Endo

JOURNAL OF ENDODONTICS Copyright © 2001 by The American Association of Endodontists

Printed in U.S.A. VOL. 27, NO. 4, APRIL 2001

Relationship Between Design Features of Endodontic Instruments and Their Properties. Part 3. Resistance to Bending and Fracture
Edgar Schafer, Priv-Doz Dr med dent, and Joachim Tepel, Priv-Doz Dr med dent ¨

Stainless-steel prototypes characterized by fivedifferent cross-sectional shapes (square, triangular, rhomboidal, “S”-shaped, and the cross-sectional design of H-type files) and three different number of flutes (16, 24, and 32) were used for investigation of the relationship between design features and the resistance to bending and fracture of root canal instruments. Both resistance to bending (bending moment) and resistance to fracture (angulardeflection and torque) were determined in accordance to ISO 3630-1. Numbers 15, 25, and 35 prototypes were tested with a sample size of 10 instruments each. Whereas prototypes with the rhombus-shaped cross-sectional design displayed the lowest resistance to bending, those of the square cross-section showed the greatest. In general, S-shaped prototypes and those with the H-type cross-section achievedlower angular deflection than all other prototypes, whereas those with the triangular cross-sectional shape and 32 flutes reached the greatest angular deflection. Overall, rhombus-shaped prototypes showed lower torque values than all other prototypes. Results indicate that bending and torsional properties of endodontic instruments are mainly influenced by their cross-sectional design. Moreover,these properties are also affected by the number of flutes and the manufacturing process (twisted or milled) of endodontic instruments.

In the ISO standard 3630-1 (1), as well as in ANSI/ADA standards 28 and 58 (2, 3), several mechanical requirements for root canal instruments are listed (e.g. resistance to bending and resistance to fracture). To determine the resistance to bending of a rootcanal instrument, the instrument is fixed at its tip over 3 mm and bent. The bending moment at a bending angle of 45 degrees is measured (1). To investigate the torsional properties of endodontic instruments, the instrument is fixed at its tip and its shaft, and then
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rotated clockwise until fracture, whereas the rotation angle and torque are recorded continuously. From these records, thetorsional angle at which the instruments fractures and the torque at this point can be calculated (1). The resistance to bending of root canal instruments influences the results of instrumentation in curved canals. Instruments with increased flexibility cause fewer undesirable changes in the shape of curved canals than those with greater resistance to bending. This increase in flexibility is achievedeither by different design features of the instruments or by the use of nickel-titanium alloys (4, 5). Likewise, differences in bulk of the different variations in crosssectional design have an effect on the flexibility and other physical properties of endodontic instruments (4). Several authors have observed that instruments of the square cross-section display greater resistance to bending thanthose with the triangular crosssectional shape or instruments with the H-type cross-section (4, 6). However, little is known about whether the resistance to bending is influenced also by the number of flutes of the instruments. Because fracture of root canal instruments is an occasional problem in root canal treatment (7), the resistance to fracture of endodontic instruments has been investigatedin great detail. Fracture of a root canal instrument occurs under torsional loading when it cuts into the canal wall. The torque has a clinical impact with respect to nonelastic deformation, because instruments with low torque values tend to unwind at lower forces than those with high torque values (4 – 6). Concerning the influence of the cross-section of root canal instruments on the resistance...