With the step-back technique, the diameter of the root canal at the apical level of instrumentation is kept as small as possible to resist extrusion of filling material beyond the canal. In addition, the apical part of the canal is given a moderately tapered form in an attempt to retain the obturating materials within the canal. Further coronally, the canal is flared as much as necessary according to the anatomy of the canal and to facilitate the obturation of the canal. The flare of the canal is especially pronounced with the use of thermoplasticized gutta-percha obturation techniques which require the insertion of rather large and rigid instruments to the apical region of the canal.
The degree of instrumentation most apically in the root canal is determined by the size of the first K-file that binds in the canal at the apical level of instrumentation. The canal is then enlarged an additional two instrument sizes at this level. This means that if a no. 15 file binds, the canal is enlarged to a size 25. The last file used most apically, in this instance no. 25, is called the master apical file. The apical taper is then accomplished by a step-back use of instruments of increasing sizes. Between each change of file, the full length of the canal is recapitulated with the master apical file. Further coronally, the canal is then flared with hand or engine-driven instruments to give it the desired continuous tapered shape.
The step-back technique can be used in all teeth. However, if the root canal is wide apically so that little natural resistance is offered, a more definite shelf may have to be prepared in the canal wall to help prevent overfilling during the obturation phase. Also, when the step-back technique is used, it must be remembered that the root canal in many teeth is considerably wider in a buccolingual than in a mesiodistal direction. The first instrument that binds will do so in the narrowest part of the canal, and an enlargement of two to three instrument sizes, which most often will mean 0.10-0.15 mm, may not be enough to reach the walls at the widest part of the canal. Thus, tissue commonly is left behind apically with the step-back technique, especially on the lingual aspect of the root canal wall.