Shaping of simulated root canals in resin blocks using the step-back technique with K-files manipulated in a simple in/out filling motion.

A total of 60 simulated root canals in clear resin blocks of various lengths and degree of curvature were prepared by hand with K-files using a step-back enlargement technique. The instruments were manipulated exclusively with a simple in/out filing or rasping motion with no attempt at rotation or twisting. The preparation technique was assessed by instrumentation time, deformation and fracture of instruments, and loss of working distance. The shape of the prepared canals was assessed by direct inspection and from composite photographic prints produced by superimposing negatives of the canals obtained before and after preparation. In general, the preparations were time-consuming and tedious to complete. Deformation and separation of instruments occurred mainly in those canals with short, severe curves. Loss of working distance was a universal finding and was associated to a degree with the straightening of curved canals inherent in their preparation. Prepared straight canals displayed a continuously tapering form being narrowest at their end-point. On the other hand, prepared curved canals were invariably hourglass in shape. Under the conditions of this study the manipulation of K-files in a simple in/out filing motion proved an unsatisfactory method of preparing simulated curved root canals in resin blocks.