Low-cycle fatigue of rotary NiTi endodontic instruments in hypochlorite solution.
OBJECTIVE
To examine the low-cycle fatigue (LCF) behaviour of NiTi rotary endodontic instruments in aqueous sodium hypochlorite solution.


METHODS
Four brands of NiTi rotary instrument (n=179) were subjected to a rotational-bending fatigue test at a rate of 250rpm until fracture, the instrument being immersed in 1.2% sodium hypochlorite solution. The surface strain amplitude, calculated from the curvature (from a pre-test photograph) and diameter of the fracture cross-section (from SEM photomicrograph) was plotted against the total number of revolutions to failure. An apparent fatigue-ductility exponent was determined from a regression line fitted to the LCF lives for each brand of instrument. All detached fragments were examined fractographically for crack initiation and the extent of crack extension into the cross-section. The values were compared using one-way analysis of variance (ANOVA) or chi2, where appropriate, at alpha=0.05.


RESULTS
A linear strain-life relationship was obtained for all groups; the apparent fatigue-ductility exponent was similar between various brands (ANOVA, P>0.05), but not for the number of crack origins (chi2, P<0.05). There was an inverse, linear relationship between the square root of the extension of the fatigue-crack and the strain amplitude.


CONCLUSIONS
NiTi rotary instruments fatigued in hypochlorite shows a strain-life relationship with low-cycle and high-cycle fatigue regions. The LCF behaviour is not affected by the cross-sectional shape of the instrument. There appears to be a critical extent of crack propagation for various surface strain amplitudes leading to final, catastrophic fracture of the instrument.
Cheung
Gary S P
GS
Endodontics, Faculty of Dentistry, The University of Hong Kong, Hong Kong SAR, China. spcheung@hku.hk
Darvell
Brian W
BW
D003343:Corrosion; D003722:Dental Alloys / Q000737:chemistry*; D004202:Disinfectants / Q000737:chemistry*; D004548:Elasticity; D004867:Equipment Design; D004868:Equipment Failure; D006801:Humans; D008422:Materials Testing; D008855:Microscopy, Electron, Scanning; D009532:Nickel / Q000737:chemistry*; D018583:Pliability; D018915:Root Canal Preparation / Q000295:instrumentation*; D012399:Rotation; D012973:Sodium Hypochlorite / Q000737:chemistry*; D013314:Stress, Mechanical; D013499:Surface Properties; D014025:Titanium / Q000737:chemistry*; D019415:Torque
D003722:Dental Alloys; D004202:Disinfectants; C040654:titanium nickelide; D009532:Nickel; D014025:Titanium; D012973:Sodium Hypochlorite