Root canal instruments for manual use: cutting efficiency and instrumentation of curved canals

The cutting efficiency of different endodontic hand instruments and the effects of instrumentation on curved canal shape were investigated under standardized experimental conditions using an automatic testing device. Cutting efficiency in rotary motion was assessed by determination of the maximum penetration depth of the instruments into a cylindrical canal in a special resin block (size 25 and size 35 instruments). Changes in canal shape were determined by instrumentation of standardized canals (42 degrees curvature) incrementally from size 15 to size 35. Except in the case of one instrument, size 35, in both sizes tested flexible instruments reached significantly (P < 0.05) greater maximum penetration depths than conventional reamers or K-files. Changes in the canal shape differed significantly between the different instruments in 13 of the 14 measuring points. Drastic undesirable changes in the canal shape (e.g. straightening or zip and elbow) occurred after instrumentation with reamers and K-files, but these changes were less noticeable after instrumentation with flexible instruments with conventional tips. After instrumentation with flexible instruments with modified tips there were few undesirable changes in shape. The conclusion could be drawn that flexible instruments, especially those with modified tips, were clearly superior to conventional reamers and K-files with regard to cutting efficiency and instrumentation of curved canals.