U.S. patent application number 10/353533 was filed with the patent office on 2003-06-26 for artificial bone graft implant.
Invention is credited to Bearcroft, Julie A., Brosnahan, Robert, Small, Laura C..
Application Number | 20030120348 10/353533 |
Document ID | / |
Family ID | 23880468 |
Filed Date | 2003-06-26 |
United States Patent
Application |
20030120348 |
Kind Code |
A1 |
Brosnahan, Robert ; et
al. |
June 26, 2003 |
Artificial bone graft implant
Abstract
An artificial bone graft implant for use as a replacement for
living bone material in surgical procedures requiring the use of
bone graft material includes a body formed of a microporous
material and configured to be implanted into a prepared site in a
patient's bone tissue. The body has a center portion, an outer
portion and a pair of opposed outer surfaces defining the body. The
body has a small average pore size in the center portion that
gradually changes to larger average pore size in the outer portion.
The pore size of the center portion of the implant allows for a
load bearing capacity similar to natural bone and the pore size of
the outer portion of the implant allows for the ingrowth of bone
tissue.
Inventors: |
Brosnahan, Robert;
(Germantown, TN) ; Small, Laura C.; (Memphis,
TN) ; Bearcroft, Julie A.; (Bartlett, TN) |
Correspondence
Address: |
LERNER, DAVID, LITTENBERG,
KRUMHOLZ & MENTLIK
600 SOUTH AVENUE WEST
WESTFIELD
NJ
07090
US
|
Family ID: |
23880468 |
Appl. No.: |
10/353533 |
Filed: |
January 29, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10353533 |
Jan 29, 2003 |
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09718968 |
Nov 22, 2000 |
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09718968 |
Nov 22, 2000 |
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08940660 |
Sep 29, 1997 |
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6149688 |
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08940660 |
Sep 29, 1997 |
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08473658 |
Jun 7, 1995 |
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Current U.S.
Class: |
623/23.5 ;
606/76; 623/17.11 |
Current CPC
Class: |
A61F 2002/30153
20130101; A61F 2230/0004 20130101; A61F 2230/0026 20130101; A61F
2002/2835 20130101; A61F 2230/0008 20130101; A61F 2002/30785
20130101; A61F 2002/30062 20130101; A61F 2230/0034 20130101; A61F
2/30767 20130101; A61F 2002/30158 20130101; A61F 2250/0023
20130101; A61F 2002/30125 20130101; A61F 2002/30112 20130101; A61F
2/4455 20130101; A61F 2310/00179 20130101; A61F 2002/2825 20130101;
A61F 2002/30011 20130101; A61F 2230/0019 20130101; A61F 2310/00329
20130101; A61F 2/442 20130101; A61F 2310/00293 20130101; A61F
2210/0004 20130101; A61F 2002/30187 20130101; A61F 2002/30733
20130101; A61F 2002/30777 20130101; A61F 2002/30968 20130101; A61F
2/28 20130101 |
Class at
Publication: |
623/23.5 ;
623/17.11; 606/76 |
International
Class: |
A61F 002/28; A61F
002/44 |
Claims
1. A artificial bone graft implant for use as a replacement for
living bone material in surgical procedures requiring the use of
bone graft material, the implant comprising: a body formed of a
microporous material and configured to be implanted into a prepared
site in a patient's bone tissue, said body having a center portion,
an outer portion and a pair of opposed outer surfaces defining said
body; said body having a small average pore size in the center
portion that gradually changes to larger average pore size in the
outer portion, wherein the pore size of the center portion of the
implant allows for a load bearing capacity similar to natural bone
and the pore size of the outer portion of the implant allows for
the ingrowth of bone tissue.
2. The implant of claim 1, wherein the body is comprised of a
biocompatible microporous material that bonds to bone.
3. The implant of claim 2, wherein the biocompatible microporous
material is selected from the group consisting of polymers,
ceramics and composite materials.
4. The implant of claim 3, wherein the composite material comprises
calcium phosphate, bioactive glass, and bioresorbable polymers.
5. A bone implant for use as a replacement for living bone material
comprising: a body comprising a bioabsorbable material configured
and shaped to be implanted into a prepared site in bone tissue, the
body having a center portion, an outer portion and a pair of
opposed outer surfaces defining the body; the body having a
relatively low average porosity in the center portion and gradually
changing to a relatively high average porosity in the outer
portion, wherein the pore size of the center portion provides for a
load bearing capacity generally similar to material bone and the
pore size of the outer portion permits ingrowth of bone tissue.
6. The implant as claimed in claim 5, wherein the body is
monolithically formed of a bioabsorbable material.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a continuation of application
Ser. No. 09/718,968 filed Nov. 22, 2000, which is a divisional of
application Ser. No. 08/940,660 filed Sep. 29, 1997, now U.S. Pat.
No. 6,149,688, which is a continuation of application Ser. No.
08/473,658 filed Jun. 7, 1995, now abandoned.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to artificial bone graph
implants and more specifically to artificial bone graft implants
constructed so as to allow bone ingrowth while maintaining a
load-bearing strength similar to natural bone.
[0003] There are numerous surgical situations in which bone grafts
are used as part of the surgical procedure. For example, bone
grafts are used in facial reconstruction, in repairing long bone
defects, and in spinal surgery such as intervertebral discectomy
and fusion in which a bone graft implant replaces a spinal disc in
the intervertebral space.
[0004] Bone used for bone graft implants is often removed from
another portion of a patient's body, which is called an autograft.
A significant advantage of using a patient's own bone is the
avoidance of tissue rejection, but harvesting bone also has its
shortcomings. There is a risk to the patient in having a second
surgical procedure (bone harvesting) performed at a secondary site
which can lead to infection or additional pain to the patient.
Further, the bone harvesting site is weakened by the removal of the
bone. Also, the bone graft implant may not be perfectly shaped
which can cause misplacement of the implant. This can lead to
slippage or absorption of the implant, or failure of the implant to
fuse with the bone it is in contact with.
[0005] Other options for a bone graft source is bone removed from
cadavers, called allograft, or from an animal, called xenograft.
While these kinds of bone grafts relieve the patient of having a
secondary surgical site as a possible source of infection or pain,
this option carries a high incidence of graft rejection and an
increased risk of the transmission of communicable diseases.
[0006] An alternative to using living bone graft implants is the
use of a manufactured implant made of a synthetic material that is
biologically compatible with the body. With varying success,
several synthetic compositions and various geometries of such
implants have been utilized. In some instances, the implanting
surgery of such implants is accomplished without difficulty, but
the results can be unsatisfactory because any minor dents or
imperfections in the implant can cause poor bone-to-implant bonding
or an implant having a very high porosity can collapse due to lack
of mechanical strength. In other instances, the artificial implant
requires a complex surgical procedure that is difficult to perform
and may not lead Co correction of the problem again, because of the
above discussed side effects or dislocation of the artificial
implant. Presently, no fully satisfactory artificial implant is
known that can be implanted with a relatively straightforward
procedure.
[0007] Considerable study has been devoted to the development of
materials that can be used for medical implants, including
load-bearing implants, while allowing ingrowth of new bone tissue
into the implant. To be suitable for this use, the material must
meet several criteria, namely biocompatibility, porosity which
allows tissue ingrowth and a mechanical strength suitable to
bearing loads expected of natural bone without greatly exceed the
natural bone's stiffness.
[0008] Several materials have been examined as potential implant
materials including ceramics, such as hydroxylapatite,
Ca.sub.10(P0.sub.4).sub.6(OH- ).sub.2, hardened polymers and
biocompatible metals. Hydroxylapatite (HAp) has been of particular
interest because of its similarity to natural bone mineral, but it
has only been used for low load bearing applications as pure porous
HAp itself is relatively low in mechanical strength and may not
serve as a good prosthetic material for high load bearing
implants.
[0009] Studies have been directed at improving the mechanical
strength properties of an HAp material in order to render it
suitable as a high load bearing prosthetic material. European
patent EP 0577342A1 to Bonfield et al. discloses a sintered
composite of HAp and a biocompatible glass based on CaO and
P.sub.20.sub.5 that may be used in dental and medical applications
as a replacement for unmodified HAp. To date, improvements in the
mechanical strength of HAp material has been achieved at the
expense of its porosity. Upon densification necessary to achieve
adequate load bearing strength, the HAp material has a porosity
which is insufficient to provide the desired degree of bone
ingrowth.
[0010] In a study entitled "Dense/porous Layered Apatite Ceramics
Prepared by HIP Post Sintering," Materials Science, Vol. 8, No. 10,
pp. 1203 (October, 1989), by Ioku et al., the preparation of layers
of dense HAp and porous HAp from two different types of HAp powder
is discussed. This structure is prepared by first densifying
specially produced fine crystals of HAp with a post-sintering
process employing hot isostatic pressing (HIP). Then a commercial,
coarse HAp powder is molded in layers with the densified HAp.
Despite its being of academic interest, this type of HAp structure
is not suitable for fabrication into load bearing bone prosthetic
device configurations in which natural bone ingrowth may be
achieved because of its lack of strength. However, Ioku suggests
that the addition of zirconia whiskers into the dense HAp layer
might provide some of the toughness necessary for hard-tissue
prosthetic applications.
[0011] Still desired in the art is an artificial bone graft implant
that is formed of a biocompatible mineral material similar to bone
which possesses compressive strength close to that of natural bone
while providing for in growth of bone tissue for permanent
fixation.
SUMMARY OF THE INVENTION
[0012] The present invention provides an artificial bone graft
implant formed of a biocompatible mineral material which possesses
compressive strength similar to that of natural bone and allows
bone tissue ingrowth for permanent fixations. The artificial bone
graft implant is used as a replacement for living bone material in
surgical procedures requiring the use of bone graft material. The
inventive implant has a body configured to be implanted into a
prepared site in a patient's bone tissue, with the body having a
pair of opposed outer surfaces defining the body. A first and a
second porous portion form the body with the first and second
porous portions having pores of different sizes such that time
average pore size of the first porous portion is greater than the
average pore size of the second porous portion. The first porous
portion of the body is formed in the shape of a core, with the core
being in contact with the opposed outer surfaces of the body, and
the second porous portion of the body is formed in the shape of an
outer shell. The pore size of the first porous portion of the
implant allows for the ingrowth of bone tissue while the pore size
of the second portion of the implant allows for a load bearing
capacity similar to natural bone.
[0013] As will subsequently be described, the unique hybrid
structure of a dense outer shell and a porous core provides load
bearing support while simultaneously allowing bone ingrowth. The
implant of the invention may be readily implanted by established
surgical procedures, with minimal need to alter known surgical
procedures. The hybrid porous/dense construction of the implant
ensures normal load bearing and support through the eventual
ingrowth of bone tissue, and minimizes the likelihood of implant
dislocation relative to adjacent bone tissue either during surgery
or during the post-operative fusion process.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] A better understanding of the invention can be obtained when
the detailed description of exemplary embodiments set forth below
is reviewed in conjunction with the accompanying drawings, in
which:
[0015] FIG. 1 is a perspective view of the present invention in the
form of a spinal disc implant;
[0016] FIG. 1A is a plan view of the posterior end of the implant
of FIG. 1 taken along lines 1A-1A;
[0017] FIG. 1B is a cross sectional view of the implant of FIG. 1
taken along lines 1B-1B in FIG. 1A;
[0018] FIG. 1C is a cross sectional view of the implant of FIG. 1A;
taken along lines 1C-1C in FIG. 1;
[0019] FIG. 1D is a cross sectional view of the implant of FIG. 1
taken along lines 1D-1D in FIG. 1A;
[0020] FIG. 2 is a perspective view of the present invention in the
form of a femoral ring implant;
[0021] FIG. 2A is a plan view of the posterior end of the implant
of FIG. 2 taken along lines 2A-2A;
[0022] FIG. 2B is a cross sectional view of the implant of FIG. 2
taken along lines 2B-2B in FIG. 2A;
[0023] FIG. 2C is a cross sectional view of the implant of FIG. 2
taken along lines 2C-2C in FIG. 2;
[0024] FIG. 3 is a perspective view of the present invention in the
form of an alternate spinal disc implant;
[0025] FIG. 3A is a plan view of the posterior end of the implant
of FIG. 3 taken along lines 3A-3A in FIG. 3;
[0026] FIG. 3B is a cross sectional view of the implant of FIG. 3
taken along the lines of 3B-3B if FIG. 3A; and
[0027] FIG. 4 is a schematic top plan view of a human cervical
vertebra with an implanted spinal disc implant of FIG. 1 indicated
in solid lines.
DETAILED DESCRIPTION
[0028] The artificial bone graft implant of the present invention
has two basic portions, each composed of a biocompatible
microporous material. A first portion is a core element and a
second portion is a shell element that partially surrounds, and is
bonded with, the core element through an interface region. The core
is formed of a highly porous composition which allows for bone
ingrowth and the shell is formed of a low porosity dense
composition which provides for mechanical strength. This results in
the average pore size of the core being greater than the average
pore size of the shell. Also, the percent porosity of the core will
be greater that the percent porosity of the shell. The interface
region connecting the two portions may have gradient pore sizes in
which the gradient range goes from a large pore size adjacent the
core element to a small pore size adjacent the shell element. The
core element can be formed in any shape but in one preferred
embodiment it is formed of two uniformly sized cores generally
oblong in shape and spaced apart from each other. A second
preferred embodiment has one core element generally rectangular in
shape.
[0029] The shell element can also have a porous coating on its
outer surface to promote bone ingrowth over all or a portion of the
shell element in addition to the highly porous core element.
Alternatively, the shell element can be formed with a gradient of
pore sizes rather than being formed of a unitary low porosity dense
composition. In this embodiment, a center portion of the shell
element is of a dense or low porosity which gradually changes to a
high porosity outside surface. The core element remains the same in
this alternative embodiment.
[0030] The inventive implant is made from a microporous material
that, after surgical implantation, bonds to the natural bone of the
patient to form a rigid structure. Such material encompasses, but
is not limited to, biocompatible metallics, ceramics (including
hydroxylapatite), polymers, and composite materials consisting of
phosphate(s), bioactive glass(es), and bioresorbable polymer(s).
The implant is preferably made from a ceramic, most preferably a
hydroxylapatite such as calcium hydroxylapatite, having a chemical
formula Ca.sub.10(P0.sub.4).sub.6(OH).- sub.2, available from Smith
& Nephew Richards, Inc, 1450 Brooks Road, Memphis, Tenn. 08116
U.S.A. The use of such materials in implants is known in the art,
see for example U.S. Pat. No. 4,863,476, whose disclosure is
incorporated in its entirety by reference herein.
[0031] The dense portion of the preferred hydroxylapatite implant
can be formed by pressing the dry HAp powder which is followed by
sintering. The amount of pressure required for the pressing is
dictated by the shape of the implant, but is typically in the range
of 1000 to 2000 psi (6.9 to 14.8 MPa). The pressure is used to
consolidate the powder and maximize packing. The optimal sintering
protocol is dependent on the size and shape of the green
(unsintered) part. The sintering could, but does not necessarily,
include simultaneous use of heat and isostatic pressure (HIP).
Sintering atmospheres can include argon, nitrogen, air and vacuum.
The porous portion of the preferred hydroxylapatite implant
involves then addition of bubbling agents, such as hydrogen
peroxide, to a HAp slurry. The slurry is then dried and
sintered.
[0032] The hybrid dense/high porosity structure of the inventive
implant can be produced by a two-step process wherein the dense and
highly porous portions are produced separately (as described above)
then combined with the interface and sintered to produce the final
hybrid implant.
[0033] Alternatively, the hybrid dense/highly porous implant can be
produced in a one-step process by injection molding a HAp slurry
containing a binder that will burn out during sintering which will
create the different porosity of the core and shell.
[0034] In an alternative embodiment, the inventive implant can be
formed of a unitary structure having a gradient of pore sizes. The
preferred gradient consists of a dense or low porosity center which
gradually changes to a high porosity outside surface. The average
pore size of the highly porous region would range from 100 to 800
microns. Bone requires a minimum pore size before ingrowth can
occur. The maximum size would be limited by the strength
requirements of the implant. The percent porosity at which the
implant structure is considered to have high porosity is generally
between about 30% porosity (or 70% dense) to 40% porosity (or 60%
dense) which would still allow the implant to maintain the required
mechanical strength. However, the maximum amount of percent
porosity would most likely need to be lower than 40% in order to
maintain adequate strength of the implant structure.
[0035] The artificial bone graft implant of the present invention
can be formed in any desirable shape for use in surgical procedures
requiring a bone graft implant, such as facial reconstruction, the
repair of long bone defects, and spinal surgery. For example, in
spinal surgery, bone graft implants are frequently used when a
fusion is done as part of an intervertebral discectomy procedure.
During the fusion procedure, the bone graft implant is inserted
into the intervertebral space after the disc is removed.
[0036] An embodiment of the present invention as an artificial
spinal disc implant 10 is illustrated in FIGS. 1, 1A-D. As shown,
implant 10 has two opposed lateral surfaces 12, 14 with an anterior
end 16 that tapers toward a posterior end 18. The implant includes
a pair of high porosity inner cores or inserts 20, 22 that extend
between the opposed lateral surfaces 12, 14, a low porosity dense
shell 24 surrounding the cores 20, 22, and a region of gradient
porosity 26 can separate the cores 20, 22 and shell 24. The cores
20, 22 are generally oblong in shape with curved end portions and
are generally spaced equal-distant apart from each other and the
outer surface of the opposed lateral surfaces 12, 14.
[0037] A second embodiment of the invention as a femoral ring
implant 10A, is illustrated in FIGS. 2, 2A-2C. Implant 10A has one
central porous core 28 extending between the opposed. lateral
surfaces 12, 14 and a dense shell 30. The central core 28 can be
separated from the dense shell 30 by a region of gradient porosity
32. The central core 28 is generally rectangular in shape but could
also be generally oval in shape. The femoral ring implant 10A can
be used to repair long bones or it can be used as a spinal disc
replacement such as implant 10.
[0038] The inventive implant formed of a unitary structure having a
gradient of pore sizes is illustrated in FIGS. 3, 3A, 3B as implant
10B. Implant 10B has a dense or low porosity center 32 between the
opposed lateral surfaces 12, 14 which gradually changes to a high
porosity outer portion 34, as shown in FIG. 3B.
[0039] The implant 10 is illustrated in FIG. 4 as it would appear
when implanted between two cervical spinal vertebrae V (with only
one vertebrae being shown) as a replacement for a spinal disc.
[0040] In the preferred embodiment of the artificial bone graft
implant, the implant is formed of HAp including the core, shell,
and gradient regions. The implant may also be made of calcium
phosphate or other microporous ceramics, polymers and composite
materials. The dense HAp element has a mechanical strength
sufficient to bear the loads normally experienced by natural bones,
and a compressive strength similar to the natural bones of the
body. The porous HAp core element allows for natural bone ingrowth
which facilitates permanent fixation of the implant after
implantation in natural bone.
[0041] Although particular embodiments of the invention have been
described in detail for purposes of illustration, various
modifications may be made without departing from the spirit and
scope of the invention. Accordingly, the invention is not to be
limited except as by the appended claims.
* * * * *