LASER ETCHING OF ENAMEL FOR DIRECT BONDING AN INVITRO STUDY

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1 LASER ETCHING OF ENAMEL FOR DIRECT BONDING AN INVITRO STUDY Dr. Rajesh Reddy K. Authors: Dr. Rajesh Reddy K., MDS, Morth RC SEd. Associate Professor Dr. Kishore M.S.V., MDS, Morth RC SEd. Associate Professor Dr. K. Sadashiva Shetty, MDS Professor and Head Department of Orthodontics, Bapuji Dental College and Hospital, Davangere , Address for Correspondence : Dr. Rajesh Reddy K. Sowbhagya Dental Center, /1/A/B, Sowbhagya Abode, Road No. 7, Banjara Hills, Hyderabad , Andra Pradesh, Mob. : kuchkullar@yahoo.com Abstract : Key words : The aim of the Dr. study Kishore was to determine M.S.V, MDS, the shear Morth bond RC SEd. strength of mesh shaped stainless steel orthodontic brackets, Associate bonded Professor to acid etched enamel and laser etched enamel and to compare the shear bond strength following acid etching and laser etching. 50 non carious extracted premolar teeth divided in to 5 groups of 10 each were employed in the study. The buccal surfaces of group - I were subjected to conventional etching using 37% phosphoric acid for 30 seconds, while the other four groups were subjected to Nd:YAG laser etching at different power settings of 80mj, 100mj, 150mj and 200mj respectively for 15 seconds. Brackets were later bonded on to these teeth using Ultimate- light curing primer and adhesive. The shear bond strength of each sample was determined using a universal testing machine and the results were evaluated. Acid Etching, Laser Etching, Bond strength. INTRODUCTION Orthodontic clinicians have long been interested in the development of methods by which orthodontic brackets could be attached directly to the teeth. This bandless system which eliminates the placing of metal bands with attached brackets as a means for spatial control of teeth during orthodontic tooth movement has several advantages over the full band system. The classical micromechanical method of bonding resins to enamel consists of cleaning the enamel, etching it with phosphoric acid, rinsing and drying it. This acid etching method has been used to promote adhesion since Buonocore (1955), first described the technique to increase the bond strength of resins to enamel, following which Neumann in 1965 applied it to direct bonding of orthodontic brackets. However, the acid etch technique had several undesirable sequelae, which include loss of enamel, enamel cracks 29

2 and scratches, enamel decalcification, caries and retention of resin tags with indelible staining. Attempts were made to overcome these problems by crystal growth solutions and sand blasting, but, all these methods resulted in poor bond strength. Recent advances in laser technology and research into its potential have set the stage for a revolution in dental practice. Orthodontic applications of laser treatment were based on the outcome of the earlier studies on laser effects of enamel. There have been conflicting reports by researcher's regarding the viability of using laser etching as an alternative to acid etching. Laser etching produces microscopic cracks, fissures and craters in the enamel surface than conventional demineralization of enamel. It was also noted that lasers induce caries resistance, which would be of interest in orthodontic bonding. The present study is aimed at evaluating the viability of using laser etching as an alternative to routine acid etching. AIMS AND OBJECTIVES: The following objectives are accomplished by this study. 1. To determine the shear bond strength of mesh shaped stainless steel orthodontic brackets when bonded to acid etched enamel and laser etched enamel. 2. To compare the shear bond strength values of acid etched and laser etched enamel and thereby to determine if laser etching can be used as an alternative method. MATERIALS AND METHODS: Fifty sound non-carious premolar teeth extracted for orthodontic purpose were selected for this study. The extracted teeth had to meet certain criteria before use. They had to be grossly perfect on buccal surface, devoid of cracks from extraction forceps, be free of caries and have had no pretreatment with a chemical agent such as alcohol, formalin or hydrogen peroxide. These teeth were thoroughly cleaned of any soft tissue, blood and were stored immediately in distilled water to prevent dehydration. The samples were randomly divided into five groups consisting of 10 teeth in each group. They are grouped as: Group - I: Acid etched group conditioned with 37% phosphoric acid for 30 seconds. Group - II: Laser etched group irradiated at 80 milli joules. Group - III: Laser etched group irradiated at 100 milli joules. Group - IV: Laser etched group irradiated at 150 milli joules. Group - V: Laser etched group irradiated at 200 milli joules. Teeth in each group were mounted vertically on different color coded acrylic blocks for subsequent identification. The buccal surfaces of group - I were subjected to conventional etching using 37% phosphoric acid for 30 seconds, while the other four groups were subjected to Nd:YAG laser etching at different power settings of 80mj, 100mj, 150mj and 200mj respectively for 15 seconds. Since there is little surface absorption of the laser beam by enamel, it was coated with a light absorbing material as an initiator (black Indian ink), with the nylon brush over an area slightly greater than that of the bracket base. Brackets were later bonded on to these teeth using Ultimate - light curing primer and adhesive. The primer was applied on each etched surface of the teeth in their respective groups and cured. The adhesive paste was applied on to the bracket base and brackets were placed on the long axis of the tooth. Excess adhesive was removed with a sickle scaler and cured with visible light at 450 nm for a 20 second burst to each of the mesial, distal, incisal and gingival margin. The bonded specimens were stored in distilled water for one week before determining the shear bond strength. The shear bond strength of each sample was determined with a Hounsfield universal testing instrument. The cross head speed was 1mm/min and the force values at the point of bond failure of the bracket were recorded. The ultimate shear bond strength was recorded in kilograms and converted to megapascals. Shear bond strength of each sample was calculated using the formula: Bond strength = Breaking Load / Nominal area of bonding. The collected data was subjected to statistical analysis by using one way analysis of variance and the Newman-Keul's multiple range tests at 95% levels of confidence. RESULTS: One way ANOVA test revealed that different groups exhibited different bond strengths with Group - I showing the highest bond strength followed by Group-V, Group-IV, Group-III and least with Group-II. 30

3 Group-I showed significant difference in bond strength with other groups except for Group-V which is statically not significant. On the other hand there was a significant correlation between laser etching and shear bond strength. As the laser etching power increased from 80 mj to 200 mj, the bond strength also increased. On an average, for every 10 mj increase in power, the bond strength increased by 0.5 Mpa. Tables I to VIII shows the values measured and comparisons of all the groups. DISCUSSION: Acid etch technique for bonding composite resins to enamel has revolutionized the practice of esthetic dentistry. Buonocore 4 was the first to demonstrate that, the bonding of acrylic filling materials to enamel could be substantially increased by conditioning the enamel surface with 37% phosphoric acid. However, following acid etching some amount of enamel is lost making the surface more susceptible to demineralization and initiation of caries around the brackets, which is the major drawback in acid etching. The advantage of laser etching is that, laser itself induces caries resistance producing microscopic cracks, fissures and craters on the enamel that enhance mechanical inter locking with the composite.(dai.p.robert Harry 9 ). Various comparative studies between acid etching and laser etching on surface roughness and morphology of enamel have concluded that laser irradiation has caused surface changes similar to that of acid etching. (Arcoria et al 3, Shahabi et al 40, Ralph. H. Stern et al 32, John A. Hess 14, Keller & Hibst 44 and Osnat Feurstein et al 30 ). The present study evaluates the effectiveness of Nd:YAG laser in conditioning dental enamel for direct bonding of orthodontic brackets and comparing it with conventional acid etch technique. The results of the study show that, the mean shear bond strength value of the acid etched group is 9.24 Mpa, which is the highest comparable value to the laser etched groups. The mean shear bond strength values of laser etching are 2.8 Mpa, 5.51 Mpa, 7.01Mpa and 8.88 Mpa when etched at 80mj, 100mj, 150mj and 200 mj respectively. The shear bond strength of laser etched enamel increased with the power and energy output from the laser unit, a finding consistent with previously reported works. (White.J et al 46 and Vonfraunhofer J.A. et al 45 ). Surfaces etched at higher power setting achieved the bond strength which is equal to or above the minimum bond strength needed for the bracket to withstand masticatory forces. (7 mpa By Lopez. J.I. 21 and 0.6 kg/ mm 2 Vonfraunhofer J.A. 36 ). It was observed that there is an increase in the shear bond strength with an increase in the laser power. Considering this, a statistical prediction was done by taking all the laser etching values and it was found that with increase of laser power by 10 mj there was an increase of 0.5 Mpa in shear bond strength. From these findings it is seen that laser etching can be used as a viable method in preparation of enamel for direct bonding. The effect of laser irradiation on the substrate tooth, notably, the effects on enamel morphology and physiological changes within the deeper structures, await clarification. SUMMARY & CONCLUSION: In this study, an attempt was made to use lasers as an alternative method for etching enamel, which may prove a more convenient and time saving procedure in the near technological future. This in-vitro study compared the shear bond strength of brackets adhered to Nd-YAG laser etched enamel with that of brackets adhered to acid etched enamel. The summary of mean shear bond strength of the enamel surfaces etched by laser irradiation and by phosphoric acid was compared. Statistical analysis of the data indicates that, there are significant differences between shear bond strength obtained with laser irradiation and with acid etching of enamel. The bond strengths obtained at the lower power settings (80 mj, 100 mj and 150 mj) showed statistical differences while the bond strength obtained with highest laser power setting (200 mj) was less than that of acid etching but statistically insignificant. However, before concluding that, laser can be used as a substitute to acid etch technique it is necessary to know, the capability of ideal laser system performing hard tissue procedures consistently and effectively, without causing pulpal damage. More research is needed before a potential application of lasers is justified in a clinical setting. Safety and efficacy must be clearly established. ACKNOWLEDGEMENTS: The author would like to acknowledge Dr. K. Sadashiva Shetty; my esteemed mentor and tutor through out my life and my colleague Dr. Kishore M.S.V. for their help in compilation of this manuscript. 31

4 Fig. 2 : Orthophosphoric Acid Gel (37%) Fig. 1 : Specimens used in the study Fig. 3 : Nd : YAG Laser Unit Fig. 4 : Universal Strength Testing Machine (Hounsfield) 32

5 TABLE -I ENAMEL BOND STRENGTH EVALUATION OF GROUP -1 (Acid etched) Machine Parameters of Test a) Crosshead Speed = 1 mm / min b) Grip Distance = 12 mm Specimen No. KgF Mpa Mean S.D

6 TABLE - II ENAMEL BOND STRENGTH EVALUATION OF GROUP -2 (Laser etched at 80mj) Machine Parameters of Test a) Crosshead Speed = 1 mm / min b) Grip Distance = 12 mm Sample Specimen No. KgF Mpa Mean S.D

7 TABLE - III ENAMEL BOND STRENGTH EVALUATION OF GROUP -3 (Laser etched at 100mj) Machine Parameters of Test a) Crosshead Speed = 1 mm / min b) Grip Distance = 12 mm Sample Specimen No. KgF Mpa ` Mean S.D

8 TABLE - IV ENAMEL BOND STRENGTH EVALUATION OF GROUP -4 (Laser etched at 150mj) Machine Parameters of Test a) Crosshead Speed = 1 mm / min b) Grip Distance = 12 mm Sample Specimen No. KgF Mpa Mean S.D

9 TABLE - V ENAMEL BOND STRENGTH EVALUATION OF GROUP -5 (Laser etched at 200mj) Machine Parameters of Test a) Crosshead Speed = 1 mm / min b) Grip Distance = 12 mm Specimen No. KgF Mpa Mean S.D

10 TABLE - VI SHEAR BOND STRENGTH FOR ACID ETCHING AND DIFFERENT LASER POWER SETTINGS Shear Bond strength (Mpa) Difference between groups Groups Range Mean SD II III IV V I. Acid Etching P<0.01 PO.01 P<0.01 NS II. LE 80 mj PO.01 PO.01 PO.01 III. LE loomj PO.05 PO.01 IV. LE 150 mj PO.05 V. LE200mj One way ANOVA (F = 44.55, PO.01) New man - Keul's Range test (Pair wise comparisons): Minimum significant Range = 1.5 P 0.05 = 1.9 P< 0.01 I > V > IV > III > II NS Other comparisons are statistically significant *NS: Not Significant 38

11 TABLE - VII ANOVA TABLE Source of variation Sum of squares Degree of freedom Mean sum of squares Variance of ratio, F-value P-value Between groups Within Groups <0.001 Total TABLE - VIII CORRELATION BETWEEN LASER ETCHING POWER AND SHEAR BOND STRENGTH Relationship between Mean±SD Correlation coefficient V Regression coefficient 'b' Prediction equation (prediction of Shear Bond Strength for given Laser Etching Laser Etching Shear bond strength = (Laser Etching Power) Shear Bond strength (Mpa) 6.05 ±

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