All nickel-titanium instruments have both good and bad things. Theyare all called "memory"files.The good thing is that the nickel-titaniumfile has elastic and self-adaptor. The bad thing is when the rotating filestays at the curved root canal, it is still cutting the outer part of the bend,and the problem arises.The time the ledge is formed depends on thedesign of the file (the stronger the cutting ability of the file, the faster theledge formation).As the root canal tends to return to the design shape,the ledges are formed.The use of rigid hand files in root canal prepara-tion, or preparation for too rush, violence，may also lead to the forma-tion of ledges.