U.S. patent application number 10/562648 was filed with the patent office on 2007-02-01 for system and method for ankle arthroplasty.
This patent application is currently assigned to Advanced Bio Surfaces, Inc.. Invention is credited to Jeffrey C. Felt, Scott McGarvey, Mark A. Rydell.
Application Number | 20070027547 10/562648 |
Document ID | / |
Family ID | 38835071 |
Filed Date | 2007-02-01 |
United States Patent
Application |
20070027547 |
Kind Code |
A1 |
Rydell; Mark A. ; et
al. |
February 1, 2007 |
System and method for ankle arthroplasty
Abstract
An interpositional arthroplasty system for use in repairing
ginglymus joints such as the joints of the ankle, including a
tibiotalar implant (124) that provides a first major surface (130)
adapted to be positioned against a tibia (120) and a second major
surface (132) adapted to be positioned against a talus (108) and a
polymeric ankle implant that provides a first major surface adapted
to be positioned against a talus and a second major surface adapted
to be positioned against a calcaneus. Such an implant can be useful
for correcting various deformities of an ankle, as well as
increasing articulation of a joint. Also described are
biomaterials, including polymeric biomaterials having use in
preparing such implants, as well as surgical kits that include
implants and other tools adapted for their use.
Inventors: |
Rydell; Mark A.; (Golden
Valley, MN) ; McGarvey; Scott; (Edina, MI) ;
Felt; Jeffrey C.; (Greenwood, MI) |
Correspondence
Address: |
INTELLECTUAL PROPERTY GROUP;FREDRIKSON & BYRON, P.A.
200 SOUTH SIXTH STREET
SUITE 4000
MINNEAPOLIS
MN
55402
US
|
Assignee: |
Advanced Bio Surfaces, Inc.
Minnetonka
MN
|
Family ID: |
38835071 |
Appl. No.: |
10/562648 |
Filed: |
June 25, 2004 |
PCT Filed: |
June 25, 2004 |
PCT NO: |
PCT/US04/20456 |
371 Date: |
May 2, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60483499 |
Jun 27, 2003 |
|
|
|
Current U.S.
Class: |
623/21.18 ;
623/14.12 |
Current CPC
Class: |
A61F 2002/4207 20130101;
A61F 2/3094 20130101; A61F 2310/00365 20130101; A61B 90/90
20160201; A61F 2002/4205 20130101; A61F 2002/4622 20130101; A61B
17/1659 20130101; A61F 2/30965 20130101; A61F 2230/0017 20130101;
A61F 2002/30576 20130101; A61F 2002/30616 20130101; A61F 2002/4628
20130101; A61B 17/1615 20130101; A61F 2002/4233 20130101; A61B
90/00 20160201; A61F 2/4225 20130101; A61B 2090/062 20160201; A61F
2002/30148 20130101; A61B 17/562 20130101; A61B 17/8872 20130101;
A61F 2002/3071 20130101; A61F 2002/30754 20130101; A61F 2/4684
20130101; A61F 2002/4215 20130101; A61B 2090/061 20160201; A61F
2/4606 20130101; A61F 2/0095 20130101; A61F 2002/3082 20130101;
A61F 2002/4658 20130101; A61F 2/4202 20130101; A61F 2002/30881
20130101; A61F 2/4657 20130101; A61F 2002/30563 20130101; A61F
2002/30884 20130101; A61F 2250/0085 20130101 |
Class at
Publication: |
623/021.18 ;
623/014.12 |
International
Class: |
A61F 2/42 20060101
A61F002/42; A61F 2/46 20070101 A61F002/46; A61F 2/30 20070101
A61F002/30 |
Claims
1. An interpositional arthroplasty system for use in repairing
ginglymus joints such as the joints of the ankle, comprising a
tibiotalar implant that provides a first major surface adapted to
be positioned against a tibia and a second major surface adapted to
be positioned against a talus, and/or a talus-calcaneus implant
that provides a first major surface adapted to be positioned
against a talus and a second major surface adapted to be positioned
against a calcaneus.
2. An implant according to claim 1 wherein the tibiotalar implant
further comprises one or more external structures adapted to
improve retention of the implant within the joint site.
3. An implant according to claim 2 wherein the structure comprises
an integral bead shaped structure proximate its anterior side
adapted to cap and thereby engage the neck of the talus.
4. An implant according to claim 1 wherein the implant comprises a
biomaterial.
5. An implant according to claim 4 wherein the biomaterial is a
polyurethane.
6. An implant according to claim 5 wherein the polyurethane is
biocompatible with respect to cytotoxicity, sensitization,
genotoxicity, chronic toxicity, and carcinogenicity.
7. An implant according to claim 5 wherein polyurethane has a Shore
hardness of at least about 60 D or less.
8. An implant according to claim 1 wherein the talus-calcaneus
implant includes a posterior lip.
9. An implant according to claim 1 wherein the talus-calcaneus
implant includes a anterior lip.
10. A kit for a positional arthroplasty system for use in repairing
ginglymus joints such as the joints of the ankle, the kit
comprising: a) at least one implant selected from the group
consisting of a tibiotalar implant that provides a first major
surface adapted to be positioned against a tibia and a second major
surface adapted to be positioned against a talus, and a
talus-calcaneus implant that provides a first major surface adapted
to be positioned against a talus and a second major surface adapted
to be positioned against a calcaneus, and b) one or more devices
adapted to perform one or more steps selected from the group
consisting of preparing the joint to receive an implant,
determining an appropriate implant size for a particular joint,
determining an appropriate implant thickness and/or angle,
inserting the implant into the joint, and/or securing the implant
to a desired extent.
11. A kit according to claim 10 wherein the kit includes a tibial
smoother.
12. A kit according to claim 10 wherein the kit includes a talus
smoother.
13. A kit according to claim 10 wherein the kit includes an implant
gripper.
14. A kit according to claim 10 wherein the kit includes one or
more implant templates.
15. A kit according to claim 11 wherein the tibial smoother is
fenestrated.
16. A kit according to claim 11 wherein the tibial smoother is
universal in its orientation.
17. A kit according to claim 12 wherein the talus smoother is
fenestrated.
18. A kit according to claim 12 wherein the talus smoother is
universal in its orientation.
19. A method of repairing a ginglymus joint, comprising the steps
of providing and implanting according to claim 1.
20. A ginglymus joint that includes an implant according to claim
1.
21. A kit comprising a tool useful for preparing a joint to receive
an implant, an apparatus useful for determining an appropriate
implant size for the joint, an apparatus useful for determining an
appropriate implant thickness, and a tool useful for inserting the
implant into the joint and/or securing the implant to a desired
extent.
22. A device for implantation into an ankle joint space within the
body of a mammal, the device comprising a composite or monolith
structure fabricated from a biocompatible, biodurable material that
is adapted to be inserted into the joint compartment.
23. A device according to claim 22 wherein the implanted device is
substantially free of anchoring portions that need to be attached
to the bone, cartilage, ligaments or other tissue, yet by its
design is capable of being used with minimal translation, rotation,
or other undesired movement or dislocation within or from the joint
space.
24. A device according to claim 23 wherein stability of the device
within the joint space is provided by the fixation/congruency of
the device to the one or the other of the two joint members.
Description
TECHNICAL FIELD
[0001] In one aspect, this invention relates to biomaterials for
implantation and use within the body. In yet another aspect, this
invention further relates to the field of orthopedic implants and
prostheses, and more particularly, for implantable materials for
use in orthopedic joints.
BACKGROUND OF THE INVENTION
[0002] Applicant has previously described, inter alia, prosthetic
implants formed of biomaterials that can be delivered and finally
cured in situ, and/or that can be partially or fully prepared ex
vivo, for implantation into the body, e.g., using minimally
invasive techniques. See for instance, U.S. Pat. Nos. 5,556,429;
5,795,353; 6,140,452; 6,306,177; and 6,652,587, as well as US
Application Publication Nos. US-2002-015653 1; US-2002-0127264;
US-2002-0183850; and US-2004-0107000, and International
applications having Publication Nos. WO 95/30388; WO 98/20939; WO
02/17821; WO 03/053278; WO 03/061522, and WO 2004/006811 (the
disclosures of each of which are incorporated herein by
reference).
[0003] In spite of developments to date, there remains a need for a
joint prosthesis system that provides an optimal combination of
properties such as ease of preparation and use, and performance
within the body, and particularly for use in joints other than the
knee.
SUMMARY OF THE INVENTION
[0004] The present invention provides an interpositional
arthroplasty system for use in repairing ginglymus joints such as
the joints of the ankle. In some preferred embodiments, the system
includes an implant designed to be positioned in the tibiotalar
(true ankle joint) and/or in the subtalar joint. The implant can
comprise one or more biomaterials such as polymers, ceramics,
and/or metals, including combinations thereof.
[0005] In a preferred embodiment, the invention provides a
tibiotalar implant that provides a first major surface adapted to
be positioned against a tibia and a second major surface adapted to
be positioned against a talus. In a further preferred embodiment,
the implant includes one or more structures adapted to improve
retention of the implant within the joint site, e.g., by means of
an integral bead shaped structure proximate its anterior side
adapted to cap and thereby engage the neck of the talus.
[0006] In other preferred embodiments, the invention provides a
polymeric ankle implant that provides a first major surface adapted
to be positioned against a talus and a second major surface adapted
to be positioned against the calcaneus bone. In a further
embodiment, the implant includes one or more structures adapted to
improve retention of the implant within the joint site, e.g. by a
shape that conforms to the calcaneus, posterior lip, and/or
anterior lip. Such an implant can be useful for correcting various
deformities of an ankle, such as creating space between bones in
the foot to reduce bone on bone impingement, as well as increasing
articulation of a joint.
[0007] Some embodiments of the system can also include one or more
components and one or more related devices, in the form of a kit
that can be used to provide or perform some or all of the steps of
preparing the joint to receive an implant, determining an
appropriate implant size for a particular joint, determining an
appropriate implant thickness and/or angle, inserting the implant
into the joint, and/or securing the implant to a desired extent.
One or more of the various components and devices, including
optionally one or more implants themselves, can be provided or
packaged separately or in varying desired combinations and
subcombinations to provide a kit of this invention. Further, the
invention also includes a method of repairing a ginglymus joint, as
well as a ginglymus joint that includes an implant of this
invention.
[0008] In preferred embodiments, the invention provides a
prosthetic device for implantation into an ankle joint space within
the body of a mammal, the device comprising a composite or monolith
structure fabricated from a biocompatible, biodurable material that
is adapted to be inserted into the joint compartment. More
preferably, the implanted device is substantially free of anchoring
portions that need to be attached to the bone, cartilage, ligaments
or other tissue, yet by its design is capable of being used with
minimal translation, rotation, or other undesired movement or
dislocation within or from the joint space. The stability of the
device within the joint space is provided, in whole or in part, by
the fixation/congruency of the device to the one or the other, and
generally the relatively less mobile, of the two joint members.
BRIEF DESCRIPTION OF THE DRAWING
[0009] FIG. 1 is a side view of a foot and ankle region showing
implants in accordance with an embodiment of the present
invention.
[0010] FIG. 2(a) is a top view of a tool useful for preparing a
joint to receive an implant in accordance with an embodiment of the
present invention.
[0011] FIG. 2(b) is a side view of a tool useful for preparing a
joint to receive an implant in accordance with an embodiment of the
present invention.
[0012] FIG. 2(c) is a bottom view of a tool useful for preparing a
joint to receive an implant in accordance with an embodiment of the
present invention.
[0013] FIG. 3(a) is a top view of a sizing tool in accordance with
an embodiment of the present invention.
[0014] FIG. 3(b) is a side view of a sizing tool in accordance with
an embodiment of the present invention.
[0015] FIG. 4(a) is a top view of a gripping tool in accordance
with an embodiment of the present invention.
[0016] FIG. 4(b) is a side view of a gripping tool in accordance
with an embodiment of the present invention.
[0017] FIG. 4(c) is a side view of a gripping tool in accordance
with an alternate embodiment of the present invention.
[0018] FIG. 5 is a perspective view of an implant template in
accordance with an embodiment of the present invention.
[0019] FIG. 6(a) is a side view of an implant in accordance with an
embodiment of the present invention.
[0020] FIG. 6(b) is a top view of an implant in accordance with an
embodiment of the present invention.
DETAILED DESCRIPTION
[0021] A preferred embodiment will be described with reference to
the figures, where FIG. 1 is a side view of a foot 100 including a
plurality of bones 102. The bones of foot 100 include a first
phalanges 104, a metatarsal bone 106, a talus 108, and calcaneus
110. A tibia 120 is also shown in FIG. 1. As shown in FIG. 1, the
tibia 120 and talus 108 form a tibiotalar joint 112 (sometimes
referred to as a true ankle joint or TAJ). The tibiotalar joint 112
is responsible for up and down motion of the foot. The talus 108
and calcaneus 110 form a subtalar joint 114. The subtalar joint 114
allows for side to side motion of the foot.
[0022] In the embodiment of FIG. 1 a tibiotalus implant 124 is
disposed between tibia 120 and talus 108. The tibiotalus implant
124 can be useful for treating arthritic joints, replacing natural
cartilage, and/or providing a separation between the tibia 120 and
talus 108 to reduce bone on bone contact during articulation. The
tibiotalus implant 124 can have a first major surface 130 useful
for positioning against the tibia 120. The first surface 130 can be
adapted to provide an articulating surface for articulation of the
tibia 120. The tibiotalus implant can also have a second major
surface 132 adapted for positioning against the talus 108. The
second surface 132 can be useful for providing a cushioning surface
and/or congruency with the talus 108. In such embodiments, the
tibiotalar implant 124 can be adapted to provide a combination of
desirable wear resistance, congruency, and cushioning
properties.
[0023] The tibiotalus implant 124 can be provided with means for
stabilizing (e.g., fixing) the implant 124 within the joint,
wherein the means for stabilizing provides for less motion of the
implant relative to the talus 108 than the tibia 120. With
reference to FIG. 1, it will be appreciated that an exemplary means
for stabilization means includes a tibiotalus implant 124 that has
a bead shaped structure 126 proximate its anterior side that
engages the neck of the talus 108 to reduce the likelihood of
anterior and posterior movement during articulation. Of course,
other stabilization means can be provided to relatively fix the
tibiotalar implant 124 to the talus 108.
[0024] The implant 124 can comprise any shape or size that is
therapeutically useful. In some embodiments, the implant 124 may be
between about 1 mm and 7 mm thick. In a preferred embodiment, the
implant 124 is between about 2 mm and 3 mm thick. The implant 124
can also be sized to substantially cover the surface of the top
portion of the talus 108. In such embodiments, the implant 124 can
be about 40 mm to 50 mm in length.
[0025] Also in the embodiment of FIG. 1, a second implant 116 is
disposed between talus 108 and calcaneus 120. In some embodiments,
talus-calcaneus implant 116 can be relatively fixed to calcaneus
110 and allow talus 108 to articulate against it. As shown in FIGS.
6(a) and (b), implant 116 can contain an S-shaped side
cross-section useful for following the contour of the calcaneus
110. Further, implant 116 can contain a posterior lip and/or a
anterior lip, each of which are useful for engaging the calcaneus
120 to increase stabilization.
[0026] The implant 116 can comprise any shape or size that is
therapeutically useful. In some embodiments, implant 116 is between
about 2 mm and 5 mm thick. In some preferred embodiments, implant
116 is between about 2.5 mm and 3.5 mm thick. Implant 116 can be
sized to substantially cover the top surface of the calcaneus 120.
For example, the implant 116 can be about 35 mm to 45 mm in
length.
[0027] Some embodiments of the system can also include one or more
devices in the form of a kit that can be used to provide or perform
some or all of the steps of preparing the joint to receive an
implant, determining an appropriate implant size for a particular
joint, determining an appropriate implant thickness and/or angle,
inserting the implant into the joint, and/or securing the implant
to a desired extent. One or more of the various components and
devices, including optionally one or more implants themselves, can
be provided or packaged separately or in varying desired
combinations and subcombinations to provide a kit of this
invention.
[0028] In some embodiments, at least one tool is provided for
preparing the joint to receive an implant. Such a tool can comprise
a tibial smoother 200 and/or a talus smoother 202 as shown in FIGS.
2(a)-(c). Both the tibial smoother 200 and the talus smoother 202
can be provided with a proximate end 210 useful for manual or
motorized manipulation and a smoothing end 212 useful for smoothing
the surface of a bone. The smoothing end 212 can be provided with
any structure or feature that allows it to adequately remove
osteophytes, cartilage and other deposits to smooth the surface of
a bone, such as grit portion 214. In some embodiments, smoothing
end 212 is fenestrated. Such embodiments are useful for smoothing
the tibia and talus simultaneously, as well as for providing
self-cleaning properties by allowing debris to pass between the
superior and anterior sides. Grit portion 214 can be relatively
courser for removing larger osteophytes or can be relatively finer
for smoothing small osteophytes and finer finishing of the bone
surface. Smoother 200 can also be universal in its orientation,
permitting it to be used smoothing bone in both the right and left
ankles.
[0029] In some embodiments, at least one sizing tool for
determining an appropriate implant thickness and/or angle is
provided. Such a tool can comprise an implant sizer 300 as shown in
FIGS. 3(a) and (b). Sizing tool 300 can include proximate end 310
useful for manual manipulation and a sizing end 312 useful for
inserting into the body to determine an appropriate implant size.
As shown in FIGS. 3(a) and (b), the sizing end 312 can be shaped
substantially as an implant. One or more sizing tools 300 can be
provided in the form of a kit, with each tool 300 having an
identifiable shape, thickness, or angle. In some embodiments,
sizing tool 300 is provided with means for adjusting its thickness,
such as a track with one or more components that can be locked in
to increase thickness.
[0030] In some embodiments, a tool is provided for inserting an
implant into a joint and or securing the implant to a desired
extent. Such a tool can comprise an implant gripper 400 as shown in
FIGS. 4(a)-(c). Gripper 400 can be provided with a proximate end
410 useful for manual manipulation and an gripping end 412 useful
for gripping and retaining an implant 124 for placement into a
body. Gripping end 412 can include a top arm 416 and a bottom arm
418 useful for gripping and retaining an implant 124. As shown in
FIG. 4(b), top arm 416 can include a hinge 420 useful for providing
top arm 416 with a lower profile when releasing implant 124. Other
embodiments, such as the one shown in FIG. 4(c), do not include a
hinge 420.
[0031] The present invention can also include one or more implant
templates 500, as shown in FIG. 5. Implant template 500 is useful
for determining the proper implant thickness and/or angle need to
match physiological values. Implant template 500 may be provided in
a variety of thicknesses and shapes, e.g. shapes useful for the
right and left ankles. A marker, such as a dog tag 502, can be
provided to list this information. In some embodiments, implant
template 500 can be inserted with gripper 400. A band, e.g. a chain
504 can be provided to remove the implant template 500 from the
joint. In some embodiments, chain 504 can also retain dog tag 502.
Of course, the implant itself may be provided with a marker, such
as a dog tag 502 and a band 504, which can be removed at the time
of implantation.
[0032] The tools described above can be constructed of any suitable
material. For example, the tools can be constructed of stainless
steel, ceramic, and/or polymeric materials. Embodiments constructed
at least partially of stainless steel can be relatively more
suitable for providing a reusable tool, and embodiments constructed
at least partially of a polymer can be relatively more suitable for
providing a disposable tool. Further, all of the tools above can be
shaped to provide an ergonomic fit for the user. Some embodiments
provide a universal tool that can be adapted, e.g., configured, to
provide an ergonomic fit for both left and right hands.
[0033] In one exemplary ankle surgery method in accordance with the
present invention, an incision is made in the front of the foot,
anterior to the tibiotalar joint 112. The tibiotalar implant 124 is
inserted into the space between the two bones. In embodiments
provided with a bead shaped structure 126, the bead shaped
structure 126 is placed in contact with the neck of the talus 108
to reduce the likelihood of anterior and posterior movement during
articulation. The implant can be further restrained by adjacent
soft tissue. The incision is finally sutured closed.
[0034] In one exemplary ankle surgery method in accordance with the
present invention, an incision is made in the lateral side of the
foot. The subtalar implant 116 is inserted into the space between
the talus 108 and the calcaneus 110. In embodiments provided with a
posterior and/or anterior lip, such lip is placed in contact with
the calcaneus 110 to reduce the likelihood of anterior and
posterior movement during articulation. The implant can be further
restrained by adjacent soft tissue. The incision is finally sutured
closed.
[0035] The methods of repairing the joints described above can also
include the steps of preparing a joint to receive an implant,
determining an appropriate implant size for a particular joint,
determining an appropriate implant thickness, inserting the implant
into the joint, and/or securing the implant to a desired extent. In
some embodiments, these steps are performed with the use of one or
more of the tools or apparatus described above.
[0036] In some embodiments, implants 124 and 116 may be provided
with means to confirm their post-operative position. For example,
implants 124 and 116 can be radio-opaque. In such embodiments, a
radio-opaque material, such as tungsten, can be provided within the
implant in one or more locations. The implant location can then be
determined using radio-opacity techniques known in the art.
[0037] The biomaterial can be prepared from any suitable material.
Generally, a material is suitable if it has appropriate
biostability, biodurability and biocompatibility characteristics.
Typically, the materials include polymeric materials, having an
optimal combination of such properties as biostability,
biodurability, biocompatibility, physical strength and durability,
and compatibility with other components (and/or biomaterials) used
in the assembly of a final composite.
[0038] Examples of polymeric materials that may be suitable in some
applications, either alone or in combination, include polyurethane,
available from Polymer Technology Group Incorporated under the
names Bionate,.TM. Biospan,.TM. and Elasthane.TM., available from
Dow Chemical Company under the name Pellethane,.TM. and available
from Bayer Corp. under the names Bayflex,.TM. Texin,.TM. and
Desmopan;.TM. ABS, available from GE Plastics under the name
Cycolac.TM., and available from Dow Chemical Company under the name
Magnum;.TM. SAN, available from Bayer Plastics under the name
Lustran;.TM. Acetal, available from Dupont under the name
Delrin,.TM. and available from Ticona GmbH and/or Ticona LLC
(Ticona) under the name Celcon;.TM. polycarbonate, available from
GE Plastics under the name Lexan,.TM. and available from Bayer
Corp. under the name Makrolon;.TM. polyethylene, available from
Huntsman LLC, and available from Ticona under the names GUR
1020.TM. and GUR 1050;.TM. polypropylenes, available from Solvay
Engineered Polymers, Inc. under the name Dexflex;.TM. aromatic
polyesters, available from Ticona; polyetherimide (PEI), and
available from GE Plastics under the name Ultem;.TM.
polyamide-imide (PAI), available from DSM E Products under the name
Torlon;.TM. polyphenylene sulfide, available from Chevron Phillips
Chemical Company LP under the name Ryton;.TM. polyester, available
from Dupont under the name Dacron;.TM. polyester thermoset,
available from Ashland Specialty Chemical Company under the name
Aropol;.TM. polyureas; hydrogels, available from Hydromer Inc.;
liquid crystal polymer, available from Ticona under the name
Vectra;.TM. polysiloxanes, available from Nusil Technologies, Inc.;
polyacrylates, available from Rohm & Haas under the name
Plexiglas;.TM. epoxies, available from Ciba Specialty Chemicals;
polyimides, available from Dupont under the names Kapton,.TM. and
Vespel;.TM. polysulfones, available from BP Amoco Chemicals under
the name Udel,.TM. and available from BASF Corporation under the
name ULItrason;.TM. PEAK/PEEK, available from Victrex under the
name Victrex PEAK;.TM. as well as biopolymers, such as collagen or
collagen-based materials, chitosan and similar polysaccharides, and
combinations thereof Of course, any of the materials suitable for
use in a composite or single biomaterial implant may be
structurally enhanced with fillers, fibers, meshes or other
structurally enhancing means.
[0039] The present invention provides a biomaterial having an
improved combination of properties for the preparation, storage,
implantation and long term use of medical implants. The improved
properties correspond well for the preparation and use of an
implant having both weight bearing and/or articulating functions,
and preferably in the form of an implant for interpositional
arthroplasty.
[0040] In turn, a preferred biomaterial of this invention provides
an optimal combination of properties relating to wear resistance,
congruence, and cushioning while meeting or exceeding requirements
for biocompatibility, all in a manner that serves to reduce the
coefficient of friction at the major motion interface.
[0041] Wear resistance can be assessed by determining parameters
such as DIN abrasion and flexural stress strain fatigue resistance.
A preferred implant will have sufficient wear resistance to avoid
the generation of clinically significant particulate debris over
the course of the implant's use.
[0042] Congruence can be assessed by determining parameters such as
tensile modulus compressive modulus, and hardness, to determine the
manner and extent to which the implant will conform itself to
possible other components of the implant itself and/or to bone or
surrounding tissue.
[0043] Cushioning can be assessed by determining such parameters as
hardness, compressive modulus, and tensile modulus, to determine
the elastomeric nature of the material, and in turn, its
suitability for use in a weight bearing joint. More elastomeric
materials will generally provide greater comfort in weight bearing
applications, particularly if the other physical properties can be
maintained.
[0044] Applicant has discovered that improved wear resistance,
congruence, and/or cushioning toughness can be achieved without
undue effect on other desired properties, such as abrasion,
hardness, specific gravity, tear resistance, tensile strength,
ultimate elongation, and biocompatibility. Moreover, Applicant has
discovered that such properties can themselves be provided in
varying forms, as between first and second biomaterials of a
composite of the present invention.
[0045] A polymeric biomaterial of this invention can be prepared
using any suitable means, including by curing the polymer ex vivo.
The composition can be used in any suitable combination with other
materials, including other compositions of the same or similar
nature, as well as other materials such as natural or synthetic
polymers, metals, ceramics, and the like.
[0046] The invention further provides a method of preparing the
composition, a method of using the composition, implants that
comprise the composition, as well as methods of preparing and using
such implants.
[0047] The biomaterial used in this invention preferably includes
polyurethane components that are reacted ex vivo to form a
polyurethane ("PU"). The formed PU, in turn, includes both hard and
soft segments. The hard segments are typically comprised of stiffer
oligourethane units formed from diisocyanate and chain extender,
while the soft segments are typically comprised of one or more
flexible polyol units. These two types of segments will generally
phase separate to form hard and soft segment domains, since they
tend to be incompatible with one another. Those skilled in the
relevant art, given the present teaching, will appreciate the
manner in which the relative amounts of the hard and soft segments
in the formed polyurethane, as well as the degree of phase
segregation, can have a significant impact on the final physical
and mechanical properties of the polymer. Those skilled in the art
will, in turn, appreciate the manner in which such polymer
compositions can be manipulated to produce cured and curing
polymers with desired combination of properties within the scope of
this invention.
[0048] The hard segments of the polymer can be formed by a reaction
between the diisocyanate or multifunctional isocyanate and chain
extender. Some examples of suitable isocyanates for preparation of
the hard segment of this invention include aromatic diisocyanates
and their polymeric form or mixtures of isomers or combinations
thereof, such as toluene diisocyanates, naphthalene diisocyanates,
phenylene diisocyanates, xylylene diisocyanates, and
diphenylmethane diisocyanates, and other aromatic polyisocyanates
known in the art. Other examples of suitable polyisocyanates for
preparation of the hard segment of this invention include aliphatic
and cycloaliphatic isocyanates and their polymers or mixtures or
combinations thereof, such as cyclohexane diisocyanates,
cyclohexyl-bis methylene diisocyanates, isophorone diisocyanates
and hexamethylene diisocyanates and other aliphatic
polyisocyanates. Combinations of aromatic and aliphatic or arylakyl
diisocyanates can also be used.
[0049] The isocyanate component can be provided in any suitable
form, examples of which include 2,4'-diphenylmethane diisocyanate,
4,4'-diphenylmethane diisocyanate, and mixtures or combinations of
these isomers, optionally together with small quantities of
2,2'-diphenylmethane diisocyanate (typical of commercially
available diphenylmethane diisocyanates). Other examples include
aromatic polyisocyanates and their mixtures or combinations, such
as are derived from phosgenation of the condensation product of
aniline and formaldehyde. It is suitable to use an isocyanate that
has low volatility, such as diphenylmethane diisocyanate, rather
than more volatile materials such as toluene diisocyanate. An
example of a particularly suitable isocyanate component is the
4,4'-diphenylmethane diisocyanate ("MDI"). Alternatively, it can be
provided in liquid form as a combination of 2,2'-, 2,4'- and
4,4'-isomers of MDI. In a preferred embodiment, the isocyanate is
MDI and even more preferably 4,4'-diphenylmethane diisocyanate.
[0050] In one embodiment of the invention, the isocyanate is
4,4'-diphenylmethane diisocyanate (as available from Bayer under
the tradename Mondur M), from preferably about 20 to 60 weight
percent, more preferably from about 30 to 50 weight percent. The
actual amount of isocyanate used should be considered in
combination with other ingredients and processing parameters,
particularly including the amount of chain extender (such as
butanediol (BDO)) used, since the combination typically determines
the hard segment component, and in turn hardness, of the
corresponding cured polymer. Hardness correlates in a generally
proportional fashion with the combined weights of MDI and BDO, such
that compositions having between 30 and 60 total weight percent
(MDI+BDO) are generally useful, with those compositions having
between about 50 to about 60 total weight percent being somewhat
harder, and particularly useful for either the first (femoral
contacting) biomaterial and surface of a composite implant or for
implants having a single biomaterial providing both first and
second surfaces. By contrast, compositions having between about 40
to about 50 total weight percent are somewhat more congruent and
cushioning, though less wear resistant, and therefore are preferred
for use as the second biomaterial, e.g., tibial contacting surface,
of a composite implant as described herein.
[0051] Some examples of chain extenders for preparation of the hard
segment of this invention include, but are not limited, to short
chain diols or triols and their mixtures or combinations thereof,
such as 1,4-butane diol, 2-methyl-1,3-propane diol,
1,3-propane-diol ethylene glycol, diethylene glycol, glycerol,
tri-methylpropane, cyclohexane dimethanol, triethanol amine, and
methyldiethanol amine. Other examples of chain extenders for
preparation of the hard segment of this invention include, but are
not limited to, short chain diamines and their mixtures or
combinations thereof, such as dianiline, toluene diamine,
cyclohexyl diamine, and other short chain diamines known in the
art.
[0052] The soft segment consists of urethane terminated polyol
moieties, which are formed by a reaction between the polyisocyanate
or diisocyanate or polymeric diisocyanate and polyol. Examples of
suitable diisocyanates are denoted above. Some examples of polyols
for preparation of the soft segment of this invention include but
are not limited to polyalkylene oxide ethers derived form the
condensation of alkylene oxides (e.g. ethylene oxide, propylene
oxide, and blends thereof), as well as tetrahyrofuran based
polytetramethylene ether glycols, polycaprolactone diols,
polycarbonate diols and polyester diols and combinations thereof.
In a preferred embodiment, the polyols are polytetrahydrofuran
polyols ("PTHF"), also known as polytetramethylene oxide ("PTMO")
or polytetramethylene ether glycols ("PTMEG"). Even more
preferably, the use of two or more of PTMO diols with different
molecular weights selected from the commercially available group
consisting of 250, 650,1000, 1400, 1800, 2000 and 2900.
[0053] Two or more PTMO diols of different molecular weight can be
used as a blend or separately, and in an independent fashion as
between the different parts of a two part system. The
solidification temperature(s) of PTMO diols is generally
proportional to their molecular weights. The compatibility of the
PTMO diols with such chain extenders as 1,4-butanediol is generally
in the reverse proportion to the molecular weight of the diol(s).
Therefore the incorporation of the low molecular weight PTMO diols
in a "curative" (part B) component of a two part system, and higher
molecular weight PTMO diols in the prepolymer (part A) component,
can provide a two-part system that can be used at relatively low
temperature. In turn, good compatibility of the low molecular
weight PTMO diols with such chain extenders as 1,4-butanediol
permits the preparation of two part systems with higher (prepolymer
to curative) volume ratio. Amine terminated polyethers and/or
polycarbonate-based diols can also be used for building of the soft
segment.
[0054] In one embodiment of the invention, the polyol is
polytetramethyleneetherglycol 1000 (as available from E.I. du Pont
de Nemours and Co. under the tradename Terathane 1000), preferably
from about 0 to 40 weight percent, more preferably from about 10 to
30 weight percent, and perhaps even more preferably from about 22
to 24 weight percent, based on the total weight of the polymer. The
polyol disclosed above may be used in combination with
polytetramethyleneetherglycol 2000 (as available from E.I. du Pont
de Nemours and Co. under the tradename Terathane 2000), preferably
from about 0 to 40 weight percent, more preferably from about 10 to
30 weight percent, and perhaps even more preferably from about 17
to 18 weight percent, based on the total weight of the polymer.
[0055] In one embodiment, the biomaterial may include a chain
extender. For example, the chain extender may be 1,4-butanediol (as
available from Sigma Aldrich Corp.), preferably from about 1 to 20
weight percent, more preferably from 5 to 15 weight percent, to
perhaps even more preferably from 12 to 13 weight percent, based on
the total weight of the polymer.
[0056] The polyurethane can be chemically crosslinked, e.g., by the
addition of multifunctional or branched OH-terminated crosslinking
agents or chain extenders, or multifunctional isocyanates. Some
examples of suitable crosslinking agents include, but are not
limited to, trimethylol propane ("TMP"), glycerol, hydroxyl
terminated polybutadienes, hydroxyl terminated polybutadienes
(HOPB), trimer alcohols, Castor oil polyethyleneoxide (PEO),
polypropyleneoxide (PPO) and PEO-PPO triols. In a preferred
embodiment, HOPB is used as the crosslinking agent.
[0057] This chemical crosslinking augments the physical or
"virtual" crosslinking of the polymer by hard segment domains that
are in the glassy state at the temperature of the application. The
optimal level of chemical cross-linking improves the compression
set of the material, reduces the amount of the extractable
components, and improves the biodurability of the PU. This can be
particularly useful in relatively soft polyurethanes, such as those
suitable for the repair of damaged cartilage. Reinforcement by
virtual cross-links alone may not generate sufficient strength for
in vivo performance in certain applications. Additional
cross-linking from the soft segment, potentially generated by the
use of higher functional polyols can be used to provide stiffer and
less elastomeric materials. In this manner a balancing of hard and
soft segments, and their relative contributions to overall
properties can be achieved.
[0058] In one embodiment, the chemical cross-linking agent is
2-ethyl-2-(hydroxymethyl)-1,3-propanediol (also known as
trimethylolpropane, as available from Sigma Aldrich Corp.),
preferably from about 0 to 5 weight percent, more preferably from
about 0.1 to 1 weight percent, and perhaps even more preferably
from about 0.15 to 0.3 weight percent, based on the total weight of
the polymer.
[0059] Additionally, and optionally, a polymer system of the
present invention may contain at least one or more biocompatible
catalysts that can assist in controlling the curing process,
including the following periods: (1) the cure induction period, and
(2) the full curing period of the biomaterial. Together these two
periods, including their absolute and relative lengths, and the
rate of acceleration or cure within each period, determine the cure
kinetics or profile for the composition. In some embodiments,
however, a catalyst is not included. For instance embodiments in
which the biomaterial is heated in the course of curing, such as in
a heated mold in the manner described herein, can performed without
the use of a catalyst.
[0060] Some examples of suitable catalysts for preparation of the
formed PU of this invention include, but are not limited to, tin
and tertiary amine compounds or combinations thereof such as
dibutyl tin dilaurate (DBTDL), and tin or mixed tin catalysts
including those available under the tradenames "Cotin 222", "Fomrez
UL-22" (Crompton Corp.), "dabco" (a triethylene diamine from
Sigma-Aldrich), stannous octanoate, trimethyl amine, and triethyl
amine.
[0061] In one embodiment of the invention, the catalyst is
bis-(dodecylthio)-dimethylstannane (available from Crompton Corp.
as Fomrez catalyst UL-22), preferably from about 0 to 2 weight
percent, more preferably from about 0 to 1 weight percent, and
perhaps most preferably from 0.0009 to 0.002 weight percent, based
on the total weight of the polymer.
[0062] Further, a polymer stabilizer additive useful for protecting
the polymer from oxidation may be included. In one embodiment of
the invention, the additive is pentaerythritol tetrakis
(3-(3,5-di-tert-buyl-4-hydroxyphenyl)proprionate (available from
Ciba Specialty Chemicals, Inc. as Irganox 1010), preferably from
about 0 to 5 weight percent, more preferably about 0.1 to 1 weight
percent, and perhaps even more preferably about 0.35 to 0.5 weight
percent, based on the total weight of the polymer.
[0063] Optionally, other ingredients or additives can be included,
for instance, a reactive polymer additive can be included from the
group consisting of hydroxyl- or amine-terminated compounds
selected from the group consisting of poybutadiene, polyisoprene,
polyisobutylene, silicones, polyethylene-propylenediene, copolymers
of butadiene with acryolnitrile, copolymers of butadiene with
styrene, copolymers of isoprene with acrylonitrile, copolymers of
isoprene with styrene, and mixtures of the above. Other additives
may also be optionally provided. For example, catalysts such as
Dabco, antioxidants such as vitamin E, hydrophobic additives such
as hydroxyl-terminated polybutadiene, and dye green GLS, singularly
or in combination, may be included in the polymer formulation.
[0064] Suitable compositions for use in the present invention are
those polymeric materials that provide an optimal combination of
properties relating to their manufacture, application, and in vivo
use. In the uncured state, such properties include component
miscibility or compatibility, processability, and the ability to be
adequately sterilized or aseptically processed and stored. While
the composition is curing, suitable materials exhibit an optimal
combination of cure kinetics and exotherm. In the cured state,
suitable compositions exhibit an optimal combination of such
properties as abrasion, hardness, specific gravity, tear
resistance, tensile strength, ultimate elongation, and
biocompatibility.
[0065] The composition of the present invention provides a
polyurethane that can be prepared ex vivo. Particularly when formed
ex vivo, products incorporating the composition of this invention
may be made in advance of their use, on a commercial scale, and
under stringent conditions.
[0066] Polymeric biomaterials of this invention, including
preferred polyurethanes can be prepared using automated
manufacturing processes within the skill of those in the art. A
preferred manufacturing method, for instance, includes the use of
multichannel dispensing equipment to inject the polymer. Such
equipment is well suited to high precision applications, having a
variable or fixed number of channels, some have all channels
dispensing the same volume while in others the volume can be set by
channel, some have all channels dispensing the same fluid, while
others allow for different fluids in different channels. The
dispensing can be automated repetitive or manual. Suitable devices
for metering, mixing and dispensing materials such as urethanes are
commercially available from a variety of sources, including for
instance from Adhesive Systems Technology Corp., 9000 Science
Center Drive, New Hope, Minn. 55428.
[0067] Furthermore, polymeric biomaterials of this invention may be
cured in a heated mold. The mold may receive the contents of the
polymeric biomaterial before it is cured. In one embodiment, a
permanent enclosed mold is used to form at least a part of the
implant. Such a mold may be similar to a standard injection mold
and have the ability to withstand large clamping forces. Further,
such a mold may include runners and/or vents to allow material to
enter and air to exit. Such a mold may be constructed from metals,
polymers, ceramics, and/or other suitable materials. The mold may
be capable of applying and controlling heat to the biomaterial to
accelerate curing time. In some embodiments, the mold may be coated
with a release coating agent to facilitate ease of removal of the
cured biomaterial from the mold. Examples of suitable release
agents include Teflon,.TM. silicone, florinated ethylene propylene
(FEP), dichronite, gold, and nickel-Teflon combinations, various
types of which are commercially available from a variety of
sources, e.g., McLube Division of McGee Industries. In addition,
the mold may be provided in two separable parts to further
facilitate removal of the cured biomaterial.
[0068] Further, time and temperature parameters can be modified in
processing to change the characteristics of the implant. A time
temperature profile may be selected to achieve certain implant
properties. In embodiments formed with a heated mold as described
above, those skilled in the art will appreciate the manner in which
both the temperature of the mold as well as the time biomaterial is
maintained can be adjusted to change the characteristics of the
molded implant.
[0069] In the embodiment in which an ex vivo curing polymer is
used, the present invention preferably provides a biomaterial in
the form of a curable polyurethane composition comprising a
plurality of parts capable of being at least partially mixed at a
time before use, the parts including: (1) a polymer component
comprising the reaction product of one or more polyols, and one or
more diisocyanates, and (2) a curative component comprising one or
more chain extenders, one or more catalysts, and optionally, one or
more polyols and/or other optional ingredients.
[0070] In some embodiments, long term congruence of the biomaterial
is facilitated by its hydration in vivo, permitting the biomaterial
to become more pliable, and in turn, facilitate congruence with the
tibial plateau. In turn, an increase in hydration and/or changes in
temperature can improve the fit and mechanical lock between the
implant and the tibial plateau. The biomaterial may be hydrated ex
vivo and/or in vivo, both before and after the composition is
cured. Preferably, the biomaterial may be further hydrated within
the joint site after the composition in order to enhance both
conformance and performance of the implant.
[0071] Implantable compositions of this invention demonstrate an
optimal combination of properties, particularly in terms of their
physical/mechanical properties, and biocompatibility. Such
performance can be evaluated using procedures commonly accepted for
the evaluation of natural tissue, as well as the evaluation of
materials and polymers in general. In particular, a preferred
composition, in its cured form, exhibits physical and mechanical
properties that approximate or exceed those of the natural tissue
it is intended to provide or replace. Fully cured polymeric (e.g.,
polyurethane) biomaterials within the scope of this invention
provide an optimal combination of such properties as abrasion,
compressive hardness, compressive modulus hardness, specific
gravity, tear resistance, tensile strength, ultimate elongation,
tensile modulus, and biocompatibility.
[0072] Physical/Mechanical Properties and Test Methods
[0073] Various properties of the composition of this invention can
be evaluated for use in quality control, for predicting service
performance, to generate design data, to determine compliance with
established standards, and on occasion, to investigate failures.
See, for instance, Handbook of Polymer Testing: Physical Methods,
edited by Roger Brown, Marcel Dekker, Inc., New York, N.Y. (1999),
the disclosure of which is incorporated herein by reference.
Suitable properties include those dealing with a) mass, density and
dimensions, b) processability, c) strength and stiffness (including
compressive hardness, compressive modulus, tensile stress-strain,
flexural stress-strain, flexibility, and tear tests), c) fatigue
and wear (including abrasion resistance and hardness), d) time
dependent properties (such as creep, stress relaxation, compression
set, tension set), e) effect of temperature (such as thermal
expansion, shrinkage, and thermal oxidative aging), f)
environmental resistance, and g) and biocompatibility
parameters.
[0074] Of particular note are those properties that lend themselves
to the preparation, delivery and long term use of improved implants
having an articulating surface, and preferably for long term weight
bearing use.
[0075] The preferred property ranges given below are only relevant
to certain embodiments of the invention. It will be appreciated by
those reasonably skilled in the art that materials having one or
more properties outside the scope of the preferred ranges given
below are suitable for use with the present invention.
[0076] Abrasion values for a polymer can be determined with a
rotating cylindrical drum device, known as a DIN abrader. A loaded
cylindrical test piece is traversed along an abrasive cloth
attached to a rotating drum, and the mass loss is measured after a
specified length of travel. Advantages of this device include the
use of a test piece small enough to be cut from a product or a
comparatively thin sheet and a much reduced risk of abrasive
contamination caused by debris or smearing. The result can be
expressed with the abrasion resistance index, which is the ratio of
the volume loss of a black standard rubber sample to the volume
loss of the test sample.
[0077] The polymer preferably provides a DIN abrasion value of less
than about 70 mm.sup.3, more preferably less than about 60 mm.sup.3
and most preferably less than about 50 mm.sup.3, as determined by
ASTM Test Method D5963-96 ("Standard Test Method for Rubber
Property Abrasion Resistance Rotary Drum Abrader"). DIN abrasion
values of greater than about 70 mm.sup.3 tend to exhibit wear rates
that are too great for longer term use as articulating surface.
[0078] Biomaterial can be formed into standardized (e.g.,
puck-like) implant shapes and subjected to conditions intended to
replicate, while also meet and exceed physiological conditions.
Preferred biomaterials of this invention are able to withstand one
million cycles (approximately equivalent to 1 year implantation),
and more preferably greater than 5 million cycles (approximately
equivalent to 5 years) before generating unsuitable debris.
[0079] Flexural stress/strain fatigue can be measured in a variety
of ways. Using the standardized shape as described above, samples
can be compressively loaded in cycles of increasing loads, and the
stress strain fatigue can be plotted verses the number of
cycles.
[0080] As another example, flexural stress/strain fatigue can be
determined by a three point bending test, in which a standardized
implant sample shape is supported at its anterior and posterior
ends. A cyclical load is applied to the sample in an area
substantially between the two supports to provide a deflection of
approximately 4 mm, and the total number of cycles until failure is
recorded.
[0081] Biomaterials formed into implant shapes in accordance with
the present invention, under conditions intended to meet and exceed
physiological conditions, are preferably able to withstand one
million cycles (approximately equivalent to 1 year implantation),
and more preferably greater than five million cycles (approximately
equivalent to 5 years implantation) in a test similar to the one
described above.
[0082] Fracture toughness can generally be determined by a number
of methods. For example, fracture toughness can be measured by
tests similar to ASTM Test Method D5045-99.
[0083] Preferably, the polymer provides a peak load fracture
toughness of at least about 50 lbs, more preferably more than about
80 lbs, and most preferably more than about 110 lbs. Further, the
polymer preferably provides an energy to break fracture toughness
of greater than about 15 lb-in, more preferably greater than about
25 lb-in, and most preferably greater than about 30 lb-in. These
values may be obtained with tests similar to ASTM Test Method
D5045-99.
[0084] The term hardness has been applied to scratch resistance and
to rebound resilience, but for polymers it is taken to refer to a
measure of resistance to indentation. The mode of deformation under
an indentor is a mixture of tension, shear, and compression. The
indenting force is usually applied in one of the following ways:
Application of a constant force, the resultant indentation being
measured, measurement of the force required to produce a constant
indentation, or use of a spring resulting in variation of the
indenting force with depth of indentation.
[0085] A biomaterial of this invention preferably provides a
hardness value when hydrated of less than about 75 Shore D, more
preferably less than about 70 Shore D, and most preferably less
than about 60 Shore D, as determined by ASTM Test Method D2240. In
some embodiments, hydration of the biomaterial may lower the shore
hardness value.
[0086] In one method of determining specific gravity, a test piece
is provided weighing a minimum of 2.5 grams, which can be of any
shape as long as the surfaces are smooth and there are no crevices
to trap air. The test piece is weighed in air and then in water
using a balance accurate to 1 mg. The test piece can be suspended
by means of a very fine filament, the weight of which can be
included in the zero adjustment of the balance and its volume in
water ignored. The specific gravity is calculated from the
difference in measurements.
[0087] The polymer preferably provides a specific gravity of about
1 to 2 g/cm.sup.3, more preferably about 1 to 1.5 g/cm.sup.3, and
most preferably about 1.15 to 1.17 g/cm.sup.3, as determined by
ASTM Test Method D792.
[0088] A tear test may be used to measure tear strength. In a tear
test, the force is not applied evenly but is concentrated on a
deliberate flaw or sharp discontinuity in the sample and the force
to produce continuously new surface is measured. This force to
start or maintain tearing will depend in a complex manner on the
geometry of the test piece and the nature of the discontinuity.
[0089] Preferably, a biomaterial of this invention provides a tear
strength value in the Die C configuration of greater than about 400
pounds per linear inch (PLI), more preferably greater than about
600 PLI, and most preferably greater than about 800 PLI, and a
value in the Die T configuration of preferably greater than about
100 PLI, more preferably greater than about 150 PLI, and most
preferably greater than about 250 PLI, as determined by ASTM Test
Method D624.
[0090] To measure tensile modulus, tensile strength, and ultimate
elongation, a test piece of the material is stretched until it
breaks, and the force and elongation at various stages is measured.
A tensile machine is used to perform this test. Generally, the
basic elements of a tensile machine are grips to hold the test
piece, a means of applying a strain (or stress), a force-measuring
instrument, and an extensometer.
[0091] The polymer preferably provides a tensile modulus at 100%
elongation value of about 1,000 to 10,000 psi, more preferably
about 2,000 to 5,000 psi, and most preferably about 2,500 to 4,500
psi, as determined by ASTM Test method D412.
[0092] The polymer preferably provides a tensile modulus at 200%
elongation value of about 1,000 to 10,000 psi, more preferably
about 2,000 to 6,000 psi, and most preferably about 2,500 to 5,000
psi, as determined by ASTM Test method D412.
[0093] The polymer preferably provides a tensile strength value of
greater than about 6,000 psi, more preferably greater than about
6,500 psi, and most preferably greater than about 7,000 psi., as
determined by ASTM Test Method D412.
[0094] Preferably, the polymer provides an ultimate elongation of
greater than about 200%, more preferably greater than about 250%,
and most preferably greater than about 300%, as determined by ASTM
Test Method D412.
[0095] To measure compressive modulus and compressive strength, a
sample is again formed in a standardized (e.g., puck) shape and
varying compressive loads are applied to the sample in order to
develop a corresponding curve. The compressive modulus can be
determined from this curve. Compressive strength may be determined
by applying increasing loads to a sample until the sample
fails.
[0096] Preferably, the sample implant provides an compressive
modulus of greater than about 4,000 psi, more preferably greater
than about 4,500 psi, and most preferably greater than about 5,000
psi, as determined in the manner described above.
[0097] Preferably, the sample implant also provides a compressive
strength of greater than about 6,000 psi, more preferably greater
than about 7,000 psi, and most preferably greater than about 8,000
psi, as determined by a test similar to the one described
above.
[0098] Water absorption may be determined in a variety of ways. A
suitable method for measuring water absorption is to submerge a
sample of the test material, with an implant-type geometry, in a
saline solution. Once the sample and saline solution reach
equilibrium at 37 degrees Celsius, which may take a month or
longer, the sample is removed and weighed to determine its water
absorption.
[0099] Preferably, the polymer provides a water absorption value
less than about 5% at 37 C, more preferably less than about 3% at
37 C, and most preferably less than about 2% at 37 C, as determined
by a test similar to the one described above.
[0100] The medical-grade polyurethane resins were evaluated for
biocompatibility in accordance with ISO 10993: Biological
Evaluation of Medical Devices and FDA G95-1: Required
Biocompatibility Training and Toxicology Profiles for Evaluation of
Medical Devices. The biological effects of the resin, such as
cytotoxicity, sensitization, genotoxicity, implantation, chronic
toxicity, and carcinogenicity, were studied. The tests were
conducted in accordance with the FDA Good Laboratory Practice (GLP)
Regulation.
[0101] The following tests were conducted to determine if the
polymer is biocompatible: 1) ISO MEM elution using L-929 mouse
fibroblast cells; 2) ISO agarose overlay using L-929 mouse
fibroblast cells; 3) ISO acute systemic injection test; 4) ISO
intracutaneous reactivity test; 5) ISO guinea pig maximization
sensitization test; 6) Material mediated rabbit pyrogen test; 7) In
vitro genotoxicology test; and 8) ISO muscle implantation study in
the rabbit with histology-1 week. The results of the eight selected
screening biocompatibility tests above show that the polymer passes
all the tests and is considered biocompatible.
[0102] In an alternative embodiment, the implant can be provided by
any of a series of metals, including titanium, stainless steel,
cobalt chrome millithium alloys and tantalum. Other surface
materials can include various ceramics and biologic polymers.
[0103] Numerous characteristics and advantages of the invention
covered by this document have been set forth in the foregoing
description. It will be understood, however, that this disclosure
is, in many respects, only illustrative. Changes can be made in
details, particularly in matters of shape, size and ordering of
steps without exceeding the scope of the invention. The invention's
scope is, of course, defined in the language in which the appended
claims are expressed.
* * * * *