The influence of cyclic shear fatigue on the bracket–adhesive–enamel complex: An in vitro study

ObjectivesTo describe the effect of fatigue on the strength of the bracket–adhesive–enamel complex and characterize the fatigue behavior of the materials tested.MethodsUpper central incisor brackets (Discovery®, Dentaurum) were bonded with a light-curing (Transbond XT™, 3M Unitek) and a chemically-curing adhesive (Concise™, 3M Unitek) on bovine teeth embedded in cylindrical resign bases and stored in water at 37(±2) °C for 24 (±2) h. The first 15 specimens were tested with a universal testing machine ZMART.PRO® (Zwick GmbH & Co. KG, Ulm, Germany) for ultimate shear bond strength according to the DIN-13990-2-standard. The remaining three groups of 20 specimens underwent fatigue staircase testing of 100, 1000 and 3000 cycles at 1 Hz with a self-made testing machine. The survived specimens were subjected to shear strength testing.ResultsThe fatigued specimens showed decreased shear strength with both adhesives at all cycle levels. The shear strength after fatigue for 100, 1000 and 3000 cycles was in the Concise™-groups 34.8%, 59.0%, 47.3% and in the Transbond™ XT-groups 33.6%, 23.1%, 27.3% relative to the ultimate shear strength. The fatigue life of the Concise™-groups decreased with increasing stress and Transbond™ XT showed lower fatigue ratio with no obvious trend. The specimens bonded with Transbond™ XT showed typically favorable fracture modes in contrary to Concise™.SignificanceFatigue of the bracket–adhesive–enamel complex decreased its shear strength. The staircase method can provide a standardized experimental protocol for fatigue studies, however testing at various cycle numbers is recommended.