Note: Descriptions are shown in the official language in which they were submitted.
CA 02574838 2007-01-23
SCREW
The invention relates to a screw of the introductory portion of claim
1, particularly to a locking screw for an intramedullary nail with transverse
boreholes.
The locking of intramedullary nails is prior art. Locking screws are
introduced into the transverse boreholes of an intramedullary either with the
help
of an imaging process (x-ray control) or of a more or less complicated
targeting
device. In either case, a certain targeting accuracy is unavoidable, that is,
the tip
of the screw cannot be aligned precisely coaxially with the center axis of the
transverse borehole and, instead, deviates from this by a certain amount. So
that the locking screw ends up in the transverse borehole and can be brought
through the latter in spite of this targeting error, the external diameter of
the
screw is underdimensioned relative to the diameter of the transverse borehole.
If
this targeting inaccuracy remains within the boundaries of this
underdimensioning, the locking screw can be guided through the transverse
borehole without problems in despite of the targeting error. However, because
of
the underdimensioning, the locking screw now has a certain clearance relative
to
the transverse borehole.
This clearance defines the amount, by which the main bone
fragments, which are to be fixed by means of locking screws in the
corresponding locking hole, can move relative to the nail and, accordingly,
because of the rigidity of the nail, also relative to the other main bone
ffagments,
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CA 02574838 2007-01-23
which are to be fastened with the same nail. Together with the flexibility of
the
material and of the overall device, this may cumulatively assume a magnitude,
which prevents or substantially delays successful healing. Admittedly, in
order to
guarantee the applicability of the locking for the surgeon, this clearance is
unavoidable. Clinically, however, it is undesirable for certain indications,
such as
in the case of metaphysical fragments.
Even nails with a solid cross section, which may have internal
thread in the locking hole, are not free of clearance. The internal thread
only
prevents axial movement of the nail on the locking screw.
The invention is to provide a remedy here. It is an object of the
invention to provide a locking screw, with which the existing clearance
between it
and the transverse borehole in a locking intramedullary nail can be eliminated
and a non-positive tension between the screw and the intramedullary nail can
be
produced.
Pursuant to the invention, this objective is accomplished by a
locking screw, which has the distinguishing features of claim 1.
The concept of the central line is defined by the line connecting the
centers of gravity of the axially successive orthogonal cross-sectional areas
of
the core of the screw without taking into consideration any cavities and
disregarding the external thread.
The advantages, achieved by the invention, may be seen to lie
essentially therein that, due to the inventive locking screw, the clearance
between the transverse borehole of the intramedullary nail and the locking
screw
can be eliminated. The following are further advantages:
= the insertion accuracy and the time expended by the surgeon remain within
the previous limits;
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= the strength of the locking screw is retained;
= should it break, the screw can be extracted;
= the weakening of the screw core, brought about by the cavities, facilitates
the
advance of the latter through a constricted site, such as a transverse
borehole, of an intermedullary nail, because the screws, which do not have a
straight, central line, can be deformed radially to a certain degree.
For a special embodiment of the invention, the screw core has a
cavity in the form of an axial cannulation, which passes preferably
continuously
through the screw, so that a guiding wire may also be used.
The central line of the screw may extend helically or spirally,
resulting in an axial displacement of the screw, which leads to a gradual
elimination of the clearance and a gradual build-up of the tension.
The central line may lie in a single plane, in which case it is
particularly easy to manufacture the screw.
Preferably, a marking is provided at the head end of the screw.
Advantageously, this marking shows the orientation of the plane, in which any
points of inflection of the central line are located.
The central line may also not lie in one plane. By these means, it
becomes easier for the screw to wind its way through the borehole and the
stress
points do not lie on one line.
The central line may be formed, for example, of several straight
lines, which are misaligned with respect to one another.
In the case of a special embodiment, the circumferential surface of
the screw core is not formed from a family of parallel straight lines.
Accordingly,
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the circumferential surface correspond not to the circumferential surface of a
straight circular cylinder but to that of a circular cylinder, which is
deformed
according to the central line, which is not rectilinear.
The volume V,,o,, of the screw core having one or more cavities
advantageously amounts to less than 95% of the volume Vf,,, of the body
surrounded by the circumferential surface of the screw core. Typically, Vho,i
<
0.90 Vf,i, and preferably Vha,i < 0.80 Vf,ii.
With the exception of any axial exit of a cavity at the tip of the
screw, the cavities in the screw core should penetrate advantageously not more
than 1% and preferably 0% of the circumferential surface.
The tip of the screw may taper and the head end may have means
for driving the screw.
In the case of a special embodiment, the screw core does not have
a rotational axis of symmetry.
For a further embodiment, a straight line, extending between the
center of the head end and the center of the tip, is at a distance x > 0 from
the
central line at least in placers. Advantageously, the distance "x" fulfills
the
condition that 0.02 d < x < 0.6 d and preferably the condition that 0.1 d < x
< 0.4
d, "d" corresponding to the diameter of the external thread. At least on a
partial
axial segment, the distance "x" should advantageously be greater than 0.05 mm
and preferably greater than 0.50 mm.
For a further embodiment, the central line, extending between the
head end and the tip, has a length L and is divided into three sections A, B
and
C,
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A) section A extending from the head end (5) by the amount of 0.10 L
to 0.25 L in the direction of the tip,
B) section B extending from the tip the amount of 0.10 to 0.25 L in the
direction of the head end,
C) section C being disposed between the sections A and B and having
the length C = (L - A - B) and
D) the central line in the sections A and B extending essentially in a
straight line and being co-axial with one another.
This results in the advantage that the locking takes place in the opposite
corticalis by the rotational movement about the central line and that the
locking in
the corticalis at the head end takes place essentially with the proper axial
alignment.
In the case of a further embodiment, the central line is curved S-
shaped or eccentric preferably only in section C. Moreover, preferably it has
a
point of inflection only in section C. In the case of a further embodiment,
the
central line has at least two points of inflection at a distance of y from one
another preferably only in section C.
In the case of the special embodiment, the wall thickness W of the
screw core in the region of the cannulation is larger than 0.05 d and
preferably
larger than 0.1 d. Advantageously, the wall thickness W of the screw core in
the
region of the cannulation is less than 0.8 d and preferably less than 0.6 d.
The inventive screw can be used together with a locking
intramedullary nail, which has at least one transverse borehole, which has a
cross-sectional profile P with a maximal extent "a" measured in the direction
of
the central line and a maximum extent b measured perpendicular to "a", with
the
proviso that, on the one hand, a> b and, on the other, a > d < b. The cross-
sectional profile P, however, may also be circular with a = b. Advantageously,
the condition that 0.70 b < d < 0.95 b and preferably 0.8 b < d < 0.9 is
fulfilled.
CA 02574838 2007-01-23
The distance x advantageously fulfills the condition that x < (b - d +
1 mm), "b" being the diameter of the transverse borehole in mm and "d" the
diameter of the external thread in mm.
The distance x advantageously should fulfill the condition that 0.5(b
- d) < x< 0.35(b - d) and preferably the condition that 1.5(b - d) < x <2.2(b -
d),
"d" being the diameter of the external thread in mm.
The diameter D should fulfill the condition that 0.02 d < D < 0.60 d
and preferably the condition that 0.1 d < x < 0.4 d, "d" being the diameter of
the
extemal thread.
The distance y between two adjacent points of inflection should
essentially fulfill the condition that D = ny, n being an odd number and D the
diameter of the intramedullary nail.
The invention and further developments of the invention are
explained in even greater detail in the following by means of partially
diagrammatic representations of several examples, of which
Fig. 1 shows a perspective view of an inventive screw,
Fig. 2 shows a longitudinal section through the screw of Fig. 1 and
Fig. 3 shows a longitudinal section through a locking intramedullary nail with
a
transverse borehole, into which the screw of Fig. 1 has been introduced.
The screw, shown in Figs. 1 and 2, comprises a screw head 2 with
a hexagon socket 8, a screw core 3 with an external thread 7 and a central
line 4,
which extends from the.head end 5 to the tip 6 of the screw. The central line
4 is
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not a straight line, as it is in the case of the usual straight screws.
Instead, in the
example shown, in the length region C, which is defined below, it consists of
a
serpentine line, which lies in the plane of the drawing and has two points of
inflection 11, 12. The straight line 15, passing through the center of the
head end
and of the tip 6, therefore deviates in places from the central line 4 by the
variable amount x. In the example shown, the maximum deviation of x,
measured at the points of inflection of the central line 4, is 0.2 mm.
Between the center of the head end 5 and that of the tip 6, which
are at a distance L from one another, the central line 4 is divided into three
sections A, B and C. Section A extends from the center 5 at the screw head 2
by
the amount of 1/6 L in the direction of the tip 6 at the free end of the screw
core 3
in essentially a straight line. Section B extends from the tip 6 at the free
end of
the screw core 3 by the amount of 1/6 L in the direction of the center of the
end 5
of the screw head 2 and also extends essentially linearly as well as coaxially
with
section A. Section C is disposed between sections A and B and has the
curvature described above.
Instead of being serpentine or helical, the cavity 13, in the form of a
cannulation extending axially, may also be linear. Such a shape Is easier to
produce.
Fig. 3 shows how the screw 1 is introduced into the transverse
borehole 9 of an intramedullary nail 10. The external thread 7 of the screw
core
3 has a diameter "d" (Fig. 2), which is smaller than the dimension "a" of the
transverse borehole 9.
The technique of screwing the locking screw into the transverse
borehole of an intramedullary nail is explained briefly in the following:
a) the surgeon screws the screw 1 in the usual manner through the transverse
borehole 9 of the intramedullary nail 10;
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=
b) the relatively thin and soft corticalis yields as the screw 1 wriggles its
way
through, so that there is no strain over the thickness of the corticalis;
c) in the region of the intramedullary nail 10, the screw 1 is stretched and
strained somewhat because of the reaction of the wall of the transverse
borehole, so that a greater force is required to screw in the screw and an
increased holding force results;
d) the screw 1 wriggles through the inlet opening of the transverse borehole 9
of the intramedullary nail 10. Since the diameter D of the intramedullary
nail 10 is larger than the distance y between the two points of inflection 11,
12, the screw 1 is braced in the transverse borehole 9. Due to the
screwing-in torque or due to the force exerted by the surgeon on the screw
1, the latter is forcibly deformed elastically, when it takes hold of the
opposite corticalis. This leads to additional bracing of the screw 1 and to an
angularly stable locking of the intramedullary nail 10.
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