Cells and Materials


The fracture surfaces of sonic and hand scaler instruments fractured under clinical and laboratory conditions were compared to their etched sections, and analyzed for evidence revealing the process that led to their failure. Scanning electron microscopy and energy-dispersive spectroscopy indicated the quality of the martensitic stainless steel comprising the sonic scalers, all from one manufacturer, was inferior to the steel comprising the hand scalers from another manufacturer. The sonic scalers contained stringer inclusions aligned longitudinally and consisting of calcium, aluminum, silicon, and other elements up to 50 µm in length. The sonic scalers displayed brittle fracture while the hand scalers displayed mainly ductile fracture. Microcracks occurred between stringer inclusions. Microhardness for sonic scalers, although slightly higher and significantly different from hand scalers, proved ineffective for detecting a structure-property relationship. Microscopy, however, proved very useful for this purpose and also well-suited for analyzing the stress state occurring on the instruments at the time of failure. All sonic scalers were stressed by being bent inwards, while a hand scaler was pulled and torqued by twisting. Retained coarse grinding grooves also affected fracture. Recommendations are made for hand instrument standards to include checks on steel quality for inclusions of the stringer type.