A comparison of shear bond strength and mean survival time between treated and untreated mesh backed stainless steel brackets, an in-vitro assessment.

Transcription

1 (J lnd Orthod Soc 2002; 35: ) RESEARCH ARTICLE - A comparison of shear bond strength and mean survival time between treated and untreated mesh backed stainless steel brackets, an in-vitro assessment. Nitin 0. Gulve on: M. Bhagtan;"" In the present study, an anempt has been made to increase the retentive mechanism of the bracket base by means of sandblasting or chemical etching of the bracket base. From the study, it was observed that sandblasting and chemical etching of the bracket base shows a micro-roughened bonding surface, when examined under the Scanning Electron Microscope. Sandblasting or chemical etching of mesh backed stainless steel brackets produced a significant increase in shear bond strength when tested using Instron unit and showed increase in mean survival time under simulated mechanical loading using a Ballmill. Introduction: Bonding Orthodontic attachments to teeth have become an accepted clinical procedure in orthodontics. The retention of components bonded to the tooth surface is important to ensure a successful outcome for lixed orthodontic therapy. A wide variety 01 brackets are available for clinical use and a considerable amount of development has taken place to improlle their properties. The materials from which they are constructed may be stainless steel, ceramic or plastic. The most popular material in current use is stainless steel. As stainless steel brackets do not form a chemical union with any of the available bonding adhesive, mechanical interlock must be obtained. Several designs of bracket bases have been devised for this purpose, for example; perforations in bracket base, wire mesh, photoetched undercuts, horizontal undercut channels etc. Of these, meshbracket bases gained the widest acceptance.' 2 However clinical reports and numerous in-vitro studies have demonstrated that the adhesive resin usually bonds more firmly to the etched enamel than to the mesh. The weak link in the bracket adhesive Reader. Department of Orthodontics & Dent01acial Orthopaedics. M.G.V.'S DenIal College & Hospital, Nashik. o. Prolessor & Head, Depanment of Orthodontics & Denl0facial Orthopaedics, Nair Hospital DenIal College. Mumbai. system is the poor mechanical interlocking at the mesh adhesille interlace.u Current research in the field of surface treatment of metal to improve bonding in restorative dentistry may have some ramification with respect to orthodontic bonding procedure. One of the most useful devices to be introduced in recent years is the sandblaster. Sandblasting involves spraying a stream of aluminium oxide particles of 50 11m size under high pressure against the metal surface intended lor bonding, leading to microetching of metal. Etching 01 metal can also be achieved by treating the metal surlace with 9% hydrofluoric acid for two minutes in an enclosed ventilated hood, followed by cleaning with sodium hypoxide. 7 The aim of the present study were: 1) To evaluate, using a Scanning Electron Microscope, the surface 01 stainless steel mesh backed bracket base that have been sandblasted and chemicajly etched. 2) To compare, in-liitro, the shear bond strength 01 sandblasted mesh backed stainless steel bracket, chemically etched mesh backed stainless steel brackets and untreated mesh backed stainless steel brackets, using Instron Universal Testing Machine. And to determine and compare the sites of bond failure after debonding for clinical significance with magnifying glass. 124

2 (J Ind Orthod Soc 2002; 35: ) 125 3) To compare, in-vitro, the effect of simulated mechanical stress on survival time of sandblasted mesh backed stainless steel bracket, chemicall y etched mesh backed stainless steel brackets and untreated mesh backed stainless steel brackets, using a Ballmill. Materials and methods: One hundred and eight contoured stainless steel mesh backed Begg brackets were used in this study. Of these 3 were used for analysing the surface of bracket base under Scanning Electron Microscope. 75 brackets were used for bond strength analysis, while remaining 30 were used for survival lime analysis. Examination of bracket base under Scanning Etectron Microscope: Three mesh backed stainless steel brackets were examined using Scanning Electron Microscope. (Fig. 1) First bracket was untreated and used as a control sample. In the second bracket, surface of the bracket base was sandblasted with aluminium oxide particles. In the third bracket, the surface of the bracket base was treated with hydrofluoric acid. The beam of Scanning Electron Microscope was directed at 900. Subgroup 1 was bonded with untreated mesh backed stainless steel brackets, subgroup 2 was bonded with sandblasted mesh backed stainless steel brackets and subgroup 3 was bonded with chemically etched mesh backed stainless steel brackets with a self cure composite following manufacturers instructions. All specimens were then placed in an incubator at 37 C for 24 hours. An Instron Universal Testing Machine with a crosshead speed of 1 mmlmin was used to measure the force required to debond the brackets. A stainless steel wire loop was hocked in the slot of the bracket. The direction of pull was so designed that it would include shear force to simulate oral condition. In order to obtain Fig. 2.' Testing the shear bond strength Fig. 1: Scanning Electron Microscope Shear bond strength testing: 75 freshly extracted premolars were obtained from patients scheduled for extraction for orthodontic treatment. Teeth were sectioned at the cementoenamel junction with carborandum disk under running water. Crowns 01 the teeth were then embedded in acrylic resin such that the buccal surfaces of teeth were exposed. The samples were divided into 3 subgroups teeth each. Each subgroup was given colour coding. the peak stress, the peak load has to be divided by the area of bracket base, which was obtained using planometer. Following debonding, the fracture sites were visually inspected with a magnifying glass to detennine the location of bond failure. Assessing the effect of simulated mechanical stress on the survival time of brackets: Bond strength test may give an incomplete indication of clinical performance. A more accurate method of testing bond reliability may be achieved using simulated mechanical loading. The Ballmill provides such a mechanical stress, and bond Fig. 3: Sallmill

3 126 A comparison of shear bond strength and mean survival time between treated... - Nitin O. Gulve, T. M. Bhaglani survival can be checked at various time intervals. (Fig. 3). Samples of 30 teeth were divided into 3 subgroups of 10 teeth each and were colour coded, with indelible pen. Subgroup 1 was bonded with untreated mesh backed stainless steel brackets, subgroup 2 was bonded with sandblasted mesh backed stainless steel brackets, and subgroup 3 was bonded with chemically etched mesh backed stainless steel brackets with a self cure composite following manufacturers instructions. The sample was placed in a Ballmill, which contained 470 gms of ceramic spheres with diameter varying from 16.8 mm to 17.9 mm, (Fig. 4). The Sallmill was then charged with 250 ml of distilled water at 37 C. The SallmiU was subjected to 100 revolutions per minute. After every hour the mill was opened. the teeth were removed and kept aside. roughened bonding surface. (Fig. 6 A & B). The chemically etched bracket base also shows microroughened bonding surface. (Fig. 7 A & B). Fig. 6 A: Sandblasted (150 X) Fig, 6 B: Sandblasted (500 X) Fig. 4: Interior view of the Ballmill Testing was recommended. The same procedure was continued till all samples showed bond f?iture. Observations and Results: Examination of bracket base under Scanning Electron Microscope: Untreated bracket base shows the relatively smooth bonding surface. (Fig. SA & 8). The sandblasted bracket base shows the micro- Fig. 7 A: Chemically etched Fig. 7 B: Chemically etched bracket base (150 X) under SEM. (500 X) Shear bond strength testing: Table t shows the mean value, range and standard deviation for all 3 subgroups. The mean bond strength lor subgroup I was 6.59 Mpa, for subgroup 2 was 9.16 Mpa, and for subgroup 3 was 8.79 Mpa. Table-I Mean Bond Strength, Range and Standard Deviation in Megapascals Subgroup Mean Range Standard deviation 1 (Unlreated) (Sandblasled) (Chemically Elched) , Fig. 5 A: Untreated (150 X) Fig. 5 B: Untreated bracket base Under SEM. (500 X) Table II shows that there is a statistically significant increase in shear bond strength of sandblasted mesh backed stainless steel bracket and chemically etched mesh backed stainless steel bracket as compare to the untreated mesh backed stainless sleel bracket.

4 (J Ind Orthod Soc 2002; 35:124,128) 127 Table-II Statistical Comparison of Mean Bond Strengths Using a Student's 't' Test Subgroup, 2 3 (Unlle&ted) (SandblaSled) (ChemlCf,Uy Etched) 1 (UnHealed) ' 5.58' 2 (Sandblasted) (Chemically Etched) Denoles statistically significant dilferences at 10/0 significance lor 48 degree of freedom. T table Table III shows the distributive percentage 01 various bond failure sites. In subgroup I, oul of 25 samples, 20 show the fracture at bracket base - adhesive interface. In subgroup 2 and subgroup 3 out of 25 samples, 17 show fracture at adhesiveadhesive interface. There was no instance of enamel detachment for any sample. -- I (Untrealed) Table-III Bond Failure Sites Ad Adhesive Adl"leslve Bracket... (Cohesive) Ad""" """'" 20 (80%) 3(12"1.) 2 (80/. 1 Enamel """'" 0 2 (Sandblasted) 5 (20"10) 17 (68%) 3(12%) 0 3 (Chemically Etched) 6 (24,.. ) 17 (68%) 2 (8%) 0 Table-IV Number of Bond Failures per Hour in the Ball mill Hours Subgroup 1 Subgroup 2 Subgroup Total Assessing the effect of simulated mechanical stress on the survival time of brackets: Table IV shows, number of bond failure per hour in the Ballmil!. In the present study, all specimens that had not received any bracket base treatment failed with in 14 hours. Whereas. only four out of ten specimens with each of sandblasted bases and chemically etched bases had failed at the same time i.e. 14 hours. An specimens showed bond failure by the end of 20 hours. Table V shows, the mean survival time for sandblasted mesh backed stainless steel brackets to be 14.7 hours and chemically etched mesh backed stainless steel brackets was 13.8 hours as compared to 8 hours for untreated mesh backed stainless steel brackets. Table Vt shows that there is a statistically significant increase in mean survival time 01 sand Table-V Mean Survival Time (in hours) No. Subgroup 1 Subgroup 2 Subgroup 3 (Untreated) (Sandblasted (Chemically Etched) Mean Table- VI Statistical Comparison of Mean Bond Strengths using A Student's 't' Test Subgroup 1 (Un. 2 (Sand 3 (Chemically treated ) blasted) Etched) 1 (Untreated) ' 2.92' 2 (Sand blasted) (Chemically Etched) - - -, - Denotes statistically significant dilfetences at 1 % significance lor 18 degree 01 freedom. "l'table -= 2.88.

5 128 A comparison 01 shear bond strength and mean survival time between treated... - Nitin D. Gulve, T. M. Bhagtani blasted mesh backed stainless steel bracket and chemically etched mesh backed stainless steel bracket as compared to untreated mesh backed stainless steel bracket. Discussion: Since the beginning of orthodontic science, the orthodontist has always endeavored to engineer 'a handle' 9n teeth so that, they can be moved more efficiently. The stainless steel bonded brackets have become the most common medium through which orthodontic forces are applied. Any orthodontic appliance system depends on an extremely fastidious bracket placement in order to achieve excellent treatment results. The failure of a direct bonded attachment during orthodontic treatment is an unpleasant experience for both the patient as well as the orthodontist. In the present study, an attempt has been made to increase the retentive mechanism of the bracket base by means of sandblasting or chemical etching of the bracket base. From the study it was observed that, sandblasting or chemical etching of the bracket base produces a micro-roughened bonding surface, when examined under the Scanning Electron Microscope. Sandblasting of mesh backed stainless steel bracket bases produced a significant increase in the mean shear bond strength and mean survival time over that of untreated mesh backed stainless steel bracket bases. This study is in agreement with that of Millett D.T. et a1 8, Newman G. V. et. a1 9 Chemical etching of mesh backed stainless steel bracket bases also produced a significant increase in the mean shear bond strength and mean survival time over that of untreated mesh backed stainless steel bracket bases. No previous study has examined the mean shear bond strength and mean survival time of chemically etched bases, hence the result cannot be compared with other studies. Taken in conjunction with significantly higher bond strengths of treated bracket base, it may be found that the potential for enamel damage is greater with treated bracket base. However no enamel damage was observed in this study. In contemporary orthodontics. there has been a drastic reduction in the width of the bracket, which reduces the bracket base area of the bracket and hence requires an increase in the retentive property. This can be achieved by preparing the bracket base by sandblasting or chemical etching process. This bracket base preparation can also be useful in noncompliant patients and for posterior teeth. Sandblasting and chemical etching of bracket bases are not recommended for ceramic brackets and also not for adult patients presenting with enamel cracks. any enamel defect and periodontally involved teeth. Conclusion: From the result of this study one would expect sandblasted or chemically etched mesh backed stainless steel brackets to have longer clinical time as compared to untreated mesh backed stainless steel brackets. The sandblasting and chemical etching procedure can be used for situations where increased bond strength is required. References: 1) Gorelick L. Bonding. the state of the art a national survey. J Clin Orthod: 13: 39-53, ) Reynold I. R.. Von Fraunhofer J. Direct bonding in Orthodontics: a comparison of attachments Br J O"hod: 4: ) Buzzina V. A. Hallgren S. E. Powers J. M. Bond strength of Orthodontic direct bonding cementbracket system as studied in vitro. Am J Orthod: 81: ) Dickinson P. T., Powers J. M., Evaluation of 14 direct bonding Orthodontic bases. Am J Orthod: 78: ) Maijer R., Smith D. C. Variables influencing the bond strength of metal Orthodontic brackets base. Am J O"hod: 79: ) MacColi G. A., Rossouw P. E., Tilley K. C., Yamin C. The relationship between bond strength and orthodontic bracket base surface area with conventional and microetched foil-mesh bases. Am J O"hod: 113: ) Tay W. M., Resin bonded bridges, first edition, ) Millet D. T. McCabe J.F.. Gordon P. H., Role of sandblasting on retention of metallic brackets applied with glass ionomer cement. Br J. Orthod: 20: : ) Newman G. V. Newman R. A., Sun B. I. Jin-lian. Ozsoylu S. A., Adhesive promoters: theirs effect on the bond strength of metal brackets. Am J Orthod: 108: : We thank Mrs. Saraswathy and Mr. Sanjava for helping with typing and posting of the journal.