llllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllll
`
`US 20020137008Al
`
`(19) United States
`(12) Patent Application Publication
`McSpadden et al.
`
`(10) Pub. No.: US 2002/0137008 Al
`Sep. 26, 2002
`( 43) Pub. Date:
`
`(54) ENDODONTIC INSTRUMENT
`
`Publication Classification
`
`(76)
`
`Inventors: Jobn T. McSpadden, Lookout
`Mountain, GA (US); Jonathan A.
`Barney, Newport Beach, CA (US)
`
`Correspondence Address:
`lAW OFFICES OF JONATHAN A. BARNEY,
`ESQ.
`312 SIGNAL ROAD
`SUITE 200
`NEWPORT BEACH, CA 92663 (US)
`
`(21) Appl. No.:
`
`10/023,425
`
`(22) Filed:
`
`Dec. 17, 2001
`
`Related U.S. Application Data
`
`(60) Provisional application No. 60/256,530, filed on Dec.
`18, 2000.
`
`Int. Cl.7
`....................................................... A61C 5/02
`(51)
`(52) U.S. CI. .............................................................. 433/102
`
`(57)
`
`ABSTRACT
`
`An endodontic file is provided that is fabricated from a
`superelastic alloy material such as an alloy of nickel, tita(cid:173)
`nium and niobium. The superelastic alloy material is
`selected to have a relatively high loading plateau greater
`than about 500 MPa. Such alloy material allows the forma(cid:173)
`tion of precision ground flutes and cutting edges with
`reduced incidence of burrs, rolled metal deposits and other
`imperfections. Thus, the cutting edges of an endodontic file
`constructed in accordance with the invention are sharper and
`cleaner than heretofore achieved and less susceptible to
`wear. The resulting file is also stiffer than comparable files
`fabricated from conventional NiTi alloys such that improved
`tactile feedback and manipulation control are provided.
`
`Icy:;
`
`I
`
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`
`S2.
`
`GOLD STANDARD EXHIBIT 2032
`US ENDODONTICS v. GOLD STANDARD
`CASE PGR2015-00019
`
`

`
`Patent Application Publication Sep. 26, 2002 Sheet 1 of 10
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`US 2002/0137008 Al
`
`Sep.26,2002
`
`1
`
`ENDODONTIC INSTRUMENT
`
`RELATED APPLICATIONS
`
`[0001] This application claims priority under 35 § 119(e)
`to provisional application Ser. No. 60/256,530 filed Dec. 18,
`2000.
`
`BACKGROUND OF THE INVENTION
`
`[0002] 1. Field of the Invention
`
`[0003] The present invention relates generally to the field
`of dentistry and more particularly to nickel-titanium endo(cid:173)
`dontic instruments used for performing root canal therapy.
`
`[0004] 2. Description of the Related Art
`
`[0005]
`In the field of endodontics, one of the most impor(cid:173)
`tant and delicate procedures is root canal therapy or cleaning
`and extirpating a root canal to provide a properly dimen(cid:173)
`sioned cavity to receive a biologically inert filling material
`such as gutta percha. This procedure is important in order to
`enable complete filling of the canal without any voids and in
`a manner which prevents the entrapment of noxious tissue in
`the canal as the canal is being filled.
`
`[0006]
`In a typical root canal procedure, the dentist
`removes decayed and/or inflamed tissue and debris from the
`canal using an endodontic file or rasp. In performing this
`procedure the dentist must gain access to the entire canal,
`shaping it as necessary while substantially maintaining the
`central axis of the canal. But root canals normally are very
`small in diameter, and can be quite curved and/or convo(cid:173)
`luted. It is therefore often very difficult to gain access to the
`full length and all affected surfaces of a root canal using a
`conventional endodontic file.
`
`[0007] Endodontic files have historically been made from
`stainless steel. These tools provide excellent manipulation
`control and sharp, long-lasting cutting surfaces. However,
`due to the inherent limited flexibility of steel, such tools can
`pose a significant danger of breakage, especially in sharply
`curved and/or heavily calcified root canals. To alleviate the
`breakage problem, endodontic files fabricated from nickel(cid:173)
`titanium alloy (Nitinol™ or NiTi) have been introduced.
`
`[0008] Nickel-titanium has several peculiar properties that
`make it very useful in endodontic applications. In particular,
`the alloy has the unusual ability to reversibly change its
`crystalline structure from a hard, high-modulus "austentitic"
`crystalline form to a soft, ductile "martensitic" crystalline
`form upon application of pressure and/or by cooling. This
`results in a highly elastic material having a very pronounced
`pseudo-elastic strain characteristic. This pseudo-elastic elas(cid:173)
`tic strain characteristic is often described as "superelastic(cid:173)
`ity."
`
`[0009] As a result of this reversible stress-induced crys(cid:173)
`talline transformation process a very tough and rubber-like
`elasticity is provided in such alloys. These material proper(cid:173)
`ties have proven very desirable for endodontic files in
`overcoming the aforementioned breakage problems. A series
`of comparative tests of endodontic instruments made of
`nickel-titanium and stainless steel were conducted and pub(cid:173)
`lished in an article entitled "An Initial Investigation of the
`Bending and the Torsional Properties of Nitinol Root Canal
`Files," Journal of Endodontics, Volume 14, No.7, July 1988,
`pages 346-351. The Nitinol instruments involved in these
`
`tests were manufactured in accordance with fabrication
`procedures and operating parameters conventionally used in
`the machining of stainless steel endodontic instruments.
`This process involved grinding one or more helical flutes in
`a tapered shaft to form helical cutting edges. The reported
`tests demonstrated that the NiTi instruments exhibited supe(cid:173)
`rior flexibility and torsional properties as compared to
`stainless steel instruments.
`
`[0010] Based on the initial success of these and other
`similar studies, NiTi endodontic instruments have been
`commercially introduced and have become widely accepted
`in the industry. As the use of such NiTi instruments has
`proliferated, however, certain drawbacks have become
`apparent.
`
`[0011] One particularly well-documented drawback is the
`expense and difficulty of machining endodontic files from
`NiTi alloy. Slow grinding with fine-grit grinding wheels is
`the presently accepted method for machining NiTi alloys,
`but even then, special procedures and parameters must
`typically be observed to obtain reliable results. See, for
`example, U.S. Pat. No. 5,464,362 to Heath et. al., which
`describes a method of grinding a rod of a nickel-titanium
`alloy to create a fluted endodontic file. The cost of purchas(cid:173)
`ing and operating the specialized grinding machines and
`grinding wheels required, and the relatively slow grinding
`process make the endodontic files produced by this method
`inordinately expensive when compared to their stainless
`steel counterparts.
`
`[0012] Another significant drawback is the tendency of the
`NiTi material to form latent burrs, rolled metal deposits
`and/or other imperfections along the desired cutting edges
`during the machining process. If these imperfections are not
`carefully monitored and controlled, they can have deleteri(cid:173)
`ous effects on file performance. Another significant draw(cid:173)
`back is that the cutting edges of presently available NiTi
`instruments are not as sharp as their stainless steel counter(cid:173)
`parts and tend to lose their sharpness more rapidly with use.
`Another significant drawback is reduced manipulation con(cid:173)
`trol due to reduced stiffness and extreme torsional flexibility
`of presently available NiTi endodontic files as compared
`with stainless steel files.
`
`[0013] These and other drawbacks have created a demand
`for improved NiTi alloys, machining methods and instru(cid:173)
`ments fabricated therefrom that overcome the aforenoted
`drawbacks while preserving the essential advantages of the
`nickel-titanium alloy.
`
`SUMMARY OF THE INVENTION
`
`[0014]
`It is therefore an object of the present invention to
`provide an improved class of superelastic alloy materials
`particularly adapted for use in endodontic files. It is another
`object of the invention to provide an endodontic instrument
`having improved manipulation control, sharp cutting edges
`and a reduced tendency to break during use. It is another
`object of the invention to reduce the number of instruments
`necessary to enlarge a root canal. Still another object of the
`invention is to provide an endodontic instrument which can
`be more quickly and economically produced.
`
`[0015]
`In one preferred embodiment, the invention pro(cid:173)
`vides an endodontic file fabricated from a superelastic alloy
`material comprising nickel-titanium. The superelastic alloy
`
`

`
`US 2002/0137008 Al
`
`Sep.26,2002
`
`2
`
`material is selected to have a relatively high loading plateau
`greater than about 500 MPa. Such alloy material allows the
`formation of precision ground flutes and cutting edges with
`a reduced incidence of burrs, rolled metal deposits and other
`imperfections. Thus, the cutting edges of an endodontic file
`constructed in accordance with the invention are sharper and
`cleaner than heretofore achieved and less susceptible to
`wear. The resulting file is also stiffer and more responsive
`than comparable files fabricated from conventional NiTi
`alloys such that improved tactile feedback and manipulation
`control are provided.
`
`In another preferred embodiment, the invention
`[0016]
`provides an endodontic file fabricated from a superelastic
`alloy material, such as nickel-titanium, that has significant
`latent stress and crystalline deformation induced by cold
`working. Such alloy material is generally harder and stiffer
`than conventional NiTi alloys used for endodontic files.
`Therefore, such improved alloy allows the formation of
`precision ground flutes and cutting edges with a reduced
`incidence of burrs, rolled metal deposits and other imper(cid:173)
`fections. The cutting edges of an endodontic file constructed
`in accordance with the invention are sharper and cleaner
`than heretofore achieved and less susceptible to wear. The
`resulting file is also stiffer and more responsive than com(cid:173)
`parable files fabricated from conventional NiTi alloys such
`that improved tactile feedback and manipulation control are
`provided.
`
`In another preferred embodiment, the invention
`[0017]
`provides an endodontic file fabricated from a superelastic
`alloy material comprising nickel, titanium and at least 5%
`niobium by weight. Such alloy material is generally harder
`and stiffer than conventional NiTi alloys used for endodontic
`files. Therefore, such improved alloy allows the formation of
`precision ground flutes and cutting edges with a reduced
`incidence of burrs, rolled metal deposits and other imper(cid:173)
`fections. Thus, the cutting edges of an endodontic file
`constructed in accordance with the invention are sharper and
`cleaner than heretofore achieved and less susceptible to
`wear. The resulting file is also stiffer than comparable files
`fabricated from conventional NiTi alloys such that improved
`tactile feedback and manipulation control are provided.
`
`[0018] For purposes of summarizing the invention and the
`advantages achieved over the prior art, certain objects and
`advantages of the invention have been described herein
`above. Of course, it is to be understood that not necessarily
`all such objects or advantages may be achieved in accor(cid:173)
`dance with any particular embodiment of the invention.
`Thus, for example, those skilled in the art will recognize that
`the invention may be embodied or carried out in a manner
`that achieves or optimizes one advantage or group of advan(cid:173)
`tages as taught herein without necessarily achieving other
`objects or advantages as may be taught or suggested herein.
`
`[0019] All of these embodiments are intended to be within
`the scope of the invention herein disclosed. These and other
`embodiments of the present invention will become readily
`apparent to those skilled in the art from the following
`detailed description of the preferred embodiments having
`reference to the attached figures, the invention not being
`limited to any particular preferred embodiment(s) disclosed.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`[0020] Having thus summarized the general nature of the
`invention and its essential features and advantages, certain
`
`preferred embodiments and modifications thereof will
`become apparent to those skilled in the art from the detailed
`description herein having reference to the figures that follow,
`of which:
`
`[0021] FIG. 1 is a section view of a tooth and associated
`root structure illustrating the use of an endodontic file for
`performing a typical root canal procedure;
`
`[0022] FIGS. 2A-H are various views illustrating a typical
`prior art fluted endodontic file fabricated from a nickel
`titanium alloy;
`
`[0023] FIG. 3A is a comparative stress-strain diagram of
`various nickel-titanium alloys versus stainless steel;
`
`[0024] FIG. 3B is a comparative energy-absorption or
`toughness diagram of various nickel-titanium alloys versus
`stainless steel;
`
`[0025] FIGS. 4A-H are various views of one preferred
`embodiment of a fluted endodontic file having features and
`advantages in accordance with the present invention;
`
`[0026] FIGS. 5A-H are various views of a first alternative
`preferred embodiment of a fluted endodontic file having
`features and advantages in accordance with the present
`invention; and
`
`[0027] FIGS. 6A-H are various views of a second alter(cid:173)
`native preferred embodiment of a fluted endodontic file
`having features and advantages in accordance with the
`present invention.
`
`DESCRIPTION OF THE PREFERRED
`EMBODIMENTS
`
`[0028] FIG. 1 is a partial cross section of a tooth 50 and
`supporting root structure illustrating the use of an endodon(cid:173)
`tic file 80 to carry out a standard root canal procedure. The
`root canal 56 of a tooth houses the circulatory and neural
`systems of the tooth. These enter the tooth at the terminus 52
`of each of its roots 54 and extend through a narrow, tapered
`canal system to a pulp chamber 58 adjacent the crown
`portion 60 of the tooth. If this pulp tissue becomes diseased
`or injured, it can cause severe pain and trauma to the tooth,
`sometimes necessitating extraction of the tooth. Root canal
`therapy involves removing the diseased tissue from the canal
`56 and sealing the canal system in its entirety. If successful,
`root canal therapy can effectively alleviate the pain and
`trauma associated with the tooth so that it need not be
`extracted.
`
`[0029] To perform a root canal procedure, the endodontist
`first drills into the tooth 50 to locate the root canal(s) 56 and
`then uses an endodontic file or reamer instrument 80 to
`remove the decayed, injured or dead tissue from the canal.
`These instruments are typically elongated cutting or abrad(cid:173)
`ing instruments which are rotated and/or reciprocated within
`the root canal either by hand or using a slow speed drill. The
`primary goal is to remove all of the decayed or injured nerve
`while leaving the integrity of the root canal walls relatively
`unaffected. Preserving the integrity of the root canal 56 is
`important in order to allow proper filling of the root canal
`void in a homogenous three dimensional manner such that
`leakage or communication between the root canal system
`and the surrounding and supporting tissues of the tooth 50 is
`prevented. Once as much of the diseased material as prac(cid:173)
`ticable is removed from the root canal, the canal 56 is sealed
`
`

`
`US 2002/0137008 Al
`
`Sep.26,2002
`
`3
`
`closed, typically by reciprocating and/or rotating a con(cid:173)
`denser instrument in the canal to urge a sealing material such
`as gutta -percha into the canal.
`[0030] One of the primary challenges in performing root
`canal therapy is that the root canals are not necessarily
`straight and are often curved or convoluted. Therefore, it is
`often difficult to clean the canal while preserving its natural
`shape. Many instruments (particularly the older, stainless
`steel instruments) have a tendency to straighten out the canal
`or to proceed straight into the root canal wall, altering the
`natural shape of the canal. In some extreme cases, the
`instrument may transport completely through the canal wall
`causing additional trauma to the tooth and/or surrounding
`tissues. Also, the openings of many root canals are small,
`particularly in older patients, due to calcified deposits on the
`root canal inner walls. Thus the files or reamers must be able
`to withstand the torsional load necessary to penetrate and
`enlarge the canal opening without breaking the instrument,
`as may also occasionally occur with the older stainless steel
`endodontic files.
`[0031] To alleviate the transportation and breakage prob(cid:173)
`lems, highly flexible endodontic files fabricated from nickel(cid:173)
`titanium alloy (Nitinol™ or NiTi) were introduced and have
`become the preferred choice for many experienced endo(cid:173)
`dontists. However, as the acceptance and use of such NiTi
`instruments have proliferated certain drawbacks have
`become apparent, as will be addressed herein.
`[0032] FIGS. 2A-H are various views of a typical fluted
`endodontic file fabricated from a NiTi alloy. See, e.g. U.S.
`Pat. No. 5,882,198, incorporated herein by reference. The
`file 100 generally comprises a shaft 102 having a shank
`portion 104 and an elongated working portion 106. The
`working portion 106 extends from a proximal end 107
`adjacent the base of the shank 104 to a distal end 108
`terminating in a chisel tip 150. The shank portion 104 may
`typically include a fitting portion 109 for mating with the
`chuck of a dental handpiece (not shown). The fitting portion
`109 includes a generally !-shaped fiat side 182 which defines
`a step 184 and a generally semicircular disk 186 above and
`adjacent to a generally semi-circular groove 188. Such
`fitting 109 is typical of those employed in the dental industry
`for connecting or interfacing a dental tool with dental drill
`or handpiece.
`[0033] Alternatively and/or in addition to the fitting por(cid:173)
`tion 109, the shank portion 104 may include a knurled or
`otherwise treated surface (not shown) or handle to facilitate
`hand manipulation of the file 100. Thus, the instrument 100
`may either be used by manipulating the instrument manually
`in a rotating or reciprocating action, or the instrument may
`be manipulated by attaching the fitting portion 109 of the
`instrument to a motorized handpiece for effecting more rapid
`removal of tissue from the root canal, as desired.
`[0034] Helical flutes 124 and 126 are formed in the
`working portion 106 extending from the distal end 108
`adjacent the tip 150 and exiting at the proximal end 107
`(sometimes called the "flute exit" or "exit"), as shown in
`FIG. 2A. These flutes are typically formed by specialized
`slow-speed grinding operations using a 3-axis or 6-axis
`grinding machine in accordance with well-documented
`manufacturing techniques. Any number of such flutes may
`be formed in this manner, as desired.
`[0035] Helical lands 116 and 118 are typically provided
`generally extending between adjacent flutes 124 and 126.
`
`The helical flutes 124, 126 and helical lands 116, 118
`intersect one another to define leading edges 128, 132 and
`trailing edges 130, 134 with respect to clockwise rotation of
`the instrument (see, e.g. FIG. 2G). The leading edges 124,
`126 are typically sharpened to provide a cutting edge for
`removing tissue from the root canal as the instrument is
`rotated and/or reciprocated. The trailing edges 130, 134 may
`be sharpened or not, depending upon the particular file
`geometries desired and manufacturing conveniences. Rake
`angles of the cutting edges 128, 132 may be positive,
`negative, or neutral, depending upon manufacturing conve(cid:173)
`nience and/or clinical purpose. Typical rake angles range
`from about +20 degrees to about -35 degrees measured with
`respect to a radial line passing through the cutting edge
`perpendicular to a line tangent to the periphery of the
`working portion.
`
`[0036] As shown in FIGS. 2D and 2G the helical lands
`117, 118 are typically formed so as to define outer peripheral
`land portions 116, 120 having width w 1 (sometimes called
`the "margin width") and optional recessed land portions 112,
`114 having width w 2 (sometimes called the "relief width").
`The combined width w 1 +w 2 is sometimes called the "land
`width." The recessed land portions 112, 114 are at a first
`predetermined radial distance R1 from the cross-sectional
`center of the working portion 106. The outer land portions
`116, 120 lie at the outer periphery of the working portion 106
`at a second predetermined radial distance R2 from the center
`of the working portion 106, typically about 4 to 30 percent
`greater than the radial distance R1 .
`
`[0037] The working portion 106 of the instrument 100
`typically has a length ranging from about 3 mm to about 18
`mm. A commonly preferred length is about 16 mm. The
`working portion 106 may have a constant cross-sectional
`diameter or, more typically, it is tapered from the proximal
`end 107 to the distal end 108, as shown. In the particular
`embodiment shown, the taper is substantially uniform-that
`is, the rate of taper is constant along the working portion
`106. A typical taper rate may range from about 0.01 mm/mm
`to about 0.08 mm/mm. The web thickness "tw"-that is the
`thickness of the "web" of material between opposed flutes
`124, 126-is also typically tapered from the proximal end
`107 of the working portion to the distal end 108. The web
`taper rate is typically between about -0.01 mm/mm to about
`0.08 mm/mm.
`
`[0038] The tip 150 of the instrument 100 may assume any
`number of a variety of possible configurations, depending
`upon the preference of the endodontist and manufacturing
`conveniences. In the illustrated embodiment, the tip 150 is
`formed as a chisel edge or chisel tip, as shown in more detail
`in FIGS. 2E and 2F. The chisel tip 150 generally comprises
`two or more facets 151, 153 which intersect to define a chisel
`edge 154. The chisel edge 154 is typically substantially
`linear and substantially orthogonal to a longitudinal axis of
`the working portion 106, although such configuration is not
`necessary. Additional sharpened cutting edges 155, 157 are
`formed at the tip 150 by the intersection of the facets 151,
`153 with the flutes 124, 126. Upon rotation of the instrument
`in a root canal the chisel edge 154 loosens diseased or
`decayed tissue while the cutting edges 155, 157 cut away
`and remove the tissue as the file is operated in the canal.
`
`[0039] The chisel tip 150 is typically formed by grinding
`fiats or facets 151, 153 into the tip of the instrument 100, as
`
`

`
`US 2002/0137008 Al
`
`Sep.26,2002
`
`4
`
`shown, forming the chisel edge 154. The facets 151, 153
`define an included point angle ~ typically between about
`45-100 degrees, as shown in FIG. 2E. The chisel edge 154
`is typically canted from center by a primary angle y of
`between about 5-25 degrees, as shown in FIG. 2F. As
`illustrated, The facets 151, 153 of the chisel tip 150 formed
`apices with the cutting edges 128, 132 and additional cutting
`edges 155, 157.
`
`[0040] The endodontic instrument shown and described in
`connection with FIG. 2 above is made from a superelastic
`alloy, such as SE508 nickel-titanium wire manufactured by
`Nitinol Devices and Components, Inc. of Fremont, Calif.
`This is a typical binary nickel-titanium alloy used for
`endodontic files and comprises about 56% nickel and about
`44% titanium by weight. Table 1, below, summarizes certain
`selected material properties of the SE508 NiTi alloy:
`
`TABLE 1
`
`SE508 MATERIAL PROPERTIES
`
`PHYSICAL PROPERTIES
`
`Melting Point
`Density
`Electrical Resistivity
`Modulus of Elasticity
`Coefficient of Thermal Expansion
`MECHANICAL PROPERTIES
`
`Ultimate Tensile Strength (UTS)
`Total Elongation
`SUPERELASTIC PROPERTIES
`
`Loading Plateau Stress @ 3% strain
`Superelastic Strain (max)
`Permanent Set (after 6% strain)
`Transformation Temperature (Af)
`COMPOSITION
`
`Nickel (nominal)
`Titanium (nominal)
`Oxygen (max)
`Carbon (max)
`
`1310° c.
`6.5 g/cm3
`82 .uohm-cm
`75 x 10.6 MPa
`11 X 10-6;c C.
`
`1150 MPa
`10%
`
`450 MPa
`8%
`0.2%
`5-18° c.
`
`55.8 wt.%
`44.2 wt.%
`0.05 wt. % (max)
`0.02 wt. % (max)
`
`[0041] Fluted endodontic instruments fabricated from
`NiTi SE508 and similar NiTi alloys have been commercially
`introduced and have become widely accepted in the industry.
`As the use of such NiTi instruments has proliferated, how(cid:173)
`ever, certain drawbacks have become apparent.
`
`[0042] As noted above, one particularly well-documented
`drawback is the expense and difficulty of machining endo(cid:173)
`dontic files from such NiTi alloys. SE508 NiTi alloy, for
`example, is a very difficult material to process using con(cid:173)
`ventional machining operations. Therefore, slow and expen(cid:173)
`sive grinding operations must typically be used to create the
`desired fluting and cutting edges.
`
`[0043] Another significant drawback is the tendency of the
`SE508 NiTi alloy and similar alloys to form latent burrs,
`rolled metal deposits and/or other imperfections along the
`desired cutting edges during the machining process. This is
`illustrated in more detail in FIG. 2H. Notably, it may be seen
`that a burr or rolled metal deposit 160 (not necessarily drawn
`to scale) extends outward from the cutting edge 132. If such
`burrs or other similarly occurring imperfections are not
`carefully monitored and controlled, they can have deleteri(cid:173)
`ous effects on file performance. Another significant draw-
`
`back is that the cutting edges of presently available NiTi
`instruments are typically not as sharp as their stainless steel
`counterparts and tend to lose their sharpness more rapidly
`with use. Another significant drawback is reduced manipu(cid:173)
`lation control due to reduced stiffness and extreme torsional
`flexibility of presently available NiTi endodontic files as
`compared with stainless steel files.
`[0044] The present invention provides an improved class
`of superelastic alloys and manufacturing techniques particu(cid:173)
`larly suited and adapted for forming endodontic files and
`which overcome the afore noted drawbacks while preserving
`and enhancing the essential advantages of the superelastic
`alloy in minimizing file breakage and canal wall transpor(cid:173)
`tation. In particular, it has been discovered that by increasing
`the loading plateau of a superelastic alloy, its machinability,
`cutting-edge sharpness and sharpness holding-ability, and
`manipulation control are improved resulting in increased
`clinical efficacy and manufacturing economy.
`[0045] The concepts and teachings of the present inven(cid:173)
`tion are particularly applicable to nickel-titanium alloys and
`endodontic instruments (files, reamers, obturators, drill bits
`and the like) fabricated therefrom. However, the invention
`disclosed herein is not limited specifically to endodontic
`instruments fabricated from NiTi alloys, but may be prac(cid:173)
`ticed with a variety of dental instruments using any one of
`a number of other suitable alloy materials having the desired
`superelastic properties, such as Silver-Cadmium (Ag-Cd),
`Gold-Cadmium (Au-Cd) and Iron-Platinum (Fe3Pt), to
`name but a few.
`[0046] FIGS. 3A and 3B are comparative graphs illus(cid:173)
`trating stress-strain curves and energy absorption/toughness
`for selected NiTi alloys as compared to traditional stainless
`steel material. FIG. 3A shows a typical stress-strain curve
`163 for stainless steel as compared to similar curves 165,
`170 for selected NiTi alloys. As can be discerned from the
`graph, each of the NiTi alloys has a significantly larger strain
`to failure than stainless steel. This is largely because the NiTi
`alloys exhibit a superelastic property that enable them to
`undergo significant strain at a substantially constant stress or
`load level, sometimes called the "loading plateau"168, 175.
`[0047] Curve 165 indicates a stress-strain curve and load(cid:173)
`ing plateau 168 for a typical NiTi alloy, such as SE508, used
`to fabricate endodontic files. Curve 170 indicates a stress(cid:173)
`strain curve and loading plateau 175 for an improved class
`of superelastic alloys having features and advantages in
`accordance with the present invention. In particular, those
`skilled in the art will recognize that the curve 170 indicates
`a higher loading plateau 175 than the loading plateau 168 of
`curve 165.
`[0048] The increased loading plateau 175 is desirable for
`several reasons. For a given file design and diameter, a
`higher loading plateau increases the apparent stiffness of the
`file (in both bending and torsion) and therefore its respon(cid:173)
`siveness and ease of manipulation by endodontists. Files
`formed from conventional NiTi alloys can often feel overly
`flexible and non-responsive and, thus, exhibit reduced tactile
`feedback and difficult manipulation control-particularly in
`the smaller diameter files. Endodontic files fabricated from
`improved superelastic alloys having increased stiffness in
`accordance with one preferred embodiment of the invention
`provide improved responsiveness and manipulation control
`without significantly adversely increasing the risk of file
`breakage or canal wall transportation.
`
`

`
`US 2002/0137008 Al
`
`Sep.26,2002
`
`5
`
`[0049] A higher loading plateau can, in some alloys, also
`increase the toughness or amount of energy required to
`permanently deform or break the file. FIG. 3B shows a
`typical toughness or energy absorption characteristic for
`stainless steel (area under curve 163) as compared to the
`toughness of a typical NiTi alloy used for endodontic files
`(area under curve 165). As can be discerned from the graph,
`NiTi is a tougher material than stainless steel as it absorbs
`more energy before ultimately failing. Curve 170 indicates
`a stress-strain curve for an improved class of NiTi alloys
`having features and advantages in accordance with the
`present invention. Those skilled in the art will recognize that
`the area under curve 170 is larger than the area under curve
`165. Therefore, it is anticipated that endodontic files fabri(cid:173)
`cated from such alloy may exhibit improved toughness and
`resistance to breakage. Of course, it is not required that this
`relationship hold true in all cases, as it is contemplated that
`suitable superelastic alloys having some or all of the advan(cid:173)
`tages disclosed herein may still be clinically advantageous
`for use in endodontic files, albeit possibly not quite as tough
`as present NiTi alloys.
`[0050] There are several ways in which an improved
`superelastic alloy may be realized. One way is by cold
`working a superelastic material, such as conventional NiTi
`alloy, by drawing or rolling it. Conventional NiTi alloys for
`endodontic file use, such as SE508 NiTi, are drawn down in
`wire form from larger diameter wire to smaller and smaller
`diameter wire as dictated by the particular instrument size
`desired to be achieved. Typically this is done without the
`application of heat and, thus, the material undergoes "cold(cid:173)
`working" or permanent deformation of the material crystal(cid:173)
`line str