The Study of Shear Bond Strength with Acidulated Phosphate Fluoride Applied after Acid Etching - Sem Investigation

Introduction: The most significant development in the field of orthodontics is the acid etches technique that has been used in the direct bonding of orthodontic brackets. In patients with poor oral hygiene decalcification and decay have been observed around the bonded brackets. . The topical fluoride is effective in increasing resistance to dental caries or enamel decalcification. These include pretreatment with topical fluoride before etching, incorporation of fluoride in the etching solution, and topical application of fluoride after etching. The fluoride application may affect the bond strength. Hence the study is to evaluate the shear bond strength with acidulated phosphate fluoride after acid etching and detect the changes on the enamel surface after debonding with scanning electron microscope. Materials and Methods: Forty human premolar teeth were extracted from teenagers (9 to 16 years of age). The teeth were divided into two groups of 20 teeth each. One for control group and the other for evaluating shear strength after acid etching. The specimens were mounted on an acrylic block of dimension (4x2x1cm) to make it compatible to the jigs of the Hounsfield universal testing machine. Beggs bracket with the bracket base measurement of 3mm X 3mm manufactured by TP orthodontic Inc (USA) were used in this study. The buccal surfaces of the crown were cleaned with pumice using polishing brush for 10 seconds then washed with water and dried with an air spray. The specimens of Group 1 (Control Group) the etchant solution was applied on the buccal surface of each crown for 15 seconds then rinsed with water for 10 seconds and dried with an air spray. The etched surface became chalky white in appearance. In the group II the etchant was applied for 15 seconds, rinsed and dried as in the control group. However the crowns of the teeth were immersed in APF gel for 4 minutes after etching. The buccal surfaces of the teeth were abundantly washed with a water spray for 10 seconds then dried again etched enamel and bracket base were coated with a sealant and the composite resins (3M Unitek Co, USA) immediately applied to the bracket base. The brackets were accurately pressed to the demarcated etched buccal enamel with a placement scalar and to test the shear strength, the blocks were mounted with the bracket slots perpendicular to the floor. Stainless steel ligature wire (.009) was threaded through the bracket and passed upward to the movable cross head and twisted tightly. So that the load would be applied directly over the center of the base. The cross head speed of 2mm/minute was used. The force at which the bond failed was recorded as breaking load and the bond strength. The values were recorded and statistically evaluated. The debonded interface in each group were observed with an SEM. The samples were coated all over to a thickness of about 20pm with gold as the conducting material. The specimens were observed on the screen at various magnification and the photographs were taken for evaluation. Results: The Mean shear strength of control group Group I is 10.76 MPa. Group II APF application after acid etching is 7.405 MPa. Conclusion: The results showed that there is significant decrease in bond strength with APF application after acid etching compared to the control group. In the SEM APF gel produced surface coating appeared to consists of densely packed small spherical globular structure present after acid etching