The purpose of this ´study is to evaluate the effects of mechanical and thermal fatigue on the shear bond strength(S$S) in orthodontic brackets bonded to human premolars with chemically cured adbesive(Mono-Lok2 Rocky Mountain Orthodontics). Two types of metal brackets (Ormesh, Microioc) and three types of ceramic brackets ( Fascination, Starfire, Transcend 2000) were used in this study. The 106 loadcycles of 17.4 X 1(lzsin2 fti g ? cm and the 1,000 thermocycles of 15 second dwell time each in 5 IC and 55 C baths were acturated as mechanical and thermal fatigue stress, and SBS were measured after each fatigue test. The fracture sites were examined by stereoscope and scanning electron microscope.
The results obtained were summarized as follows ;
1. In static shear _boxed test, Fascination brackets showed the maximum SBS(2a.78±3.45 MPa) and Mrcroloc brackets showed the minimum SBS(1428}3.10 MPa). Fascination and Starfre brackets shouted significantly greater SBS than Microloc brackets(P<0.05).
2. In mechanical fatigue test, Fascination brackets showed the maximum SBS (20.19 X45 MPa) and Starfire brackets showed the minimum SfS(9.10 x.33 MPa). The SBS of Transcend 2000 brackets(301) and Starfire brackets(P<0.05) significantly de ceased after 1()s loadcycles.
3. In thermocyding test, Onnesh brackets showed the maximum SBS (19.36±2.76 MPa) and Starfire brackets showed the minimum SBS(1L94}6.86 MPa). The SBS of Transcend 2000(P<0.01), Microloc and Starfire brackets(P<0.05).siguificantly decreased after 103 then mcycles,
4. Failure sites of thermocyciing groups were similar to those of static groups but after mechanical fatigue test, Ormesh and Transcend 2000 brackets failed at the bracket/resin interface and Mrcroloc brackets failed within adhesive. Facination brackets failed at the enameVresin interface irrespective of experimental condition.
|