CONTROLLING ADHESION OF ORTHODONTIC ADHESIVES THROUGH ADJUSTMENT OF THE INTERPHASE MECHANICAL-PROPERTIES

Adhesives for ceramic orthodontic brackets are so strong that instance s of enamel fracture and bracket fracture have occurred during removal . Our approach to minimize the potential enamel damage was to modify t he mechanical properties of the adhesive, a BIS GMA-silica composite, by use of diethyl phthalate which is a common plasticizer. The plastic izer, used in amounts up to 20% of the adhesive weight, significantly decreases the adhesive modulus and tensile strength. One objective of this research is to evaluate plasticizer stability in the adhesive via functional testing in a simulated oral environment. A second objectiv e was to simulate, by use of finite element analysis, clinical loading conditions during orthodontic treatment and removal. The finite eleme nt analysis determined the changes in computed stresses due to plastic ization. After 25 days in an artificial saliva solution held at 60 deg rees C, the bracket removal torque was lower for the 10% plasticized a dhesive group than that for the non-plasticized group. The 3-D linear elastic finite element analysis found that plasticization should not l ead to premature failure when typical treatment loadings were applied. The torsional loading conditions simulating bracket removal reported peak stresses in excess of the plasticized adhesive tensile strength i n the corner regions. Thus, modelling of the adhesive as a layer with distinct mechanical properties appears reasonable.