Note: Descriptions are shown in the official language in which they were submitted.
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Title: Support orthopaedic device for a knee joint
DESCRIPTION
Field of application
The present invention relates to a support orthopaedic device for a knee
joint.
In particular, the device is of the type comprising a proximal connector
and a distal connector, articulated to each other and intended to be
respectively associated to a femoral bone and to a tibial bone of the lower
limb
of a patient, these bones being connected to each other through a knee joint.
Prior art
Knee joint compartments may be subject to surgical operations for
trauma or corrective treatment. Such operations, as in the general case of any
orthopaedic operation, are characterized by a post-operational period in which
the ossification process takes place; in this phase, it is generally necessary
to
ensure that the joint is not subjected to excessive loads.
The unloading of the joint may be obtained by total immobilization of the
knee; on the other hand, this solution has substantial therapeutic drawbacks,
and is therefore not often used.
A known alternative solution foresees the use of an orthopaedic device of
the type mentioned above, which is provided for allowing partial or total
transmission of loads through the operated joint.
In these cases, the proximal and distal connectors are generally
associated to anchoring devices, which are respectively associated to the
femoral and tibial bones of the patient.
A primary drawback of known orthopaedic devices is the articulation
between the two connectors. In fact the relative movement imparted to the two
connectors has to be unitary and above all has to reproduce in the most
faithful manner possible the movements of the anchoring bones, which are
physiologically determined by the natural joint. Such functional conditions
are
not ensured by simple and efficient constructive morphology. In fact, the
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coupling provided by the knee joint foresees a combination of rotations and
relative slipping among the two affected bones, whose kinematics are difficult
to reproduce with a reliable and small-sized mechanism. In fact, such a
coupling cannot be reproduced by a simple hinge having a fixed rotational
axis.
In any case, a system is provided which allows the distraction of the joint,
by
moving the articulation contact surfaces farther away from each other in order
to allow a minimal rotational movement, even if the device does not faithfully
reproduce traditional joint movement.
A second problem arising from the therapeutic use of orthopaedic devices
according to the known art is the precise positioning of the same device with
respect to the bone structure of the patient. In particular, it is of
fundamental
importance that the articulation of the two connectors is provided on a plane
that is normal to the articulation axis of the lower limb. The known anchoring
devices are associated, by means of screws, to suitable bone sites on the
femur
and tibia. The bone surface in these sites is not normal with respect to the
articulation axis; as a result there is no morphological reference that may
guide
the fixing operation of the orthopaedic device. Moreover, the attachment sites
are relatively remote from the joint compartment, further complicating the
problem regarding a proper positioning of the orthopaedic device.
The technical problem which the present invention has to solve is
therefore to provide an orthopaedic device having a structure that overcomes
said drawbacks of the known art.
Summary of the invention
This technical problem is solved by an orthopaedic device of above said
type, also comprising a first rod and second rod that are hinged to each other
along hinging axes that are normal to a median excursion plane of the
orthopaedic device 1 itself, to the proximal connector and to the distal
connector, so as to form with them a four-bar linkage. The four-bar linkage is
planarly mobile along a plane parallel to the excursion plane, between a main
configuration corresponding to an extended position of the knee joint and a
plurality of secondary configurations corresponding to different degrees of
bending of the knee joint; in particular, the relative motion imposed on the
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proximal and distal connectors by the four-bar linkage is consistent with the
physiological movement of the knee joint.
In general, the idea of the present invention is to reproduce the flexing
and extending movements of the knee joint by means of a four-bar linkage, two
opposed members of which are provided by the connectors. This constructive
solution allows for a reduced size and complexity of the orthopaedic device,
while on the other hand ensuring the relative movement of the different
elements to reproduce, in a sufficiently faithful manner, the physiological
movement of the knee joint.
An advantage of the invention is the fact that the device according to the
present invention is easily positioned and modularly assembled with other
known orthopaedic devices, ensuring a fast and precise implant.
Further characteristics and advantages will be more clearly understood
from the following detailed description of a preferred embodiment of the
present invention, which is non-limitingõ with reference to the attached
figures, which are illustrative but not limiting in scope.
Brief description of the drawings
Fig. 1 shows an isometric view of an orthopaedic device according to the
invention,
Fig. 2 shows an exploded view of various components of the orthopaedic
device of fig. 1,
Fig. 3 shows an isometric view of the orthopaedic device of fig. 1,
associated to a knee joint, which is extended by means of known anchoring
devices,
Fig. 4 shows an isometric view of elements represented in fig. 3, from a
different perspective,
Fig. 5 shows an isometric view of elements represented in fig. 3, with a
flexed knee joint,
Fig. 6 shows a front view of a tibia-femur group, to which the orthopaedic
device of fig. 1 is associated by means of known anchoring devices,
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Fig. 7 shows a diagram reproducing the kinematics of the four-bar
linkage facilitating articulation of the device of fig. 1.
Detailed description
With reference to the appended figures, reference numeral 1 indicates a
support orthopaedic device for a knee joint. The primary task of such a
device,
as already said, is to transmit the same loads that would otherwise act on the
joint, allowing for proper healing during the post-surgery period and at the
same time ensuring an at least partial mobility of the affected lower limb.
The orthopaedic device 1 comprises a proximal connector 2 and a distal
connector 3, which are articulated to each other and are provided to be
respectively associated to a proximal bone 100 and distal bone 101 of a lower
limb, which are connected to each other by means of the knee joint. In
particular, in the preferred embodiment of the appended figures the proximal
bone 100 is the femur of the patient, whereas the distal bone 101 is
advantageously the tibia.
Said orthopaedic device 1 also comprises a first rod 4a and a second rod
4b, said rods 4a, 4b being hinged, along hinging axes that are normal to a
median excursion plane of the same device, to proximal connector 2 and distal
connector 3, in order to form a four-bar linkage with them.
The four-bar linkage, which is schematically shown in fig. 7, is therefore
comprised of a frame operatively provided by proximal connector 2, two
balance beams which are provided by rods 4a and 4b and by a push rod, which
is provided by distal connector 3.
The four-bar linkage is planarly mobile according to a plane which is
normal to the excursion plane between a main configuration corresponding to
an extended position of the knee joint and a plurality of secondary
configurations corresponding to different degrees of bending of the knee
joint;
the relative motion imparted to the proximal and distal connector 2, 3 by the
four-bar linkage is consistent with the physiological movement of the knee
joint.
As can be seen in attached figures 3-6, the proximal connector 2 of
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orthopaedic device 1 is provided for direct connection to a first anchoring
device 5, which is provided for being fixedly associated to the proximal bone
100, i.e. the femur; whereas the distal connector 3 is provided for direct
connection to a second anchoring device 6, provided for being fixedly
5 associated to the distal bone 101, i.e. the tibia.
The first anchoring device 5, in the preferred embodiment, comprises a
longitudinal rail 50, provided with at least a first clamp 51 which may be
associated to a femoral surface by means of an assembly of bone screws. The
bone screws are advantageously inserted into a femur, which, as previously
said, extends along a longitudinal axis x that is not perpendicular to the
joint
axis y of the limb (in particular, the angle between the two is approximately
81 ). The first clamps 51 may be adjusted with respect to the longitudinal
rail
50. The attachment site of the bone screws is preferably located on a lateral
surface of the femur; therefore device 1, which is rigidly fixed to the distal
end
of the longitudinal rail 50, is in a lateral position with respect to the
lower limb.
In an alternative embodiment of present invention, not shownõ the first
anchoring device 5 may comprise a ring, or a ring portion, which is connected
to proximal connector 2.
As can readily be seen in fig. 6, a compressor 70 may be associated to
first anchoring device 5, in order to distract both distal 3 and proximal
connectors 2, in order to create a space in the joint. Once the joint has been
distracted, indicatively by 2-10 mm, the compressor may be removed.
The second anchoring device 6 comprises a rigid half ring 60 and a
second clamp 61 that can be associated to a tibial surface by means of bone
screws. Thanks to the provision of the rigid half ring it is possible to
associate
the bone screws of the second clamp 61 to the front surface or middle-front
surface of the tibia, taking advantage of the most suitable sites for such an
implant.
As discussed before, for a proper operation of the orthopaedic device the
excursion plane of the device has to be normal to the articulation axis of the
lower limb, identified by x in fig. 6. In order to ensure these conditions of
the
orthopaedic device 1, it comprises means for precise positioning of at least
one
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of the proximal or distal connector 2, 3 with respect to the corresponding
bone,
i.e. the femur for proximal connector 2 or the tibia for distal connector 3.
In particular, said means for precise positioning comprise a first portion
20a of one of the connectors 2, 3, said portion 20a having at least a
reference
hole 20 to temporarily receive a guide thread 200 which is fixed to the bone
of
the corresponding connector; advantageously, the reference hole 20 is normal
to the excursion plane.
In the preferred embodiment shown, the first portion 20a is provided on
the proximal connector 2, and the reference hole 20 is positioned on a distal
end of connector 2, with respect to the proximal bone 100 to which it is
associated. In this way, as shown in fig. 4, the guide thread 200 is suitably
introduced into a cannula screw that is inserted in the medial epicondyle 102
of the patient. By means of morphological references near the epicondyle the
cannula screw can be readily positioned so that it is parallel to the
articulation
axis y of the knee. The guide thread 200 is also parallel to the articulation
axis
y and its insertion into the reference hole 20 causes the excursion plane of
orthopaedic device 1 to be orthogonal to this axis. The guide thread 200 has
no
structural purpose and is removed once the orthopaedic device 1 has been
implanted.
From an operational point of view, a method for fixing the device
according to the invention to the bone structure of a patient comprises the
following steps: an insertion step of a thread in the femoral channels in a
direction parallel to the articulation axis y of the limb, followed by
anchoring of
the proximal connector 2 and the distal connector 3 to the bone structure of
patient, by means of said anchoring means 5, 6.
The diagram of fig. 7 shows the positions of the members forming the
four-bar linkage in the case of the main configuration and of a secondary
configuration corresponding to bending at a substantially right angle of the
knee joint . In the diagram the trajectory of a point of the distal connector
3
corresponding to a change of the bending angle of the tibia, with a fixed
position of the proximal connector, is also shown. Such a trajectory, which is
substantially the same as the trajectory provided by the articular movement of
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a healthy knee, is obtained by the following constructive means.
First of all, the four-bar linkage used is a Grashof mechanism, in which
the relative excursion of links is however operatively limited at least by the
anchoring to the bone structures of the lower limb.
The first and second rod 4a, 4b which comprise the four-bar linkage,
cross each other at least in their main configuration. In particular, the four-
bar
linkage remains inverted for the better part of its operative configurations,
wherein by operative configurations one refers to the main configuration and
secondary configurations which are operatively feasible. The inverted position
of the four-bar linkage allows the instant centre of rotation of the device to
be
held inside the four-bar linkage's volume, thereby constraining the instant
axis
of rotation near the knee joint. In fact, as per Chasles' theorem, the instant
centre of rotation is at the crossing C of the longitudinal axes z', z", shown
in
fig. 7, of the two rods 4a, 4b, which form the balance rods of the four-bar
linkage.
In particular it would be advantageous if an inversion configuration, i.e.
a dead point configuration, were to form part of the group of secondary
configurations; in this case such a configuration corresponds to a bending at
a
substantially right angle of the knee joint.
Advantageous dimensional characteristics of the four-bar linkage
according to the invention comprise: a ratio of the distance between hinging
axes and the first and second rods 4a, 4h of distal connector 3 and the
distance between hinging axes and the first and second rods 4a, 4b of the
proximal connector 2 higher than 1, and preferably comprised between 3,5 and
4,3 (the value of the ratio in the illustrated embodiment is 43/11); a ratio
of the
distances of the opposed hinging axes in relation to first and second rods 4a,
4b between 0,9 and 1,1 and preferably equal to 1, as in the illustrated
embodiment; ratio of distances of the opposed hinging axes of first rod 4a and
the distance between the hinging axes of first and second rods 4a, 4b of
distal
connector 3 higher than 1 and preferably lower than 1,2 (the value of the
ratio
in the illustrated embodiment is 47/43). Obviously, all these values are
absolutely non-limiting with reference to the present invention.
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The orthopaedic device 1 according to the invention advantageously
comprises: first locking means 30 provided for locking the four-bar linkage in
the main configuration or in any one of its secondary configurations; second
locking means 34 provided for locking the four-bar linkage in a finite
plurality
of possible locking configurations; stop means 37 adjustable for unilaterally
limiting the excursion of the four-bar linkage in order to inhibit the bending
of
the knee joint to which the device 1 is associated.
At least one of first and second rod 4a, 4b comprises a first transversal
threaded hole 40a, with axis normal to the excursion plane of device 1; the
first
locking means 30 comprise a locking screw 31, insertable into the first
transversal hole 40a and into a slot 32 of a first septum 33, integral with
one of
the distal or proximal connectors 2, 3. Therefore the locking screw 31 may be
clamped against the wall of the first septum 33 peripheral to slot 32,
allowing
the four-bar linkage to be locked in a desired configuration.
At least one of the first and second rods 4a, 4b features a second
transversal hole 40b with axis normal to the excursion plane of the device 1.
The second locking means 34 comprise a second septum 36 integral with one
of the distal or proximal connectors 2, 3, which features a plurality of
locking
holes 35a, 35b (although, as said, the real locking of the device is
preferably
obtained by means of locking screw 31 of first locking means 30, and these
holes are mainly used as references for the degree of articulation bending).
Each of the locking holes 35a, 35b is aligned with the second transversal hole
40b at a possible locking configuration. In these configurations, it suffices
to
insert a pin through the two holes to lock the rod to the connector, therefore
locking the four-bar linkage in the desired locking configuration.
In the preferred embodiment shown, the two locking holes 35a, 35b allow
the four-bar linkage to be locked in above said main configuration, and in a
secondary configuration corresponding to a right angle bend of the limb
associated with device 1.
The stop means 37 comprise a stop screw 38, insertable into a threaded
stop hole 39. The stop hole 39 is integral with one of the connectors 2, 3 and
has an axis which is parallel to the excursion plane of device 1. The end of
the
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stop screw 38 is provided in order to interfere with the movement of at least
one of the rods 4a, 4b through contact, limiting the excursion of the four-bar
linkage. In this case, it is provided for interfering with the rotation of the
second rod 4b.
A rotation of the locking screw 38 about its axis corresponds to its
traversing along the axis of locking hole 39; in this way it is possible to
modify
the position of the end of the screw provided for limiting the angular
excursion
of rod 4a, 4b, by adjusting the maximum bending angle of the device.
Advantageously, the screw and the four-bar linkage are formed in order to
provide a substantially linear relationship between the angle of rotation of
the
screw and variation of the maximum excursion angle of the limb associated to
the device 1.
Structurally, the first septum 33, the second septum 36 and stop hole 39
are all integral with the distal connector 3. In particular, this distal
connector
comprises a base 3a, from which two lateral walls laterally rise, forming a
hinging bracket for first rod 4a and second rod 4b. These lateral walls are
parallel to the excursion plane of device 1 and are contiguous to both hinged
rods 4a, 4b. The first septum 33 and second septum 36, as shown in the
appended figures, are part of one of these walls.
The first transversal hole 40a is provided on the first rod 4a, whereas the
second transversal hole 40b is provided on the second rod 4b. The rods are
preferably identical, with a first segment hinged to distal connector 3,
having a
transversal extension equal to the distance between the lateral walls of the
connector; a second segment, hinged to proximal connector 2, having a
reduced transversal extension in order to allow the two members to cross
without interference.
Slot 32 has a circular arc shape with centre on the hinging axis of first
rod 4a to distal connector 3. The terminal portion of locking screw 31, when
unclamped, slides within this circular arc; in any case, slot 32 limits the
angular excursion of first rod 4a.
The two lateral walls of distal connector 3, which are inferiorly joined by
base 3a, are joined on the upper side by cross beams 3b. Such cross beams are
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lateral with respect to the free central space inside of which the first and
second rod 4a, 4b may move.
One of the beams 3b, in particular the one nearer to the second rod 4b,
has a stop hole 39 for inserting the stop screw 38. This beam also has a hole
5 with an axis that is normal to that of the stop hole 39, inside which a
screw is
inserted that interacts with the stop screw 38 in order to lock it in the
desired
position.
The base 3a of the distal connector 3 is provided for association with the
above-mentioned rigid half ring 60, which is mounted in a distal position
10 underneath the base 3a. To this end, the base 3a provides suitable
connecting
means, i.e. a central hole 300 on the bottom of the bracket defined by the
lateral walls, as well as longitudinal slots 301 provided on a portion of the
base
which is not surmounted by lateral walls. These openings allow insertion of
fixing screws of the rigid half ring 60, as shown in figs. 3-6.
The proximal connector 2 comprises, at its distal end (always referring to
the femoral bone, to which it is associated) two side-by-side brackets, into
which the second segments of first rod 4a and second rod 4b are inserted and
hinged, according to parallel but offset axes. Reference hole 20, which is
preferably a through hole, is provided on the distal end of both brackets, in
a
lateral position with respect to the central free space inside of which first
and
second rod 4a, 4b may move.
The proximal portion of the proximal connector 2 is advantageously
arranged in such a way as to be associated to a longitudinal rail 50 of above
said type. To this end, it comprises suitable connection means, i.e. two
shaped
and parallel grooves 21 and at least one main threaded seat 22 for inserting a
locking member 23 of the longitudinal rail.
Obviously, in order to meet contingent and specific needs a person
skilled in the art may introduce various modifications and variations to above
said orthopaedic device, which are all within the scope of protection of the
invention, as defined by the following claims.
