Abstract : This study compared how preparation of mesial root canals of human mandibular molars is affected by the use of either the crown-down preparation technique with a GT Rotary File and ProFile nickel-titanium instruments, or the step-back technique using a stainless-steel K-file.

 These root canals were scanned before and after being prepared with the Ni-Ti file, or the conventional method, with a micro computed tomography scanner. We measured and compared 11 points such as size of apical end, differences of working length, straightening of outer and inner curvature canals, and differences in dentine volume.

 Unlike the crown-down technique, the step-back method showed a tendency to turn the inner angle of the canal curve into a straighter line. The outer angle of the canal curve increased with both preparation techniques.

 At the curved part, no modification was observed with the crown-down technique, but the step-back technique tended to make the distance from the apical stop to orifice longer, while shortening the distance from the apical stop to canal curve.

 The amount of dentine removed with the crown-down technique was more important than with the step-back technique.

 Nickel-titanium rotary instruments produced good canal shapes in curved canals, compared to the conventional method.

Abstract : This research analyzed trace elements of crystal contained in powdered mineral trioxide aggregate (MTA) with SEM-EDS analysis. The results of this analysis were compared with trace elements, which have the possibility of appearing in the mineralization formation in dentine. Research has shown that some trace elements in the mineralization mechanism include Ca, P, S, Mg, Si, and Al in the root formation of dentin ; include Ca, P, S, and Mg at the completed root of dentin ; and Si and Al were replaced by Ca and P with the formation at the maturation of root. It is assumed that these trace elements of MTA are necessary for the mineralization process of dentine. We suppose that a vital reaction is further promoted by using these elements as a compound.