This document discusses non-surgical re-treatment of root canals and compares different file systems for removing gutta-percha during re-treatment. It outlines causes of root canal treatment failure and methods for non-surgical re-treatment, including hand files and rotary nickel-titanium (NiTi) file systems like R-Endo and ProTaper. Two studies are reviewed that compared the effectiveness and efficiency of manual versus rotary instrumentation for removing filling material. The studies found no significant differences in removal based on filling material, but rotary files like ProTaper and K3 were more effective than manual files at debris removal. In severely curved canals, all file systems left debris, showing room for