U.S. patent application number 10/654260 was filed with the patent office on 2004-06-24 for precision cast dental instrument.
Invention is credited to Mason, Robert M..
Application Number | 20040121283 10/654260 |
Document ID | / |
Family ID | 31978636 |
Filed Date | 2004-06-24 |
United States Patent
Application |
20040121283 |
Kind Code |
A1 |
Mason, Robert M. |
June 24, 2004 |
Precision cast dental instrument
Abstract
An endodontic file (200) is provided particularly adapted for
the removal of tooth structure, decayed or damaged nerve tissues or
dentine material on the interior walls of a root canal or dentine
and/or enamel from the external tooth wall. The endodontic
instrument includes a shaft (202) having a shank portion (204) and
a generally elongated working portion (206). The working portion
preferably includes cutting or abrading features (232) adapted upon
rotation and/or reciprocation of the instrument to cut, abrade or
remove tissue from the interior walls of a root canal or dentine
and/or enamel from the external tooth wall. The working portion
extends from a proximal end (207) adjacent the shank portion to a
distal end (208) terminating at a tip (250). The entire instrument
and/or at least the working portion thereof is formed of an
amorphous or essentially amorphous material having no or
essentially no detectable crystalline structure and/or from a
nanocrystalline material having an average crystalline grain size
less than about 1 .mu.m. The instrument may be formed by
conventional grinding operations or by direct casting, forging or
molding, in a manner producing an integral as-molded instrument
having one or more sharp cutting edges. The instrument is
inexpensive to manufacture and exhibits improved cutting-edge
sharpness, wear resistance, lubriciousness and resistance to
breakage.
Inventors: |
Mason, Robert M.;
(Tallahassee, FL) |
Correspondence
Address: |
LAW OFFICES OF JONATHAN A. BARNEY, ESQ.
312 SIGNAL ROAD
SUITE 200
NEWPORT BEACH
CA
92663
US
|
Family ID: |
31978636 |
Appl. No.: |
10/654260 |
Filed: |
September 3, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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60408584 |
Sep 6, 2002 |
|
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Current U.S.
Class: |
433/102 |
Current CPC
Class: |
Y10T 29/49567 20150115;
A61C 5/42 20170201; A61C 3/02 20130101; B23B 51/0081 20130101; B23B
2251/406 20130101 |
Class at
Publication: |
433/102 |
International
Class: |
A61C 005/02 |
Claims
What is claimed is:
1. A dental instrument adapted for the removal of tooth structure,
decayed or damaged nerve tissues or dentine material on the
interior walls of a root canal or dentine and/or enamel from the
external tooth wall, said dental instrument comprising a shaft
having a shank portion and a working portion having one or more
cutting or abrading features thereon adapted for the removal of
said tooth structure, decayed or damaged nerve tissues or dentine
material on the interior walls of a root canal or dentine and/or
enamel from the external tooth wall upon rotation and/or
reciprocation of the instrument, said working portion extending
from a proximal end adjacent the shank portion to a distal end
terminating at a tip, the working portion or at least the cutting
or abrading features thereof being formed of an amorphous material
having no or essentially no detectable crystalline structure and/or
a nanocrystalline material having an average crystalline grain size
less than about 1 .mu.m.
2. The dental instrument of claim 1 where said cutting or abrading
features comprise one or more of the following: helical cutting
edges, non-helical cutting edges, burr-like barbs or projections,
disjointed flutes and cutting edges, cutting blades, and/or
micro-projecting cutting features.
3. The dental instrument of claim 1 wherein said tip comprises a
chisel tip.
4. The dental instrument of claim 1 wherein said shank portion is
secured to a formed fitting or handle portion for mating with the
chuck of a dental handpiece.
5. The dental instrument of claim 4 wherein said fitting or handle
portion includes a generally I-shaped flat side having a step and a
generally semicircular disk above and adjacent to a generally
semi-circular groove for mating with the chuck of a dental
handpiece.
6. The dental instrument of claim 1 wherein substantially the
entire working portion is formed of an amorphous material having no
or essentially no detectable crystalline structure and/or a
nanocrystalline material having an average crystalline grain size
less than about 1 .mu.m.
7. The dental instrument of claim 1 wherein substantially the
entire dental instrument is formed of an amorphous material having
no or essentially no detectable crystalline structure and/or a
nanocrystalline material having an average crystalline grain size
less than about 1 .mu.m.
8. The dental instrument of claim 7 wherein substantially the
entire dental instrument is formed by die casting, forging or
metal-injection molding in a manner producing an integral as-molded
instrument having one or more sharp cutting edges.
9. The dental instrument of claim 1 wherein substantially the
entire working portion is formed by die casting, forging or
metal-injection molding in a manner producing an integral as-molded
instrument having one or more sharp cutting edges.
10. The dental instrument of claim 1 wherein said amorphous or
nanocrystalline material comprises a metal alloy comprising one or
more of the following elements: zirconium, beryllium, titanium,
copper, or nickel.
11. The dental instrument of claim 10 wherein said metal alloy
comprises one or more of the following alloys: Zr--Ti, Ni--Ti,
Cu--Ti--Zr, Ln-Al-TM, Mg-Ln-TM, Zr--Al-TM, Hf--Al-TM, Ti--Zr-TM,
Zr--Al--Co--Ni--Cu, Zr--Ti--Al--Ni--Cu, Zr--Ti--Nb--Al--Ni--Cu,
and/or Zr--Ti--Hf--Al--Co--Ni--Cu, where Ln is a lanthanide metal,
and TM is a transition metal of the Groups VI to VIII.
12. The dental instrument of claim 10 wherein said amorphous or
nanocrystalline material is formed by rapidly cooling said alloy
from its molten state to a solid state such a manner that an
ordered crystalline structure is not formed.
13. The dental instrument of claim 10 wherein said amorphous or
nanocrystalline material is formed by cooling said alloy from its
molten state to a solid state in such a manner that the average
crystalline grain size of the solidified material is less than
about 1 .mu.m.
14. The dental instrument of claim 1 wherein said working portion
is formed by pouring or injecting a molten metal alloy into a die
or mold and then rapidly cooling or quenching the molten alloy at a
rate greater than about 10.degree. C. per second to form a desired
instrument shape as imparted by the die or mold.
15. The dental instrument of claim 1 wherein the one or more
cutting features comprise a pair of helical cutting edges defined
by the intersection of a pair of helical flutes and a first
corresponding pair of helical lands located at a peripheral surface
defined by a first predetermined radial distance R1 from the
cross-sectional center of the working portion, said working portion
further comprising a second pair of land portions located at a
peripheral surface defined by a second predetermined radial
distance R2 from the cross-sectional center of the working portion
and wherein R1 is about 4 to 30 percent greater than R2.
16. An endodontic instrument adapted for the removal of tooth
structure, decayed or damaged nerve tissues or dentine material on
the interior walls of a root canal or dentine and/or enamel from
the external tooth wall, said instrument comprising a shaft having
a shank portion and a working portion adapted for the removal of
said tooth structure, decayed or damaged nerve tissues or dentine
material on the interior walls of a root canal or dentine and/or
enamel from the external tooth wall upon rotation and/or
reciprocation of the instrument, said working portion being formed
of a glassy metal alloy having no or essentially no detectable
crystalline structure and/or having an average crystalline grain
size less than about 1 .mu.m.
17. The endodontic instrument of claim 16 where said working
portion further comprises cutting or abrading features comprising
one or more of the following: helical cutting edges, non-helical
cutting edges, burr-like barbs or projections, disjointed flutes
and cutting edges, cutting blades, and/or micro-projecting cutting
features.
18. The endodontic instrument of claim 16 wherein said shank
portion is secured to a formed fitting or handle portion for mating
with the chuck of a dental handpiece.
19. The endodontic instrument of claim 18 wherein said fitting or
handle portion includes a generally I-shaped flat side having a
step and a generally semicircular disk above and adjacent to a
generally semi-circular groove for mating with the chuck of a
dental handpiece.
20. The endodontic instrument of claim 16 wherein substantially the
entire instrument is formed of said glassy metal alloy having no or
essentially no detectable crystalline structure and/or a
nanocrystalline material having an average crystalline grain size
less than about 1 .mu.m.
21. The endodontic instrument of claim 20 wherein substantially the
entire dental instrument is formed by die casting, forging or
metal-injection molding in a manner producing an integral as-molded
instrument having one or more sharp cutting edges.
22. The endodontic instrument of claim 16 wherein substantially the
entire working portion is formed by die casting, forging or
metal-injection molding in a manner producing an integral as-molded
instrument having one or more sharp cutting edges.
23. The endodontic instrument of claim 16 wherein said glassy metal
alloy comprises one or more of the following elements: zirconium,
beryllium, titanium, copper, or nickel.
24. The endodontic instrument of claim 16 wherein said glassy metal
alloy comprises one of the following base alloy systems: Zr--Ti,
Ni--Ti, Cu--Ti--Zr, Ln-Al-TM, Mg-Ln-TM, Zr--Al-TM, Hf--Al-TM,
Ti--Zr-TM, Zr--Al--Co--Ni--Cu, Zr--Ti--Al--Ni--Cu,
Zr--Ti--Nb--Al--Ni--Cu, and/or Zr--Ti--Hf--Al--Co--Ni--Cu, where Ln
is a lanthanide metal, and TM is a transition metal of the Groups
VI to VIII.
25. The endodontic instrument of claim 24 wherein said glassy metal
alloy is formed by rapidly cooling said alloy from its molten state
to a solid state in such a manner that an ordered crystalline
structure is not formed.
26. The endodontic instrument of claim 16 wherein said glassy metal
alloy comprises a bulk vitreous alloy formed by cooling said alloy
from its molten state to a solid state in such a manner that the
average crystalline grain size of the solidified material is less
than about 1 .mu.m.
27. The endodontic instrument of claim 16 wherein said working
portion is formed by pouring or injecting said glassy metal alloy
in its molten state into a die or mold and then rapidly cooling or
quenching the molten alloy at a rate greater than about 10.degree.
C. per second to form a desired instrument shape as imparted by the
die or mold.
28. A dental instrument adapted for the shaping and/or removal of
tooth structure, said instrument comprising a shaft having a shank
portion and a working portion, said working portion having one or
more sharp cutting features thereon adapted for the removal of said
tooth structure upon rotation and/or reciprocation of the
instrument, said dental instrument or at least the working portion
thereof being formed by die casting, forging or metal-injection
molding in a manner producing an integral as-molded instrument
having said one or more sharp cutting features.
29. The instrument of claim 28 where said sharp cutting or abrading
features comprise one or more of the following: helical cutting
edges, non-helical cutting edges, burr-like barbs or projections,
disjointed flutes and cutting edges, cutting blades, and/or
micro-projecting cutting features.
30. The instrument of claim 28 wherein said shank portion is
integrally formed with a fitting or handle portion for mating with
the chuck of a dental handpiece.
31. The instrument of claim 30 wherein said fitting or handle
portion includes a generally I-shaped flat side having a step and a
generally semicircular disk above and adjacent to a generally
semi-circular groove for mating with the chuck of a dental
handpiece.
32. The instrument of claim 28 wherein substantially the entire
instrument is formed of a glassy metal alloy having no or
essentially no detectable crystalline structure and/or a
nanocrystalline material having an average crystalline grain size
less than about 1 .mu.m.
33. The instrument of claim 32 wherein said glassy metal alloy
comprises one or more of the following elements: zirconium,
beryllium, titanium, copper, or nickel.
34. The instrument of claim 32 wherein said glassy metal alloy
comprises one of the following base alloy systems: Zr--Ti, Ni--Ti,
Cu--Ti--Zr, Ln-Al-TM, Mg-Ln-TM, Zr--Al-TM, Hf--Al-TM, Ti--Zr-TM,
Zr--Al--Co--Ni--Cu, Zr--Ti--Al--Ni--Cu, Zr--Ti--Nb--Al--Ni--Cu,
and/or Zr--Ti--Hf--Al--Co--Ni--Cu, where Ln is a lanthanide metal,
and TM is a transition metal of the Groups VI to VIII.
35. The instrument of claim 32 wherein said glassy metal alloy is
formed by rapidly cooling said alloy from its molten state to a
solid state in such a manner that an ordered crystalline structure
is not formed.
36. The instrument of claim 32 wherein said glassy metal alloy
comprises a bulk vitreous alloy formed by cooling said alloy from
its molten state to a solid state in such a manner that the average
crystalline grain size of the solidified material is less than
about 1 .mu.m.
37. The instrument of claim 28 wherein said working portion is
formed by pouring or injecting a metal alloy in its molten state
into a die or mold and then rapidly cooling or quenching the molten
alloy at a rate greater than about 10.degree. C. per second to form
the desired instrument shape.
Description
RELATED APPLICATIONS
[0001] This application claims priority under 35 U.S.C. .sctn.
119(e) to U.S. provisional application Ser. No. 60/408,584 filed
Sep. 6, 2002.
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 rotating or oscillating cutting
instruments and tools useful for the removal of tooth structure,
including decayed or damaged nerve tissues and dentine material on
the interior walls of the root canal and dentine and enamel from
the external tooth wall.
[0004] 2. Description of the Related Art
[0005] In the field of endodontics, one of the most important and
delicate procedures is that of cleaning or extirpating a root canal
to provide a properly dimensioned cavity while essentially
maintaining the central axis of the canal. This step 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 root canal procedure, the dentist removes injured
tissue and debris from the canal prior to filling the canal with an
inert filling material. In performing this procedure the dentist
must gain access to the entire canal, shaping it as necessary. But
root canals normally are very small in diameter, and they are
usually quite curved. It is therefore very difficult to gain access
to the full length of a root canal.
[0007] Many tools have been designed to perform the difficult task
of cleaning and shaping root canals. Historically, dentists have
used a wide multitude of tools to remove the soft and hard tissues
of the root canal. These tools, usually called endodontic files,
have been made by three basic processes. In one process, a file is
created by twisting a prismatic rod of either square or triangular
cross section in order to create a file with helical
cutting/abrading edges ("K-file"). The second process involves
grinding helical flutes into a circular or tapered rod to create a
file with one or more helical cutting edges ("Hedstrom file"). The
third method involves "hacking" or rapidly striking a circular or
tapered rod with a blade at a given angle along the length of the
rod, thus creating an endodontic file characterized by a plurality
of burr-like barbs or cutting edge projections ("barbed file" or
"broach"). Each of these methods produces an instrument having
unique attributes, advantages, and disadvantages.
[0008] Endodontic files have historically been made from
medical-grade stainless steels. But steel is inherently stiff and
brittle and, thus, these tools can sometimes pose a significant
danger of breakage in the curved root canal, particularly if
over-torqued or fatigued. Modern endodontic instruments are often
formed from exotic alloys such as nickel-titanium commonly known as
"Nitinol.TM." or "NiTi." A series of comparative tests of
instruments made of nickel-titanium alloy and stainless steel were
conducted and published 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 reported tests demonstrated that
the NiTi instruments exhibited superior flexibility and torsional
properties as compared to stainless steel instruments.
[0009] Based on the initial success of these and 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. 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 reliably obtain clinically acceptable instruments. See, for
example, U.S. Pat. No. 5,464,362 to Heath et. al., which describes
a method of slow grinding a rod of a nickel-titanium alloy to
create a fluted endodontic file. The cost of purchasing and
operating the required specialized 6-axis grinding machines and
other grinding/machining equipment and the time consumption of the
grinding process itself make endodontic files produced by this
method inordinately expensive when compared to their stainless
steel counterparts.
[0010] Another significant drawback is the extreme 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 deleterious effects on file
performance. Another significant drawback is that the cutting edges
of presently available NiTi instruments are not as sharp as their
stainless steel counterparts and tend to lose their sharpness more
rapidly with use. Another significant drawback is reduced
manipulation control due to reduced stiffness (excessive
"rubberiness") and extreme torsional flexibility of presently
available NiTi endodontic files as compared with stainless steel
files. Another drawback is increased heat generation created by
bare or oxidized NiTi surfaces rubbing against root canal
walls.
[0011] These and other drawbacks have limited the growth of NiTi
instruments and have created demand for improved manufacturing
methods, alloys and instruments that overcome the aforenoted
drawbacks.
SUMMARY OF THE INVENTION
[0012] The present invention provides an improved class of alloys
and precision manufacturing techniques particularly suited and
adapted for forming endodontic files, reamers, drills and similar
cutting instruments having improved lubriciousness and resistance
to wear and breakage.
[0013] In one preferred embodiment, the invention provides an
endodontic file fabricated from one or more amorphous metal alloys.
Such instruments exhibit greatly improved cutting-edge sharpness,
wear resistance, lubriciousness and resistance to breakage from the
propagation of cracks caused by crystalline defects.
[0014] In another preferred embodiment, the invention provides an
endodontic instrument fabricated by direct casting, forging or
molding. Most preferably, the endodontic instrument is fabricated
as a single integral unit, including the working or cutting portion
and the handle or attachment end thereof, from an amorphous metal
alloy. Commercial quantities of such instruments can be quickly and
economically manufactured using modified metal injection molding
and/or amorphous metal die-casting techniques.
[0015] In another preferred embodiment, the invention provides an
endodontic file fabricated from a bulk amorphous or nanocrystalline
alloy material. The alloy material is preferably selected to have a
relatively high ultimate tensile strength and a relatively low
modulus of elasticity. The endodontic file is preferably formed by
forcing an amorphous alloy under pressure into a suitably designed
mold or die to form a cast integral body having one or more helical
or non-helical flute patterns, cutting edges and/or any number of
other desired working surface features. The endodontic file
constructed in accordance with the invention possesses sharper and
cleaner cutting edges than heretofore achieved using conventional
grinding processes and is further capable of receiving an increased
range of desired working surface features, such as cutting edges,
barbs, projections, recesses and the like. The endodontic file
constructed from amorphous alloy in accordance with the invention
is also sharper, more lubricous and resistant to wear and breakage
than instruments manufactured from conventional crystalline
alloys.
[0016] 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 accordance 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
advantages as taught herein without necessarily achieving other
objects or advantages as may be taught or suggested herein.
[0017] 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
[0018] 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:
[0019] 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;
[0020] FIGS. 2A-H are various views illustrating a typical prior
art fluted endodontic file fabricated from a nickel titanium
alloy;
[0021] FIGS. 3A-H are various views of one preferred embodiment of
an endodontic file having features and advantages in accordance
with the present invention;
[0022] FIG. 4 is a chart illustrating differences in material
strength and elasticity among various selected microcrystalline
materials and illustrating the superior properties of amorphous
metal "glassy" alloys; and
[0023] FIGS. 5A-C are various views of a second alternative
preferred embodiment of an endodontic file having features and
advantages in accordance with the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0024] FIG. 1 is a partial cross section of a tooth 50 and
supporting root structure illustrating the use of a typical fluted
endodontic 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.
[0025] 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 abrading 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 pulp tissue while leaving the integrity of the
central axis of the root canal relatively unaffected. Proper
cleaning and shaping of the root canal 56 is important in order to
allow complete 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
practicable is removed from the root canal, the canal 56 is sealed
closed, typically by reciprocating and/or rotating a condenser
instrument in the canal to urge a sealing material such as
gutta-percha into the canal.
[0026] 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.
[0027] To alleviate the transportation and breakage problems,
highly flexible endodontic files fabricated from nickel-titanium
alloy (Nitinol.TM. or NiTi) were introduced and have become widely
accepted. See, e.g. U.S. Pat. No. 5,882,198, incorporated herein by
reference. But conventional fluted instrument designs are difficult
to manufacture from Nitinol alloys, often requiring expensive
grinding operations and specialized 6-axis grinding machines to
create the desired continuous helical fluting and sharp cutting
edges.
[0028] 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. Typically, the shank portion
104 is securely gripped or otherwise permanently secured to a
formed plastic or metal fitting or handle portion 109 for mating
with the chuck of a dental handpiece (not shown). The fitting
portion 109 includes a generally I-shaped flat 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.
[0029] Alternatively and/or in addition to the fitting portion 109,
the shank portion 104 may include a knurled or otherwise treated
surface (not shown) or plastic or metal 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.
[0030] Helical flutes 124 and 126 are typically 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.
[0031] 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 128, 132 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 convenience 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.
[0032] 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.sub.1 (sometimes called the "margin width")
and optional recessed land portions 112, 114 having width w.sub.2
(sometimes called the "relief width"). The combined width
w.sub.1+w.sub.2 is sometimes called the "land width." The recessed
land portions 112, 114 are at a first predetermined radial distance
R.sub.1 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
R.sub.2 from the center of the working portion 106, typically about
4 to 30 percent greater than the radial distance R.sub.1
[0033] 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 "t.sub.W"--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.
[0034] The tip 150 of the instrument 100 may assume 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 and remove the tissue as the file is
operated in the canal.
[0035] The chisel tip 150 is typically formed by grinding flats or
facets 151, 153 into the tip of the instrument 100, as shown,
forming the chisel edge 154. The facets 151, 153 define an included
point angle .mu. typically between about 45-100 degrees, as shown
in FIG. 2E. The chisel edge 154 is typically canted from center by
a primary angle .gamma. 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.
[0036] The endodontic instrument shown and described in connection
with FIG. 2 is formed by grinding away selected portions of
material from a tapered rod of a metallic alloy material, such as
stainless steel or nickel-titanium ("NiTi"), thereby creating the
desired working surface features 116, 118, 124, 126, 128, 130, 132
and 134. The metallic alloy materials presently used, including
stainless steel and NiTi, all have crystalline grain structures
consisting of a plurality of crystal formations or grains 190
varying in size from about 15 .mu.m to about 200 .mu.m and arranged
in a microstructure (see, e.g., FIGS. 2B, 2G & 2F). Such
crystalline microstructures are naturally produced by the
nucleation and growth of crystalline phases from the molten alloy
during cooling and solidification and/or subsequent annealing or
other heat processing steps. Essentially, the atoms in the
structure arrange themselves in an ordered manner, in which a small
repeat unit called a "unit cell" can be identified. This unit cell
is repeated in all three directions, and in this way, an ordered
crystalline structure is built up.
[0037] But crystalline microstructures inherently contain a large
number of missing atoms, impurities, and misaligned planes of atoms
or dislocations and/or other defects in the arrangement of atoms
within the crystalline solid. These defects create material
weaknesses which limit the ultimate tensile strength and
flexibility of the metal alloy. When exposed on outer surfaces of a
formed structure, such defects also tend to create sites prone to
stress-induced crack formation, increased friction, material wear
and edge degradation.
[0038] Crystalline microstructures are also typically malleable
and, thus, tend to plastically stretch and/or deform when exposed
to large stresses. For example, copper wire is easy to bend because
the crystalline micro-structure contains planes of atoms which can
slip easily past one another. In the context of cutting tools, the
more malleable a material is the more difficult it is to achieve
and maintain sharp cutting edges. Thus, the formation of sharp
cutting edges in a conventional metal alloy (especially NiTi
alloys) is often plagued by the formation of latent burrs, rolled
metal deposits and/or other imperfections created along the desired
cutting edges by stresses induced 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 serious deleterious effects
on instrument performance.
[0039] FIGS. 3A-H are various views of a fluted endodontic file
having features and advantages of the present invention. Except for
the modifications discussed below, all other physical and geometric
aspects of the instrument 200 are substantially as illustrated and
discussed above in connection with FIGS. 2A-H. The file 200
generally comprises a shaft 202 having a shank portion 204 and an
elongated working portion 206. The working portion 206 extends from
a proximal end 207 adjacent the base of the shank 204 to a distal
end 208 terminating in a chisel tip 250. Preferably, the shank
portion 204 is integrally formed with or securely gripped by a
fitting portion 209 for mating with the chuck of a dental handpiece
(see, e.g. FIG. 3B). For this purpose the fitting portion 209
includes a generally I-shaped flat side 282 which defines a step
284 and a generally semicircular disk 286 above and adjacent to a
generally semi-circular groove 288. Such fitting 209 is typical of
those employed in the dental industry for connecting or interfacing
a dental tool with dental drill or handpiece.
[0040] In contrast to the crystalline micro-structure of the
instrument 100 illustrated and described above in connection with
FIGS. 2A-H, the file 200 is preferably formed substantially
entirely of an amorphous or nanocrystalline metal alloy (see, FIGS.
3B, 3F & 3G). Such alloys allow for production of exceedingly
durable and wear-resistant endodontic cutting instruments having
sharper cutting edges and greater flexibility than conventional
steel and NiTi alloys. For purposes of the present description and
the appended claims, the term "amorphous" refers to any solid
material having no (or essentially no) detectable crystalline
structure. The term "nanocrystalline" refers to any solid material
having an average crystalline grain size in the nanometer range (or
less than about 1 .mu.m).
[0041] Amorphous alloys result from the fact that certain metals,
if rapidly cooled, undergo a "glass transition" and can actually
freeze as vitreous or semi-vitreous solids. Unlike conventional
metals and metal alloys, these "glassy" alloys lack the normal
grain boundaries that serve as points of weakness in ordinary
crystalline microstructure materials. Such materials possess
desirable material properties such as high strength and low modulus
of elasticity superior to the limits of conventional
microcrystalline materials. Thus, as illustrated by FIG. 4, glassy
alloys are typically much stronger than their crystalline metal
counterparts (by factors of 2 or 3), are quite tough (much more so
than ceramics), and have relatively higher strain limits. These and
other desirable properties make amorphous alloys ideally suited for
constructing endodontic instruments and similar cutting
instruments. Typical physical and mechanical properties are
outlined below in TABLE 1.
1 TABLE 1 Property Measured Value (typ.) Density ("as cast") 7.19
(g/cc) Vicker's Hardness 900 (50 g Load) Tensile Strength 1.0-1.7
(GPa) Elastic Modulus 100-110 (GPa) Lamination Factor >79 (%)
Thermal Expansion 2-7 (ppm/.degree. C.) Crystallization Temperature
510 (.degree. C.) Continuous Service Temp. 150 (.degree. C.)
Corrosion Resistance Excellent Wear-Resistance Excellent
[0042] As noted above, the atoms in an amorphous material are not
arranged in any particular pattern or ordered structure. Rather
they have a tightly-packed, highly random arrangement. Amorphous
materials can be formed by cooling a molten metal alloy quickly
enough to prevent crystallization--in other words, fast enough that
the atoms do not have time to arrange themselves into an ordered
structure. For example, Vitreloy is an amorphous or glassy alloy
containing five elements differing dramatically in atomic size and
having the approximate elemental composition: 41.2% zirconium,
22.5% beryllium, 13.8% titanium, 12.5% copper, and 10.0% nickel.
Because of the varying atomic sizes, and their random arrangement
in the vitrified solid, there are no groups of atoms that can
easily move past one another. Because there are no planes of atoms
in an amorphous material, the atoms are effectively gridlocked into
the glassy structure, making the movement of groups of atoms very
difficult.
[0043] As FIG. 4 illustrates, most conventional solid materials are
either: (i) very hard and strong, like steel, but have limited
elastic recovery ability; (ii) highly elastic, like plastics and
polymers, but have limited hardness and strength; or (iii) a
compromise, like titanium, but being superior in neither strength
nor elasticity. Advantageously, amorphous alloys, such as Vitreloy,
are demonstrably superior in both strength/hardness and elasticity,
making such alloys an ideal material for a bendable cutting tool
200. The hardness and relative non-malleability of the material
provides for sharp and durable cutting edges 232 and other surface
features (FIG. 3H). Cutting edges 232 are easily formed without the
aforenoted problem of rolled metal deposits and other defects
experienced with grinding or machining of conventional
microcrystalline alloy materials. At the same time, the high
elastic recovery of the amorphous material allows for a high degree
of bending and flexibility, particularly desirable in endodontic
applications for accessing and removing hard dentine material deep
from within the inner walls of a root canal.
[0044] The combination of high strength, hardness and excellent
elasticity make Vitreloy and similar amorphous alloys ideal
materials for endodontic instruments and similar cutting
instruments requiring efficient cutting and removal of dentine and
enamel from and within tooth structures and curved root canal
surfaces. Other suitable amorphous alloy materials include, without
limitation, binary alloy systems including alloys containing
zirconium and/or titanium, including NiTi alloys; ternary alloy
systems including Cu--Ti--Zr, Ln-Al-TM, Mg-Ln-TM, Zr--Al-TM,
Hf--Al-TM and Ti--Zr-TM (wherein Ln is a lanthanide metal, and TM
is a transition metal of the Groups VI to VIII); and various other
complex alloy systems including Zr--Al--Co--Ni--Cu,
Zr--Ti--Al--Ni--Cu, Zr--Ti--Nb--Al---Ni--Cu, and
Zr--Ti--Hf--Al--Co--Ni--Cu. Most of these alloy systems
(particularly the ternary and more complex alloys) have low
critical cooling rates for glass formation and, thus, can be easily
produced in bulk shapes with thicknesses of 10-50 mm or more.
[0045] In addition to amorphous metal alloys, those skilled in the
art will readily appreciate that certain nanocrystalline materials
may also find favorable application in the present invention.
Nanocrystalline materials have an ultra-fine crystalline grain
structure that produces a significantly higher percentage of atoms
at grain boundaries. Whereas in conventional polycrystalline
materials grain boundaries typically account for less than 1% of
the material volume, in nanocrystalline materials they can occupy
as much as 50% or more. The predominance of intercrystalline atomic
structures at grain boundaries provide marked improvements in
material performance and properties very similar to those achieved
in the amorphous metal materials described above. However, some
nanocrystalline materials can be processed or formed so as to
retain certain desired properties of conventional microcrystalline
structures, such as malleability and material toughness or
energy-absorbing capability. Suitable nanocrystalline materials can
be formed in a similar fashion as described above for amorphous
metal materials (e.g., by rapid cooling of molten alloy) and/or by
the addition of one or more trace elements or impurities selected
to provide catalyzing sites for nanocrystalline formation.
Preferred nanocrystalline materials have about 5% or greater atoms
located at grain boundaries. More preferably at least about 25% of
atoms are located at grain boundaries and most preferably at least
about 50% to 75% of atoms are located at grain boundaries. A
suitable amorphous (or essentially amorphous) material may comprise
anywhere from 75% to 100% of atoms located at grain boundaries.
[0046] Fabrication of the endodontic instrument 200 may be readily
accomplished using any combination of conventional machining and/or
grinding techniques well know to those skilled in the art. Thus,
the instrument 200 may be fabricated from a round or tapered rod or
blank of selected material comprising amorphous metal alloy and/or
nanocrystalline alloy material. Because of the relative hardness of
the selected material, the formation of sharp cutting edges and
other fine or detailed features by conventional grinding operations
can be easily achieved at normal grinding speeds and normal-to-high
production throughputs. Thus, an improved, high-quality cutting
instrument 200 may be reliably manufactured without significantly
altering existing manufacturing processes, time or costs.
[0047] Alternatively, the instrument 200 may be readily formed at
higher production rates by metal injection molding ("MIM") using
one or more powdered amorphous or nanocrystalline alloys suspended
in a liquid polymer binder. The MIM process is very similar to
plastic injection molding and can provide much the same shapes,
part geometries and surface features. Very fine metal powder
combined with a binder material is injected into a mold. After the
binder solidifies, the part is removed from the mold and the part
is sintered (solid state diffused) in a controlled atmosphere
furnace. The binder is melted or dissolved by the intense heat and
vacuum pressure. The part is then further sintered at a controlled
temperature and pressure until the powdered metal particles fuse to
one another, leaving the resulting part at 94-99% of theoretical
density. Subsequent machining operations or processing steps may be
employed, as desired, to refine and sharpen cutting edges 232
and/or other surface details as necessary. Advantageously, very
high production rates can be achieved through the use of
multi-cavity tooling, high-speed injection molding techniques and
the like.
[0048] Most preferably, the entire instrument 200 is formed as a
single, integral molded structure using a modified high-pressure
die-casting technique. A selected molten amorphous metal alloy is
preferably injected into a die or mold under pressure and is then
rapidly cooled or quenched at a rate greater than about 10.degree.
C. per second to form the desired solid vitreous metal structure
having the desired instrument shape as imparted by the die or mold.
Advantageously, it has been discovered that metallic glass alloys
can be die-cast in this manner with very high detail and precision
as compared to conventional die casting with crystalline metal
alloys. For example, Liquidmetal Technologies of Lake Forest,
Calif. has developed an amorphous metal alloy material,
Liquidmetal.TM., that can be die-cast with the greatest of
precision, for example, down to 1 micron. The solidification of the
Liquidmetal alloy exhibits extremely small shrinkage when compared
to solidification of ordinary metals. This results largely from the
lack of phase transformation in the Liquidmetal alloy from its
molten state to its solid state upon cooling and solidification. As
such, it is advantageously possible and desirable to die-cast an
entire cutting instrument 200 or at least the working portion 207
thereof in virtually whatever intricate or sophisticated design(s)
thereof that may be desired (including "as-cast" sharp cutting
edges and/or other detailed working surface features) with minimal
or no post-finishing processes. Of course, subsequent machining
operations or processing steps may be employed, as desired, to
deflash, refine and/or hone cutting edges 232 and/or other details
as desired. The highest possible manufacturing throughput and
efficiency can be achieved through the use of multi-cavity dies,
high-speed casting techniques and the like.
[0049] Advantageously, such precision die-cast instruments are not
only less expensive to manufacture, but the design and physical
geometries thereof are not limited by the normal requirements
imposed by conventional machining operations. Thus, the
manufacturing method in accordance with one embodiment of the
invention facilitates the possibility of even more advanced and
more sophisticated file geometries, including, for example,
multiple tapers, intertwined and/or disjointed flutes and cutting
edges, cutting blades, micro-projecting cutting features and/or
other desired features in the working surface of an endodontic
instrument or similar instrument.
[0050] FIG. 5 illustrates one possible embodiment of an advanced
endodontic file 300 having features and advantages in accordance
with the present invention. The file 300 generally comprises a
tapered working portion 307 having multiple projecting cutting
blades 315 integrally formed thereon and each having an outer-most
exposed cutting edge 325. The entire file structure, including
cutting blades 315, sharp cutting edges 325 and other possible
desired features (not shown), is preferably die-cast substantially
entirely of an amorphous or nanocrystalline metal alloy. The
multiple cutting blades 315 preferably are spaced and arranged
along the working portion 307 in an irregular, non-symmetric and/or
uneven pattern. Preferably, the projecting cutting blades are also
formed with different and/or alternating helix angles, rake angles,
orientations and projections along substantially the entire length
of the working portion 307. Desirably, such cutting blade
geometries provide non-continuous, non-uniform cutting edges 325,
which, when the instrument is rotated and/or reciprocated within a
curved root canal, help promote a continuous canal shape while
minimizing the risk of canal penetration. The tip 308 of the
instrument 300 preferably is smooth and bullet-shaped, as
illustrated in FIG. 5C, with a small notch 330 formed therein,
defining at least one sharp cutting edge 340. Of course, those
skilled in the art will appreciate that the particular features and
geometries described above can be varied to produce additional
desired effects without departing from the essential teachings of
the invention disclosed herein.
[0051] Advantageously, cast endodontic instruments fabricated in
accordance with the invention are highly uniform in size and
geometry. They have precision formed flutes, cutting edges and
other working surface features that are substantially identical
with substantially little or no variation from instrument to
instrument. Such precision-cast root canal instruments have
tremendous clinical advantages and improved efficacy. Furthermore,
these instruments reliably perform and behave more predictably and
produce more uniform results than can be obtained with presently
available machined instruments. It is well known that conventional
endodontic instruments vary significantly in flute geometries,
taper rates, helix angles, cutting edge orientations and sharpness,
and other critical features as a result of normal material sizing
variations, machining tolerances and tolerance stacking. Metal
injection molding or die casting of endodontic instruments with
sharp cutting edges and other working surface features intact
eliminate many if not all of these consistency problems.
[0052] The concepts and teachings of the present invention are
particularly applicable to amorphous alloys and nanocrystalline
alloys and endodontic instruments (files, reamers, obturators,
drill bits and the like) fabricated therefrom. However, the
invention is not limited specifically to endodontic instruments
fabricated from the disclosed alloys, but may be practiced with a
variety of dental instruments using any one of a number of other
suitable alloy materials. Those skilled in the art will also
recognize that a variety of well known machining techniques for
making conventional endodontic instruments may also generally be
applied to the manufacture of instruments as disclosed herein with
various known or later developed improvements in materials or
processing. For example, suitable instruments may be ground from a
straight or tapered rod, twisted, and/or drawn to a taper with or
without grinding. Suitable grinding techniques include those
described in standard metallurgical texts for grinding various
metals. Those skilled in the art will further appreciate that while
the particular instruments illustrated and described herein are
reamers or files, similar instruments can also be configured for
use as condensers or compactors by reversing the direction of twist
of the helical flutes and lands and/or reversing the direction of
rotation of the instrument.
[0053] Although this invention has been disclosed in the context of
certain preferred embodiments and examples, it will be understood
by those skilled in the art that the present invention extends
beyond the specifically disclosed embodiments to other alternative
embodiments and/or uses of the invention and obvious modifications
and equivalents thereof. Thus, it is intended that the scope of the
present invention herein disclosed should not be limited by the
particular disclosed embodiments described above, but should be
determined only by a fair reading of the claims that follow.
* * * * *