U.S. patent application number 13/374253 was filed with the patent office on 2012-05-03 for threadform for medical implant closure.
Invention is credited to Roger P. Jackson.
Application Number | 20120109220 13/374253 |
Document ID | / |
Family ID | 45997509 |
Filed Date | 2012-05-03 |
United States Patent
Application |
20120109220 |
Kind Code |
A1 |
Jackson; Roger P. |
May 3, 2012 |
Threadform for medical implant closure
Abstract
A thread located on a cylindrical closure for an open headed
medical implant. The thread has a leading surface and a trailing
surface that both slope rearwardly from an interior edge to an
exterior edge thereof.
Inventors: |
Jackson; Roger P.; (Mission
Hills, KS) |
Family ID: |
45997509 |
Appl. No.: |
13/374253 |
Filed: |
December 19, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09644777 |
Aug 23, 2000 |
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13374253 |
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Current U.S.
Class: |
606/305 |
Current CPC
Class: |
A61B 17/7032
20130101 |
Class at
Publication: |
606/305 |
International
Class: |
A61B 17/86 20060101
A61B017/86 |
Claims
1. A medical implant having a head with a pair of spaced arms and
an implant closure sized to be operably threadedly received between
said arms; said closure having a threadform thereon that is sized
and shaped to be threadedly received in a mating threadform on said
arms; said closure has a direction of advancement along an axis of
rotation relative to said head; said threadform comprising: a) a
leading surface that has an inner edge and an outer edge; b) a
trailing surface that has an inner edge and an outer edge; and
wherein c) intersections of a plane passing through said axis of
rotation with both said leading surface and said trailing surface
slope rearwardly relative to the direction of advancement from the
respective inner edges to the outer edges thereof; and d) the
leading surface outer edge and the trailing surface outer edge are
spaced apart.
2. The implant according to claim 1 wherein: a) the intersection of
said trailing surface with a plane passing through said axis of
rotation is at a first angle of from about 1.degree. to about
45.degree. relative to a line perpendicular to said axis of
rotation.
3. The implant according to claim 2 wherein: a) said first angle is
between about 5.degree. and 20.degree..
4. The implant according to claim 2 wherein: a) said first angle is
between about 7.degree. and 15.degree..
5. The implant according to claim 2 wherein: a) the intersection of
said leading surface with a plane passing through said axis of
rotation is at a second angle of from about 30.degree. to about
75.degree. relative to a line perpendicular to said axis of
rotation.
6. The implant according to claim 2 wherein: a) said second angle
is in the range from 40.degree. to 50.degree..
7. The implant according to claim 1 wherein: a) said trailing and
leading surfaces are nonparallel.
8. A medical device comprising: a) a first implant having a head
with a channel sized and shaped to receive a rod member and a pair
of spaced arms on opposite sides of said channel; b) a second
closure implant for closing between said arms; said closure implant
having an axis of rotation and a thread on an outer surface
thereof; said thread being in a helical pattern on said closure
implant and having a leading surface and a trailing surface; said
leading surface having inner and outer edges and said trailing
surface having inner and outer edges; intersections of both said
leading surface and said trailing surface with a plane passing
through said axis of rotation slope from respective inner to outer
edges rearwardly with respect to a direction of advancement of said
closure implant in closing said first implant; c) the outer edges
of the leading and trailing surfaces are spaced to increase the
strength of the outermost part of the thread; and d) each of said
arms include a threadform on inner facing surfaces thereof sized
and shaped to matingly and threadedly receive the thread of said
closure implant;
9. The device of claim 8 wherein: a) the inner and outer edges of
both said leading surface and said trailing surfaces are each
spaced from the axis of rotation at substantially the same radius
over substantially the entire length of the thread.
10. The device of 8 wherein: a) said inner edges of both said
leading and trailing surfaces are substantially spaced and said
outer edges of both said leading and trailing surfaces are in close
proximity to each other throughout the length of the thread such
that the thread is generally triangular in cross-section.
11. The device according to 10 wherein: a) said cross-section has
the general shape of an obtuse triangle.
12. In a medical implant having a head with a pair of spaced
upright arms and a closure operably located between the arms, a
thread positioned on the closure and being sized and shaped to be
threadedly received in a mating thread located on the pair of
upright arms; said thread having an axis of rotation with a leading
surface and a trailing surface relative to advancement along the
axis of rotation; the improvement comprising: a) said trailing
surface having an inner and an outer edge; said trailing surface
sloping rearwardly from the inner edge to the outer edge thereof;
and said inner edge having a generally constant radius over an
entire length of said thread, such that, as said closure is
advanced and applies force on a bottom side thereof, said thread
resists splaying of said arms; the leading surface and the leading
surface having an outer edge that is spaced apart from the trailing
surface outer edge.
13. In a medical implant sized and shaped for closing between a
pair of arms of a head of a bone screw and having a lower surface
adapted to engage and apply pressure to a rod received in the head;
said implant further having a cylindrical shaped outer surface with
a thread wound in a helical pattern about said outer surface and
wherein said thread has a leading surface and a trailing surface
relative to advancement of the implant along an axis of rotation;
the improvement comprising: a) said trailing surface having an
inner and an outer edge; any intersection of said trailing surface
with a plane passing through the axis of rotation slopes rearwardly
from an inner radius to an outer radius of said trailing surface
over substantially the entire length of said trailing surface; the
leading and having an outer edge that is spaced from the trailing
surface outer edge.
14. In a medical implant having a head with a pair of spaced arms
and a closure for positioning between and closing between the arms,
the closure having a thread located thereon that is operably
received in a mating thread located on said spaced arms; said
thread having a leading surface and a trailing surface relative to
advancement about an axis of rotation; the improvement comprising:
a) both said leading and trailing surfaces having respective inner
and outer edges with the outer edges being spaced apart; said
trailing surface sloping rearwardly from the inner edge to the
outer edge thereof; said trailing surface and leading surface inner
edges being spaced.
15. The implant according to claim 14 wherein: a) said
cross-section is generally in the shape of an obtuse triangle.
16. In a medical implant having a cylindrical shaped outer surface
with a thread helically wound about said outer surface and wherein
said implant is sized and shaped to be threadedly received between
a pair of arms of a bone screw head and has a bottom surface
adapted to abut against a rod received in said head; said thread
having a leading surface and a trailing surface relative to
advancement of the implant along an axis of rotation and further
wherein both said trailing surface and said leading surface have
respective inner and outer edges; the improvement comprising: a)
said leading and trailing surfaces both sloping rearwardly from
respective inner to outer edges thereof with the outer edges being
spaced apart; said trailing surface and leading surface inner edges
being spaced.
17. In an orthopedic medical implant having a head with a pair of
spaced arms and including a closure for operably closing between
the arms, the closure having a thread thereon and the arms having a
mating thread; said thread having a leading surface and a trailing
surface relative to advancement about an axis of rotation; the
improvement comprising wherein: a) both of the intersections of
said leading surface and said trailing surface with a plane passing
through the axis of rotation slope rearwardly from a radially inner
edge to outer edge thereof; b) a first angle between the leading
surface intersection and a line perpendicular to the axis of
rotation is substantially greater than a second angle between the
trailing surface intersection and a line perpendicular to the axis
of rotation; and c) the leading and trailing surfaces have
respective outer edges that are spaced apart.
18. The implant according to claim 17 wherein: a) said second angle
is between about 1.degree. and 45.degree..
19. The implant according to claim 18 wherein: a) said first angle
is greater than 30.degree..
20. The implant according to claim 17 wherein: a) said first angle
is in the range from about 30.degree. to 45.degree. and said second
angle is in the range from about 5.degree. to 20.degree..
21. In a medical implant having a head with a pair of spaced arms
and including a closure for closing between said arms; said closure
having thereon a thread sized and shaped to be threadedly received
in a mating thread on said arms; said thread having a leading
surface and a trailing surface relative to advancement about an
axis of rotation; the improvement comprising: a) said leading
surface and said trailing surface being non-parallel; b) an
intersection of a plane with said trailing surface slopes
rearwardly from an inner edge to an outer edge of said trailing
surface; and c) the leading and trailing surfaces have respective
outer edges that are spaced apart.
22. A medical implant closure having a threadform; said closure
being adapted to be threadedly receivable in a second medical
implant bone screw head between spaced arms wherein said closure
has a direction of advancement along an axis of rotation relative
to said head; said threadform comprising: a) a leading surface that
has an inner edge and an outer edge; b) a trailing surface that has
an inner edge and an outer edge; and wherein c) intersections of a
plane passing through said axis of rotation with both said leading
surface and said trailing surface slope rearwardly relative to the
direction of advancement from the respective inner edges to the
outer edges thereof; and d) the outer edges of the leading and
trailing surfaces are spaced apart.
23. The closure according to claim 22 wherein: a) said threadform
is helically wound about said cylindrical shaped body.
24. The closure according to claim 22 wherein: a) said threadform
is continuous.
25. The closure according to claim 22 wherein: a) said threadform
is in a helical pattern, but is discontinuous.
26. The closure according to claim 22 further including: a) the
second implant having a receiving thread on an inner surface
thereof; said receiving thread being sized and shaped to matingly
and threadably receive said threadform.
27. A medical implant for locking a rod in a bone anchor, the
implant comprising: a) a closure body; b) the body having a
helically wound thread with leading and trailing surfaces that
slope rearwardly and outwardly and remain spaced apart at a crest
thereof, and c) the body having a central opening for receiving a
tool, the opening being restricted at a lower end thereof to block
the tool from extending therethrough.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of pending U.S. patent
application Ser. No. 09/644,777, filed Aug. 23, 2000, incorporated
by reference herein.
BACKGROUND OF THE INVENTION
[0002] The present invention is directed to a threadform for use in
threadedly joining together two elements and, in particular, to a
threadform for joining together medical implants. The threadform
includes a leading surface and a trailing surface, both of which
slant rearwardly and away from the direction of advancement from an
inner edge to an outer edge thereof.
[0003] Medical implants present a number of problems to both
surgeons installing implants and to engineers designing them. It is
always desirable to have the implant be strong and unlikely to fail
or break during usage. It is also desirable for the implant to be
as small and lightweight as possible so that it is less intrusive
on the patient. These are normally conflicting goals, and often
difficult to resolve.
[0004] One particular type of implant presents special problems. In
particular, spinal bone screws, hooks, etc. are used in many types
of back surgery for repair of injury, disease or congenital defect.
For example, spinal bone screws of this type are designed to have
one end that inserts threadably into a vertebra and a head at an
opposite end thereof. The head is designed to receive a rod or
rod-like member which is then both captured in the head and locked
in the head to prevent relative movement between the various
elements subsequent to installation.
[0005] There are two different major types of bone screws and
similar devices. The types are closed head and open head. The
closed head devices are highly effective at capturing the rod since
the rod is threaded through an opening in the head. Unfortunately,
closed head devices are very difficult to work with in actual
surgery as the spine is curved and the rods are also curved in
order to follow the spine. Consequently, the more heads that the
rod must pass through, the more difficult it is to thread it.
[0006] The second type of head is an open head wherein a channel is
formed in the head and the rod is simply laid in an open channel.
The channel is then closed with a closure. The open headed bone
screws and related devices are much easier to use and in some
situations must be used over the closed headed devices.
[0007] While the open headed devices are often necessary and often
preferred for usage, there is a significant problem associated with
them. That is, the open headed devices conventionally have two
upstanding arms that are on opposite sides of a channel that
receives the rod member. In order to lock the rod member in place,
significant forces must be exerted on a relatively small device.
The forces are required to provide enough torque to insure that the
rod member is locked in place relative to the bone screw so that it
does not move axially or rotationally therein. This typically
requires torques on the order of 100 inch pounds.
[0008] Because the bone screws, hooks and the like are relatively
small, the arms that extend upwardly at the head can be easily bent
by radially outward directed forces due to the application of
substantial forces required to lock the rod member. Historically,
early closures were simple plugs that were threaded and which
screwed into mating threads on the inside of each of the arms.
However, conventionally threaded plugs push the arms radially
outward upon the application of a significant amount of torque
which ends up bending the arms sufficiently to allow the threads to
disengage and the closure to fail. To counter this various
engineering techniques were applied to allow the head to resist the
spreading force. For example, the arms were significantly
strengthened by increasing the width of the arms by many times.
This had the unfortunate effect of substantially increasing the
weight and the size of the implant, which was undesirable. Many
prior art devices have also attempted to provide rings or some
other type of structure that goes about the outside of the arms to
better hold the arms in place while the center plug is installed.
This additional structure has typically caused the locking strength
of the plug being reduced which is undesirable. Also, the
additional elements are unfavorable from a point of view of
implants, as it typically desirable to maintain the number parts
associated with the implants at a minimum.
[0009] Consequently, a lightweight and low profile closure plug was
desired that resists spreading of the arms while also not requiring
additional elements that circle around the outside of the arms so
as to hold the arms in place.
SUMMARY OF THE INVENTION
[0010] A threaded closure for use in conjunction with an open
headed medical implant wherein the thread associated with the
closure exerts forces that draw the arms radially inward toward the
closure rather than outward from the closure during installation.
In this manner the arms do not spread substantially during
installation of the closure under the torque required to lock a rod
member within the head of the implant.
[0011] The thread is preferably helically wound about a cylindrical
outer surface of the closure and preferably has an inner radius and
outer radius that remain constant over substantially the entire
length of the thread. The thread has both a leading surface and a
trailing surface that have inner edges that are spaced from one
another. Preferably the outer edges of the leading and trailing
surfaces are in close proximity to one another such that the thread
has a generally obtuse triangular shaped cross-section, with minor
reduction or rounding at the outer tip.
[0012] Whereas in V-shaped thread forms, the leading surface slopes
rearwardly from the inner edge and the trailing surface slopes
frontwardly from the leading edge, and in buttress-type threads,
the leading surface slopes rearwardly from the inner edge and the
trailing surface slopes slightly frontwardly or has no slope, the
thread of the present invention is such that both the leading
surface and the trailing surface slope rearwardly with respect to
the direction of advancement from the respective inner edges to
outer edges thereof. That is, the intersections of a plane passing
through an axis of rotation of the closure with the leading and
trailing surfaces both slope rearwardly from the respective inner
edges of the leading and trailing surfaces relative to the
direction of advancement of the closure in the open-headed
implant.
[0013] The inner facing surfaces of the arms are likewise threaded
with a mating threadform that is sized and shaped to mate with the
thread on the closure. The mating threadform on the implant arms is
discontinuous between the arms.
[0014] Because of the configuration of the thread on the closure
and the mating thread on the arms, forces applied to the closure,
during installation of the closure between the arms, produce a
reactive axial force on the arms of the implant, but also produce a
somewhat inward force thereon. Therefore, the arms are urged toward
the closure during installation rather than away from the closure
during installation. In this manner the thread and mating thread
function in a gripping manner between the opposed elements to hold
them together, rather than force them apart.
Objects and Advantages of the Invention
[0015] Therefore, the objects of the present invention are: to
provide a closure for an open headed lightweight and low profile
medical implant wherein the implant has a pair of spaced arms and
the closure closes between the arms; to provide such a closure
which is threaded and which does not substantially space the arms
during insertion, so as to reduce the likelihood of failure of the
implant and closure system during use; to provide such a closure
having a threadform that includes leading and trailing surfaces,
both of which surfaces slope rearwardly from inner edges to outer
edges thereof; to provide such a closure wherein the inner edges of
both the trailing and leading surfaces have substantially constant
radius over an entire length of the thread; to provide such a
closure which can be installed at comparatively high torques so as
to lock a rod member in the open head of the implant; and to
provide such a closure and implant that are relatively easy to use
and especially well adapted for the intended usage thereof.
[0016] Other objects and advantages of this invention will become
apparent from the following description taken in conjunction with
the accompanying drawings wherein are set forth, by way of
illustration and example, certain embodiments of this
invention.
[0017] The drawings constitute a part of this specification and
include exemplary embodiments of the present invention and
illustrate various objects and features thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is an exploded perspective view of an open headed
bone screw, rod and closure for the bone screw in accordance with
the present invention.
[0019] FIG. 2 is a fragmentary side elevational view of the bone
screw, rod and closure installed in the bone screw.
[0020] FIG. 3 is a fragmentary cross-sectional view of the closure,
taken along line 3-3 of FIG. 1.
[0021] FIG. 4 is a highly enlarged and fragmentary side elevational
view of the bone screw, rod and closure with a right hand arm of
the bone screw shown in phantom lines in order to better illustrate
features of the closure.
DETAILED DESCRIPTION OF THE INVENTION
[0022] As required, detailed embodiments of the present invention
are disclosed herein; however, it is to be understood that the
disclosed embodiments are merely exemplary of the invention, which
may be embodied in various forms. Therefore, specific structural
and functional details disclosed herein are not to be interpreted
as limiting, but merely as a basis for the claims and as a
representative basis for teaching one skilled in the art to
variously employ the present invention in virtually any
appropriately detailed structure.
[0023] The reference 1 generally indicates a thread form or thread
in accordance with the present invention that is located on a
medical implant closure 5 that is used in conjunction with a rod
member 6 and an open headed medical implant 7.
[0024] Describing the elements in reverse order, the illustrated
open headed medical implant 7 is a bone screw for use in spinal
surgery. The implant 7 includes a shank 11 having a bone engaging
and implantation thread 12 thereon. The implant 7 also includes an
open head 14. The head 14 is U-shaped having a base 16 and a pair
of upstanding spaced arms 17 and 18. The arms 17 and 18 are spaced
by a channel 20 having a seat 21 at the bottom thereof. The arms 17
and 18 have facing surfaces 24 and 25 that are sides of the channel
20. Each of the surfaces 24 and 25 have facing threaded sections 28
and 29 respectively.
[0025] While the medical implant 7 shown here in is an open headed
bone screw, it is foreseen that the present invention can be easily
used and adapted with other types of open headed implants such as
hooks and the like.
[0026] The rod member 6 is typically simply an elongated
cylindrical rod which may be bent by benders to conform with the
desired curvature of the spine. The rod member may be smooth or
knurled. The rod member 6 may also include other types of similar
structures such as connectors having a cylindrical or rod like
nipple associated therewith for insertion into the bone screw head
14.
[0027] The illustrated closure 5 is a cylindrical shaped plug
having a generally cylindrical shaped radially outer surface 32, a
flat bottom 33 and a flat top 34. The closure 5 has an axis of
rotation, generally indicated by the reference numeral A. The axis
of rotation A is at the radial center of the closure 5. A bore 37
that is co-axial with the axis of rotation A extends through the
top 34 and partially though the closure 5. The bore 37 is
polyfaceted so as to have a hexagonal cross section such that
closure 5 can be installed or removed with an alien type wrench
that fits the bore 37.
[0028] Although a particular closure 5 has been illustrated herein,
it is foreseen that the invention can be used in conjunction with
plugs and set screws of various types and configurations. For
example, the closure 5 may include a break off head for insertion
and various types of structure for removal, as opposed to the bore
37. The closure 5 may also include structure to assist in engaging
and securing the rod member 6, such as a depending point, a
roughened under surface, or a cutting ring. Finally, although the
closure of the present invention is illustrated in use in
conjunction with an open headed implant, it is foreseen that the
closure 5 could be utilized in conjuncture with closed bores,
either as a plug or set screw.
[0029] The thread 1 winds about the outer surface 32 of the closure
5 in a generally helical pattern or configuration, which is typical
of threads and can have various pitches, be counterclockwise
advanced or vary in most of the ways that conventional threads
vary. The thread 1 has a leading surface 40 and a trailing surface
41. As used herein the terms leading trailing refer to the
direction of advancement of the closure 5 when used to close the
implant 7 which is downward or in the direction of the rod member 6
in FIG. 4. In the illustrated embodiment, advancement is produced
by clockwise rotation. The leading surface 40 has an inner edge 44
and an outer edge 45. The trailing surface 41 also has an inner
edge 48 and an outer edge 49.
[0030] With reference to FIG. 3, the leading surface inner edge 44
and trailing surface inner edge 48 are substantially spaced. Both
the leading surface inner edge 44 and trailing surface inner edge
48 have substantially constant radius with respect to the axis of
rotation A, preferably throughout the length of the thread 1 and at
least throughout substantially most of the thread 1. The leading
surface outer edge 45 and trailing surface outer edge 49 are
closely spaced relative to one another and may be slightly relieved
as shown so as to have a slight connecting wall 50 that decreases
the sharpness of the thread 1 and increases the strength thereof.
As can be seen in FIG. 3, the general shape of the cross section of
the thread 1 is that of a obtuse triangle with the outer sharpened
edge slightly reduced. It can also be seen that the intersection of
the leading surface 40 and the trailing surface 41 with a plane
passing through the axis of rotation A which is essentially what is
shown in the front or closest surface shown in FIG. 3 both slope
rearwardly, as indicated by the arrow shown FIG. 3 from inner edges
44 and 48 to outer edges 45 and 49 thereof.
[0031] The angle indicated by the reference numeral B is between
the intersection D of a plane passing through the axis of rotation
A and the leading surface 40 and a radius perpendicular to the axis
of rotation A. The angle indicated by the reference numeral C is
between the intersection E of a plane passing through the axis of
rotation A and the trailing surface 41 and a radius perpendicular
to the axis of rotation A. The angle B is substantially greater
than the angle C. The angle C will normally be between about 1 and
45.degree. with the preferred angle being between 5.degree. and
20.degree. and with the most preferred angle being between being 7
to 15.degree.. Greater angles than 45.degree. may be utilized, but
the thread decreases in strength as the angle C increases which
increases the likelihood that the thread may break in use. The key
feature of the trailing surface 41 is that the surface 41 slopes
rearwardly from inside to outside. The angle B will vary with
desired thread strength and width of wall 50, but will always be
greater than angle C. Preferably the angle B is in the range from
30.degree. to 70.degree. and it is preferred that the angle B be in
the range from 40.degree. to 50.degree. In the illustrated
embodiment angle C is approximately 45.degree. and angle B is
approximately 15.degree..
[0032] As is best seen in FIG. 4, the threaded sections 28 and 29
of the arms 17 and 18 respectively are provided with a threadform
53 that is sized and shaped to threadedly receive the thread 1. The
threadform 53 is discontinuous, as it extends over the threaded
sections 28 and 29. The threadform 53 has a first surface 55 that
abuts against the leading surface 40 and a second surface 56 that
abuts against the trailing surface 41 during use. It is noted that
as torque is applied to closure 5 in a clockwise manner so as to
advance the closure 5 in the implant 7, the trailing surface 41
engages and pushes against the second surface 56 associated with
implant 7. The force exerted on the closure 5 by this process is
countered by a reactive force acting on the implant 7 that has a
first component that is axial, that is parallel to the axis of
rotation of the closure 5, and second component that has a radial
inward vector, that is toward the axis of rotation of the closure
5. The surfaces 40 and 41 are non parallel to each other.
[0033] It is foreseen that the thread 1 can be continuous or
discontinuous, as is threadform 53.
[0034] It is to be understood that while certain forms of the
present invention have been illustrated and described herein, it is
not to be limited to the specific forms or arrangement of parts
described and shown.
* * * * *