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Scientific Section |
P.O. Box 22490, Riyadh 11426, Kingdom of Saudia Arabia
Abstract
Objectives: To compare the effect of five in-office bracket reconditioning methods on: (i) bracket slot width and interwing gap measurements; (ii) the appearance of the bracket bases under scanning electron microscope (SEM), and; (iii) shear/peel bond strength (SPBS).
Setting: Ex vivo study.
Method: One hundred and twenty-five brackets were initially bonded and were divided into five experimental groups and reconditioning by the following methods: (i) adhesive grinding using green stone (Gp II); (ii) sandblasting (Gp III); (iii) direct flaming (Gp IV); (iv) using the BigJane machine (Gp V), and; (v) application of Buchman method (Gp VI).
Outcomes: Distortion of the brackets. Scanning electron miscropy of three representative specimens from each group. The remaining brackets were rebonded, then shear/peel forces to failure were measured (SPBS).
Results: The ANOVA and multiple comparison test exhibited a statistical, but not clinical, significant increase in the bracket measurements of Group VI. There was a significant reduction (28%) in the SPBS of Group II. Under the SEM, the wire mesh structure was maintained; however, the amount of adhesive remnants greatly varied among the groups.
Conclusions: Although none of the in-office reconditioning methods employed adversely affected the bracket base and/or the bracket measurements, reconditioning with a green stone was not effective. Sandblasting method and direct flaming are recommended because of simplicity and time-saving advantages.
Key words: Bracket Base, Bracket Recycling, Bracket Slot, In-office Reconditioning, Stainless Steel Brackets.
Introduction
Orthodontists are commonly faced with the decision of what to do with ‘loose’ brackets, and/or with inaccurately located brackets that need respositioning during treatment (Wright and Powers, 1985
; Regan et al., 1993). One solution is to recycle the brackets. However, the efficiency of the orthodontic treatment will be affected by any distortion of the bracket base, change in the slot size, and/or reduction in bracket bond strength produced during the reconditioning process. As a result, when brackets are recycled, the method used should completely remove the bonding material from the bracket without distorting the bracket. Importantly, the slot tolerance of the recycled bracket should not only be changed, but also the potential for good bonding should not be reduced. While there are several commercial recycling methods available, these are impractical to perform at the chairside. As a result, several in-office bracket reconditioning methods have been introduced. These include a variety of mechanical methods (e.g. handpieces with rotary burs or chairside sandblasting), a variety of thermal methods (e.g. direct flaming or heating in a furnace), and a combination of both mechanical and thermal methods (e.g. the Buchman method which consists of direct flaming to burn off the composite, followed by sandblasting and electropolishing). The effectiveness of these methods has been evaluated in several investigations. For example, a reduction in the bracket bond strength was reported after grinding the adhesive with a green stone to the surface of the mesh base (Wright and Powers, 1985). Alternatively, when the resin surface of the bracket was roughened with a green stone, the rebond bond strength was not changed (Egan et al., 1996). In addition, a study by Regan et al., (1993) revealed that the difference between the bond strength results obtained following bracket base preparation with a green stone and a more complicated process, i.e. Buchman method, was not significant. When sandblasting techniques using a high-speed stream of aluminum oxide particles propelled by compressed air were evaluated by Sonis, (1996). Millet et al. (1993) and MacColl et al. (1998), it was found that sandblasting increased the bond strength and the survival time of the new brackets. In addition, when the shear bond strengths of previously failed bonded metal brackets subjected to air abrasion was compared with new brackets, no significant differences between the two groups were found (Sonis, 1996). Another method was introduced by the Esmadent recycling company, which has advertised a BigJane machine, that can be purchased for bracket recycling in the office (Buchman, 1980). It was found that, one recycling using this machine is of negligible clinical importance without compromising retention or mechanical precision of the edgewise mechanism (Wheeler and Ackerman, 1983).
The aim of this study was to evaluate and compare the effects of five in-office reconditioning methods of metallic brackets on: (i) the bracket slot width and inter-wing gap dimensions; (ii) the bracket base appearance under the SEM; and (iii) the shear/peel bond strength (SPBS) of the bracket.
Materials and Methods
Preparation of brackets for recycling
One-hundred-fifty new metallic lower incisor brackets (full size diamond standard edgewise twin bracket, 0•022-inch slot, Cat. #3420500, ORMCO Corporation, Glendora, California, USA), with 9•68-mm2 bracket base surface area, were divided into six groups, one control (Group I ) and five experimental groups (Groups II–VI), each was composed of 25 brackets. Experimental brackets were initially bonded to a flat translucent polytetrafluoroethylene sheet (PTFE) using a light-cured highly filled orthodontic adhesive (Transbond XT, 3M Unitek Corporation/3M, Monrovia, California, USA), in strict accordance with the manufacturer's instructions. This step was carried out without the benefit of etching, so that predictable plastic sheet/adhesive separation would occur on debonding.
Both the plastic sheet and the bracket bases were coated with a thin layer of primer, which was thinned with a gentle stream of oil- and moisture-free air, then light-cured for 10 seconds (Elipar Highlight light curing unit ESPE Dental-Medizin GmbH and Co. KG, D-82229 Seefeld, Germany). The adhesive was applied to the bracket base. The bracket was then positioned on the PTFE sheet and seated under a standard force (500 g weight) (Basudan, 1998). The excess resin flash around the base was removed with a dental explorer. Light was then applied for 10 seconds on each of the proximal sides of the bracket to cure the adhesive. The brackets were easily debonded using a tweezers (Hommacher, Solinqen, Germany stainless HSC 014-05) to either the mesial or distal tie wings, then examined visually and microscopically (Swift Institute, International model 7819551, Tokyo, Japan) at x20 magnification to ensure that failures were at the resin/plastic sheet interface. Following bracket debonding, five different reconditioning methods were applied on the experimental groups to remove the resin layer attached to the bracket base prior to rebonding (Table 1). The control group was neither bonded initially nor reconditioned.
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). One examiner carried out all measurements. The measurements of the control group were taken at two different times in order to assess the intra-examiner method error. Any differences were evaluated with the student t-test for paired samples, double determination method error, and coefficient of reliability (Houston, 1983).
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Shear/Peel Bond Strength Testing
The remaining 22 brackets of each of the experimental groups were rebonded, while the Control brackets were bonded for the first time, to modified acrylic cylinders with holes filled with a light-cure composite material (Restorative Z100, 3M Scotchbond, St. Paul, MN 55144-1000, USA; Basudan,1998). Bond strength testing was carried out on a universal testing machine (Instron, Model 8500 PLUS Dynamic Testing System, USA, 100 Royall Street, Canton, MA 02021-1089) using a customized mounting jig (Basudan, 1998). An occlusogingival load at 0•5 mm/min cross-head speed was applied to the bracket by moving the lower jaw upwards producing shear force at the bracket adhesive interface and parallel to the bracket base (Figure 2). The load required for debonding was recorded and converted to the maximum shear/peel stress in megapascals.
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). A one-way analysis of variance (ANOVA) was used to evaluate the effect of reconditioning methods on debonding shear stress Any significant differences revealed by ANOVA were further investigated using a Tukey B test. Significance for all statistical tests was set at 5 per cent (P 
0•05). Results
Change in Bracket Dimensions
The error analysis revealed that the method of measurement was reliable. Table 2 shows slot width and inter-wing gap values for each group. Data analysis revealed that there was a difference in both measurements between groups (ANOVA, P < 0•00001). Tukey tests showed that both the slot width and the inter-wing gap means of Group VI (Buchman method) were significantly different from those of other groups at the 0•05 level of significance.
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). Viewing the brackets in the SEM after reconditioning showed variation of surface differences between the groups. Figure 3b shows one of the representatives of Group II that had been reconditioned mechanically using green stone. It is clear that all available undercuts were filled with the adhesive with a nearly continuous resin coverage above the level of the wire mesh intersections. The sandblasted bracket bases were dull and rough with an intact multi-stranded structure, and the retentive areas were less well-defined (Figure 3c). Some of brackets that were directly flamed, were similar to the control, while others had some of the retentive areas filled with the adhesive (Figure 3d). With the naked eye, only three brackets in Group V had a few white specks, that is adhesive remnants attached to the bracket base. The remaining 22 brackets appeared very similar to the control bracket, but with very few adhesive-filled retentive areas (Figure 3e). None of the Buchman group brackets had adhesive remnants when viewed under the light microscope. However, the bases were slightly dull and rough when viewed under SEM (Figure 3f).
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. When this data were analysed with a one-way ANOVA, a significant difference in the mean SPBS of the groups (P < 0•00001) was found. Multiple comparison range testing revealed that that the mean SPBS of the group of brackets prepared by grinding (Group II) is significantly different from all the other groups, at the 0•05 level of significance.
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One of our findings was that Buchman method resulted in an increase 21•06 and 28•14 µm for slot width and inter-wing gap, respectively. However, these amounts can be considered clinically insignificant.
It was evident from the analysis of the shear/peel bond strength (SPBS) data that the mean SPBS of Group II (grinding with green stone) is significantly lower than all other methods. From a mechanical point of view, this is not surprising because preparing the brackets for rebonding by removal of the adhesive with a green stone, leaves a composite surface devoid of undercuts (Figure 3b
).
The optimal bond strength required for orthodontic clinical use is as yet unknown. Ideally, the brackets should be easily bonded to the enamel, not undergo any in-service bond failures and yet be easily removed at the end of treatment without damage to the enamel surface (Ireland and Sheriff, 1997). The highest number given as an optimal bond strength required clinically, 7•85 Mpa, was cited by Reynolds (1975). In this study, all brackets tested cleared this requirement. However, extrapolation of laboratory data to the clinical situation should always be done with caution.
Which is the Most Effective Method of Chairside Conditioning?
When we consider the clinical value of our finding. It appears that mechanical adhesive grinding is quick, simple and easy to perform as a chairside in-office procedure. Unfortunately, this results in a reduction in bond strength. When the Buchman method or BigJane machine is used, the procedure is complex and takes more time. Sandblasting and direct flaming methods appear to offer the clinician a viable, simple, easy method to immediately reuse previously failed brackets. However, it should be emphasized that the composite incineration process is known to produce toxic fumes that might be inhaled (Klaassen, 1996). Nevertheless, the amount of adhesive remnants burned during the in-office bracket reconditioning process is small and with wearing a facemask in an open room space, the produced vapour is considered as a very low hazardous material.
Conclusions
Acknowledgments
The authors would like to thank the CDRC (College of Dentistry Research Center, King Saud University) for supporting this research, Dr Fouad Salama for his constant guidance, and Professor Hamdi Mohammed-AlTahawi for his help in editing.
Notes
E-mail: aisha_basudan{at}yahoo.com
References
Basudan, A. M. (1998)A comparison of in-office reconditioning methods of metallic direct-bond orthodontic brackets,Master's Thesis, King Saud University.
Buchman, D. J. (1980) Effects of recycling on metallic direct-bond orthodontic brackets, American Journal of Orthodontics, 77, 654–668.[Medline]
Egan, R. R., Alexander, S. A. and Cartwright, G. E. (1996) Bond strength of rebonded orthodontic brackets, American Journal of Orthodontics and Dentofacial Orthopedics, 109, 64–70.[Medline]
Houston, W. J. B. (1983) The analysis of errors in orthodontic measurements, American Journal of Orthodontics, 83, 382–390.[Medline]
Ireland, A. J. and Sheriff, M. (1997) The effect of timing of archwire placement on in vivo bond failure, British Journal of Orthodontics, 24, 243–254.[Abstract]
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MacColl, G. A., Rossouw, P. E., Titley, K. C. and Yamin, C. (1998) The relationship between bond strength and orthodontic bracket base surface area with conventional and micro-etched foil-mesh bases, American Journal of Orthodontics and Dentofacial Orthopedics, 113, 276–281.[Medline]
Millet, D , 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]
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.
Reynolds, I.R. (1975) A review of direct orthodontic bonding, British Journal of Orthodontics, 2, 171–178.
Sonis, A. L. (1996) Air abrasion of failed bonded metal brackets: a study of shear bond strength and surface characteristics as determined by scanning electron microscopy, American Journal of Orthodontics and Dentofacial Orthopedics, 110, 96–98.[Medline]
Wheeler, J. J. and Ackerman, R. J. (1983) Bond strength of thermally recycled metal brackets, American Journal of Orthodontics, 83, 181–186.[Medline]
Wright, W. L. and Powers, J. M. (1985) In vitro tensile bond strength of reconditioned brackets, American Journal of Orthodontics, 87, 247–252.[Medline]
Zar, J. H. (1996)Biostatistical Analysis, 3rd edn,Prentice-Hall Inc., New Jersey.
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