ABSTRACT
Adhesives for ceramic orthodontic brackets are so strong that instances of enamel fracture and bracket fracture have occurred during removal. Our approach to minimize the potential enamel damage was to modify the mechanical properties of the adhesive, a BIS GMA-silica composite, by use of diethyl phtha-late which is a common plasticizer. The plasticizer, 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 objective was to simulate, by use of finite element analysis, clinical loading conditions during orthodontic treatment and removal. The finite element analysis determined the changes in computed stresses due to plasticization. After 25 days in an artificial saliva solution held at 60°C, the bracket removal torque was lower for the 10% plasticized adhesive group than that for the non-plasticized group. The 3-D linear elastic finite element anlaysis found that plasticization should not lead 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 in the corner regions. Thus, modelling of the adhesive as a layer with distinct mechanical properties appears reasonable.
