It has been shown that rotary nickel-titanium files cannot be used indefinitely. Researchers and clinicians have not been able to agree on how many times a file can be used before being discarded, except if a file has fractured or become visibly distorted. This study used ISO size 20 files of 0.04 taper in the curved canals of extracted mandibular molars. The canals had been previously instrumented to an ISO size 15 with stainless steel hand files. The irrigant used during rotary and hand instrumentation was Glyde®. The rotary files were closely examined with scanning electron microscopy before use to detect any defects. They were then reexamined after each of five uses to document deterioration. An electric handpiece was configured to rotate at 150 rpm and secured to the testing device, which also held the extracted tooth. The testing device controlled the load at 8 N, the depth of penetration for each canal and the rate of penetration (12 mm/min). Used instruments demonstrated surface fatigue wear and cracking. Torsional moment at failure was determined on a torsiometer for used and new instruments. Data (n = 10) were analyzed by analysis of variance. The torsional moment for used and new instruments was not affected by use (p = 0.25).
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