OBJECTIVETo investigate the shaping efficiency of three instrumentation techniques in the C-shaped canals.

METHODSSixty extracted mandibular second molars with C-shaped canals were selected and cross-sectioned at 2, 5, 8 mm from the apex by using a muffle system as described by Bramante. The teeth containing C-shaped canals were randomly divided into three groups, respectively instrumented using stainless steel K-files in step-back and step-down techniques, and ProTaper series in crown-down technique. The digital images of canal cross-sections pre- and post-instrumentation were photographed to evaluate the proportion and position of uninstrumented area Working length loss and perforation were recorded. Data were analyzed using ANOVA.

RESULTSStep-down technique gained smaller uninstrumented area in the coronal third of the canals, while step-beck technique gained the same results in the apical third (P < 0.05). Uninstrumented proportion in apical part was significantly higher than in the middle and the coronal (P < 0.05). Instrumenting type I canal as 2 or 3 separated canals was likely to reduce the miss. There were three perforations in manual stainless steel K-file groups.

CONCLUSIONSEarly opening the coronal part of C-shaped canal, shaping C-shaped canal as two or three separated canals and instrumenting the apical part with step-back technique, seemed to be the effective methods to avoid miss.