Restor Dent Endod.  2012 May;37(2):74-78.

Effect of internal stress on cyclic fatigue failure in K3

• Kim JY
• Kim JW
• Cho KM
• Park SH

Abstract

OBJECTIVES
This study aimed to evaluate the relationship between the cyclic fatigue of a K3 file and internal stress intentionally induced until the activation of the auto-stop function of the torque-controlled motor.
MATERIALS AND METHODS
K3 (Sybron Endo) .04 and .06 taper, size 25, 30, 35, 40 and 45 were used in this study. To give the internal stress, the K3 files were put into the .02 taper Endo-Training-Bloc (Dentsply Maillefer) until the activation of the auto-stop function of the torque-controlled motor. The rotation speed was 300 rpm and torque value was 1.0 N.cm. K3 were grouped by the number of induced internal stress and randomly distributed to 4 experimental groups (n = 10, Stress 0 [control], Stress 1, Stress 2 and Stress 3). For measuring the cyclic fatigue failure, the K3 files were worked against a sloped glass block and time for file separation was recorded. Data was statistically analyzed Statistical analyses were performed using two-way ANOVA and Duncan post-hoc test at p < 0.05 level.
RESULTS
Except .04 taper size 30 in Stress 1 group, there were statistically significant differences in time for file separation between control and all experimental groups. K3 with .04 taper showed higher cyclic fatigue resistance than those of .06 taper.
CONCLUSION
In the limitation of this study, the cyclic fatigue of the K3 file was influenced by the accumulated internal stress from use until the auto-stop function was activated by the torque-controlled motor. Therefore, clinicians should avoid the reuse of the K3 file that has undergone auto-stops.

Keyword

Auto-stop; Cyclic fatigue failure; Internal stress; K3; Separation; Torsional stress

MeSH Terms

Fatigue
Glass
Intention
Torque

Figure

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  Figure 1 Representative image of cyclic fatigue test (inclined glass plane).

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  Figure 2 FE-SEM view of K3. (a) Torsional fracture of K3 .04/#25. Central dimples and circular abrasion mark (white arrow) (×200); (b) Torsional distorsion of K3 .04/#25; (c) Torsional distorsion of K3 .04/#30 (×30); (d) Cyclic fatigue fracture of K3 .04/#45. Radial striations linked at a point on the edge (black arrow) (×120). The surfaces of cyclic fatigue fracture in all experimental groups showed same patterns. FE-SEM, field emission scanning electron microscope.

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