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Páginas: 23 (5651 palabras) Publicado: 13 de agosto de 2012
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Author Manuscript
Int Endod J. Author manuscript; available in PMC 2005 October 26.

NIH-PA Author Manuscript

Published in final edited form as:
Int Endod J. 2005 June ; 38(6): 364–371.

Effect of cryogenic treatment on nickel-titanium endodontic
instruments
J. W. Kim1, J. A. Griggs2, J. D. Regan1, R. A. Ellis1, and Z. Cai2
1Departments of Graduate Endodontics and2Biomaterials Science, Baylor College of Dentistry, Texas A&M
University System, Health Science Center, Dallas, TX, USA

Abstract
Aim—To investigate the effects of cryogenic treatment on nickel-titanium endodontic instruments.
The null hypothesis was that cryogenic treatment would result in no changes in composition,
microhardness or cutting efficiency of nickel-titanium instruments.NIH-PA Author Manuscript

Methodology—Microhardness was measured on 30 nickel-titanium K-files (ISO size 25) using a
Vicker’s indenter. Elemental composition was measured on two instruments using X-ray
spectroscopy. A nickel-titanium bulk specimen was analysed for crystalline phase composition using
X-ray diffraction. Half of the specimens to be used for each analysis were subjected to a cryogenictreatment in liquid nitrogen (−196 °C) for either 3 s (microhardness specimens) or 10 min (other
specimens). Cutting efficiency was assessed by recording operator choice using 80 nickel-titanium
rotary instruments (ProFile® 20, .06) half of which had been cryogenically treated and had been
distributed amongst 14 clinicians. After conditioning by preparing four corresponding canals, each
pairof instruments were evaluated for cutting efficiency by a clinician during preparation of one
canal system in vitro. A Student’s t-test was used to analyse the microhardness data, and a binomial
test was used to analyse the observer choice data. Composition data were analysed qualitatively.
Results—Cryogenically treated specimens had a significantly higher microhardness than the
controls (P <0.001; β > 0.999). Observers showed a preference for cryogenically treated instruments
(61%), but this was not significant (P = 0.21). Both treated and control specimens were composed
of 56% Ni, 44% Ti, 0% N (by weight) with a majority in the austenite phase.

NIH-PA Author Manuscript

Conclusions—Cryogenic treatment resulted in increased microhardness, but this increase was not
detectedclinically. There was no measurable change in elemental or crystalline phase composition.
Keywords
cryogenic; cutting efficiency; energy-dispersive X-ray spectroscopy; microhardness; nickeltitanium; X-ray diffraction

Introduction
In 1988 an orthodontic wire alloy, Nitinol (nickel-titanium or NiTi) was described in the
endodontic literature (Walia et al. 1988). Nickel-titanium alloy exhibitsthe unique properties
of shape memory and pseudoelasticity. Nickel-titanium instruments show considerably greater
flexibility and resistance to torsional fracture when compared with stainless steel instruments
(Walia et al. 1988). In addition, nickel-titanium alloy was shown to have a lower modulus of

Correspondence: J. A. Griggs, 3302 Gaston Avenue, Dallas, TX 75246, USA (Tel.: +1 214 8288307; fax: +1 214 828 8458; e-mail:
jgriggs@bcd.tamhsc.edu)..

Kim et al.

Page 2

NIH-PA Author Manuscript

elasticity, a wider range of elastic deformation and a greater overall strength (Andreasen et
al. 1985, Walia et al. 1988). However, due to the pseudoelastic property of NiTi alloy, NiTi
instruments must be machined rather than twisted (Thompson 2000). This machining process
maylead to surface defects within the cutting surfaces of the instrument, which have been
implicated in the relatively low cutting efficiency of the nickel-titanium instrument (Thompson
2000). Nickel-titanium instruments also exhibit a lower microhardness (303–362 VHN) than
stainless steel instruments (522–542 VHN) (Brockhurst & Denholm 1996, Brockhurst & Hsu
1998). Consequently, surface...
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