SummaryThe purpose of this study was to compare, in vitro, the machining efficiency of four brands of nickel‐titanium files and two brands of stainless steel K‐type files. Instruments sizes 15 to 40 were tested in a linear motion simulating the clinical motion used to remove a file from the canal. The tips of the loaded files were in contact with a resin block. The load applied increased with file size. An indentation varnish caliper was used to measure the depth of the groove after 100 back‐and‐forward motions. The stainless steel instruments with a triangular cross‐section were more efficient than the stainless steel instruments with a square cross‐section. There was a significant discrepancy between the machining ability of the nickel‐titanium K files. The Maillefer instruments were the m
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