Purpose: To study the anatomical features of curved root canals.
Methods: According to Nagy's classification,the root canals were divided into 4 types:type I (straight), type C (continuously curved), type J (apically curved) and type S (multicurved). Each type had a sample selected from extracted permanent teeth. Digital radiographs were taken at buccolingual and mesiodistal directions with a K-file inserted into the root canal. The images were treated by Photoshop7.0 and CorelTRACE10 softwares, after that a CAD/CAM software UG NX V1.0 was applied to extract the 2D curves of the root canal axis from the images,then the 3D curves of the root canal axis were built up accordingly. The curvature of the curves was analysed and the geometric parameters were measured by UG.
Results: The curvature values of type I root canal were very low; the curvature of type C root canal reached a peak on the middle part,it decreased slowly towards the two ends; type J root canal had a curvature peak near the root apex, and the middle 1/3 and neck 1/3 parts of the root canal were "curvature flat zones". Type S root canal had a curvature inflection,and beside it were 2 curvature peaks at opposite directions. The geometric parameter values of the same root canal's 2D and 3D axis were different.
Conclusion: Each point on the root canal axis had a curvature value and a vector of tangent, different root canal forms had different rules of curvature distribution, just 1 or 2 parameters couldn't describe a curved root canal completely.