JoURNI>J. OF ENDODONTICS
`Copyright @ 2001 by The American Association of Endodontists
`
`Printed In U.S.A.
`VoL. 27, No. 8, AuGUST 2001
`
`Influence of Structure on Nickel-Titanium
`Endodontic Instruments Failure
`
`Gregoire Kuhn, Bruno Tavernier, and Laurence Jordan
`
`The purpose of this work was to investigate the pro·
`cess history on fracture life of nickel-titanium end(cid:173)
`odontics files. The results are based on microstruc·
`tural investigations of nickel-titanium engine-driven
`rotary instruments based on X-ray diffraction, scan·
`ning electron microscopy, and microhardness tests.
`Endodontic files are very work-hardened, and there
`is a high density of defects in the alloy that can
`disturb the phase transformation. The microhard·
`ness Vickers confirmed these observations (disloca(cid:173)
`tions and precipitates). The X-rays show that exper(cid:173)
`imental spectrum lines are extended, typical of a
`distorted lattice. The surface state of the endodontic
`files (scanning electron microscopy} is an important
`factor in failure and fracture initiation.
`
`Nickel-titanium (NiTi) endodontic instruments were introduced to
`facilitate instrumentation of curved canals. Ni-Ti instruments are
`superelastic and will flex far more than stainless-steel instruments
`before exceeding their elastic limit (I, 2). Despite this increased
`flexibility many investigators have reported unexpected fractures
`(3). The fractures of the files can seriously jeopardize the therapy.
`Difficult endodontic treatments are related to abruptness of
`canal curvature. The risk with traditional files is plastic deforma·
`tion and fracture due to stress and strain in the canals.
`Shape memory alloys (SMAs) are characterized by a ther(cid:173)
`moelastic martensitic transformation. The transformation is revers(cid:173)
`ible, accompanied by a hysteresis, and can be induced by variation
`of either temperature or stress (4). In the case of temperature(cid:173)
`induced martensitic transformation the formation of the martensitic
`phase starts at temperature Ms (Martensite Start) and ends at
`temperature Mf (Martensite Finish) (Fig. 1 ). The reversible trans(cid:173)
`formation occurs between temperatures frequently noted as As
`(Austenite Start) and Af (Austenite Finish).
`In the case of stress-induced martensitic transformation the process
`is driven by superelasticity; this effect is linked to structural changes
`at certain temperatures (Fig. 2). Austenite is transformed to martensite
`during loading and reverts back to austenite when unloaded. At the
`beginning of the strain the alloy is fully austenitic (Fig. 2a), at a
`particular stress ( ap) that depends on temperature, the martensitic
`transfonnation is observed (martensite is stable with stress). The
`plateau has been explained as being caused by orientation of the
`
`%of
`transformed
`phase
`
`100%
`Austenite
`
`Tp
`
`100%
`Martensite +---1"-""--'--
`
`Temp. °C
`
`As Ms
`Af
`Mf
`FrG 1. Theoretical thermogram of martensitic transformation tem(cid:173)
`peratures. For example at Tp, two phases are present: 50% aus(cid:173)
`tenite + 50% martensite. Mf, Martensite Finish; As, Austenite Start;
`Ms. Martensite Start; Af, Austenite Finish; Tp, theoretical point.
`
`cr
`
`Austenite
`
`FrG 2. Schematic stress-strain curve of superelasticity; CT = stress;
`ap = particular stress; • = strain.
`
`martensite variants along the direction of the strain (Fig. 2b), the "easy
`deformation" is a consequence of a stress-induced phase transforma(cid:173)
`tion. Without acting strain (curvamre of the root canal) or stress the
`martensite is unstable at root canal temperature, and specimens re(cid:173)
`cover their original shape after unloading (Fig. 2c) (4). If austenite is
`not present a transformation cannot take place (superelasticity behav(cid:173)
`ior does not exist).
`Austenite and martensite have somewhat different atomic arrange(cid:173)
`ments known as crystallographic structures (5). The crystallographic
`
`516
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`GOLD STANDARD EXHIBIT 2017
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`
`

`
`VoL 27, No. 8, August 2001
`
`Influence of Structure on NiTi Failure
`
`517
`
`XRD
`
`A Philips diffractometer was used to detect the phase present at
`room temperature. The wavelength of the CuK01 radiation used in
`this study was 0.154051 nm.
`
`RESULTS
`
`XRD
`
`Results ofXRD of Hero and Profile files showed that the alloys
`are fully austenite at room temperature. The four principal peaks
`indicate the existence of an austenitic phase. Table I shows char(cid:173)
`acteristic (hkl) planes of this phase.
`The XRD scans of all specimens show a ( 11 0) texture. Peak
`width is an indication of cold work. Narrow peaks indicate easy
`cold work; wide ones, hard cold work.
`From an enlargement of the peak (110) (Fig. 3) it is apparent
`that the experimental spectrum lines are extended; this shows that
`the lattice has been distorted and therefore that the alloys are
`work-hardened (9).
`
`Micro hardness
`
`This test compared data relating to each instrument and shows
`a statistically significant difference among these instruments (p <
`0.005).
`Table 2 shows the average value for each instrument.
`For size 20 Hero 0.06 taper and size 20 Profile 0.06 taper there
`was no statistically significant difference (p = 0.0179). After
`
`TABLE 1. X-ray data of Hero and Profile at room temperature
`
`6 (deg)
`Hero
`
`21.1682
`38.7098
`46.3636
`53.9729
`
`6 (deg)
`Profile
`
`21.1682
`38.7736
`46.3017
`53.9651
`
`Planes
`(hkl)
`
`(110)
`(211)
`(220)
`(310)
`
`Structure
`
`Austenite
`Austenite
`Austenite
`Austenite
`
`3500
`
`30011
`
`2SOO
`
`2000
`
`1500
`
`1000
`
`soo
`
`. - .. • theoretical peale
`
`--Hero
`
`- - .Pro1i1.:
`
`I
`II
`
`, .. F
`
`:'·~ (110)
`1(.1 ;
`,. 1:
`I;
`~ ;
`I
`'
`I·
`
`-: \
`•
`I
`•
`~
`l
`I
`1
`I
`~
`I
`· I
`;I
`: ~
`I
`I
`
`·.,
`
`structure of austenite is cubic, whereas that of martensite is more
`complex (monoclinic). X-ray diffraction (XRD) is a valuable tool for
`the determination of the crystallographic structure of materials. Planes
`(hld) of atoms constructively interfere with X -rays and diffraction
`occurs: Bragg's law,..\= 2d sinl:l, allows the calculation ofinterplanar
`spacings or d-spacings from the angular location of XRD peaks, l:l
`(degree). Comparison of XRD date to known standards is used to
`identifY phases. Miller indices, hid integers, are assigned to XRD
`peaks. Miller indices describe the orientation of planes of atoms to the
`unit cell of a material's crystal structure (6).
`Some authors, Miyazaki et al. (7), found superelasticity to be
`greatly dependent on the thermal history of the material. They
`showed various heat treatments can produce or eliminate super(cid:173)
`elasticity behavior. Thermal treatments are involved in promoting
`some systemic changes in the mechanical properties and transfor(cid:173)
`mation characteristics. When the metal is heated up thermally
`activated diffusion and partial annihilation of lattice defects will
`occur and the stored energy will be released in the form of heat
`(recovery). For the recrystallization process a different mechanism
`starts to operate (i.e. grain growth) with healing of defects (8).
`The aim of the present work was to investigate fatigue charac(cid:173)
`teristics of superelastic NiTi and subsequently the process history
`of fracture life.
`
`MATERIALS AND METHODS
`
`The engine-driven rotary instruments that were studied are
`produced by Maillefer (ProFile) and Micro-Mega (Hero) in many
`geometrical shapes. The studied files had a 25 mm length, a taper
`ranging between 0.04 and 0.06 mm per mm length, and sizes 20 to
`40, representing the diameter of the tip base of the file given in
`hundredths of a millimeter.
`
`Thermal Treatments
`
`Different thermal treatments were investigated. The heat treat(cid:173)
`ments consisted of an annealing at 350, 400, 450, 510, 600, and
`700 degrees Celsius in salt baths for 10 min, and at 600° and 700°C
`for 15 min with the same process, and subsequent water quench.
`
`Methodologies
`
`Results are based on microstructural investigations of NiTi
`engine-driven rotary instruments with scanning electron micros(cid:173)
`copy (SEM), XRD, and microhardness.
`
`Microhardness
`
`For mechanical characterization the microhardness Vickers was
`measured with a weight less than 1.96 N. These tests were con(cid:173)
`ducted at room temperature and only on the inactive part of the file
`(without machining). Microhardness characterizes penetration re(cid:173)
`sistance of a material. Because the number of samples was small
`(10) the analysis of the results was performed with a Kruskai(cid:173)
`Wallis test.
`
`SEM
`
`Size 20 Hero and Profile 0.04 and 0.06 tapers were observed by
`SEM to study the surface states. A JEOL T330 was used, and EDS
`analysis was performed on a Tracor TN 5500.
`
`21
`
`21,5
`
`e (deg)
`
`22
`
`0
`20 1S
`20
`FIG 3. XRD (11 0) of the austenite phase for two endodontic files. A
`notable difference between scans of NiTi files and theoretical peak
`is the width of the (11 0) peak.
`
`

`
`518
`
`Kuhn et al.
`
`Journal of Endodontics
`
`TABLE 2. Microhardness results
`
`Endodontics Files
`
`Average (HV)
`
`Hero 6 20
`Profile 6 20
`Profile 4 20-350"C 1 0'
`Profile 4 20-400"C 1 0'
`Profile 4 20-450"C 10'
`Profile 4 20-51 O"C 1 0'
`Profile 4 20-600"C 10'
`Profile 4 20-600"C 15'
`Profile 4 20-700"C 10'
`Profile 4 20-700"C 15'
`
`HV, Hardness Vickers. Prime indicates minutes.
`
`421
`475.2
`407.2
`420
`401.6
`372.4
`258
`258
`254.4
`254.2
`
`FIG 5. A size 20 Profile 0.06 taper, noncurved body region that shows
`striation patterns resulting from machining.
`
`FIG 4. Presentation ofthe system that maintains files curved for SEM
`examination.
`
`comparison of two types of heat treatments (superior to 550°C and
`inferior to 550°C), no significant difference was found with each
`type but there is a statistical difference among each group (p =
`0.0001). Another difference appears between size 20 Profile 0.06
`taper and size 20 Profile 0.04 taper that had heat treatments inferior
`to ssooc (p = 0.0013).
`Our samples, before any use, were measured at a hardness above
`400 HV. These samples are already work-hardened, probably with
`precipitates. The samples annealed at temperature below 600°C
`had a lesser density of defects (dislocations). For temperatures
`above 600°C the alloy will recrystallize and the precipitates will
`partially disappear.
`
`fiG 6. A size 20 Profile 0.06 taper, maximum curved area, at less
`magnification (x 1 000). The cracks are still easy identifiable when the
`file is under stress.
`
`For size 20 Hero 0.06 taper SEM shows less machining damage. In
`deep flutes, we can observe some beginning cracks (Fig. 8), which
`seem to be becoming bigger near the most curved part (Fig. 9).
`
`SEM
`
`DISCUSSION
`
`For SEM examination the endodontic files, selected before any
`use, were maintained curved to generate stress-strain on the most
`curved part of the file (Fig. 4). SEM microphotographs of the
`noncurved body regions show for size 20 Profile 0.06 taper sig(cid:173)
`nificant machining marks along the faces of the flutes (Fig. 5). For
`the curved body region of size 20 Profile 0.06 taper the cutting
`edges and ridges show irregularities and cracks. These cracks are
`easily identifiable because of the curvature of the instrument (Fig.
`6). At this higher magnification (Fig. 7), on the top of the curva(cid:173)
`ture, real sinuous-shape cracks appear.
`
`Some authors (10) studied the mechanical fatigue of NiTi al(cid:173)
`loys. Fatigue crack growth rates were measured and found to be
`lower than predicted from the phenomenological law relating
`growth rates to the elastic modulus. By comparison a deviation of
`factor 3 is observed with conventional metals or alloys (Ti-, AI-,
`and Fe-based alloys). This decrease in normalized crack growth
`rate may be a consequence of reversible martensitic deformation
`processes (superelasticity) leading to less accumulation of damage
`per cycle compared with more conventional materials.
`
`

`
`Vol. 27, No.8, August 2001
`
`Influence of Structure on NiTi Failure
`
`519
`
`FIG 7. A size 20 Profile 0.06 taper, at a higher magnification {X5000).
`Real sinuous shape cracks appear (arrows), which mean beginning
`risks of fracture.
`
`FIG 8. A size 20 Hero 0.06 taper, noncurved area. Cracks are also
`present (arrow) between every striation.
`
`But users and the literature report "premature" failures. More(cid:173)
`over manufacturer's advise not to use each file on more than I 0 to
`12 root canals.
`The purpose of the present paper was therefore to explain why
`these endodontic instruments are prematurely broken and the pro(cid:173)
`cess history of fracture life.
`The mechanical properties and the various phase transformation
`temperatures of NiTi SMAs are known to be very dependent on
`thermomechanical processing. When the material is subjected to
`deformation or stress by machining a high density of lattice defects
`is produced as dislocations. XRD and the microhardness Vickers
`results show that alloys are very work-hardened, thus the density
`of dislocations is very important. After Treppmann (II) a sample
`of NiTi fully recrystallized and without any precipitation has a
`microhardness measure at 220 HV. The dislocations present in the
`matrix influence the reorientation processes for superelasticity
`
`FIG 9. A size 20 Hero 0.06 taper, maximum curved area. The defects
`appear very extended under strain.
`
`(12). Both the defects and internal stresses can act as a negative
`factor to the mobility of martensite interfaces (8). When the an(cid:173)
`nealing temperature is above 600°C the recrystallization process
`takes place, decreasing the density of lattice defects and internal
`stress produced by work-hardening (13).
`SEM results show a high incidence of machining marks on the
`surface as more or less deep scratches. In clinical conditions the
`curve of canals distorts the endodontic instruments; cyclic fatigue
`is caused by repeated tensile-compressive stress. The maximum of
`this stress is in the surface of the curve. Crack nucleation and
`propagation stages appear mostly on the half of the instrument that
`is in tension (outside of the curve). The crack nucleation stage is
`facilitated by the high density of surface defects and then the
`fatigue failure is largely a crack propagation process.
`
`CONCLUSION
`
`Manufacture of NiTi alloys by machining in endodontic files
`promotes work-hardening, which defects contribute to the degra(cid:173)
`dation of mechanical properties of these alloys. Cold work and heat
`treatments are important variables to be controlled during the
`manufacture of endodontic files products. In superelasticity on
`repeated cycling reorientation of the martensite under stress leads
`to gradual defect accumulation. It might be expected that these
`dislocations are generated at the interface between different mar(cid:173)
`tensite colonies. A higher dislocation density influences the reori(cid:173)
`entation processes and crack growth: the files become brittle. The
`phenomenon of repeated cyclic metal fatigue, caused by canal
`curvatures, may be the most important factor in instrument sepa(cid:173)
`ration (14).
`In these applications it is critical to predict the service life based
`on theoretical modeling. Some suggestions could be advanced to
`improve the lifetime of endodontic files:
`
`• Apply some thermal treatments (recovery) before machining to
`decrease the work-hardening of the alloy
`• Choose machining conditions adapted to this NiTi SMA
`• An electropolishing procedure could be used by the manufac(cid:173)
`turer to reduce the machining damage on the file surface.
`
`

`
`520
`
`Kuhn etal.
`
`Journal of Endodontics
`
`The authors are grateful to Dr. Luc Robbiola for SEM support and Profes(cid:173)
`sor Pierre Machtou for materials support.
`
`Drs. Kuhn, Tavernier, and Jordan are affiliated with UFR d'Odontologie,
`Paris, France. Drs. Kuhn and Jordan are affiliated with the Laboratolre de
`Metallurgle Structurale, ENSCP, Paris, France. Dr. Jordan is affiliated with
`CECM, CNRS, Vitry sur Seine, France. Address requests for reprints to Dr.
`Gregoire Kuhn, Laboratoire de Metallurgle Structurale, ENSCP, 11 Rue Pierre
`et Marie Curie, 75231 Paris Cedex 05, France.
`
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`
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