The objective of this study was to evaluate the shaping characteristics of different
root canal instruments in teeth with different root canal forms. A total of 420 extracted
human roots were embedded in resin blocks. The embedded roots were divided into three
groups, i.e., roots with (i) straight (I form), (ii) apically curved (J form), and
(iii) entirely curved canals (C form) with reference to the Schneider's angle and
the length of the radius of arc of the curvature of the root canals. Each of the three
groups containing 140 roots were randomly divided into seven subgroups prior to preparation.
The canals were enlarged manually with traditional and flexible hand instruments,
three different engine driven instruments and sonically and ultrasonically powered
instruments. The shortcomings of the seven preparation methods were assessed by superimposition
of projected radiographs taken in bucco-lingual and mesio-distal views before and
after preparation. Results showed that observation of perforation, incidence of elbow
and of asymmetrical preparation of the root canals depended on root canal configuration.
It was striking that the coronal transposition of the apical stop, uneven wall contour,
ledge formation and incidence of zip were independent of root canal morphology. Evaluation
of data of asymmetry of preparation (canal transposition) revealed significant differences
(p<0.05). Asymmetry of preparation was the most frequent aberration of the prepared
root canals and it was possible to study it with precision. It was concluded that
this measurement should be a priority for future instrument testing. The shaping characteristics
of hand and engine driven instruments was modified by root canal morphology, but that
of sonic and ultrasonic instruments was less dependent on the original anatomical
form of root canals.