U.S. patent application number 11/879536 was filed with the patent office on 2009-01-22 for bone screws and particular applications to sacroiliac joint fusion.
Invention is credited to John G. Stark.
Application Number | 20090024174 11/879536 |
Document ID | / |
Family ID | 40260242 |
Filed Date | 2009-01-22 |
United States Patent
Application |
20090024174 |
Kind Code |
A1 |
Stark; John G. |
January 22, 2009 |
Bone screws and particular applications to sacroiliac joint
fusion
Abstract
Procedures for the fusion of the sacroiliac joint advantageously
make use of an implant selected to distract the joint upon
insertion and to maintain or increase tension upon insertion. The
implant can have a varying structure along its length. In some
method described herein for fusing the sacroiliac joint using an
implant, an implant is screwed into the sacroiliac joint between
the sacrum bone and the iliac bone. The implant comprises a shaft,
a tool engagement flange at top end of the shaft, a pointed tip
comprising no more than about 20 percent of the length of the
screw, and threads spiraling around the shaft. For screws of
particular interest, the volume displacement perpendicular to the
shaft increases at least about 5 percent from a point adjacent the
tip to a point near the top of the shaft. Some of the desirable
screw designs can be used in other orthopedic application,
especially situations involving varying bone hardness. Useful
filler material can be formed from a blend of bone powder and
bioactive agents.
Inventors: |
Stark; John G.; (Deephaven,
MN) |
Correspondence
Address: |
DARDI & ASSOCIATES, PLLC
220 S. 6TH ST., SUITE 2000, U.S. BANK PLAZA
MINNEAPOLIS
MN
55402
US
|
Family ID: |
40260242 |
Appl. No.: |
11/879536 |
Filed: |
July 17, 2007 |
Current U.S.
Class: |
606/321 ;
128/898; 606/301; 606/79; 606/93 |
Current CPC
Class: |
A61B 17/8625 20130101;
A61B 17/7055 20130101 |
Class at
Publication: |
606/321 ;
128/898; 606/79; 606/301; 606/93 |
International
Class: |
A61B 17/04 20060101
A61B017/04; A61B 17/58 20060101 A61B017/58; A61B 19/00 20060101
A61B019/00; A61B 17/00 20060101 A61B017/00 |
Claims
1. A method for fusing the sacroiliac joint, the method comprising:
drilling a bore into the joint between the sacrum bone and iliac
bone to prepare the joint for insertion of an implant; selecting a
threaded implant based on the prepared joint that distracts the
joint upon insertion and maintains or increases tension upon
insertion, wherein the implant has a varying structure along its
length; and inserting the implant into the bore within the
sacroiliac joint between the sacrum bone and iliac bone to
immobilize the joint.
2. The method of claim 1 wherein the drilling process comprises the
use of a drill tool and wherein the drill tool is tapered.
3. The method of claim 1 wherein the implant has a diameter at
least about 0.5 millimeters wider than the drilled bore at full
implantation into the joint.
4. The method of claim 1 wherein the selecting process is performed
using a threaded sizing element engaging a torque wrench wherein
the threaded element is tightened to reach a selected value of
torque.
5. The method of claim 1 wherein the selecting process comprises
choice of a thread design based on bone hardness around the
bore.
6. The method of claim 1 wherein the implant is tapered and
headless.
7. The method of claim 1 further comprising placement of bone
material within a joint space between the sacrum bone and iliac
bone.
8. The method of claim 1 wherein the threads of the implant vary
along the length of the implant.
9. The method of claim 1 wherein implant comprises two sets of
interwoven threads.
10. The method of claim 8 wherein implant has a tip and top end at
the opposite end from the tip, the top end comprising a tool
engagement flange and wherein the lateral extent of the threads
increases toward the top end of the implant relative to the thread
structure near the tip.
11. A method for fusing the sacroiliac joint using an implant, the
method comprising screwing an implant into the sacroiliac joint
between the sacrum bone and the iliac bone wherein the implant
comprises a shaft, a tool engagement flange at top end of the
shaft, a pointed tip comprising no more than about 20 percent of
the length of the screw, and threads spiraling around the shaft,
wherein the volume displacement perpendicular to the shaft
increases at least about 5 percent from a point adjacent the tip to
a point near the top of the shaft.
12. The method of claim 11 wherein the shaft is tapered to increase
in diameter toward the top relative to the tip over at least a
portion of the shaft.
13. The method of claim 11 wherein the shaft has a cylindrical
section between the tip and the top and wherein the increase in
differential volume results from an increase in thread diameter
from the tip to the top.
14. The method of claim 11 wherein the thickness of the thread
increases from the tip to the top.
15. The method of claim 11 wherein the thickness and diameter of
the threads increases from the tip to the top.
16. The method of claim 11 wherein the thread spacing decreases
from the tip to the top.
17. A medical implantable screw comprising a headless shaft, a tool
engagement flange at top end of the shaft, a pointed tip comprising
no more than about 20 percent of the length of the screw, and
threads spiraling around the shaft, wherein the volume displacement
of the thread increases at least about 5 percent from a point
adjacent the tip relative to a point near the top of the shaft and
wherein the screw comprises one or more biocompatible
materials.
18. The medical implantable screw of claim 17 wherein the thread
thickness increases from the tip to the top of the shaft.
19. The medical implantable screw of claim 17 wherein the lateral
extent of the thread increases from the tip to the top of the
shaft.
20. The medical implantable screw of claim 17 wherein the thread
spacing decreases from the tip to the top of the shaft.
21. A method for inducing bone in-growth for joint fusion or bone
repair, the method comprising placing a blend of a bone powder and
a bioactive agent that induces bone growth into a joint or bone
fracture.
22. The method of claim 21 wherein the bioactive agent comprises
bone morphogenic protein.
23. The method of claim 21 wherein the bone powder comprises
demineralized bone powder.
24. The method of claim 21 wherein the bone powder comprises a
synthetic bone material.
25. A composition comprising a blend of bone powder and a bioactive
agent that simulates bone growth.
26. The composition of claim 25 further comprising a biocompatible
carrier.
27. A method for inserting an orthopedic implant into a bone, the
method comprising placing a sizing element into a prepared location
for a bone screw and tightening the sizing element with a torque
wrench to evaluate the size of appropriate bone screw for
implantation into the site.
28. An orthopedic implant comprising a bone replacement materials
and a bone growth stimulating biologically active agent, wherein
the bone replacement material is a bio-resorbable polymer, a
natural or synthetic bone composition or a combination thereof and
wherein the biologically active agent is blended into the bone
replacement material composition.
29. The orthopedic implant of claim 28 wherein the bone replacement
material comprises demineralized bone powder, crushed bone, coral,
hydroxyapatite, calcium phosphate, calcium sulfate or a combination
thereof and the biologically active agent is bone morphogenic
protein.
Description
FIELD OF THE INVENTION
[0001] The invention relates to methods for fusing the sacroiliac
joint using anchors that distract the joint and immobilize the
joint in its distracted position. The invention further relates to
bone screw designs that are versatile with respect to firm
anchoring in softer or harder bone structures through increasing
gripping of the structure as the screw is advanced.
BACKGROUND OF THE INVENTION
[0002] Lower back pain is a common ailment among the population and
results in pain and suffering as well as loss of work time. Thus,
approaches for the treatment of back pain can both relieve
suffering as well as reduce employee sick time. Since back pain
results in considerable employee absenteeism, effective treatments
for lower back pain have both economic benefits as well as the
benefit of alleviating considerable suffering.
[0003] The sacroiliac joint is located at the juncture of the
ilium, the upper bone of the pelvis, and the sacrum at the base of
the spine. While the sacroiliac joint has a limited range of
motion, dysfunction of the joint has been identified. The joint is
supported by a range of ligaments including, for example, the
sacroiliac ligament at the base of the joint and the anterior
sacroiliac ligament at the top of the joint. The joint is in the
vicinity of the passage of a large number of blood vessels and
nerves that pass from the torso to the lower extremities. Any
procedures near the joint should avoid damage to the adjacent
vessels and nerves.
SUMMARY OF THE INVENTION
[0004] In a first aspect, the invention pertains to a method for
fusing the sacroiliac joint in which the method comprises drilling,
selecting an implant and inserting the implant. In particular a
bore is drilled into the joint between the sacrum bone and iliac
bone to prepare the joint for insertion of an implant. A threaded
implant is selected based on the prepared joint in which the
implant or screw is selected to distract the joint upon insertion
and maintains or increases tension upon insertion. The implant can
have a varying structure along its length. The selected implant can
be inserted into the bore within the sacroiliac joint between the
sacrum bone and iliac bone to immobilize the joint. In some
embodiments, material is also placed into the joint or as a
component of the implant to promote bone growth before, during or
following completion of the procedure to deliver an implant.
[0005] In further aspects, the invention pertains to a method for
fusing the sacroiliac joint using an implant. The method can
comprise screwing an implant into the sacroiliac joint between the
sacrum bone and the iliac bone in which the implant comprises a
shaft, a tool engagement flange at top end of the shaft, a pointed
tip comprising no more than about 20 percent of the length of the
screw, and threads spiraling around the shaft. In some embodiments,
the volume displacement of the threads perpendicular to the shaft
increases at least about 5 percent from a point adjacent the tip to
a point near the top of the shaft.
[0006] In additional aspects, the invention pertains to a medical
implantable screw comprising a headless shaft, a tool engagement
flange at top end of the shaft, a pointed tip comprising no more
than about 20 percent of the length of the screw, and threads
spiraling around the shaft. In some embodiments, the volume
displacement of the thread increases at least about 5 percent from
a point adjacent the tip relative to a point near the top of the
shaft and wherein the screw comprises one or more biocompatible
materials.
[0007] In other aspects, the invention pertains to a method for
inducing bone in-growth for joint fusion or bone repair. The method
comprises placing a blend of a bone powder alone or in combination
with a bioactive agent that induces bone growth into a joint or
bone fracture. In some embodiment, the bioactive agent induces bone
growth.
[0008] Furthermore, the invention pertains to a composition
comprising a blend of bone powder and a bioactive agent that
simulates bone growth.
[0009] In addition, the invention pertains to a method for
selecting an orthopedic implant. The method comprises placing a
sizing element into a prepared location for a bone screw and
tightening the sizing element with a torque wrench to evaluate the
size of appropriate bone screw for implantation into the site.
[0010] In further aspects, the invention pertains to an orthopedic
implant comprising a bone replacement materials and a bone growth
stimulating biologically active agent. The bone replacement
material is a bio-resorbable polymer, a natural or synthetic bone
composition or a combination thereof. The biologically active agent
generally is blended into the bone replacement material
composition
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a side view of an embodiment of an implant having
a head and a pointed tip.
[0012] FIG. 2 is a top view of the implant of FIG. 1.
[0013] FIG. 3 is a side view of an implant core having an
approximately constant core diameter over the length of the
implant.
[0014] FIG. 4 is a side view of a cannulated implant core having an
approximately constant core diameter in which the path of the core
channel is outlined with phantom lines.
[0015] FIG. 5 is a top view of the core of FIG. 4.
[0016] FIG. 6 is a side view of a hollow, fenestrated implant.
[0017] FIG. 7 is a side view of a headless implant with a tapered
core.
[0018] FIG. 8 is a top view of the implant of FIG. 7.
[0019] FIG. 9 is a side view of an implant with a generally
cylindrical core and tapered threads.
[0020] FIG. 10 is side view of a tapered implant with a changing
thread pitch.
[0021] FIG. 11 is a side view of a tapered implant with two sets of
co-axial threads.
[0022] FIG. 12 is a sectional view of the sacroiliac joint.
[0023] FIG. 13 is a side view of the sacroiliac joint with hidden
vertebrae and the sacroiliac joint shown in phantom lines.
[0024] FIG. 14 is a front view of a model of the sacroiliac joint
exposed from the tissue with a drill bit positioned to drill a bore
into the joint between the sacrum bone and the iliac bone.
[0025] FIG. 15 is a front view of a model of the sacroiliac joint
exposed from the tissue with a screw positioned for placement into
a bore drilled into the joint between the sacrum bone and the iliac
bone.
[0026] FIG. 16 is a side view depicting the sacroiliac joint with
the implant and filler material inserted into the joint between the
sacrum bone and iliac bone.
[0027] FIG. 17 is a sectional view through the sacroiliac joint
showing the implant placed into the joint.
DETAILED DESCRIPTION OF THE INVENTION
[0028] Improved fusion of the sacroiliac joint can be accomplished
with implants, e.g., screws, that are designed to distract the
joint with spreading forces as they are screwed into the joint. In
particular, the sacroiliac joint can be successfully fused through
the insertion of an implant directly into the gap of the joint
between the iliac bone and sacral bone with the implant gripping
bone on the respective sides of the joint. If the implant is
designed properly to increase distraction of the joint as it is
advanced into the joint, the implant can be selected to screw
tightly into the joint to stably fuse the joint. The placement of
the implant can be prepared through drilling, impacting or the
like. An implant can be selected that screws into the resulting
bore and tightens with a desired amount of tension. In some
embodiments, the threads of the improved screws have threads that
increase in volume displacement from the tip to the head to provide
greater spreading forces as the screw is inserted.
[0029] The sacroiliac joint has some properties that make the joint
challenging for fusion. Pioneering work has demonstrated that
fusion of the sacroiliac joint using implants and the like placed
into the joint between the sacrum bone and iliac bone can alleviate
chronic debilitating lower back pain where there is no other
identifiable source. While the sacroiliac joint does not move large
distances, wear, damage and/or concentration of forces to the joint
evidently can result in intense pain. Due to the complexity of the
joint, inaccessibility to the joint surfaces and the major nerve
and vascular structures passing adjacent the joint, the joint is
not presently amenable to joint replacement. Similarly, the joint
is not amenable to revision, realignment or reconstruction. The
joint is held together by a network of ligaments that hold the
joint together to resist distracting forces.
[0030] The sacroiliac joint generally can have significantly
differing bone characteristics across the joint. At points where
the load is carried, the bone can be hard while closely neighboring
bone can be relatively soft. Thus, a fusion procedure can encounter
significantly different bone on different occasions and even at
different locations across the placement of the implant during a
single procedure. While the procedures described herein comprise
identification of the boundaries of the joint, placement of the
implant within the joint generally is selected without information
on the character of the underlying bone since the locations of the
force load across the joint cannot be easily measured.
[0031] In some embodiments, design of improved screws for the
immobilization of the sacroiliac joint can provide desirable screw
configurations for general bone screws. In particular, the screws
can be selected to provide good gripping whether or not the bone
encountered is relatively hard or relatively soft. For implantation
into the sacroiliac joint, it is desirable for the bone to distract
the joint through the direction of spreading forces from the screw.
Thus, in some embodiments, the screw can be designed to have
characteristics that differ at different positions along the length
of the screw. For example, the distraction of the joint can
increase as the screw is implanted into the joint. In some
embodiments it is desirable to select the shape of the screw, e.g.,
the core and/or the threads, such that the screw grips more
strongly as the screw is advanced. Other portions of the procedure,
such as drilling and post-implantation processing of the joint, can
also be adapted for the immobilization procedure to take better
advantage of the procedure's objectives of distracting and
stabilizing the joint.
[0032] In general, bone screws can be used to stabilize a crushed
bone structure, attached fractured bone elements to each other,
attach an object, such as a ligament or tendon, to a bone and/or
immobilize a bone joint. A considerable amount of development has
been devoted to spinal screws to stabilized crushed or damaged
spinal disks. In spinal applications, the purpose of a spine cage
can be space filing. However, other applications generally can
involve gripping by the screw based on spreading forces extending
radially outward from the axis of the screw. A head on the screw
may or may not be desired for facilitating transfer of axial forces
into spreading forces.
[0033] In order for the screw to grip more tightly as the screw
advances, it can be desirable for the displacement of the screw to
increase as the screw is driven into the bone/joint. This increased
displacement can be accomplished through an increase in the core
diameter, an increase in the thread displacement or both. The
threads may or may not change along the length of the screw. For
example, the thread thickness can increase from the screw tip to
the screw head, the lateral extent of the threads can increase from
the tip to the head and/or the thread spacing can decrease from the
tip to the head. Similarly, there may or may not be a second set of
threads between a first set of threads. With a screw that provides
increasing displacement as the screw is inserted, the screw tightly
grips as it is inserted into the sacroiliac joint.
[0034] Screws have a general structure with a tip and a top at the
opposite end from the tip. The top may or may not include a head.
However, the top generally comprises a flange to engage a driving
tool. Threads spiral around a core of the implant, although threads
can be discontinuous in some embodiments without significantly
changing their function. The tip may or may not have cutting
flutes. The core may or may not be porous, fenestrated, hollow
and/or cannulated. In some embodiments, the character of the screw
changes along the length of the screw from the tip to the top. The
character may or may not change uniformly or monotonically, but on
average the implant can be designed to maintain or increase tension
upon insertion.
[0035] In some embodiments, the improved implant for the sacroiliac
joint is a tapered screw. Tapered screws for sacroiliac
immobilization/fusion are also described in the present inventor's
copending U.S. patent application Ser. No. 10/797,481, filed on
Mar. 10, 2004, entitled "Sacroiliac Joint Immobilization,"
incorporated herein by reference. In general, the tapered screws
can have tapered cores, tapered threads or both. While a taper can
increase the displacement of the implant from the tip toward the
head, other parameters can similarly increase the thread
displacement along the length of the implant. For example, threads
can have increased displacement through an increase in thickness
that correspondingly increases displacement resulting from
increased thread volume.
[0036] The improved procedures and tools described herein can be
used for either open or closed procedures. In open procedures, the
joint is surgically opened to visual observation of the joint. Once
the boundaries of the joint are identified, the location for the
implant can be drilled out, such as with a powered mechanical
reamer or drill. Care should be taken to stay within the joint to
avoid contacting any blood vessels or nerves, of which there are
significant members that pass close to the sacroiliac joint. The
size of the reamer or drill bit can be selected based on the size
of the individual or other suitable parameters based on an
examination of the patient.
[0037] An implant is then selected to distract and fuse the joint.
As noted above, implants of particular interest have structures
that vary along the length of the implant to maintain or increase
tension as the implant is inserted into the bore that is formed by
the drilling. The implant is screwed into the joint to provide the
desired amount of distraction forces at its full placement. The
selection of the screw can comprise the use of a sizing element,
which can be used with a wrench, such as a torque wrench, to select
a correctly sized and characterized implant to screw into the joint
within a desired range of torque parameters.
[0038] In some embodiments, a sizing system can comprise a set of
exchangeable sizing elements that have an outer shape approximating
the outer shape of the implants. The set of sizing elements can
include, for example, an element corresponding to each size of
available implants. The sizing elements can be made from a material
that can be easily sterilized, such as stainless steel or other
metal, so that the sizing elements can be sterilized and reused. In
some embodiments, a torque wrench can provide a display output of
the applied torque for monitoring by the health care professional
or the wrench can be a torque limiting wrench that limits the
amount of torque that can be applied so that the applied torque
does not exceed an upper limit. Suitable torque limits for implant
placement can be in some embodiments from about 0.5 Newton-meters
to about 12 Newton-meters and in other embodiments form about 0.75
Newton-meters to about 8 Newton-meters. Torque limiting wrenches
are described further in U.S. Pat. No. 6,162,053 to Hollander,
entitled "Analog Dental Wrench," and U.S. Pat. No. 6,807,885 to
Loper, entitled "Torque Limiting Wrench for an Ultrasonic Medical
Device," both of which are incorporated herein by reference.
[0039] The sizing elements can be inserted into the prepared joint
at a desired amount of torque. If the sizing element holds at the
selected torque, the correspondingly sized implant can be used
following removal of the sizing element. In this way, the health
professional can avoid the accidental selection of an implant that
is too small. If an implant is selected that is too small, the
implant may strip out when it is being implanted, which would then
require the subsequent use of a larger implant. Since the implants
generally cannot be reused, the initially selected implant that was
too small can be wasted. Thus, the use of the sizing system can
result in the waste of fewer implants since the correct size
implant can be selected more accurately.
[0040] After completing the insertion of the implant, bone
materials can be placed into the joint to promote bone in growth
that further supports immobilization. In particular, crushed bone
material, demineralized bone matter, synthetic bone material or
compositions or corresponding putties can be placed into the joint.
Additionally or alternatively, bone morphogenic protein or other
similar bone growth stimulating compositions can be placed into the
joint to stimulate bone growth to immobilize the joint. In
particular, it can be desirable to blend powdered, natural or
synthetic bone material with a bone growth composition, such as
bone morphogenic protein, for placement into the joint or other
bone fracture to stimulate bone growth with the bone material as a
foundation.
[0041] In the closed or less invasive procedures, a cannula can be
placed into the joint as guided by a pin or the like. The position
of a pin to guide the implant can be placed once other pins have
been used to identify the boundaries of the joint. Various imaging
approaches can be used to guide the process of finding the
boundaries of the joint without opening the joint up for visual
inspection. The cannula exposes a small opening to the joint at the
selected location of the implant placement. The cannula can then
guide the drilling and/or implant insertion steps in a procedure
that is less disruptive to the patient.
[0042] In general, approaches to the sacroiliac joint lack easily
dissectible tissue planes that provide dissection between muscles
or between nerves. Large adjacent nerves, such as the sciatic
nerves, and arteries, such as the iliac vessels, limit the approach
to the joint and preclude certain access. The nearby vessels and
nerves also greatly magnify the risk of error. Similarly, the close
location of the spine also provides limits to access as well as
further contributing to the risks.
[0043] The sacroiliac joint bone anatomy has a structure
specifically for the non-intuitive passage of force through the
joint with the weight of the torso transmitted to the hips. This
transfer of forces generally takes place whether seated or
standing. The bone mass associated with the sacroiliac joint is
limited and is not amenable to build up or reconstitution. Thus,
procedures are generally designed to avoid sacrificing unnecessary
amounts of bone support. In particular, drilling or impacting a
bore within the joint should provide appropriate gripping of the
implant without excessively weakening of the surrounding bone.
[0044] The bones around the joint exhibit complex coupled motions
of angulation, rotation and squirm. There is often little or no
significant sliding or translation of surfaces over each other.
Articular cartilage on one surface rests against fibrocartilage on
the other surface, which is unique in the body. In contrast, there
are closely coupled combinations with small degrees of relative
motion of generally less than one degree in rotation or one
millimeter in translation. Periarticular ligamentous structure
combined with distant encompassing structure controls the joint. At
the same time, most muscles cross the joint in a complex way. Most
muscles cross several joints and/or disk spaces and transfer forces
and balance forces between the plurality of joints at the same
time. Also, many muscles that technically do not cross the joint
help to control the joint.
[0045] Pain can result from the joint due to one or more
pathologies. Congenital defects, such as smallness or malformation,
can result in pain. Acquired laxity of the joint can result from
pregnancy or due to congenital conditions. Trauma, such as falls,
can result from direct incongruities that are secondary to pelvic
or sacrial fractures or from ligamentous disruption. Furthermore,
inflammatory syndromes, such as septic arthritis or rheumatic
conditions can similarly result in pain in the sacroiliac joint.
Also, age results in an increasingly irregular topography of each
side of the joint with progressive changes in the surface
topography distributing unusual forces across the joint. Following
performance of the procedure, the fused joint is fixed in a manner
suitable to resist weight bearing stresses.
[0046] The procedures described herein are selected to improve the
uniformity of results from sacroiliac fusion procedures. Similarly,
the procedures generally can be used with more consistent results
by a medical professional with less training and experience than
procedures using less well designed procedures. The implants are
more specifically designed for use in the sacroiliac joint or for
other orthopedic applications that encounter bone structures of
varying type during the procedure. These implants yield more
reproducible results relative to other implants, such as spinal
implants, that are designed for other types of application, which
can be inserted into the sacroiliac joint in a fusion
procedure.
[0047] As noted above, the procedures and implants described here
can be used advantageously for other orthopedic applications in
which different types of bone structures can be encountered, such
as a hard outer bone and a soft inner bone. For example, the
implants can be used for the reattachment of severed ligaments
where the implant is directed into a bone structure to perform the
reattachment. In these applications, it can be desirable to have an
implant with changing structure along the length of the
implant.
Screw Structures and Materials
[0048] Screws of particular interest have changing parameters from
the tip to the head. These changing parameters can be selected to
result in the maintenance or increase of tension upon insertion
into properly prepared boned or joints. In some embodiments, the
screws are tapered. Various thread designs are appropriate to form
desired screws that have desired retention while providing desired
amounts of dissection with spreading forces, which are desirable
for insertion into, for example, a sacroiliac joint.
[0049] Referring to FIG. 1, an embodiment of an orthopedic screw
100 is shown schematically. Screw 100 comprises head 102, core 104,
threads 106 and tip 108. Head 102 is optional. Head 102 has its
conventional meaning as a terminal element having a lateral extent
larger than the core adjacent the head. Generally, head 102
comprises a driving tool engagement flange 110, as shown in FIG. 2.
Tool engagement flange 1 10 can have any reasonable shape to engage
a driving tool, such as a straight channel, a cross channel, a
hexagonal depression or appropriate extensions. As described
further below, a tool engagement flange can be located along the
top surface of the core for embodiments lacking a head. In some
embodiments, the tool engagement flange can extend significantly
into the core of the screw to provide an extended interface for
engaging a drive tool.
[0050] Core 104 connects head 102 with tip 108 and supports threads
106. The core is the portion of the body of the screw without the
threads. As shown in FIG. 1, core 104 is tapered. An embodiment of
a constant diameter core 116 is shown in FIG. 3. Core 104 can be
tapered independent of the overall screw. In particular, if the
core is generally cylindrical, the threads can have changing
lateral extent along the length of the screw to result in a tapered
screw. Similarly, if the core is tapered, the lateral extent of the
threads can be constant to lead to the same taper for the screw,
the lateral extent of the threads can change along the length of
the screw to contribute further to the taper of the screw or the
threads can change along the length of the screw to decrease or
eliminate the overall taper of the screw, as shown, for example, in
FIG. 1.
[0051] Core 104 can be, for example, solid, hollow or cannulated.
In the cannulated embodiments, a channel 118 ends through the
entire length of core 120 such that the screw can be delivered over
a pin or the like, as shown in FIGS. 4 and 5. Channel 118 would
similarly extend through a head or tip if present. Hollow cores can
also be fenestrated to provide for bone in growth into the hollow
core. For example, referring to FIG. 6, core 124 has openings 126
into the hollow interior of core 124. The top of a hollow core
screw can be reversably openable, such as being threaded, such that
bone material or other similar materials, can be inserted to
promote bone in growth. Fenestrated spinal fusion cages that can be
filed with bone material is described, for example, in U.S. Pat.
No. 4,961,740 to Ray et al., entitled "V-Thread Fusion Cage and
Method of Fusing a Bone Joint," and U.S. Pat. No. 5,669,909 to
Zdeblick et al., entitled "Interbody Fusion Device and Method for
Restoration of Normal Spinal Anatomy," both of which are
incorporated herein by reference.
[0052] In general, threads 106 can have a variety of
characteristics. In some embodiments, the threads have changing
characteristics along the length of the implant. As shown in FIG.
1, threads 106 have a smaller lateral extent from the tip to the
head, such that the edge of the threads form a generally
cylindrical outer surface of the screw even though core 104 has a
diameter that increases toward the head. Some of the thread
characteristics of particular interest are described in detail
below with respect to some specific embodiments.
[0053] Referring to FIG. 1, tip 108 generally can be identified by
an abrupt change in structure at a boundary 130. Tip 108 can have a
point 132. Also, tip 108 can comprise cutting flutes 134 or the
like to facilitate insertion of the screw. If it is ambiguous
whether or not a particular implant has a separate tip, the 15
percent of the length of the implant away from the head/top can be
considered the tip adjacent the threaded core. Tip 108 may or may
not be threaded.
[0054] For implantation into a sacroiliac joint between the sacrum
bone and iliac bone, it can be desirable to use an implant that is
headless such that there is no head sticking up from the joint
after the procedure that can cause irritation to the patient.
However, headless screws do not transfer axial forces to spreading
forces when the top of the screw reaches the surface of bone. Thus,
the screw should be designed to provide spreading forces to
distract the joint without the need for a head to contact the bone
surface.
[0055] An embodiment of a headless screw is shown in FIG. 7. Screw
140 comprises core 142, threads 144 and drive flange 146, shown in
FIG. 8. Drive flange 146 is located along top surface 148 at the
top of core 142. In this embodiment, core 142 is tapered such that
the core has a smaller diameter near the tip of the screw and a
larger diameter near the top of the screw adjacent the drive
flange. Threads 144 spiral around core 142 and have a roughly
symmetrical shape relative to the top surface of the thread and the
bottom surface of the thread. Also, threads 144 have a roughly
constant lateral extent from the core. The lateral extent "L", as
shown in FIG. 7, is the distance from the core to the edge of the
thread. Thus, the diameter of the threads in screw 140 increases
from the tip to the head of the screw due to the taper of the core
and not due to a change in the lateral extent of the threads. In
alternative embodiments, the core can be tapered over a portion of
the length of the screw with a constant or counter tapered shape
over other portions of the screw while providing desired degrees of
joint distraction.
[0056] Another embodiment of a tapered bone screw is shown in FIG.
9. In this embodiment, screw 150 comprises a core 152 and threads
154. The top 156 of core 152 has a drive flange, which can be, for
example, the same as shown in FIG. 8. In this embodiment, core 152
has an approximately constant diameter along the length of the
screw extending along the axis of the screw. However, the lateral
extent of the threads increases from the tip to the top of the
screw. Thus, the diameter of the threads and the overall screw
diameter correspondingly increase as a result of the increased
thread lateral extent even though the core has a constant diameter.
Threads 154 have an asymmetric shape with a flatter top surface
relative to a more angled lower surface. Asymmetric threads are
discussed in more detail below. In alternative embodiments, the
core is tapered and the lateral extent of the threads changes such
that there are two contributions to the overall taper of the
screw.
[0057] Another embodiment of a tapered bone screw is shown in FIG.
10. Screw 160 is also a headless screw comprising a core 162 and
threads 164. Top surface 166 of core 162 has a driver flange, which
can be, for example, the same as shown in FIG. 8. Core 162 has a
taper in this embodiment, although in other embodiments the core
can have a constant diameter. In this embodiment, threads 164 have
two segments 166, 168 with different pitch. In particular, threads
168 have adjacent threads closer to each other than threads 166. In
this embodiment, the thread pitch change is relatively abrupt,
although in other embodiments the thread pitch can change
gradually. Similarly, the other embodiments, the threads can have
three or more sections with different pitch from each other.
[0058] An embodiment of a tapered bone screw 180 is shown in FIG.
11 in which the screw has two sets of interwoven, coaxial threads.
Specifically, screw 180 comprises a core 182, first threads 184 and
second threads 186. Second threads 186 spiral around the core
between the threads of first threads 184. The properties of the two
sets of threads can be selected as desired. In general, two sets of
threads can be combined with other screw features described herein,
such as headed or headless screws, tapered cores or cylindrical
cores, etc.
[0059] In general, for appropriate embodiments the screws should
have appropriate dimensions for insertion into the sacroiliac joint
between the sacrum bone and iliac bone. The screw should provide
desired immobilization of the joint without damaging the
surrounding bone. For embodiments with a screw head the head can
have any reasonable dimension based on the overall screw
dimensions.
[0060] For insertion into the sacroiliac joint, the length should
be selected such that the implanted screw, except possibly for an
optional screw head should be contained with in the joint. For a
typical adult human, the screws then can have a length from about
10 millimeters (mm) to about 45 mm, and in other embodiments from
about 15 mm to about 35 mm. For other orthopedic applications,
suitable lengths of the screw generally range from about 10 mm to
about 80 mm. A person of ordinary skill in the art will recognize
that other ranges of screw lengths within the explicit ranges above
are contemplated and are within the scope of the present
disclosure.
[0061] Similarly, for insertions within a sacroiliac joint, the
screw should have a diameter that is consistent with distracting
the bone without being too large such that the bone is damaged
through its insertion. For a typical adult human, appropriate screw
would have an average diameter from about 6 mm to about 28 mm, and
in other embodiments from about 8 mm to about 25 mm. A person of
ordinary skill in the art will recognize that additional ranges of
diameters within the explicit ranges above are contemplated and are
within the scope of the present disclosure. If the screw is
tapered, the leading diameter and trailing diameter correspond with
the amount of taper and the average diameter.
[0062] In general, the screw taper can be determined as the angle
of the screw edge relative to the screw axis. In general, the taper
can be at least about 1 degree, in further embodiments at least
about 2 degrees, in other embodiments from about 3 degrees to about
12 degrees, and in additional embodiments from about 3.5 degrees to
about 10 degrees. As noted above, the taper can result from a core
taper, a variation in the threads of the lateral extent or both.
The range of core tapers can generally range over the same ranges
of angles given above for the overall taper of the screw. Of
course, a taper does not need to be linear, so that the surface of
the screw can be curved. For curved tapers, the angle can be
estimated from threads adjacent the tip and the top to provide a
reasonable estimate of the angle of the overall taper.
[0063] In general, the threads can have several parameters to
characterize the nature of the threads. For example, the thickness
of the thread can be evaluated at the point half way from the edge
of the thread to the core. This thickness "T" is noted in FIG. 7
for reference. The thickness can be related to the sharpness of the
thread. The edge of the thread can be sharp, but generally it is
desirable for the edge of the thread to be rounded. As shown in
FIG. 7, the threads meet the core along smooth curves. Smooth
transitions at the edges of the elements avoid the undesirable
concentration of forces along the bone that can result in a poor
interface between the bone and screw. The cross sectional shape of
the threads can be approximately symmetrical or in other
embodiments asymmetrical relative to the top and the bottom of the
threads.
[0064] The top of the thread is oriented toward the top of the
screw while the bottom of the thread is oriented toward the tip of
the screw. The leading and trailing edges generally have different
functions so that it may be desirable for these surfaces to have
different shapes from each other. For example, the top thread
surface may push outward on the bone while the lower surface may
cup or compress the bone longitudinally as it pulls the screw
deeper.
[0065] The thread dimensions can be selected to achieve the desired
distraction and gripping properties of the screw upon implantation.
The average lateral extent of the screw can be 0.25 to 2.5
millimeters and in other embodiments from about 0.4 to about 2.0
millimeters. In some embodiments, the lateral extent of the thread
tapers along the length of the screw with a greater lateral extent
of the thread near toward the top of the screw, such as shown in
FIG. 9. For these embodiments, the lateral extent of the tapered
threads can be at least about 0.1 mm and in further embodiments at
least about 0.25 for the smaller threads, and the larger lateral
extent of the threads of the screw can be no more than about 3.5 mm
and in further embodiments no more than about 3 mm. In some
embodiments, the variation in the lateral extent of the threads is
monotonic along the length of the screw optionally excluding the
last turn of the threads at the top and/or at the tip.
[0066] The average pitch of the threads can be generally from about
1 mm per turn to about 4 mm per turn, and in further embodiments
from about 1.5 mm per turn to about 3.5 mm per turn. As noted
above, the pitch of the threads can be constant over the length of
the screw or the pitch of the threads can change over the length of
the screw or a portion thereof. In general, it is desirable for the
threads to have smooth surfaces at the edge of the threads as well
as at the meeting of the threads with the core. Thus, while the
threads can be sharp, smooth surfaces of the threads can avoid
discontinuities that can result in undesirable concentration of
forces. However, in some embodiments, discontinuous thread surfaces
can result in a desirable concentration of forces.
[0067] In general, the implants/screws can be formed from any
suitable biocompatible material, which is non-toxic. The material
can be biologically effectively inert or can impart specific
desired biological effects, such as through the elution of bone
morphogenic protein. Suitable biocompatible materials can include,
for example, metals, such as stainless steel, tantalum and
titanium, rigid polymers, such as polycarbonates and
polyetheretherketone (PEEK), ceramics, such as alumina, or
composites, such as carbon composites or carbon fiber composites.
In some embodiments, the screws can comprise a bioresorbable
polymer, such as poly(hydroxyacids), poly(epsilon-caprolactone),
polylactic acid, polyglycolic acid, poly(dimethyl glycolic acid),
copolymers thereof and mixtures thereof. The screws can be formed,
for example, using conventional machining, molding or the like. The
screw or its surface can be porous. For example, porous tantalum is
commercially available for forming the screw. In addition,
synthetic bone materials and/or sterile bone materials, either
allograft or xenograft materials, can be used to form the
implantation elements. Suitable synthetic bone material includes,
for example, coral and calcium compositions, such as
hydroxyapatite, calcium phosphate and calcium sulfate.
[0068] In some embodiments, the implant can be formed from a
bio-resorbable polymer a natural or synthetic bone material or a
combination thereof and a bioactive agent that stimulates bone
development, such as BMP. The BMP can be blended with the material
prior to molding, casting or otherwise formed into the implant or
portion thereof. Generally, if a portion of the implant is formed
from the BMP blended with bioresorbably polymer or bone material,
this portion can be a support portion, i.e., a portion that
provides mechanical integrity to the implant. In appropriate
embodiments, as the resorbable polymer biodegrades, bone replaces
the implant material. Similarly, for implants formed from the bone
material, the implant becomes incorporated into the new bone that
forms as a result of the bioactive agent.
[0069] Optionally, a bioactive agent can be coated on the surface
of the immobilization element. To coat the immobilization device
with the bioactive agent, the device can be dipped in a composition
comprising the bioactive agent, sprayed with a composition
comprising the bioactive agent, painted with the bioactive agent,
and/or coated with other processes, such as those generally known
in the art. If the coating composition comprises a solvent, the
solvent can be allowed to evaporate after applying the coating
composition. The bioactive agent can be applied alone as a coating
composition or with another agent to control the elution of the
agent. The agent can be applied from a solution with a solvent that
can evaporate following the application of the coating solution.
Also, the bioactive agent can be combined with a control release
agent, such as a biodegradable polymer that gradually releases the
bioactive agent as the polymer degrades within the patient.
Biocompatible, biodegradable polymers are known in the art, such as
polylactic acid, poly(glycolic acid) and copolymers and mixtures
thereof. A binder may or may not be included to control the elution
from the coating. Furthermore, the bioactive agent can be injected
or otherwise delivered in the vicinity of the immobilization
device. The bioactive agent can be combined with a suitable
biocompatible carrier, such as commercially available buffered
saline or glycerol.
[0070] Suitable biologically active agents include, for example,
bone morphogenic protein (BMP) and cytokines. BMP mediates the
formation and healing of bone, cartilage, tendon and other bone
related tissues. One human BMP polypeptide is described in detail
in Published U.S. Patent Application Serial Number 2003/032098 to
Young et al., entitled "Bone Morphogenic Protein," incorporated
herein by reference. Suitable cytokines include, for example, human
chemokine alpha 2, which is effective to stimulate bone marrow
growth. A human cytokine, human chemokine alpha 2, is described in
U.S. Pat. No. 6,479,633 to Ni et al., entitled "Chemokine Alpha 2,"
incorporated herein by reference.
Sacroiliac Joint Fusion Procedure with Distracting Screws
[0071] The sacroiliac joint can be successfully fused using a
distracting implant that is inserted into the joint between the
sacrum bone and the iliac bone with a screw design that maintains
or increases tension as the screw is tightened. In general, the
joint is prepared by selecting the location of the implant within
the joint and exposing the area either through an open procedure or
through a cannula for a less invasive procedure. The location
selected for the implant can be drilled to prepare the joint for
the implant. Following implantation of the screw, filler material
can be placed within the joint to further stabilize the joint and
promote bone in growth. Additionally or alternatively, bioactive
compositions can be used to stimulate bone in growth.
[0072] The appropriate approach to the sacroiliac joint for the
insertion of an implant is from the patient's back, at or just
above the buttocks and slightly displaced form the patient's center
line running along their spine. The approach is angled outward back
to front. The two openings into the joint are displaced with one
accessible from the left and the other from the right. Generally,
the health care professional selects one side or the other for
immobilization based on an examination of the patient, although in
some embodiments, at least one implant is placed on each side of
the patient.
[0073] The selected side of the joint can be accessed with an open
procedure or through a less invasive procedure. In the open
procedure, a significant incision is made to expose the joint, and
the exposed area is cleared out for desired exposure to the joint.
In contrast, for the less invasive procedure, a cannula can be used
to form an opening to the joint. Pins can be used to locate the
limits of the joint as well as a location for the implant
placement. Visualization techniques, such as x-ray visualization,
can be used to facilitate pin placement. Drilling and implant
placement can be performed through the cannula. Closed procedures
for immobilization of the sacroiliac joint through placement of an
implant into the joint is described further in Applicant's
copending U.S. patent application Ser. No. 10/797,481 filed on Mar.
10, 2004, entitled "Sacroiliac Joint Immobilization," incorporated
herein by reference.
[0074] Referring to FIG. 12, portion of the sacroiliac joint is
shown. As noted above, the sacroiliac joint 200 is located between
the sacrum 202 at the base of the spine and the ilium 204, the
upper bone of the pelvis. As shown in FIG. 12, various ligaments
206 support the joint. Referring to FIG. 13, walking and other
movement apply torque on the sacroiliac joint 200. As shown in FIG.
13, sacroiliac joint 200 is shown with phantom lines between the
spine 208 and the pelvis 110. This torque on the sacroiliac joint
can result in pain if there is injury or disease.
[0075] Once the selected section of the joint is exposed with an
open or a less invasive approach, the site can be prepared using a
drill or reamer or impactor. Commercial orthopedic drills are
available with a selectable range of drill bits. A powered impactor
that can be adapted for the insertion of a sizing element is
discussed in U.S. Pat. No. 7,001,393 to Schwenke et al., entitled
"Servo-Controlled Impacting Device For Orthopedic Implants,"
incorporated herein by reference. In addition, impaction of the
joint can be performed using a manual or power drill operated with
the drill bit rotating backwards so that bone is not removed, but
the joint is distracted through impaction by the rotating drill bit
in preparation for the placement of an implant. Referring to FIG.
14, a site 220 is identified within the sacroiliac joint 222
between the sacrum bone 224 and the iliac bone 226. A drill bit 228
is shown in FIG. 14 positioned above the drill site 220. During the
drilling process, care should be taken not to drill past the joint
to avoid injuring any blood vessels or ligaments. The hole can be
drilled to have a smaller dimension than the average dimension of
the screw so that the screw firmly anchors in place. If the screw
has a pointed tip, the drilled bore can have a diameter
approximately equal to the smaller diameter of threads along a
tapered core.
[0076] Referring to FIG. 15, once bore 230 is drilled into the
joint area between sacrum bone 224 and ilium bone 226, a screw 232
can be screwed into bore 230. As described herein, in some
embodiments, screw 232 can be selected to distract the joint during
placement. The distraction involves the slight movement away of
sacrum bone 224 from ilium bone 226 while stabilizing the joint. As
shown FIGS. 14 and 15, an implant is inserted into the left
sacroiliac joint. In other embodiments, an implant is inserted into
right sacroiliac joint 234, or separate implants are placed into
both the left and right sacroiliac joints. The sacroiliac joint
with the representative implant is shown in FIGS. 16 and 17 along
with additional stabilizing material 240.
[0077] Once the screw/implant is inserted into the joint,
additional stabilizing material can be placed into the joint. This
stabilizing material can be selected to promote bone growth into
the joint to further contribute to bone immobilization. Suitable
materials include, for example, synthetic bone materials and/or
sterile bone materials, either allograft or xenograft materials.
Suitable synthetic bone material includes, for example, coral and
calcium compositions, such as hydroxyapatite, calcium phosphate and
calcium sulfate. The bone material can be placed into the joint as
a powder. Suitable material includes, for example, demineralized
bone powder, which is commercially available. Gels and putty of
demineralized bone powder suspended in glycerol are also
commercially available.
[0078] Suitable biologically active agents include, for example,
bone morphogenic protein (BMP) and suitable cytokines. BMP is
involved in formation and healing of bone related tissue, including
bone, cartilage and tendon. Suitable cytokines include, for
example, human chemokine alpha 2, which is effective to stimulate
bone marrow growth. Furthermore, the bioactive agent can be
injected or otherwise delivered in the vicinity of the
immobilization device.
[0079] The biologically active agents can be coated onto an implant
for delivery. In some embodiments, it has been found that desirable
results are obtained through a blend of biologically active agent,
such as BMP, and bone powder, such as demineralized bone powder or
crushed bone material, although synthetic materials can be used
similarly. The material can be blended and then deposited into the
joint, or the materials can be layered into the joint.
[0080] The embodiments above are intended to be illustrative and
not limiting. Additional embodiments are within the claims.
Although the present invention has been described with reference to
particular embodiments, workers skilled in the art will recognize
that changes may be made in form and detail without departing from
the spirit and scope of the invention. The incorporations by
reference above are intended to incorporate the full disclosures of
the references to the extent that the incorporated subject matter
is not inconsistent with the explicit disclosure herein, which will
not be altered by any incorporation by reference, as well as to
incorporate the disclosures with respect to the specific issues
referenced in the incorporation.
* * * * *