Root canal treatments, like any medical treatment, does have a risk of complications. However, this risk is very minor and easy to fix if it does occur.
A file is a small tool used in a root canal to remove any infected tissue within the damaged tooth. Because of its small size, it is very fragile and can sometimes be broken. The chance of a file breaking during a root canal is only at 3%. A broken file is most likely to occur in a situation where there is a curved or narrow canal, making it more difficult to use the file to remove tissue. If this is the case, specialised root canal treatment may be recommended to reduce the chance of a file breaking.
Files are made of materials such as nickel-titanium and stainless steel, which are both very sterile medical-grade materials. They are completely stable, and will not cause any pain or harm to the tooth or tissue.
If a file does break, it is either left if it is not going to do any harm or damage, or retrieved if it is safe to do so. A broken file is very unlikely to have any impact on the long-term effects or survival of the tooth, and more often the not the file will just be incorporated as part of the root filling materials.