U.S. patent application number 16/704755 was filed with the patent office on 2020-04-09 for bone screw with apertures.
The applicant listed for this patent is Eric D. Kolb. Invention is credited to Eric D. Kolb.
Application Number | 20200107868 16/704755 |
Document ID | / |
Family ID | 54354337 |
Filed Date | 2020-04-09 |
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United States Patent
Application |
20200107868 |
Kind Code |
A1 |
Kolb; Eric D. |
April 9, 2020 |
BONE SCREW WITH APERTURES
Abstract
A bone screw includes a main body having a proximal end and a
distal end. The main body extends along a longitudinal axis. The
main body has an externally threaded surface that includes at least
one helically extending thread having a minor diameter and a major
diameter, which major diameter is greater than the minor diameter.
The thread includes at least two flank surfaces extending between
the minor diameter and the major diameter. The minor diameter of
the thread defines a central portion of the main body. The thread
may include at least one aperture that extends along an aperture
axis through the thread between the flank surfaces.
Inventors: |
Kolb; Eric D.; (Sandy Hook,
CT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kolb; Eric D. |
Sandy Hook |
CT |
US |
|
|
Family ID: |
54354337 |
Appl. No.: |
16/704755 |
Filed: |
December 5, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14699458 |
Apr 29, 2015 |
10531904 |
|
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16704755 |
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61986602 |
Apr 30, 2014 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
A61B 17/8625 20130101;
A61B 17/863 20130101; F16B 39/30 20130101; F16B 33/02 20130101;
A61B 2560/00 20130101; A61B 2017/8655 20130101 |
International
Class: |
A61B 17/86 20060101
A61B017/86 |
Claims
1. A bone screw, comprising: a main body having a proximal end and
a distal end, which main body extends along a longitudinal axis;
the main body having an externally threaded surface that includes
at least one helically extending thread having a minor diameter and
a major diameter, which major diameter is greater than the minor
diameter, and which thread includes at least two flank surfaces
extending between the minor diameter and the major diameter;
wherein the minor diameter of the thread defines a central portion
of the main body; and wherein the thread includes at least one
aperture that extends along an aperture axis through the thread
between the flank surfaces; and an insert disposed within the at
least one aperture.
2. The bone screw of claim 1, wherein the insert comprises at least
one of an osteoconductive material, an osteoinductive material, or
an osteogenic material.
3. The bone screw of claim 2, wherein the insert comprises at least
one of tricalcium phosphate, hydroxyapatite, bone morphogenic
proteins, absorbable polymers, autograft, allograft, xenograft, or
demineralized bone matrix.
4. The bone screw of claim 1, wherein the insert comprises at least
one of an antimicrobial material or an antibacterial material.
5. The bone screw of claim 1, wherein the insert comprises an
adhesive material.
6. The bone screw of claim 1, wherein the at least two flank
surfaces extending between the minor diameter and the major
diameter include a first flank surface extending between the minor
diameter and the major diameter and a second flank surface
extending between the minor diameter and the major diameter, the
first flank surface being disposed on the opposite side of the
thread from the second flank surface.
7. The bone screw of claim 6, wherein the at least one aperture
extends through the thread from the first flank surface to the
second flank surface, and the at least one aperture is defined by
an outer radial surface, an inner radial surface, a first lateral
surface, and a second lateral surface, and the inner radial surface
of the at least one aperture is coincident with the minor diameter
of the at least one helically extending thread.
8. The bone screw of claim 7, wherein the outer radial surface of
the at least one aperture is disposed radially inside of the major
diameter, and the at least one aperture has a closed perimeter.
9. The bone screw of claim 8, wherein the at least one aperture is
configured to have a substantially trapezoidal shaped
cross-sectional area.
10. The bone screw of claim 1, wherein the externally threaded
surface of the main body extends a length, and the at least one
aperture includes a plurality of the apertures disposed in the
thread at positions located throughout substantially all of the
externally threaded surface length, and each of the at least one
apertures includes an insert.
11. The bone screw of claim 1, wherein the at least one aperture is
defined by an outer radial surface, an inner radial surface, a
first lateral surface, and a second lateral surface, and the inner
radial surface of the at least one aperture is coincident with the
minor diameter of the at least one helically extending thread; and
wherein the outer radial surface extends a first circumferential
distance between the first lateral surface and the second lateral
surface, and the inner radial surface extends a second
circumferential distance between the first lateral surface and the
second lateral surface, and the first circumferential distance is
greater than the second circumferential distance; and wherein the
at least one aperture includes a height extending between the outer
radial surface and the inner radial surface, the height is less
than second circumferential distance.
12. A bone screw system, comprising: a bone screw comprising: a
main body having a proximal end and a distal end, which main body
extends along a longitudinal axis; the main body having an
externally threaded surface that includes at least one helically
extending thread having a minor diameter and a major diameter,
which major diameter is greater than the minor diameter, and which
thread includes at least two flank surfaces extending between the
minor diameter and the major diameter; wherein the minor diameter
of the thread defines a central portion of the main body; and
wherein the thread includes at least one aperture disposed in at
least one of the flank surfaces, the at least one aperture
configured to receive an insert; and at least one insert configured
to be disposed within the at least one aperture.
13. The bone screw system of claim 12, wherein the insert comprises
at least one of an osteoconductive material, an osteoinductive
material, or an osteogenic material.
14. The bone screw system of claim 12, wherein the insert comprises
at least one of an antimicrobial material or an antibacterial
material.
15. The bone screw of claim 12, wherein the insert comprises an
adhesive material.
16. The bone screw system of claim 12, wherein the at least two
flank surfaces include a first flank surface extending between the
minor diameter and the major diameter and a second flank surface
extending between the minor diameter and the major diameter, the
first flank surface being disposed on the opposite side of the
thread from the second flank surface; and wherein the at least one
aperture extends through the thread from the first flank surface to
the second flank surface.
17. The bone screw system of claim 12, wherein the at least one
apertures is a recess disposed in at least one of the flank
surfaces.
18. A bone screw, comprising: a main body having a proximal end and
a distal end, which main body extends along a longitudinal axis;
the main body having an externally threaded surface that includes
at least one helically extending thread having a minor diameter and
a major diameter, which major diameter is greater than the minor
diameter, and which thread includes at least two flank surfaces
extending between the minor diameter and the major diameter;
wherein the minor diameter of the thread defines a central portion
of the main body; and wherein the thread includes at least one
recess disposed in one of the flank surfaces; and an insert
disposed within the at least one recess.
19. The bone screw of claim 18, wherein the insert comprises at
least one of an osteoconductive material, an osteoinductive
material, or an osteogenic material.
20. The bone screw of claim 18, wherein the insert comprises at
least one of an antimicrobial material or an antibacterial
material.
21. The bone screw of claim 18, wherein the insert comprises an
adhesive material.
Description
[0001] The present application is a divisional of U.S. patent
application Ser. No. 14/699,458 filed Apr. 29, 2015, which is
entitled to the benefit of and incorporates by reference essential
subject matter disclosed in U.S. Provisional Patent Application
Ser. No. 61/986,602, filed Apr. 30, 2014.
BACKGROUND OF THE INVENTION
1. Field of Invention
[0002] The present invention relates generally to surgical
implants, and more specifically to a bone screw with features that
permit bone ingrowth.
2. Background Information
[0003] Fasteners are used in a variety of surgical applications
including affixing implants to tissue and affixing tissue to
tissue. A common surgical fastener is a bone screw, which comprises
threads designed to achieve mechanical purchase within bone. Bone
screws exist in a variety of styles including pedicle screws,
fracture fixation screws, and screws for securing bone plates. Bone
screws are widely used because they are easy to insert, easy to
produce, and under certain conditions provide adequate mechanical
purchase. However, in applications involving highly porous,
osteopenic or osteoporotic bone the efficacy of bone screws is
substantially reduced. Design optimization of bone screws for these
applications is challenged by competing constraints; the need for
larger (i.e. coarser) threads versus the need for adequate bending
strength (e.g., provided by the minor diameter of a screw) versus
the maximally allowable major diameter (often limited by anatomy or
mating component geometry). Further, bone screws function in a
dynamic biological environment in which the supporting bone and
tissue undergoes constant remodeling which can adversely affect the
stability of the bone screw through time.
[0004] Design features aimed at improving the efficacy of bone
screws without affecting the conventional thread form have focused
on enhancing the implant-bone interface. Proposed enhancement have
included use of biologically favorable metals (e.g. titanium),
application of favorable coatings (e.g. hydroxyapatite), and
surface texturing.
[0005] Other design solutions departing from conventional thread
forms have incorporated features including expanding tips,
trabecular metal, etc. These solutions may compromise the
mechanical integrity of the screw.
[0006] Despite these proposed solutions, screw failure (e.g.
loosening, breaking) is still a frequent clinical occurrence. Thus,
the need exists for a bone fixation device that can be delivered as
easily as a conventional bone screw, allows for bone ingrowth, and
maintains adequate mechanical performance (e.g., pullout,
bending).
SUMMARY
[0007] According to an aspect of the present disclosure, a bone
screw is provided that includes a main body having a proximal end
and a distal end, which main body extends along a longitudinal
axis. The main body has an externally threaded surface that
includes at least one helically extending thread having a minor
diameter and a major diameter, which major diameter is greater than
the minor diameter. The thread includes at least two flank surfaces
extending between the minor diameter and the major diameter. The
minor diameter of the thread defines a central portion of the main
body. The thread includes at least one aperture that extends along
an aperture axis through the thread between the flank surfaces.
[0008] According to another aspect of the present disclosure, a
bone screw is provided that includes a main body having a proximal
end and a distal end, which main body extends along a longitudinal
axis. The main body has an externally threaded surface that
includes at least one helically extending thread having a minor
diameter and a major diameter, which major diameter is greater than
the minor diameter. The thread includes at least two flank surfaces
extending between the minor diameter and the major diameter. The
minor diameter of the thread defines a central portion of the main
body. The thread includes at least one recess disposed within one
of the flank surfaces.
[0009] According to another aspect of the present invention, a bone
screw is provided that includes a main body having a proximal end
and a distal end, which main body extends along a longitudinal
axis. The main body has an externally threaded surface that
includes at least one helically extending thread having a minor
diameter and a major diameter, which major diameter is greater than
the minor diameter. The thread includes at least two flank surfaces
extending between the minor diameter and the major diameter. The
minor diameter of the thread defines a central portion of the main
body, and the central portion includes a porous material.
[0010] The various bone screw embodiments described herein can be
used for a variety of surgical applications including use as
pedicle screws and fracture fixation screws.
[0011] In certain of the disclosed embodiments the apertures
provide space into which bone can grow. In other embodiments the
apertures define a structure that allows the threads to locally
deform and accommodate local changes in bone density. In other
embodiments the apertures accommodate an insert formed of an
osteoconductive, osteoinductive or osteogenic material.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a perspective view of a bone screw embodiment.
[0013] FIG. 2 is a cross-sectional view of the embodiment in FIG.
1.
[0014] FIG. 3 is a perspective view of a bone screw embodiment.
[0015] FIG. 3A is a distal end view of the bone screw embodiment
shown in FIG. 3.
[0016] FIG. 4 is a perspective view of a bone screw embodiment.
[0017] FIG. 4A is a distal end view of the bone screw embodiment
shown in FIG. 4.
[0018] FIG. 5 is a perspective view of a bone screw embodiment.
[0019] FIG. 5A is a distal end view of the bone screw embodiment
shown in FIG. 5.
[0020] FIG. 6 is a perspective view of a bone screw embodiment.
[0021] FIG. 6A is a distal end view of the bone screw embodiment
shown in FIG. 6.
[0022] FIG. 7 is a perspective view of a bone screw embodiment.
[0023] FIG. 7A is a distal end view of the bone screw embodiment
shown in FIG. 7.
[0024] FIG. 8 is a perspective exploded view of a bone screw
embodiment.
[0025] FIG. 8A is a perspective view of the bone screw embodiment
with inserts disposed in the thread.
[0026] FIG. 9 is a perspective view of a bone screw embodiment.
[0027] FIG. 10 is a perspective view of a bone screw
embodiment.
[0028] FIG. 11 is a diagrammatic sectional view of a threaded
section portion.
[0029] FIG. 11A is a diagrammatic planar view of a threaded section
portion.
[0030] FIG. 12 is a diagrammatic planar view of a threaded section
portion.
[0031] FIG. 13 is a diagrammatic sectional view of a threaded
section portion.
DETAILED DESCRIPTION
[0032] Referring to FIG. 1, a bone screw 1 according to the present
disclosure includes a main body 22 having a proximal end 2 and a
distal end 3. A longitudinal axis 4 extends between the proximal
end 2 and the distal end 3. The main body 22 includes an externally
threaded surface 24 disposed between the proximal end and the
distal end, which externally threaded surface 24 includes at least
one helically extending thread 9 as will be described below.
[0033] The bone screw 1 may include a head 5 that is configured to
allow the bone screw 1 to be rotationally driven (e.g., into a bone
substrate). The present disclosure is not limited to any particular
head configuration, and acceptable examples include a head 5 that
can be driven by a screw driver, and hex key, etc. The bone screw
head 5 may also be configured to mate with a complimentary element
such as those used within bone fixation devices (e.g., a pedicle
screw), plates, etc. The embodiment shown in FIG. 1, for example,
includes a recess 6 configured to receive a hex key driver.
[0034] The portion of the main body 8 that includes the externally
threaded surface 24 may be cylindrically--shaped; i.e., the major
diameter of the thread 9 disposed within the threaded surface 24
(described below) is constant throughout substantially all the
externally threaded surface 24 as is shown in FIGS. 1-10. The
present disclosure is not limited to having a cylindrically-shaped
externally threaded surface 24, however; e.g., the externally
threaded surface 24 may be tapered.
[0035] The main body 8 may include a tip portion 7 contiguous with
the distal end 3, which tip portion 7 is configured to facilitate
entry of the bone screw 1 into a substrate (e.g., bone).
Non-limiting examples of a tip portion 7 configured to facilitate
entry of the bone screw 1 into a substrate (e.g., bone) include a
tip portion 7 having a tapered region (i.e., a region that
decreases from a first diameter to a smaller second diameter
located at the distal end 3), which tapered region may be threaded,
a tip portion having a self-tapping feature, or a tip portion
having a self-drilling feature, or combinations thereof.
[0036] In some embodiments the bone screw 1 may include a shank
portion 28 disposed between the proximal end 2 and the externally
threaded surface 24.
[0037] The externally threaded surface 24 includes at least one
helically extending thread 9 having a minor diameter 11 and a major
diameter 12, which major diameter 12 is greater than the minor
diameter 11. The major diameter 12 is disposed at the outer most
radial point of the thread 9, and the minor diameter 11 is disposed
at the inner most radial point of the thread 9. The outer most
radial point of the thread 9 may be referred to as a "peak 15" of
the thread 9, and the inner most radial point of the thread 9 may
be referred to as a "root 32" of the thread 9. In those embodiments
wherein the externally threaded surface 24 is cylindrically-shaped,
an imaginary line drawn to intersect with the major diameter points
of the helical thread 9 is parallel with the longitudinal axis 4 of
the bone screw 1. In those embodiments wherein the externally
threaded surface 24 is tapered, an imaginary line drawn to
intersect with the major diameter points of the helical thread 9 is
non-parallel with the longitudinal axis 4 of the bone screw 1. The
thread 4 is further defined by at least two flank surfaces 34a, 34b
extending between the minor diameter 11 and the major diameter 12,
which flank surfaces 34a, 34b are disposed on opposite sides of the
thread 9. The minor diameter 11 of the externally threaded surface
24 defines a central portion 36 of the main body 8.
[0038] The cross-sectional geometry of the helical thread 9 may be
defined by parameters such as the major and minor diameters 11, 12,
the flank surfaces 34a, 34b, shape, and the pitch 12 of the helical
thread 9. In regards to the "shape" of the thread 9, the
cross-sectional geometry of the thread 9 may reflect, for example,
a symmetrical thread 9 having flank surfaces 34a, 34b, each
disposed at the same angle relative to the longitudinal axis 4, or
flank surfaces 34a, 34b disposed at different angles, etc. As
another example, the flank surfaces 34a, 34b may intersect one
another at the thread peak 15 (to form a pointed thread peak 15, or
may not intersect one another (e.g., a truncated thread peak 15).
The present disclosure is not limited to any particular thread
cross-sectional geometry.
[0039] The particular cross-sectional geometry of the helical
thread 9 may be selected in view of the application. For example,
one embodiment of the present bone screw 1 may have a helical
thread 9 with a cross-sectional geometry that is well-suited for
mechanical engagement with a first type of substrate (e.g., porous
bone, osteopenic bone, osteoporotic bone, etc.), while another
embodiment of the present bone screw 1 may have a helical thread 9
with a cross-sectional geometry that is well-suited for mechanical
engagement with a second type of substrate (e.g., dense bone,
etc.). Embodiments of the present bone screw 1 may include more
than one type of helical thread 9, or a helical thread 9 that has a
plurality of cross-sectional geometries, each located at a
different position.
[0040] To facilitate the description of the present disclosure, the
various bone screw embodiments depicted in the drawings are shown
as having single-lead right-handed threads 9. The present
disclosure is not limited to bone screws 1 having a single-lead
right-handed thread 9. For example, embodiments of the present bone
screw 1 may have a left-handed thread 9 and/or multiple lead
threads 9.
[0041] In some embodiments of the present bone screw 1, the
helically extending thread 9 includes at least one aperture 13
extending through the thread 9. The aperture(s) 13 extends along an
aperture axis 38 through the thread 9, from flank surface 34a to
flank surface 34b of the thread 9 thereby providing an open passage
through the body of the thread 9. The present bone screw is limited
to any particular orientation of the aperture axis 38 (e.g.,
relative to the longitudinal axis 4). To illustrate a couple of
exemplary aperture axis orientations, consider the present bone
screw in terms of a pair of orthogonal axes X and Y. Consider
further that the longitudinal axis 4 is coincident with the X axis,
and the Y axis extends radially outward from the X-axis. Referring
to FIGS. 11 and 11A, a first aperture axis 38 may be described as
extending along a line parallel to the longitudinal axis 4 (i.e.,
parallel to the X-axis;) at a particular distance from the
longitudinal axis 4 (i.e., at a radial distance 48 out from the
centrally located longitudinal axis 4--see FIG. 11). FIG. 11A shows
axis 38 and axis 4 as the same because they are in the same radial
plane. In FIG. 11A the radial distance 48 between axis 38 and axis
4 extends perpendicular (i.e., into) the 2-dimensional plane of the
figure, and is therefore not shown. At the radial distance 48, the
aperture axis 38 (and therefore the aperture 13) passes through the
flank surfaces 34a, 34b of the thread 9. In this example, because
the aperture axis 38 is parallel with the longitudinal axis 4, the
aperture axis 38 is skewed by an angle .beta. (i.e., the aperture
axis 38 is non-perpendicular) to the centerline 50 of the helically
extending thread 9.
[0042] Now referring to FIG. 12, a second aperture axis example may
be described as extending along a line skewed from the longitudinal
axis 4 at a particular distance from the longitudinal axis 4 (i.e.,
at a radial distance 48 (e.g., see FIG. 11) out from the centrally
located longitudinal axis 4). Here again, at the radial distance
the aperture axis 38 (and therefore the aperture 13) passes through
the flank surfaces 34a, 34b of the thread. In this example, the
aperture axis 38 is skewed from the longitudinal axis 4 by an angle
.alpha. which orients the aperture axis 38 as perpendicular to the
centerline 50 of the helically extending thread 9.
[0043] In some embodiments, the apertures 13 are disposed
completely within the thread(s) 9, and therefore do not extend
radially inward of the minor diameter 11 of the threaded surface
portion 24 (i.e., no portion of an aperture 13 is disposed within
the central portion 36). In these embodiments because the entirety
of the central portion 36 is maintained, the mechanical properties
(e.g., bending strength) of the bone screw 1 are not locally
compromised by an aperture 13 traversing the cylindrical central
portion 36 defined by the minor diameter 11. In some embodiments,
one or more apertures 13 may extend radially inside of the minor
diameter 11 and therefore into the central portion 36 of the
threaded surface portion 24 of the bone screw 1 (i.e., a portion of
an aperture 13 is disposed within the central portion 36).
[0044] FIGS. 1-10 depict bone screw 1 embodiments having apertures
13 disposed within the helical thread 9 over substantially the
entire externally threaded surface portion 24. In alternate
embodiments the apertures 13 may be disposed within the helical
thread 9 within less than the entirety of the externally threaded
surface portion 24. For example, in some embodiments a portion of
the helical thread 9 disposed adjacent the head 5 may not include
apertures 13. The specific placement of the apertures 13 can be
chosen to customize the bone screw 1 for the application at hand;
e.g., positioning the apertures 13 to provide increased or
decreased strength or deflection thread zones, zones where more
bone affixation is desired, zones where inserts 618 (described
below) are desired, etc.
[0045] In some embodiments (e.g., see FIGS. 1 and 2), the apertures
13 are circular shaped cross-sectional area. The circular apertures
13 are not limited to any particular diameter. For example, in many
applications a bone screw 1 having circular apertures 13 with a
diameter of approximately 1-2 mm is useful. The circular apertures
13 may, however, have smaller or larger diameters, or combinations
thereof. The range of acceptable aperture diameters is bounded by
the minimum feature size into which new bone would grow (.about.100
microns) and a maximum feature size that provides adequate fastener
bending strength.
[0046] Referring to FIGS. 3 and 3A, in another embodiment the
apertures 113 of bone screw 101 have a substantially trapezoidal
shape. A substantially trapezoidal shape is defined as having a
circumferentially extending width 140 and a radially extending
height 142 (e.g., see FIG. 3A). The trapezoidal aperture 113
configuration shown in FIG. 3 has a width 140 greater than a height
142, thereby providing apertures 113 with a greater cross-sectional
area as compared to the circular apertures 13 depicted in FIGS. 1
and 2. Here apertures 113 are arranged circumferentially about the
bone screw, with adjacent apertures 113 circumferentially separated
from one another by a thread portion hereinafter referred to as a
strut 114. The struts 114 function to provide support for the
thread peak 115, which in this embodiment runs relatively
uninterrupted along the threads 109 and over the full length of the
threaded surface 124. The strut 114 has a centerline 144. In some
embodiments, a strut 114 may have a centerline 144 that extends
along a line that extends radially outward from the longitudinal
axis 4 (e.g., see FIG. 3A). In some embodiments, a strut 414 may
have a centerline 444 that extends along a line that extends at an
angle skewed from a radial line extending outward from the
longitudinal axis 4 (e.g., see FIG. 6A). In certain embodiments a
relatively uninterrupted thread peak 115 is advantageous in that it
provides a smooth bone contacting surface that facilitates
insertion of the bone screw 101. In this embodiment the apertures
113 extending longitudinally along the length of the main body 8
are coaxial to one another. In an alternate embodiment the
apertures 113 are not positioned in a coaxial fashion, but rather
staggered such that when viewed on end an unobstructed line of
sight through all the apertures 113 is not possible.
[0047] The embodiment shown in FIGS. 4 and 4A has substantially
trapezoidal-shaped apertures 213, each having a width 240 less than
a height 242, thereby providing apertures 213 with a smaller
cross-sectional area. In this configuration, the size and number of
apertures 213 relative to the struts 214 provide a relatively stiff
thread peak 215. The central portion 236 is shown in FIG. 4A.
[0048] FIGS. 5 and 5A depict a bone screw 301 embodiment in which
the apertures 313 have a greater cross-sectional area than the
apertures 113 shown in FIG. 3; e.g., semi-annular shaped apertures.
Here the size and number of apertures 313 relative to struts 314
provides relatively more volume into which bone can grow. In
certain embodiments there may be less than one strut per revolution
of the helical thread 309.
[0049] Still referring to FIGS. 5 and 5A, the individual aperture
size (i.e., the cross-sectional area) and total number of apertures
313 results in a relatively flexible thread peak 315. The flexible
thread peak 315 is advantageous in that during insertion of the
bone screw 301 fastener it can locally deflect to accommodate the
geometry of the space it is filling. For example, if the bone into
which the bone screw 301 is being inserted has a localized region
of greater bone density, the flexible thread peak 315 can
elastically deforms as it passes that region. After passing that
region, the flexible thread peak 315 elastically returns to its
nominal undeformed state. In this way the bone screw 301 is affixed
to bone of non-homogeneous density and/or structure. In certain
applications it may be permissible or even advantageous for the
flexible thread peak 315 to plastically deform. The central portion
336 is shown in FIG. 5A.
[0050] Now referring to FIGS. 6 and 6A, in some embodiments, the
apertures 413 of bone screw 401 are shaped such that adjacent
apertures 413 form struts 414 favorably oriented to accommodate
deformation of the flexible thread peak 415. For example, as can be
seen in FIG. 6A, adjacent apertures 413 can be configured to form
struts 414 that extend at an angle skewed from a radial line
extending out from the longitudinal axis. In this configuration,
under locally applied radial forces 416 the struts 414 are
primarily subject to bending forces rather than compressive forces,
allowing greater plastic deformation of the flexible thread peak
415.
[0051] In some embodiments an aperture of bone screw may be defined
by an "open geometry" or an "open perimeter"; i.e., the aperture
breaks through the peak of the thread. For sake of clarity, an
aperture having a "closed geometry" or a "closed perimeter" is one
in which the circumferential perimeter of the aperture is
continuous and therefore cannot be accessed through the peak of the
thread. FIG. 7, for example, shows a bone screw 501 with apertures
513 having slits 517 that interrupt the flexible thread peak 515
and allow portions of flexible thread peak 515 to deform relatively
independently of other portions of the flexible thread peak 515.
The slits 517 can be relatively thin, and in certain embodiments
provide physical contact or near contact between adjacent portions
of the flexible thread peak 515. In this way one portion of the
flexible thread peak 515 provides displacement constraint for
another portion of the flexible thread peak 515. In another
embodiment, the slits 517 may be configured to allow the entirety
of the flexible thread peak 515 to deform and achieve an effective
smaller major diameter over the entire length of the shaft. When
the bone screw 501 of this type having a diameter "X" is inserted
into a hole with a diameter less than "X", the flexible thread peak
deforms to create an effectively smaller major diameter. Once fully
inserted, the flexible thread peak 515 provides a restorative
radial force (i.e., a radially outward biasing force) that acts to
enhance mechanical purchase.
[0052] One function of the various aperture embodiments is to
provide a volume into which bone can grow and form a mechanical
interlock that resists fastener loosening. Referring to FIGS. 8 and
8A, in an alternate embodiment the apertures 613 of bone screw 601
may be filled with a material other than the material of which the
bone screw 1 is comprised. The material disposed in the apertures
is referred to hereinafter as an "insert 618". The present
disclosure is not limited to any particular manner of disposing the
inserts 618 within the apertures. For example, the inserts 618 may
be affixed with the apertures 613 by the manufacturer of the bone
screw 601. Alternately, the inserts 618 may be affixed to the bone
screw at a time subsequent to the manufacture of the bone screw;
e.g., at the time of surgery. As diagrammatically shown in FIG. 8,
the inserts 618 may have a shape approximating the shape of the
corresponding aperture 613. One or more nominal dimensions of the
inserts 618 may be larger than the corresponding dimension of the
apertures 613 such that the inserts 618 are affixed to the bone
screw 601 via an interference fit. Alternately, the inserts 618 may
be affixed using an adhesive or a mechanical interlock. In other
embodiments the pre-insertion shape of the insert 618 does not
approximate the shape of the aperture 613. For example, an
amorphous volume of granular autograft material may be pressed into
the apertures 613.
[0053] In some embodiments, the inserts 618 may be constructed to
enhance the stability of the fastener within the bone, either
immediately upon insertion or through time. The inserts 618 may be
constructed from a variety of known materials that provide or carry
osteoconductive, osteoinductive or osteogenic elements including
but not limited to tricalcium phosphate, hydroxyapatite, bone
morphogenic proteins, absorbable polymers, autograft, allograft,
xenograft, and demineralized bone matrix. In some embodiments the
inserts 618 may be constructed of materials that provide
antimicrobial or antibacterial characteristics, including but not
limited to drugs, silver, and bioglass. In some embodiments the
insert 618 can be formed from an adhesive material, for example
polyurethane bone cement. Upon insertion of the fastener, the
adhesive adheres to both the bone screw 601 and the bone, thereby
enhancing initial bone screw stability. In another embodiment the
insert 618 expands upon insertion, filling voids within the bone
and thereby enhancing initial bone screw stability. The present
disclosure includes embodiments wherein all of the inserts 618 are
the same material and/or configuration, and also embodiments where
there are inserts 618 of a first type material and/or
configuration, and other inserts of a different type material
and/or configuration.
[0054] The embodiment shown in FIG. 8 comprises apertures 613
positioned at a single location along the perimeter of the shaft
608. In this and similar embodiments with fewer or smaller
apertures the thread peak 615 is relatively inflexible. Here the
thread peak 615 and other thread portions 609 act to protect the
insert 618 from damage or displacement during fastener
insertion.
[0055] Referring the FIG. 9, the struts 114 of bone screw 101 are
comprised of leading edges 119 and trailing edges 120. During
rotational insertion of the fastener, the leading edges 119 follow
the apertures 113 and the trailing edges 120 follow the struts 114.
In the embodiment shown in FIG. 9, both the leading edges 119 and
trailing edges 120 of the struts 114 are configured with a sharp
edge (e.g., a pointed edge formed by the respective strut surfaces
intersecting at an approximately 90-degree corner). In some
embodiments the leading edges 119 may be formed such that the
respective strut surfaces intersect to form a rounded or chamfered
edge that is less likely to catch or cut into adjacent bone. In
some embodiments the trailing edges 120 may be formed to such that
the respective strut surfaces intersect at an acute angle such that
the trailing edge 120 is effective at cutting bone. In some
embodiments, both the leading edges 119 and trailing edges 120 are
formed by an acute angle within the strut 114 such that they are
effective at cutting bone.
[0056] The specific aperture shape embodiments described above
reflect some but not all of the aperture shapes contemplated by the
present disclosure. Consequently, the present disclosure is not
limited to any particular aperture shape, and is not limited to
just those aperture shapes described above. In addition, a bone
screw according to the present disclosure may include more than one
type of bone screw apertures. For example, a first region of a bone
screw 1 proximate the head 5 of the bone screw 1 may include one or
more apertures 13 disposed within the helical thread 9 of a first
type (e.g., a first shape), and a second region of a bone screw 1
proximate the tip 7 of the bone screw 1 may include one or more
apertures 13 disposed within the helical thread 9 of a second type
(e.g., a second shape). Using a second type of aperture 13 near the
distal end 3 of the bone screw 1 where the tip 7 tapers and the
thread major diameter 12 and minor diameter 11 vary can be
beneficial. Hence, the present disclosure includes bone screw
embodiments having a plurality of aperture types disposed at
various positions along the externally threaded surface,
interdispersed aperture types, etc.
[0057] Although the embodiments shown in the FIGS. 1-12 depict
apertures (i.e. hole features that pass completely through the
threads), alternate embodiments may include one or more recesses 46
disposed within a flank surface 34a, 34b of the helical thread 9
(e.g., see FIG. 13). A recess 46 may be defined as a void open at a
flank surface 34a, 34b, that extends into the flank surface but
does not extend entirely through the helical thread 9 (i.e., it
does not extend between flank surfaces to create a passage through
the helical thread 9). These recesses 46 are configured (e.g.,
depth, width, height, and/or geometry) such that once the bone
screw 1 is inserted, over time bone (or other tissue) may extend
into the recess 46 and thereby provide a mechanical interlock
resisting screw loosening or backout. These recesses 46 may be
oriented substantially parallel to the longitudinal axis 4 of the
bone screw 1. In some embodiments of the present disclosure, the
bone screw 1 may include both recesses 46 and apertures 13 disposed
within the helical thread 9.
[0058] Referring to FIG. 10, in another embodiment the central
portion of the externally threaded surface includes an aperture
network 713 that includes pathways that are oriented in multiple
directions. For example, the central portion 736 (defined by the
minor diameter) may include a porous structure favorable to bone
growth into the pathways. Examples of an acceptable material that
can be used to form a porous structure part of the central portion
include, for example, Porocoat.RTM. coating (available from DePuy
Synthes, Inc.) or Trabecular Metal.TM. surface technology
(available from Zimmer, Inc.). The threads 709 that overlay this
aperture network 713 may be formed from a substantially solid
material. Thus, as the solid bone screw threads 709 engage a hole
within a corresponding substrate (e.g., bone tissue), the threads
709 provide a smooth substrate contact surface that facilitates
insertion. The aperture network 713, which is in less intimate
contact with bone during insertion, provides later access for bone
ingrowth. Also, the solid threads 709 enhance the rigidity and
strength to the bone screw 701. In some embodiments the pathway
network created by the porous structure is disposed in both a
portion of the cylindrical region of shaft approximately bounded by
the minor thread diameter and a portion of the threads. In this
embodiment the thread peaks 715 are uninterrupted (i.e. without
pores), providing a smooth bone interface surface.
[0059] Fabrication of the various embodiments may benefit from
additive manufacturing techniques (i.e. 3D printing). However, the
scope of invention is not limited to a specific manufacturing
technology. Embodiments may be manufactured from stainless steel,
cobalt chromium, titanium, polymers, ceramics and other suitable
materials.
[0060] In one fabrication method the bone screw 1 is comprised
wholly of a material provided by an additive manufacturing process;
e.g., printing. In another fabrication method a portion of the bone
screw 1 is formed by a first process comprising conventional
manufacturing techniques (e.g. machining, casting, rolling,
stamping) and a portion of the bone screw 1 is formed by a second
process comprising additive manufacturing techniques. By way of
example, in one embodiment the central portion 36 is formed using
conventional manufacturing techniques and the thread 9 is formed
using additive manufacturing techniques.
[0061] Lastly, although this present disclosure is described herein
as a bone screw, the present disclosure device is not limited to
bone screw applications. These applications include but are not
limited to other medical applications (e.g. soft tissue anchors)
and non-medical applications (e.g. screws for fastening wood,
drywall and concrete).
[0062] Although the invention has been shown and described with
respect to specific detailed embodiments thereof, it will be
understood by those skilled in the art that various changes in form
and detail thereof may be made without departing from the spirit
and the scope of the invention.
* * * * *