U.S. patent application number 12/917273 was filed with the patent office on 2011-06-23 for self shortening fastener.
This patent application is currently assigned to DEGIMA, GmbH. Invention is credited to Randolf von Oepen.
Application Number | 20110147996 12/917273 |
Document ID | / |
Family ID | 35450011 |
Filed Date | 2011-06-23 |
United States Patent
Application |
20110147996 |
Kind Code |
A1 |
Oepen; Randolf von |
June 23, 2011 |
SELF SHORTENING FASTENER
Abstract
Disclosed is a fastener that can be mounted to a bone of a
patient and can foreshorten and swell of a desired period of time.
The fastener can include a head portion and a body portion
extending from the head portion. At least one of the head portion
and the body portion has a first width that changes to a second
width greater than the first width and collectively the head
portion and the body portion have a first length that changes to a
second length shorter than the first length upon the head portion
and the body portion being exposed to a temperature below a glass
transition temperature of a polymeric material forming the head
portion and the body portion.
Inventors: |
Oepen; Randolf von; (Los
Altos Hills, CA) |
Assignee: |
DEGIMA, GmbH
Pinneberg
DE
|
Family ID: |
35450011 |
Appl. No.: |
12/917273 |
Filed: |
November 1, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11146433 |
Jun 6, 2005 |
7824434 |
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12917273 |
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60577640 |
Jun 7, 2004 |
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Current U.S.
Class: |
264/327 ;
264/328.1 |
Current CPC
Class: |
A61B 17/866 20130101;
A61B 2017/00004 20130101 |
Class at
Publication: |
264/327 ;
264/328.1 |
International
Class: |
B29C 45/73 20060101
B29C045/73; B29C 45/00 20060101 B29C045/00 |
Claims
1. A method of manufacturing an implantable fastener, the method
comprising: injection molding a biocompatible polymeric composition
into a polymeric body within an injection mold cavity, wherein at
least a portion of said polymeric body is configured to be an
implantable fastener; and removing said polymeric body from said
injection mold, wherein said polymeric body has an amount of
polymeric molecules oriented in substantially a first direction so
that said polymeric body foreshortens and swells upon being
introduced into a moist environment having a temperature below a
glass transition temperature of the biocompatible polymeric
composition.
2. The method as recited in claim 1, wherein said injection molding
comprises passing said polymeric composition through an inlet in a
master mold that is configured to orientate macromolecules of the
biocompatible polymeric composition in substantially a first
direction.
3. The method as recited in claim 1, wherein said injection molding
comprises passing said polymeric composition through an inlet into
said injection mold cavity, said inlet imparting a shear stress to
said polymeric composition to orientate macromolecules of said
biocompatible polymeric composition in substantially a first
direction generally parallel to a longitudinal axis of said
polymeric body.
4. The method as recited in claim 3, wherein said inlet has a
cross-sectional length from about 10% to about 60% of an average
cross-sectional length of said injection mold cavity or a
runner.
5. The method as recited in claim 3, wherein said inlet has a
cross-sectional length from about 20% to about 50% of an average
cross-sectional length of said injection mold cavity or a
runner.
6. The method as recited in claim 3, wherein said inlet has a
cross-sectional length from about 30% to about 40% of an average
cross-sectional length of said injection mold cavity or a
runner.
7. The method as recited in claim 1, further comprising orienting
the polymeric molecules so that said implantable fastener
foreshortens greater than about 2% of an original dimension in 10
days when maintained in a fluid at 37 degrees Celsius.
8. The method as recited in claim 1, wherein said implantable
fastener is biodegradable within a fluid medium maintained at 37
degrees Celsius.
9. The method as recited in claim 1, selecting a master mold and
one or more sub-molds, each of said one or more sub-molds
comprising said injection mold cavity.
10. The method as recited in claim 1, wherein the biocompatible
polymeric composition is a biodegradable polymeric composition.
11. The method as recited in claim 10, wherein the biodegradable
polymeric composition is comprised of at least one polymer selected
from the group consisting of a poly(alpha-hydroxy esters),
polylactic acids, polylactides, poly-L-lactide, poly-DL-lactide,
poly-L-lactide-co-DL-lacti-de, polyglycolic acids, polyglycolide,
polylactic-co-glycolic acids, polyglycolide-co-lactide,
polyglycolide-co-DL-lactide, polyglycolide-co-L-lactide,
polyanhydrides, polyanhydride-co-imides, polyesters,
polyorthoesters, polycaprolactones, polyesters, polyanydrides,
polyphosphazenes, polyester amides, polyester urethanes,
polycarbonates, polytrimethylene carbonates,
polyglycolide-co-trimethylen-e carbonates, poly(PBA-carbonates),
polyfumarates, polypropylene fumarate, poly(p-dioxanone),
polyhydroxyalkanoates, polyamino acids, poly-L-tyrosines,
poly(beta-hydroxybutyrate), polyhydroxybutyrate-hydroxyvaler-is
acids, high-density polyethylenes, ultra-high-density
polyethylenes, low-density polyethylenes, polypropylenes,
polyacrylates, polymethylmethacrylates, polyethylmethacrylates,
polysulfones, polyetheretherketones, polytetrafluoroethylenes,
polyurethanes, polystyrenes, polystyrene-co-butadienes, and
combinations thereof.
12. The method as in claim 11, wherein the biodegradable polymeric
composition is comprised of at least one polymer selected from the
group consisting of polylactides, poly-L-lactide, poly-DL-lactide,
poly-L-lactide-co-DL-lactide, polyglycolic acids, polyglycolide,
polylactic-co-glycolic acids, polyglycolide-lactide,
polyglycolide-co-DL-lactide, polyglycolide-co-L-lactide, and
combinations thereof.
13. The method as recited in claim 1, further comprising at least
one of: mixing said polymeric composition in a mixer; extruding
said polymeric composition as a thermoplastic extrudate; heating
said polymeric composition before being introduced into said
injection mold cavity; introducing said polymeric composition into
said injection mold cavity under pressure; cooling said polymeric
body in said injection mold cavity; separating said implantable
fastener form said polymeric body; or finishing said implantable
fastener.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This patent application is a divisional of U.S. patent
application Ser. No. 11/146,433 filed Jun. 6, 2005 and entitled
"SELF FORESHORTENING FASTENER," which claims the benefit of and
priority to U.S. Provisional Patent Application Ser. No. 60/577,640
filed Jun. 7, 2004, and entitled "Self Foreshortening Screw", with
Randolf Von Oepen as inventor, the disclosures of which are
incorporated in their entireties herein by reference. Additionally,
this United States patent application cross-references other United
States patent applications filed simultaneously with U.S. patent
application Ser. No. 11/146,433 on Jun. 6, 2005, entitled "Fastener
Having Torque Optimized Head" with Randolf Von Oepen as inventor,
U.S. patent application Ser. No. 11/145,692, and "Polymeric Plate
Bendable Without Thermal Energy and Methods of Manufacture" with
Randolf Von Oepen and Alexander Tschakaloff as inventors, U.S.
patent application Ser. No. 11/146,454. The disclosure of each of
the foregoing cross-referenced United States patent applications is
incorporated herein by reference in its entirety.
BACKGROUND OF THE INVENTION
[0002] 1. The Field of the Invention
[0003] This application relates generally to fasteners. More
specifically, the present invention relates to a medical fastener
that can shorten in length and increase in width over time.
[0004] 2. The Relevant Technology
[0005] Bones are a vital skeletal feature and provide the frame and
structural support for holding associated muscles and other tissue.
Additionally, bones, such as the skull bones and ribs, are
responsible for protecting vital organs such as the brain, heart
lungs, and the like. While bones are structurally strong, they tend
to break for various reasons when subjected to excessive forces.
Usually, the healing process includes a medical professional
aligning the bones on each side of the break so that the
regenerated bone material provides a structurally sound mended
bone.
[0006] In addition to aligning the bone, various stabilizing
techniques have been used to retain the broken bone in proper
alignment during the healing process. Traditionally, casts have
been used to stabilize minor breaks that do not need structural
reinforcement at the bone. On the other hand, some complicated
fractures or breaks can be susceptible to falling out of alignment
during the healing process. As such, plates and fasteners can be
used to stabilize the broken bones or fix bone structures. Use of
these kinds of structural reinforcement systems during healing have
been known to provide bone regeneration and mending.
[0007] Due to excellent strength and stability profiles, metallic
fasteners and plates have dominated the market for reinforcing
breaks or fractures during healing. The most accepted metallic
fasteners and plates are biocompatible titanium and/or titanium
alloys; however, other types of metallic materials have also been
used. Nevertheless, metallic fasteners and plates can be
problematic and have some disadvantages
[0008] One disadvantage of implanted metallic fasteners and plates
arises from being treated as a foreign body, which sometimes
requires the fasteners and plates to be removed. This can occur
even if the metallic fastener and plate system is initially well
tolerated. As such, the subsequent surgery to remove the metallic
fastener and plate system can cause additional trauma to the
patient, and adds additional costs to the health care system;
especially when the patient has to be hospitalized after the
procedure. Additionally, if the metallic fastener and plate system
includes an iron component, the irons released from the metallic
implant may be found in other organs, which can cause long-term
problems.
[0009] Another major disadvantage of metallic fastener and plate
systems arises from being much stronger than the bone being
supported. As such, a broken bone that is fixed with a metallic
fastener and plate system may not experience proper loading during
the healing process. This is because the metallic repair system can
carry a large portion of the load that is normally carried by the
bone. As a result, the bone can become weaker over time when the
metallic repair system is left in place. Accordingly, after removal
of the metallic repair system, the repaired bone may be susceptible
to fracturing around the region that was previously supported. Even
though the metallic repair system provides structural reinforcement
to the healing bone, the bone may develop decreased stability.
[0010] Additional problems arise because bone is a living
structure. When a metallic fastener is drilled into bone, the
compact pressure that results, for example, on a plate, is very
high and leads to very good initial stability. Under the stress
exerted by the fastener and the plate, the bone will change its
structure and the fastener may loosen over time. This causes
significant problems with maintaining bone alignment during bone
regeneration because the plate can move as the fastener
loosens.
BRIEF SUMMARY OF THE INVENTION
[0011] The present invention overcomes the problems described above
by providing a fastener that mounts securely within bone over time.
The present invention relates to a biodegradable polymer fastener
that foreshortens in length and swells in diameter over time to
securely mount within bone.
[0012] In one configuration, the fastener includes a head portion
and a body portion extending from the head portion. The head
portion and the body portion collectively have a first length that
changes to a second length shorter than the first length upon the
head portion and the body portion being exposed to a body
temperature below a glass transition temperature of a polymeric
material forming the head portion and the body portion.
Alternatively, the polymeric material can have a glass transition
temperature that is lower than normal body temperature, but a glass
transition temperature higher than the normal body temperature can
provide increased stability. The shortening in length can occur
before the fastener begins to degrade. Therefore, the width of at
least one of the head portion and the body portion can increases in
diameter or swell upon the head portion and the body portion being
exposed to the temperature below the glass transition temperature
of the polymeric material.
[0013] According to one aspect, the glass transition temperature is
higher than a temperature of a patient's body. For instance, the
glass transition temperature can be between about 37 degrees
Celsius and about 60 degrees Celsius.
[0014] According to another aspect, the polymeric materials can be
biodegradable and have a polymer molecule orientation so that the
increase in width of at least one of the head portion and the body
portion is greater than about 2% after the head portion and said
the body portion are immersed within a fluid maintained at about 37
degrees Celsius for 10 days. In other configurations, the increase
in width can be from about 3% to about 6% or from about 4% to about
5%.
[0015] According to another aspect, the polymeric material forming
the fastener can have a polymer molecule orientation that includes
less than about 40% of the polymer molecules being oriented in
substantially one direction. In another configuration, about 10% to
about 30% or the polymer molecules can be oriented in substantially
one direction, or about 15% to 25% oriented in substantially one
direction.
[0016] In another configuration, a method of manufacturing an
implantable fastener is provided. The method can include injection
molding a biocompatible polymeric composition into a polymeric body
within an injection mold cavity of an injection mold. At least a
portion of the polymeric body is configured to be an implantable
fastener. Following injection molding, the method can include
removing the polymeric body from the injection mold, wherein the
polymeric body has an amount of polymeric molecules oriented in
substantially a first direction so that the polymeric body
foreshortens and swells at a temperature below a glass transition
temperature of the biocompatible polymeric composition.
[0017] The method can also include passing the polymeric
composition through an inlet in a master mold that is configured to
orientate macromolecules of the biocompatible polymeric composition
in substantially a first direction. The inlet can impart a shear
stress to the polymeric composition to orientate macromolecules of
the biocompatible polymeric composition in substantially the first
direction generally parallel to a longitudinal axis of the
polymeric body. The inlet can have a cross-sectional length
(diameter) from about 10% to about 60% of an average
cross-sectional length (diameter) of said injection mold cavity or
runner feeding the inlet. In other configurations the inlet
diameter or cross-sectional length can range from about 20% to
about 50% or from about 30% to about 40% of the mold cavity average
cross-sectional length (diameter) or runner cross-sectional length
(diameter). As used herein, the term "cross-sectional length" is
meant to refer to the diameter of a circular cross-sectional area
or width of polygonal cross-sectional area.
[0018] According to another aspect, the injection mold can include
a master mold and one or more sub-molds. Each of the one or more
sub-molds can include the injection mold cavity and the type of
sub-mold used with the master mold can be varied based upon the
number and type of fastener to be made.
[0019] According to another aspect, the method can further include
a least one of (i) mixing the polymeric composition in a mixer,
(ii) extruding the polymeric composition as a thermoplastic
extrudate, (iii) heating the polymeric composition before being
introduced into the injection mold cavity, (iv) introducing the
polymeric composition into the injection mold cavity under
pressure, (v) cooling the polymeric body in the injection mold
cavity, (vi) separating the implantable fastener form the polymeric
body, or (vii) finishing the implantable fastener.
[0020] These and other objects and features of the present
invention will become more fully apparent from the following
description and appended claims, or may be learned by the practice
of the invention as set forth hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] To further clarify the above and other advantages and
features of the present invention, a more particular description of
the invention will be rendered by reference to specific embodiments
thereof which are illustrated in the appended drawings. It is
appreciated that these drawings depict only typical embodiments of
the invention and are therefore not to be considered limiting of
its scope. The invention will be described and explained with
additional specificity and detail through the use of the
accompanying drawings in which:
[0022] FIG. 1 illustrates a perspective view of a fastener
according to the present invention;
[0023] FIG. 2 schematically illustrates one process for
manufacturing the fastener according to the present invention;
[0024] FIG. 3 illustrates one configuration of a master mold usable
with the process for manufacturing the fastener of FIG. 2; and
[0025] FIG. 4 schematically represents the illustrative methods
steps for manufacturing the fastener according to the present
invention.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0026] The present invention generally relates to a fastener that
mounts tightly and securely within bone as the bone changes its
structure under the stress applied by the fastener. The present
invention also relates to a fastener made of a biodegradable
polymer that shortens in length and swells in diameter to maintain
a tight and secure fit within bone during the time when the bone
regenerates and degrades following repair of the bone.
[0027] Turning to FIG. 1, illustrated is a fastener 10 according to
one aspect of the present invention. The fastener 10, such as a
screw, pin, bone anchor, suture material, bone pins, meniscus
repair systems, clamps or the like, can be used during a medical
procedure to aid with positioning bone or fixing other medical
devices to patient bone. A driver (not shown) can be used either
alone or in combination with an electric drill or other device for
rotating the driver to mount the fastener 10 into the bone.
Additional information regarding the driver and the manner of
mounting the fastener 10 into the bone can be found in co-pending
U.S. patent application Ser. No. ______, entitled "Fastener Having
Torque Optimized Head", filed Jun. 6, 2005 (Attorney Docket No.
16406.3.1), which is incorporated herein by this reference.
[0028] The illustrated fastener 10 can include a fastener head or
head portion 12 and a body portion 14 extending from the head
portion 12. The head portion 12 can include a recess 16 to receive
the driver (not shown), while a thread 18, such as a raised helical
rib, winds around the body portion 14 and can mount or engage with
a patient's bone or tissue when the fastener 10 is driven into the
bone or tissue. Since the head portion 12 has a diameter greater
than the body portion 14, the head portion 12 prevents excessive
mounting of the fastener 10 to the bone of a patient, i.e., the
head portion 12 prevents the fastener 10 from being driven too
deeply into the bone or passing through a mounting hole in a plate
mountable to the patient's bone.
[0029] With continuing reference to FIG. 1, the head portion 12 can
have a curved portion 22 and a generally tapered portion 24. It
will be understood, however, that each of the curved portion 22 and
the tapered portion 24 can have other configurations. For example
either or both of the curved portion 22 and the tapered portion 24
can be planar. Similarly, although the head portion 12 is
illustrated as having a generally circular peripheral edge, one
skilled in the art can appreciate that the peripheral edge can be
polygonal, oval, or any other configuration.
[0030] Similarly, while the body portion 14 is illustrated as
having a generally uniform cross-section along its length, it will
be understood that the body portion 14 can have a tapered
configuration or some other configuration so long as the body
portion 14 can engage with the patient's bone or other structure
within which the fastener 10 is driven. In additional, it will be
understood that the head portion 12 and the body portion 14 can
have various other configuration that are typically associated with
a screw and more generally a threaded fastener, i.e., a fastener
including one or more threads to aid in mounting the fastener to a
structure.
[0031] The thread 18 of the body portion 14 can extend from a first
end 30 toward a second tapered end 32. Alternatively, the body
portion 14 can extend from a first end 30 to second end 32, wherein
the first end and/or the second end can be of a constant dimension
or tapered. Interrupting the thread 18 can be a channel 34 having
an open end at the second tapered end 32. This channel 34 provides
clearance at the end of the fastener 10 to collect particles, such
as scale of bone, particles and blood, which are within a hole,
optionally tapped, receiving the fastener 10. By collecting the
scale of bone, particles and blood, the channel 34 eliminates the
possibility that scale of bone, particles and blood can press
between the fastener 10 and the hole's wall or threads and prevent
the fastener or screw from being driven into the hole. Including
the channel 34 can reduce the frictional contact between the
fastener 10 and the hole's wall or threads, thereby making it
easier to mount the fastener 10 to the patient's bone.
[0032] Although the description of the present invention will be
directed generally to the shortening and swelling of the fastener
10 for medical procedures, it will be understood by those skilled
in the art that the shortening and swelling features of the present
invention can apply to other situations and other types of
fasteners. Consequently, the presently described invention may be
used in other situations outside the medical arts and other types
of fasteners.
[0033] To alleviate many of the problems associated with existing
metallic fasteners, such as the loosening of a metallic fastener in
bone over time, the above-described fastener 10 can be manufactured
from a biodegradable polymer that foreshortens and swells in
diameter over time to maintain a tight and secure fit within bone.
The fastener 10 increases in diameter over time to tightly mount to
the bone as the stressed bone changes its structure. The fastener
10 can, therefore, maintain desired stability for a period of time
longer than existing metallic fasteners or screws. Optionally, the
fastener 10 can be manufactured from a polymeric composition
comprised of biodegradable, inert, and/or natural polymers.
[0034] To achieve the desired foreshortening and swelling, the
fastener 10 can be fabricated from a polymer having a high degree
of polymer chain orientation, where polymer chain orientation
describes the amount of polymer macromolecules that are aligned in
one direction. This type of polymer can be referred to as a high
orientation polymer. With the polymer being highly orientated, the
fastener 10 would be expected to maintain its dimensions when the
fastener 10 is exposed to temperatures below its glass transition
temperature, or the temperature above which the polymer changes
from a hard or brittle condition to a flexible and elastomeric
condition. It has been found, however, that when manufactured from
a highly orientated polymer the fastener 10 relaxes at temperatures
of approximately 37 degrees Celsius (the typically temperature
within a patient's body), even if the glass transition temperature
is much greater, such as above 50 degrees Celsius or between about
45 degrees Celsius to about 60 degrees Celsius. This relaxing
causes the fastener 10 to shorten or shrink in the direction of the
orientation of the polymer macromolecules. Aligning the polymer
macromolecules with the longitudinal axis of the fastener 10
results in the fastener 10 shortening or shrinking in length. Since
the total volume of the fastener 10 remains the same, at least
before the fastener 10 begins to biodegrade, the fastener 10 swells
as it shortens or shrinks This swelling maintains the fastener 10
securely within the bone as the bone changes structure under the
stress applied by the fastener 10.
[0035] The degree of shortening and swelling can be chosen by
selecting particular materials and the process used to create the
fastener 10. For instance, the fastener 10 can be configured and
fabricated to have less than about 6% or 3% shrinkage in length
over a period of 10 days after being placed in a 37.degree. C.
bath. Alternatively, the fastener 10 can be configured to shrink
greater than 2% in length under the same conditions. Moreover, the
fastener can swell from about 0.5% to about 10% of an original
dimension under the same conditions. It will be understood that
other degrees of shrinkage and/or swelling can be appropriate and
higher or lower than the identified 3% and 10%.
[0036] As described, the shrinkage and/or swelling of the
structural element after implantation in a subject can be highly
depended on the method of manufacturing, but also of the material.
It has been observed that pure amorphous materials have the
tendency to shrink more than materials which or by its nature
crystalline or semi-crystalline. Thus, amorphous materials can have
shrinkage from about 5% to about 50%, or from about 10% to about
30% in other configurations.
[0037] Additionally, crystalline or semi-crystalline materials can
have a shrink in length from about 0.5% to about 15%, or about 1%
to about 10% in other configurations. Accordingly, the swelling in
width can be from about 0.5% to about 20%, or from about 1% to
about 15% in other configurations. Also, it should be understood
that the swelling and foreshortening do not have to be uniform over
the width or length of the screw or fastener. Moreover, by varying
the dimensions of the fastener (i.e., tapering) the shrinkage and
swelling can be adjusted accordingly.
[0038] Various types of polymers can be employed in preparing the
fastener 10 in accordance with the present invention. The polymers
can include a wide range of biocompatible materials that can be
implanted within body of a living animal, such as a human, dog,
cat, horse, cow, and the like. Additionally, the polymers can be
combined and blended in order to achieve compositions that have
high initial strengths, shortens and widens over time, and can
degrade within a living body over time.
[0039] In one embodiment, a polymer composition for use in
injection molding the fastener 10 can include at least one
biodegradable polymer. For example, the biodegradable polymer
composition can include at least one of poly(alpha-hydroxy esters),
polylactic acids, polylactides, poly-L-lactide, poly-DL-lactide,
poly-L-lactide-co-DL-lactide, polyglycolic acids, polyglycolide,
polylactic-co-glycolic acids, polyglycolide-co-lactide,
polyglycolide-co-DL-lactide, polyglycolide-co-L-lactide,
polyanhydrides, polyanhydride-co-imides, polyesters,
polyorthoesters, polycaprolactones, polyesters, polyanydrides,
polyphosphazenes, polyester amides, polyester urethanes,
polycarbonates, polytrimethylene carbonates,
polyglycolide-co-trimethylene carbonates, poly(PBA-carbonates),
polyfumarates, polypropylene fumarate, poly(p-dioxanone),
polyhydroxyalkanoates, polyamino acids, poly-L-tyrosines,
poly(beta-hydroxybutyrate), polyhydroxybutyrate-hydroxyvaleric
acids, combinations thereof, or the like. Additionally, these
polymers can be used at a wide range of molecular weights, which
can range from less than about 25,000 MW to over 1,000,000 MW. More
particularly, the molecular weight can vary depending on the type
of polymer, initial strength, shortening and swelling rate,
degradation rate, and the like. Additional information on the
tensile strength, tensile modulus, flexural modulus, and
elongations at yield and at break of various biocompatible and
biodegradable polymers can be found with Engelberg and Kohn;
Physico-mechanical Properties of Degradable Polymers Used in
Medical Applications: A Comparative Study; Biomaterials; 1991;
12:292-304, which is incorporated herein by reference.
[0040] In one embodiment, a polymer composition for use in
injection molding the fastener can include at least one inert
polymer. For example, the inert polymer can include at least one of
high-density polyethylenes, ultra-high-density polyethylenes,
low-density polyethylenes, polypropylenes, polyacrylates,
polymethylmethacrylates, polyethylmethacrylates, polysulfones,
polyetheretherketones, polytetrafluoroethylenes, polyurethanes,
polystyrenes, polystyrene-co-butadienes, epoxies, and the like.
Such inert polymers can be used at a wide range of molecular
weights in order to impart various mechanical strengths and
shortening and swelling rate to the fastener 10.
[0041] In one embodiment, the polymer composition for use in
injection molding a biocompatible fastener 10 can include at least
one natural polymer that can be derived from a natural source.
Natural polymers can include polysaccharides, proteins, and the
like. Examples of some suitable polysaccharides include
methylhydroxyethylcellulose, hydroxymethylethylcellulose,
carboxymethylcellulose, methylcellulose, ethylcellulose,
hydroxyethylcellulose, hydroxyethylpropylcellulose, amylopectin,
amylose, seagel, starches, starch acetates, starch hydroxyethyl
ethers, ionic starches, long-chain alkylstarches, dextrins, amine
starches, phosphate starches, and dialdehyde starches, alginic
acid, phycocolloids, agar, gum arabic, guar gum, locust bean gum,
gum karaya, gum tragacanth, and the like. Examples of some
proteinaceous materials include collagens, caseins, and the like.
Moreover, these natural polymers can also impart biodegradable
characteristics to the fastener 10.
[0042] In one embodiment, the biodegradable polymers can be
reinforced with fibers comprised of magnesium, wherein such fibers
can significant strength the fastener. For example, short fibers,
which are added to the polymer during the injection molding
process, can be oriented in the direction of the flow so as to
significantly improve the mechanical properties. Additionally, the
magnesium fibers can be pretreated with corona plasma or other
well-known method to improve the interface between the polymers and
fiber. Since pure magnesium can be highly reactive with water or
body fluids, the polymer matrix can act as a shield and protect
against fast degradation and magnesium reactions. It can also be
understood that optionally the fastener can be formed completely
from magnesium and subsequently coated with a polymer coating to
shield and protect against fast degradation.
[0043] In one embodiment, short fibers of biodegradable micro or
nano-porous silicon materials, biodegradable ceramics, organic
materials can be added to the polymer and fastener. The short
fibers, which are added to the polymer during the injection molding
process, can be oriented in the direction of the flow and
significantly improve the mechanical properties of the resulting
fastener. Optionally, these degradable fibers can be pretreated
with corona plasma or other well-known method to improve the
interface between the polymer matrix and the fiber. Also, the rate
of fiber biodegradation can be slowed by being encapsulated within
the polymer matrix.
[0044] The addition of fibers into the fastener can improve many of
the mechanical and/or strength characteristics. In part, this can
arise from the nature of the fibers, and/or being oriented with the
polymer molecules. For example, the fibers can increase the Young's
modulus and increase the strength.
[0045] In another embodiment, the fastener can be fabricated from
magnesium, biodegradable micro or nano-porous silicon materials,
biodegradable ceramics, or organic materials. Optionally, the
fastener made from one or more of these materials and ceramics can
be coated or covered with a polymer or polymer matrix.
[0046] In yet another embodiment, the biodegradable polymers of the
fastener can be admixed with a drug for being delivered into the
body after implantation. This can include mixing a drug into the
polymer composition before being injection molded, or applying a
drug-containing polymeric coating onto the fastener. In any event,
a portion of the fastener, either the bulk biodegradable polymer or
a biodegradable coating can be configured to deliver drugs into the
body after being implanted. Accordingly, any drug can be included
into the fastener, including but not limited to, analgesics,
anti-inflammatory, anti-microbial, and like drugs.
[0047] In one embodiment, the biodegradable polymers, inert
polymers, natural polymers, magnesium fibers, and/or porous silicon
fibers can be prepared into a polymeric blend that is comprised of
different types of polymers and materials. As such, a polymeric
blend can be configured to achieve injection moldability, polymer
molecule orientation, high initial strength, and desired shortening
and swelling rates. Moreover, the biodegradable polymers and/or
natural polymers can be blended in order to achieve the fastener 10
that can degrade over time after being implanted.
[0048] In order to achieve the desired swelling and foreshortening,
the polymer molecules can be oriented within the fastener 10 to
have a desired amount of orientation. For example, when the polymer
is biodegradable, such orientation includes less than about 40% of
the polymer molecules oriented in substantially one direction. In
other configurations, the orientation can be from about 10% to
about 30% oriented in substantially one direction, or from about
15% to 25% oriented in one direction. However, when other polymers
or additives are included, variations in the amount of orientation
can be achieved and still retain the foreshortening
characteristic.
[0049] In one embodiment, it can be preferred to prepare the
fastener with a biodegradable polymer and another material such as
an inert polymer, natural polymer, magnesium fiber, and/or silicon
fiber. In one aspect, this can be beneficial to allow
biodegradability over time and still retain some structural support
after the degradable portion has been depleted. As such, this can
be favorable for complex bone reconstructions that may need some
long-term support. That is, an initially high amount of support can
be provided that decreases over time until a final amount of
support is obtained, which allows the bone to reform and strengthen
as the biodegradable portion is depleted. Alternatively, additional
biodegradable materials can enhance the biodegradability of the
fastener. For example, the biodegradable polymer to other material
ratio can range from about 10 to about 1, from about 8 to about 4
in other configurations, from about 6 to about 4 in yet other
configurations, and vice versa depending on the characteristic
desired.
[0050] Moreover, an embodiment of the fastener 10 can be configured
to shrink in a water bath maintained at about 37.degree. C. As
such, the fastener can be configured to have a dimension, such as
length, that foreshortens greater than about 1% of its original
dimension in a period of 10 days, greater than about 2%, or greater
than 4% in other configurations. Additionally, the fastener 10 can
be configured to swell in width to be greater than about 2% of its
original dimension in a period of 10 days, greater than about 3%,
or greater than 5% in other configurations.
[0051] In one embodiment, a fabrication system and process can be
employed to prepare the fastener 10 (FIG. 1) having features in
accordance with the present invention, i.e., impart the desired
amount of polymer molecule orientation. Such a fabrication system
can include the use of an injection mold configured to prepare the
fastener 10 (FIG. 1) having the characteristics described herein.
The injection molding process can impart a shear stress to the
polymer molecules that results in the desired amount of
orientation, which is usually in the direction of the flow within a
mold of the injection molding system. The process of injection
molding is a controllable process that results in the fastener 10
(FIG. 1) having the directional molecular orientation to achieve
the desired strength and flexibility so that it will not break,
fracture, or fatigue during use and will shorten and swell when
mounted within bone.
[0052] Common elements of an injection molding system can include,
but not limited to, the runners, runner network, flow dividers,
cold wells, gate regions, gates, and a mold having a mold cavity.
By varying the configuration of each of these and manipulating or
changing the mold cavity orientation, vents, mold temperature,
polymer composition and temperature, and flow or injection rates of
an injection mold system the desired amount of polymer molecule
orientation can be obtained. For instance, the gate or injection
port within the injection mold can be adapted to orient the
molecules by the shear stresses that are imparted to the polymeric
melt when the injection mold cavity is being filled. A smaller gate
can provide a high shear stress and result in high polymer molecule
orientation. A cross-sectional length, such as a diameter, of this
small gate can be from about 10% to about 60% of the cavity average
cross-sectional length (diameter) or runner cross-sectional length
(diameter) to provide optimal polymer orientation. In other
configurations, the cross-sectional length of the small gate can be
from about 12% to about 25%, or from about 15% to about 20%. In
still other configurations the small gate diameter or
cross-sectional length can range from about 20% to about 50% or
from about 30% to about 40% of the mold cavity average
cross-sectional length (diameter) or runner cross-sectional length
(diameter).
[0053] Through optimizing the level of polymeric molecule
orientation, the injection molding system can impart mechanical
stability to the fastener 10 (FIG. 1), while creating the
properties that allow the fastener 10 (FIG. 1) to foreshorten and
swell overtime. By optimizing the injection molding conditions
(e.g., polymer composition, mold configuration, polymer melt
temperature, flow rates, gate configuration, shear stress, etc.),
the process can provide the fastener that when implanted in a
patient will securely mount to the bone of a patient as it shortens
in length and swells in width during the time that the bone changes
its structure because of the stress applied by the fastener 10
(FIG. 1).
[0054] Turning to FIG. 2, is a schematic diagram illustrating an
embodiment of the injection molding system 50 in accordance with
the present invention. In general, the injection molding system 50
can be configured to yield an implantable fastener to secure bone
or mount a plate to structurally reinforce bone. The injection
molding system 50 can include a mixer 52 configured to receive
polymeric materials, such as biodegradable and/or inert polymeric
materials, in order to form a substantially homogenous polymeric
composition. Additionally, the mixer 52 can be configured to
receive other types of polymeric materials, plasticizers,
rheology-modifying agents, fillers, and the like in order to
provide various other characteristics to the fastener 10 (FIG. 1)
fabricated with the injection molding system 50.
[0055] Optionally, the injection molding system 50 can include an
extruder 54. As such, the polymeric composition mixed and
formulated within the mixer 52 can be supplied into the extruder 54
for further mixing, compacting, heating, and/or extruding. The
extruder 54 can be a single screw extruder, double screw extruder,
or piston-type extruder. Additionally, the extruder 54 can include
heating elements in order to take advantage of the thermoplastic
characteristics of some embodiments of the polymeric composition
and heat the composition past its softening point, melting point,
and/or glass-transition temperature. In any event, the extruder 54
can extrude the composition through a die head to an extrudate of
any shape, which can optionally be pelleted before injection
molding.
[0056] After being extruded from the extruder 54 or mixed within
the mixer 52, the polymeric composition can be introduced into a
pre-mold 60. The pre-mold 60 can be a compartment, container, tube,
conduit, injection line, or the like in fluid communication with
the injection mold 62 that can hold the polymeric material before
being injection molded. Alternatively, the composition can be
provided directly into the injection mold 62 from the extruder 54
or mixer 52.
[0057] Additionally, a dryer 16 can dry the polymeric material
while in the pre-mold 20. Sometime the polymeric material can
absorb moisture during processing, wherein the moisture can be
counter-effective to a resulting plate; especially when a
biodegradable polymer, which can cause the plate to prematurely
degrade. As such, the dryer 16 can be configured to remove moisture
from the polymeric material.
[0058] Additionally, a pressurizer 18 can pressurize the pre-mold
20 so that polymeric composition can be injected into the injection
mold 22 under high pressure. For example, the injection molding
process can be performed at about 10 atm to about 2500 atm or from
about 100 atm to about 1500 atm.
[0059] The heater 56 and the pressurizer 58 may be optional because
the pre-mold 60 and/or the injection mold 62 may be outfitted with
such components or otherwise provide these functionalities. In
addition to the above, other well-known injection molding equipment
may be utilized in conjunction with the pre-mold 60 so as to
prepare the polymeric composition for injection molding.
[0060] Following heating in the pre-mold 60, the polymeric
composition can be injected into the injection mold 62 in order to
form the fastener 10 (FIG. 1). For instance, the polymeric
composition can be injected into a master mold within a cavity of
the injection mold 62, the master mold having one or more cavities
that define the configuration of the fastener 10 (FIG. 1). Usually,
the process includes injecting the polymeric composition under high
pressure and/or heat so that the composition can flow through the
various pathways and compartments within the injection mold 62
until it reaches the cavities that define the fastener 10 (FIG.
1).
[0061] In the described configuration of the injection molding
system 50, the master mold of the injection mold 62 can be cooled
by water, air, or fluid flowing through conduits in the master mold
or other portions of the injection molding system 50. In this
manner, the polymeric composition can be cooled quickly following
injection into the master mold so that the orientation of the
polymer macromolecules can be fixed and prevented from changing
during the molding process.
[0062] While general features of the injection molding system 50
have been described in connection with injection molding, various
other processes or techniques can be utilized in order to prepare
the fastener 10 (FIG. 1) in accordance with the present
invention.
[0063] Turning to FIG. 3, illustrated is one half of a master mold,
identified by reference numeral 70. The following discussion will
be directed to the illustrated half, but it will be understood that
the discussion also applies to the other half of the master mold
since it will have generally a similar or complementary
configuration and function. As shown, the master mold 70 has a
generally octagonal configuration with a cold runner 74 disposed at
its center 72. Spaced around the center 72 are individual sub-molds
76a-76h that define the configuration of the fastener 10 to be
formed using the injection molding system 50 (FIG. 3). Each
sub-mold 76a-76h can include a cavity 78a-78h that defines the
particular configurations of the fastener moldable using that
particular sub-mold. Although the illustrated master mold 70 is
illustrated as having eight different sub-molds 76a-76h, it will be
understood that in other configurations any combination of
sub-molds 76a-76h are possible. For instance, the master mold 70
can include eight or less than eight of any one of the sub-molds
76a-76h.
[0064] The following discussion will be directed to the sub-mold
76a. However, a similar discussion can be made with each of the
other sub-molds 76b-76h. As illustrated, the sub-mold 76a includes
the cavity 78a that communicates with the cold runner 74 by way of
a channel 80. A tapered end 82 of the channel 80 provides the inlet
to the cavity 78a for the polymer composition. This tapered end 82
acts as a gate having a small diameter to provide a high shear
stress to the polymer composition that results in high polymer
molecule orientation. Depending upon the particular configuration
of the fastener, the tapered end 82 can have a cross-sectional
length (diameter) of from about 5% to about 30% of the cavity
average cross-sectional length (diameter) or runner cross-sectional
length (diameter) to provide optimal polymer orientation. In other
configurations, the cross-sectional length (diameter) of the
tapered end 82 can be from about 10% to about 25% or from about 15%
to about 20% of the cavity average cross-sectional length
(diameter) or runner cross-sectional length (diameter). More
generally, the cross-sectional area of the tapered end 82 can be
sufficiently sized to highly orientate the polymer macromolecules,
i.e., apply the level of shear stress to the polymer composition to
achieve high polymer molecular orientation. Examples of some
cross-sectional shapes for the tapered end 82 can include circles,
rectangles, squares, octagons, pentagons, and the like, wherein
various polygons can be used in order to provide the proper polymer
molecule orientation.
[0065] Since the master mold 70 is cooled by water, air, or fluid
surrounding and/or passing through conduits in the master mold 70,
the high polymer molecule orientation is fixed or frozen during the
injection molding process. This results in the fasteners formed by
the master mold 70 having the desired high polymer molecule
orientation that causes the shorting and swelling of the fastener
in the patient's body and below the glass transition
temperature.
[0066] It will be understood that the injection molding system 50
(FIG. 3) described herein is only one example of a possible
injection molding system that can be used to form the fasteners of
the present invention. It can be understood that various other
types of injection molding system can be used. For instance, the
injection molding system 50 (FIG. 3) can include a two body, three
body, or multi-body mold, and can be operated with cold or hot
runners. Additionally, various modifications can be made to the
exemplary mold described herein.
[0067] The present invention can also relate to a method of
manufacturing the fastener having the features described in
accordance with the present invention. Such a method of
manufacturing can employ the foregoing compositions, equipment,
systems, and processes as previously described. An exemplary method
of manufacturing is described in more detail below.
[0068] FIG. 4 illustrates an embodiment of an implantable fastener
fabrication method 100. Such a fastener fabrication method 100 can
include and utilize any of the equipment, components, and processes
described in connection to FIG. 1 through FIG. 3 and otherwise know
to those in the injection molding art. Accordingly, the fastener
fabrication method 100 includes preparing a polymer to have the
thermoplastic characteristics and resulting fastener strength and
flexibility profiles as described above, as represented by block
102. By preparing the polymer composition to have the proper
components and concentrations, the implantable fastener can be
prepared to have the preferred foreshortening, swelling, and
structural features.
[0069] In one embodiment, the polymer composition can be extruded,
as represented by block 104. Extruding the polymer composition can
be beneficial in order to provide the proper configuration,
consistency, temperature, and the like before injection molding.
This can include further mixing and/or compaction of the polymeric
materials as well as heating the polymer past its softening point,
melting point, and/or glass transition temperature.
[0070] In any event, the polymer can be supplied into a polymer
pre-mold, as represented by block 106. Within the pre-mold, the
polymer composition can be pressurized so as to have the proper
pressure for being injected into the injection mold, as represented
by block 108. Additionally, the polymer composition can be dried in
the pre-mold to remove moisture as needed, as represented by block
110.
[0071] After being properly conditioned, the polymer composition
can be introduced into the injection mold, such as the master mold,
for injection molding, as represented by block 112. The polymer
molecules in the composition can be oriented by the tapered end 82
(FIG. 3) of the channel 80 leading to the cavity 78 of the master
mold 70 to achieve a desired amount of orientation, as represented
by block 114. The injection molded body can then be removed from
the mold and cooled, as represented by block 116, and the fastener
subsequently separated from the polymeric runners or other
polymeric features, as represented by block 118. More specifically,
when the molded body is formed, which typically includes molded
runners, vents, dividers, cold wells, and plate regions, the
fastener can be separated from the other features. In any event,
the separation can be performed by cutting, pressing, stamping, or
otherwise removing the polymer features from the plates.
[0072] Moreover, after the fastener has been separated from other
polymeric features, the fastener can be finished, as represented by
block 120. Finishing can include grinding, surfacing, sanding or
otherwise removing anomalies or other surface features on the
fastener. Also, the finishing can include providing a coating to
the molded fastener, if desired. Additionally, any other well-known
process for finishing a molded article can be used in connection
herewith in order to substantially finish the fastener into a
useable and implantable condition. However, the fastener can be
ready for use after injection molding without any further
finishing.
[0073] The present invention may be embodied in other specific
forms without departing from its spirit or essential
characteristics. The described embodiments are to be considered in
all respects only as illustrative and not restrictive. The scope of
the invention is, therefore, indicated by the appended claims
rather than by the foregoing description. All changes which come
within the meaning and range of equivalency of the claims are to be
embraced within their scope.
* * * * *