Note: Descriptions are shown in the official language in which they were submitted.
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BONE PLATE WITH CONICAL SCREW THREADS
FIELD OF THE INVENTION
The present invention relates generally to devices for fixation of parts of a
fractured bone and more specifically, to bone plates and systems for
stabilization and/or
compression of parts of a fractured bone.
BACKGROUND OF THE INVENTION
Bone plates may generally be utilized to carry out two different types of
osteosynthesis, namely "rigid osteosynthesis" and "flexible osteosynthesis."
Rigid
osteosynthesis is used for medical care of joint fractures, simple shaft
fractures (where
nailing is impossible) as well as for osteotomies. Aside from the possibility
of anatomical
repositioning, the bone itself supports and stabilizes the osteosynthesis,
which allows for
the possibility of putting stress on the extremity earlier and without pain.
Additional
advantages of the medical care of stable fractures can be observed when the
blood
circulation in the bone is greatly diminished due to trauma. For treating
"nonunions" or in
the case of existing infection, the fracture must be kept stable in order to
make bone
healing possible and so as not to irritate the infection further by
instability of the fracture
gap.
Flexible osteosynthesis, also known as "biological osteosynthesis," may be
desirable in the medical treatment of comminuted fractures in the shaft region
of tubular
bones. In the case of these fractures, it is an objective to mairitain the
proper length of the
bone and to fix the bone ends (joints) in their proper anatomic positions with
respect to one
another. With flexible osteosynthesis, the fracture zone is not directly
affixed or
manipulated, and consequently, the blood circulation in this area is not
inhibited. Bone
plates designed for flexible osteosynthesis thus operate similarly to a
locking,
intramedullary nail, which is anchored only in the metaphyses.
Since fractures cannot always be treated with one type of osteosynthesis,
surgeons must frequently compromise because a bone plate, which allows him to
combine
the two types of osteosynthesis discussed above, is not available. Such a
combination
would be beneficial, for example, when a joint fracture can be compressed with
traction
screws through the bone plate and the whole of the joint may be connected to
the diaphysis
over an internal fixative with angularly stable screws. Another illustrative
application
concerns porotic bones, where a bone plate with axially and angularly stable
screws can be
anchored in the metaphysial fragment, with a stable plate-affixation being
undertaken in
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the diaphyseal range with the assistance of a plate traction screw through the
fracture. A
primary fracture stabilization can be achieved by this type of procedure.
This situation has led to the development and marketing of bone implants
for both types of osteosynthesis. The two types of implant group, however, are
designed
specifically for their respective method. Thus, the disadvantages of these two
systems lies
in the difficulty in combining them.
Thus, a need exists for improved bone plates that provide for both rigid and
flexible osteosynthesis.
SUMMARY OF THE INVENTION
The present invention is directed to a bone plate that is adapted to be used
for both rigid and flexible osteosynthesis, without compromising the plates
ability to be
used for either type of osteosynthesis. Accordingly, the bone plate of the
present invention
may be used as a compression plate or as an internal fixative.
According to one embodiment of the invention, the bone plate includes an
upper surface, a bone contacting surface, and a plurality of holes extending
through the
upper and bone contacting surfaces. At least one of the holes is elongated in
a direction
substantially aligned with a longitudinal axis of the plate, and includes a
threaded portion
and a non-threaded portion. The threaded portion may extend over a range of
greater than
about 180 with respect to a central axis of the hole. The threaded portion of
the hole is
dimensioned and configured to engage a threaded head of a bone screw, and fix
the bone
screw at a predetermined angle with respect to the bone plate. Preferably, the
threaded
portion extends through the full thickness of the bone plate, i.e., from the
upper surface to
the bone contacting surface, thus maximizing the stability of the bone screw
to bone plate
interface.
With the threaded screw head fixed in the threaded portion of the elongated
hole, the bone plate may be used as an internal fixative. Use in this
configuration,
however, creates high stresses at the interface of the bone plate and screw
head because the
plate is not forced against the bone, and therefore, the bone fracture is
fixed primarily by
friction between the plate and the bone. This increase in stress is taken into
account by the
threaded portion of the hole extending over a range of at least about 180
with respect to a
central axis of the hole, and thereby enclosing the screw head in at least
this angular range.
This feature of the bone plate is especially advantageous where thin bone
plates are
involved. Preferably, the threaded portion is disposed on one of the two
longitudinal ends
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of the hole. This positioning allows for the threaded portion to extend over a
larger
angular range. For example, the threaded portion may extend over a range of
between
about 190 and about 280 , and preferably over a range of between about200 to
250',
thus maximizing the strength of the bone screw to bone plate interface.
According to another embodiment of the present invention, at least one of
the holes may include a threaded portion that is angled or tapered with
respect to a central
axis of the hole. More specifically, the threaded portion may conically taper
inward
towards the bone-contacting surface of the bone plate. A bone screw to be
rigidly fixed to
the bone plate may include a threaded screw head that is tapered to
substantially match the
tapered shape of the threaded portion of the hole. Thus, the bone screw may be
rigidly
fixed to the bone plate by engagement between the matching conical threads.
This method
of attachment is especially advantageous when self-drilling screws are to be
used since,
due to the conical shape of the matching threads, the screw may be inserted
into the bone
independently of the plate. More specifically, the screw head becomes rigidly
clamped to
the plate only as the threaded screw head penetrates the threaded portion of
the hole.
Despite any initial misalignment between the threads on the screw head (the
position of
which are initially dictated by the orientation of the bone screw in the bone)
and the
threads on the bone plate, the conical shape of the mating threads ensures
that the threads
on the screw head will ultimately align with the threaded portion of the hole.
When the
conical thread screw head is tightened into the threaded portion of the hole,
the screw head
creates radial forces in the plate hole. Thus, the bone plate must be
dimensioned and
configured to withstand these high radial forces, e.g., to withstand flexing
of the walls of
the screw holes in the bone plate.
The threaded portion preferably tapers at a cone angle of between about 50
and about 20 . Preferably, the thread tapers at a cone angle of about 10 .
In the case where the threaded portion of the hole is tapered, as discussed
above, the threaded portion may extend through a different angle when measured
at the
upper surface than when measured at the bone-contacting surface. For example,
when
measured at the upper surface, the threaded portion may extend through a first
angle of
between about 200 and about 270 , while when measured at the bone-contacting
surface,
the threaded portion may extend through a second angle of between about 180
and about
230 .
According to another aspect of the present invention, at least one of the
holes may be dimensioned and configured to receive a ball shaped head of a
bone screw
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and provide for compression of two fractured bone fragments. For example,
according to
one. embodiment, the non-threaded portion of the elongated hole, discussed
above, may
include a concave, substantially spherical recess at the upper surface. The
recess may be
dimensioned and configured to accommodate the spherical head of a conventional
bone
screw. Such an arrangement may be especially useful when the bone screw is put
in place
eccentrically with respect to the hole, as is necessary for attaining
compression of a
fracture. Additionally, the non-threaded portion of the hole may flare outward
in the area
of the bone contacting surface to provide for increased angulation of the bone
screw with
respect to the bone plate.
The threaded portion of the hole may be positioned closer to the end of the
elongated hole that is closer to the center of the plate, thus avoiding any
undesirable effects
on the compression capability of the non-threaded portion. Thus, when the bone
plate is
used as a compression plate, the geometry of the non-threaded portion
(compression
portion) is not adversely affected by the presence of the threaded portion.
As discussed above, the hole may be elongated. Thus, the elongated hole
may define first and second dimensions on the bone contacting surface, wherein
the first
dimension DL is substantially parallel to the longitudinal axis of the bone
plate and the
second dimension DQ is substantially perpendicular to the longitudinal axis.
The ratio of
DL/DQ may be within the range of about 1.1 to 3, and preferably is in the
range of about
1.1 to 1.5. This ratio follows from the combination of the threaded portion
(locking
portion) with the non-threaded portion (compression portion), which requires a
clamping
path for the screw head. This ration of DIJDQ represents a preferred
compromise between
compression and plate weakening due to the presence of the hole.
According to another embodiment of the invention, the underside of the
bone plate may be concave, thus allowing the plate to conform to the rounded
cross-
section of the tibia, femur, humerus, lower arm bone, and other bones with
which the
present invention may be used. The concave configuration of the underside also
allows a
conventional bone screw to be inserted obliquely through the plate hole. This
feature may
be especially important when gripping a small bone fragment, which must be
pulled up to
the plate.
The present invention is also directed to a bone plating system including at
least one bone screw having a screw head with a thread disposed thereon, the
thread being
configured and dimensioned to engage the threaded portion of the above-
described bone
plate holes. Preferably, the bone screw is self-tapping and/or self-drilling.
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BRIEF DESCRIPTION OF THE DRAWINGS
To facilitate an understanding of the characteristics, structure and operation
of the invention, preferred features of the invention are described in the
accompanying
discussion, wherein similar reference characters denote similar elements
throughout the
several views or embodiments, and wherein:
FIG. 1 is a top view of a segment of a bone plate according to one
embodiment of the present invention;
FIG. 2 is a top view of a segment of a bone plate according to another
embodiment of the present invention;
FIG. 3 is a longitudinal cross-sectional view showing a threaded hole of the
bone plate of FIG. 1;
FIG. 4 is a longitudinal cross-sectional view of a partially-threaded,
elongated hole of the bone plate of FIG. 2; and
FIG. 5 is a perspective view of the segment of the bone plate of FIG. 2, with
a bone screw inserted into the partially-threaded, elongated hole.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
One embodiment of a bone plate according to the present invention is
shown in FIG. 1. The bone plate includes an upper surface 1, a bone-contacting
surface 2,
and defines a longitudinal axis 3. The bone plate further includes two holes
4A and 4B,
which are generally located along the longitudinal axis 3, and extend through
the bone
plate from upper surface 1 to bone-contacting surface 2. Holes 4A and 4B are
dimensioned and configured to receive the heads of bone screws. As shown in
FIG. 1,
arrow 7 indicates a longitudinal direction toward one end of the bone plate
and arrow 8
indicates a longitudinal direction toward a central portion of the bone plate.
Still referring to FIG. 1, hole 4A, which is located closer to the central
portion of the bone plate, is elongated along the longitudinal axis 3 of the
bone plate.
More specifically, hole 4A defines first and second dimensions on bone-
contacting surface
2. First dimension DL is substantially parallel to longitudinal axis 3, and
second dimension
DQ is substantially perpendicular to longitudinal axis 3. First dimensiori DL
is preferably
larger than second dimension DQ. According to one preferred embodiment, DL is
5.2 mm
and DQ is. 3 mm, however other dimensions are possible. Elongated hole 4A may
be
provided with a concave and preferably spherical recess 6 near upper surface
1. Recess 6
may be dimensioned to receive a bone screw having a substantially spherical-
shaped head.
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Hole 4B, which is located closer to one end of the bone plate; includes a
thread 5 which extends through an angle of 360 around the hole 4B. As shown
in the
schematical representation of FIG. 1, hole 4B assumes the shape of a cone
tapering inward
in a direction toward bone contacting surface 2, and accordingly thread 5 also
tapers
inward toward bone contacting surface 2. As shown in FIG. 3, hole 4B and
thread 5
preferably taper inward with respect to central axis 20 of hole 4B by a cone
angle 22 of
about 10 , however other value of cone angle 22 are-possible. In addition,
thread 5 is
preferably a double thread. As shown in FIG. 3, thread 5 of hole 4B may run
along the full
thickness of the bone plate from the upper surface 1 to the bone-contacting
surface 2.
In one preferred embodiment of the invention shown in FIGS. 2 and 4, the
two bone-plate holes 4A and 4B of FIG. 1 may be combined to form a combination
hole
4C. The combination hole 4C is thus provided with a threaded portion which
includes a
thread 5, and a non-threaded portion which has no threads disposed thereon.
The threaded
portion is preferably located at the end of the hole 4C which is nearer to the
central portion
of the bone plate.
With reference to FIG. 2, when measured at upper surface 1, the threaded
portion (thread 5) extends over a first angle 9 with respect to the central
axis of hole 4C,
and when measured at bone-contacting surface 2, the threaded portion (thread
5) extends
over a second angle 10 with respect to the central axis. Preferably, first
angle 9 is about
223 and second angle 10 is about 256 , however other values of first and
second angles 9,
10 are possible.
The table below displays, for illustrative purposes only, preferred
parameters which may be used depending on the diameter of thread 5.
Thread Diameter 3.0 mm 4.0 mm 5.0 mm
Double Thread yes yes yes
Thread Pitch 0.7 mm 0.9 mm 1.0 mm
Thread Depth (measured as'/2 of outside/inside 0.2025 mm 0.2575 mm 0.2810 mm
d i ameter-d i fferenti al )
Angular Range (at upper surface) 200 200 190
Angular Range (at bone-contacting surface) 260 240 250
Shape of Threaded Portion Conical Conical Conical
Referring to FIG. 5, combination hole 4C is shown with a bone screw 11
received therein. The bone screw preferably has a screw head 13 wi-th a thread
12
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disposed thereon. As shown in FIG. 5, thread 12 may substantially match thread
5 of
combination hole 4C. Preferably, bone screw 11 is self-drilling and/or self-
tapping.
While preferred embodiments and features of the present invention have
been disclosed herein, it will be appreciated that numerous modifications and
embodiments may be devised by those skilled in the art. It is intended that
the appended
claims cover all such modifications and embodiments as fall within the true
spirit and
scope of such claims and that the claims not be limited to or by such
preferred
embodiments or features.
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