.14 .009**

Equilibrium 2 S5 S1 Victory Series 28.90 .878 1.41 .992

S2 Mini Dyna-Lock 157.10 .000*** 13.37 .001***

S3 Mini Sprint 101.70 .015** 3.93 .730

S4 Topic 87.20 .050* 11.14 .009**

a The mean difference is significant at *P .05; **P .01; ***P .001 levels.

Table 3. Descriptive Statistic of Shear Bond Strength (MPa) and Nominal Area (mm2)

Bracket Area (mm2) Mean (MPa) SD SE

95% CI for Mean

Lower

Bound

Upper

Bound Minimum Maximum

Victory Series S1 8.97 30.48 7.99 2.53 24.76 36.19 21 44

Mini Dyna-Lock S2 9.25 15.70 5.67 1.79 11.64 19.75 9 27

Mini Sprint S3 5.9 33.00 6.00 1.90 28.71 37.29 24 40

Topic S4 12 17.92 6.73 2.13 13.11 22.74 3 25

Equilibrium 2 S5 10.4 29.07 8.67 2.74 22.86 35.27 14 40

Table 4. ARI Scores in Percentagea

Value Criterion Interpretation S1 S2 S3 S4 S5

ARI 0 No adhesive left on the tooth (10%) Adhesive fracture at ce-

ment-enamel interface

50 10 50 10 20

ARI 1 Less than half of the adhesive left on the tooth Mixed fracture 30 10 50 10 20

ARI 2 More than half of the adhesive left on the tooth 20 30 40 20

ARI 3 All adhesive left on the tooth (90%), with distinct impression of the

bracket base

Adhesive fracture at brack-

et-cement interface

50 40 20

a ARI indicates adhesive remnant index.

Application of Tukeys HSD post hoc test showed thatS5 and S1 presented significantly greater shear bond

strength in comparison with the other samples (Table2).

The mean bonding force per area squared is shown

in Table 3. The Tukeys HSD post hoc test demon-strated significant differences between the brackets

evaluated (Table 2): S1, S3, and S5 showed a signif-icantly greater bonding force when the data were ex-

pressed in megapascals.

The location of the fracture for each test sample wasevaluated with the ARI index (Table 4). Three possible

types of fractures were observed: cohesive fracture,within the body of the cement; adhesive fracture, atthe cement-bracket base or enamel-cement interface,

and mixed fracture.8 For all the brackets, 4050% ofbreakage was a mixed fracture. S1 and S3 showed

50% of fractures at the cementenamel interface, andS2 showed 50% of breakage at the bracket-cement

interface. Regarding S4, 40% of the fractures occurred

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854 COZZA, MARTUCCI, DE TOFFOL, PENCO

Angle Orthodontist, Vol 76, No 5, 2006

FIGURE 1. Victory Series: photomicrograph at 20.

FIGURE 2. Mini Dyna-Lock: photomicrograph at 20.

FIGURE 3. Mini Sprint: photomicrograph at 20.

FIGURE 4. Topic: photomicrograph at 20.

at bracket-cement interface. S5 showed greater vari-ability of fracture sites, and in 20% of the specimens

the fracture occurred within the enamel.

DISCUSSION

With the exception of S1, all brackets used in thisstudy were integral brackets, in which the body and

the retentive base are a unique piece.S1 presents with an 80-G mesh foil, which seems

to be the most retentive size, providing large spacesfor the penetration of the adhesive and the curing

light.9,14,15 However, some authors consider that themesh size does not influence the retention significantlyor that it depends on the filler content of the adhesive

used5,16 (Figure 1).In S2, the retentive system, the retention groove

base, is characterized by horizontal undercut channelsopen at the medial and distal extremities, with a V

grooved pattern running vertically on the surface of the

base. In theory, such a design should reduce the

chances of air entrapment because excess materialcan escape. The retention of this kind of base does

not seem to be completely satisfying10 (Figure 2).S3 presents the smallest base surface among those

selected in this study (5.9 mm2). Retention is providedby a waffle base, consisting of metallic indentationscoming out from the bottom of the bracket. Scanning

electron micrograph shows that each indentationseems to be tipped occlusogingivally, creating ade-

quate undercuts. The free volume among the inden-tations allows the escape of air and excess resin (Fig-

ure 3).S4 is characterized by a laser-structured base in

which the retention is obtained with many hole-shaped

cavities on the bottom of the brackets that are realizedby a laser beam scanned over the base surface. Pho-

tomicrograph shows the presence of projecting metal-lic margins, probably derived from the laser beam ac-

tion (Figure 4).

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FIGURE 5. Equilibrium 2: photomicrograph at 20.

The perfectly quadrangular base shape is the main

characteristic distinguishing these brackets from S5,which presents a smaller and more concave and an-atomic base shape (Figure 5).

The analysis of the shear bond strength indicatesthat all the retentive systems used in the brackets test-

ed provide the clinically acceptable bond force levels(68 MPa) suggested by Reynolds and von Fraunho-

fer.7 However, this is an in vitro study and care shouldbe taken in extrapolating the results to those that mightbe obtained in the oral environment, in which moisture

contamination dramatically reduces adhesion.6 In fact,the aim of this study was to determine the retention

capacity, without considering in vivo real conditions.

To compare the retention capacity of brackets se-lected, it is necessary to express the shear bondstrength (in N) and the bonding force (in MPa). Thevalues in newtons describe the shear bond strength

considering the retentive base surface, whereas thevalues in megapascals, obtained by dividing the val-

ues in newtons for the base areas, exclude the influ-ence of the millimetric extension of the base and re-

flect strictly the effectiveness of the retention mecha-nism. S5 showed the highest shear bond strength.This value is not significantly greater than S1, but it is

significantly greater than the values shown by the oth-er specimens. When the values are expressed in me-

gapascals, S3, S1, and S5 demonstrate a similar andsignificantly greater bonding force with respect to the

others.Comparing the results with the extension in square

millimeters of the base surfaces, it is evident that the

latter factor can influence the shear bond strength: in-creasing the surface area of the bracket, the load car-

rying capacity increases, as observed in previousstudies.5,9

Interestingly, S4 showed significantly lower values

(both in N and in MPa) with respect to the similar S5.

This phenomenon is probably due to the greater baseextension, to the consequent decreased adaptability of

S4, and to the smaller number of laser shots per areafor S4 than for S5. Moreover, S4 presents a particular

anatomic design, probably incompatible with the bo-

vine incisor profile.Finally, the ARI index analysis showed a consider-

able variability in the fracture sites. In S1 and S3, 50%of bond failures were located at the enamel-adhesive

interface (ARI 0) and 50% were mixed fractures (ARI1). No ARI 2 and ARI 3 cases were present for S3 and

no ARI 3 cases were present for S1. This seems tobe a confirmation of the high retention of these bracketbases.

In S2 and S4, most of the bond failures were locatedat the bracket-adhesive interface (50% and 40%, re-

spectively), with variable percent values in the otherARI scores. This finding is comparable to those found

at the Instron machine test, expressing the lower re-tention capacity or their lower adaptability. S4 showedan equal distribution in all the ARI index scores and

two cases of crown fracture. In general, ARI resultsshould be interpreted with caution because they are

subjective.17

CONCLUSIONS

All brackets tested provided acceptable bond forcelevels.

Victory Series and equilibrium 2 brackets showed

the highest shear bond strength when values are ex-

pressed in newtons. When values are expressed in megapascals, the foil

mesh base of Victory Series, the waffle base of Mini

Sprint, and the laser-structured base of equilibrium2 brackets demonstrated a similar and significantlygreater bonding force compared with the others

specimens. The enlargement of the surface area of the bracket

can increase the load carrying capacity, but it caus-es an adaptability drop. The results of this study

showed that probably the retentive base extensioncan be lower than the 7 mm2 proposed as the min-imal area in previous studies.11

The ARI index values showed considerable variabil-ity.

REFERENCES

1. Buonocore MG. A simple method of increasing the adhesionof acrylic filling materials to enamel surfaces. J Dent Res.1955;34:849853.

2. Newmann GV. Epoxy adhesives for orthodontic attach-ments: progress report. Am J Orthod. 1965;51:901912.

3. Mitchell DL. Bandless orthodontic bracket. J Am Dent As-soc. 1967;74:103110.

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