REVIEW
`An overview of nickel-titanium alloys used
`in dentistry
`
`S. A. Thompson
`Department of Adult Dental Health, University of Wales College of Medicine, Cardiff, UK
`
`Abstract
`
`Thompson SA. An overview of nickel-titanium alloys used in
`dentistry. International Endodontic Journal, 33, 297-310, 2000.
`
`Literature review The nickel-titanium alloy Nitinol
`has been used in the manufacture of endodontic instru(cid:173)
`ments in recent years. Nitinol alloys have greater
`strength and a lower modulus of elasticity compared
`with stainless steel alloys. The super-elastic behaviour
`of Nitinol wires means that on unloading they return to
`
`their original shape following deformation. These prop(cid:173)
`erties are of interest in endodontology as they allow
`construction of root canal instruments that utilize these
`favourable characteristics to provide an advantage
`when preparing curved canals. This review aims to pro(cid:173)
`vide an overview of Nitinol alloys used in dentistry in
`order for its unique characteristics to be appreciated.
`
`Keywords: endodontics, nickel-titanium, root canals.
`
`Received 13 October 19 9 9; accepted 7 December 19 9 9
`
`Introduction
`
`In the early 1960s, a nickel-titanium alloy was
`developed by W F. Buehler, a metallurgist investigating
`nonmagnetic, salt resisting, waterproof alloys for the
`space programme at the Naval Ordnance Laboratory
`in Silver Springs, Maryland, USA (Buehler et al. 1963).
`The thermodynamic properties of this intermetallic
`alloy were found to be capable of producing a shape
`memory effect when specific, controlled heat treat(cid:173)
`ment was undertaken (Buehler et al. 19 6 3). The alloy was
`named Nitinol, an acronym for the elements from
`which the material was composed; ni for nickel, ti
`for titanium and nol from the Naval Ordnance Labor(cid:173)
`atory. Nitinol is the name given to a family of inter(cid:173)
`metallic alloys of nickel and titanium which have been
`found to have unique properties of shape memory
`and super-elasticity.
`The super-elastic behaviour of Nitinol wires means
`that on unloading they return to their original shape
`before deformation (Lee et al. 1988, Serene et al. 1995).
`
`Correspondence: Dr Shelagh Thompson, Department of Adult
`Dental Health. University of Wales College of Medicine, Heath Park.
`Cardiff CF14 4XY, Wales, UK (fax: +44 (0)2920 742479; e-mail:
`thompsonsa@cardiff.ac.uk).
`
`As the alloy has greater strength and a lower modulus
`of elasticity compared with stainless steel (Andreasen
`& Morrow 1978, Andreasen et al. 1985, Walia et al. 1988),
`there may be an advantage in the use of NiTi instru(cid:173)
`ments during the preparation of curved root canals,
`because the files will not be permanently deformed
`as easily as would happen with traditional alloys
`(Schafer 1997).
`
`Metallurgy of nickel-titanium alloys
`
`The nickel-titanium alloys used in root canal treatment
`contain approximately 56% (wt) nickel and 44% (wt)
`titanium. In some NiTi alloys, a small percentage (<2%
`wt) of nickel can be substituted by cobalt. The resultant
`combination is a one-to-one atomic ratio (equiatomic)
`of the major components and, as with other metallic
`systems, the alloy can exist in various crystallographic
`forms (Pig. 1). The generic term for these alloys is 55-
`Nitinol; they have an inherent ability to alter their type
`of atomic bonding which causes unique and signi(cid:173)
`ficant changes in the mechanical properties and
`crystallographic arrangement of
`the alloy. These
`changes occur as a function of temperature and stress.
`The two unique features that are of relevance to
`clinical dentistry occur as a result of the austenite to
`
`© 2000 Blackwell Science Ltd
`
`International Endodontic Journal, 33, 297-310, 2000
`
`GOLD STANDARD EXHIBIT 2037
`US ENDODONTICS v. GOLD STANDARD
`CASE PGR2015-00019
`
`

`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`IEJ339.fm Page 298 Saturday, June 10, 2000 8:50 AM
`
`Overview of NiTi alloys
`
`Thompson
`
`Figure 3 Diagrammatic representation of the super-elasticity
`effect of NiTi alloy.
`
`that when it is cooled through a critical
`transformation
`
`temperature range (TTR), the alloy shows dramatic
`changes in its modulus of elasticity (stiffness), yield
`strength and electric resistivity as a result of changes in
`electron bonding. By reducing or cooling the temperature
`through this range, there is a change in the crystal
`structure which is known as the
`
`martensitic transformation;
`the amount of this transformation is a function of the
`start (Ms) and finish (Mf) temperature. The phenomenon
`causes a change in the physical properties of the alloy
`
`et al. 1972) and gives rise to the
`(Wang
`shape memory
`characteristic. The hysteresis of the martensitic trans-
`formation is shown in Fig. 4.
`The transformation induced in the alloy occurs by
`a shear type of process to a phase called the
`martensitic
`or daughter phase (Fig. 1), which gives rise to
`twinned
` (Fig. 1) that forms the structure of a closely
`martensite
`packed hexagonal lattice (Fig. 1). Almost no macro-
`scopic shape change is detectable on the transformation,
`unless there is application of an external force. The
`martensite shape can be deformed easily to a single
`orientation by a process known as de-twinning to
`de-
`, when there is a ‘flipping over’ type
`twinned martensite
`of shear. The NiTi alloy is more ductile in the martens-
`itic phase than the austenite phase. The martensitic
`transformation and the shape memory effect is shown
`in Fig. 1.
`The deformation can be reversed by heating the alloy
`above the TTR (reverse transformation temperature
`range or RTTR) with the result that the properties of the
`NiTi alloy revert back to their previous higher temperature
`values (Fig. 1). The alloy resumes the original parent
`structure and orientation as the body-centred cubic,
`high temperature phase termed
`
`austenite with a stable
`energy condition (Fig. 1). The total atomic movement
`between adjacent planes of atoms is less than a full
`interatomic distance when based on normal atomic
`lattice arrangements. This phenomenon is termed
`shape
` (Fig. 2) and allows the alloy to return to its
`memory
`
`Figure 1 Diagrammatic representation of the martensitic
`transformation and shape memory effect of NiTi alloy.
`
`Figure 2 Diagrammatic representation of the shape memory
`effect of NiTi alloy.
`
`martensite transition in the NiTi alloy; these charac-
`teristics are termed
`
`shape memory and
`super-elasticity
`(Figs 2 and 3).
`
`Structure of nickel–titanium
`
`The crystal structure of NiTi alloy at high temperature
`ranges (100
`C) is a stable, body-centred cubic lattice
`
`austenite phase or parent
`which is referred to as the
`phase (Fig. 1). Nitinol has the particular characteristic
`
`298
`
`International Endodontic Journal,
`, 297–310, 2000
`33
`
`© 2000 Blackwell Science Ltd
`
`(cid:176)
`

`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`IEJ339.fm Page 299 Saturday, June 10, 2000 8:50 AM
`
`Figure 4 Hysteresis of martensitic
`transformation.
`
`previous shape, by forming strong, directional and
`energetic electron bonds to pull back displaced atoms to
`their previous positions; the effect of this transforma-
`tion is instantaneous.
`It is possible using the shape memory effect to educate
`or place the NiTi alloy into a given configuration at a
`given temperature. This can be carried out at lower
`temperatures which deform the NiTi with a very low
`force and results in the ‘twins’ all occurring in the
`same direction. When the NiTi is heated through its
`transformation temperature it will recover its original
`‘permanent’ shape (Fig. 2). The application of shape
`memory to orthodontics is discussed later. In terms of
`endodontology, this phenomenon may translate to the
`ability to remove any deformation within nickel–
`titanium instruments by heating them above 125
`C
`
`et al. 1995).
`(Serene
`The transition temperature range for each nominal
`55-Nitinol alloy depends upon its composition, as this
`causes considerable variability in the number of electrons
`available for bonding to occur and is constant for a par-
`ticular NiTi alloy composition. The TTR of a 1 : 1 ratio
`C.
`of nickel and titanium is in the range of –50 to +100
`Reduction in the TTR can be achieved in several ways;
`in the manufacturing process both cold working and
`thermal treatment can significantly affect TTR, as
`can altering the nickel : titanium ratio in favour of
`excess nickel or by substituting cobalt for nickel, atom
`for atom. Cobalt substitution produces alloys with
`the composition NiTi
`Co
`. The TTR can be lowered
`x
`1-x
`progressively by continued substitution of cobalt for
`nickel as cobalt possesses one less electron than nickel,
`
`Thompson
`
`Overview of NiTi alloys
`
`thus lowering the total number of bonding electrons.
`However, formation of a detrimental second phase
`NiTi
` occurs if excess nickel is added in attempts to
`3
`lower the TTR.
`
`Stress-induced martensitic transformation
`
`•
`
`The transition from the austensitic to martensitic phase
`can also occur as a result of the application of stress,
`such as occurs during root canal preparation. In most
`metals, when an external force exceeds a given amount
`mechanical slip is induced within the lattice causing
`permanent deformation; however, with NiTi alloys a
`
`stress-induced martensitic transformation occurs, rather
`than slip. This causes:
`•
`a volumetric change associated with the transition
`from one phase to the other and an orientation
`relation is developed between the phases
`the rate of the increase in stress to level off due to
`progressive deformation (Fig. 5) even if strain is
`added due to the martensitic transformation. This
`
`super-elasticity (Fig. 4), a
`results in the so-called
`movement which is similar to slip deformation. The
`differences between the tensile behaviours of NiTi
`and stainless steel alloy can be seen in Fig. 6.
` when the stress decreases or stops
`springback
`without permanent deformation occurring (Fig. 3).
`Springback is defined as load per change in deflec-
`tion (Andreasen & Morrow 1978), to the previous
`shape with a return to the austenite phase, pro-
`vided the temperature is within a specific range
`(Fig. 4).
`
`•
`
`© 2000 Blackwell Science Ltd
`
`, 297–310, 2000
`International Endodontic Journal,
`33
`
`299
`
`(cid:176)
`(cid:176)
`

`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`IEJ339.fm Page 300 Saturday, June 10, 2000 8:50 AM
`
`Overview of NiTi alloys
`
`Thompson
`
`Figure 5 NiTi phase transformation.
`
`Figure 6 Diagrammatic representation
`of the tensile behaviour of stainless steel
`and NiTi super-elastic alloy and
`mechanisms of elastic deformation.
`
`The plastic deformation that occurs in NiTi alloys
`within or below the TTR is recoverable, within certain
`limits, on reverse transformation. It is this phenome-
`non of crystalline change which gives rise to the
`shape memory effect of the material and the super-
`elastic behaviour. The part of the RTTR in which
`‘shape
` occurs is termed the
`shape recovery temperature
`recovery’
`
`range (SRTR). This has also been termed ‘mechanical
`
`memory’ (Buehler & Wang 1968). This is unlike conven-
`tional metallic stress-strain behaviour where elastic
`response in conventional alloys is recoverable, but is
`small in size; and where larger strains are associated
`with plastic deformation, that
`is not recoverable
`(Fig. 7).
`
`300
`
`International Endodontic Journal,
`, 297–310, 2000
`33
`
`© 2000 Blackwell Science Ltd
`
`

`
`
`
`
`
`
`
`
`
`
`
`
`
`IEJ339.fm Page 301 Saturday, June 10, 2000 8:50 AM
`
`Thompson
`
`Overview of NiTi alloys
`
`Figure 7 NiTi strength curve.
`
`Figure 8 Stress-strain curve: stainless
`steel and nickel–titanium.
`
`The super-elasticity of nickel–titanium allows deforma-
`tions of as much as 8% strain to be fully recoverable
`(Fig. 8), in comparison with a maximum of less than
`1% with other alloys, such as stainless steel. Although
`other alloys such as copper–zinc, copper–aluminium,
`gold–cadmium and nickel–niobium have been found to
`have super-elastic properties (Buehler & Wang 1968),
`nickel–titanium is the most biocompatible material and
`has excellent resistance to corrosion.
`An alloy system is an aggregate of two or more
`
`metals which can occur in all possible combinations. As
`such, a second group of Nitinol alloys can be formed if
`the NiTi alloy contains more nickel and as this approaches
`60% (wt) Ni an alloy known as 60-Nitinol forms. The
`shape memory effect of this alloy is lower, although
`its ability to be heat-treated increases. Both 55 and 60-
`Nitinols are more resilient, tougher and have a lower
`modulus of elasticity than other alloys such as stainless
`steel, Ni–Cr or Co–Cr (Fig. 8). Table 1 shows the values
`for typical properties of Nitinol alloys.
`
`© 2000 Blackwell Science Ltd
`
`, 297–310, 2000
`International Endodontic Journal,
`33
`
`301
`
`

`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`IEJ339.fm Page 302 Saturday, June 10, 2000 8:50 AM
`
`Overview of NiTi alloys
`
`Thompson
`
`Property
`
`Physical
`Density (gm cm3)
`Melting point ((cid:176) C)
`Magnetic permeability
`Coefficient of thermal expansion (· 106/(cid:176) C)
`Electrical resistivity (ohm-cm)
`Hardness 950 (cid:176) C (Furnace cooled)
`Hardness 950 (cid:176) C (Quenched-R.T. water)
`Mechanical
`Young’s modulus (Gpa)
`Yield strength (Mpa)
`Ultimate tensile strength (Mpa)
`Elongation
`Shape memory
`Transformation temperature ((cid:176) C)
`Latent heat of transformation
`Shape memory recoverable strain
`Super-elastic recoverable strain
`Transformation fatigue life
`at 6% strain
`at 2% strain
`at 0.5% strain
`
`55-Nitinol
`austenite
`
`6.45
`1310
`<1.002
`11.0
`100 · 10 - 6
`89 RB
`89 RB
`
`120
`379
`690–1380
`13–40%
`
`–50 to +100
`10.4 BTU lb–1
`6.5–8.5%
`up to 8%
`several hundred
`cycles
`105cycles
`107cycles
`
`55-Nitinol
`martensite
`
`Table 1 Typical properties of Nitinol
`alloys
`
`6.6
`80 · 10 - 6
`
`50
`138
`
`Figure 9 Diagrammatic representation of
`the manufacturing process of Nitinol
`alloy.
`
`Manufacture of Nitinol alloy
`
`Nickel–titanium alloy production is a very complex
`process that consists of:
`•
`vacuum melting/casting
`•
`press forging
`•
`rotary swaging
`•
`rod/wire rolling
`In the past, NiTi alloys with near stoichiometric
`composition have been produced satisfactorily by both
`arc and induction melting methods (Buehler & Wang
`
`1968). One of the problems encountered with arc-melting
`was that it required multiple remelts to ensure chemical
`homogeneity; however, importantly this process pro-
`duces only minimum contamination of the alloy. Current
`manufacture involves the use of vacuum induction
`melting in graphite crucibles (Fig. 9) that ensures effective
`alloy mixing by simple means, with slight carbon con-
`tamination in the melt forming TiC (Buehler & Cross
`1969). The presence of oxide impurities does not effect
`the unique properties of 55-Nitinol, as these appear to
`be evenly distributed within the NiTi matrix.
`
`302
`
`International Endodontic Journal,
`, 297–310, 2000
`33
`
`© 2000 Blackwell Science Ltd
`
`

`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`Thompson
`
`Overview of NiTi alloys
`
`Figure 10 SEM photomicrograph of milling marks and roll-
`over on a rotary NiTi instrument.
`
`deformation. Attempts to twist instruments in a con-
`ventional way would probably result in instrument
`fracture (Schäfer 1997). The instrument profile (design)
`has to be ground into the Nitinol blanks. Further
`difficulties during production include elimination of
`surface irregularities (milling marks) and metal flash
`(roll-over) on the cutting edges that may compromise
`the cutting ability of these instruments and potentially
`cause problems with corrosion (Fig. 10).
`The composition of Nitinol used to construct endo-
`dontic instruments is 56% (wt) nickel and 44% (wt)
`titanium. Although only one manufacturer (Dentsply,
`Maillefer Instruments SA, Ballaigues, Switzerland) has
`released the absolute composition and manufacturing
`details of the nickel–titanium used to construct their
`instruments, it would appear that this is the only alloy
`composition that can utilize the super-elastic properties
`of the alloy.
`It is possible to vary the composition of NiTi alloy in
`order to give rise to wires with two different characteristics;
`either to be a super-elastic alloy or to have the shape
`memory property. The differences between the alloys are
`in their nickel content and the transitional temperature
`range for the given alloy. Various parameters affect the
`transformation temperature; a decrease in the trans-
`formation temperature occurs with an increase in nickel
`content or by substituting trace elements such as cobalt
`as discussed previously, whilst an increase in annealing
`temperature increases the transformation temperature.
`Ideally, for the manufacture of root canal instruments
`the ultimate tensile strength of the alloy should be as
`high as possible to resist separation (Fig. 7), whilst the
`elongation parameters must be suitable for instrument
`
`IEJ339.fm Page 303 Saturday, June 10, 2000 8:50 AM
`
`The double vacuum melting manufacturing process
`ensures purity and quality and maintains the mechanical
`properties of the alloy. The raw materials are carefully
`formulated before the alloy is melted by vacuum induction.
`After this process, vacuum arc remelting takes place to
`improve the alloy chemistry, homogeneity and structure.
`The double melted ingots are hot worked and then cold
`worked to a variety of shapes and sizes according to
`product specifications, i.e. Nitinol wires, bars, etc. as
`described earlier. Alloys for orthodontic or medical
`use can be produced as drawn or with mechanically
`cleaned surfaces.
`Hot and cold working can be undertaken on Nitinol
`alloys, below the crystallization temperature. The alloy
`composition is important to the manufacturing process
`and it appears that 55-Nitinol can be processed by all
`forms of hot working more easily than 60-Nitinol.
`Strengthening of the alloy occurs through low temper-
`ature deformation and maintains a minimum of 12%
`tensile elongation. Some NiTi alloys appear to be sensitive
`to the effects of heat treatment that can effect both shape
`memory and the pseudo-elastic behaviour; however,
`NiTi alloys of near stoichiometric composition such
`as are used in dentistry do not appear to be affected
`
`
`(Saburi et al. 1982, Mercier & Torok 1982).
`Gould (1963) studied the machining characteristics
`of nickel–titanium alloys and found that tool wear was
`rapid and the cutting speed, feed, tool material, tool
`geometry and type of cutting fluid had an effect on the
`results of the manufactured Nitinol. Specifically, these
`alloys could be turned 10–20 times faster with carbide
`tools than with high speed steel tools. Light feeds of
`1
`0.003–0.005 in rev
` are recommended in turning
`Gould 1963) and in order to maximize the tool life, 55-
`–1
`Nitinol should be cut at a speed of 220 ft min
`. An
`active highly chlorinated cutting oil is advised to obtain
`a reasonable drill-life along with the use of silicon
`carbide wheels to grind the surface of Nitinol alloys. The
`speeds at which cutting tools should be operated vary
`according to the composition of the alloy. Therefore, it
`appears that sharp carbide cutting tools are required to
`machine Nitinol alloys using techniques involving light
`feeds and slow speeds.
`
`Construction of root canal instruments
`
`The manufacture of NiTi endodontic instruments is
`more complex than that of stainless steel instruments,
`as the files have to be machined rather than twisted.
`The super-elasticity of the alloy means that it cannot
`maintain a spiral as the alloy undergoes no permanent
`
`© 2000 Blackwell Science Ltd
`
`, 297–310, 2000
`International Endodontic Journal,
`33
`
`303
`
`-
`

`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`IEJ339.fm Page 304 Saturday, June 10, 2000 8:50 AM
`
`Overview of NiTi alloys
`
`Thompson
`
`Figure 11 Diagrammatic representation
`of the production of finished Nitinol wire.
`
`flexibility, (Fig. 8) thereby decreasing canal transportation,
`and to allow high resistance to fatigue.
`Once the alloy has been manufactured, it undergoes
`various processes before the finished wire can be machined
`into a root canal instrument (Fig. 11). Essentially, the
`casting is forged in a press into a cylindrical shape prior
`to rotary swaging under pressure, to create a drawn
`wire. The wire is then rolled to form a tapered shape with
`even pressure from a series of rollers applied to the wire.
`During the construction phase, other processes are carried
`out on the rod-rolled wire including drawing the wire
`onto a cone, annealing the wire in its coiled state, descal-
`ing and further fine wire drawing followed by repeated
`annealing with the wire in a straight configuration. This
`stage is followed by drawing the actual profile or cross-
`sectional shape of the wire, e.g. imparting either a round,
`square or oblong shape prior to a cleaning and conditioning
`process. The finished wire is stored on reels prior to
`machining. This process is illustrated in Fig. 11.
`
`Uses of nickel–titanium alloy
`
`Nitinol wire was first used in the self-erectile action of a
`disc on rod antenna that recovered its prefolded shape
`above the transition temperature of the alloy (Buehler &
`Wang 1968). The unique ‘mechanical memory’ of 55-
`Nitinol allowed it to recover its original shape after
`mechanical distortion by heating it above the transition
`temperature. The rate of recovery of the antenna was
`determined by the rate at which the critical temperature
`was reached, which depended on the thermal con-
`
`ductivity and the mass of the material. The corrosion
`resistance of Nitinol alloys was evaluated by Buehler
`& Wang (1968) who reported that they performed
`adequately in a marine environment.
`Duerig (1990) described applications for shape memory
`NiTi alloys grouped according to the primary function of
`
`free recovery was
`the memory element. An example of (i)
`NiTi eyeglass frames, (ii)
`
`constrained recovery was couplings
`for joining aircraft hydraulic tubing and electrical con-
`
`work production was actuators both electrical
`nectors, (iii)
`and thermal and (iv)
`
`super-elasticity was orthodontic
`archwire, guidewires and suture anchors.
`Further uses of the super-elastic properties of NiTi
`wire were described by Stoeckel & Yu (1991). As super-
`elasticity is an isothermal event, applications with a well
`controlled temperature environment are most successful, e.g.
`in the human body. NiTi wire has been used as ortho-
`®
`dontic archwire and springs, in Mammalok
` needle wire
`localizers (to locate and mark breast tumours), guidewires
`for catheters, suture needles and anchors, the temples and
`bridges for eyeglasses, and, in Japan, underwire brassieres.
`
`Use of nickel–titanium alloys in dentistry
`
`Orthodontics
`
`Initially NiTi alloys were used in the construction of
`orthodonic archwires. Extensive research published
`in the materials science and orthodontic journals has
`allowed the properties of the material to be appreciated
`and used in an appropriate clinical manner. Many of
`
`304
`
`International Endodontic Journal,
`, 297–310, 2000
`33
`
`© 2000 Blackwell Science Ltd
`
`

`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`IEJ339.fm Page 305 Saturday, June 10, 2000 8:50 AM
`
`these studies have relevance to an understanding of
`NiTi alloys used in endodontology and a brief descrip-
`tion of this literature is described.
`NiTi wires were first used in orthodontics by Andreasen
`& Hilleman (1971), who observed differences in the
`physical properties of Nitinol and stainless steel
`orthodontic wires that allowed lighter forces to be used.
`The strength and resilience of NiTi wires meant there
`was a reduction in the number of arch wire changes
`necessary to complete treatment. Rotations of teeth could
`be accomplished in a shorter time, without increasing
`patient discomfort. Nitinol wires showed better resistance
`to corrosion so were felt more appropriate for intraoral
`use than stainless steel.
`Andreasen & Morrow (1978) observed the unique
`properties of Nitinol, including its outstanding elasticity
`(which allows it to be drawn into high-strength wires)
`and its ‘shape memory’ (which allows the wire when
`deformed, to ‘remember’ its shape and return to its original
`configuration). The most important benefits of Nitinol
`wire were its construction as a resilient, rectangular
`wire that allowed simultaneous rotation, levelling, tipping
`and torquing movements, to be accomplished early in
`treatment. Limitations to the use of the material were
`noted, such as the time taken to bend the wires, the
`necessity of not using sharp-cornered instruments that
`could lead to breakage and the inability to be soldered or
`welded to itself. Overall, the authors felt the material
`represented a significant improvement over conventional
`arch wire and was a valuable addition to the orthodontist’s
`armamentarium.
`Burstone & Goldberg (1980) observed beneficial
`characteristics such as low modulus of elasticity
`combined with a high tensile strength that allowed
`wires to sustain large elastic deflections due to the very
`high springback quality. Limitations such as restricted
`formability and the decrease of springback after bending
`prompted investigations into other alloys, such as beta
`titanium. Drake
`
`et al. (1982) concluded that Nitinol
`wire was suitable mainly for pretorqued, preanglulated
`brackets.
`et al. (1986) tested a new Japanese NiTi alloy
`
`Miura
`and compared it to stainless steel, Co–Cr–Ni and Nitinol
`wires. Japanese NiTi alloy exhibited super-elastic properties
`and was least likely to undergo permanent deformation
`during activation. The Nitinol wire showed less per-
`manent deformation and excellent springback qualities in
`comparison with the stainless steel and Co–Cr–Ni wires,
`however, load and deflection were almost proportional,
`indicating lack of super-elastic qualities. This may have
`been due to the fact that Nitinol was manufactured by a
`
`Thompson
`
`Overview of NiTi alloys
`
`work-hardening process, thus affecting the properties of
`shape memory or super-elasticity.
`Kusy & Stush (1987) observed a discrepancy between
`the stated dimensions of wires of 10 Nitinol and seven beta
`titanium wires; the sizes were smaller 95% of the time
`and neither wire obeyed simple yield strength relationships.
`The ultimate tensile strength of Nitinol wires increased
`with decreasing cross-sectional area and also appeared
`more ductile with increasing cross-sectional area.
`Yoneyama
`
`et al. (1992) assessed the super-elasticity
`and thermal behaviour of 20 commercial NiTi ortho-
`dontic arch wires. A three-point bending test allowed
`load-deflection curves to be determined and differential
`scanning calorimetry was used to determine thermal
`behaviour due to phase transformation of the alloy.
`Substantial differences were noted between the wires;
`some showed super-elasticity (which occurs with the
`stress induced martensitic transformation), whilst other
`wires only had good springback qualities. Super-elasticity
`was only exhibited by wires showing high endothermic
`energy in the reverse transformation from the martensitic
`phase to the parent phase and with low load/deflection
`ratios. These wires showed nearly constant forces in
`the unloading process, a desirable physiological property
`for orthodontic tooth movement; the lower the L/D ratio,
`the less changeable is the force which the wire can display.
`Clearly, there are differences in the mechanical
`properties and thermal behaviour of NiTi alloy which
`vary with composition, machining characteristics and
`differences in heat treatment during manufacture. The
`need for correct production of NiTi alloys was stressed
`by Yoneyama
`. (1992) in order that the desired
`et al
`clinical characteristics could be obtained.
`Evans & Durning (1996) reported the differences in
`formulations of nickel–titanium alloy and their applica-
`tions in archwire technology. A review article published
`by Kusy (1997) described the properties and characteristics
`of contemporary archwires from initial development
`to their current use in variable modulus orthodontics
`as advocated by Burstone (1981). The variation in
`composition of nickel–titanium alloys was discussed
`together with the influence this had on the properties
`of the resultant alloy.
`
`Corrosion behaviour of NiTi orthodontic wires
`
`The corrosion behaviour of Nitinol orthodontic wires
`was compared with stainless steel, cobalt-chrome and
`-titanium by Sarkar
`
`et al. (1979). The wires were
`exposed to a 1% NaCl solution via an electrochemical
`cyclic polarization
`technique. Scanning electron
`
`© 2000 Blackwell Science Ltd
`
`, 297–310, 2000
`International Endodontic Journal,
`33
`
`305
`
`b
`

`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`IEJ339.fm Page 306 Saturday, June 10, 2000 8:50 AM
`
`Overview of NiTi alloys
`
`Thompson
`
`microscopy and energy dispersive X-ray analysis was
`used to determine differences between pre- and postpo-
`larized surfaces. The Nitinol alloy was the only specimen
`to exhibit a pitting type corrosion attack, which the
`authors concluded warranted further investigation.
`Sarkar & Schwaninger (1980) studied the
`in vivo
`corrosion characteristics of seven Nitinol wires in clinical
`use for 3 weeks to 5 months. Scanning electron micros-
`copy revealed the presence of numerous, round-bottomed,
`corrosion pits interspersed with corrosion products rich
`in titanium. This was presumed to be a mixed oxide of
`titanium and nickel. Fractured surfaces of Nitinol wires
`showed small equiaxed dimples that resulted from
`microvoid coalescence within the grain-boundary zones.
`There appears to be correlation with
`
`in vitro (Sarkar
`
`
`
`et al. 1979) and in vivo corrosion of Nitinol.
`The performance of wires used in orthodontics can be
`affected by corrosion in the mouth. Edie & Andreasen
`(1980) examined Nitinol wires under SEM as received
`from the manufacturer and following 1 month to
`1 year’s use in the mouth. They found no corrosion
`of the Nitinol wires with maintenance of a smooth,
`undulating surface. In contrast, stainless steel wires
`showed sharp elevations of metal particles on their surface.
`
`et al. (1981) used polarization resistance and
`Clinard
`zero resistance ammetry to study the corrosion beha-
`viour of stainless steel, cobalt–chromium, beta-titanium
`and Nitinol orthodontic springs. They also studied the
`effect of coupling the wires to stainless steel brackets.
`In orthodontic treatment, the corrosion behaviour of
`the wires was affected by coupling to the brackets.
`The effects of beta-titanium and cobalt–chromium were
`comparable showing less corrosion than the other
`wires, however, Nitinol was shown to be inferior to
`stainless steel, with a tendency to pitting attack. The
`authors concluded that over the relatively short period
`of orthodontic treatment, the effect of the corrosion did
`not appear to be deleterious to the mechanical pro-
`perties of the wires, and should not significantly effect
`the outcome of treatment.
`et al. (1981) com-
`
`To assess corrosion potential, Edie
`pared Nitinol wire with stainless steel wire in clinical
`use for periods ranging from 1 to 8 months. Scanning
`electron microscopy was used to assess surface charac-
`teristics; qualitative chemical information was obtained
`with X-ray spectrometry to indicate oxide prevalence
`and organic contamination of the wires. Unused Nitinol
`wires exhibited large variations in surface texture with
`an undulating ‘bubbling’ or mottled ‘caked’ appearance In
`comparison, stainless steel wires were generally smoother,
`but showed small metallic prominences. Obvious pits
`
`occurred on electrolytically corroded Nitinol wires, with
`loosely bound corrosion products; however, after clinical
`use, no differences in surface characteristics were obvious
`when comparing pre- and postoperative SEM photographs.
`There was no significant difference between the surface
`oxygen content of Nitinol compared to stainless steel, which
`would suggest that there were no differences in the clinical
`performance of the two wires, in terms of corrosion.
`
`Effects of sterilization on NiTi orthodontic wires
`
`Mayhew & Kusy (1988) examined the effects of dry
`heat, formaldehyde-alcohol vapour and steam autoclave
`sterilization on the mechanical properties and surface
`topography of two different nickel–titanium arch wires.
`The wires were being reused clinically, due to their high
`cost. After sterilization, the elastic modulus and tensile
`properties were determined for Nitinol and Titanal wires
`(Lancer Pacific, Carlsbad, CA, USA); laser scanning
`was employed to detect surface alterations caused by
`tarnish, corrosion or pitting.
`No detrimental effects were noted, and the nickel–
`titanium arch wires maintained their elastic properties,
`had excellent resilience and low load-d