Adorno CG, Yoshioka T, Suda H. The effect of working length and root canal preparation technique on crack development in the apical root canal wall. International Endodontic Journal, 43, 321–327, 2010.AbstractAim  To evaluate the effects of working length and root canal preparation technique on crack development in the apical root canal wall.Methodology  Seventy extracted mandibular premolars were mounted in a resin block with simulated periodontal ligaments and divided into seven groups according to preparation technique and working length: group A, step‐back preparation with stainless steel files with working length set at the apical foramen and defined as root canal length (CL); group B, same as for A, except that the working length was CL–1 mm; group C, crown‐down preparation with Profile instruments followed by an apical enlargement sequence with CL as working length and group D, same as for C, except that the working length was CL–1 mm. Groups E, F and G served as controls. Groups E and F were prepared only with the crown‐down sequence up to CL and CL–1 mm, respectively. Group G was left unprepared. Digital images of the apical root surface (AS) were recorded before preparation, immediately after instrumentation and after removing the apical 1 mm (AS–1 mm) and 2 mm (AS–2 mm) of the root end.Results  Working length significantly affected crack development at AS (P < 0.05). Preparation technique significantly affected crack development at AS–1 mm (P < 0.05). At AS–2 mm, there was no significant difference between preparation technique and working length in terms of crack development on the canal wall.Conclusion  Root canal preparation alone, regardless of the technique used, can potentially generate cracks on the apical root canal wall as well as the apical surface. Working 1‐ mm short of the apical foramen might produce fewer cracks in the apical region.