effect of impression technique on bond strength

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ORIGINAL ARTICLE Effect of impression technique on bond strength Jiaping Su, BDS, a Ross S. Hobson, BDS, MDS, PhD, FDS, Morth RCPS, ILTM, b and John F. McCabe, BSc, PhD, DSc c Suzhou, China, and Newcastle upon Tyne, United Kingdom If the effects of surface preparation (eg, acid etching, laser preparation, crystal growth) are to be investigated on the same tooth from which the bond strength is recorded, a method of surface replication is required that does not affect the subsequent bond. This study investigated the effect of 2 different methods of taking impressions on bond strength. Three groups of 11 mandibular incisors were used. The labial enamel was etched with 37% phosphoric acid for 30 seconds. Group A (control) had no impression taken; in group B (silicone), impressions were taken with silicone impression material before bonding; in group C (polyether), an impression was taken with polyether before bonding. After the impressions were taken, GAC brackets (A Company, San Diego, Calif) were bonded to the labial surfaces of the etched enamel with Transbond XT light-cured composite (3M Unitek, Monrovia, Calif). Teeth with bonded brackets were stored in water at 37°C for 24 hours, and then bond strength was measured on a testing machine. The adhesive remnant index (ARI) was also recorded. The lowest bond strength was found after silicone replication (mean [standard deviation]: 8.6 [1.7] MPa) and the highest in the control group (21.2 [4.0] MPa). There was no significant difference between the control group and the polyether replication group (19.1 [4.7] MPa). The surface detail replications of polyether and silicone were found to be identical. It was concluded that polyether had no significant effect on bond strength and was suitable for surface replication before bonding. Polyether allows replication of the enamel surface without a significant effect on bond strength, and this technique could be used to examine the relationship between enamel preparation techniques and subsequent bond strength between composite and enamel. (Am J Orthod Dentofacial Orthop 2004;125:51-5) E ver since Buonocore 1 prescribed the acid etch- ing of enamel as a way of increasing the bond strength of acrylic to the enamel surface, there has been a major research drive to increase bond strength between dental materials and dental hard tissues. Many authors 2-9 have suggested that bond strength might be affected by the extent, quality, and quantity of surface preparation. Studies by Hobson and others 10-13 have suggested that the enamel etch pattern itself might influence the bond strength to enamel; however, as yet no study has examined the relationship between the bond strength and the etch pattern on the same teeth. To assess the enamel etch pattern achieved and record the bond strength on the same tooth, a replica- tion or impression technique is required. The use of silicone impression material has been described by Beynon 14 for the replication of enamel ex vivo. This technique was modified by Hobson et al 13 in their attempt to study the relationship between bond survival and etch patterns. They reported extremely high failure rates when impressions had been recorded before bond- ing in vivo, and this might have been related to the use of silicone impression material before the placement of orthodontic bonds. The present study was undertaken to investigate how bond strength is affected by 2 impres- sion materials used for replication of the etched enamel surface before bonding. MATERIAL AND METHODS Thirty-three human mandibular incisors were col- lected over a 12-month period from a fluoridated area in northeast England from male and female subjects between 12 and 25 years old. The teeth had been extracted for orthodontic reasons and were stored in 0.5% chloramine T solution at 5°C before experimen- tation. All teeth were bonded and tested within 6 months of extraction. 15 The surfaces of each tooth were cleaned with pumice-and-water slurry and a rotating brush for 20 seconds, washed for 20 seconds, and air dried with a a No. 3 Hospital of Suzhou, Suzhou, People’s Republic of China. b Department of Child Dental Health, Newcastle Dental School, Newcastle upon Tyne, United Kingdom. c Department of Dental Materials, Newcastle Dental School. Reprint requests to: R. S. Hobson, Newcastle Dental School, Child Dental Health, Framlington Place, Newcastle upon Tyne, United Kindom; e-mail, [email protected]. Submitted, July 2002; revised and accepted, December 2002. 0889-5406/$30.00 Copyright © 2004 by the American Association of Orthodontists. doi:10.1016/j.ajodo.2002.12.001 51

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Page 1: Effect of impression technique on bond strength

ORIGINAL ARTICLE

Effect of impression technique on bondstrengthJiaping Su, BDS,a Ross S. Hobson, BDS, MDS, PhD, FDS, Morth RCPS, ILTM,b andJohn F. McCabe, BSc, PhD, DScc

Suzhou, China, and Newcastle upon Tyne, United Kingdom

If the effects of surface preparation (eg, acid etching, laser preparation, crystal growth) are to be investigatedon the same tooth from which the bond strength is recorded, a method of surface replication is required thatdoes not affect the subsequent bond. This study investigated the effect of 2 different methods of takingimpressions on bond strength. Three groups of 11 mandibular incisors were used. The labial enamel wasetched with 37% phosphoric acid for 30 seconds. Group A (control) had no impression taken; in group B(silicone), impressions were taken with silicone impression material before bonding; in group C (polyether),an impression was taken with polyether before bonding. After the impressions were taken, GAC brackets (ACompany, San Diego, Calif) were bonded to the labial surfaces of the etched enamel with Transbond XTlight-cured composite (3M Unitek, Monrovia, Calif). Teeth with bonded brackets were stored in water at 37°Cfor 24 hours, and then bond strength was measured on a testing machine. The adhesive remnant index (ARI)was also recorded. The lowest bond strength was found after silicone replication (mean [standard deviation]:8.6 [1.7] MPa) and the highest in the control group (21.2 [4.0] MPa). There was no significant differencebetween the control group and the polyether replication group (19.1 [4.7] MPa). The surface detailreplications of polyether and silicone were found to be identical. It was concluded that polyether had nosignificant effect on bond strength and was suitable for surface replication before bonding. Polyether allowsreplication of the enamel surface without a significant effect on bond strength, and this technique could beused to examine the relationship between enamel preparation techniques and subsequent bond strengthbetween composite and enamel. (Am J Orthod Dentofacial Orthop 2004;125:51-5)

Ever since Buonocore1 prescribed the acid etch-ing of enamel as a way of increasing the bondstrength of acrylic to the enamel surface, there

has been a major research drive to increase bondstrength between dental materials and dental hardtissues. Many authors2-9 have suggested that bondstrength might be affected by the extent, quality, andquantity of surface preparation. Studies by Hobson andothers10-13 have suggested that the enamel etch patternitself might influence the bond strength to enamel;however, as yet no study has examined the relationshipbetween the bond strength and the etch pattern on thesame teeth.

To assess the enamel etch pattern achieved andrecord the bond strength on the same tooth, a replica-tion or impression technique is required. The use of

silicone impression material has been described byBeynon14 for the replication of enamel ex vivo. Thistechnique was modified by Hobson et al13 in theirattempt to study the relationship between bond survivaland etch patterns. They reported extremely high failurerates when impressions had been recorded before bond-ing in vivo, and this might have been related to the useof silicone impression material before the placement oforthodontic bonds. The present study was undertaken toinvestigate how bond strength is affected by 2 impres-sion materials used for replication of the etched enamelsurface before bonding.

MATERIAL AND METHODS

Thirty-three human mandibular incisors were col-lected over a 12-month period from a fluoridated area innortheast England from male and female subjectsbetween 12 and 25 years old. The teeth had beenextracted for orthodontic reasons and were stored in0.5% chloramine T solution at 5°C before experimen-tation. All teeth were bonded and tested within 6months of extraction.15

The surfaces of each tooth were cleaned withpumice-and-water slurry and a rotating brush for 20seconds, washed for 20 seconds, and air dried with a

aNo. 3 Hospital of Suzhou, Suzhou, People’s Republic of China.bDepartment of Child Dental Health, Newcastle Dental School, Newcastleupon Tyne, United Kingdom.cDepartment of Dental Materials, Newcastle Dental School.Reprint requests to: R. S. Hobson, Newcastle Dental School, Child DentalHealth, Framlington Place, Newcastle upon Tyne, United Kindom; e-mail,[email protected], July 2002; revised and accepted, December 2002.0889-5406/$30.00Copyright © 2004 by the American Association of Orthodontists.doi:10.1016/j.ajodo.2002.12.001

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3-in-1 air/water syringe. The buccal surface was thenetched for 30 seconds with 37% phosphoric acidsolution (Transbond XT, 3M Unitek, Monrovia, Calif).The etched surfaces were then washed for 30 secondswith water and air dried. The teeth were then dividedinto 3 groups of 11 teeth, as follows.

In group A (control), GAC .018 stainless steelbrackets (A Company, San Diego, Calif) were bondedwith Transbond XT orthodontic adhesive (3M Unitek),which was used and light cured according to themanufacturer’s instructions. The mesial and distal sur-faces of the brackets were light cured for 20 secondseach, for a total of 40 seconds of light curing.

In group B (silicone), the impression techniqueused was described by Hobson et al.13 It was amodification of that originally described by Beynon14

for the replication of the acid etch pattern of hominidenamel ex vivo. This is a 2-stage technique in whichPresident (Coltene, Altstatten, Switzerland) siliconeputty was used to produce a customized putty “tray.”When the putty had set, the tray was removed, and thelabial surface of the putty impression was cut awaywith a scalpel to leave a “box” over the labial surface ofthe tooth. President light-bodied silicone material wasthen placed into the putty tray, and the tray andimpression were placed on the labial surface of thetooth with gentle finger pressure. Once set, the impres-sion was removed, dried, and inspected with a hand-held (�2 magnification) magnifying glass. Any impres-sion that had visible defects was discarded, and theimpression was repeated. After recording the siliconeimpression, the labial surface was cleaned thoroughlywith 50% alcohol with a camel-hair brush for 30seconds to remove any residue from the tooth surfacethat might affect the resulting bond. The tooth was thendried and bonded with identical brackets, material, andtechnique as were used for group A.

In group C (polyether), the impression was re-corded with a single-stage technique with polyether(Impregum, ESPE, Seefeld, Germany) impression ma-terial. Once the impression had set, it was removedfrom the tooth surface, and the labial surface wascleaned thoroughly with acetone applied with a camel-hair brush for 30 seconds. Acetone was recommendedby the manufacturer, and a pilot study confirmed itseffectiveness at removing polyether residue. The toothwas then air dried and bonded as for groups A and B.

After bonding, all teeth were stored in distilledwater at 37°C for 24 hours before bond strength testing.Testing was done on an Instron 5567 testing machine(Instron, High Wycombe, United Kingdom), as de-scribed by Fox et al.15 The tooth root was mountedvertically in a block of cold-cured denture base acrylic

resin, which was then clamped vertically in the base ofthe testing machine. A shear force from a bladeattached to the upper member of the testing machinecrosshead was used to debond the bracket. The bladecontacted the upper edge of the bracket near the enamelsurface, and debonding force was applied parallel to theenamel surface at a crosshead speed of 1 mm/minute.The projected bonding surface area for all brackets wasmeasured before bonding with a vernier micrometer,and the surface area in millimeters squared was used tocalculate the bond strength in megapascals. Afterdebonding, the enamel surface was examined with �10magnification to ascertain the adhesive remnant index(ARI),16 which describes the amount of compositeadhesive remaining on the surface of the tooth: 0, noadhesive on the surface; 1, less than a third of thesurface covered; 2, more than one third but less thantwo thirds of the surface covered; 3, more than twothirds but less than the whole surface covered; 4, wholesurface covered; 10, enamel fracture occurred. The ARIscore indicates the site of bond failure: an ARI score of4 suggests that failure occurred either at the bracket-composite interface or cohesively within the composite.A score between 1 and 3 indicates mixed sites offailure, at the enamel-composite and the composite-bracket interfaces. A score of 0 indicates that the site offailure was at the enamel-composite interface; 10 indi-cates that the enamel had fractured.

The silicone and polyether replicas were convertedto positive replicas with Spurrs epoxy resin (AgarScientific, Stanstead, United Kingdom) and gold coatedin preparation for scanning with an electron micro-scope.13,14 The images were examined quantitativelyfor comparison of the quality of replication.

Statistical analysis of the bond strength data wasperformed with analysis of variance and post hocTukey tests for bond strength, along with Weibullanalysis to examine the probability of failure.

RESULTS

The bond strength results are given in Table I. Themean bracket surface area was 13.42 mm2, and this wasused to calculate the bond strengths in megapascals.The highest bond strength was found in the controlgroup. The lowest bond strength was found with groupB (silicone). The polyether group had a bond strengththat was not significantly different from that of thecontrol group. It can be seen that the mean bondstrength of group B is less than half of the mean bondstrength for group A (control).

Analysis of variance showed that group B had asignificantly lower bond strength than groups A and C

American Journal of Orthodontics and Dentofacial OrthopedicsJanuary 2004

52 Su, Hobson, and McCabe

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(P � .05), whereas groups A and C were not signifi-cantly different from each other (P � .05).

The ARI data (Table II) indicated that the siliconereplication technique resulted in bond failure at theenamel surface in almost all cases. In contrast, both thecontrol and the polyether groups failed partially at thecomposite-bracket and the enamel-composite inter-faces. There was a single incident of enamel fracture inboth the control group and the polyether group.

The ability of the chosen impression technique toreplicate the enamel surface was examined qualita-tively with scanning electron microscopy (SEM) at�1070 magnification. It can be seen from Figures 1and 2 that both the silicone and the polyetherreplication techniques reproduced the enamel surfaceeffectively.

Weibull plots describing probability of failure as afunction of stress are shown in Figure 3. It can be seenthat the control and polyether groups are very similar inshape and form, whereas the silicone group is signifi-cantly shifted to the left, indicating a much higherprobability of failure at low levels of stress.

DISCUSSION

This study investigated the effect of replicationtechniques on the subsequent bond strength of bracketsbonded to enamel surfaces. Mandibular incisors wereused because they have been found to have the mostconsistent etch pattern.10,12

The replication techniques (silicone and polyether)differed only in the materials used and the methodsused to clean up the enamel surface before bonding.

Silicone impression techniques have already beenshown to be capable of replicating to within 0.2 mmand to have an extremely good dimensional stabili-ty.13,14 The 2-stage technique prevents excess displace-ment of the light-bodied impression material and en-sures high resolution replication of the etched enamelsurface. This silicone replication technique has previ-ously been used successfully to replicate enamel etchpatterns in vivo and has been shown to accuratelyreplicate the pattern of the enamel surface itself.10-13

However, the silicone replication technique has beenimplicated in a higher bond failure rate in vivo.13 Thismight be due to silicone residue left on the enamelsurface after replication and before bonding. The clean-ers (acetone for polyether and alcohol for silicone) wereselected after consultation with the impression materialmanufacturers and confirmed by pilot studies of theeffect of a number of potential cleaners on subsequentbond strength to be the best available.

The study was not designed to compare the accuracyof replication techniques. The images look very similar intheir ability to replicate the etched enamel surface, but noquantitative analysis was undertaken. However, previouswork has shown that silicone rubber can accuratelyreproduce etched enamel surfaces.10-14 In this study, thepolyether technique could accurately reproduce theenamel surface to the same extent as the silicone, but it didnot have the significant effect on bond strength seen withthe silicone replication technique.

The effect of the silicone replication technique onbond strength was further confirmed by the ARIscores on the enamel surfaces, which showed that thefailure occurred almost exclusively at the enamelinterface after silicone impression taking. It seemsthat residues from the silicone impression materialwere able to prevent intimate contact between etchedenamel and resin adhesive. On the other hand, the

Table I. Bond strength (MPa) recorded after control,silicone, and polyether impressions, with mean bondstrength and standard deviations

Specimennumber Control Silicone Polyether

1 24.72 5.87 18.672 20.07 10.88 22.333 19.04 10.70 23.034 23.87 8.23 17.935 25.82 6.00 13.286 13.91 9.64 13.407 22.60 7.64 17.508 19.78 9.99 17.169 15.99 8.81 27.9210 20.86 8.40 14.9311 26.62 8.52 23.81Mean 21.21 8.61 19.11SD 4.0 1.7 4.7

Table II. Adhesive remnant index for each toothsurface after bond strength testing, along with themedian values

Specimennumber Control Silicone Polyether

1 3 0 12 0 0 13 1 0 14 1 0 15 1 0 16 10 0 17 3 0 08 3 1 19 0 0 110 0 0 1011 0 0 1Median 1 0 1

Score of 10 indicates enamel fracture.

American Journal of Orthodontics and Dentofacial OrthopedicsVolume 125, Number 1

Su, Hobson, and McCabe 53

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ARI scores for polyether and control groups indi-cated that a mixed adhesive failure at the enamel-composite and bracket-composite interfaces oc-curred. These findings agree with those ofothers.11,13 Enamel fracture occurred on a singleoccasion for both control and polyether groups. Thisis obviously a highly undesirable clinical event, andits occurrence in this study suggests that, undercertain circumstances, the composite-adhesive bondstrength might be greater than that of enamel. Thisphenomenon warrants further investigation.

Although this study was not designed to examinethe accuracy of the replication technique, qualitatively,the replicated surfaces were extremely similar. Theadvantage of the polyether replication was that it didnot have an effect on subsequent bond strength.

CONCLUSIONS

Polyether impression materials allow replication ofthe enamel surface without a significant effect on bondstrength, and this technique might be used to examinethe relationship between enamel preparation techniquesand subsequent bond strength between composite andenamel.

REFERENCES

1. Buonocore MG. A simple method of increasing the adhesion ofacrylic filling materials to enamel surfaces. J Dent Res 1955;34:849-53.

2. Baharav HC, Pilo R, Helft M. The efficacy of liquid and gel acidetchants. J Prosthet Dent 1988;60:545-7.

3. Brannstrom MNK, Malmgren O. The effect of various pretreat-ment methods of the enamel in bonding procedures. Am J Orthod1978;74:522-30.

Fig 1. SEM micrograph (�1070 magnification) ofenamel surface replicated by silicone impression mate-rial.

Fig 2. SEM micrograph (�1070 magnification) ofenamel surface duplicated by polyether impressionmaterial.

Fig 3. Weibull distribution plots for 3 groups: control,no impression before bonding; silicone, impression withsilicone impression material before bonding; and poly-ether, impression with polyether impression materialbefore bonding.

American Journal of Orthodontics and Dentofacial OrthopedicsJanuary 2004

54 Su, Hobson, and McCabe

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4. Franchi MBL. Effects of acid etching solutions on human enameland dentine. Quintessence Int 1995;26:431-5.

5. Galil KA, Wright GZ. Acid etching patterns on buccal surfacesof permanent teeth. Paediatr Dent 1979;1:230-4.

6. Guba CJ, Cochran MA, Swartz ML. The effects of varied etchingtime and etching solution viscosity on bond strength and enamelmorphology. Oper Dent 1994;19:146-53.

7. Garcia-Godoy F, Gwinnett AJ. Effect of etching times andmechanical pretreatment on the enamel of primary teeth: an SEMstudy. Am J Dent 1991;4:115-8.

8. Johnston CD, Hussey DL, Burden DJ. The effect of etch durationon the microstructure of molar enamel: an in vitro study. Am JOrthod Dentofacial Orthop 1996;109:531-4.

9. Oliver R. The effects of differing etch times on the etch pattern onenamel of unerupted and erupted teeth examined using the scanningelectron microscope. Br J Orthod 1986;14:105-7.

10. Mattick CR, Hobson RS. A comparative micro-topographic

study of the buccal enamel of different tooth types. J Orthod2000;27:143-8.

11. Hobson RS, McCabe JF, Hogg SD. Bonding strength to surfaceenamel for different tooth types. Dent Mater 2001;17:184-9.

12. Hobson RS, Rugg-Gunn AJ, Booth TA. Acid-etch patterns on thebuccal surface of human permanent teeth. Arch Oral Biol2002;47:407-12.

13. Hobson RS, McCabe JF, Rugg-Gunn AJ. The relationshipbetween acid etch patterns and bond survival in vivo. Am JOrthod Dentofacial Orthop 2002;121:502-9.

14. Beynon A. Replication technique for studying microstructure infossil enamel. Scanning Microsc 1987;1:663-9.

15. Fox NA, McCabe JF, Buckley JG. A critique of bond strengthtesting in orthodontics. Br J Orthod 1994;21:33-43.

16. Artun J, Bergland S. Clinical trials with crystal growth condi-tioning as an alternative to acid etch enamel pretreatment. Am JOrthod 1984;85:333-40.

Editors of the International Journal of Orthodontia (1915-1918),International Journal of Orthodontia & Oral Surgery (1919-1921),International Journal of Orthodontia, Oral Surgery and Radiography(1922-1932), International Journal of Orthodontia and Dentistry ofChildren (1933-1935), International Journal of Orthodontics and OralSurgery (1936-1937), American Journal of Orthodontics and OralSurgery (1938-1947), American Journal of Orthodontics (1948-1986),and American Journal of Orthodontics and Dentofacial Orthopedics(1986-present)

1915 to 1932 Martin Dewey1931 to 1968 H. C. Pollock1968 to 1978 B. F. Dewel1978 to 1985 Wayne G. Watson1985 to 2000 Thomas M. Graber2000 to present David L. Turpin

American Journal of Orthodontics and Dentofacial OrthopedicsVolume 125, Number 1

Su, Hobson, and McCabe 55