The smear layer produced when root canals are instrumented is known to affect the prognosis of endodontic treatment. Clinicians are now moving towards flexible nickel titanium instruments in hand and rotary forms. It is important for them to be aware of the amount of smear layer produced by the various instrumentation systems. Objective: To observe and compare the mean smear layer scores following instrumentation of root canals with ISO standardized stainless steel K files, ProTaper hand and rotary systems under the scanning electron microscope.(SEM) Design: 45 single rooted human teeth with conical roots and type1 canals were divided into three groups of 15 each. The root canals in group 1 were prepared by ISO standardized stainless steel K files, group 2, by the hand ProTaper files and group 3, by the rotary ProTaper files. Irrigation was carried out using saline and 3% sodium hypochlorite. Following instrumentation, the coronal portion of the teeth was sectioned at the CEJ. The roots were then sectioned longitudinally and examined under the SEM. The SEM photographs were scored by two blinded examiners. Statistical analysis: Consistency between the two examiners was checked using the Kappa tests. The data was analysed using Kruskal Wallis one way analysis of variance. The significance was set at a P value <0.01. Results: The difference between group 1 and group 2 was not significant. P = 0.82 There was significant difference between group 2 and group 3. P<0.01 There was significant difference between group 1 and group 3. P<0.01. Conclusion: Under the conditions of the study, the group 2 showed the most smear layer, followed by the group 1. Group 3 showed the least presence of smear layer.
