U.S. patent application number 13/936495 was filed with the patent office on 2013-10-24 for spinal implant system and method.
The applicant listed for this patent is Zimmer Spine, Inc.. Invention is credited to Wayne Gray.
Application Number | 20130282127 13/936495 |
Document ID | / |
Family ID | 39563612 |
Filed Date | 2013-10-24 |
United States Patent
Application |
20130282127 |
Kind Code |
A1 |
Gray; Wayne |
October 24, 2013 |
SPINAL IMPLANT SYSTEM AND METHOD
Abstract
Embodiments of the present invention provide a spinal implant
system and method. One embodiment of the present invention includes
a spinal implant comprising, a first implant plate, a second
implant plate, a spacer member coupled between the first implant
plate and the second implant plate and an end plate coupled to the
spacer member, the end plate configured to couple to adjacent
vertebrae. The implant plates can include spacer channels that
receive the spacer member and insertion tool channels that receive
an insertion tool. Preferably, on each implant plate, the spacer
channel is near the center of the plate and the insertion tool
channels are on either side of the spacer channel. The spacer
channels and insertion tool channels can be dovetailed or otherwise
shaped to capture a portion of the spacer member and insertion
tool. Mating connectors can prevent removal of the spacer member
from the spacer channels.
Inventors: |
Gray; Wayne; (Pflugerville,
TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Zimmer Spine, Inc. |
Minneapolis |
MN |
US |
|
|
Family ID: |
39563612 |
Appl. No.: |
13/936495 |
Filed: |
July 8, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11752200 |
May 22, 2007 |
8480715 |
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13936495 |
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Current U.S.
Class: |
623/17.16 |
Current CPC
Class: |
A61F 2002/30616
20130101; A61F 2002/3092 20130101; A61B 17/8047 20130101; A61B
17/8057 20130101; A61F 2/442 20130101; A61F 2002/305 20130101; A61F
2220/0025 20130101; A61F 2002/30878 20130101; A61F 2002/4681
20130101; A61F 2002/4627 20130101; A61F 2/4455 20130101; A61F
2002/30387 20130101; A61F 2002/4628 20130101; A61F 2002/30401
20130101; A61F 2002/30604 20130101; A61F 2230/001 20130101; A61F
2002/3013 20130101; A61B 17/8052 20130101; A61B 17/8033 20130101;
A61F 2002/30772 20130101; A61F 2/4611 20130101; A61F 2002/2835
20130101; A61B 2017/0256 20130101; A61F 2002/30578 20130101; A61F
2002/3082 20130101; A61F 2002/4687 20130101; A61F 2310/00976
20130101; A61B 17/7062 20130101; A61F 2310/00407 20130101; A61F
2310/00796 20130101 |
Class at
Publication: |
623/17.16 |
International
Class: |
A61F 2/44 20060101
A61F002/44 |
Claims
1. (canceled)
2. A spinal implant comprising: a first support member capable of
engaging a first vertebral body; a second support member capable of
engaging a second vertebral body, the first and second support
members capable of moving relative to each other; an attachment
member including a spacer and an end plate, the spacer capable of
being disposed between the first and second support members to
maintain a relative position of the first and second support
members, the end plate defining a through hole capable of receiving
a bone screw inserted through the end plate and into one of the
adjacent vertebral bodies.
3. The spinal implant of claim 1, wherein the spacer is capable of
being inserted between the first and second support members after
the first and second support members are inserted between adjacent
vertebrae.
4. The spinal implant of claim 2, wherein the first and second
support members are capable of being moved away from each other
after being inserted between first and second adjacent
vertebrae.
5. The spinal implant of claim 2, wherein the spacer and end plate
are formed of a single piece of material.
6. The spinal implant of claim 2, further comprising a retention
member disposed within the through hole, the retention member
capable of retaining the bone screw therein.
7. The spinal implant of claim 2, wherein the first and second
support members each have a channel on an inner face to receive the
spacer.
8. The spinal implant of claim 7, wherein the channels are located
at approximately the center of the first and second support
members.
9. The spinal implant of claim 7, wherein each of the first and
second support members comprise one or more detents and the spacer
comprises one or more indents to mate with the detents, the detents
and indents positioned to prevent full removal of the spacer from
the first and second channels.
10. The spinal implant of claim 2, wherein the end plate defines a
set of through holes including at least one through hole positioned
to be proximate to a first of the adjacent vertebrae and a second
through hole positioned to be proximate to a second of the adjacent
vertebrae.
11. The spinal implant of claim 2, wherein at least a portion of
the spinal implant is formed of a biocompatible material.
12. The spinal implant of claim 2, wherein the first and second
support members each have outer surfaces capable of promoting
osseointegration.
13. The spinal implant of claim 2, wherein the first and second
support members are shaped to substantially conform to the shape of
the adjacent vertebrae.
14. The spinal implant of claim 2, wherein outer surfaces of the
first and second support members are sloped.
15. The spinal implant of claim 2, wherein the first and second
support members each have holes to promote bone growth.
16. The spinal implant of claim 2, wherein the first and second
support members and the spacer form an assembly having a width that
is greater than a height of the assembly.
17. A spinal implant comprising: a first support member capable of
engaging a first vertebral body; a second support member capable of
engaging a second vertebral body, the first and second support
members capable of moving relative to each other after being
inserted between adjacent first and second vertebral bodies; a
spacer capable of being inserted between the first and second
support members to maintain a relative position of the first and
second support members, wherein the spacer is capable of being
inserted between the first and second support members after the
first and second support members are inserted between adjacent
vertebrae; and an end plate capable of being coupled to the spacer,
the end plate defining at least one through hole capable of
receiving a bone screw inserted through the end plate and into one
of the adjacent vertebral bodies.
18. The spinal implant of claim 17, wherein the spacer and end
plate are formed of a single piece of material.
19. The spinal implant of claim 17, further comprising at least one
retention member disposed within the at least one through hole, the
retention member capable of retaining the bone screw therein.
20. The spinal implant of claim 17, wherein the first and second
support members each have a channel on an inner face to receive the
spacer.
21. The spinal implant of claim 17, wherein the first and second
support members are shaped to substantially conform to the shape of
the adjacent vertebrae.
Description
CROSS-REFERENCES TO RELATED APPLICATION
[0001] The present application is a continuation of and claims
priority to U.S. patent application Ser. No. 11/752,200, filed May
22, 2007, the complete disclosure of which is herein incorporated
by reference.
TECHNICAL FIELD OF THE INVENTION
[0002] Embodiments of the present invention relate to spinal
implants. Even more particularly, embodiments of the present
invention relate to spinal implants that utilize implant plates and
spacers.
BACKGROUND OF THE INVENTION
[0003] An intervertebral disc may be subject to degeneration due to
trauma, disease, and/or aging. Treatment of a degenerated disc may
include partial or full removal of the intervertebral disc. This
may destabilize the spinal column resulting in subsidence or
deformation of vertebrae and possible alteration of the natural
separation distance between adjacent vertebrae. During spinal
fixation surgery, a spinal implant can be inserted in the space
created by the removal or partial removal of the intervertebral
disc between adjacent vertebrae. The spinal implant may maintain
the height of the spine and restore stability to the spine.
Maintaining the appropriate distance between the vertebrae helps
reduce the pressure applied to nerves that pass between the
vertebral bodies, thereby reducing pain and nerve damage.
[0004] Various types of spinal implants may be inserted into the
space provided by the discectomy. The spinal implant may be a
fusion device that allows bone growth to fuse the implant to the
adjacent vertebrae. One type of implant used to promote fusion
includes a pair of engaging plates and struts. The engaging plates
engage the vertebrae and the struts separate the engaging plates to
provide the appropriate separation. The engaging plates can be
selected to achieve a desired lordotic angle. Implants having
engaging plates and struts are described in U.S. Pat. No. 6,045,579
by Hochschuer et al., U.S. Provisional Patent Application No.
60/363,219 by Landry et al. and U.S. patent application Ser. No.
10/387,361 by Landry et al., each of which is fully incorporated by
reference herein.
[0005] Spinal implants as described above can provide the proper
lordotic alignment and vertebral separation for a particular
patient. Such implants, however, typically rely on the compressive
forces of the spine to hold them in place. The spinal implant,
however, may move laterally causing the implant to become
misaligned.
SUMMARY OF THE INVENTION
[0006] Embodiments of the present invention provide a spinal
implant system and method. One embodiment of the present invention
includes a spinal implant comprising a first implant plate, a
second implant plate, a spacer member coupled between the first
implant plate and the second implant plate and an end plate coupled
to the spacer member, the end plate configured to couple to
adjacent vertebrae. The implant plates can include spacer channels
that receive the spacer member and insertion tool channels that
receive an insertion tool. Preferably, on each implant plate, the
spacer channel is near the center of the plate and the insertion
tool channels are on either side of the spacer channel. The spacer
channels and insertion tool channels can be dovetailed or otherwise
shaped to capture a portion of the spacer member and insertion
tool. Mating connectors can prevent removal of the spacer member
from the spacer channels.
[0007] The implant plates can have various sizes. Additionally, the
implant plates can have various slopes to achieve a particular
lordotic angle when implanted. The spacer member can also have a
selected shape to achieve a desired separation between the implant
plates and lordotic angle. According to one embodiment, the end
plate and spacer member can be a single piece of material.
[0008] Another embodiment of the present invention can include a
spinal implant comprising a first implant plate, a second implant
plate, a spacer member at least partially inserted between the
first implant plate and the second implant plate and an end plate
integrated with the spacer member configured to couple to adjacent
vertebrae. The first implant plate can include a first spacer
channel, a first insertion tool channel and a second insertion tool
channel. The first spacer channel can be least partially defined by
sidewalls configured to capture at least a first portion of the
spacer member. The first insertion tool channel and the second
insertion tool channel are positioned on opposite sides of the
first spacer channel and can be at least partially defined by
side-walls configured to capture respective portions of the
insertion tool. The second implant plate can comprise a second
spacer channel, third insertion tool channel and fourth insertion
tool channel. The second spacer channel can be at least partially
defined by sidewalls configured to capture at least a second
portion of the spacer member. The third insertion tool channel and
fourth insertion tool channel are positioned on opposite sides of
the second spacer channel and can be at least partially defined by
sidewalls configured to capture respective portions of the
insertion tool.
[0009] Another embodiment of the present invention can include a
method of forming a spinal implant comprising inserting a first
implant plate having a first spacer channel and a second implant
plate having a second spacer channel in a space between adjacent
vertebrae, distracting the first implant plate and second implant
plate from an initial position to a second position with an
insertion tool, moving an end plate and spacer member to insert the
spacer member in the first spacer channel and the second spacer
channel, and fastening the end plate to the adjacent vertebrae. The
spacer member is guided to the first spacer channel and second
spacer channel using the insertion tool.
[0010] Yet another embodiment of the present invention can include
a spreader for forming an implant between adjacent bone structures
comprising, a first arm configured to couple to first implant plate
and a second arm configured to couple to a second implant plate.
The first arm and second arm are configured to distract to move the
first implant plate and second implant from an initial position to
a distracted position and are shaped to guide an end plate and
spacer member from a first position to a second position in which
the spacer member is coupled to the first implant plate and the
second implant plate.
BRIEF DESCRIPTION OF THE FIGURES
[0011] A more complete understanding of the present invention and
the advantages thereof may be acquired by referring to the
following description, taken in conjunction with the accompanying
drawings in which like reference numbers indicate like features and
wherein:
[0012] FIG. 1 is a diagrammatic representation of a side view of
one embodiment of a spinal implant;
[0013] FIG. 2 is a diagrammatic representation of an oblique view
of one embodiment of a spinal implant;
[0014] FIG. 3 is a diagrammatic representation of an inside surface
of one embodiment of an implant plate;
[0015] FIG. 4 is a diagrammatic representation of an end view of
one embodiment of an implant plate;
[0016] FIG. 5 is a diagrammatic representation of various
embodiments of channel shapes;
[0017] FIG. 6 is a diagrammatic representation of a cross-sectional
view of one embodiment of an integrated end plate and spacer
member;
[0018] FIG. 7 is a diagrammatic representation of a top view of one
embodiment of an integrated end plate and spacer member;
[0019] FIG. 8 is a diagrammatic representation of and end view of
one embodiment of an integrated end plate and spacer member;
[0020] FIG. 9 is a diagrammatic representation of another end view
of one embodiment of an integrated end plate and spacer member;
[0021] FIG. 10 is a diagrammatic representation of one embodiment
of an insertion tool;
[0022] FIG. 11 is a diagrammatic representation of one embodiment
of an arm of an insertion tool;
[0023] FIGS. 12a-12c are diagrammatic representations of an
insertion tool and spinal implant in various stages of an insertion
process;
[0024] FIG. 13 is a diagrammatic representation of one embodiment
of a bone screw attached to an end plate;
[0025] FIG. 14 is a diagrammatic representation of another
embodiment of a bone screw attached to an end plate;
[0026] FIG. 15 is a diagrammatic representation of an embodiment of
multiple bone screws attached to an end plate;
[0027] FIG. 16 is a diagrammatic representation of another
embodiment of multiple bone screws attached to an end plate;
[0028] FIG. 17 is a diagrammatic representation of another
embodiment of a bone screw and end plate;
[0029] FIG. 18 is a diagrammatic representation of yet another
embodiment of a bone screw and end plate;
[0030] FIG. 19 is a diagrammatic representation of an embodiment of
a ring;
[0031] FIGS. 20a-20b are diagrammatic representations of
cross-sectional views of various embodiments rings;
[0032] FIG. 21a-21b are diagrammatic representations of
cross-sectional views of various embodiments of bone screw heads;
and
[0033] FIG. 22 is a diagrammatic representation of another
embodiment of a bone screw and end plate.
DETAILED DESCRIPTION OF THE INVENTION
[0034] Preferred embodiments of the invention are illustrated in
the FIGURES, like numerals being used to refer to like and
corresponding parts of the various drawings.
[0035] Embodiments of the present invention provide spinal implants
and methods. The spinal implant may be a fusion device that allows
bone growth to fuse the implant to the adjacent vertebrae.
According to one embodiment, a spinal implant can include implant
plates to engage adjacent vertebrae and a spacer to maintain
separation between the implant plates. The connection between the
implant plates and spacer can be a frictional or interference
connection that prevents the implant plates from fully disengaging
from the spacer. According to one embodiment, for example, each
engaging plate can include a dovetailed channel that receives a
complementarily shaped portion of the spacer. The dovetails (or
shaped surface) prevent the implant plates from vertically
separating from the spacer during use. The friction and/or
interference connection can limit relative motion of the spacer and
implant plates to prevent the spacer and implant plates from
disengaging. The spacer can be connected to or integrated with an
end plate that attaches to the adjacent vertebral bodies using
fasteners. The end plate can reduce stress on the spacer and
prevent the spacer from exiting the space between the vertebral
bodies. The spinal implant, according to one embodiment, can be
adapted for anterior procedures such that a surgeon inserts the
engaging plates from an anterior position and fastens the end plate
to an anterior side of the vertebral bodies.
[0036] The implant plates can also be adapted to receive an
insertion tool. Preferably, the insertion tool can be used to both
separate the implant plates during insertion and guide the spacer
to channels in the implant plate. According to one embodiment, the
insertion tool can include two arms with one arm supporting the
upper implant plate and the other arm supporting the lower implant
plate. Each arm can include two or more prongs that are received by
complementary channels in the respective implant plate. According
to one embodiment, the prongs of the insertion tool are spaced so
that they can straddle a portion of the end plate and act as guides
to guide the end plate and the spacer to the implant plates. The
end plate is positioned between the prongs so that the end plate
can move towards the implant plates.
[0037] During a procedure, a surgeon can select the appropriate
implant plates, spacer or end plate based on size, desired lordotic
angle or other factors. The surgeon can position the end plate
between the prongs of each arm of the insertion tool so that the
end plate is unable to fall out of the insertion tool when prongs
are parallel with the ground, but is able to move towards the end
of the insertion tool. The surgeon can connect the implant plates
to the end of each arm. The insertion tool can then be inserted
into the body so that the engaging plates are in the cavity formed
by the removal or partial removal of the vertebral disc. The
surgeon can separate the arms of the insertion tool to distract the
implant plates. When the implant plates are suitably positioned,
the surgeon can move the end plate towards the implant plates
causing the spacer to couple to the implant plates to complete the
implant. The surgeon can further fasten the end plate to the
vertebral bodies using suitable fasteners.
[0038] FIG. 1 is a diagrammatic representation of a side view one
embodiment of an implant 100. Implant 100 can include implant
plates 110 and 112 to contact the vertebrae, a spacer member 115 to
maintain a vertical distance between the adjacent vertebrae and an
end plate 120 to secure the implant. Spacer member 115 can be
coupled to end plate 120 (e.g., as an integrated piece, using a
fastener or through other suitable mechanism). Implant 100 can
comprise any biocompatible material, including, but not limited to,
titanium, titanium allow, stainless steel, ceramic material, bone,
polymers or combinations thereof. In one embodiment, implant 100 is
formed of a titanium and aluminum alloy, such as Ti6A14V-Eli.
[0039] Implant plates 110 and 112 may have a variety of different
form factors and sizes. For example, outer face 125 may be angled
relative to inner face 130 and outer face 127 may be angled
relative to inner face 132 so that a desired alignment of the
adjacent vertebrae is achieved when implant 100 is in place. In
other words, the outer faces of the implant plates may be sloped to
allow an anterior side height to differ from a posterior side
height. In another embodiment, spacer member 115 may be sloped to
achieve a similar result. In addition to a slope, outer faces 125
and 127 may be curved.
[0040] This curvature may allow outer faces 125 and 127 to
substantially conform to the shapes of vertebral surfaces,
particularly the anatomical domes of the respective vertebra.
Preferably, outer faces 125 and 127 achieve at least 75% contact
with the corresponding vertebrae.
[0041] Various surfaces of implant plates 110 and 112 can be
treated to promote osseointegration. For example, outer faces 125
and 127 can be coated with titanium plasma spray, bone morpohogenic
proteins, hydroxyapatite and/or other coatings. In addition to or
instead of coating outer faces 125 and 127, outer faces 125 and 127
may be roughed by processes such as, but not limited to, chemical
etching, surface abrading, shot peening, electric discharge
roughening or embedding particles in the surface.
[0042] Implant plates 110 and 112 may include a number of
protrusions 135 that can extend into adjacent vertebrae to better
hold implant plates 110 and 112 in place. Protrusions 135 can be
arranged in radial rows or other arrangements with any number of
protrusions. Protrusions 135 can extend any distance, but
preferably extend from 0.2 mm to 1 mm from the respective outer
face.
[0043] Surgical kits for implant 100 can include any number of
implant plates. For example, a surgical kit for implant 100 can
include a number of small, medium and large implant plates with
various slopes from that, as an example, range from 0 to 9 degrees
in approximately three degree increments. This allows the surgeon
to form implant 100 to have the appropriate sized plates for a
patient and to achieve lordotic adjustment from about 0 degrees
(where both implant plates have 0 degree slopes) to 18 degrees
(where both implant plates have 9 degree slopes). In other
embodiments, plates with different slopes can be selected (e.g., a
lordotic adjustment of 9 degrees can be achieved by selecting an
Implant plate with a 0 degree slope and an Implant plate with a 9
degree slope). In yet another embodiment, the surgeon can select
spacer members and implant plates with various slopes to achieve
the desired lordotic adjustment. The implant plates can be color
coded and/or include other indicia to indicate size, slope and
other parameters.
[0044] Implant plates 110 and 112 can couple to spacer member 115
using, for example, fasteners, chemical bonding, a friction fit,
mating connectors or other suitable connection. In one embodiment,
for example, a friction fit may be formed between spacer member 115
and implant plates 110 and 112 to couple implant plates 110 and 112
to spacer member 115. Channels that hold spacer member 115 may
include projections that fit within indentions in spacer member 115
to form an interference fit when the spacer member 115 is fully
inserted in the channel. Alternatively, the channels may include
indentions that mate with projections extending from spacer member
115 when spacer member 115 is fully inserted into the channel of
implant plates 110 and 112. In this case, implant plates 110 and
112 are held in place or limited in movement relative to spacer
member 115 by both the friction fit and the mating connector.
According to other embodiments, spacer member 115 or implant plates
110 and 112 may deform during attachment. A threshold amount of
force may be required to connect implant plates 110 and 112 to
spacer member 115 to inhibit unintentional full insertion of spacer
member 115 into implant plates 110 and 112 and to inhibit removal
of spacer member 115 once in place.
[0045] Spacer member 115 can be connected to or be integrated with
plate 120. The size of spacer member 115 can be selected to provide
the appropriate distance between implant plates 110 and 112 and
hence the appropriate vertical distance between the vertebrae
between which implant 100 is implanted. Spacer member 115 can also
be shaped to limit the distance that implant plates 110 and 112 are
inserted into the cavity between the vertebrae. Additionally,
spacer member 115 can be shaped so that a desired lordotic angle is
achieved when implant 100 is inserted. For example, spacer member
115 can include be a partial wedge shape with so that the anterior
height of spacer member 115 is different than the posterior height
of spacer member 115.
[0046] According to one embodiment, the center of spacer member 115
is a cavity (better shown in FIG. 7, discussed below). This cavity
can be packed with bone growth material. By way of example, but not
limitation, the bone growth material can include autograft bone
(such as bone from the patient's lilac crest), allograft bone,
synthetic bone growth material or combinations thereof.
[0047] End plate 120 can be flat, curved or have any suitable form
factor for spinal surgery. Generally, end plate 120 includes holes
for fasteners that allow plate 120 to be attached to the
appropriate vertebrae. Examples of fasteners include, but are not
limited to, bone screws, nails, rivets, trocars, pins, barbs or
other threaded or non-threaded member which is securable within or
to bone. According to one embodiment, bone screws can be attached
to plate 120 in a manner that allows for polyaxial rotation prior
to attachment to the bone. One example of a mechanism for attaching
a plate to vertebrae that allows for polyaxial rotation of bone
screws is described in U.S. patent application Ser. No. 10/036,012,
entitled "System and Method for Stabilizing the Human Spine with a
Bone Plate," by Wagner et al., filed Dec. 26, 2001, which is hereby
fully incorporated by reference herein. End plate 120 may be
attached to the spine with any number of fasteners.
[0048] End plate 120 and spacer member 115, according to one
embodiment, can be formed of a single piece of material. End plate
120 can include a passage that opens to outer surface 143 of end
plate 120 and the center of spacer member 115. The passage both
strengthens end plate 120 under compressive loads and provides
access to the center of spacer member 115.
[0049] FIG. 2 is a diagrammatic representation illustrating an
oblique view of one embodiment implant 100 showing end plate 120,
spacer member 115 and implant plates 110 and 112, FIG. 2 emphasizes
outer surface 125 of implant plate 110 and showing features such as
protrusions 135 discussed above. Additionally, FIG. 2 illustrates
an entrance to passage 140 that can lead to the cavity at the
center of spacer member 115, FIG. 2 further illustrates that
implant plates 110 and 112 can include holes defined there through
(e.g., hole 144, for example). According to one embodiment, the
holes of implant plate 110 can align with the holes of implant
plate 112 when implant 100 is assembled. These holes can allow bone
to pass as bone growth occurs, thereby allowing the vertebrae to
fuse together. FIG. 2 further illustrates holes 145 for fasteners
to attach plate 120 to the adjacent vertebrae.
[0050] FIG. 3 is a diagrammatic representation illustrating one
embodiment of inner face 130 of implant plate 110. Inner face 130
can include recessed portions to define channels 155 and channel
160. According to one embodiment, channels 155 receive an insertion
tool. The sidewalls of channels 155 can angled (e.g., dovetailed)
or otherwise shaped to mate and form a frictional connection with
the complementarily shaped insertion tool. Channels 155 and the
complementary portion of the insertion tool can be shaped so that
the insertion tool is only removed from implant plate 110 by
sliding the insertion tool out of channels 155 in the opposite
direction from which it was inserted into channels 155. The depth
of insertion of the insertion tool can be limited by the length of
channels 155, a stop in the channels, a stop on the insertion tool
or by other mechanism.
[0051] Channel 160 can be shaped and sized to engage with spacer
member 115. The sidewalls of channel 160 can also be angled (e.g.,
dovetailed) or otherwise shaped to capture spacer member 115.
Implant plate 110 can include detents 165 with protrusions 170 that
help prevent spacer member 115 from sliding out of channel 160.
Detents 165 can be formed so that they return to approximately
their original positions if pushed outward from the center of
channel 160. As spacer member 115 slides into channel 160, detents
165 can push away from the center of channel 160 until protrusions
170 fit in complementary indentions in spacer member 115 (shown in
FIG. 7). Protrusions 170 and the complementary indentions mate to
limit movement of implant plate 110 (and implant 112) relative to
spacer member 115. Preferably, movement is limited so that spacer
member 115 can not be easily removed from implant plate 110. In
other words, the mating connection (or other connection) prevents
implant plate 110 from sliding off of spacer member 115 during
expected use as a spinal implant. The depth of insertion of spacer
member 115 can be limited by the length of channel 160, a stop in
channel 160, a stop on spacer member 115 or end plate 120 or by
other suitable mechanism.
[0052] FIG. 4 is a diagrammatic representation illustrating an end
view of one embodiment of implant plate 110. Assuming implant plate
110 is used in an anterior approach procedure, FIG. 4 is an
anterior end view. As shown in FIG. 4, implant plate 110 includes
channels 155 open to the anterior end with dovetailed walls 175.
Similarly, implant plate 110 includes channel 160 open to the
anterior end with dovetailed walls 180. Walls 175 and 180 are
angled so that the respective channels are wider closer to the
outer face than the inner side of implant plate 110. Channels 155
and 160 can be otherwise shaped to respectively receive the
insertion device and spacer member 115.
[0053] FIG. 5 is a diagrammatic representation of end views of
other example channel shapes for receiving the insertion device or
spacer member 115. Keyhole, "T", and partial "T" shapes are shown.
The embodiments of FIG. 5 are provided by way of example and not
limitation.
[0054] FIG. 6 is a diagrammatic representation of a cross-sectional
view of one embodiment of an integrated end plate 120 and spacer
member 115. End plate 120 can have be flat, curved or other
suitable form factor for spinal surgery. Spacer member 115 can be
connected to or be integrated with plate 120. The size of spacer
member 115 can be selected to provide the appropriate distance
between implant plates 110 and 112 and depth of insertion.
Additionally, spacer member 115 can be shaped so that a desired
lordotic angle is achieved when implant 100 is inserted. For
example, spacer member 115 can be a partial wedge shape so that the
anterior height of spacer member 115 is different than the
posterior height of spacer member 115. Spacer member 115 can
include a cavity 195 that can be packed with bone growth material.
End plate 120 can include a passage 140 that opens to outer surface
143 of end plate 120 and the center of spacer member 115 to allow
access to cavity 195.
[0055] FIG. 7 is a diagrammatic representation of a top view of
integrated end plate 120 and spacer member 115. Spacer member 115
can include indents 190 to capture protrusion 170 of implant plate
110 (shown in FIG. 3). As illustrated in FIG. 7, spacer member 115
can also form a cavity 195. Bone growth or other material can be
packed in cavity 195.
[0056] FIG. 8 is a diagrammatic representation of an end view of
integrated end plate 120 and spacer member 115. For an anterior
procedure, FIG. 8 represents a posterior view. Spacer member 115,
according to one embodiment, has a complementary shape to channel
160 of the implant plates 110 and 112 so that a portion of spacer
member 115 is captured by the sidewalls of channel 160. For
example, spacer member 115 can include tapered (or other shaped)
flanges 197 that are captured by the dovetailed sidewalls of the
respective channels 160. When in place, the joint formed by
channels 160 and flanges 197 prevent the implant plates from
vertically separating from spacer member 115.
[0057] End plate 120 can have a "bow" shape in which the upper and
lower portions of end plate 120 are wider than the center portion.
As discussed below in conjunction with FIG. 11, this allows end
plate 120 to limit the separation distance of an insertion
tool.
[0058] FIG. 9 is a diagrammatic representation of an end view of
end plate 120. For an anterior procedure, FIG. 9 represents an
anterior view. End plate 120 can include fastener holes 145 to
allow end plate 120 to be fastened to adjacent vertebrae.
Additionally, end plate 120 can include an opening to passage 140
to allow access to cavity 195 (shown in FIG. 7). End plate 120 can
also include recessed feature 205 that can aid in alignment of a
driver during insertion of spinal implant 100.
[0059] FIG. 10 is a diagrammatic representation of one embodiment
of a separator 200 (i.e., a portion of an insertion device) to
insert implant 100. Separator 200 can include arms 210 and 212.
Arms 210 and 212 can include respective attachment portions that
couple separator 200 to respective arms of a spreader using a
friction fit, mating fit or other suitable connection mechanism.
Arm 210 couples to implant plate 110 and arm 212 couples to implant
plate 112 (e.g., through frictional connections or other
connections). End plate 120 and spacer member 115 are movably
captured in a channel formed in separator 200 so that end plate 120
and spacer member 115 can slide toward implant plates 110 and
112.
[0060] FIG. 11 is a diagrammatic representation of a bottom view of
one embodiment of arm 212. Arm 212 can include prongs 215 shaped to
fit corresponding channels in implant plate 112 (e.g., channels 155
shown in FIG. 3). According to one embodiment, prongs 215 can be
separated by a distance that is sufficient to straddle the center
portion of end plate 120 but not the top and bottom portions of end
plate 120. Put another way, the gap between prongs 215 is greater
than the width of the center of end plate 120 but less than the
width of the top and bottom portions of end plate 120.
Consequently, prongs 215 (and the corresponding prongs on the arm
210) form a channel down which end plate 120 and the integrated or
connected spacer member 115 can move. The bow shape or other shape
of end plate 120 limits the separation distance of arms 210 and
212.
[0061] Arm 212 can define a passage 225. This passage can allow a
driver (e.g., a slap hammer or other driver) access to end plate
120. The driver can assert a force on end plate 120 to move end
plate 120 into position during implantation. Additionally, passage
225 allows materials to be added to end plate assembly 120. For
example material can be injected through passage 225, passage 140
(shown in FIG. 2) into cavity 195 formed by spacer member 115
(shown in FIG. 7).
[0062] FIGS. 12a-c illustrate one embodiment of separator 200 and
implant 100 during various stages of insertion. FIG. 12a
illustrates separator 200 from one side and FIGS. 12b and 12c
illustrate separator 200 from the opposite side during the
procedure. In operation, a surgeon can make an incision on the
anterior side of the body during a discectomy procedure. Implant
plates 110 and 112, end plate 120 and spacer member 115 can be
loaded on separator 200 (FIG. 12a) and separator 200 inserted into
the body using a spreader such that implant plates 110 and 112 are
inserted in the space created by removal or partial removal of a
vertebral disc. Arms 210 and 212 are separated to separate implant
plates 110 and 112 (FIG. 12b). The separation distance can be
limited by the geometry of end plate 120 or through another
suitable mechanism. When implant plates 110 and 112 are separated,
spacer member 115 can be moved to join with implant plates 110 and
112 (FIG. 12c). End plate 120 can then be attached to the vertebral
bodies using fasteners. Separator 200 can be removed from implant
plates 110 and 112. Various portions or all of separator 200 and
implant 100 may be radiopaque or include radiopaque markers to
allow viewing with medical imaging devices to ensure proper
placement of implant 100.
[0063] As discussed above, end plate 120 can be attached to the
bones using fasteners that rotate to allow better alignment of end
plate 120. FIG. 13 depicts a cross-sectional view of an embodiment
of one of the holes 145 (also shown in FIG. 2) in which screw 320
is disposed. Hole 145 is preferably substantially spherical in
shape so that a head 332 of screw 320 may be rotated and moved to
various positions within borehole 312. Ring 318 is preferably sized
to fit into hole 145 between plate 120 and head 332. The outer
surface of ring 318 is preferably curved to permit movement of the
ring within hole 145. The combination of ring 318 and hole 145 is
like that of a ball and socket since ring 318 may be rotated both
horizontally and vertically in clockwise and counterclockwise
directions within hole 145. Ring 318 may also be rotated in
directions that are angled away from the horizontal and vertical
directions. In FIG. 13, ring 318 at least partially surrounds head
332 of screw 320 which is positioned within hole 145. A shank 334
of bone screw 320 preferably has threading 336 to allow the screw
to be inserted into a bone when it is rotated in a clockwise
direction. Head 332 preferably includes a cavity 342 that extends
from the top of the head to an inner portion of the head. Cavity
342 may be shaped to receive the end of any fastening device e.g.,
a socket wrench that may be used to turn screw 320. Screw 320 may
be simultaneously screwed into a bone and moved to its desired
position. The inner surface of ring 318 and the outer surface of
head 332 are preferably tapered and shaped to mate with each other.
The bottom portion of head 332 is preferably smaller than the upper
portion of ring 318. As screw 320 is inserted into a bone, head 332
preferably applies a radial force to ring 318, thereby causing the
ring to expand within the hole and increase the size of the gap in
ring 318 that allows ring 318 to expand. An interference fit may
form between screw head 332, ring 318, and plate 120 in which these
elements fit so tightly together that they obstruct the movements
of each other. The hoop stress of ring 318 on head 332 may fixedly
attach screw 320 to plate 120. Also during insertion of screw 320,
screw head 332 and ring 318 may be positioned within hole 145 such
that their left sides are at a higher elevation than their right
sides. FIG. 13 shows that positioning screw head 332 in this
configuration may result in a centerline 338 of shank 334 being
obliquely angulated with respect to plate 120. In fact, centerline
338 may be positioned where it is at an angle ranging from 0 to 15
degrees with respect to an imaginary axis 340 which is
perpendicular to plate 120. FIG. 13 demonstrates shank 334 of screw
320 being angled to the left of imaginary axis 340 while FIG. 14
demonstrates shank 334 being angled to the right of imaginary axis
340. Screw 320 is not limited to these positions and can be angled
in various directions, such as into the page.
[0064] FIGS. 15 and 16 depict different embodiments of end plate
120 with fasteners inserted. FIG. 15 shows that screws 320 may be
positioned within holes 145 such that they extend in converging
directions with respect to each other. The screws 320 depicted in
FIG. 16, on the other hand, are shown as being positioned such that
their shanks 334 extend in diverging directions with respect to
each other. Screws 320 may be moved to such positions as described
above. Since bone screws 320 may be placed in diverging or
converging directions through holes 145 at both ends of plate 120,
screw backout may be greatly reduced. Further, the use of rings 318
to fixedly attach screws 320 to plate 120 may prevent damage to
tissue structures by any screws that are able to escape from the
bone. Rings 318 preferably do not extend above the upper surface of
plate 120, and thus advantageously do not contact tissue
structures. Screw 320 may be placed in a uni-cortical position
within the bone since the problem of screw backout is greatly
reduced by the diverging or converging screw configurations.
[0065] According to one embodiment, end plate 120 is prepared for
surgical implantation by pre-positioning of rings 318 within holes
145. During the actual surgical procedure, holes may be drilled and
tapped into the bones to which plate 120 is to be attached. Plate
120 may then be positioned adjacent to the bones when spacer member
115 is coupled to implant plate 110 and implant plate 112. Each of
the screws 320 may be screwed into the bone holes while they are
being positioned within their corresponding holes 145. Each pair of
screws 320 at opposite ends 120 may be positioned so that shanks of
the screws are at oblique angles relative to the plate. The
insertion force of each screw 320 into each ring 318 preferably
causes the ring to exert a compressive force on the screw head,
thereby fixably connecting the screws to plate 120.
[0066] A side view of another embodiment of a spinal plate 120 and
fasteners is shown in FIG. 17. This embodiment includes a bone
screw 420 and a ring 418. Plate 120 may be used to stabilize a bony
structure such as the spine to facilitate a bone fusion (e.g., a
spinal fusion). The bone screw 420 may be used to connect plate 120
to a bone such as a vertebra. Ring 418 preferably fixes bone screw
420 to plate 120 at a selected angle that depends upon the
patient's anatomy.
[0067] In this embodiment, each hole 145 preferably has a curvate
inner surface 413 for engaging the outer surface 423 of ring 418.
The inner surface 413 preferably has the shape of a portion of an
outer surface of a sphere. Hole 145 has a width that is defined
across the inner surface 413 of the borehole. The width of the
borehole may vary in a direction axially through the borehole. For
example, the width of the holes preferably increases from a surface
of the plate to about the middle of the plate. The width of the
hole 145 preferably decreases from about the middle of the plate to
an opposite surface of the plate such that the hole has a maximum
width near the midpoint between the surfaces.
[0068] The outer surface 423 of ring 418 is preferably curvate for
engaging the inner surface 413 of the borehole. The shape of
surfaces 423 and 413 preferably allow ring 418 to swivel within the
borehole. The swiveling action may be similar to that of a ball and
socket joint. The ring preferably surrounds at least a portion of
the head 425 of a bone screw. The enlarged end 427 disposed on head
425 is optional and need not be included if it inhibits angulation
of the bone screw. The swiveling of the ring within the borehole
preferably enables the shank 435 of the bone screw 420 to rotate in
a substantially conical range of motion. In this manner, the head
is preferably movable within the borehole, and the shank is
adjustably positionable at a plurality of angles substantially
oblique to the plate.
[0069] In an embodiment, the surfaces 423 and 413 are preferably
shaped to provide a conical range of motion to the shank that is
within a preferred range of angles. The head is preferably movable
within the borehole such that the shank can be positioned at a
selected angle relative to an imaginary axis running perpendicular
to the plate proximate borehole 145. The selected angle is
preferably less than about 45 degrees, more preferably less than
about 30 degrees, and more preferably still less than about 15
degrees.
[0070] Ring 418 preferably has an outer width that is less than or
about equal to the width of hole 145 at a location between the
surfaces of plate 120. In this manner, ring 418 may be positioned
within hole 145 proximate the middle of the hole to enable the bone
screw 420 to extend substantially perpendicularly from the bone
plate 120. Prior to surgery, rings 418 are preferably
pre-positioned within holes 145 of plate 120, "Pre-positioned" is
taken to mean that the rings are capable of swiveling within the
borehole but are preferably inhibited from falling out of the
borehole because of the reduced width of the borehole proximate the
upper and lower surfaces. The width of the borehole proximate the
upper and lower surfaces of plate 120 is preferably less than or
about equal to the outer width of the ring to inhibit the ring from
falling out of the borehole. In this manner, the surgeon may use a
plate 120 having rings 418 pre-positioned within the holes 145 such
that the rings will not fall into the surgical wound when implant
100 is installed.
[0071] Alternately, the rings 418 can be manually positioned within
holes 145 during surgery. Ring 418 preferably includes one or more
slots or gaps. The slot preferable allows the ring to be contracted
or expanded. Contraction of ring 418 may allow the ring to be
positioned within the borehole during surgery. Once positioned
within the borehole the ring preferably expands and is inhibited
from falling out of the borehole.
[0072] The ring 418 is preferably capable of being swiveled such
that one portion of the ring is adjacent to one surface of plate
120 while another portion of the ring lies adjacent to the opposite
surface of plate 120. Ring 418 is preferably sufficiently thin to
allow it to reside within the borehole without extending from the
borehole beyond the surfaces of plate 120. Generally, it is
preferred that the ring and screw head remain within the hole 145
to minimize the profile of implant 100. In some embodiments,
however, the bone screw 420 may be capable of being angulated
relative to the plate 120 such that ring 418 extends from the hole
145 beyond a surface of the plate 120.
[0073] The head 425 is preferably screwed into ring 418 to create a
fixed connection between bone screw 420 and plate 120 at a selected
angle. In an embodiment depicted in FIG. 18, screw head 425
preferably contains head threading 421 on its outer surface that is
complementary to ring threading 419 contained on the inner surface
of ring 418. The head threading 421 preferably mates with the ring
threading 419 to enhance the connection between the bone screw 420
and the ring 418. The head 425 preferably has a cavity 442 formed
on its upper surface for receiving a driving tool such as a screw
driver or an allen wrench.
[0074] It is believed that using a threading engagement between the
head 425 and ring 418 increases the hoop stress exerted on head
425, resulting in a stronger connection between the bone screw 420
and the plate 120. Moreover, if bone threading 436 becomes loose
within a bone, screw back-out from plate 120 will tend to be
resisted by the threaded connection between the screw head 425 and
the ring 418. Thus, even if the shank 435 loosens within the bone,
the head will tend to remain within the borehole of the plate so as
not to protrude from the plate into surrounding body tissue. As
shown in FIG. 18, the head threading 421 on the head 425 and the
ring threading 419 on the inner surface of ring 418 is preferably
substantially fine relative to the threading 436 on bone screw 420.
That is, the pitch of the head threading 421 and ring threading 419
is preferably smaller than that on bone screw 420. The ring
threading 419 preferably has multiple starts to facilitate
connection of the bone screw and the ring. In one embodiment, the
ring threading 419 has a double start such that the head can be
started into the ring threading at either one of two orientations
offset by 180 degrees. In another embodiment, the ring threading
has a triple start such that the head can be started into the ring
threading at any one of three orientations offset by 420
degrees.
[0075] The ring threading 419 and head threading 421 are preferably
pitched to a substantially similar degree to the threading 436 on
the bone screw 420. Preferably, the ring threading 419 and head
threading 421 are pitched such that the head 425 causes expansion
of the ring 418 while the bone screw 420 is being inserted into the
bone.
[0076] During the surgical procedure for attaching the plate 120 to
a bone, holes may be drilled and tapped into the bones to which
plate 120 is to be attached. Plate 120 may then be positioned
adjacent to the bones. A ring 418 may be positioned within the
borehole. A bone screw 420 may be positioned through ring 418 such
that the head threading 421 of head 425 engages the ring threading
419 of ring 418. The bone screw 420 may then be rotated to insert
the bone screw into the bone. As the screw is rotated the head
threads and ring threads preferably interact such that the head is
moved into the ring. Movement of the head 425 into the ring 418
preferably causes the ring to expand such that the orientation of
the bone screw 420 relative to the plate 120 is fixed. Preferably,
the ring threading and head threading is pitched such the
orientation of the bone screw 420 is fixed after plate 120 engages
the bone.
[0077] The bone screws may be used in pairs to prevent screw
backout. The bone screws are preferably positioned into the bone in
substantially converging or substantially diverging directions
relative to one another.
[0078] The outer surface of the head 425 is preferably tapered so
that screwing the head into the ring causes a change in width
(e.g., expansion) of the ring 418 to fix the bone screw 420 in
position relative to the plate 120. The inner surface of the ring
418 may also be tapered to substantially match the taper on the
outer surface of the head. At least a portion of the head 425
preferably has a width greater than the inner width of the ring
418. As the screw head is screwed into the ring 418, the ring
preferably expands outwardly from its inner surface to accommodate
the increasing width of the screw head 425. The ring 418 may
contain a slot or gap as previously described to facilitate
expansion of the ring against the inner surface 413 of the hole
145. The slot is preferably widened as a result of force received
from head 425. The force exerted by head 425 against the inner
surface of ring 418 preferably presses the ring into a fixed
engagement against inner surface 413 of hole 145.
[0079] Alternatively, ring 418 may contain one or more partial
slots 445, as depicted in FIG. 19. Each partial slot 445 preferably
extends from a top 447 or bottom 449 of ring 418 into the ring.
Partial slots may extend up to about midpoint 448 of ring 418. In
one embodiment, a plurality of slots 445 may be oriented about the
ring such that alternate slots extend from the top 447 and/or the
bottom 449 of ring 418, as depicted in FIG. 19. These alternating
partial slots preferably facilitate the expansion and contraction
of ring 418.
[0080] Cross-sectional views of two embodiments of ring 418 having
threaded section 419 are shown in FIGS. 20A and 20B. The ring may
contain an inner surface that is tapered (as shown in FIG. 20A) or
that is substantially untapered (as shown in FIG. 20B).
Cross-sectional views of two embodiments of screw 420 are shown in
FIGS. 21A and 21B. The head 425 may have a substantially untapered
outer surface (as shown in FIG. 21A) or a substantially tapered
outer surface (as shown in FIG. 21B). It is to be understood that
each of the heads of the screws depicted in FIGS. 21A and 21B may
be used in combination with either of the rings 418 depicted in
FIGS. 20A and 20B. It is also to be appreciated that the head of
the screw may include an outer surface having a substantially
untapered portion along with a tapered portion proximate its end
for expanding the ring 418.
[0081] As described herein, a "ring" is taken to mean any member
capable of fitting between the inner surface 413 of a fastener hole
and the bone screw 420 to connect the bone screw to the plate 120.
The ring is preferably substantially circular to surround head 425,
but the ring may instead have a non-circular shape. The ring may be
made of a number of biocompatible materials including metals,
plastics, and composites.
[0082] In an embodiment, a stronger connection between the bone
screw 420 and the plate 120 may be formed by texturing either outer
surface 431 of head 425 of bone screw 420 or inner surface 433 of
ring 418, as depicted in FIG. 22. Preferably, both surfaces are
textured to inhibit movement of the bone screw with respect to the
plate. During typical manufacturing procedures, outer surface 431
of head 425 and inner surface 433 of ring 418 may be formed as
relatively smooth surfaces. While the friction between these smooth
surfaces tends to be sufficient to maintain bone screw 420 in a
fixed position with respect to plate 120, under stressful
conditions the bone screw may be forced out of ring 418. By
providing at least one textured surface, the coefficient of
friction of the surface may be increased so that a large amount of
force is needed to overcome the frictional connection between head
425 of bone screw 420 and ring 418. This increase in friction
between bone screw 420 and ring 418 may further inhibit screw
backout from plate 120.
[0083] A number of textured surfaces may be used to increase the
coefficient of friction between ring 418 and head 425 of bone screw
420. In general, any process which transforms a relatively smooth
surface into a roughened surface having an increased coefficient of
friction may be used. Methods for forming a roughened surface
include, but are not limited to: sanding, forming grooves within a
surface, ball peening processes, electric discharge processes, and
embedding of hard particles within a surface.
[0084] In one embodiment a plurality of grooves may be formed in
outer surface 431 of head 425 of bone screw 420 or inner surface
433 of ring 418. Preferably, a plurality of grooves is formed in
both outer surface 431 and inner surface 433. While it is preferred
that both outer surface 431 and the inner surface 433 (is the lead
line for 433 in the right place?) be textured, texturing of only
one of the surfaces may be sufficient to attain additional
resistance to movement.
[0085] In another embodiment, the frictional surface may be created
by an electrical discharge process. An electrical discharge process
is based on the principle of removal of portions of a metal surface
by spark discharges. Typically a spark is generated between the
surface to be treated and an electrode by creating potential
differential between the tool and the electrode. The spark produced
tends to remove a portion of the surface disposed between the
electrode and the surface. Typically, the electrode is relatively
small such that only small portions of the surface are removed. By
moving the electrode about the surface numerous cavities may be
formed within the surface. Typically these cavities are somewhat
pyramidal in shape. Various patterns may be formed within the
surface depending on how the electrode is positioned during the
discharge. Electric discharge machines are well known in the art. A
method for forming a frictional surface within a metal surface
using an electric discharge process is described in U.S. Pat. No.
4,964,641 to Miesch et al. which is incorporated by reference as if
set forth herein.
[0086] A variety of patterns may be formed using an electric
discharge machine. Preferably a diamond pattern or a waffle pattern
is formed on either inner surface 433 of ring 418 or outer surface
431 of head 425 of bone screw 420.
[0087] In another embodiment, inner surface 431 of ring 418 and/or
outer surface 433 of head 125 of bone screw 120 may be textured by
the use of a shot peening process. A shot peening process for
forming a textured surface is described in U.S. Pat. No. 5,526,664
to Vetter which is incorporated by reference as if set forth
herein. In general, a shot peening process involves propelling a
stream of hardened balls, typically made of steel, at a relatively
high velocity at a surface. To create a pattern upon an area of the
surface the stream is typically moved about the surface. The speed
by which the stream is moved about the surface tends to determine
the type of textured surface formed.
[0088] Preferably, the stream is moved such that a pattern
resulting in a textured surface having ridges and valleys is formed
on inner surface 433 of ring 418 and outer surface 431 of head 425
of bone screw 420. When the textured inner surface 431 of ring 418
and the textured head 425 of bone screw 420 are coupled together
the ridges and valleys may interact with each other to provide
additional resistance to movement in either a longitudinal
direction or a direction perpendicular to the longitudinal
axis.
[0089] In another embodiment, the textured surface may be produced
by embedding sharp hardened particles in the surface. A method for
embedding sharp hardened particles in a metal surface is described
in U.S. Pat. No. 4,768,787 to Shira which is incorporated by
reference as if set forth herein. The method of Shira involves
using a laser or other high energy source to heat the surface such
that the surface melts in selected areas. Just before the molten
area re-solidifies, a stream of abrasive particles is directed to
the area. In this manner some of the particles tend to become
embedded within the molten surface. The particles typically have a
number of sharp edges that protrude from the surface after the
particles have been embedded within the surface.
[0090] Any of the above methods of texturing may be used in
combination with another method. For example, outer surface 431 of
head 425 of bone screw 420 may be textured using a pattern of
grooves. Inner surface of ring 418, however, may be textured using
an electrical discharge method. When coupled together the textured
surfaces of bone screw 420 and ring 418 may interact with each
other to provide additional resistance to movement in either a
longitudinal direction or a direction perpendicular to the
longitudinal axis.
[0091] Textured surfaces may also be formed on any of the other
surfaces of the plate system. The formation of textured surfaces
preferably increases the frictional resistance between the various
components of the plate system.
[0092] While the present invention has been described with
reference to particular embodiments, it should be understood that
the embodiments are illustrative and that the scope of the
invention is not limited to these embodiments. Many variations,
modifications, additions and improvements to the embodiments
described above are possible. It is contemplated that these
variations, modifications, additions and improvements fall within
the scope of the invention as detailed in the following claims.
* * * * *