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British Journal of Orthodontics, Vol. 26, No. 2, 141-146, June 1999
© 1999 British Orthodontic Society

A Laboratory Investigation to Compare Enamel Preparation by Sandblasting or Acid Etching Prior to Bracket Bonding

Refereed Paper

A. E. Sargison, B.D.S., M.SC., F.D.S., M.ORTH.

Present address: Department of Orthodontics, Sunderland Royal Hospital, Kayll Road, Sunderland, Tyne and Wear, UK.

J. F. McCabe, D.S.C.

Departments of Child Dental Health, and Dental Materials Science, Newcastle Dental School, Framlington Place, Newcastle Upon Tyne NE2 4BW, UK

D. T. Millett, B.D.SC., D.D.S., F.D.S., M.ORTH.

Department of Orthodontics, Glasgow Dental School, 378, Sauchiehall Street, Glasgow, G2 3JZ, UK


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 Conclusions
 References
 
A laboratory investigation to compare the mean shear debonding force and mode of bond failure of metallic brackets bonded to sandblasted and acid-etched enamel is described. The buccal surfaces of 30 extracted human premolars were sandblasted for 5 seconds with 50 µ alumina and the buccal surfaces of a further 30 human premolars were etched with 37 per cent phosphoric acid for 15 seconds. Following storage for 24 hours at 37°C in distilled water, shear debonding forcewas measured using an Instron Universal Testing Machine with a cross-head speed of 10 mm/minute. Mean shear debonding force was significantly lower for brackets bonded to sandblasted enamel compared to acid etched enamel (P<0.001). Weibull analysis showed that at a given stress the probability of failure was significantly greater for brackets bonded to sandblasted enamel. Brackets bonded to etched enamel showed a mixed mode of bond failure whereas following sandblasting, failure was adhesive at the enamel/composite interface (P<0.01).

Key words: Debonding Force, Etching, Orthodontic Brackets, Sandblasting


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 Conclusions
 References
 
Since the work of Newman (1965Go) orthodontic brackets have been cemented to teeth with resin adhesives following enamel pretreatment with acid. However, the acid etch technique has several undesirable sequelae. These include loss of enamel due to prophylaxis, etching, debonding, and clean up; enamel cracks and scratches following debonding and retention of resin tags or indelible staining (Evans and Oliver, 1991Go).

A further disadvantage is the difficulty in confining the etchant to the area covered by the bracket base. In an effort to overcome the potential risk of decalcification which this poses, several studies have investigated the effects of reduced etching times or etchant concentrations on bracket bond strengths and failure rates. It would appear that etching for shorter durations or reducing acid concentrations results in less enamel loss (Barkmeier et al., 1985 ;Go Legler et al., 1990Go) and does not compromise bracket retention (Carstensen, 1986;Go Labart et al., 1988;Go Legler et al., 1989;Go Sadowsky et al., 1990;Go Surmont et al., 1992Go). However, the acid concentration should not be so low as to result in the formation of insoluble dicalcium phosphate dihydrate precipitates which are likely to interfere with the bonding process (Chow and Brown, 1973Go).

To overcome some of the problems presented by the acid etch technique, crystal growth solutions have been investigated (Pizarro et al., 1994Go). These result in the formation of long needle-shaped crystals on the enamel surface, but bracket bond strengths tend to be inadequate (Artun and Bergland, 1984;Go Maijer and Smith, 1986Go).

Another possible means of enamel preparation is sandblasting. This technique has been used in orthodontics for treating the fitting surfaces of bands and brackets to enhance bond strength (Millett et al., 1993Go) and for the removal of cement from failed brackets prior to recementation (Regan et al., 1993Go). Only one study (Reisner et al., 1997Go) to date has evaluated sandblasting as a method of enamel preparation prior to bracket bonding. With the development of miniature intra-oral sandblasters it would seem timely to explore this possibility further.

The aims of this study were:

  1. In a pilot study, to examine the appearance of enamel surfaces, using scanning electron microscopy, following sandblasting and etching for similar time intervals, and to ascertain the optimal means of enamel preparation by sandblasting.
  2. To determine the mean shear debonding force of orthodontic brackets following enamel preparation with either sandblasting or etching.
  3. To analyse the mode of bond failure with both methods of enamel preparation.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 Conclusions
 References
 
Pilot study
The buccal surfaces of premolar teeth were either etched with 37 per cent phosphoric acid or sandblasted for, 5, 15, 30, 45, or 60 seconds. They were then observed using scanning electron microscopy to examine the morphology of the prepared enamel surfaces.

These preliminary electron microscopy results showed that the etched enamel surfaces demonstrated a classical, well defined `honeycomb' or `cobblestone' appearance (Figure 1). In this study, shorter etching times produced a cobblestone pattern and preferential dissolution of the prism peripheries, whereas longer etching times produced a honeycomb appearance and preferential dissolution of the prism cores, with greater depth of etching. Fifteen seconds etching was selected because significant etching was demonstrated on the scanning electron microscope photographs at this stage. Also, review of the orthodontic literature showed that, when using this relatively short etching time, bond strengths are not significantly reduced and enamel loss is more conservative.



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  		FIG. 1 Acid etching for 15 seconds.

 
Sandblasting produced a less well defined pattern on the enamel with irregular grooving of the enamel surface (Figure 2). Scanning electron microscopy showed that there was no apparent relationship between morphology and sandblasting time. Five seconds sandblasting was chosen because this would mimic what would be acceptable in the clinical situation.



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  		FIG. 2 Sandblasting for 5 seconds.

 
Main study
Sound extracted human premolars were stored in distilled water in a refrigerator following decontamination in formalin. They were divided onto two groups of 30, each comprising 15 mandibular and 15 maxillary premolars The roots were grooved to aid retention and then mounted in polyester resin blocks with their long axes vertical and their crowns protruding. Following removal from the moulds, the buccal enamel surfaces were cleaned with a pumice slurry, washed in water, dried and treated by one of the following methods:

  1. Etching: the buccal surfaces were etched with 37 per cent phosphoric acid solution for 15 seconds, washed, and dried thoroughly.
  2. Sandblasting: the buccal surfaces were sandblasted with a micro-etcher (Ventura Oral Systems Limited, Halifax, UK) using 50 µ alumina for 5 seconds and then blown with air to remove any residual contamination.

The prepared buccal surfaces were bonded with a mesh backed, premolar stainless steel bracket (`A' Company, Orthologic, UK) using Right-on ® (T.P. Orthodontics, Leeds, UK). This is a self-cured, lightly filled dimethacrylate resin. Excess composite was removed with a probe and the specimens were allowed to bench cure for 10 minutes. They were then immersed in distilled water in a humidifier at 37°C for 24 hours.

The shear debonding force required to debond the brackets was measured in Newtons using a cross-head speed of 10 mm/minute. A close fitting stainless steel wire loop was placed around the gingival tie wings and connected to the load cell of an Instron (Figure 3) using the method described by Fox et al. (1991Go).



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  		FIG. 3 Specimen for testing using an Instron.

 
The debonded brackets and enamel surfaces were examined for mode of bond failure under a stereomicroscope and several specimens were also observed using a scanning electron microscope. The most predominant site of bond failure was recorded for each specimen, the categories recorded being; adhesive failure at the enamel/composite interface, adhesive failure at the composite/bracket interface, or cohesive failure in enamel or composite. The percentage of specimens failing at each site was calculated for the sandblasted and etched specimens.

Statistical Analysis
Mean debonding force, standard deviation, and standard error were calculated for each sample, and analysed using a t-test. Weibull analysis was used to calculate probabilities of failure at given values of applied force. Chi-squared analysis was used to compare the mode of bond failure.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 Conclusions
 References
 
Debonding force data for etching and sandblasting are shown in Table 1. The mean debonding force for 15 seconds etching was 64.7 N and for sandblasting was 27.4 N. A t-test showed that the debonding force for etching was significantly higher (P< 0.001. Weibull analysis, which equates probability of bond failure with applied shear force has previously been used in bond strength studies (McCabe and Carrick, 1986;Go Fox et al., 1991Go). Weibull data are also shown inTable 1 and demonstrated graphically in Figure 4. The Weibull moduli are 2.4 and 1.8, respectively, for etching and sandblasting. The lower value of modulus for sandblasting demonstrates less reliability for this method of enamel preparation. The high values of correlation coefficient of linearized least square fit indicate that the data closely fits the Weibull distribution function. The graph illustrates that for a given probability of failure significantly more force would be required to dislodge a bracket cemented with Right-on ® following 15 seconds enamel etching than with 5 seconds sandblasting.


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  		TABLE1 Bonding characteristics for methods of enamel preparation
 


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  		FIG. 4 Weibull curves for etched and sandblasted surfaces.

 
Examination of mode of bond failure showed that the etched specimens tended to fail by a mixed mode of bond failure with some composite cement remaining on the enamel and some on the bracket (Figure 5). However, the sandblasted specimens showed adhesive bond failure at the enamel/composite interface (Figure 6) with minimal cement remaining on the enamel. Table 2 shows the data for mode of bond failure. Recording the predominant site of bond failure, all of the sandblasted specimens failed at the enamel/composite interface, whereas 19 (63 per cent) of the etched specimens failed at the enamel/composite interface and 11 (37 per cent) at the composite bracket interface. Recording the predominant site there were no cohesive failures within enamel or composite. Statistical analysis showed that these results were highly significant (Chi-squared = 13.47 on 1 df; P< 0.01).



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  		FIG. 5 Mixed mode of bond failure for etched specimen.

 


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  		FIG. 6 Failure at the cement/bracket interface for sandblasted specimen.

 

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  		TABLE 2 Predominant site of bond failure (per cent) for each specimen
 

   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
Conclusions
 References
 
The mean shear debonding force for brackets bonded following 15 seconds etching was significantly greater than for 5 seconds sandblasting. Several factors may account for this difference, the morphology of the prepared enamel being the most likely.

Acid etching provides micromechanical attachment by a variety of means, ranging from preferential dissolution of the prism cores resulting in a honeycomb appearance to preferential dissolution of the prism peripheries resulting in a cobblestone appearance (Carstensen, 1992Go). Pitted or smooth etching patterns may also be produced. Preferential dissolution of the prisms can occur to a depth of 5-25 µ with the diameter of the defect ranging from 5-6 µ (Reynolds, 1975Go). On premolar teeth, honeycomb or cobblestone etch patterns are mainly exhibited in the central areas with pitted or smooth etching morphologies more commonly found in the cervical areas (Carstensen, 1992Go). Varying the phosphoric acid concentration from 5 to 37 per cent has been shown to have a significant effect on the etch pattern with a reduced acid concentration resulting in less depth of etch (Bryant et al., 1987;GoLegler et al., 1990Go). However, there is little effect on bond strength (Legler et al., 1989;GoSadowsky et al., 1990Go). Altering the etching times from 15 to 60 seconds, whilst maintaining a phosphoric acid concentration of 37 per cent also results in significantly less depth of etch (Barkmeier et al., 1985;Go Legler et al., 1990;Go Surmont et al., 1992Go). Bond strength of bonded brackets is not compromised by the reduced etching times (Barkmeier et al., 1985;Go Carstensen, 1986;Go Labart et al., 1988;Go Legler et al., 1989Go).

In relation to sandblasting, there has only been one previously published account describing its effect on enamel, apart from when described as air abrasion which appears to be synonomous. This uses a high velocity stream of alumina and air, producing a uniform roughness of the enamel up to 5 µ in depth, although individual defects ranging in width from 1 to 20 µ have been described (Laurell and Hess, 1995Go). On the other hand, air powder polishing uses a stream of air, water, and sodium bicarbonate (Barnes et al., 1987Go), and produces non-uniform roughening of enamel (William et al., 1980Go). A similar appearance was produced by sandblasting in the present study.

In this study sandblasting produced irregular grooving of the enamel with a less regularly defined pattern than that demonstrated with etching. Varying the particle size of the alumina and the duration of sandblasting could influence the morphology produced. We used a particle size of 50 µ provided and recommended by the manufacturer of the micro-etcher. A larger particle size may give different results. In relation to the duration of sandblasting, no difference was noted under SEM between enamel sandblasted for 5, 15, 30, 45, or 60 seconds. As a 5-second sandblast would be the only reasonable duration clinically (unless special precautions, e.g. rubber dam isolation were undertaken) this time interval was chosen for specimen preparation.

Following debonding, the brackets bonded to sandblasted enamel showed less composite remaining on the enamel surface than with acid etching. This is consistent with the mode of bond failure following crystal growth (Artun and Bergland, 1984Go) or with very low acid etch concentrations (Carstensen, 1993Go). Although this mode of bond failure would facilitate clean up following debonding and reduce the possibility of iatrogenic damage to enamel following this procedure, the weak bond strengths recorded with sandblasting enamel precludes its use clinically.

The mean debonding force for brackets bonded to sandblasted enamel was less than half that recorded for brackets bonded to acid etched enamel (27.4 and 64.7 N, respectively). Weibull analysis showed that there would be a 5 per cent chance of failure for brackets bonded to sandblasted enamel at a low force magnitude of 10 N. As forces applied clinically are likely to be well in excess of this, brackets bonded to sandblasted enamel would have an unacceptable clinical failure rate.

From this study the main disadvantages of sandblasting enamel are the unacceptably low debonding force in comparison to acid etching and the increased probability of bond failure at low levels of applied force. Sandblasting enamel is not recommended as a means of enamel preparation for orthodontic bonding, but is a useful technique to increase bond strengths when bonding to porcelain, amalgam or to gold (Zachrisson and Buyukyilmaz, 1993Go).


   Conclusions
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Conclusions
References
 

  1. Brackets cemented with Right-on ® following 15 seconds etching showed significantly higher mean shear debonding forces than following 5 seconds sandblasting.
  2. Weibull analysis showed that for a given applied stress, the probability of bond failure was significantly less for etched enamel than for sandblasted enamel.
  3. There was a mixed mode of bond failure for the brackets cemented following etching. Following sandblasting the specimens failed cleanly at the enamel/composite interface.


   Acknowledgments
 
The authors would like to acknowledge the assistance of Dr T. E. Carrick in specimen testing, Ventura Oral Systems, Halifax, UK, for supplying the micro-etcher and Orthologic, UK for the provision of the `A' Company brackets.


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Conclusions
 References
 
Artun, J. and Bergland, S. (1984) Clinical trials with crystal growth conditioning as an alternative to acid etch pretreatment,American Journal of Orthodontics , 77, 201 –208.

Barkmeier, W. W., Gwinnett, S. J. and Shaffer, S. E. (1985) Effects of enamel etching time on bond strength and morphology,Journal of Clinical Orthodontics , 16, 36 –37.

Barnes, C. M., Hayes, E. F. and Leinfelder, K. F. (1987) Effects of an air abrasive polishing system on restorative surfaces,Journal of General Dentistry , 35, 186 –189.

Bryant, S., Retief, D. H., Russell, C. M. and Denys, F. R. (1987) Tensile bond strengths of orthodontic bonding reins and attachments to etched enamel,American Journal of Orthodontics and Dentofacial Orthodontics , 92, 225 –231.

Carstensen, W. (1986) Clinical results after direct bonding of brackets using shorter etching times,American Journal of Orthodontics , 89, 70 –72.[Medline]

Carstensen, W. (1992) The effect of different phosphoric acid concentrations on surface enamel,Angle Orthodontist , 61, 51 –57.

Carstensen, W. (1993) Clinical effects of reduction of acid concentration on direct bonding of brackets,Angle Orthodontist , 63, 221 –224.[Medline]

Chow, L. C. and Brown, W. E. (1973) Phosphoric acid conditioning of teeth for pit and fissure sealants,Journal of Dental Research , 52, 1158 .[Free Full Text]

Evans, R. and Oliver, R. (1991) Orthodontic bonding using glass ionomer cements: an in-vitro study,European Journal of Orthodontics , 13, 493 –500.[Abstract/Free Full Text]

Fox, N. A., McCabe, J. F. and Gordon, P. H. (1991) Bond strengths of orthodontic bonding materials; an in-vitrostudy,British Journal of Orthodontics , 18, 125 –130.[Abstract]

Labart, W. A., Barkmeier, W. W. and Taylor, M. H. (1988) Bracket retention after 15 seconds acid conditioning,Angle Orthodontist , 22, 254 –255.

Laurell, K. L and Hess, J. A. (1995) Scanning electron micrographic effects of air-abrasion cavity preparation on human enamel and dentin,Quintessence International , 26, 139 –14.,[Medline]

Legler, B. S., Retief, D. H., Bradley, E. L., Denys, F. R. and Sadowsky, P. L. (1989) Effects of phosphoric acid concentration and etch duration on the shear bond strength of an orthodontic bonding resin to enamel—an in-vitro study,American Journal of Orthodontics and Dentofacial Orthodontics , 96, 485 –492.

Legler, B. S., Retief, D. H. and Bradley, E. L. (1990) Effects of phosphoric acid concentration and etch duration on enamel depth of etch: an in vitro study,American Journal of Orthodontics and Dentofacial Orthodontics , 98, 154 –160.

Maijer, R. and Smith, D. C. (1986) Crystal growth on the outer enamel surface—an alternative to acid etching,American Journal of Orthodontics and Dentofacial Orthopaedics , 90, 195 –198.

McCabe J. F. and Carrick T. E. (1986) A statistical approach to the mechanical testing of dental materials,Dental Materials , 2, 139 –142.[Medline]

Millett, D. T., McCabe J. F. and Gordon, P. H. (1993) The role of sandblasting on the retention of metallic brackets applied with glass ionomer cement,British Journal of Orthodontics , 20, 117 –122.[Abstract]

Newman, G. V. (1965) Epoxy adhesives for orthodontic attachments,American Journal of Orthodontics , 51, 901 .[Medline]

Pizarro, K. A., Jones, M. L. and Knox, J. (1994) An in vitro study of the effects of differing crystal growth solutions on the topography of the enamel surface,European Journal of Orthodontics , 16, 11 –17.[Abstract/Free Full Text]

Regan, D., LeMasney, B. and Van-Noort, R. (1993) The tensile bond strength of new and rebonded stainless steel orthodontic brackets,European Journal of Orthodontics , 15, 125 –135.[Abstract/Free Full Text]

Reynolds, I. R. (1975) A review of direct orthodontic bonding,British Journal of Orthodontics , 2, 171 –178.

Reisner, K. R., Levitt, H. L. and Mante, F. (1997) Enamel preparation for orthodontic bonding: a comparison between the use of a sandblaster and current techniques,American Journal of Orthodontics and Dentofacial Orthopaedics , 111, 366 –373.

Sadowsky, P. L., Retief, D. H., Cox, P. R., Hermandez-Orsini, R., Rape, W. G. and Bradley, E. L. (1990) Effects of etchant concentration and duration on the retention of orthodontic brackets: an in vivo study,American Journal of Orthodontics and Dentofacial Orthopaedics,98, 417 –421.

Surmont, P., Dermant, L. and Moors, M. (1992) Comparison of shear bond strength of orthodontic brackets between five bonding systems related to different etching times; an in vitro study,American Journal of Orthodontics and Dentofacial Orthopaedics , 101, 414 –419.

William, D., Norling, B. and Johnson, W. (1980) A new prophylaxis instrument; effect on enamel alterations,Journal of the American Dental Association ,101, 923 .[Abstract]

Zachrisson, B. U. and Buyukyilmaz, T. (1993) Recent advances in bonding to gold, amalgam and porcelain,Journal of Clinical Orthodontics , 27,661 –675.




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