Precurving endodontic files

 When I was taught at uni by the clinical instructors in endodontics I was given a mix of advice on how to negotiate canals with hand files. One instructor said she never precurved hand files, another said to do it when there is a curve in the canal and push apically with the curve in different directions until you can bypass the curved area. Some provided no instruction at all. In hindsight, my endodontic instruction at university was misguided, limited and in some instances completely counter to sound endodontic principles.  

The best way to solve a ledge is prevention and the best way to cause a ledge is to enter a canal with files that are too stiff to early and to put them in without precurving then. Below is a photo I got off the internet that shows a precurved file. You want a gentle curve as a sharp curve will concentrate stress and have more risk of breakage. You don't want too much of a curve as it will straighten itself out when you put it into the canal. A slight curve allows it to remain curved all the way down to the end of the canal. You will use different curves in different situation, a smaller, sharper curve for negotiating ledges for example but the below photo shows a good general curve for scouting canals.

 

 

Having some experience now, I have a good idea what works in my hands and why things are done a certain way. I would say it is safer to always precurve hand files even in straight looking canals. I don't always do this but it is certainly safer to do so. The reason this is is that canal anatomy is never perfectly straight like a hand file is perfectly straight. There will always be some deviation away from a straight path and therefore a straight instrument entering the canal will contact the walls at a certain point. If the angle of contact is too sharp and the file is aggressive enough to dig into the wall then the start of a ledge will have begun. 

One of the instructors at uni taught me to start every case with a size 15 K file and just keep pushing it in different directions until it reached the apex. Subsequently I ledged a lot of canals before I realised that it is safer to start with smaller diameter files. A smaller diameter file will have more flex so is more likely to deform than dig into the wall. If the canals look tight on the xray I will start with an 06k file as a standard. If the canals look normal I will generally start with a 10k file. If the canals look wide and relatively straight I may start with a 15k file as it would give a more predictable result with the apex locator and the risk of ledging is pretty minimal in these cases.

Another instructor told me that she never precurves files and when I couldn't negotiate a particulator curved lower premolar, she told me to try a smaller file and when an straight 06k file wouldn't pass through the curve she said the case can't be done. Now I would put a precurved 06k file into the canal and watchwinding it down till the curve was passed. The watch winding motion is extremely important as it allows the file to pass without apical pressure and it is actually the apical pressure that is more likely to dent into the walls.