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BACKGROUND OF THE INVENTION
This invention relates to an intramedullary catheter. More specifically,
the present invention is directed toward an improved device and method for
allowing repetitive delivery or withdrawal of fluids to or from the
vascular system of bone marrow through a catheter device placed within a
patient's bone.
Repetitive delivery of fluid into a patient's vascular system often entails
an intravenous device. When a patient requires fluid during each of
numerous treatment sessions, an intravenous device must be inserted into
the patient's vein. Each time the device is inserted, the physician runs
the risk of missing the vein and injecting fluid outside the vein.
Moreover, physicians often find it difficult to find a vein, or once they
do so, numerous injections into that vein may cause its rapid
deterioration. In an effort to solve the above problems, numerous devices
are commercially available which comprise a catheter commonly inserted
into a large vein and having a self-sealing septum through which
repetitive injections can be made into the catheter. Thus, instead of
repetitively placing a needle into a vein, the needle can be repetitively
placed through a septum and into a port attached to a conduit placed
within the vein.
Intravenous catheters represent substantial improvements in the art,
however, when used over a long period of time, they can cause infection
and clotting in the vein near the area where the catheter is placed into
the vein. Recently, a device and method were developed for repetitively
placing fluid into the vascular system via bone marrow. Such a device
incorporated herein as U.S. Pat. No. 4,772,261 comprises an intramedullary
catheter placed into a tapped bore within the patient's bone and into the
bone marrow. This device allows placement under the skin and the closing
of the skin over the device such that the portion of the catheter
extending outside the bone remains hidden under the skin. Although
catheters placed into a patient's bone marrow represent improvements in
the art, they are often difficult to place, and once in place, are
difficult to find. It is important, when placing a bone catheter, that the
head or outer-most member of the catheter be large enough to be easily
detectable (or palpable) by the physician so that he or she can target the
injection needle into the septum of the catheter. Furthermore, it is
important that once a catheter is in place, it be securely held within the
bone marrow and will not cause pain during normal patient movement. It is
also important that the catheter-bone interface be secure or tight enough
so as not to leak fluid outside the bone and into the surrounding tissue.
However, the base of the catheter must be designed or shaped not to cause
necrosis of the underlying periosteum resulting in a nidus for infection.
Still further, it is important that a device be provided for repetitive
harvesting or withdrawing of fluid from bone marrow as well as repetitive
delivery of fluid to bone marrow.
SUMMARY OF THE INVENTION
Accordingly, it is desirable to produce an improved intramedullary catheter
which can be rapidly placed and securely held in a patient's bone. The
improved catheter of the present invention can either be implanted
underneath the patient's skin or can reside partially above the skin in a
percutaneous embodiment. If implanted, the catheter is placed within a
bore made through the patient's bone and into underlying bone marrow. A
physician can feel for the catheter residing underneath the patient's skin
and thereby insert a needle through the skin and into the catheter for
delivery or withdrawal of fluid to or from the patient's vascular system.
If the device is not implanted, but is placed percutaneous, the physician
can simply visually detect the head of the catheter and gain access to the
vascular system above the patient's skin.
The present invention includes a conduit having threads extending along the
length of the catheter from a conically shaped head to a distal end that,
when placed, resides within the patient's bone marrow. The threads act to
securely hold the device within bone as the patient is undergoing normal
activity. The improved device is securely held in the bone with a
conically shaped head extending either inside (implanted) or outside
(percutaneous) the patient's skin. If implanted, the conically shaped head
allows sealing engagement with the bone adjacent the bore to prevent
infection from entering the bone marrow and to prevent fluid from leaking
outside the bone into the surrounding tissue.
In accordance with one embodiment of the present invention, a novel device
is provided which can be implanted underneath a patient's skin. The
implanted device is adapted to allow repetitive passing of fluid to a
patient's vascular system via bone marrow. The implanted device comprises
a tubular conduit with threads extending along the length of the conduit
from a head placed at one end to the tip at the other end. By drilling a
bore into the bone, the threaded conduit can be screwed into the bore by
rotational movement of a tool placed over the head. When fully implanted
into the bone, the tip of the conduit resides within the bone marrow of
the bone and the head sealingly abuts the outer surface of the bone. A
seal means is provided and adapted to retain a sealing membrane on the
head of the device. The seal means includes a silicon elastomer which
permits repetitive insertion and withdrawal of a needle without exposing
bone marrow to infection. The head is generally conically-shaped having
interior walls defining a saucer-shaped cavity covered by the seal means.
The cavity is of sufficient size to receive a needle tip and allow a fluid
access reservoir between the needle tip and the conduit (and subsequently
the bone marrow). The seal means, or port to the septum, is of sufficient
size to be easily detected by the physician when placing the needle in the
implanted head. Although a physician cannot see the implanted device, he
or she can target the needle by feeling for the underlying head and
sealing membrane. The sealing membrane is dome shaped to aid the physician
in palpably detecting the target area.
In accordance with another embodiment of the present invention, there is
provided a device which is only partially implanted. The head portion of
the device remains outside the patient's skin in a percutaneous
embodiment. The percutaneous device is similar to the implanted device in
that it has threads on the outside of a conduit extending from a conically
shaped head to the distal tip. However, unlike the implanted device, the
percutaneous device can be inserted directly into the bone without having
to first drill a bore into the bone. The percutaneous conduit having
cutting threads placed along the outer surface of the conduit and a
cutting tip at the distal tip of the threads. The threaded conduit is
screwed into bone by rotating a tool placed over the head of the device.
As the device is being screwed into the bone, cutting tip and cutting
threads form a bore simultaneous with the insertion of the device. Thus,
the percutaneous device can be placed directly into the bone without
having to pre-drill a bore as in the implanted embodiment. Placement of
the percutaneous device is therefore quickly performed to allow emergency
delivery or withdrawal of fluid to or from the patient's bone marrow.
In either the implanted or percutaneous embodiments, sealing engagement is
made with the bone to prevent infection from entering the bone marrow and
to prevent fluid from leaking from the bore. If implanted, the conically
shaped portion of the head sealingly abuts the bone surface adjacent the
bore. Thus, infection is prevented from entering the bore between the head
and the bone surface. Conversely, if the head is configured above the
patient's skin as in the percutaneous embodiment, a butting member or
protrusion sealing abuts the bone surface to prevent infection from
entering the bore. Thus, as is shown here and throughout the following
discussion, both embodiments have provisions which prevent infection from
entering the bone marrow while also preventing fluid leakage from the
bore. Prevention of infection and leakage is provided by a combination of
the conically shaped, sealing portion of the head as well as the sealing
membrane which covers the head. Furthermore, the threads are radially
dimensioned to provide a relatively tight fit with the bore such that
little or no passage exists between the bone marrow and the outside air or
overlying tissue.
In accordance with the instant invention, there is also provided a novel
method of passing fluid to and from the vascular system of a patient
through bone. The method includes placing an implanted device underneath
the skin in communication with the patient's bone marrow. Once placed,
passage of fluid through the device and overlying skin is easily achieved
by placing a needle into the device and injecting or withdrawing fluid
therethrough. The method comprises the steps of providing a device having
an elongated, tubular conduit with threads extending the length of the
conduit; drilling a bore into the bone; implanting the device into the
bore with the conduit in operable communication with the bone marrow;
injecting fluid through the elastomer and into the conduit for delivery
through the conduit into the bone marrow and transport to the vascular
system; repeating the injecting step for repetitive delivery of fluid to
the vascular system; withdrawing fluid through the elastomer from the bone
marrow; and, repeating the withdrawing step for repetitive, relatively
long-term drawing of fluid from the patient's vascular system. The
drilling step comprises making the bore of sufficient diameter to receive
and securely hold the threads of the conduit. The implanting step
comprises forming mating threads to the bore in response to rotating
movement of the conduit within the bore. The drilling step comprises
drilling the bore extending from the surface of the bone to the bone
marrow, or extending from the skin covering the bone to the bone marrow.
The implanting step comprises rotatable insertion and sealing a small
portion of the outer surface of the head against the surface of the bone
to prevent infection from entering the bore between the outer surface of
the head and the surface of the bone.
In accordance with the instant invention, there is also provided a novel
method of passing fluid to or from the bone marrow wherein the head of the
device is not implanted underneath the skin. In this percutaneous
embodiment, a device is provided with cutting threads extending the length
of the conduit with a cutting tip attached to the distal end of the
conduit and a head attached to the proximal end. An elongated tool is
placed over the head and the tool is rotated such that the cutting tip and
threads form a bore directly into the bone simultaneous with the placement
of the device within the bone. Upon full insertion of the device, a needle
can be placed through the sealing membrane to pass fluid to or from the
bone marrow. Moreover, when the device is fully inserted, a protrusion
placed on the conduit sealingly abuts the outer surface of the bone to
prevent infection from entering the bore while also preventing fluid from
exiting the bore. It is important to note that a bore need not be
pre-drilled when using the percutaneous device since the percutaneous
conduit is similar in function to a drill bit. By attaching the rotating
tool to the distal end and screwing the device into the bone, the device
functions as both a catheter and drill bit for simultaneous placement
within the bone.
If, for aesthetic reasons or for a desire to decrease the exposure of
subcutaneous tissue to infection, the device is implanted beneath the
skin, then a pilot bore must be predrilled. A drill bit is provided having
a cutting shaft with a cutting or piercing tip placed at one end and a
countersink attached near the other end. The tip is sharp to ensure
precise placement of the bore within the bone. Moreover, the cutting shaft
increases in diameter from the tip to provide a smooth bore which is
countersunk at the bone surface by the countersink attached to the cutting
shaft. The countersink produces a pilot bore entrance having a conical
shape which matches the conically shaped head such that a fairly precise
and smooth fit exists between the countersink bore and the head. The
matching of the countersink bore and the head prevents contamination or
leakage therebetween. Collinear with the axis of rotation of the cutting
shaft and attached to the countersink is a coupling shaft which can
accommodate a rotating means such as a drill. Rotational movement provided
by the drill will impart rotational cutting movement upon the cutting
shaft and tip.
It is further appreciated that the present invention, including the
conduit, head, threads, etc. can be coated or impregnated on the inside
and/or outside with certain materials which can promote or inhibit certain
biological characteristics. For example, Heparin bearing material may be
placed on the inner surface of the conduit and saucer cavity to help
prevent clotting within the fluid passage. Also, e.g., Titanium oxide can
be placed on the outer surface of the buttress threads and head to promote
fixation of the present device to the bone to further secure it to the
bone and help prevent fluid leakage. Antibiotics may be placed on the
inner and/or outer surface to prevent infection. Still further, proteins
may be placed on the distal end of the conduit to prevent new bone or
tissue formation on said tip which could occlude the conduit. It is
understood that any of these materials can be either coated onto or
impregnated directly into the inner or outer surface of the present device
without departing from the scope and spirit of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of the percutaneous intramedullary
catheter of the present invention secured within bone and extending
partially above the patient's skin.
FIG. 2 is a top plan view of the present invention along plane 2--2 from
FIG. 1.
FIG. 3 is a cross-sectional view of the implanted, adult-sized
intramedullary catheter of the present invention secured within bone and
extending entirely beneath the patient's skin.
FIG. 4 is a sectional view of the present invention along plane 4--4 from
FIG. 3.
FIG. 5 is a perspective view of the disassembled intramedullary catheter of
the present invention.
FIG. 6 is a perspective view of the assembled, pediatric-sized
intramedullary catheter of the present invention.
FIG. 7 is a bottom plan view along plane 7--7 from FIG. 6.
FIG. 8 is a perspective view of a twist drill bit used to produce a bore
within bone through which the implanted intramedullary catheter of the
present invention is installed.
FIG. 9 is a perspective view of a paddle drill bit used to produce a bore
within bone through which the implanted intramedullary catheter of the
present invention is installed.
FIG. 10 is a cross-sectional view of a rotating tool for engaging the
intramedullary catheter of the present invention.
FIG. 11 is a cross-sectional view along plane 11--11 from FIG. 10.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to the drawings, FIG. 1 illustrates a cross-sectional view of the
improved intramedullary catheter 10 partially implanted beneath a
patient's skin 12. The implanted device 10 shows head 14 extending above
the skin and conduit 16 extending beneath the skin. The device is securely
held in place by threads 18 placed along conduit 16 and engaged with
cortical bone 20 and cancellous bone 22. Contained within cancellous bone
22 are spongy bony networks filled with venous sinusoidal spaces which
forms bone marrow 24.
The embodiment shown in FIG. 1 illustrates device 10 having a catheter
placed through skin 12 in a percutaneous arrangement. A physician can
insert catheter 10 through the skin and directly into underlying bone. The
distal tip 26 is designed to have one or more cutting edges for piercing
subcutaneous tissue 28 and underlying bone 20 and 22. As tip 26 enters the
bone similar to a drill bit entering a solid medium, threads 18 further
cut the bone as conduit 16 enters the passage formed by tip 26 and threads
18. Once tip 26 penetrates the more dense cortical bone 20, threads 18
become seated in cortical bone 20 thereby allowing for easier penetration
of tip 26 into the less dense cancellous bone 22. After device 10 is
sufficiently screwed into bone 20 and 22, tip 26 will reside within the
cancellous portion of bone 22 in communication with bone marrow 24.
Moreover, when fully inserted, head 14, having a conical portion 30, will
abut the outer surface of skin 12. Conical portion 30 will engage with the
outer surface of skin 12, leaving a small gap between the major portion of
head 14 and skin 12. Such a gap is important to prevent entrapment of
bacteria between the head and skin. Furthermore, conical portion 30
functions to seal against the skin and prevent infection from entering
subcutaneous tissue 28 via the outside environment. Also, conical portion
30 maintains elevation of head 14 above skin 12. The percutaneous
configuration of FIG. 1 allows a physician to quickly insert device 10
through skin 12 and into underlying bone 20 and 22 without the necessity
of pre-drilling a pilot hole or bore. Device 10, having cutting threads
18, functions as a drill bit for simultaneous drilling and placement of
conduit 16 into underlying bone 20 and 22. When fully inserted, conical
portion 30 of head 14 provides a seal thereby preventing infection from
entering subcutaneous tissue 28 and/or marrow 24 between skin 12 and head
14. A radially extending protrusion 35 of conical shape at the proximal
end of the threaded conduit acts as a sealing "stop" to prevent the
catheter from being further advanced into the bone. Moreover, protrusion
35, being of larger radial dimension than threads 18, provides a seal
against leakage of medications or fluids from the intramedullary space to
outside the bone. Thus, protrusion 35 prevents medications or fluids from
seeping from marrow 24 to subcutaneous tissue 28.
The importance of the percutaneous application of the present invention
shown in FIG. 1 is further appreciated by physicians in the field. Often,
in emergency situations, it is desirable to set forth immediate fluid
delivery or withdrawal procedures to or from the patient's vascular
system. In doing so, many physicians, in their haste, prefer the
intravenous route since access to a vein is quicker and easier than access
to bone marrow. However, as shown in the present invention, emergency
access to bone marrow can be quickly achieved by the percutaneous
embodiment. Device 10 can be quickly screwed into bone 20 and 22 in a one
step procedure leaving head 14 exposed. Not only can device 10 be quickly
inserted, but repetitive, subsequent fluid delivery or withdrawal can be
achieved without having to continually enter a vein as in conventional
art. Device 10 remains in place after emergency insertion for repetitive
delivery or withdrawal of fluid to and from the patient's vascular system
without incurring the disadvantages found in intravenous delivery systems.
FIG. 2 illustrates a top plan view of head 14 shown along plane 2--2 from
FIG. 1. The outer portion of head 14 is shown having a substantially
circular body 32, preferably stainless steel, plastic, titanium, etc. used
to encompass a sealing membrane 34. As shown in FIGS. 1 and 2, membrane 34
is dome-shaped on the exposed outer surface to enable a physician, nurse,
or patient to easily palpate the location of septum or membrane 34. This
is important so that a needle can be accurately inserted into membrane 34
with less chance of misguiding the needle and injecting fluid outside of
device 10. Conventional intravenous catheters utilize a membrane or septum
of the present invention, but do not have a palpable, easily detectable
dome-shaped membrane or septum of the present invention. Often, in
intravenous catheters, a physician may have to insert a needle many times
in order to successfully find and/or enter the device port.
By being dome-shaped and having sufficient radial dimension, membrane 34 is
easily located and quickly accessed by a physician. The diameter of
membrane 34 must be large enough to provide an adequate target, but not so
large as to cause discomfort or unsightliness. Preferably, the diameter of
the domed portion of membrane 34 is approximately 12 mm in an adult and 10
mm in a pediatric model. It is found by the Applicant that such a
preferred size provides good target area for physicians to access device
10. However, diameter of membrane 34 can vary substantially without
deviating from the scope of this invention.
FIG. 3 illustrates another embodiment showing device 10 fully implanted
beneath skin 12. In circumstances where it is not essential that device 10
be placed immediately, device 10 can be implanted under the skin to
provide a more aesthetic appearance and to further protect the body from
the possibility of infection entering subcutaneous tissue 28. Depending
upon the depth of tissue 28, device 10 once secured in place will allow
varying degrees of palpability. If tissue is relatively shallow, the
dome-shaped portion of membrane 34 protrudes against skin 12 or tissue 28
and beyond the plane of skin 12. Thus, in shallow-tissue areas, the
dome-shaped membrane 34 is visually detected by the physician to enable
insertion of a needle into the membrane. However, in areas where
subcutaneous tissue 28 is relatively deep, palpable detection is easily
achieved due to the large radial dimensions of head 14. It is understood
that either embodiment, implanted or percutaneous, allows the physician to
easily pass fluid to and from the bone marrow vascular system.
Illustrated in FIG. 3 is conical portion 30 which sealingly abuts against
the outer surface of cortical bone 20. In addition to providing sealing
arrangement, conical portion 30 is dimensioned to slightly elevate head 14
above the outside surface of bone 20 such that only a small portion of the
inside surface of head 14 touches bone 20. Pain during normal movement is
minimized by having only a small portion of head 14 pressing on the
nerve-rich periosteum. Furthermore, minimizing the contact area between
head 14 and the surface of bone 20 allows normal circulation of the
periosteum under the head thereby preventing substantial cell death and
necrosis. Thus, when fully inserted and implanted underneath a patient's
skin 12, device 10 is configured to prevent infection from entering bone
marrow from either the outside environment or tissue 28 overlying bone 22.
Membrane 34 and conical portion 30 ensure such infections from happening.
Further, membrane 34 and conical portion 30 prevent fluid from leaking
from underlying bone marrow 24 to tissue 28.
Shown in FIG. 3 is the distal end of conduit 16 which includes self-tapping
threads. Self-tapping threads may be notched in a tetrahedral shape at the
distal point thus permitting maximum cutting through cancellous bone 22.
The self-tapping feature is advantageous in that it will save a step in
the implantation procedure and will make it simpler and faster to insert
device 10 into bone 20 and 22. Instead of having to tap threads into the
bore, the self-tapping threads can make their own threads as threaded
conduit 16 is screwed into the pre-drilled bore. The implanted device 10
requires only that a bore be drilled and that threads can be made within
the bore by the self-tapping feature of threads 18. It is important to
note that although self-tapping threads shown in FIGS. 3 and 4 utilize a
tetrahedral design, any standard self-tapping design can be used, which
may also include but is not limited to a three-point notch design shown in
FIG. 7.
As shown in FIGS. 1, 3, and 5, threads 18 which are self-tapping, provide
quick and secure implantation of device 10 within bone 20 and 22. Buttress
threads can be used, and are often called ASNIS cancellous bone threads
commonly found in the orthopedic industry. The threads are preferably ten
pitch, or 2.5 mm per revolution and measure 6.5 mm at the crest and 4.0 mm
at the root (basic shaft diameter). The inside diameter of conduit 16 is
preferably 13 gauge or 2.4 mm. Buttress threads are useful in withstanding
uni-directional stress and have nearly ten times the holding power
(pull-out strength and tightening torque) than standard machine threads
placed in bone. Furthermore, 13 gauge conduit provides a generous size
that is more than adequate for the infusion of fluids and medications or
for the aspiration of blood and marrow. Thus, not only is it important
that the inside diameter of conduit 16 be sufficiently large to carry
fluid, but it must also be sufficiently dimensioned to carry marrow if
device 10 is used for repeated bone marrow harvesting. Bone marrow
harvesting, useful in bone marrow transplants and/or monitoring of the
bone marrow, is achieved by aspirating through membrane 34 blood, fluids
or particulate matter from bone marrow 24. In applications wherein
medications can suppress the bone marrow or in patients with immune
deficiency disease, or who require frequent monitoring of bone marrow
activity, the present invention provides a useful technique for correctly
withdrawing portions of the patient's bone marrow. In addition, the bone
marrow may be repeatedly harvested for the determination of complete blood
counts and all routine blood chemistries without the necessity of having
to access the vein.
FIG. 5 is a perspective view of various components of device 10. Membrane
34 is sealed between body 32 and conical portion 30. The inside surface of
body 32 is circular to surround the outer perimeter of membrane 34. In
addition, body 32 includes a lip 36 which engages membrane 34 against the
outer surface of conical portion 30 when assembled. After device 10 is
assembled, the dome portion of membrane 34 protrudes through the circular
opening within body 32. The inner portion of head 30, which underlies
membrane 34, forms a reservoir or cavity defined by saucer 38. The
reservoir or cavity is of sufficient size to accommodate the tip of a
needle inserted through membrane 34. If fluid is injected from the needle
into the cavity, it is forced into the inner passage of conduit 16.
Membrane 34 permits repeated access of a needle to the reservoir such that
when the needle is withdrawn, the hole created by the needle is sealed to
prevent infection from entering the reservoir. The dome-shaped portion of
membrane 34 and the reservoir are of sufficient size to accommodate
successful introduction of a needle from a wide range of angles. The floor
of the reservoir is saucer-shaped to direct the needle towards the conduit
16.
FIG. 6 illustrates another embodiment of self-tapping threads having a
three-point notch design rather than the tetrahedral design shown in FIGS.
3 and 4. Applicants have found that the notch design provides good
self-tapping characteristics in cortical bone. Also, FIG. 6 illustrates
device 10 of smaller dimensions, i.e., having a radially smaller head 14
and shorter conduit 16 to accommodate pediatric/small bone application.
Smaller catheters are designed primarily to accommodate the smaller size
bones found in pediatric population and in small animals weighing
approximately 30-90 pounds. The length of conduit usable in smaller-bone
applications can vary drastically, however, 9 mm being a preferred size. A
9 mm tip will usually allow the distal end of conduit 16 to enter marrow
24 without impinging on the opposite wall or perforating through the
opposite side of cortical bone 20. It is understood, however, that conduit
16 can be of any dimension (length or width) depending upon the particular
bone size and/or fluid delivery rate. If a bone is relatively large, then
longer catheters are preferred. Conversely, if a bone is relatively small,
then a smaller catheter with a shorter conduit is preferred. In either
case, the optimal length of catheter is chosen such that the distal end
resides within marrow 24 and somewhere between the opposing walls of
cortical bone 20.
It is further appreciated that the present device 10, including the conduit
16, head 14, tip 26, threads 18, conical portion 30, body 32, lip 36 and
saucer 38 can be coated or impregnated on the inside and/or outside with
certain materials which can promote or inhibit certain biological
characteristics. For example, Heparin bearing material may be placed on
the inner surface of the conduit 16 and saucer cavity 38 to help prevent
clotting within the fluid passage. Also, e.g., Titanium oxide can be
placed on the outer surface of conduit 16, buttress threads 18, head 14
and tip 26 to promote fixation of the present device to the bone to
further secure it to the bone and help prevent fluid leakage. Antibiotics
may be placed on the inner and/or outer surface of conduit 16, head 14,
threads 18, etc to help prevent infection. Still further, proteins may be
placed on the distal end of conduit 16 or tip 26 to prevent new bone or
tissue formation on said tip which could occlude the conduit. It is
understood that any of these materials can be either coated onto or
impregnated into the inner or outer surface of selective portions of the
present device 10 without departing from the scope and spirit of the
present invention.
FIG. 7 illustrates a bottom plan view along plane 7--7 from FIG. 6.
Self-tapping threads 30 are shown with the three-point notch design
typically used for implantation with cortical bone. Also shown is conical
portion 30 extending radially outward from conduit 16 toward the
underneath side of head 14. The outer portion of conical section 30 is
understood to engage against skin 12 (in the percutaneous embodiment) or
bone 14 (in the implanted embodiment) to provide a sealing arrangement
necessary to prevent infection or fluid leakage.
FIG. 8 illustrates a dedicated drill bit 40 specifically used for the
implanted embodiment. Bit 40 serves to bore the proper size hole for
optimum fitting of the device's self-tapping threads 18 into bone 20 and
22. Twist drill bit 40 comprises an elongated cutting shaft 42 having
spiral cutting edges which cut and remove cortical and cancellous bone 20
and 22, respectively, in response to rotational movement placed upon shaft
44. As the cutting shaft 42 forms a bore within bone 20 and 22, a collar
47 functions to stop the drilling at the proper depth so as not to drill
completely through the bone and out the other side. Thus, collar 47
prevents the distal tip of cutting shaft 42 from penetrating the opposite
wall or through the opposite wall. Countersink 46, functions to create the
proper angle of the bone onto which the conical portion of the head can
seat and form a tight seal. Without this countersinking step, the process
of making threads, either with a tap or with the self-tapping threads, may
damage the entrance into the bone to where it may leak fluid from the bone
marrow and into the surrounding tissue. The countersink dresses the bone
in a smooth circular fashion to promote a sealing fit. In addition, if the
countersinking step takes place separate from the drilling, the angle of
the countersink may be different from the angle of the bore, again causing
an irregularity with respect to device 10, which provides a space for
leaking. Shaft 44, shown at the top of twist drill | | |
