U.S. patent application number 10/256751 was filed with the patent office on 2003-06-19 for bulk metallic glass medical instruments, implants, and methods of using same.
Invention is credited to Horton, Joseph A. JR., Parsell, Douglas E..
Application Number | 20030111142 10/256751 |
Document ID | / |
Family ID | 25175927 |
Filed Date | 2003-06-19 |
United States Patent
Application |
20030111142 |
Kind Code |
A1 |
Horton, Joseph A. JR. ; et
al. |
June 19, 2003 |
Bulk metallic glass medical instruments, implants, and methods of
using same
Abstract
MRI-compatible medical instruments and appliances are made using
bulk metallic glass alloys. MRI-guided methods include the use of
articles that include bulk metallic glass alloys.
Inventors: |
Horton, Joseph A. JR.; (Oak
Ridge, TN) ; Parsell, Douglas E.; (Ridgeland,
MS) |
Correspondence
Address: |
UT-Battelle, LLC
111 Union Valley Rd.
PO Box 2008, Mail Stop 6498
Oak Ridge
TN
37831
US
|
Family ID: |
25175927 |
Appl. No.: |
10/256751 |
Filed: |
September 27, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10256751 |
Sep 27, 2002 |
|
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09799445 |
Mar 5, 2001 |
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Current U.S.
Class: |
148/561 |
Current CPC
Class: |
A61B 10/02 20130101;
A61B 17/866 20130101; C22C 14/00 20130101; C22C 9/00 20130101; A61B
17/3211 20130101; A61L 31/026 20130101; C22C 45/10 20130101; A61L
31/18 20130101 |
Class at
Publication: |
148/561 |
International
Class: |
C22C 045/00 |
Goverment Interests
[0002] This invention was made with Government support under
Contract No. DE-AC05-00OR22725 awarded by the United States
Department of Energy. The Government has certain rights in the
invention.
Claims
What is claimed is:
1. A method of carrying out a medical procedure comprising the
steps of: a. providing an article selected from the group
consisting of medical instruments and biomedical appliances, at
least a portion of said article comprising a bulk metallic glass;
and b. using said article to carry out a medical procedure wherein
an MRI image of at least part of said article is made.
2. A method in accordance with claim 1 wherein said medical
procedure is an orthopedic procedure.
3. A method in accordance with claim 1 wherein said medical
procedure is an endodontic procedure.
4. A method in accordance with claim 1 wherein said medical
procedure comprises an MRI-guided procedure.
5. A method of carrying out an MRI-guided procedure comprising the
steps of: a. providing an article, at least a portion of said
article comprising a bulk metallic glass; and b. using said article
to carry out an MRI-guided procedure.
6. A method in accordance with claim 5 wherein said MRI-guided
procedure further comprises a medical procedure.
7. A method in accordance with claim 5 wherein said MRI-guided
procedure further comprises a non-medical procedure.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional application of U.S. patent
application Ser. No. 09/799,445, filed on Mar. 5, 2001, the
entirety of which is incorporated herein by reference.
FIELD OF THE INVENTION
[0003] The present invention relates to medical, surgical, and
dental hardware, especially medical instruments and biomedical
appliances, and particularly to those instruments and appliances at
least partially constructed of a bulk metallic glass (BMG), and to
methods of using the same.
BACKGROUND OF THE INVENTION
[0004] The concept of magnetic susceptibility is central to many
current research and development activities in magnetic resonance
imaging (MRI). For example, the development of MR-guided surgery
has created a need for surgical instruments and other devices with
susceptibility tailored to the MR environment; susceptibility
effects can lead to position errors of up to several millimeters in
MR-guided stereotactic surgery; and the variation of magnetic
susceptibility on a microscopic scale within tissues contributes to
MR contrast and is the basis of functional MRI. The magnetic
aspects of MR compatibility are discussed in terms of two levels of
acceptability: Materials with the first kind of magnetic field
compatibility are such that magnetic forces and torques do not
interfere significantly when the materials are used within the
magnetic field of the scanner; materials with the second kind of
magnetic field compatibility meet the more demanding requirement
that they produce only negligible artifacts within the MR image and
their effect on the positional accuracy of features within the
image is negligible or can readily be corrected. Several materials
exhibiting magnetic field compatibility of the second kind have
been studied and a group of materials that produce essentially no
image distortion, even when located directly within the imaging
field of view, is identified. Because of demagnetizing effects, the
shape and orientation, as well as the susceptibility, of articles
within and adjacent to the imaging region is important in MRI, but
the use of literature values for the susceptibility of materials is
often difficult because of inconsistent traditions in the
definitions and units used for magnetic parameters--particularly
susceptibility.
[0005] Thus, methods and apparatus have long been sought to permit
surgical procedures involving surgical instruments and/or
surgically implanted appliances to be guided or monitored by
MRI.
[0006] For this to be possible, a new implant material has long
been needed which has a low MRI signature. In addition, it would be
desirable for such materials to also have high hardness, tensile
strength, and toughness. A desirable material would have a lower
elastic modulus and an extremely high elastic limit of about 2%
compared to that of a typical metal, namely about 0.2%. Bone has an
elastic limit of about 1%. Such material would be unique in its
ability to flex elastically with the natural bending of the bones
and so distribute stresses more uniformly. Faster healing rates
would result from reduced stress shielding effects while minimizing
stress concentrators. Because of these unique mechanical
properties, screws could have a thinner shank and deeper threads
yielding greater holding power. Applications where such material is
desirable would include such as fracture fixation screws, rods,
pins, knee and hip joint wear surfaces and shafts, and aneurysm
clips. A large variety of other applications, changes and
modifications would be obvious to those skilled in the art.
[0007] Current implant materials produce a distortion or blooming
(enlargement) in the MRI image. Larger implants are even internally
heated during an MRI. This is especially important for aneurysm
clips where later imaging is often needed and where no movement of
the clip as a result of an MRI is essential.
[0008] Definitions:
[0009] A bulk metallic glass (BMG) is defined for purposes herein
as an amorphous metallic alloy that is cast in bulk form. A BMG is
known to be inclusive of amorphous thin-film materials such as
those that are typically deposited on surfaces.
[0010] A medical instrument is defined for purposes herein as any
device used by medical and/or dental personnel in any surgical
and/or dental procedure.
[0011] A biomedical appliance is defined for purposes herein as any
medically functional device that is configured for disposition on
or inside a living body, including surgically implanted orthopedic
devices, dental implants, and the like.
OBJECTS OF THE INVENTION
[0012] Accordingly, objects of the present invention include at
least the following:
[0013] provision of new and improved medical instruments and
biomedical appliances having at least one of: desirably high
elastic limit, hardness, strength, toughness, and ability to hold a
cutting edge;
[0014] provision of new and improved MRI-compatible medical
instruments and surgically implantable orthopedic appliances that
allow surgeries to be guided in real time by MRI imaging;
[0015] provision of new and improved surgically implantable
appliances (e.g., orthopedic, endodontic, etc.) with mechanical
properties compatible with those of bone, low corrosion rate, and
good biocompatibility (lack of rejection by human or animal
tissue);
[0016] provision of new and improved articles configured for use as
tools, instruments, and parts used for maintaining, inspecting,
modifying, operating, or exploring both man-made and
naturally-occurring structures which are internally or externally
viewable by MRI; and
[0017] provision of new and improved methods of performing medical
and dental procedures utilizing the aforementioned instruments and
appliances.
[0018] Further and other objects of the present invention will
become apparent from the description contained herein.
SUMMARY OF THE INVENTION
[0019] In accordance with one aspect of the present invention, the
foregoing and other objects are achieved by an article, at least a
portion of which includes a bulk metallic glass having magnetic
properties suitable for producing an MRI image, the article having
a shape suitable for producing an MRI image and being configured
for use as a medical instrument and/or a biomedical appliance.
[0020] In accordance with another aspect of the present invention,
a method of carrying out a medical procedure includes the steps of:
providing a medical instrument or a biomedical appliance, at least
a portion of which includes a bulk metallic glass; and using the
medical instrument or biomedical appliance to carry out a medical
procedure.
[0021] In accordance with a further aspect of the present
invention, a method of carrying out an MRI-guided procedure
includes the steps of: providing an article, at least a portion of
which includes a bulk metallic glass; and using the article to
carry out an MRI-guided procedure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] In the drawings:
[0023] FIG. 1 is a full-scale MRI image of a 7 mm diameter rod of a
conventional copper alloy (Cu-4 Cri-2 Nb) in salt water.
[0024] FIG. 2 is a full-scale MRI image of a 7 mm diameter rod of
BAM-11 in salt water in accordance with the present invention.
[0025] FIG. 3 is a full-scale MRI image of a 7 mm diameter rod of
Ni-free BMG in salt water in accordance with the present
invention.
[0026] FIG. 4 is a full-scale MRI image of a bullet nosed 6 mm
diameter rod of Ti-6 Al-4.
[0027] FIG. 5 is a full-scale MRI image of a bullet nosed 6 mm
diameter rod of BAM-11 in salt water in accordance with the present
invention.
[0028] FIG. 6 is a view of a typical conventional orthopedic bone
screw.
[0029] FIG. 7 is a view of an orthopedic bone screw of an improved
design made possible by the use of BMG in accordance with the
present invention.
[0030] For a better understanding of the present invention,
together with other and further articles, advantages and
capabilities thereof, reference is made to the following disclosure
and appended claims in connection with the above-described
drawings.
DETAILED DESCRIPTION OF THE INVENTION
[0031] It has been discovered that certain bulk metallic glass
(BMG) alloys have a very low MRI signature. BMG materials have high
hardness, tensile strength, and toughness. The present invention is
based on the discovery that the unique properties of BMG alloys
make them especially suitable for biomedical implant applications
as well as for medical instruments.
[0032] The invention described and claimed herein involves both
apparatus and methods for the use of medical instruments and
surgically implantable devices made of bulk metallic glasses during
surgical and dental procedures in an intervention MRI in which the
MRI guides the surgery in real time. The bulk metallic glass has an
excellent MRI signature due to its amorphous structure. An accurate
signature of the metallic implant or instrument is of paramount
importance for accurate position information of the instrument or
implant device.
[0033] It is recognized that a large range of specific compositions
of BMG are known to and may be used by the skilled artisan in this
invention. . For example, see U.S. Pat. No. 5,735,975 issued on
Apr. 7, 1998 to Lin, et al. entitled "Quinary Metallic Glass
Alloys" and U.S. Pat. No. 5,803,996 issued on Sep. 8, 1998 to
Inoue, et al. entitled "Rod-Shaped or Tubular Amorphous Zr Alloy
Made by Die Casting and Method for Manufacturing said Amorphous Zr
Alloy". The patents of both Lin and Inoue are incorporated herein
by reference. The specific composition of the alloy used in
examples described herein is Zr -17.9 Cu -14.6 Ni -5 Ti -10 Al (at
%).
[0034] It is further recognized that attempts have been made in the
past to provide medical instruments which cause reduced or enhanced
artifact on diagnostic images such as MRI images. One such example
is U.S. Pat. No. 5,895,401, issued Apr. 20, 1999,
"Controlled-Artifact Magnetic Resonance Instruments" by Daum et al.
However, Daum teaches a crystalline alloy comprising at least 85%
titanium and does not teach an alloy which is amorphous such as the
bulk metallic glass taught herein. Conversely, the bulk metallic
glass as taught herein contains no more than 12 at. % titanium.
Thus the present invention and Daum teach grossly different
materials.
[0035] Because of the unique atomic structure in an amorphous
metallic glass, this material possesses unique magnetic properties
that allow the excellent MRI signature.
[0036] The composition tested contains substantial amounts of a
ferromagnetic element, Ni. In a crystalline structure, the presence
of Ni produces substantial blooming in an MRI image. All of the
other elements in this alloy, namely Zr, Ni, Ti, and Al all have
susceptibilities substantially higher than that of human tissues
thereby producing a positional error in the MRI image. Only copper
has susceptibility similar to human or animal tissues. Comparison
images show that the image of the BMG is better even than a copper
alloy. All bulk metallic glasses that are not expressly designed
for good soft or hard ferromagnetic properties are expected to have
this good MRI signature.
[0037] BMG alloys have a lower modulus and an extremely high
elastic limit of about 2% as compared to that of a typical metal,
namely about 0.2%. Bone has an elastic limit of about 1%. BMGs are
unique in their ability to flex elastically with the natural
bending of bones and so distribute stresses more uniformly. Faster
healing rates result from reduced stress shielding effects while
minimizing stress concentrators. Because of the unique mechanical
properties of the BMGs, screws can have a thinner shank and deeper
threads yielding greater holding power. Compared to the lowest
modulus developmental titanium alloy, for a given load, the BMG
will require 1/4 the cross section to carry the load and will
undergo twice the deflection. Compared to stainless steel, the area
will be 1/3 to carry the load and the BMG will have 5 times the
deflection. Potential applications include fracture fixation
screws, rods, pins, hip joint wear surfaces and shafts, aneurysm
clips, endodontic files and orthodontic arch wires as well as
components of devices such as pacemakers, neurostimulators,
medicine-metering pumps, and equipment for remotely-viewed
microsurgery.
[0038] FIGS. 1 and 2 show comparison MRI images of a copper alloy
and a BMG alloy (Zr -17.9 Cu -14.6 Ni -5.0 Ti -10.0 Al, referred to
here as BAM-11 (all compositions are in atomic %) in a flask of
salt water showing much less bloom with the BMG alloy. This is a
surprising result, since one would expect the susceptibility to be
close to that of zirconium as the major constituent and the nickel
in the alloy should make it higher. While copper is the metal with
a susceptibility closest to that of living human or animal tissue,
it is too soft for many uses, especially medical instruments and
implants. The toxicity of beryllium all but prohibits the use of
the harder Be--Cu alloys. For these reasons, the unique magnetic
properties of BMG materials yield the ability to fabricate
MRI-friendly implant devices as well as a new class of medical
instruments for use within the interventional MRI environment.
Minimally invasive procedures not possible via conventional
surgical and dental techniques can be successfully performed
through interventional MRI, indicating a critical need for
instruments to facilitate these procedures.
[0039] Prior to the recent development of these bulk metallic
glasses, rapid solidification such as melt spinning or gas
atomization producing thin ribbons or powders (<150 mm) was
required to achieve the sufficiently high cooling rates necessary
for glass formation. Due to their unique amorphous microstructure
with a number of different elements present, the BMGs exhibit a
number of exceptional properties. For example, alloy BAM-11 has a
yield strength of 1900 MPa, an elastic limit of 2 to 2.2%, Young's
Modulus of 90 GPa, Vickers hardness of 590 kg/mm2, and a toughness
of 55 to 60 MPa{square root}M. Table 1 lists mechanical properties
of BMGs and compares them to that of the three most commonly used
implant materials and to one of the new low-modulus titanium alloys
under development. Table 2 lists some potential applications for
BMG alloys.
[0040] (Table 1 begins next page)
1TABLE 1 Mechanical properties of bulk metallic glass compared to
the three leading implant materials Experimen- tal Ti- BMG Ti-6Al-
35Nb-5 Property (BAM 11) Co-Cr 4V 316L-CW Bone Ta-7 Zr Tensile
Yield 1900 450 830 690 Compressive 547 Strength, MPa 130-150
(cortical) Elastic Strain 2-2.2 0.18 0.67 0.34 1% 0.9 Limit, %
Plastic Strain 1 8 10 12 -- 19 to failure, % Young's Modulus, 90
248 124 200 17* 55 GPa Hardness, Vickers, 590 350-390 320 365 -- --
Kg/mm.sup.2 Toughness, Mpa m.sup.1/2 55-60 -- 57 100 -- -- Fatigue
load for failure -- 310 520 240 -- 265 at 10.sup.7 cycles, MPa
Density, g/cc 5.9 8.5 4.4 8 -- -- Biocompatibility Good, Good/ Good
Good/ -- good initial question- question- evaluation able able
Magnetic Very Probably Ti is Austenitic is Human tissue Probably
Susceptibility compat- ferro- 182 .times. 3-6 .times. is -11 to
similar to ible magnetic 10.sup.-6 10.sup.-3 -7 .times. Ti CW is
10.sup.-6 ferro- magnetic *Bone modulus in GPa Cortical
Bone.about.15, Subchondral Bone.about.2, Trabecular Bone
.about.0.1
[0041]
2TABLE 2 Summary of potential health field application for bulk
metallic glasses Critical Property Application Unique BMG Property
Measurement Comments Fracture Fixation and Fusion Lower modulus,
high Fatigue, crack initiation at Some plates require plastic
Plates strength drill holes, corrosion, deformation to adjust fit
at biocompatability time of insertion which BMG could not
accommodate. Screws Toughness, high strength Fatigue, corrosion,
Less wear debris will lead to biocompatability less immune system
response. Hip joints-wear surface Low coefficient of friction,
Wear, corrosion, Less wear debris will lead to hardness
biocompatability less immune system response. Hip joints-shaft Low
modulus Corrosion, biocompatability Lower modulus distributes load
better Cutting tools, scalpel, bone Toughness, high elastic Edge
holding, susceptibility May have longer life than biopsy osteotome,
limit, hardness SS that quickly dull. endodontic files Aneurysm
clips High elastic limit, good MRI Creep, corrosion, Can do
emergency MRI signature biocompatability later without possible
fatal results Orthodontist Arch Wires High elastic limit, low
Processing to preform or Need to slide in mounts coefficient of
friction cast to proper shape (hard enough to be compatible with
ceramic brackets), maintain force over large elastic deformations
MRI interventional surgical Good MRI signature, Edge holding,
susceptibility A unique material for the instruments toughness,
hardness next generation of surgical care in an interventional
MRI
EXAMPLE I
[0042] Initial screening tests on BAM-11 performed at the
Biomaterials and Orthopedic Research Department at the University
of Mississippi Medical Center have shown biocompatibility
comparable to current implant materials. For initial
biocompatibility screening, two cell lines were selected:
microphage and fibroblast. Because these cell types are key in
inflammation and encapsulation processes they are generally
predictive of soft tissue biocompatibility. Four analyses of
biocompatibility were conducted for each cell type: 1) cellular
viability, 2) catalase activity, 3) TNF beta cytokine concentration
and 4) lactate dehydrogenase concentration. The results are
presented in Table 3.
3TABLE 3 Initial Biocompatability Tests of BAM-11 compared to two
control specimens* BAM-11 Ti, commercially pure Polyethylene
Macrophage Viability, 85 90 91 % Macrophage/Catalase 18 22 12
Activity/standardized activity/protein quan- tity
Macrophage/Lactate 6 8 6 Dehydrogenase, stan- dardized
activity/pro- tein quantity Macrophage/Cytokine, 19 18 11
picograms/protein quantity Fibroblast Viability, % 95 90 92
survival Fibroblasts/Catalase 9 5 5.5 Activity standardized
activity/protein quantity Fibroblast/Lactate 6.5 4 5 Dehydrogenase,
stan- dardized activity/ protein quantity
EXAMPLE II
[0043] Screening tests on BAM-11 specimens performed at the
Biomaterials and Orthopedic Research Department at the University
of Mississippi Medical Center have shown corrosion resistance
comparable to current implant materials. All specimens were wet
ground with SiC paper, 80, 240, 320, 600, and 1500 grit followed by
ultrasonic cleaning in distilled water for 5 min. The titanium was
additionally passivated in 40% HNO3 for 30 min according to an ASTM
standard. Cyclic polarization tests were conducted on triplicate
samples of the alloys in Ringer's solution (9.0 g/L-NaCl, 0.42
g/L-KCl, 0.25 g/L-CaCl.sub.2). Specimens were allowed to reach an
open-circuit potential (Ecorr) for a period of one hour. A
potential scan increasing at a rate of 0.1667 mV/s (ASTM G5) was
then initiated at 100 mV below Ecorr and continued until a current
threshold of 1.times.10-2A/cm2 was reached. At this point the scan
was reversed and decreased in the same rate until Ecorr was
reached. The results are presented in Table 4.
4TABLE 4 Initial corrosion tests of BAM-11 Alloy E.sub.corr (mV)
R.sub.br (mV) I.sub.corr (na/cm.sup.2) Titanium -51 .+-. 61.5 None
recorded 8.2 .+-. 3.4 316L SS -72.7 .+-. 20 323 .+-. 66.4 14.1 .+-.
6.7 BAM11 -228.3 .+-. 38.6 -65.3 .+-. 53.0 56.1 .+-. 32.8
[0044] While currently used surgically implantable orthopedic
appliances contain nickel, we have developed compositions that
eliminate nickel due to long-term biocompatibility concerns.
EXAMPLE III
[0045] Alloys with composition of Zr-32.5 Cu-5 Ti-10 Al (at. %)
were arc cast into a water-cooled copper mold and as cast were 99%
amorphous. FIG. 3 shows an MRI image of this new alloy compared to
a Cu alloy (FIG. 1) and the BAM-11 alloy (FIG. 2). It is evident
that without Ni an even better MRI image is obtained. This is
attributed to both removal of Nickel and the high percentage of
amorphous material.
[0046] It is believed that removal of the nickel is not necessary
for use of the BMGs as instruments and temporary fixation devices
such as immobilizing screws. Instruments such as bone biopsy tools,
scalpel blades, and the like have been fabricated by well-known,
conventional techniques such as casting, machining, laser welding,
grinding, and polishing.
[0047] Magnetic properties of BMGs are also of interest. One of the
first commercial uses of rapidly solidified amorphous metals was
the Fe--B based alloy for read-write heads and now transformer
cores. The BMGs also have been found to have unusual magnetic
properties including a soft magnetic alloy with zero
magnetostriction, a hard magnetic alloy with only 30% Fe that does
not saturate at 15 Tesla and has the highest ever measured
coercivity, 8.4T, although at liquid helium temperature. In
general, the magnetic susceptibility is related to density of
electronic states at the Fermi energy. Knowledge of the composition
dependence of the susceptibility would provide important input in
refining models of the amorphous state through first principles
calculations of the electronic structure and hence the Fermi energy
density of states. The preliminary result presented here concerning
a good MRI image was surprising considering the composition and
especially the presence of nickel in the alloy. Actual direct
measurements of the magnetic susceptibility by a squid magnetometer
yield 109.times.10-6 for BAM-11 versus 182.times.10-6 for Ti-6 Al-4
V agreeing with the MRI results.
[0048] Mechanical properties of the BMG described herein are
superior for many surgically implantable appliances and medical
instruments than currently used materials. These properties include
higher yield strength, higher elastic strain limit, lower Young's
modulus (better for reducing stress shielding), higher hardness,
and comparable toughness. For example, Because of these unique
mechanical properties, screws could have a thinner shank and deeper
threads yielding greater holding power. For example, FIG. 6 shows
of a typical conventional orthopedic bone screw design, while FIG.
7 shows an orthopedic bone screw of an improved design made
possible by the use of BMG in accordance with the present
invention.
[0049] BMG is uniquely suited for MRI compatible medical
instruments and surgically implantable devices because of minimal
generation of image distortion. Information readily available from
MRI images is critically helpful for many types of surgical
procedures. MRI images not only function to guide the surgeon's
tools to the location of the procedure along the least damaging
path, but also to differentiate and define tissue types for
facilitation of more efficient and complete procedures. BMG
constructed surgical tools allow for a host of procedures to be
performed in the MRI environment. BMG offers the best
MRI-compatible cutting instruments available. As such, procedures
involving the cutting of bone could be MRI-guided. Examples of such
procedures include craniotomy procedures involving tumors,
embolisms or strokes and orthopedic procedures involving femoral
avascular necrosis and vertebral fusions.
[0050] A broad range of other non-medical, even non-biological
applications is available to the skilled artisan. Remote viewing
and control of diagnostic apparatus and apparatus used to maintain
or repair any structure which is viewable using MRI technology is
enabled by using instruments and implants constructed of BMG, with
minimal invasion of the structure, minimal disturbance of the
system the structure functions within, and minimal disruption of
processes. Applications might include diagnosis, maintenance, and
repair of such structures as electronic structures, composite
structures used in aerospace, marine, and other endeavors, or
remotely disarming of hazardous structures such as bombs, mines,
and other explosives.
[0051] It should be noted that ferromagnetic metallic glasses are
not suitable for MRI imaging and are specifically excluded from the
scope of the present invention. See, for example, U.S. Pat. No.
5,976,274 issued on Nov. 2, 1999 to Inoue, et al. entitled "Soft
Magnetic Amorphous Alloy and High Hardness Amorphous Alloy and High
Hardness Tool Using the Same" and U.S. Pat. No. 4,653,500 issued on
Mar. 31, 1987 to Osada, et al. entitled "Electrocardiographic
amorphous alloy electrode".
[0052] While there has been shown and described what are at present
considered the preferred embodiments of the invention, it will be
obvious to those skilled in the art that various changes and
modifications can be made therein without departing from the scope
of the inventions defined by the appended claims.
* * * * *