Endodontic instrument of predetermined flexibility

An endodontic instrument for use in root canal therapy which comprises an inner core of a metal having a specific flexibility, and at least one outer shell of a metal having a different flexibility. By selecting the ratio of the two metals, the instrument may be designed to have a predetermined desired flexibility which is sufficient to assure proper operation of the instrument, yet not so stiff as to risk damage to the tooth canal. A kit which is composed of several such instruments of increasing diameter is also disclosed, and with the instruments being designed to be used sequentially in root canal therapy, and wherein all of the instruments possess substantially the same degree of flexibility. The instruments can thus be used with predictable and consistent results.

BACKGROUND OF THE INVENTION 
The present invention relates to the field of endodontic instruments 
adapted for use in performing root canal therapy on teeth, and which are 
characterized by high flexibility and high resistance to torsional 
breakage. 
Root canal therapy is a well-known procedure wherein the crown of a 
diseased tooth is opened so as to permit the canal to be cleaned and then 
filled. More particularly, after opening the crown, a series of very 
delicate, flexible, finger-held instruments or files are used to clean out 
and shape the root canal, and each file is manually rotated and 
reciprocated in the canal by the clinician. Files of increasingly larger 
diameter are used in sequence, to achieve the desired cleaning and 
shaping. When the canal is thus prepared, it is solidly filled with a 
filling material, which typically comprises a waxy, rubbery compound known 
as gutta percha. In one procedure, the gutta percha is positioned on an 
instrument called a compactor, and the coated compactor is inserted into 
the prepared canal and rotated and reciprocated to compact the gutta 
percha therein. The clinician then fills the tooth above the gutta percha 
with a protective cement, and lastly, fits a crown to the tooth. 
Endodontic files of the described type are commonly supplied to the 
clinician in kits which comprise several files of increasing diameter. In 
particular, and in accordance with ANSI/ADA Specification No. 28-1988, 
files are provided in diameters which range from 0.08 mm at the tip (size 
08) to 1.40 mm at the tip (size 140), and the files are provided in kits 
which contain a number of files of increasing diameter so that the files 
from a particular kit may be used in sequence by the clinician in 
accordance with the requirements of the particular canal being cleaned. 
As is well-known by clinicians, procedural errors occasionally occur during 
root canal therapy. These errors result in changes to the root canal 
morphology such as the perforation of the canal, and the formation of a 
ledge in the wall of the canal. Also, the instrument may fracture. These 
errors usually occur because the endodontic instruments, which are 
typically formed of stainless steel, lack the requisite flexibility 
particularly in the larger diameter instruments. Thus, upon being inserted 
into the root canal of a tooth, the stainless steel instrument is often 
unable to conform to the sometimes compound curvature or outline of the 
canal. In particular, the instrument tends to cut into the inside edges of 
the curved canal (referred to as transportation), which can lead to the 
perforation of the wall of the canal, and ledges may be formed at the 
pilot end of the instrument by reason of a lateral movement of the pilot 
end. 
Depending upon the severity of these errors, repair measures might be 
needed. For instance, if a perforation of the wall results, the 
perforation must be repaired to prevent bacteria from collecting and the 
possibility of saliva leaking into the cavity. 
Recently, endodontic files composed of a nickel-titanium alloy have been 
introduced, which provide a high degree of flexibility in both bending and 
torsion, and superior resistance to fracture, as compared to stainless 
steel instruments. In this regard, reference is made to the 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, at pages 346-351. 
While the new nickel-titanium instruments have been found to reduce 
transportation and the formation of ledges in the canal, in the smaller 
sizes, the highly flexible nature of the instruments reduces their filing 
or cleaning efficiency. Also, the tips of the instruments tend to deflect 
back upon themselves in severely curved canals. 
It is accordingly an object of the present invention to provide an 
endodontic instrument which has a predetermined desired degree of 
flexibility for a given size or diameter of the instrument. 
It is a further object of the present invention to provide a kit comprising 
a plurality of endodontic instruments wherein each instrument in the kit 
exhibits a predetermined desired degree of flexibility, and with the 
flexibilities of the instruments being chosen so as to provide an 
effective and predictable operational result for each of the instruments 
in the kit. 
It is a more particular object of the present invention to provide a kit of 
a plurality of endodontic instruments of progressively increasing 
diameter, and wherein the instruments of smaller diameter have sufficient 
stiffness for effective filing and cleaning of the canal, and the 
instruments of larger diameter have sufficient flexibility to minimize 
transportation and ledge formations. 
SUMMARY OF THE INVENTION 
The above and other objects and advantages of the present invention are 
achieved in the embodiment illustrated herein by the provision of an 
endodontic instrument which comprises an elongate shank having a proximate 
end and an opposite pilot end and so as to define a working length 
adjacent said pilot end. At least one continuous helical flute is formed 
in the shank so as to extend along the working length, and the shank 
comprises a core of a first material, and at least one outer shell 
coaxially surrounding the core and comprising a second material. The first 
and second materials in the shank have differing flexibilities such that 
the relative amounts of said first and second materials define a 
predetermined desired flexibility for the shank. 
Preferably, the first and second materials are each metallic, and in one 
embodiment the first metallic material is nickel-titanium alloy, and the 
second material is selected from the group consisting of stainless steel, 
titanium alpha alloy, titanium beta alloy, and titanium alpha beta alloy. 
In another embodiment, each of the first and second materials is selected 
from the group consisting of pure titanium, nickel-titanium alloy, and 
niobium-titanium alloy. 
Instruments of the above construction may be provided in a kit, with the 
instruments in the kit having progressively increasing diameters. Also, 
the shanks of the instruments are composed of the first and second 
metallic materials which are selected and configured so that the shanks of 
all of the instruments have a predetermined desired flexibility. In the 
preferred embodiment, the flexibility of all of the instruments in the kit 
is between about 80 and 375 gm. cm., and the flexibility is substantially 
the same for all of the instruments in the kit. Also, the flexibility may 
vary from kit to kit, so that the clinician can easily predict the 
performance of each instrument with respect to a given root canal 
configuration, and as a result, the performance of effective, skilled root 
canal therapy is enhanced. 
The kit of instruments of the present invention is provided, in the most 
preferred embodiment, with a plurality of instruments in conventional 
sizes as set forth in the above referenced ANSI/ADA specifications. That 
is, endodontic files are typically provided in kits with increasing 
working length diameters to precisely perform various therapies. The 
consistent and known flexibility of the individual instruments within the 
kit is maintained by proportioning the ratio of the more flexible 
component with respect to the more stiff component. Smaller instruments 
require a greater amount of the more stiff component in proportion to the 
more flexible component while larger instruments would require a greater 
proportion of the more flexible component.

DETAILED DESCRIPTION OF THE DRAWINGS 
Referring more particularly to the drawings wherein like numerals reference 
like parts, an endodontic instrument or file which embodies the features 
of the present invention is indicated generally at 10, and in FIG. 1 the 
file is illustrated in an operative position in a typical root canal in a 
tooth 11. The file 10 comprises a shank 12 which is composed of a 
composite metal as further described below, and which typically has a 
length of about 30 mm. The shank 12 also includes an outer or proximate 
end which mounts a conventional handle 13. The portion of the shank 
immediately below the handle is cylindrical and has a diameter of between 
about 0.5 and 1.6 mm, and this shank portion includes calibrated depth 
markings 15 of conventional design. The shank further includes an opposite 
distal or pilot end 18, shown best in FIG. 2, and a working length 19 
which is defined adjacent the pilot end 18. The working length may be 
cylindrical (not shown), or as illustrated it may be slightly tapered 
toward the pilot end 18 at an included angle of between about one half and 
four degrees. 
As best seen in FIG. 3, the shank 12 comprises a cylindrical core 20 of a 
first metallic material, and an outer shell 21 coaxially surrounding the 
core and which is composed of a second metallic material. The first 
material of the core 20 and the second material of the outer shell 21 have 
differing flexibilities, and such that relative amounts of the first and 
second materials define a predetermined desired flexibility for the shank. 
In one preferred embodiment, the first metallic material of the core 20 has 
a relatively high flexibility, and it is composed of nickel-titanium alloy 
which has a very low modulus of elasticity, only one-fourth to one-fifth 
the value for stainless steel, and a very wide range of elastic 
deformation. Most preferably "55-Nitinol" alloy is used for the core 
material which contains 54-56 weight percent nickel with the balance 
comprising titanium. This alloy possesses unique mechanical memory, is 
non-magnetic, is corrosion resistant and has a relatively low density of 
0.234 lb. per cu. in. 
The second metallic material of the outer shell 21 has a relatively low 
flexibility and may comprise for example stainless steel, titanium alpha 
alloy, titanium beta alloy, and titanium alpha beta alloy. The 
compositions of the three above noted titanium alloys are well known, and 
a further definition may be obtained from U.S. Pat. No. 4,197,643 and 
2,797,996, the disclosures of which are incorporated herein by reference. 
Also, the following represent a specific example of each of the alloys: 
(a) Titanium alpha alloy: Titanium alloy with 5% aluminum and 2.5% tin. 
(b) Titanium beta alloy: Titanium alloy with 13% vanadium, 11% chromium, 
and 3% aluminum. 
(c) Titanium alpha beta alloy: Titanium alloy with 6% aluminum, 2% tin, 4% 
zirconium, and 2% molybdenum. 
In accordance with the present invention, the proportion of the first and 
second materials may be varied to permit the file 10 to possess a 
predetermined desired flexibility. Thus for example, the percentage of the 
material of the core 20 is reduced for the small diameter files to impart 
more stiffness, whereby the percentage of the material of the core is 
increased for the larger diameter files to impart more flexibility. 
In another preferred embodiment of the invention, each of the first and 
second materials is selected from the group consisting of pure titanium, 
nickel-titanium alloy, and niobium-titanium alloy. The nickel-titanium 
alloy preferably comprises "55-Nitinol" alloy as described above, and the 
niobium-titanium alloy preferably comprises between about 40 to 50% 
niobium and the balance titanium. These materials may be selected and 
configured so as to provide a desired flexibility to the instrument. The 
following are specific examples of several suitable configurations: 
1) A core of pure titanium and an outer shell of nickel-titanium alloy. 
2) A core of niobium-titanium alloy and an outer shell of nickel-titanium 
alloy. 
3) A core of pure titanium, a first outer shell of nickel-titanium alloy, 
and a second outermost shell of niobium-titanium alloy. 
4) A core of niobium-titanium alloy, a first outer shell of nickel-titanium 
alloy, and a second outermost shell of niobium-titanium alloy. 
In the illustrated embodiments, the working length 19 of the instrument is 
tapered toward the pilot end at an included angle of between about 1/2 and 
4 degrees. Preferably, and in accordance with the above identified 
ANSI/ADA Specification, the taper is 0.02 mm difference in diameter per 
millimeter of the working length. Also, the working length 19 further 
comprises two continuous helical flutes 30 which extend along its length. 
The flutes are preferably machined in the outer surface of the outer shell 
material in the manner further described in U.S. Pat. No. 4,934,934. This 
machining operation may result in a cross section as seen in FIG. 4. More 
particularly, each of the two flutes 30 defines a curved concave wall 31 
when viewed in transverse cross section, and a helical land 32 is 
positioned between axially adjacent flute sections. Alternatively, a 
machining operation may be employed which produces a triangular or 
quadrangular cross section (not shown). 
The instruments of the present invention may be provided in kits, which 
facilitate their use by the clinician. In particular, the kit comprises a 
plurality of several instruments 10 as schematically illustrated in FIG. 
5, and which are adapted to be serially used in performing root canal 
therapy. The shanks of the instruments in the kit have progressively 
increasing diameters, as indicated by the size designations printed on the 
handles 13. Also, the shanks 12 having different proportions of the first 
and second materials so that all of the instruments have a predetermined 
desired flexibility. Preferably, the shanks are designed so that the 
flexibility of all of the shanks in the kit is substantially the same. 
The term "flexibility" as used herein refers to the "stiffness" as defined 
in the above referenced ANSI/ADS Specification. As there defined, the 
flexibility or stiffness is determined by holding the instrument in a 
torque meter, bending the shank at an angle of 45.degree., and then 
measuring the required torque. With the present invention, the flexibility 
of the shanks of all of the instruments in the kit is preferably between 
about 80 and 375 gm. cm. 
As noted above, the flexibility of all of the instruments in a kit may be 
substantially the same. However, the flexibility from kit to kit may vary, 
so that the clinician may select a kit with a particular flexibility to 
best meet the needs of the canal being processed. For example, in the case 
of a highly curved canal, a kit of instruments of relatively high 
flexibility would be selected, whereas in the case of a relatively 
straight canal, a more stiff kit would be selected. 
The instrument 10 is fabricated by a process wherein a bar of the material 
of the outer shell 21 is provided which has a diameter of several inches, 
e.g. 4 to 6 inches. The bar is through bored, and a rod of the core 20 
material is fitted within the bore in a close fitting relationship. Next, 
the composite member is rolled or drawn to the desired final diameter by 
the same process presently used to produce finished wire. Thereafter, the 
portion of the composite member which is to form the working length of the 
instrument may be tapered by a suitable grinding operation, and the flutes 
30 are then ground in the tapered surface. In this regard, it will be 
understood that the radial thickness of the outer shell 21 should be 
sufficient to receive the full depth of the flutes 30. Finally, the handle 
13 is assembled in a conventional manner. 
In the case of instruments having more than one outer shell, a bar of the 
material of the outermost shell is bored to coaxially receive a previously 
bored sleeve of the material of the inner shell, and a rod of the material 
of the core is fitted within the sleeve. The composite member is then 
rolled or drawn to the desired final diameter in the manner described 
above. 
In the drawings and specification, there has been set forth preferred 
embodiments of the invention, and although specific terms are employed, 
they are used in a generic and descriptive sense only and not for purposes 
of limitation.