U.S. patent application number 11/505632 was filed with the patent office on 2007-03-01 for high precision manufacture of polyurethane products such as spinal disc implants having gradual modulus variation.
This patent application is currently assigned to Ranier Technology Ltd.. Invention is credited to Geoffrey Andrews, Martin Cable, Scott Johnson, Robert Snell.
Application Number | 20070050037 11/505632 |
Document ID | / |
Family ID | 9945504 |
Filed Date | 2007-03-01 |
United States Patent
Application |
20070050037 |
Kind Code |
A1 |
Snell; Robert ; et
al. |
March 1, 2007 |
High precision manufacture of polyurethane products such as spinal
disc implants having gradual modulus variation
Abstract
A process for making a polymeric product having a gradual
variation in modulus through at least a portion of the product is
disclosed together with an artificial spinal disc formed using the
process.
Inventors: |
Snell; Robert; (Suffolk,
GB) ; Andrews; Geoffrey; (Cambridge, GB) ;
Cable; Martin; (Cambridgeshire, GB) ; Johnson;
Scott; (Cambridgeshire, GB) |
Correspondence
Address: |
WOLF GREENFIELD & SACKS, PC
FEDERAL RESERVE PLAZA
600 ATLANTIC AVENUE
BOSTON
MA
02210-2206
US
|
Assignee: |
Ranier Technology Ltd.
Cambridge
GB
|
Family ID: |
9945504 |
Appl. No.: |
11/505632 |
Filed: |
August 17, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10530919 |
Apr 8, 2005 |
|
|
|
PCT/GB03/04352 |
Oct 8, 2003 |
|
|
|
11505632 |
Aug 17, 2006 |
|
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|
Current U.S.
Class: |
623/17.16 ;
264/255; 264/328.6 |
Current CPC
Class: |
A61F 2310/00796
20130101; A61F 2/3094 20130101; A61F 2/30756 20130101; A61F
2002/30807 20130101; A61F 2310/00365 20130101; A61F 2/442 20130101;
A61F 2/32 20130101; C08G 18/0895 20130101; A61F 2250/0018 20130101;
C08G 2120/00 20130101; A61F 2230/0015 20130101; A61F 2002/30133
20130101; C08G 18/40 20130101; A61F 2/389 20130101; B29C 41/20
20130101; A61F 2002/30014 20130101 |
Class at
Publication: |
623/017.16 ;
264/255; 264/328.6 |
International
Class: |
A61F 2/44 20060101
A61F002/44; B29C 45/00 20060101 B29C045/00; B29C 45/16 20070101
B29C045/16 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 8, 2002 |
GB |
0223327.8 |
Claims
1. A method of forming a polymeric article comprising: injecting a
first polymeric material having a first stoichiometry into a mold;
injecting a second polymeric material having a second stoichiometry
into the mold; and molding a polymeric material article having a
pre-determined portion exhibiting a gradual variation in Young's
modulus.
2. A method according to claim 1, wherein the first polymeric
material is a first polyurethane and the second polymeric material
is a second polyurethane.
3. A method according to claim 2, wherein the first and the second
polyurethanes are formed by reacting at least a multifunctional
isocyanate and a polyol.
4. A method according to claim 2, wherein relative amounts of the
multifunctional isocyanate and the polyol that react to form the
second polyurethane are different than relative amounts of the
multifunctional isocyanate and the polyol reacting to form the
first polyurethane.
5. A method according to claim 2, further comprising mixing at
least the multifunctional isocyanate and the polyol to form a first
reactive mixture and mixing at least the multifunctional isocyanate
and the polyol to form a second reactive mixture.
6. A method according to claim 5, further comprising intensively
mixing at least the multifunctional isocyanate and the polyol to
form a first reactive mixture and intensively mixing at least the
multifunctional isocyanate and the polyol to form a second reactive
mixture.
7. A method according to claim 5, further comprising introducing
the first reactive mixture into an extruder and introducing the
second reactive mixture into an extruder.
8. A method according to claim 7, comprising introducing the first
reactive mixture into a first extruder followed by introducing the
second reactive mixture into the first extruder.
9. A method according to claim 7, wherein polymerization reactions
that form the first polyurethane and the second polyurethane occur
in the extruder.
10. A method according to claim 1, comprising forming the article
in the mold.
11. A method according to claim 1, comprising injecting the first
polymeric material into the mold followed by injecting the second
polymeric material into the mold.
12. A method according to claim 1, wherein the first polymeric
material and the second polymeric material are injected
simultaneously into the mold.
13. A method according to claim 1, wherein the first polymeric
material and the second polymeric material are injected into the
mold at different injection rates.
14. A method according to claim 1, wherein different amounts of the
first polymeric material and the second polymeric material are
injected into the mold.
15. A method according to claim 1, wherein polymerization reactions
continue in the mold.
16. A method according to claim 15, wherein polymerization
reactions between the first polymeric material and the second
polymeric material occur in the mold.
17. A method according to claim 1, wherein the first polymeric
material and the second polymeric material are formed in the same
apparatus.
18. A method according to claim 1, wherein the first polymeric
material and the second polymeric material are formed in different
apparatus.
19. A method according to claim 1, further comprising arranging a
first end plate in the mold and forming the article including the
first end plate bonded to a surface of the article.
20. A method according to claim 19, wherein a second end plate is
bonded to a surface of the article.
21. A method according to claim 20, wherein the first end plate and
the second end plate are covalently bonded to polymeric material at
surfaces of the article.
22. A method according to claim 1, wherein the article is an
implant.
23. A method according to claim 22, wherein the article is a spinal
disc.
24. A method according to claim 1, wherein the article includes a
nucleus region and an annulus region.
25. A method according to claim 24, wherein the nucleus region
surrounds the annulus region.
26. A method according to claim 24, wherein a Young's modulus of
the nucleus region is less than a Young's modulus of the annulus
region.
27. A method according to claim 26, wherein the pre-determined
portion is between the nucleus region and the annulus region.
28. A method according to claim 1, wherein the Young's modulus
varies continuously across the portion of the article.
29. An artificial spinal disc according claim 24, wherein the
pre-determined portion comprises a polymeric material having a
gradual variation in stoichiometry.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 10/530,919, filed on Apr. 8, 2005, which claims the benefit of
and priority to PCT Application No. PCT/GB2003/004352, filed on
Oct. 8, 2002, the entire contents of each of which are incorporated
herein by reference.
[0002] The present invention relates to a method of manufacturing a
polymeric product and to a product for medical use made in
accordance with the method. An example of a component made from the
material and using the method of the invention is an artificial
spinal disc or disc spacer used to replace a displaced or damaged
intervertebral disc in the spine of a patient although the
invention is also applicable to the manufacture of replacement
joints in addition to other surgical instrumentation and components
for the healthcare industry.
[0003] Although reference will now be made primarily to the
application of the invention in the manufacture of an artificial
spinal disc, the invention is not limited to an artificial spinal
disc or the specific method of making such a disc, as has already
been mentioned above.
[0004] Approximately one third to a quarter of the length of an
adult human spine is occupied by the vertebral discs. Each disc
comprises an annular wall (annulus fibrosus) that surrounds and
contains a central nucleus (nucleus pulposus) filled with
gelatinous material that occupies approximately 30 to 50% of the
cross sectional area of the disc. The annular wall is a
concentrically laminated structure containing aligned collagen
fibres and fibrocartilage and provides the major stabilizing
structure to resist torsional and bending forces applied to the
disc. The discs are contained between vertebral endplates comprised
of hyaline cartilage that act as an intermediate layer between the
hard vertebrae and the softer material of the disc.
[0005] The joints and musculoskeletal tissues of the human body are
subject to traumatic injury and disease and degenerative processes
that over a period of time can lead to the deterioration or failure
of the joint causing severe pain or immobility. Generally, the
ability of a load bearing joint to provide pain free articulation
and carry load is dependent upon the presence of healthy bone,
cartilage and associated musculoskeletal tissues that provide a
stable joint. With reference to the spine, spinal disc
degeneration, characterised by features such as loss of fluid,
annular tears and myxomatous changes can result in discogenic pain
and/or disc bulging or herniation of the nucleus in which the disc
protrudes into an invertebral foramen comprising spinal nerves
resulting in back pain and/or sciatica. This condition is more
commonly referred to as a "slipped" disc.
[0006] To alleviate the condition described above, the damaged
spinal disc may be surgically removed from the spine and the two
adjacent vertebrae either side of the damaged disc fused together
(arthrodesis). Although this technique successfully eliminates the
symptoms of pain and discomfort and improves joint stability, it
results in a total loss of movement of the fused vertebral joint
and increases the stress placed on the adjacent joints leading to
collateral damage of these joints and associated soft tissues. The
degenerative cycle then begins anew.
[0007] A more desirable solution is to replace the damaged spinal
disc with an artificial implant (arthroplasty) that allows full,
pain free movement of the vertebrae and which mimics the function
of a healthy spinal disc. Artificial spinal discs currently exist
for use in such a procedure. However, the development of existing
artificial discs has been limited, despite advances in
biomaterials, because they lack the complexity of structure and
cannot adequately mimic the biomechanics of a normal healthy human
spinal disc.
[0008] Conventional artificial discs articulate using a bearing
surface manufactured using metals, alloys or durable polymers
including ultra-high molecular weight polyethylene. However, the
use of hard, non-deformable bearing surfaces render the implant
non-compliant and unable to replicate the compliant load bearing
capacity provided by the natural disc. As a result, adjacent spinal
levels are still exposed to increased mechanical stresses resulting
in a high risk of further degeneration.
[0009] Compliant artificial spinal discs are generally either
manufactured using a material of single uniform hardness (single
durometer) or using two materials of differing hardness (dual
durometer), in which case the material has a lower modulus core
contained within a higher modulus shell. The former requires a
compromise in material specification to balance strength and wear
resistance with compliance whereas the latter often generates
problems caused by a progressive failure along the interface
between the two materials over a period of use. An artificial
spinal disc of the latter type is known from U.S. Pat. No.
5,171,281.
[0010] A need therefore remains for an artificial spinal disc
implant which can be surgically inserted in place of a damaged
spinal disc and which will enable full, pain-free movement of the
affected vertebral joint, which is durable enough to withstand the
loads and wear imposed upon it in use without failing, and at the
same time exhibit biomechanics which are as similar as possible to
that of the body's own natural spinal discs and so can withstand
both compression and torsional loading. If these requirements are
not adequately met, and the artificial disc is too stiff, it will
not deform sufficiently during movement and excessive deformation
of the adjacent natural discs will occur. On the contrary, if the
disc does not have the required degree of stiffness, excessive
movement of the disc will occur causing it to bulge out resulting
in pain and discomfort for the patient.
SUMMARY
[0011] According to an aspect of the invention, there is provided a
process for making a polymeric product having a gradual variation
in modulus through at least a portion of the product, comprising
the steps of:
[0012] (a) reacting a multifunctional isocyanate, a polyol and,
optionally, a chain extender, wherein at least two reagents
selected from the isocyanate, the polyol, the chain extender, any
mixture thereof and any pre-polymer formed therefrom, are
intensively mixed to form a first polyurethane having a
predetermined stoichiometry and thermal history;
[0013] (b) reacting a multifunctional isocyanate, a polyol and,
optionally, a chain extender, wherein at least two reagents
selected from the isocyanate, the polyol, the chain extender, any
mixture thereof and any pre-polymer formed therefrom, are
intensively mixed to form a second polyurethane having a
predetermined stoichiometry and thermal history which is different
to the stoichiometry and thermal history of the first polyurethane;
and
[0014] (c) injecting the first and second polyurethanes into a
mould defining the polymeric product before the polymerisation
reactions associated with the production of the first and second
polyurethanes are complete so that polymerisation reactions between
the first and second polyurethanes occur in the mould.
[0015] Although reference is made to first and second
polyurethanes, it will be appreciated that the invention also
includes within its scope a process involving more than two
polyurethanes, each of which has a different predetermined
stoichiometry and thermal history.
[0016] The modulus referred to is the modulus of elasticity or
tensile modulus, also referred to as Young's Modulus, and is the
ratio of stress to strain below the elastic limit. The Young's
Modulus is calculated by dividing the strain into stress and
provides a measure of the stiffness of the material.
[0017] The process preferably involves the step of injecting the
first and second polyurethanes into the mould simultaneously. The
polyurethanes may be injected at the same rate into the mould or,
the relative rate of injection of the two polyurethanes into the
mould may be altered so that more of one polyurethane is injected
into the mould than the other thus providing a material that
exhibits a graduation in modulus.
[0018] In one embodiment, the process includes the step of mixing
the first and second polyurethanes prior to injection into the
mould via a common injection port. Even when the polyurethanes are
mixed prior to injection, it is possible to vary the relative rates
of injection by varying the rate of delivery of each
polyurethane.
[0019] Advantageously, the length of the common injection port may
be altered to control the degree of interfacial mixing of the first
and second polyurethanes prior to injection into the mould.
[0020] In one embodiment, the first and second polyurethanes may be
formed simultaneously in separate apparatus. In this method, known
as the dual head technique, two output streams, having different
compositions, from two PPM machines (described in more detail
below) are fed to a mould with two separate inputs at the same
time. The input ports of the mould are so arranged to cause the
mould to fill in a prearranged manner so that interfacial mixing of
the two streams occurs resulting in the formation of a graded
modulus structure. Alternatively, the two separate streams may be
mixed prior to injection into the mould in which case they are
injected through a common injection port.
[0021] A single PPM machine for producing the polyurethane may also
be employed in which case the method includes the step of forming
the first polyurethane, delivering it to an intermediate vessel and
perturbing the relative amounts of the reagents being mixed by the
machine to form the second polyurethane with a different
stoichiometry.
[0022] The method may include the step of delivering the second
polyurethane to an intermediate vessel.
[0023] The first and second polyurethanes are preferably injected
into the mould from said intermediate vessels simultaneously.
[0024] The process preferably includes the step of controlling the
temperature of each intermediate vessel to impart a known thermal
history to the first and second polyurethanes contained
therein.
[0025] According to another aspect of the invention, there is
provided a process for making a polymeric product having a variable
modulus through at least a portion of the product comprising the
steps of:
[0026] (a) reacting a multifunctional isocyanate, a polyol and,
optionally, a chain extender, wherein at least two reagents
selected from the isocyanate, the polyol, the chain extender, any
mixture thereof and any pre-polymer formed therefrom, are
intensively mixed to form a polyurethane having a predetermined
stoichiometry and thermal history;
[0027] (b) perturbing the relative amounts of said at least two
reagents during the course of the reaction to continuously vary the
modulus of the polyurethane so formed and,
[0028] (c) injecting the polyurethane into a mould defining the
polymeric product before the polymerisation reactions associated
with the production of the polyurethane is complete so that
polymerisation continues in the mould.
[0029] The polyurethane may be passed through an extruder to be
reactively extruded therein in the method according to either the
first or second aspects of the invention.
[0030] The polyurethane may advantageously undergo thermal
profiling during the reactive extrusion step.
[0031] According to another aspect of the invention, there is
provided an apparatus for making a polymeric product having a
gradual variation in modulus through at least a portion of the
product comprising:
[0032] (a) a first delivery system for quantitatively dispensing at
least two reagents selected from an isocyanate, a polyol, a chain
extender, any mixture thereof and any pre-polymer formed therefrom;
mixing means for intensively mixing said at least two reagents to
form a first polyurethane having a predetermined stoichiometry.
[0033] (b) a second delivery system for quantitatively dispensing
at least two reagents selected from an isocyanate, a polyol, a
chain extender, any mixture thereof and any pre-polymer formed
therefrom; mixing means for intensively mixing said at least two
reagents and reactive extrusion means to form a second polyurethane
having a different predetermined stoichiometry to that of the first
polyurethane, and
[0034] (c) means for injecting the first and second polyurethanes
into a mould before the polymerisation reactions associated with
the formation of the first and second polyurethanes are complete so
that polymerisation reactions between the first and second
polyurethanes occur in the mould.
[0035] In one embodiment, the apparatus includes a common injection
port for injecting the first and second polyurethanes into the
mould simultaneously.
[0036] The apparatus may also include means for varying the
relative amounts of the first and second polyurethanes injected
into the mould or, the relative rates of injection of the first and
second polyurethanes into the mould.
[0037] According to another aspect of the invention, there is
provided an apparatus for making a polymeric product having a
gradual variation in modulus through at least a portion of the
product comprising:
[0038] (a) a delivery system for quantitatively dispensing at least
two reagents selected from an isocyanate, a polyol, a chain
extender, any mixture thereof and any pre-polymer formed therefrom;
mixing means for intensively mixing said at least two reagents to
form a first polyurethane having a predetermined stoichiometry.
[0039] (b) an intermediate vessel into which the first polyurethane
is directed whilst the delivery system is used to quantitatively
dispense at least two reagents selected from an isocyanate, a
polyol, a chain extender, any mixture thereof and any pre-polymer
formed therefrom; mixing means for intensively mixing said at least
two reagents and reactive extrusion means to form a second
polyurethane having a different predetermined stoichiometry to that
of the first polyurethane, and
[0040] (c) means for injecting the first and second polyurethanes
into a mould before the polymerisation reactions associated with
the formation of the first and second polyurethanes are complete so
that polymerisation reactions between the first and second
polyurethanes occur in the mould.
[0041] In one embodiment, the apparatus includes a second
intermediate vessel into which the second polyurethane is directed
so that the first and second polyurethanes are injected from their
respective vessels into the mould simultaneously. Alternatively,
the first polyurethane is injected into the mould from an
intermediate vessel and the second polyurethane is injected into
the mould directly from the delivery system.
[0042] The apparatus may include a mix head to mix the first and
second polyurethanes prior to injection into the mould. The mix
head may also comprise means for altering the rate of injection
into the mould, or rate of mixing, of each of the
polyurethanes.
[0043] According to another aspect of the invention, there is
provided an artificial spinal disc comprising a solid body of
polymeric material that exhibits at least a portion having a
gradual variation in modulus.
[0044] In a preferred embodiment, the modulus varies substantially
linearly through said portion.
[0045] The artificial spinal disc preferably comprises a nucleus
surrounded by an annulus region, said portion being located in a
region between the nucleus and annulus regions.
[0046] The invention also provides an artificial spinal disc
manufactured according to the process of the invention.
[0047] In one preferred embodiment for the manufacture of an
artificial spinal disc, the two-part mould has a retractable
central portion, the central portion being retracted after
injection of polyurethane having a first stoichiometry and thermal
history into the first part of the mould so as to form the annulus
region so that the polyurethane having a second stoichiometry and
thermal history can be injected into the second part of said mould
so that polymerisation reactions between the polyurethanes injected
into the first and second parts of the mould can occur in the mould
to form a region between the two polyurethanes that exhibit a
gradual variation in modulus.
[0048] The present invention also seeks to protect the use of the
process according to the invention in the manufacture of an
artificial spinal disc or any surgical device or implant.
[0049] A method of manufacturing polyurethane having a high degree
of consistency making them suitable for use in the medical product
industry is known as precision polyurethane manufacture (PPM) and
is described in detail in the Applicant's own earlier International
Application No. PCT/GB01/03441 (Publication No. WO 02/11975), to
which reference is hereby made.
[0050] The PPM process enables the reagent stoichiometry and
thermal profile to be controlled dynamically to reduce batch to
batch property variation and WO 02/11975 specifically states that
the reaction stoichiometry can be controlled within 0.01-2%,
preferably within 0.05-1%, and most preferably within 0.1-0.2% and
that the thermal profile of the resultant polyurethane can be
controlled within 0.01-2.degree. C., preferably within
0.05-1.degree. C., and most preferably within 0.1-0.5.degree. C.,
preferably using a computer. However, to date, no reference has
been made to the application of the PPM process in the manufacture
of a polymer product in which the reagent stoichiometry and thermal
profile is purposively varied so as to produce a polymer product
which exhibits a gradual variation in its modulus through the whole
or part of that product.
[0051] In one embodiment, the polymer product comprises an
artificial spinal disc. The graduation in the modulus of the
material provides an artificial disc having all the benefits of a
dual material design without any of the problems associated with
the bonding of two dissimilar, separate components. The disc
contains and constrains excessive deformation whilst maintaining
the normal physiological motions of the spinal segment.
[0052] In a preferred embodiment, the artificial disc incorporates
a set of polymeric end plates that exhibit a convex surface that
engages with the surface of an adjacent vertebral body. In this
preferred embodiment, the end-plates are manufactured as part of
the complete device. Therefore, a portion of the device exhibits a
gradual change in modulus as a function of distance from the
surface of the body in an axial direction such that there are no
interfacial bonds between the end-plates and the flexible core
region of the spinal disc.
[0053] In an alternative embodiment, the end-plates are made from
rigid materials such as biocompatible metal or polymer. In one
embodiment, the variable modulus core may be firmly bonded to the
end plates so that the end-plates are incorporated into the mould
process and are not separate parts but actually covalently bonded
with the core to provide a complete device. Conversely, the
variable modulus core may be located, but not fixed, between the
two end-plates to enable the core to slide between the
end-plates.
BRIEF DESCRIPTION OF THE DRAWINGS
[0054] Embodiments of the present invention will now be described,
by way of example only, with reference to the following drawings,
in which:
[0055] FIG. 1 illustrates a front view of the reactive injection
moulding apparatus that comprises four injection lances;
[0056] FIG. 2 illustrates a side view of one of the injection
lances of the reaction injection moulding apparatus of FIG. 1;
[0057] FIG. 3 illustrates a front view of a PPM apparatus that
include lances and extruder;
[0058] FIG. 4 shows a perspective view of an artificial spinal disc
according to an embodiment of the invention together with endplates
above and below the disc;
[0059] FIG. 5 shows a plan view of the disc shown in FIG. 4;
[0060] FIG. 6 shows a cross-sectional view along the line X-X in
FIG. 5;
[0061] FIG. 7 is a graph showing the relationship between the
modulus and distance through the disc in a radial direction
indicated by X-X;
[0062] FIG. 8 is a graph showing the relationship between the
modulus and the distance through the disc in a radial direction
indicated by Y-Y in FIG. 5;
[0063] FIG. 9 is a graph showing the relationship between the
modulus and the distance through the disc in an axial direction
indicated by Z-Z in FIG. 6.
[0064] FIGS. 10A, 10B & 10C show the distribution of material
properties in a mould during the manufacture of an artificial
spinal disc according to some specific examples;
[0065] FIG. 11 is a graph to show how a material that has a gradual
modulus can be formed by varying the amounts of two materials
injected into a mould, and
[0066] FIG. 12 is a graph to show how a material that has a gradual
modulus can be formed by having a step or incremental change in the
relative amounts of materials injected into the mould.
DETAILED DESCRIPTION OF EMBODIMENTS
[0067] Referring firstly to FIG. 4, there is shown an artificial
disc 1 together with a pair of end plates 2 used in conjunction
with the disc 1. The disc 1 is a single unitary component having a
soft elastomeric core region that mimics the function of the
natural spinal disc nucleus and varies in volume by the total
elastomer volume, and a peripheral region surrounding the core
region exhibiting a graded modulus structure in which the modulus
increases as a function of the distance from the nucleus region or
decreases as a function of the distance from the surface of the
disc 1. It will be appreciated that changing the relative volume of
nucleus and annulus regions alter the overall mechanical
performance of the complete device. Therefore, the device structure
can be modified to achieve optimum physiological performance.
[0068] In the preferred illustrated embodiment, the modulus varies
from the surface of the disc to the nucleus. However, it will also
be appreciated that the modulus may vary for only a portion of that
distance. Although reference is made to a disc having a peripheral
region and a core region, it will be appreciated that these regions
are not separate or discrete and the disc is formed from one body
of material.
[0069] FIG. 5 shows a plan view of the spinal disc illustrated in
FIG. 4 and FIG. 6 shows a cross-section through the disc along the
line marked X-X in FIG. 5. Both views have been marked with contour
lines showing the change in modulus through the disc, the modulus
being greater where the lines lie close together. As can be seen
from FIG. 5, the modulus increases as a function of the distance
from the core region in a radial direction such as the directions
indicated by X-X and Y-Y in FIG. 5, and this is shown in the plots
of FIGS. 7 and 8. FIG. 7 illustrates a plot showing the
relationship between the modulus (y-axis) against distance through
the disc (x-axis). In FIG. 7, this distance is the radial distance
along the line X-X in FIG. 5. In FIG. 8, the distance is the radial
distance along the line Y-Y in FIG. 5. FIG. 9 also shows the change
in modulus with respect to the distance through the article in an
axial direction along the line Z-Z in FIG. 6. It can be seen from
these graphs, that the material of the disc is anisotropic in that
the modulus is different depending on the direction of measurement
of the modulus through the disc.
[0070] In FIG. 4, the end plates 2 are shown separated from the
disc for the purpose of the drawing only and are in intimate
contact with upper and lower surfaces of the disc respectively when
the disc is in use. The end plates 2 are usually bonded to the disc
1. However, they can also be unbonded but in close contact with the
disc 1. The end plates 2 are constructed from any suitable metallic
material or alloy that possesses sufficient stiffness to contain
the disc and suitable fatigue strength for that purpose. In other
embodiments, the end plates 2 may be provided with a structured
surface with channels and bores suitable to promote bone ingrowth.
The end plates 2 may also be coated with an osteoconductive ceramic
such as hydroxy apatite. In a preferred embodiment the end-plates
are formed as part of the complete device such that the end-plates
are covalently bonded with the core. Thus, there is a change in
modulus between the end-plates and core region See FIG. 6 in which
the incorporated end-plates are labelled `X`.
[0071] A disc 1 having the properties described above exhibits
responses to compression and compression-torsion testing under
simulated biomechanical loads that are similar to those exhibited
by the natural spinal disc during movement of a human being and has
mechanical properties (force penetration, recovery, creep) that
ensures that the disc and the end plates deliver physiological
appropriate motion (flexion, extension and torsion) to the adjacent
vertebrae similar to those of a healthy spinal disc.
[0072] Although in a preferred embodiment the polymer product made
using the method of the invention is an artificial spinal disc, it
is envisaged that other devices or components for medical use could
also be formed using one or other of the methods of the invention.
One particular component is a variable modulus bearing surface that
would replace at least a portion of damaged or degenerate articular
cartilage within a synovial joint. This may include replacement of
part of the adjacent bone to provide a reconstructed bearing
surface. Examples include, but are not restricted to, an acetabular
cup for use in hip arthroplasty procedures and a bearing surface to
replace worn cartilage on the tibial plateau for arthroplasty
procedures of the knee. These variable modulus bearing surfaces
provide cushioning to the joint and increases congruency between
articular surfaces during load bearing resulting in improved stress
distribution and reduced contact stresses that will improve the
fatigue life of the implant.
[0073] It is also envisaged that a variable modulus polymer could
be used to replace or provide additional support to a complete bone
or part of a bone in surgical procedures carried out for cosmetic
as well as medical reasons.
[0074] An alternative embodiment for use in the medical industry is
in the manufacture of an intravenous or urethral catheter which
must have the required degree of stiffness to enable it to be
passed through bodily conduits to reach the site of an occlusion
but at the same time be flexible enough to prevent unnecessary
trauma or collateral injury to the patient during an invasive
procedure. It is envisaged that at least a portion of the elongate
catheter may be formed from material having a gradual variation in
modulus along its length.
[0075] The PPM process will now be described followed by an
explanation of its application to the manufacture of a polymer
product which exhibits a gradual variation in modulus.
[0076] The PPM process comprises reacting a multifunctional
isocyanate, a polyol and, optionally, a chain extender, wherein at
least two reagents selected from the isocyanate, the polyol, the
chain extender, any mixture thereof and any pre-polymer formed
therefrom, are intensively mixed prior to being reactively
extruded, to form a polyurethane having a predetermined
stoichiometry and thermal history. In the context of the present
invention, the term "polyurethane" is understood to include any
polymer which contains multiple urethane linkages and includes, for
example, polyurethane-ureas. In an embodiment, at least a portion
of the polyol or the chain extender is reacted with the isocyanate
prior to extensive mixing, so as to "end-cap" the polyol or the
chain extruder with isocyanate groups, thus facilitating subsequent
reaction. In a modified arrangement, at least a portion of the
isocyanate is reacted with the polyol or the chain extender prior
to extensive mixing. Preferably, at least a portion of the chain
extender is mixed with the polyol prior to intensive mixing.
Advantageously, the process allows absolute control of reagent
stoichiometry whilst intimate mixing of the reagents at the
molecular level allows the manufacture of tailored linear
polyurethanes of narrow molecular weight distribution or narrow
polydispersity. Moreover, a known and reproducible thermal history
can be imparted to the polymer during synthesis, whilst overall
thermal degredation can be minimised by reducing the number of melt
cycles for polyurethane components. Such a process allows the
integrated manufacture of a polyurethane resin, finished product or
aqueous polyurethane dispersion.
[0077] The multifunctional isocyanate may be any suitable aromatic,
aliphatic or cycloaliphatic polyisocyanate, but is most preferably
an organic diisocyanate. Preferred organic diisocyanates include
4,4'-diisocyanatodiphenylmethane, 2,4'-diisocyanatodiphenylmethane,
isophorone diisocyanate, p-phenylene diisocyanate, 2,6-toluene
diisocyanate, polyphenyl polymethylene polyisocyanate,
1,3-bis(isocyanatomethyl)cyclohexane, 1,4-diisocyanatocyclohexane,
1,6-hexamethylene diisocyanate, 1,5-naphthalene diisocyanate,
3,3'-dimethyl-4,4'-biphenyl diisocyanate,
4,4'-diisocyanatodicyclohexylmethane,
2,4'-diisocyanatodicyclohexylmethane, and 2,4-toluene diisocyanate,
or combinations thereof. The polyol may be any suitable polyhydroxy
compound, but is generally a hydroxy-terminated ester, ether or
carbonate diol. Preferred polyalkylene ether glycols include
polyethylene ether glycols, poly-1,2-propylene ether glycols,
polytetramethylene ether glycols, poly-1,2-dimethylethylene ether
glycols, poly-1,2-butylene ether glycol, and polydecamethylene
ether glycols. Preferred polyester polyols include polybutylene
adipate and polyethylene terephthalate. Preferred polycarbonate
diols include polytetramethylene carbonate diol, polypentamethylene
carbonate diol, polyhexamethylene carbonate diol,
polyhexane-1,6-carbonate diol and poly[1,6-hexyl-1,2-ethyl
carbonate]diol. However, many other suitable polyhydroxy compounds
can also be used depending upon the desired application. The
polymerisation reaction may be carried out in the presence of an
activating amount of a suitable catalyst, for example, an organotin
catalyst such as stannous octanoate. However, the presence of a
catalyst is not usually necessary, due at least in part to the
efficiency of the intensive mixing step, or even desirable,
depending upon the intended application of the polyurethane, such
as, for example, in implantable devices. For some applications, the
multifunctional isocyanate and polyol are not reacted with a chain
extender. In most cases, however, a chain extender will be included
to effect chain extension or cross-linking of the urethane-linked
pre-polymer as it forms. Any suitable polyol, polythiol or
polyamine or mixture thereof that is suitable for this purpose may
be used, such as, for example, mixed diols comprising a
2,4-dialkyl-1,5-pentanediol and a 2,2-dialkyl-1,3-propanediol.
Specific examples of 2,4-dialkyl-1,5-pentanediol include
2,4-dimethyl-1,5-pentanediol, 2-ethyl-4-methyl-1,5-pentanediol,
2-methyl-4-propyl- 1,5-pentanediol, 2,4-diethyl- 1,5-pentanediol,
2-ethyl-4-propyl-1,5-pentanediol, 2,4-dipropyl-1,5-pentanediol,
2-isopropyl-4-methyl-1,5-pentanediol,
2-ethyl-4-isopropyl-1,5-pentanediol,
2,4-diisopropyl-1,5-pentanediol,
2-isopropyl-4-propyl-1,5-pentanediol, 2,4-dibutyl-1,5-pentanediol,
2,4-dipentyl-1,5-pentanediol, 2,4-dihexyl-1,5-pentanediol, and the
like. Specific examples of 2,2-dialkyl-1,3-propanediol include
2,2-dipentyl-1,3-propanediol, 2,2-dihexyl-1,3-propanediol and the
like. Especially preferred chain extenders include 1,4-butanediol,
1,2-ethylene diamine, hydrazine and triethylamine. However, many
other suitable classes of polyols and amines are known to those
skilled in the art are also included. In the context of the present
invention, the phrase "intensively mixed" generally means that the
two or more reagents selected from the isocyanate, the polyol, the
chain extender, any mixture thereof and any pre-polymer formed
therefrom, are intimately mixed at the molecular level. In a
preferred embodiment, the two or more reagents are intensively
mixed by vortexing, such that two or more reagent streams are
caused to coincide and flow together in a spiral fashion. In
another embodiment, the two or more reagents are transported via a
swash plate pump or a gear pump. Most preferably, the two or more
reagents are intensively mixed via a reactive injection processing
technique, of the kind used in conventional RIM or SRIM processes.
In a preferred embodiment, the synthesis technique of the current
invention utilises an impingement mixing head, similar to those
used in a RIM machine, and fitted with two or more reagent streams.
Each different stream can be programmed to deliver the required
amount of reagent continuously and with high accuracy not
associated with current RIM or REX techniques. Moreover, as the
reagents are intensively mixed at a molecular level, the reaction
generally starts spontaneously, thereby avoiding the need for
inclusion of reaction catalysts altogether or greatly reducing the
amounts of such catalysts required. Since many of the catalysts
normally used in such reactions are potentially highly toxic in
vivo, the inventive process is particularly suitable for making
polyurethane medical devices intended for implantation. The
mix-head is preferably self-cleaning, to avoid having to clean the
apparatus after each used. In an embodiment, the two or more
reagents are delivered to the mix-head under pressure, preferably
via one or more injection lances. For example, a first injection
lance may contain isocyanate and a second injection lance may
contain polyol and chain extender, or a first injection lance may
contain a first isocyanate, a second injection lance may contain a
second isocyanate mixed with an aliquot of a first polyol to effect
end-capping, a third injection lance may contain a second polyol
and a fourth injection lance may contain a chain extender. There
are clearly many possible combinations and permutations of the
various reagents and all of these combinations are intended to be
included within the scope of the present invention. Preferably, the
mixing step is substantially instantaneous, most preferably
occurring within a time period of fraction of a second to a few
seconds at most. It is especially desirable that the resultant
mixture is substantially homogeneous immediately after mixing,
although the subsequent composition of the mixture will obviously
change as polymerisation proceeds. In the context of the present
invention, the term "reactively extruded" is understood to mean
that the physical and chemical properties of the polyurethane
mixture are modified in a continuous flow stirred tank reactor
(CSTR) or an extruder, preferably by at least one of the various
forms of modification described above in relation to conventional
REX procedures.
[0078] The resultant mixture may be fed directly into an extruder,
the latter preferably being close coupled to the mix-head. The
extruder may be immediately adjacent to and is, preferably,
directly connected to the mix-head, such that the resultant
polyurethane pre-polymer mixture exiting the mix-head passes
straight into the extruder. The mixture may be fed into an extruder
via a rheometer, a densitometer, a spectrophotometer or any
combination thereof. This permits an instantaneous "snapshot" of
the viscosity, density or composition of the reaction mixture to be
taken before the reactive extrusion process begins and allows for
any adjustments thereto to be made. Alternatively, the resultant
mixture is fed into the extruder via a reaction chamber,
preferably, a stirred reaction chamber, to allow further
polymerization reactions to occur. Preferably, the mixture is fed
into the extruder at a rate of 0.01-25 kg/s, preferably 0.1-10
kg/s, and most preferably 1-5 kg/s. The role of the extruder is
essentially two fold. Firstly, the temperature of sections along
the length of the extruder can be controlled, thereby controlling
reaction temperature, which in turn dictates the progress of the
polymerisation reaction. Secondly, as polymerisation reactions are
taking place in the extruder, additional reagents can be introduced
at the extruder and can participate in the polymerisation reaction
to give modified polyurethanes of specific value, such as
polyurethanes where the end group is different from the groups
within the polymer chain. For example, a chain extender can be
mixed with the pre-polymer at this stage, to promote chain
extension or cross-linking and increase viscosity and molecular
weight. Sensors can be placed along the length of the extruder
barrel to monitor the reaction as it progresses, whilst temperature
control means can be used to ensure that the reaction processes
occur within defined temperature ranges. In a preferred embodiment,
therefore, the polyurethane can be made to undergo thermal
profiling during the reactive extrusion process, such that it has a
well-defined thermal history. The extruder may also comprise a
rheometer, a densitometer, a spectrophotometer or any combination
thereof at a pre-determined point along its length or at its exit,
such that the physical and chemical properties of the forming
polymer and the final polyurethane can be closely monitored and, if
necessary, adjusted during the reactive extrusion process.
[0079] It is also known to provide an apparatus for making a
polyurethane using the PPM process comprising: a delivery system
for quantitatively dispensing at least two reagents selected from
an isocyanate, a polyol, a chain extender, any mixture thereof and
any pre-polymer formed therefrom; mixing means for intensively
mixing said at least two reagents; and reactive extrusion means.
The delivery system comprises one or more injection lances,
preferably controlled by the use of linear transducers. The
reactive extrusion means comprises a barrel extruder, preferably
having one or more entry ports for introduction of various
reagents. Preferably, the reactive extrusion means is a twin screw
extruder, most preferably a co-rotating twin screw extruder. Any
commercially available twin screw extruder may be used, such as an
APV Baker MP2030 with a 30 mm screw diameter (L:D=40/l; D=30 mm)
co-rotating twin screws and sixteen heating zones, which is
available from APV Baker, Speedwell Road, Parkhouse East,
Newcastle-under-Lyme, United Kingdom. The extruder can be provided
with at least one thermocouple, to monitor the temperature of
extruded material in the immediate vicinity of the thermocouple.
The extruder may also be provided with temperature regulating
means, to control the temperature of the extruded material in the
region adjacent thereto and the apparatus may include measurement
means adapted to measure a physical or chemical parameter or
property of the polyurethane mixture. Preferably, the measurement
means comprise a rheometer, a densitometer, a spectrophotometer or
any combination thereof located between the mixing means and the
reactive extrusion means, so as to enable an instantaneous
assessment of the properties of the polyurethane or pre-polymer
mixture to be made, prior to reactive extrusion. The apparatus also
comprises at least one measurement means within or at the exit of
the reactive extrusion means to monitor the properties of the
polyurethane mixture and the final polymer. The apparatus may
further comprise means for controlling the rate at which the
delivery means dispenses said at least two reagents in response to
a first control signal; means for controlling the rate at which the
resultant mixture is fed to the reactive extrusion means in
response to a second control signal; means for controlling the
temperature of the polyurethane mixture in the reactive extrusion
means in response to a third control signal; means for detecting a
property of the polyurethane mixture during reactive extrusion and
being adapted to generate a detector signal; and a processor
adapted to generate said first, second and third control signals in
response to said detector signal. This enables the inventive
process to be automated and any necessary adjustments required to
produce a polyurethane of well defined or pre-determined
stoichiometry and thermal history to be accurately controlled. In
this last mentioned embodiment, the control and detection means are
preferably programmable means, such that their operation can be
controlled by a program run on a computer.
[0080] The PPM apparatus 3 is illustrated in FIG. 1 and comprises
four injection lances 5, a four reagent stream mix head 6 and a
mould 7. It will be appreciated that the process is not limited to
the use of 4 lances and that there may be 5 or 6 or more depending
on the number of materials to be manufactured. This means that more
than just two polyurethanes with different stiochiometries can be
made and mixed together to get a more complex variable modulus
product. It will be appreciated that may involve more than just two
intermediate vessels.
[0081] Each lance 5 is numbered for ease of identification and
comprises a hydraulic cylinder 8, a lance pump 9, an inlet pipe 10,
a non-return valve 11 and an outlet pipe 13. The mix head 6 is
adapted so that both pairs of reagent streams from the four outlet
pipes 13 directly oppose each other. The mix head 6 comprises a
cylindrical mix-pin 14 having four vertical grooves (not shown)
that are inscribed into the surface of the cylinder at equal
intervals and run along 3/8ths of its length from the mid point to
within 1/8.sup.th of its length from the bottom face of the pin
14.
[0082] Referring to FIG. 2, each lance 5 is supplied with reagent
from mix tank 15 which is stirred by a paddle stirrer 16. Reagent
is supplied to lance 5 via inlet pipe 10, passing through
non-return valve 11. Reagent is drawn from mix-tank 15 into lance 5
by raising lance pump 9 and subsequently ejected from lance 5 by
depression of lance pump 9 through the action of hydraulic cylinder
8 controlled by means of a linear transducer 17 controlled by means
of a personal computer (not shown). Reagent is supplied to mix-head
6 via outlet pipes 13. When mix pin 10 is fully inserted into mix
head 6, the grooves align with the outlet pipes 9 to provide
channels to the return pipes 18, such that the reagent streams are
recycled to mix-tanks 15 without being able to enter the mould 7.
When mix-pin 10 is retracted so that its lower face sits between
outlet pipes 13 and the return pipes 18, the reagents are
impingement mixed before passing into mould 7. When mould 7 is
full, mix pin 10 is again fully inserted allowing excess reagents
to be recycled to mix tanks 15 via return pipes 18.
[0083] FIG. 3 illustrates the complete PPM manufacturing apparatus
that comprises four injection lances 5, a mix head 6 and an
extruder 18. The extruder 18 is joined to the mix head 6 via a
rheometer 19 and has 30 mm diameter co-rotating twin screws (not
shown), sixteen programmable heating zones 20, a second rheometer
21 and a die 22. It will be appreciated that alternative forms of
extruders can also be used depending on the application. When mix
pin is retracted, mixed reagents from mix head pass via rheometer
19 into the extruder 18. Reaction mixture passes through each of
the heating zones 20 which are programmed to maintain the mixture
at a predetermined temperature before exiting the extruder 18.
[0084] In a typical run, the mix-tanks are pre-heated to the
required temperature and then charged with the required amount of
reagents, as determined by the stoichiometry, sequence distribution
requirements and length of production run required. The reagents
are continuously stirred by paddle stirrers under nitrogen until
homogenous and at constant temperature. A "dry" run is carried out
(where the mix-head pin is not retracted) under the required
reaction conditions, that is, stream mix-ratios, mix-volume and
mix-time. The pressure of each stream is monitored during this run.
The pressure in each stream can be balanced if necessary by
adjustment of a needle valve behind the mix-head orifice (not
shown). Balancing the pressure of each stream is essential, as it
facilitates good mixing as well as preventing the reaction from
taking place in the supply lines, which will occur if the pressure
of one stream is significantly greater than the other streams,
thereby filling the stream feed line. The reagents in each tank are
circulated using the lances, with the lances drawing the reagents
from the holding tank and ejecting them around the circuit and back
into the holding tank. When ready to run, the control software is
launched. The size (volume) of each lance is programmed into the
software, and can be changed if required. Several pieces of
information are requested by the software, namely: streams to be
used, mix-volume, mix-time, and mix-ratio. The "streams to be used"
information activates the required lances. The mix-volume is the
total amount of product required at the end of the run, with the
maximum volume being the sum of the selected stream volumes. The
mix-time determines the time that the mix-pin is retracted and the
velocity of the lances. The mix ratio determines the ratio of the
velocities of the lances. From this information, the software
calculates the ideal dispensing requirements. For example, using
the following parameters for 2 lances of equal size, 1 L
mix-volume, 4 seconds mix-time and a mix-ratio of 1:1, 0.5 L from
each stream is mixed over a period of 4 seconds. If a mix-ratio of
3:1 were required, 0.75 L of the first stream would be impingement
mixed with 0.25 L of the second stream (with the velocity of the
first stream being three times that of the second stream). The
software becomes invaluable when using more streams of different
sizes and different mix-ratios. The position of each lance is
measured with the linear transducer. After the above information is
entered into the software, the contents of the lances are ejected
and the "empty" reading from the transducers taken. The lances are
then filled completely, and the "full" reading taken.
[0085] When the apparatus is ready to run, the lances 5 eject
reagents into the respective transfer lines. The velocity of each
lance 5 is monitored by software and, when constant, the mix pin 14
is retracted allowing impingement mixing of the reagents in the mix
head 6. The mixed reagents undergo rapid polymerisation reactions
as they pass from the mix head 6 into the close coupled twin screw
extruder via an in-line rheometer 19. The reacting mixture passes
through the extruder 18 in a predetermined time and follows a
predetermined thermal profile dictated by the combination of the
running speed of the extruder 18 and the temperature settings of
each of the programmed temperature zones 20. An in-line rheometer
21 is fitted at the output end of the extruder 18 to give real time
measurement of Theological properties which are used as a signal to
control aspects of the operation of the process, such as extruder
speed, temperature of the extruder zones, lance speed,
stoichiometry, reagent temperature, in accordance with an algorithm
operated by a computer, to give real time control of the
rheological properties of the polyurethane.
[0086] A product having a gradual variation in modulus is produced
according to one embodiment of the invention by forming two
polyurethanes each having a different stoichiometry and/or thermal
history and formed either one after the other using the same PPM
machine or, simultaneously using two separate PPM machines. If the
polyurethanes so produced are intensively mixed prior to injection
into a mould, or are injected separately into the mould so that
they mix within it prior to completion of the polymerisation
reactions taking place, the reactions will continue between the two
polyurethanes once in the mould resulting in a product exhibiting a
variation in modulus through it. The polyurethanes formed using the
same or different machines may be stored in an intermediate vessel
prior to injection into the mould. If the same machine is being
used to produce both polyurethanes, the first polyurethane may be
stored in an intermediate vessel whilst the second polyurethane is
formed by perturbing the relative amounts of the reagents to change
the stoichiometry of the polyurethane emerging from the machine.
The second polyurethane may be injected directly into the mould
together with the first polyurethane from the intermediate vessel.
Alternatively, the second polyurethane may be stored in another
intermediate vessel so that both polyurethanes may be injected into
the mould from their respective vessels. The temperature of each
vessel may be independently controlled to impart a different
thermal history and/or viscosity to each of the polyurethanes
stored in the intermediate vessels. As already mentioned above,
each of the polyurethanes can be fed through an extruder having
independently controllable temperature sections do that the
reaction temperature and hence the progress of the polymerisation
can be controlled. The polyurethane can also be made to undergo
thermal profiling during the reactive extrusion process such that
it has a well defined thermal history.
[0087] Of particular interest to the Applicants lies in the
evaluation of the flow of reacting polyurethane systems into a
three-dimensional (3-D) mould cavity that is suitable for the
manufacture of spinal disc prostheses. Consequently, the aim is to
provide accurate 3-D placement of material within the moulding
cavity with the ideal result being a peripheral (annulus) material
surrounding a central core (nucleus) region with the nucleus
material being symmetrical from the mid-point on the vertical plane
but with a significant lateral distribution, as has already been
described with reference to FIGS. 2 to 6. Several parameters which
affect this distribution are outlined below: [0088] 1. Material
properties--viscosity, contact angle of the material. [0089] 2.
Injection Speed--With reference to a spinal disc, the injection
speed of annulus and nucleus materials are different to achieve the
correct distribution. [0090] 3. Injection port geometry--this will
influence the flow of material into the mould. With reference to a
spinal disc, injection port geometry will influence the
distribution of the nucleus material.
[0091] Many of these parameters can be varied to achieve optimum
conditions for the control of the 3-D distribution of materials
within a mould. For example, material viscosity can be influenced
by the temperature of the mould and/or intermediate vessel,
injection rate is determined by the dispense rate from the
intermediate vessel and an optimum injection port geometry can be
machined to provide the desired conditions. In contrast, the
contact angle is dependent upon the properties of the injected
material and the material used to manufactured the mould e.g.
aluminium, steel etc. Furthermore, a commercially available release
agent e.g. silicone, will also influence the interaction between
injected material and mould walls. Understanding the influence of
the contact angle will enable the injection rate and injection port
geometry to be modified accordingly to achieve the desired 3-D
distribution of material properties.
[0092] Several examples of how these parameters influence the 3-D
distribution of material properties within a mould for the
manufacture of variable modulus device such as an artificial spinal
disc prostheses will now be given.
[0093] An axisymmetric finite element (FE) model of a flat mould
(10 mm height, 20mm radius) was constructed and validated
experimentally using moulds with the same geometry and volume. The
following parameters were used (values given are approximate):
[0094] Viscosity Material A=5000 cP
[0095] Viscosity Material B=15000 cP
[0096] Contact angle of material with mould surface=60 degrees.
[0097] Injection Speeds=0.100 ml/s and 0.010 ml/s
[0098] For the purpose of this first example, Material `A` will
from the annulus and Material `B` will form the nucleus and the
injection rate will be altered to illustrate the change in 3-D
distribution of material within the mould. The resulting
distribution of material properties using a constant injection rate
of 0.010 ml/s and 0.100 ml/s are illustrated in FIG. 10A/Table 1
and FIG. 10B/Table 2 respectively. The annulus and nucleus regions
are indicated in the diagrams together with an intermediate region
that exhibits a variation in modulus between the nucleus and
annulus properties. For the purpose of this example the variation
in modulus is generated by interfacial mixing within the injection
port prior to the bulk flow reaching the mould (not shown in this
example). TABLE-US-00001 TABLE 1 Nucleus Viscosity Annulus
Injection Speed Contact Angle (cP) Viscosity (cP) (ml/s) (Degree)
15000 5000 0.010 60
[0099] TABLE-US-00002 TABLE 2 Nucleus Viscosity Annulus Injection
Speed Contact Angle (cP) Viscosity (cP) (ml/s) (Degree) 15000 5000
0.100 60
[0100] It is clear from the illustrations that a low nucleus
injection speed provides a nucleus region that is located lower in
the mould (FIG. 10A) whereas higher injection speeds provide a
nucleus region located at the top of the mould (FIG. 10B). It will
be appreciated that an optimum speed for a given mould geometry and
material properties will provide a product that exhibits symmetry
in the vertical plane. Furthermore, injection rate will also
determine the distribution of material properties in a radial
direction. For the example provided, slower injection speeds
provide a nucleus material that exhibits a greater radial
distribution compared to higher injection speeds. With reference to
a spinal disc, optimisation of the injection speed for annulus and
nucleus materials would provide a nucleus material that is located
at the centre of the mould encapsulated by annulus material. In the
same way, the distribution of material that exhibits a gradual
variation in modulus between annulus and nucleus regions can also
be precisely controlled.
[0101] It will be appreciated that the extent of material
distribution within the product will also be influenced by mould
geometry, injection port geometry and material properties and the
injection rate should be modified accordingly to achieve the
desired material distribution.
[0102] A further example using the same model demonstrates the
influence of nucleus and annulus material properties. FIG. 10C and
Table 3 illustrates the distribution of material properties in a
mould that consists of a nucleus (Material `A`) and annulus
(Material `B`) and results from using an injection speed that is
equivalent to that used in FIG. 10B. It will be appreciated that
despite the vertical momentum provided by the bulk flow of material
through the injection port, the lower viscosity nucleus is unable
to penetrate through the high viscosity annulus material at the
surface. Consequently, the nucleus material is distributed in a
radial direction to provide a nucleus region that exhibits greater
vertical and radial symmetry in comparison to the material
distribution illustrated in FIG. 10B. TABLE-US-00003 TABLE 3
Nucleus Viscosity Annulus Injection Speed Contact Angle (cP)
Viscosity (cP) (ml/s) (Degree) 5000 15000 0.100 60
[0103] It will be appreciated from the foregoing examples that the
manufacture of a variable modulus product requires precise control
of the speed of material delivery; particularly when fabricating
devices or components with a relatively small volume such as a
spinal disc prosthesis.
[0104] Whilst the example provided describes the distribution of
properties within a mould suitable for the manufacture of variable
modulus prostheses, the same principles and inventive technology
are applicable to other mould geometries and variable modulus
products that require accurate 3-D placement of the material.
[0105] The following section provides some more detail of how a
variation in modulus can be achieved as a result of varying the
relative contributions of different materials from two intermediate
vessels.
[0106] Manufacture of a device that exhibits a gradual variation in
modulus from one material (A) to another (B) can be achieved by
varying the relative amounts of materials dispensed from the
intermediate vessels. An example of this process is represented by
the graph of FIG. 11 in which the Y-axis is the percentage of
material and the X-axis is the time. At the start of the injection
process only material A (solid line) is injected into the mould.
After a given time, injection of material A is retarded whilst the
injection of material B (dashed line) begins such that the relative
amounts of both materials is continuously varied. Alternatively,
the variation in modulus between two or more materials may be
generated by having a step or incremental change in the relative
amounts of materials injected into the mould as represented in the
graph of FIG. 12.
[0107] It will be appreciated that the extent of the region of
graduated modulus will be governed by the rate at which the
injection of materials A and B are varied.
[0108] With reference to the manufacture of a spinal disc, material
A is injected first to form the annulus. Injection of material A is
then retarded as the flow of material B is increased to generate a
variation in material properties prior to injection of 100% of
material B to form the nucleus region of the spinal disc.
Manufacturing parameters described above are optimised for the
spinal disc mould geometry and material properties and a suitable
injection speed for annulus and nucleus materials chosen
accordingly to provide the desired 3-D distribution of material
properties within the mould.
[0109] In the method, known as step perturbation, the relative
amounts of the at least two reagents are perturbed during the
course of the reaction so as to continuously vary the modulus of
the polyurethane so formed. The polyurethane is then injected into
a mould defining the polymeric product.
[0110] In another proposed method, the output from the reaction
injection moulding machine may be continuous and of a fixed
composition. However, the PPM process may include an extruder
through which the polymer is passed. The extruder may be provided
with multiple ports along its length and mould may be filled with
material sampled from each port. The material from each port may be
processed differently to provide materials that possess a different
modulus. A layer of material from each port is then fed into the
mould and the layers diffuse into each other to provide a final
component having a variable modulus. The component may also undergo
a post compression stage at high temperature to form the final
product and increase the degree of diffusion of the layers into
each other to provide a product having a more gradual change in
modulus.
[0111] In accordance with the methods of the invention, a region
that exhibits a variation in modulus between the two materials can
arise through several processes. First, as mentioned previously,
materials are injected one after the other and a variation in
modulus arises through interfacial mixing between the materials.
Second, materials are injected through a common injection port that
allows interfacial mixing within the injection tube prior to flow
reaching the mould. In this regard, the extent of the graded
modulus may be modified according to the length of the injection
nozzle which determines the extent of mixing between materials
prior to the flow entering the mould. An alternative method
involves passing continuously varied amounts of each material
through a static mixer or length of tube, to achieve a resultant
material that exhibits a continual change in properties. This
method relies upon precise control of the material dispense
cylinders to achieve a variation in the relative amount of material
dispensed from each delivery stream.
[0112] Alternatively, the intermediate vessel may be a mould or
carousel. The intermediate vessel is maintained at adequate
temperature and the material reacts to form a slug. At the required
moment this slug can be injected into a cavity mould having the
shape of the required component. As the material is held in the
carousel, it enables the reactant to consolidate and a known
thermal history to be imparted into the product.
[0113] These materials may be produced using one PPM machine that
switches between the manufacture of different materials to maintain
the level of material contained within the dispense cylinders.
However, these methods are not confined to use with a single PPM or
Reaction Injection Moulding machines. Alternatively, if larger flow
rates are required, two PPM machines maybe employed to manufacture
each material separately to ensure a continual flow of materials
into the dispense cylinders. It will be appreciated that other
material manufacturing methods or arrangements of the PPM apparatus
could be used to feed the dispense cylinders to enable manufacture
of devices and components that exhibit a variation in modulus.
[0114] The intermediate vessel may alternatively be an injection
moulding machine barrel or syringe. The material may then be
injected directly into the required mould.
[0115] Many modifications and variations of the invention will be
apparent to those skilled in the art and the foregoing description
should be regarded as a description of the preferred embodiments
only.
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