Note: Descriptions are shown in the official language in which they were submitted.
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THREADED WASHER
This is a divisional application of Canadian Patent
Application Serial No. 2,242,479 filed on December 3, 1996.
It should be understood that the expre~~sion '°the invention"
and the like encompasses the subject matter of both the
parent and the divisional application.
This invention concerns a threaded washer and an apparatus
for the fixation of small bone fractures with such a
threaded washer.
The invention relates generally to the use of small screws
in the reduction and compression of fractures and non-
unions where the bones or bone fragments are small. These
screws must provide sufficient stability and compression
along the fracture line to promote healing. For intra-
articular applications, the screw head must not protrude
above the bone surface in order to avoid interference with
joint movement and with surrounding tissue. The invention
specifically relates to the use of a vaasher with external
tapered threads to position the head of the small screw
securely in the layer of cancellous material below the
surface of the bone.
In orthopaedics, successful treatment of nonunion and
fracture of small bone fragments often involves the use of
small screws. These screws are installed through both
fragments in a direction substantially normal to the
fracture plane. The small screw must compress the
fragments sufficiently to promote healing.
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Fractures resulting in small bone fragments often occur in bones
adjoining infra-articular regions, such as the scaphoid, other
carpal bones, ends of long bones, and bones of the spine. It is
important in these applications that the head of the bone screw
does not grotrude from the outer surface of the bone, where the
screw head could interfere with joint movement or damage
surrounding tissue during articulation.
Standard small bone screws having threads only on the leading
portion of the shank have peen tried for this indication_ These
screws are threaded into pre-drilled holes until the head
contacts the outer fragment. The screw is then turned until a
force is generated coanpressing the fragments between the screw
head and the threads, which engage the inner fragment. In
intra-articular applications, the pre-drilled hole must be
countersunk to grevent the head of the screw from protruding
from the bone surface. The depth of this counterbore often
exceeds the thickness of the.relatively thin bone cortex and
penetrates to the softer cancellous bone below. When tightened,
the small screw head can sink further into the cancellous bone,
providing insufficient compressive forces for proper union of
the fragments. .
Another approach to this problem is disclosed in U.S. Fatent No.
4,175,555 to Herbert, which shows a small, headless screw
utilizing threads of different pitch at the leading and trailing
ends. The .differential in pitch draws the bone fragments
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together as the screw is tightened. Be=cause the screw head is
replaced by the threads on the trailing sand, the screw does not
protrude from the bone and does not require countersinking. The
threads on the trailing portion of i:.he screw provide some
purchase in the cancellous bone material. These screws,
however, provide insufficient compres:~ion in many clinical
applications. Furthermore, a surgeon installing the screw must
exercise care in starting each set of threads into the bone so
that proper fracture reduction and compression occur
simultaneously with the screw. head reaching the proper depth.
Otherwise, optimum fracture compression may occur too early,
with the trailing end protruding from the bone, or too late,
with the trailing screw threads engaging part of the inner
fragment.
U.S. Patent No. Re. 33,34 to bower di:>closes a hip screw with
an unthreaded shaft section and a leadir.'g threaded portion. A
threaded sleeve member is retained on and slidable along the
unthreaded shaft portion. The threads of the sleeve are
designed to engage the relatively th3.ck bone cortex of the
femur. The threads on the leading portion and on the sleeve
have different pitches. The assembled screw is installed as a
unit, with compressive.forces being generated as a result of the
pitch differential as in Herbert '555. The two-piece design
permits the unthreaded shaft section of the screw to back out
through the sleeve member when bone absorption occurs, solving
the problem of prior hip screws being forced in the opposite
direction, into the hip joint capsule.
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Complemental sheet to the description 12.11.7
The invention disclosed in FR-A 2 674 119 to Fixano concerns a
device that serves to guide and center the gliding of the screw
of osteosynthesis in case of intra-capsulary fractures of the
femoral collar. The device consists of a cylinder inserted around
the distal cylindrical part of a screw without head. The cylinder
is fastened by means of a screwable connection into the orifice
of~the bone. The screwable connection is realized by means of a
thread on the external surface of th.e distal section of the
cylinder whereby the thread has an increasing height in the
direction proximo-distal.
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A method frequently used for the installation of small bone
screws is to first install a guide wire in the bone in the
location and orientation decided upon for the screw_ The wire
passes through the surrounding soft tissue, providing a guide
for tools, such as screwdrivers, reamers and drills, and a guide
for the orthopaedic implants, such as screws and washers. 'I'ools
and implants used in such a procedure are cannulated, i.e.
provided with a central bore through the long axis for placement
over the wire.
It is an object of the present invention to provide a threaded
washer for countersinking a small bone screw below the surface
of an outer bone fragment without compi:omising the compressive
forces that the bone screw can exert between the inner and outer
fragments.
It is a further object to provide a~ threaded washer that
provides sufficient purchase in cancellous bone to support the
head of a small bone screw.
It is a further object of the invention to provide a threaded
washer to support a small bone screw, wherein the washer three_-"_
is tapered to increase the purchase in cancellous bone and to '
prevent the washer from advancing while inserting the screw.
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It is a further object of the invention to provide a method
for immobilizing and compressing bone fragments using a
threaded washer that is countersunk into the cancellous
material of the outer fragment to support the head of a
small bone screw beneath the surface of the fragment in an
intra-articular region.
In accordance with the invention, the noted drawbacks of
existing means of installing small bone Screws are overcome
by use of a threaded washer (I) for use with a bone screw
(60) in the reduction and fixation of bone fragments,
comprising a cylindrical body (2) havizag a leading edge (4)
and a trailing edge (3), a central bore (5} for insertion
of a bone screw (60), an external thrE:ad (9} increasing in
diameter toward the trailing edge (3), and driving means
(8) for inserting the threaded washer (1) in a bone. The
external thread (9) is self-tapping by means of cutting
flutes (14) and the central bore (5} is larger than the
major diameter (62} of the bone screw threads.
More specifically, the present invention provides an
apparatus for the fixation of small bone fractures,
comprising a washer with a central bore, the central bore
having a diameter, and an external tapered thread for
engaging an outer bone fragment, and a bone screw having a
shaft with a thread with a major dia.mete~ less than the
diameter of said central bore for engaging an inner bone
fragment, and having a screw head lar<~er than the central
bore, wherein an upper portion near said screw head of the
shaft is disposed within the central bore, and the screw
head is disposed exterior the washer and engages a portion
of the washer.
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5a
The present invention also provides an apparatus for the
fixation of small bone fractures, comprising a washer, the
washer having a first pitch, a central bore, the central
bore having a diameter, an external tapered thread for
engaging an outer bone fragment, and driving means for
inserting the washer in the outer bone fragment, and, a bone
screw disposed within the central bore of the washer, the
bone screw having a second pitch, a thread with a major
diameter less than the diameter of the central bore for
engaging an inner bone fragment, and a screw head larger
than the central bore for engaging the washer.
The present invention also provides a bone implant kit for
the fixation of small bone fractures comprising a washer,
with a central bore having a diameter and an external
tapered thread, for engaging an outer bone fragment, and a
bone screw, with a threaded shaft having a diameter less
than the diameter of the central bore, and a screw head
with a diameter larger than said diameter of said central
bore, for engaging an inner bone fragment, wherein the
washer is adapted to receive the bone scrrew.
In one aspect of the invention, the threaded washer is used
with a cannulated screw. A guide wire is first inserted
into the bone at the location selected for the screw. A
cannulated, reamer is then used to form a hole sufficiently
deep to accommodate the threaded washer below the surface
of the bone. The threaded washer is then inserted using a
cannulated driver, and driven into the cancellous bone
material until the tapered thread is fully engaged in the
hole. The cannulated screw is then installed through the
central bore of the threaded washer, and driven using a
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5b
cannulated driver until the head of the screw contacts the
threaded washer. Further turning of the cannulated screw
has no effect on the position of the tapered washer, but
serves to reduce the fracture and apply compressive forces
across the fracture plane.
The advantages of the threaded washer according to the
invention reside partly in its universal applicability,
since it can be used together 'with standard cannulated
screws.
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The threaded washer according to the inventian can be used in
clinical applications where the bone screw must be inserted into
the bone through a wall adjoining an intra-articular region, as
as typically the case in scaphaid fractures. In such clinical
applications, it is important to countersink the head of the
screw into the bone to avoid interference with joint
articulation. This must be done without compramising the
compressive forces applied across the fracture plane. With its
improved purchase in the cancellous material surrounding the
counterbore, the threaded washer of the invention provides this
added compressive strength while still allowing the use of
miniature screws with small screw heads.
Preferably the threaded washer and the lbone screw have the same
pitch. An advantage of the threaded washer over the use of
existing screws having a pitch differential between the leading
and trailing threads is that fracture reduction is controlled
independently of the depth of the screw head beneath the bone
surface_ In the present invention, once the head of the bone
screw contacts the threaded washer, all further rotation of the
screw reduces the fracture or applies compressive forces across
the fracture plane, without changing the' depth of the screw head
beneath the bone surface.
The invention will be disclosed more fully in connection with
the accompanying drawings in which:
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FIG. 1 is a side elevation view in partial cross section of a
threaded washer according to the invention;
FIG. 2 is a top view of the threaded washer of FIG. 1;
FIG. 3 is a longitudinal section through a pardon the threads
of the threaded washer;
FIG. 4 is a side elevation sectional v:i.ew of a threaded washer
according to the invention with a cannulated screw shaven in the
installed position;
FIG. S is a cross-sectional view of a fractured bone after
inserting a guide wire according to the invention;
FIG. 6 is a cross-sectional view of <~ fractured bone after
reaming a hole for the threaded waaher according to the
invention;
FIG. 7 is a cross-sectional view of a.fractured bone after
inserting a threaded washer according to the invention; and
FIG. 8 is a cross-sectional view of a fractured bone after
installing and tightening a cannulated screw according to the
invention.
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FIGS. 1 and 2 show a threaded washer 1 according to the
invention. As best seen in FIG. 18 the threaded washer ~.
comprises a hollow cylindrical body 2 with a trailing edge 3 and
a leading edge 4. A central unthreaded bore 5 extends along a
central axis s of the cylindrical body ~.
In the embodiment shown in FIGS. I & 2, a hexagonal socket 8 is
provided in the central bore 5 at t:he trailing edge 3 for
driving the threaded washer with a hexagonal drive !not shown).
Those skilled in the art will recognize that other driving
means, such as a double pin or a slot, could be used.
A tapered, external thread 9 is provided on the outer surface of
the hollow cylindrical body 2. The thread form, including the
tip 10 and root 12, is gradually reduced in size' from the
trailing edge 3 to the leading edge 4, to form a taper. In one
embodiment of the invention, the thread 9 is tapered 4,5° per
side. The taper provides increased purchase in cancelious bone
material, and prevents tyre threaded washer from advancing while
turning a bone screw that has been inserted through bore 5.
In a preferred embodiment, the tapered thread 9 of the washer
has cutting flutes z4, shown in FIG. I, to make the washer 1
self-tapping. The form of the thread 9 is a cancellous thread
form; that is, a form designed for purchase in cancellous bone.
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FIG. 3 shows a cancellous thread form used in one embodiment of
the invention. The thread leading face 50, which transfers the
compressive force between the threaded washer and the
surrounding cancellous bone tissue, has a small leading face
angle 51 of for example 10°. The trailing face 54 has a
trailing face angle 55 greater than the leading face angle, for
example 30°. This embodiment has a major diameter 20 of 5,5 mm
and a whole tooth depth 58 of x,625 mm.. It will be understood,
however, that the invention is in no way limited to this or any
other thread configuration.
External chamfer 12, shown in FIG. 1, is. provided on the tapered
thread 9 Where it intersects the leading edge 4~ in order to
facilitate insertion of the threaded washer, as described
below. A second external chamfer 13 is provided on the trailing
edge of the washer to break the sharp corner.
In one embodiment, the threaded washer of the invention is
fabricated from a biologically inert metal such as titanium, a
titanium alloy, or an implant quality stainles s steel, for
example ASTM-316L. A metallic washer and screw are often
removed in a subsequent surgical procedure after the fracture
has healed_ For example, the washer and screw may be removed to
relieve discomfort caused by continued bone compression across
the healed fracture plane.
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In an alternative embodiment, the washer may be fabricated from
a bio-absorbable material. In this case, no subsequent implant
removal procedure is contemplated, and bone material is
permitted to form around the metallic: screw as the washer is
absorbed by the patient°s system. Fux-thermore, compression on
the bone fragments is gradually reduced as the washer is
absorbed.
Resorbable or degradable materials that may be used for
fabricating the washer include ceramics based, for example, on
tricalcium phosphate, hydroxyapatite, calcium carbonate or
combinations thereof.
Other materials which can be used for the washer according to
the invention include polymers such as highly purified
polyhydroxyacids, polyamines, polyaminoacids; copolymers of
amino acids and glutamic acid, polyorthoeste~s, polyanhydrides,
amides, polydioxanone, polydioxanediones, polyesteramides,
polymalic acid, polyesters of diois and oxalic and/or succinic
acids, polycaprolactone, copolyoxalates, polycarbonates or
poly(glutamic-co-l2ucine). Preferably used polyhydroxyacids
comprise polycaprolactone polylactides ira their various chemical
canfiguration [e. g. poly(L-lacticle), poly(D-lactide),
poly(L/D-lactide), poly(L/DL-lactide)], polyglycolide,
copolymers of lactide and glycolide of various compositions,
copolymers of said Iactides and/or glycolide with other
polyesters, copolymers of glycolide and trimethylene carbonate,
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poly(glycolide-co-trimethylene carbonat.e), polyhydroxybutyrate,
polyhydroxyvalerate, copolymers of hydroxybutyrate and
hydroxyvalerate of various compositions.
Further materials to be used as additives are composite-systems
containing resorbable polymeric matrix and resorbable glasses
and ceramics based, for example, on tricalcium phosphate,
hydroxyapatite, and/or calcium carbonate admixed to the polymer
before processing.
FIG. 4 shows a threaded washer 1 o:E the invention with a
cannulated bone screw 60. The screw shown in FIG. 4 is a 2,~ mm
cannulated screw sold by Synthes fUSA). Those skilled in the art
will recognize that other standard bone screws, both cannulated
and non-cannulated, can be used with the threaded washer of the
invention.
The bone screw 6o has a thread 61 designed for purchase in
cancellous bone material. The bone screw thread 61 has a major
diameter 62. The central bore 5 of the threaded washer 2 is
slightly larger than the major diametew 62 of the bone screw
threads, so that bone screw 60 can be inserted into the threaded
washer l without contacting the internal wall of the washer. In
the embodiment shown in FIG. 4, the central bore S of the washer
1 is approximately 0,3 mm larger than the major diameter 62 of
the bone screw thread 61.
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Bone screw head 67 has a spherical undersurface 68. The
spherical undersurface tangentially engages the corners formed
by the driving means 8 in the form of a hexagonal socket and
the top edge 3. Between the bone screw thread 61 and the bone
screw head 67 is an unthreaded shaft portion 65. Self-drilling
flutes 70 at the distal end of the bone screw thread 61 permit
the screw to be inserted without pre-drilling or tapping.
The cannulated bone screw 60 shown in :EIG. 4 furthermore has a
central bore 66 for receiving a guide wire 112 (shown in FIGS_
5-7), used during the implantation procedure. The bone screw
head 67 has a cruciform slot 69 for receiving a cruciform screw
driver (not shown). The bone screw 60 is preferably fabricated
from implant quality stainless steel.
FIGS. 5-8 illustrate a procedure for using the threaded washer
1 of the invention together with a cannulated bone screw 60, for
reduction and fixation of a small bone fracture< Those skilled
in the art will recognize that other procedures for implanting
the threaded washer, using cannulated o~r non-cannulated screws,
are possible. It will furthermore be understood that the use of
the washer is not limited to the use illustrated.
FIG. 5 shows the initial steps in ;preparing the bone for
implantation of the threaded washer and cannulated screw.
Fracture plane 101 is shown separating outer fragment 105 from
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inner fragment 106. At the location selected for the
implantation, the fracture plane 101 is within the cancellous
region 102 of the bone, beneath the cortex 1U3.
A self-drilling guide wire 112 is inserted into the bone using a
drill (not shown) in an orientation substantially perpendicular
to the fracture plane 101. The guide wire has a fluted tip 110
to facilitate drilling. The diameter of the guidewire 112 is
chosen t~o be the proper diameter for the cannulated screw 60a
for example, with a 3,0 mm cannulated screw, a 1,1 mm guide wire
may be used. The guide wire 112 passes through the cortex 103,
through the outer fragment 1.05r and penetrates the inner
fragment 106 sufficiently to permit the thread 61 of the
cannulated screw 60 (FIG. 4) to completely engage the inner
fragment. A drilling depth gauge (not shown) may be used to
insure a proper guide wire depth.
As shown in FIG. 6, a tapered counterbore 120 is reamed in a
position concentric to the guide wire 112 using a cannulated
tapered reamer snot shown). The reamer is guided using the
guide wire 112 in the manner known in the art. The depth of the
counterbore may be controlled using a gauging mark on the reamer
(not shown) that aligns with the outer surface of the cortex
103.
The threaded washer 1 is installed in the counterbore 120 as
shown in FIG. 7. The threaded washer 1 is first assembled over
guide wire 112_ A cannulated hexagonal driver (not shown) is
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then assembled over the guide wire 112 above the threaded washer
1 and engaged with the driving means 8 in the form of a
hexagonal socket of the threaded~washE,r. Using the hexagonal
driver, the threaded washer is rotated in the counterbore 120,
engaging the thread ~ with the bone material. Cutting flutes 14
(FIG. 1) permit the tapered thread 9 to cut through cortex 103
and cancellous bone 102. The washer 1 is rotated until thread 9
is fully engaged in tapered hole 120.
In the embodiment in which the washer 1 is fabricated of a
bioabsorbable material, the use of a self-tapping thread with
cutting flutes may not be possible. In that case, the
counterbore is tapped using a tapered, cannulated tap before
implanting the threaded washer.
FIG. 8 shows the threaded washer and cannulated screw after
installation is complete. The cannulated screw 60 is assembled
over the guide wire (not shown), and a cannulated cruciform
screwdriver (not shown) is then assemb7Led over the guide wire
above the cannulated screw and in engagement with the cruciform
slot 69 of the cannulated screw. sliding on the guide wire 112,
the cannulated screw is:passed through the central bore 5 of
threaded washer 1 until screw threads 61. engage the cancellous
bone material 102 of the outer fragment 105 surrounding the
guide wire_ The cruciform screwdriver is then used to rotate
the cannulated screw 60 until threads 61 pass through the outer
fragment 105 and are entirely engaged in inner fragment 106.
The unthreaded shaft portion 65 of the cannulated bone screw
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passes through the washer 1 without coritacte The shaft portion
65 fully contacts the bone material immediately above and-below
the fracture plane 101, preventing lateral movement between the
inner and outer fragments.
After the cannulated bone screw 60 is driven into the bone, the
undersurface 68 of the screw head 67 contacts the threaded
washer 1 at the carnets formed by the driving means 8 in the
form of a hexagonal socket and the top edge 3. The fixed depth
of threaded washer 1 below the outer surface of the bone ensures
that, the cannulated screw head 67 will be countersunk to permit
joint articulation and~prevent soft tissue injury.. Further
rotation of the cannulated bone screw 6t3 draws the inner
fragment 106 upward toward the threaded washer 1, reducing the
fracture 101. The cannulaeed bone screw &0 is then rotated
until the proper compressive force is applied across the
fracture plane 1.01.
The compressive force can be increased or decreased without
affecting the axial pasition of the cannulated bone screw 60 in
the outer fragment 105 of the bone. Because the washer thread 9
is tapered, the washer -1 is restrained from advancing due to
friction while the bone screw 60 is tightened against the
threaded washer 1.
