Failure mechanism of Hedstroem endodontic files in vivo

Intracanal fracture of endodontic Hedstroem files (H-files) jeopardizes the outcome of endodontic therapy. The aim of this study was to evaluate the fracture mechanism of H-files after multiple in vivo uses. Five each of size #20 and #25 ISO H-files fractured during clinical use were collected from different dental clinics. Four fractured files of each size and three unused files of each corresponding size were embedded in a resin along their longitudinal axis and after metallographic grinding and polishing studied under an incident light microscope. One specimen in the fractured group was investigated under a scanning electron microscope. Light microscopy results showed that unused files were free of structural defects (cracks, pores, etc.), whereas fractured files demonstrated the presence of many cracks mostly located at the flute depth region. Secondary cracks were identified beyond the fracture plane of fractured files both with a light microscope and a scanning electron microscope. Experimental results demonstrated that fatigue is closely associated with the fracture mechanism of H-files (#20, #25) under clinical conditions.