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[Evaluation of the shaping ability of four Ni-Ti rotary instruments in the preparation of simulated curved root canals].

PURPOSE
To evaluate the effect of four Ni-Ti rotary instruments on the preparation of L-shaped resin root canals.


METHODS
40 L-shaped resin root canals (curvature=38.5°) were divided randomly into four groups: Hero642,Protaper, K3 and Mtwo. Resin root canals were prepared according to the protocols. Preparation time was recorded. Original and postoperative canal images were scanned and superimposed. The preparation effect was evaluated by computer software, such as resin removal amount in inner and outer wall of root canals, change of curvature degree and radius, etc. The data was analyzed with SPSS13.0 software package.


RESULTS
The preparing time of K3 was the longest. Mtwo was the shortest(P<0.05). Resin removal amount showed significant difference among the four instruments in inner root canal wall at BC, O and HO as well as in outer root canal wall at AC, HO and O. Protaper achieved the largest tapered canal,but was the worst for centering ability in the bend. Apical transportation increased near to apical foramen. The four Ni-Ti rotary instruments showed no significant difference regarding safety, undesirable shape, change of root canal's curvature degree and radius except two instrument fracture cases (K3 and Protaper) and two step cases(K3). Postoperative working length tended to be shorter,and root canals were straightened.


CONCLUSIONS
Mtwo prepares simulated resin curved root canals fast. Postoperative root canal's taper by Protaper is significantly larger than Hero642. The 4 Ni-Ti rotary instruments perform good cutting ability and keep root canal's original curvature well.