U.S. patent application number 12/800219 was filed with the patent office on 2011-04-28 for radiolucent spinal fusion cage.
Invention is credited to Bret M. Berry, Darrel S. Brodke, Ashok C. Khandkar, Ramaswamy Lakshminarayanan, Mahendra S. Rao.
Application Number | 20110098818 12/800219 |
Document ID | / |
Family ID | 23104513 |
Filed Date | 2011-04-28 |
United States Patent
Application |
20110098818 |
Kind Code |
A1 |
Brodke; Darrel S. ; et
al. |
April 28, 2011 |
Radiolucent spinal fusion cage
Abstract
An improved bone graft is provided for human implantation,
particularly such as a spinal fusion cage for implantation into the
inter-vertebral space between two adjacent vertebrae. The improved
spinal fusion cage includes a substrate block of high strength
biocompatible material having a selected size and shape to fit the
anatomical space, and a controlled porosity analogous to natural
bone. The substrate block may be coated with a bio-active surface
coating material such as hydroxyapatite or a calcium phosphate to
promote bone ingrowth and enhanced bone fusion. Upon implantation,
the fusion cage provides a spacer element having a desired
combination of mechanical strength together with osteoconductivity
and osteoinductivity to promote bone ingrowth and fusion, as well
as radiolucency for facilitated post-operative monitoring. The
fusion cage may additionally carry one or more natural or synthetic
therapeutic agents for further promoting bone ingrowth and
fusion.
Inventors: |
Brodke; Darrel S.; (Salt
Lake City, UT) ; Berry; Bret M.; (Sandy, UT) ;
Khandkar; Ashok C.; (Salt Lake City, UT) ;
Lakshminarayanan; Ramaswamy; (Salt Lake City, UT) ;
Rao; Mahendra S.; (Timonium, MD) |
Family ID: |
23104513 |
Appl. No.: |
12/800219 |
Filed: |
May 10, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10941620 |
Sep 14, 2004 |
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12800219 |
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10137108 |
Apr 30, 2002 |
6790233 |
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10941620 |
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60287824 |
May 1, 2001 |
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Current U.S.
Class: |
623/17.11 |
Current CPC
Class: |
A61L 27/30 20130101;
A61F 2/30767 20130101; A61F 2002/30677 20130101; A61F 2002/30593
20130101; A61F 2230/0069 20130101; A61L 27/3608 20130101; A61F
2002/30011 20130101; A61F 2002/4629 20130101; A61F 2002/30064
20130101; A61F 2310/0097 20130101; A61F 2002/3082 20130101; A61F
2002/30968 20130101; A61F 2002/30016 20130101; A61F 2002/30616
20130101; A61F 2230/0026 20130101; A61L 27/3804 20130101; A61F
2002/30827 20130101; A61F 2002/30881 20130101; A61F 2002/30789
20130101; A61F 2/4603 20130101; A61F 2002/30224 20130101; A61L
27/3847 20130101; A61L 2430/38 20130101; A61F 2250/0019 20130101;
A61F 2002/30822 20130101; A61F 2310/00239 20130101; A61F 2/442
20130101; A61F 2002/30158 20130101; A61F 2002/30261 20130101; A61F
2002/30774 20130101; A61F 2/4611 20130101; A61F 2002/30266
20130101; A61F 2002/30891 20130101; A61F 2002/4648 20130101; A61F
2310/00203 20130101; A61L 27/56 20130101; A61F 2002/30892 20130101;
A61F 2310/00928 20130101; A61F 2002/30179 20130101; A61F 2220/0025
20130101; A61F 2230/0019 20130101; A61F 2310/00976 20130101; A61F
2310/00796 20130101; A61F 2230/0082 20130101; A61F 2002/30896
20130101; A61F 2250/0023 20130101; A61F 2002/30166 20130101; A61L
27/365 20130101; A61F 2/447 20130101; A61F 2002/30329 20130101;
A61F 2002/4627 20130101; A61L 27/32 20130101; A61F 2/446 20130101;
A61F 2230/0028 20130101; A61F 2230/0058 20130101; A61F 2002/30777
20130101; A61F 2002/3028 20130101; A61F 2002/30235 20130101; A61F
2002/2835 20130101; A61F 2002/3085 20130101; A61L 27/10 20130101;
A61F 2002/30153 20130101; A61F 2230/0063 20130101; A61F 2002/30733
20130101; A61L 2430/02 20130101; A61F 2002/3092 20130101 |
Class at
Publication: |
623/17.11 |
International
Class: |
A61F 2/44 20060101
A61F002/44 |
Claims
1-84. (canceled)
85. An implantable prosthetic component for ingrowth attachment
with adjacent spinal tissue, comprising: a substrate block
consisting essentially of a ceramic material, and having surfaces
for ingrowth attachment with adjacent spinal tissue, the substrate
block comprising: a top surface configured for ingrowth attachment
with a first vertebral body; a bottom surface configured for
ingrowth attachment with a second vertebral body adjacent to the
first vertebral body; and a substantially radiolucent body visible
under X-ray imaging, but having a density allowing for passage of
X-rays through the ceramic material of the body under
medial-lateral X-ray imaging thus facilitating post-operative
assessment of bone ingrowth from the first and second vertebral
bodies into the body through the top and bottom surfaces.
86. The implantable prosthetic component of claim 85 wherein said
substrate block includes a first, lower porosity region and a
second, higher porosity region, said second region including said
top and bottom exterior surfaces for ingrowth attachment with said
first and second vertebral bodies.
87. The implantable prosthetic component of claim 86 wherein said
second region has a porosity ranging from about 30% to about 80% by
volume.
88. The implantable prosthetic component of claim 87 wherein the
pore sizes in the second region range from about 100 microns to
about 500 microns.
89. The implantable prosthetic component of claim 86 wherein said
first region has a porosity of less than about 5% by volume.
90. The implantable prosthetic component of claim 86 wherein said
first region has a relatively high strength corresponding
substantially with cortical bone, and wherein said second region
has a porosity corresponding substantially with cancellous bone.
Description
[0001] This is a continuation-in-part of U.S. Ser. No. 10/137,108,
filed Apr. 30, 2002, which in turn claims the benefit of U.S.
Provisional Application No. 60/287,824, filed May 1, 2001.
BACKGROUND OF THE INVENTION
[0002] This invention relates generally to improvements in bone
grafts such as spinal fusion cages of the type designed for human
implantation between adjacent spinal vertebrae, to maintain the
vertebrae in substantially fixed spaced relation while promoting
interbody bone ingrowth and fusion therebetween. More particularly,
this invention relates to an implantable bone graft such as a
spinal fusion cage having an improved combination of enhanced
mechanical strength together with osteoinductive and
osteoconductive properties, in a device that additionally and
beneficially provides visualization of bone growth for facilitated
post-operative monitoring.
[0003] Implantable interbody bone grafts such as spinal fusion
devices are known in the art and are routinely used by spine
surgeons to keep adjacent vertebrae in a desired spaced-apart
relation while interbody bone ingrowth and fusion takes place. Such
spinal fusion devices are also used to provide weight bearing
support between adjacent vertebral bodies and thus correct clinical
problems. Such spinal fusion devices are indicated for medical
treatment of degenerative disc disease, discogenic low back pain
and spondylolisthesis. These conditions have been treated by using
constructs, typically made from metals such as titanium or cobalt
chrome alloys such as used in orthopedic implants, and allograft
(donor) or autograft (patient) bone to promote bone ingrowth and
fusion.
[0004] Typical interbody spinal fusion devices, such as plugs for
example, have hollow or open spaces that are usually filled with
bone graft material, either autogenous bone material provided by
the patient or allogenous bone material provided by a third party
donor. These devices also have lateral slots or openings which are
primarily used to promote ingrowth of blood supply and grow active
and live bone. These implants may also have a patterned exterior
surface such as a ribbed or serrated surface or a screw thread to
achieve enhanced mechanical interlock between adjacent vertebrae,
with minimal risk of implant dislodgement from the site. See, for
example, U.S. Pat. Nos. 5,785,710; and 5,702,453. Typical materials
of construction for such interbody spinal fusion devices include
bio-compatible carbon fiber reinforced polymers, cobalt chrome
alloys, and stainless steels or titanium alloys. See, for example,
U.S. Pat. No. 5,425,772.
[0005] Most state-of-the-art spinal fusion implants are made from
titanium alloy and allograft (donor) bone, and have enjoyed
clinical success as well as rapid and widespread use due to
improved patient outcomes. However, traditional titanium-based
implant devices exhibit poor radiolucency characteristics,
presenting difficulties in post-operative monitoring and evaluation
of the fusion process due to the radio-shadow produced by the
non-lucent metal. There is also clinical evidence of bone
subsidence and collapse which is believed to be attributable to
mechanical incompatibility between natural bone and the metal
implant material. Moreover, traditional titanium-based implant
devices are primarily load bearing but are not osteoconductive,
i.e., not conducive to direct and strong mechanical attachment to
patient bone tissue, leading to potential graft necrosis, poor
fusion and stability. By contrast, allograft bone implants exhibit
good osteoconductive properties, but can subside over time as they
assimilate into natural bone. Further, they suffer from poor pull
out strength resulting in poor stability, primarily due to the
limited options in machining the contact surfaces. Allograft bone
implants also have variable materials properties and, perhaps most
important of all, are in very limited supply. A small but finite
risk of disease transmission with allograft bone is a factor as
well. In response to these problems some developers are attempting
to use porous tantalum-based metal constructs, but these have met
with limited success owing to the poor elastic modulii of porous
metals.
[0006] A typical titanium alloy spinal fusion device is constructed
from a hollow cylindrical and externally threaded metal cage-like
construct with fenestrations that allow communication of the
cancellous host tissue with the hollow core, which is packed with
morselized bone graft material. This design, constrained by the
materials properties of titanium alloys, relies on bony ingrowth
into the fenestrations induced by the bone graft material. However,
the titanium-based structure can form a thin fibrous layer at the
bone/metal interface, which degrades bone attachment to the metal.
In addition, the hollow core into which the graft material is
packed may have sub-optimal stress transmission and
vascularization, thus eventually leading to failure to incorporate
the graft. Mechanical stability, transmission of fluid stress, and
the presence of osteoinductive agents are required to stimulate the
ingrowth of vascular buds and proliferate mesenchymal cells from
the cancellous host tissue into the graft material. However, most
titanium-based spinal fusion devices in use today have end caps or
lateral solid walls to prevent egress of the graft outwardly from
the core and ingress of remnant disc tissue and fibroblasts into
the core.
[0007] Autologous (patient) bone fusion has been used in the past
and has a theoretically ideal mix of osteoconductive and
osteoinductive properties. However, supply of autologous bone
material is limited and significant complications are known to
occur from bone harvesting. Moreover, the costs associated with
harvesting autograft bone material are high, requiring two separate
incisions, with the patient having to undergo more pain and
recuperation due to the harvesting and implantation processes.
Additionally, autologous cancellous bone material has inadequate
mechanical strength to support intervertebral forces by itself,
whereby the bone material is normally incorporated with a
metal-based construct.
[0008] Ceramic materials provide potential alternative structures
for use in spinal fusion implant devices. In this regard,
monolithic ceramic constructs have been proposed, formed from
conventional materials such as hydroxyapatitie (HAP) and/or
tricalcium phosphate (TCP). See, for example, U.S. Pat. No.
6,037,519. However, while these ceramic materials may provide
satisfactory osteoconductive and osteoinductive properties, they
have not provided the mechanical strength necessary for the
implant.
[0009] Thus, a significant need exists for further improvements in
and to the design of bone grafts such as spinal fusion implant
devices, particularly to provide a high strength implant having
high bone ingrowth and fusion characteristics, together with
substantial radiolucency for effective and facilitated
post-operative monitoring.
[0010] Hence, it is an object of the present invention to provide
an improved bone graft such as an interbody spinal fusion implant
or cage made from a bio-compatible open pore structure, which has a
radiolucency similar to that of the surrounding bone. It is also an
object of the present invention to provide a substrate of high
bio-mechanical strength for carrying biological agents which
promote intervertebral bone ingrowth, healing and fusion. It is a
further objective of the present invention to provide an interbody
fusion device which has mechanical properties that substantially
match that of natural bone.
SUMMARY OF THE INVENTION
[0011] In accordance with the invention, an improved bone graft
such as a spinal fusion cage is provided for human implantation
into the space between a pair of adjacent vertebrae, following
removal of disc material between endplates of the adjacent
vertebrae, to maintain the adjacent vertebrae in a predetermined
and substantially fixed spaced relation while promoting interbody
bone ingrowth and fusion. In this regard, the improved spinal
fusion cage of the present invention is designed for use in
addressing clinical problems indicated by medical treatment of
degenerative disc disease, discogenic lower back pain, and
spondylolisthesis.
[0012] The improved bone graft, as embodied in the form of the
improved spinal fusion cage, comprises a substrate block formed
from a bio-compatible material composition having a relatively high
bio-mechanical strength and load bearing capacity. This substrate
may be porous, open-celled, or dense solid. A preferred composition
of the high strength substrate block comprises a silicon nitride
ceramic material. The substrate block may be porous, having a
porosity of about 10% to about 80% by volume with open pores
distributed throughout and a pore size range of from about 5 to
about 500 microns. When the substrate is porous, the porosity of
the substrate block is gradated from a first relatively low
porosity region emulating or mimicking the porosity of cortical
bone to a second relatively higher porosity region emulating or
mimicking the porosity of cancellous bone. In a second embodiment,
the substrate block is a dense solid comprised of a ceramic, metal
or polymer material. This dense solid substrate would then be
attached to a second highly porous region emulating or mimicking
the porosity of cancellous bone. Preferably, the porous region
would be formed around the substrate.
[0013] In the method where a dense, solid material is used as the
substrate block, the block will be externally coated with a
bio-active surface coating material selected for relatively high
osteoconductive and osteoinductive properties, such as a
hydroxyapatite or a calcium phosphate material. The porous portion
is internally and externally coated with a bio-active surface
coating material selected for relatively high osteoconductive and
osteoinductive properties, such as a hydroxyapatite or a calcium
phosphate material. The porous region, however, may be in and of
itself a bio-active material selected for relatively high
osteoconductive and osteoinductive properties, such as a
hydroxyapatite or a calcium phosphate material.
[0014] The thus-formed bone graft can be made in a variety of
shapes and sizes to suit different specific implantation
requirements. Preferred shapes include a generally rectangular
block with a tapered or lordotic cross section to suit the required
curvature of the inter-vertebral space, in the case of a spinal
fusion device. The exterior superior and inferior surfaces of the
rectangular body may include ridges or teeth for facilitated
engagement with the adjacent vertebrae. Alternative preferred
shapes include a generally oblong, rectangular block which may also
include serrations or the like on one or more exterior faces
thereof, and/or may have a tapered or lordotic cross section for
improved fit into the inter-vertebral space. A further preferred
shape may include a crescent shape block which may also include
serrations or the like on one or more exterior faces thereof,
and/or may have a tapered or lordotic cross section for improved
fit into the inter-vertebral space. The bone graft may desirably
include notches for releasable engagement with a suitable insertion
tool. In addition, the bone graft may also include one or more
laterally open recesses or bores for receiving and supporting
osteoconductive bone graft material, such as allograft (donor) or
autograft (patient) material.
[0015] Further alternative bone graft configurations may include a
dense substrate region substantially emulating cortical bone, to
define a high strength loading bearing zone or strut for absorbing
impaction and insertion load, in combination with one or more
relatively high porosity second regions substantially emulating
cancellous bone for contacting adjacent patient bone for enhanced
bone ingrowth and fusion.
[0016] The resultant bone graft exhibits relatively high mechanical
strength for load bearing support, for example, between adjacent
vertebrae in the case of a spinal fusion cage, while additionally
and desirably providing high osteoconductive and osteoinductive
properties to achieve enhanced bone ingrowth and interbody fusion.
Importantly, these desirable characteristics are achieved in a
structure which is substantially radiolucent so that the implant
does not interfere with post-operative radiographic monitoring of
the fusion process.
[0017] In accordance with a further aspect of the invention, the
bone graft may additionally carry one or more therapeutic agents
for achieving further enhanced bone fusion and ingrowth. Such
therapeutic agents may include natural or synthetic therapeutic
agents such as bone morphogenic proteins (BMPs), growth factors,
bone marrow aspirate, stem cells, progenitor cells, antibiotics, or
other osteoconductive, osteoinductive, osteogenic, or any other
fusion enhancing material or beneficial therapeutic agent.
[0018] Other features and advantages of the invention will become
more apparent from the following detailed description, taken in
conjunction with the accompanying drawings which illustrate, by way
of example, the principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] The accompanying drawings illustrate the invention. In such
drawings:
[0020] FIG. 1 is a perspective view depicting the spinal fusion
cage in the inter-vertebral space;
[0021] FIG. 2 is a perspective view showing one preferred
embodiment of the spinal fusion cage;
[0022] FIG. 3 is a perspective view showing the load bearing
portion of the device of FIG. 2 with anterior and posterior load
bearing walls connected by a strut, relieved in the superior and
inferior aspects;
[0023] FIG. 4 is a perspective view depicting one alternative
preferred and generally rectangular bone graft such as a spinal
fusion cage;
[0024] FIG. 5 is a perspective view depicting the load bearing
portion of the device of FIG. 4 with anterior and posterior load
bearing walls connected by a strut, relieved in the superior and
inferior aspects;
[0025] FIG. 6 is a perspective view showing still another
alternative preferred form of the invention, comprising a generally
oblong, rectangular bone graft such as a spinal fusion cage;
[0026] FIG. 7 is a perspective view depicting the load bearing
portion of the device of FIG. 6 with anterior and posterior load
bearing walls connected by a strut, relieved in the superior and
inferior aspects;
[0027] FIG. 8 is an axial view of still another alternative form of
the invention, taken generally on the load being axis of the spine,
comprising a generally crescent shaped device conforming to the
natural vertebral body shape;
[0028] FIG. 9 is a perspective view of the device of FIG. 8,
showing a porous posterior margin;
[0029] FIG. 10 is a perspective view of the load bearing portion of
the device of FIG. 8, showing a anterior and lateral load bearing
walls connected by a central strut, relieved in the superior and
inferior aspects;
[0030] FIG. 11 is an axial view of a further preferred alternative
embodiment of the invention, comprising of a generally rectangular
shape with macro-pores;
[0031] FIG. 12 is a perspective view of the device of FIG. 11
showing the interconnection of the macro-pores; and
[0032] FIG. 13 is a sectional view of the device of FIG. 11 taken
generally along the mid-transverse plane 6-6 of FIG. 11 of the
device.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0033] As shown in the exemplary drawings, a radiolucent bone graft
such as a spinal fusion cage referred to generally in FIGS. 1-3 by
the reference numeral 10 is provided for seated implantation
between a pair of adjacent patient bones such as spinal vertebrae
12 (FIG. 1) to maintain the vertebrae in spaced relation while
promoting interbody bone ingrowth and fusion. In general, the
improved bone graft 10 comprises a bio-compatible substrate having
a porous construction to define an open lattice conducive to
interbody bone ingrowth and fusion, while providing a strong
mechanical load bearing structure analogous to the load bearing
properties of cortical and cancellous bone. This open-celled
substrate is coated internally and externally with a bio-active
surface coating selected for relatively strong osteoconductive and
osteoinductive properties, whereby the coated substrate provides a
scaffold conducive to cell attachment and proliferation to promote
interbody bone ingrowth and fusion attachment. The substrate may
also carry one or more selected therapeutic agents suitable for
bone repair, augmentation and other orthopedic uses.
[0034] FIGS. 1-3 illustrate the improved bone graft in the form of
an improved spinal fusion cage 10 in accordance with one preferred
embodiment, in the shape of a generally rectangular body having
ridges formed on the exposed top and bottom ends or faces 14. The
lateral, anterior, and posterior walls of the body having notches
18 for the releasable engagement with an insertion tool.
[0035] The preferred substrate composition comprises a relatively
high strength block 16 (FIG. 3). In accordance with one preferred
form of the invention, this substrate block comprises a relatively
dense 16 silicon nitride composition having a controlled porosity
and having a suitable size and shape for seated implantation, such
as into the inter-vertebral space in the case of the spinal fusion
cage 10. In a preferred form, the remainder of the substrate is
comprised of a relatively porous silicon nitride 20 (FIG. 2) having
an open-celled controlled porosity. One preferred silicon nitride
ceramic material, comprises a doped silicon nitride of the type
disclosed in copending U.S. Ser. No. 10/171,376, which is
incorporated by reference herein.
[0036] Moreover, in the preferred form, the pores are arranged with
a variable porosity gradient to define a first region of relatively
low or reduced porosity (less than about 5%) substantially
mimicking cortical bone structure and a second region of relatively
large or increased porosity (ranging from about 30% to about 80%)
substantially mimicking cancellous bone structure. In one preferred
configuration, the outer or external surfaces of the reticulated
substrate block comprise the first or low porosity region for
improved load bearing capacity, while the interior surfaces of the
substrate block comprises the second or high porosity region
mimicking cancellous bone for enhance bone ingrowth and fusion.
[0037] This high strength substrate block is surface-coated
internally and externally with a bio-active organic or inorganic
surface coating material selected for relatively strong
osteoconductive and osteoinductive properties to provide a nutrient
rich environment for cellular activity to promote interbody bone
ingrowth and fusion attachment. Preferred surface coating materials
comprise a resorbable material such as hydroxyapatite or a calcium
phosphate ceramic. Alternative glassy (amorphous) materials having
a relatively rich calcium and phosphate composition may also be
used, particularly wherein such materials incorporate calcium and
phosphate in a ratio similar to natural bone or hydroxyapatite.
Such glassy compositions may comprise a partially or fully
amorphous osteoinductive material comprising a composite of a glass
and osteoinductive calcium compound, with a composition varying
from about 100% glass to 100% osteoinductive calcium compound. The
surface coating may also comprise autologous bone marrow
aspirates.
[0038] The resultant bone graft 10 thus comprises the substrate
block formed from the high strength Material having bio-mimetic
properties and which is nonresorbable, or slowly or infinitely
slowly resorbable when implanted into the patient, in combination
with the bio-active surface coating which is comparatively rapidly
resorbable to promote rapid and vigorous bone ingrowth
activity.
[0039] The substrate block may also advantageously be coated or
impregnated with one or more selected therapeutic agents, for
example, such as autologous, synthetic or stem cell derived growth
factors or proteins and growth factors such as bone morphogenic
protein (BMP) or a precursor thereto, which further promotes
healing, fusion and growth. Alternative therapeutic agents may also
include an antibiotic, or natural therapeutic agents such as bone
marrow aspirates, and growth factors or progenitor cells such as
mesenchymal stem cells, hematopoietic cells, or embryonic stem
cells, either alone or as a combination of different beneficial
agents.
[0040] The resultant illustrative spinal fusion cage 10 exhibits
relatively high bio-mechanical strength similar to the load bearing
characteristics of natural bone. In addition, the spinal fusion
cage 10 exhibits relatively strong osteoconductive and
osteoinductive characteristics attributable primarily to the
surface coating, again similar to natural bone. Importantly, the
fusion cage 10 is also substantially radiolucent, so that the
fusion cage does not interfere with post-operative radiological
analysis of interbody bone ingrowth and fusion.
[0041] The relatively dense, high strength portion 16 is preferably
formed in a manner and with exposed faces or ends 14 with which to
withstand the axial loading of the spine. In the preferred
embodiment as shown, the anterior and posterior walls of the device
are formed as part of this high strength portion, each with exposed
upper and lower ends or faces 14. This is done to allow the high
strength region to interface with the cortical ring of the adjacent
vertebral body 12. Additionally, a strut 22 of the high strength
material extends between the anterior and posterior walls, which
beneficially provides a load bearing structure capable of
withstanding impaction and insertion loading in the
anterior-posterior direction. Consequently, the relatively porous
portion is formed in-between the dense anterior-posterior walls and
around the central strut. The porous portion thereby forms the
remainder of the device, including a large region of the superior,
inferior, and lateral aspects. The porous portion, being less dense
in nature than the high strength regions of the device, is
increasingly radiolucent, thus allowing for assessment of bone
growth and bony attachment to the adjacent vertebral body.
[0042] FIGS. 4-10 illustrate alternative configurations for
improved bone grafts such as spinal fusion cages constructed in
accordance with the present invention, it being recognized and
understood that the bone graft can be constructed in a wide range
of different geometric sizes and shapes. FIG. 4 shows a spinal
fusion cage 110 having a generally rectangular shape similar to the
fusion cage 10 shown and described in FIGS. 1-3, but the form is
elongated, as for use in replacing an entire vertebral body. As
shown, the spinal fusion cage 110 (FIG. 5) has a relatively dense
structure defined by a high strength substrate block 112 (as
previously described) coated with the bio-active surface coating
material, but wherein the relatively dense interior structure is
defined multiple struts 116 with high strength for withstanding
impaction and insertion loading in an anterior-posterior direction
between anterior and posterior walls with exposes upper and lower
ends or faces. The multiple struts 116 additionally create interior
openings which provide for lateral fluid transmission and optimize
bone growth laterally through the center of the implant. FIG. 5
shows multiple dense struts, thereby demonstrating that the porous
region is able to make contact with the adjacent superior and
inferior vertebrae. The porous region 114 is more radiolucent than
the surrounding dense portion and therefore provides enhanced
visualization for analysis of bone growth and subsequent fusion
with the adjacent vertebrae. Each of the embodiments depicted in
FIGS. 1-13 has a height dimension and may be tapered or lordotic in
shape for enhanced anatomical fit, for example, into the
inter-vertebral space or the like.
[0043] FIGS. 6-7 depicts still another alternative preferred
embodiment of a generally oblong, rectangular geometry 410 having
both a high strength, dense region 40, as well as a relatively
porous region 44 for bone in-growth. This geometry would be useful
for surgical approaches in which it is necessary to place two
implants next to each other in the intervertebral space. More
particularly, FIGS. 6-7 show a generally oblong, rectangular bone
graft such as a spinal fusion cage 410 having a tapered height
dimension in the anterior-posterior direction. The substrate block
is formed with the first region 40 of relatively low porosity
substantially mimicking cortical bone to extend across the anterior
and posterior faces and further to include at least one
interconnecting load bearing strut 42 shown in the illustrative
drawings to extend centrally in an anterior-posterior direction
within the body of the substrate block. The remainder of the
substrate block comprises the second portion 44 of relatively high
porosity substantially mimicking cancellous bone. The harder first
region 40 including the central strut 42 beneficially provides a
hard and strong load bearing structure analogous to that shown and
described with respect to FIGS. 1-5, and capable of withstanding
impaction and insertion forces in the anterior-posterior direction
without damage to the implant, while the softer second region 44
presents an exposed and large surface area for substantially
optimized interknitting ingrowth and fusion with adjacent patient
bone. In a spinal fusion cage application, the medial-lateral faces
of the implant are advantageously defined by the softer second
region 44, wherein these regions are thus exposed to traditional
medial-lateral X-ray imaging for post-operative radiological
analysis of the implant/bone interface. Persons skilled in the art
will recognize and appreciate that alternative configurations for
the load bearing strut or struts 42 may be used, such as an
X-shaped strut configuration extending in a cranial-caudial
direction, in combination with or in lieu of the exterior faces 40
and/or the anterior-posterior central strut as shown.
[0044] FIGS. 8-10 depict a further alternative preferred form of
the invention, with a generally crescent shaped geometry 510. The
substrate block is formed of a relatively dense, high strength
region 50 substantially mimicking cortical bone extending along the
anterior and lateral walls and including exposed upper and lower
ends or faces. The dense portion 50 once again beneficially
provides a strong load bearing structure capable of withstanding
axial loads in the spine. Also, the high-strength region 50 is
located along the anterior of the substrate, thereby interfacing
with the load bearing cortical bone of the adjacent vertebral body.
An integral dense strut 52 extends between the dense lateral walls
providing a load bearing structure for impaction and insertion
forces exhibited in a lateral approach. The superior, inferior, and
posterior portions of the substrate are formed with a relatively
porous material 54. This provides for bone growth and increased
radiolucency.
[0045] FIGS. 11-14 depict a still further alternative preferred
embodiment which is formed entirely of a relatively low porosity,
high-strength substrate 610. The subsequent porous structure 60 is
created by drilling or boring a plurality of macro-pores 62 into
the superior, inferior, and lateral faces of the device. This
method allows the anterior and posterior walls to remain intact and
thus be able to withstand the loading of the spinal column. The
macro-pores are oriented in both the axial direction of the spine,
as well as between the lateral walls of the device, thereby
allowing bone to grow in the direction of the spinal loading and
laterally through the substrate. The macropores are positioned in
such a manner as to allow for continuous interconnection 70,
thereby creating a meshwork of pores for bony ingrowth into the
device. The macropores extend either from one face of the device to
the opposite face 64, or towards the center of the device, extended
to a certain depth, and terminated therein 66. The blind macropores
66 in-turn create a portion in the center of the device which
remains solid and is therefore a load bearing strut 68 extending
from the anterior wall to the posterior wall and capable of
withstanding impaction and insertion loads in the
anterior-posterior direction. This macropore method can also be
utilized with geometries similar to those depicted in FIGS. 6-10,
such as the oblong rectangular 410 and the crescent 510.
[0046] In all of the embodiments of FIGS. 1-13, the substrate block
comprises a high strength porous ceramic as previously described,
and is coated with the bio-active surface coating material, again
as previously described, to enhance bone ingrowth and fusion. The
substrate block may also include one or more therapeutic agents.
Persons skilled in the art will recognize and appreciate that the
relatively low and high porosity regions 16 and 20 shown in FIGS.
2-3 will be integrally joined by a suitable albeit relatively
narrow gradient region wherein the porosity transitions
therebetween.
[0047] The improved bone graft such as the illustrative spinal
fusion cage of the present invention thus comprises an open-celled
substrate block structure which is coated with a bio-active surface
coating, and has the strength required for the weight bearing
capacity required of a fusion device. The capability of being
infused with the appropriate biologic coating agent imparts
desirable osteoconductive and osteoinductive properties to the
device for enhanced interbody bone ingrowth and fusion, without
detracting from essential load bearing characteristics. The
radiolucent characteristics of the improved device beneficially
accommodate post-operative radiological examination to monitor the
bone ingrowth and fusion progress, substantially without
undesirable radio-shadowing attributable to the fusion cage. The
external serrations or threads formed on the fusion cage may have a
variable depth to enable the base of the device to contact the
cortical bone for optimal weight bearing capacity. In addition to
these benefits, the present invention is easy to manufacture in a
cost competitive manner. The invention thus provides a substantial
improvement in addressing clinical problems indicated for medical
treatment of degenerative disc disease, discogenic low back pain
and spondylolisthesis.
[0048] The bone graft of the present invention provides at least
the following benefits over the prior art: [0049] [a] a porous
osteoconductive scaffold for enhanced fusion rates; [0050] [b] a
bio-mimetic load bearing superstructure providing appropriate
stress transmission without fatigue failure; [0051] [c] a pore
structure and size suitable for ingrowth and vascularization,
[0052] [d] the ability to absorb and retain an osteoinductive agent
such as autologous bone marrow aspirate or BMPs; [0053] [e]
bio-inert and bio-compatible with adjacent tissue and selected for
ease of resorption; [0054] [f] fabricatable and machinable into
various shapes; [0055] [g] sterilizable; and [0056] [h] low
manufacturing cost.
[0057] A variety of further modifications and improvements in and
to the spinal fusion cage of the present invention will be apparent
to those persons skilled in the art. In this regard, it will be
recognized and understood that the bone graft implant can be formed
in the size and shape of a small pellet for suitable packing of
multiple implants into a bone regeneration/ingrowth site.
Accordingly, no limitation on the invention is intended by way of
the foregoing description and accompanying drawings, except as set
forth in the appended claims.
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