Purpose: This article is concerned with efficacy of method of instrumentation used for enlarging finely curved canals, primarily the buccal root of maxillary molars, the mesial root of mandibular molars, the mandibular incisors and the maxillary lateral incisors.
Schilder stated that in vitro and in vivo experiments confirmed that a minimum apical opening equivalent to No.3 file is desirable in a final preparation for gutta percha or silver points. the old no.3 file is roughly equivalent to #25 file in the standardized system.
Murphy and Tracy determined the maximum file size that could adapt to a given degree of curvature of a root canal. the formula stated that the size of the file to which the canal could be safely enlarged =110 – the angle divided by 2. they found size #25 file would negotiate most curved canals.
There are three types of deviation from the original curvature of the canal: perforation, ledge and zip.
Several studies compared various techniques for preparing curved canals and found that step-back technqiue is the one that resulted in least deviation of the original canal curvature.
Step-back technique consists of two phases: instrumentation and step-back. instrumentation is the enlargement of the canal at the working length to #25. step back involves shortening of #30, #35 and #40 by 1, 2, 3 mm to produce coronal taper.
The preparation with step back allows adequate coronal space for obturation with lateral condensation.