Effects of thermal and mechanical fatigue stress on bond strength in bracket base configurations

Kim, Jong Ghee; Kim, Sang Cheol

Abstract

The purpose of this study is to evaluate the effects of mechanical and thermal fatigue stress on the shear, tensile and shear-tensile combined bond strengths(SBS, TBS, CBS) in various orthodontic brackets bonded to human premolars with chemically cured adhesive(Ortho-one, Bisco, USA). Five types of commercially available metal brackets with various bracket base configurations of Photoetched base(Tomy, Japan), Non-Etched Foil Mesh base(Dentaurum, Germany), Micro-Etched Foil Mesh base(Ortho Organizers, USA), Chessboard base(Daesung, Korea), and Integral base(3M Unitek, USA) were used. Samples were divided into 3 groups, the first group was acted with shear-tensile combined loads(45 degrees) of 200g for 4 weeks(mechanical fatigue stress), the second group was subjected to the 5,000 thermocycles of 15 second dwell time each in 5 degrees C and 55 degrees C baths(thermal fatigue stress), and the third group was the control. Bond strengths were measured at the crosshead speed of 0.5 mm/min. The cross-section of bracket base/adhesive interface and the fracture surface were examined with the stereoscope and the scanning electron microscope. The resin remnant on bracket base surface was assessed by ARI(Adhesive Remnant Index). The obtained results were summarized as follows; 1. In static bond strength, Photoetched base bracket showed the maximum bond strength and Integral base bracket showed the minimum bond strength(p<0.05). In all brackets, shear bond strength(SBS) was in the greatest value and shear-tensile combined strength(CBS) was in the least value(p<0.05). 2. After mechanical fatigue test, Photoetched base bracket showed the maximum bond strength and Integral base bracket showed the minimum bond strength(p<0.05). In Photoetched base bracket and Micro-Etched Foil Mesh base bracket, shear bond strength(SBS), tensile bond strength(TBS) and shear-tensile combined strength(CBS) were decreased after mechanical fatigue test(p<0.05). 3. After thermocycling, Integral base bracket showed the minimum bond strength. The SBS, TBS and CBS of Photoetched base, Chessboard base, and Micro-Etched Foil Mesh base bracket were decreased after 5,000 cycled thermocycling(p<0.05). 4. In static test, bond failure sites were mainly between bracket and adhesive. After thermocycling, ARI scores were greater than those in static test. 5. SEM micrographs of cross-section of bracket bonding site showed that many micro-cracks in bracket/resin interface and in enamel/resin interface were found in all brackets after thermocycling.