HK1178040A - Improved orthopaedic device - Google Patents

Description

Improved orthopedic device

Technical Field

The present invention relates to an improved orthopaedic device, and in particular to an artificial orthopaedic device designed to assist or replace a damaged joint.

Background

Artificial orthopaedic devices are widely used in the field of medical surgery, broadly divided into external prostheses or limb prostheses, orthotics or orthoses and internal prostheses or joint prostheses.

In a joint (meaning here any common joint, although reference to the knee joint is preferred for convenience) there is contact between the joint surfaces which move relative to each other.

This relative motion is substantially defined by relative rolling motion and relative sliding motion between the articular surfaces.

Relative motion is ensured by joint structures such as ligaments, muscles and cartilage tissue that hold the joint surfaces in the correct relative position.

Endoprostheses can be used to restore joint function when the joint surface is damaged due to degenerative disease.

Joint prostheses attempt to replicate and replace degenerated natural joint surfaces with cams, working surfaces, and intermediate elements (e.g., plastic inserts) that replicate the relative motion of the natural joint.

In particular, replication of the original motion will depend in part on the shape of the mating surfaces that come into contact in the prosthesis (which are configured to mimic the typical shape of natural surfaces), and in part on the natural joint structure that is not removed during the surgical procedure required to implant the prosthesis.

Indeed, it should be noted that during the implantation of a joint prosthesis, the primary joint structure at the joint to be replaced (for example the cruciate ligament in a knee prosthesis) is often cut in order to allow the insertion of the prosthesis.

In many cases, the prosthesis is inserted in a state that does not allow the optimal relative position between the articular surfaces to be maintained.

In general, in most prior art prostheses, the mating surfaces that come into contact during movement will slide against each other, resulting in rapid wear due to the resulting friction and high unit loads.

To overcome these drawbacks, prostheses such as those described in international patent application WO2007/074387 have been developed. These prostheses are based on the particular shape of the mating surfaces. In particular, the co-operating surfaces described in the international patent application are defined as part of a regular surface (ruledssurface) having a curved directrix with polar coordinates of planar relative motion as directrix, or are made to coincide with an centroidal surface of spherical relative motion.

The planar or spherical relative motion of one degree of freedom approximates the actual spatial motion of a natural joint, so that the relative motion between the centroidal surfaces is only a rolling motion, with no sliding motion.

However, such prostheses described in the above-mentioned application WO2007/074387 require more removal of bone material from the femur, particularly in knee prostheses.

Further different types of prior art prostheses are described in documents US-A-5413604, WO2007/119173 and US-A-3969773.

Document WO2007/119173 describes a prosthesis comprising a spherical joint with three degrees of freedom, which is located in the centre of the medial condyle and is combined with a trajectory that does not limit the movement of the prosthesis along the setting path. As a result, the prosthesis does not closely approximate the one degree of freedom of motion of the natural knee joint and does not completely recreate the anatomical constraints of the joint, particularly when the anatomical knee structure is damaged or resected or removed to enable insertion of the prosthesis. Thus, the joint is less controlled than in the natural case, resulting in relative motion that is less anatomically correct, while placing unnatural loads on other parts of the knee structure.

The prosthesis described in document US-A-5413604, which also defines A motion with too many degrees of freedom, comprises A spherical joint with three degrees of freedom, which is located in the centre of the lateral condyle.

Document US-A-3969773 relates to A prosthesis configured to define A planar movement between two circular surfaces, the centres of which are always equidistant, and connected by two rigid parts and/or by two links.

The two surfaces effectively roll over each other, but the planar motion produced according to the proposed solution does not replicate the actual joint motion with sufficient accuracy.

Moreover, the two links do not completely limit the rotation of the prosthesis around the common normal of the contact surfaces and require additional connecting parts, which however makes the choice of a circular rolling surface important.

Disclosure of Invention

In this context, the main technical purpose of the present invention is to provide an orthopaedic device that is superior to the prior art devices.

The object of the present invention is to provide an orthopaedic device which reproduces as closely as possible the natural movements of the joint to be replaced and which is also easy to implant.

Drawings

Further characteristics and advantages of the invention will become clearer from the following non-limiting description of a preferred and non-exclusive embodiment of an orthopaedic device, illustrated in the attached drawings, wherein:

FIG. 1 is a schematic illustration of a knee joint used to define the prosthesis of the present invention;

FIG. 2 shows a schematic front view of a knee prosthesis of the invention;

FIG. 3 is a schematic front view showing a first preferred embodiment of the prosthesis of the present invention;

FIG. 4 is a schematic front view illustrating a second preferred embodiment of the orthopedic device of the present invention;

FIG. 5 is a schematic front view illustrating a third preferred embodiment of an orthopedic device of the present invention;

FIG. 6 is a schematic perspective view of a detail of the prosthesis of FIG. 5;

FIG. 7 is a schematic front view illustrating a fourth preferred embodiment of the orthopedic device of the present invention; and

fig. 8 schematically shows a detail of the orthopedic device of fig. 7.

Detailed Description

With reference to the accompanying drawings, and in particular to figures 3, 4, 5 and 7, numeral 1 indicates an orthopaedic device, or more generally a prosthesis, according to the invention.

As described in patent application WO2007/074387 (which is incorporated herein in its entirety), the movement of the joint can be approximated by a spherical movement, that is to say, reference can be made to a series of instantaneous rotations about axes of variable direction passing through points fixed on the two components.

In general, the spherical motion defined by the spherical pair has three degrees of freedom. However, spherical movements similar to knee movements (this description refers in particular to knee movements, but does not limit the scope of the invention) have only one degree of freedom.

As shown in fig. 1, the joint 2 can therefore be represented schematically with reference to an equivalent spatial mechanism with one degree of freedom.

By way of non-limiting example, the knee 3 (fig. 1) can be substantially referenced to a model 100, the model 100 comprising a first bone segment 4 connected to a second bone segment 5 by a spherical kinematic pair 6.

The actual motion of the knee 3 is constrained by natural anatomical ligament structures, such as cruciate ligaments and collateral ligaments, so that an approximately spherical motion can be considered to have only one degree of freedom.

For the sake of simplicity, the spherical pair described in the following description means a spherical kinematic pair or kinematic pair which allows three degrees of spatial freedom, that is to say rotation about three axes passing through the centre of the kinematic pair itself.

The bone segments 4 and 5 are constrained to each other by a first rod 7 and a second rod 8.

The first rod 7 is connected to the first bone segment 4 at a first anchor point 7a and to the second bone segment 5 at a second anchor point 7 b.

The second rod 8 is connected to the first bone segment 4 at a first anchor point 8a and to the second bone segment 5 at a second anchor point 8 b.

The first and second rods 7, 8 of the model 100 represent the primary natural anatomical ligament structure.

By way of a particular non-exclusive and therefore non-limiting example, the first bar 7 and the second bar 8 may represent two cruciate ligaments in the model 100. In a further particular non-exclusive and therefore non-limiting example, the first rod 7 and the second rod 8 may represent a cruciate ligament and a collateral ligament in the model 100.

The representation of the cardinal ligament structures in the model 100 by the rods 7 and 8 allows the regeneration of equidistant structures (isometry) in the spherical model 100 and in the movements it generates, which exist in natural joint movements.

Thus, the one degree of freedom spherical motion produced by the model 100 can approximate natural knee joint motion and recreate an equidistant version of at least the joint cardinal ligament structure.

The helical axis R0 of the instantaneous rotation of the spherical motion, approximating one degree of freedom of the knee joint motion, is an axis defined by the intersection of a first plane pi 1, defined by the centre "O" of the spherical pair 6 and by the two points 7a, 7b anchoring the first rod 7 to the bone segments 4, 5, and of a second plane pi 2, defined by the centre "O" of the spherical pair 6 and by the two points 8a, 8b anchoring the second rod 8 to the bone segments 4, 5.

As the fixed point "O" continues to be passed during the relative movement, the instantaneous axis of rotation R0 will change its direction due to the relative movement between the components 4 and 5.

With reference to fig. 1, the geometrical loci of the positions occupied by the instantaneous axis of rotation R0 during movement are a first conical surface S1 for the first component 4 and a second conical surface S2 for the second component 5.

In other words, the first and second surfaces S1, S2 are regular surfaces having curved directrixes, wherein the generatrix R0 moves on the corresponding curved directrix passing through the fixed point (point "O", fig. 1) during the relative movement of the parts 4 and 5.

The first and second surfaces S1, S2 are centroidal surfaces (axoid) of spherical motion that can be moved with reference to the joint 2.

The movement of the joint 2 refers in particular to a series of instantaneous rotations about axes of variable direction passing through a fixed point "O".

The motion of the joint 2 is defined entirely by the rolling of the second conical surface S2 on the first conical surface S1.

It will be appreciated that the two instant facet conical surfaces S1, S2 of spherical motion have the same apex and are in contact at each instant along a generatrix which coincides with the instant axis of rotation R0 at a given instant.

It will be appreciated that in such a movement, the instant axial surfaces of the spherical movement rotate on top of each other without slipping.

The instantaneous axial surfaces S1, S2 of motion are derived from the spatial model 100 serving as a mechanism equivalent to the joint 2.

Referring to fig. 2, for the preferred embodiment of the knee joint 3, the first bone segment 4 is a femur and the second bone segment 5 is a tibia.

The motion instant planes S1, S2 are positioned such that as shown in fig. 2, the fixation center "O" must be considered to be positioned substantially at the medial condyle 9 of the femur 4.

As shown, the center "O" is located at a medial position relative to the sagittal midplane of the knee.

In more detail, the positioning of the point "O" is an optimization of the data obtained experimentally for the sample defining the spherical movement, which simulates the actual overall movement of the natural knee.

The first preferred embodiment of the orthopaedic device 1 according to the invention (shown in figure 3) comprises a first component 10 designed to be rigidly connected to the femur 4 and a second component 11 designed to be rigidly connected to the tibia 5.

As shown, the components 10 and 11 include working or loading surfaces 10b and 11b that contact each other.

Because the motion of the knee joint 3 approximates a spherical motion with one degree of freedom, the load surfaces 10b, 11b are preferably created by the centroconic surfaces S1, S2 of the spherical motion.

More specifically, surface 10b, for example in the form of a replica of the natural condyle, is attached to the instant facet S1.

Rotation of surface S1 on surface S2 (which is an instant surface of motion) creates surface 11b associated with surface S2 itself.

Alternatively, the load surface 10b is preferably in the form of a portion of a conical surface having a curvilinear directrix with its apex at "O", approximating a natural condyle.

Preferably, the load surface 11b produced using the above-described process is also part of a conical surface having a curvilinear directrix with its apex at "O".

In other words, the load surfaces 10b, 11b are in the form of regular conical surfaces.

The regular conical surfaces 10b, 11b are thus in contact along a section, more constrained than any other surface, and the contact pressure is lower than that of a normal contact surface.

Preferably, this procedure can be reversed with respect to the procedure for establishing the tibial plate (surface 11b) and for producing the mating surface 10b corresponding to the condyle of the femur, according to the same principles.

Thus, the mating load surfaces 10b, 11b are primarily used to transfer forces between the bone segments 4, 5.

Thus, the part 10 of the prosthesis 1 associable with the femur 4 comprises a loading surface 10b, and symmetrically the part 11 of the prosthesis 2 associable with the tibia comprises a surface 11b cooperating with the first loading surface 10b of the first part 10.

In the illustrated construction, the motion of the knee 3 is constrained by natural anatomical ligament structures such as the cruciate ligaments (labeled 20 and 21) and the collateral ligaments (labeled 30 and 31), so that the approximate spherical motion has only one degree of freedom.

In effect, the relative motion of the components 10 and 11 is performed as if the components were constrained by a spherical pair and one degree of freedom constrained by the "soft" or ligamentous natural anatomy.

In a second preferred embodiment, as shown in fig. 4, the orthopaedic device 1 of the present invention also comprises a first component 10 designed to be rigidly connected to the femur 4 and a second component 11 designed to be rigidly connected to the tibia 5.

The orthopaedic device 1 of the invention also comprises a spherical pair 17 to constrain the femur 4 and the tibia 5.

The spherical pair 17 is centred at the above-mentioned fixing point "O" in the condyle 9 of the femur.

In a second preferred embodiment, represented by way of example, the spherical pair 17 comprises a socket 18 formed in the part 11, in which socket 18 a spherical portion 19 formed on the first part 10 moves.

In other words, the convex spherical portion 19 fits into the spherical concave portion of the socket 18.

This connection provides three degrees of freedom between the femur 4 and the tibia 5, and two of these degrees of freedom must be eliminated in order to obtain a spherical motion with one degree of freedom.

Preferably, the orthopedic device shown in fig. 4 includes first and second rods or connecting elements 20, 21 secured to the first and second bone segments 4, 5 or the first and second components 10, 11.

It should be noted that the connecting elements 20, 21 are preferably realized by ligament portions of natural, artificial or biological nature.

The rods 20 and 21 serve to guide the relative movement of the parts 10 and 11, that is to say, to guide the device 1.

In more detail, the four anchoring points A, B, C, D are found experimentally so as to define, in combination with the spherical pairs 17, a spherical movement of one degree of freedom, so that the orthopaedic device 1 is able to reproduce the actual movement of the knee 3.

The anchor point A, C is located on the femur 4, while the anchor point B, D is located on the tibia 5, arranged as shown in fig. 4.

As shown, points A, B, C and D are located on each bone segment 4, 5 or on component 10 and/or 11 (no distinction).

The rod 20 is fixed to the femur 4 and to the tibia 5 at anchor points a and B.

The rod 21 is fixed to the femur 4 and to the tibia 5 at anchor points C and D.

Advantageously, the anchor points A, B are connected by the rod 20 in a manner that intersects the anchor points C, D connected by the rod 21.

Preferably, the anchor points A, B, C and D are located at a cavity 12, which cavity 12 itself extends into the femur 4 between the two condyles 9, 9'.

Rods 20 and 21 pass through the cavity 12 so as to cross each other and connect the respective anchor points.

Preferably, the rods 20, 21 are located at an intermediate position of the joint 2.

Advantageously, as mentioned above, the rods 20, 21 are preferably realized by a portion of ligament.

Importantly, the fixation rods 20, 21 preferably comprise a natural cruciate ligament (when the cruciate ligament is not at risk or damaged).

Generally speaking, the rods 20, 21 are anchored to the femur 4 and tibia 5 in substantially the same manner as a natural cruciate ligament.

The ligament portions only react to traction and therefore, in order to prevent them from loosening and in order to reproduce the natural movement, the orthopaedic device 1 comprises load surfaces 10b and 11 b.

Load surfaces 10b and 11b of the type described above are built (molded) into the first and second members 10 and 11, respectively, on the side of the anatomy opposite the spherical pair 17 relative to the rods 20 and 21.

Preferably, the first load surface 10b is associated with the bone segment 4 by means of a component 10 at the condyle 9 'which matches the shape of the condyle 9'.

In other words, surface 10b is formed substantially similarly to natural condyle 9', and surface 11b is preferably a mating surface created as surface 10b (spherical motion according to one degree of freedom described above).

Alternatively, the surface 10b is formed partially like a conical surface with curved directrix, with an apex at "O" and approximating the natural condyle 9'.

Surface 11b is created substantially as a matching surface (spherical motion according to one degree of freedom described above) to surface 10 b.

In practice, as shown in fig. 4, the orthopaedic device 1 comprises a spherical pair 17 at the first condyle 9, load surfaces 10b, 11b at the second condyle 9 ', and rods 20, 21 at the cavity 12 between the first and second condyles 9, 9'.

This embodiment of the device 1 enables to preserve a healthy part of the joint 2, such as a cruciate ligament, without having to cut or treat the entire joint area.

The prosthesis 1 therefore has a spherical connection (commonly known as a "ball joint") and two ligaments which serve to constrain its movement and to reduce the degrees of freedom from three to one.

Since these ligaments and the ball and socket are not able to support high compressive loads, the prosthesis 1 comprises matching load surfaces for supporting the load without hindering the movement of the device 1.

The third preferred embodiment of the orthopaedic device 1 according to the invention, shown in figures 5 and 6, also comprises a first component 10 designed to be rigidly connected to the femur 4 and a second component 11 designed to be rigidly connected to the tibia 5.

The first and second parts 10 and 11 comprise guiding and/or contact surfaces 10a, 11a of the respective device 1, which guiding and/or contact surfaces 10a, 11a work in contact with each other.

In other words, the surfaces 10a and 11a are the surfaces of the components 10 and 11 that contact each other and are designed to guide the orthopedic device 1.

Surfaces 10a and 11a are respectively represented as a portion of the above-mentioned conical surface S1 and a portion of the above-mentioned conical surface S2.

Preferably, the parts 10 and 11 are manufactured such that the contact surfaces 10a, 11a for guiding the joint are located between the two femoral condyles 9, 9'.

In practice, the surfaces 10a and 11a are made by selecting respective portions of the instant axial surfaces S1 and S2 of motion between the condyles 9, 9' of the femur 4.

This is the preferred position since we have the above-mentioned cavity 12 in the femur 4 extending between the two condyles 9, 9' of the femur 4 itself, this cavity 12 being very close to the instant planes S1, S2 of the spherical movement (the actual movement of the joint 2 approximates this spherical movement).

The specific selection of the contact surfaces 10a, 11a at this location minimizes the work required on the bone (on the femur 4 and tibia 5).

As best shown in fig. 6, means 13 for connecting the two surfaces 10a and 11a are also provided in this position.

These connection means 13 comprise flexible elements having the same shape and function as described in patent application WO2007/074387 and briefly described below.

The connecting means 13 comprise a series of flexible elements 14, 15 and 16, preferably of the laminar type.

The elements 14, 15, 16 are fixed on the guide surfaces 10a, 11a, also at the cavity 12.

In particular, the element 14 is fixed substantially at its first end to a portion of the component 11 at the surface 11 a.

The element 14 is also fixed substantially at its second end to a portion of the component 10 at the surface 10 a.

The element 14 is positioned partially on the surface 11a from the part of the component 11 where the element 14 is fixed up to the instantaneous axis of rotation at a given instant, and this element 14 partially surrounds the surface 10a from the instantaneous axis of rotation at a given instant up to the part of the component 10 where the element 14 is fixed.

When the tibia 5 moves relative to the femur 4, the flexible element 14 opens from the surface 10a and wraps around the surface 11a, or vice versa.

The element 15 is fixed substantially at its first end to a second portion of the component 10 at the surface 10 a.

The element 15 is also fixed substantially at its second end to a second portion of the component 11 at the surface 11 a.

The element 15 wraps partially around the surface 10a from the part of the component 10 where the element 15 is fixed up to the instantaneous axis of rotation at a given instant, and the element 15 partially surrounds the surface 11a from the instantaneous axis of rotation at a given instant up to the part of the component 11 where the element 15 is fixed.

When the tibia 5 moves relative to the femur 4, the flexible element 15 opens from the surface 10a and wraps around the surface 11a, or vice versa.

Preferably, the flexible element 15 is wrapped around the guide surfaces 10a, 11a in the same way as the flexible element 14.

The flexible element 16 is positioned symmetrically with respect to the flexible element 14 with respect to the flexible element 15.

The element 16 is fixed substantially at its first end to a portion of the component 11 at the surface 11a, on the opposite side of the flexible element 14 with respect to the flexible element 15.

The element 16 is also fixed to a portion of the component 10 at the surface 10a substantially at its second end, on the opposite side of the flexible element 14 with respect to the flexible element 15.

The element 16 is positioned partially on the surface 11a from the part of the component 11 where the element 16 is fixed up to the instantaneous axis of rotation at a given instant, and this element 16 partially surrounds the surface 10a from the instantaneous axis of rotation at a given instant up to the part of the component 10 where the element 16 is fixed.

When the tibia 5 moves relative to the femur 4, the flexible element 16 opens from the surface 10a and wraps around the surface 11a, or vice versa.

The elements 14, 15 and 16 thus arranged enable the surfaces 10a and 11a (that is to say the parts 10 and 11 of the prosthesis 1) to roll over one another without sliding.

The guide surfaces 10a, 11a, which roll over each other without sliding, are preferably suitably formed also taking into account the non-zero thickness of the flexible elements 14, 15, 16.

It will be appreciated that three flexible elements are required in this configuration because the centre point "O" of the spherical motion is virtual, that is not physically defined by a spherical joint as in the previous embodiments and the embodiments described in more detail below.

Thus, the presence of the three flexible elements enables the surfaces 10a and 11a (i.e. the components 10 and 11 of the prosthesis 1) to roll over each other without sliding, while preventing relative rotation of the components 10 and 11 about the common normal of the contact surfaces 10a and 11 a.

As shown particularly in fig. 5, the parts 9 and 10 comprise working or load surfaces 10b and 11b, which working or load surfaces 10b and 11b are in contact with each other and are substantially identical to those described above.

The presence of the load surfaces 10b and 11b is particularly advantageous because the surfaces 10a and 11a for guiding the joint 2 preferably have a reduced medial-lateral extension.

Preferably, the load surfaces 10b and 11b are constructed using conical centroidal surfaces S1, S2 that move, as described above.

More specifically, the surface 10b, which is made in the form of a replica of the natural condyle, is attached to the instant axial surface S1.

Rotation of surface S1 on surface S2 (which is an instant surface of motion) creates surface 11b which is connected to surface S2 itself.

Alternatively, the load surface 10b is preferably in the form of a portion of a conical surface having a curvilinear directrix with its apex at "O", approximating a natural condyle.

Preferably, the load surface 11b produced using the above-described process is also part of a conical surface having a curvilinear directrix with its apex at "O".

Thus, the mating load surfaces 10b, 11b are primarily used to transfer forces between the bone segments 4, 5.

Looking from left to right at fig. 5, the component 10 of the prosthesis 1, which is therefore associated with the femur 4, has: a first portion comprising a load surface 10 b; a central portion, constituted by the surface 10a, defined as a portion of the centroidal surface S1 of the spherical motion; and a third portion comprising a further load surface 10 b.

In a symmetrical manner, and looking again at fig. 5 from left to right, the part 11 of the prosthesis 2 associated with the tibia has: a first portion constructed as a mating surface 11b of a first load surface 10b of the first component 10; a central portion constructed as part of an instant surface S2 of spherical motion; and a third portion built as a mating surface 11b of the second load surface 10b of the component 10.

In fact, in this embodiment, the relative movement of the parts 10 and 11 is performed as if the parts were constrained by a spherical pair and as if the relative movement were constrained by the surfaces 10a and 11a (the surfaces 10a and 11a constituting guide surfaces which allow only one degree of freedom of the device 1).

Fig. 7 shows a fourth preferred embodiment of the orthopaedic device 1 according to the invention. In a fourth preferred embodiment, the orthopaedic device 1 according to the invention also comprises a first component 10 designed to be rigidly connected to the femur 4 and a second component 11 designed to be rigidly connected to the tibia 5.

The device 1 comprises a spherical connection, that is to say a spherical pair 22, the spherical pair 22 being centred at a fixed point "O" of spherical movement in the condyles 9 of the femur 4.

In the preferred embodiment represented by way of example, the spherical pair 22 comprises a socket 23 formed in the part 11, in which socket 23 a spherical portion 24 formed on the first part 10 moves.

In other words, the spherical pair 22 comprises a spherical convex portion 24 associated with the femur 4 and a corresponding concave portion of the spherical surface 23 associated with the tibia 5, wherein the first spherical surface 24 is engaged in the second spherical surface 23 so as to form the spherical pair 22.

In order to guide and constrain the spherical pair 22 and therefore the device 1, the device 1 comprises a second pair of surfaces 25, 26, the former being associated with the femur 4 and the latter with the tibia 5.

The mating surfaces 25, 26 are part of the conical surfaces S1, S2 described above, which conical surfaces S1, S2 are centroidal surfaces of spherical motion suitably replicated in the area.

In practice, the part 10 comprises a guide surface 25 and the part 11 comprises a guide surface 26 matching the surface 25.

In other words, the guide surfaces 25, 26 fulfill the same function as the guide surfaces 10a, 11a of the embodiment shown in fig. 5.

Advantageously, the surfaces 25, 26 are located laterally of the condyles 9', preferably remaining within the natural dimensions of the knee 3.

As shown schematically in particular in fig. 8, the conical surfaces 25, 26 are constrained to roll over each other by means of a connecting device 27.

The connection means 27 comprise a pair of flexible elements 28, 29.

The flexible elements 28, 29 are positioned so that the surfaces 25 and 26 can roll over each other without slipping.

It will be appreciated that this embodiment comprises only a pair of flexible elements, since the bone segments 4, 5 are also constrained to each other by the spherical pair 22.

Elements 28, 29 are fixed to the guide surfaces 25, 26, also at the sides of the condyle 9'.

In particular, the element 28 is fixed substantially at its first end to a portion of the component 11 at the instant axial face 26.

The element 28 is also secured at substantially its second end to a second portion of the component 10 at the instant axial face 25.

The element 28 is positioned partially on the surface 26 from the part of the component 11 where the element 28 is fixed up to the instantaneous axis of rotation at a given instant, and this element 28 partially surrounds the surface 25 from the instantaneous axis of rotation at a given instant up to the part of the component 10 where the element 28 is fixed.

As the tibia 5 moves relative to the femur 4, the flexible element 28 opens from the surface 25 and wraps around the surface 26, or vice versa.

The element 29 is fixed substantially at its first end to a second portion of the component 10 at the surface 25.

The element 29 is also fixed substantially at its second end to a second portion of the component 11 at the surface 26.

The element 29 wraps partially around the surface 25 from the part of the component 10 where the element 29 is fixed up to the instantaneous axis of rotation at a given instant, and the element 29 wraps partially around the surface 26 from the instantaneous axis of rotation at a given instant up to the part of the component 11 where the element 29 is fixed.

As the tibia 5 moves relative to the femur 4, the flexible element 28 opens from the surface 25 and wraps around the surface 26, or vice versa.

Preferably, the flexible element 29 is wrapped around the guide surfaces 25, 26 in the same manner as the flexible element 28.

Preferably, the guide surfaces 25 and 26 and the flexible attachment means 27 are located on the condyle 9' side, while the spherical pair 22 is located at the first medial condyle 9.

The orthopedic device 1 shown in fig. 7 includes a pair of mating load surfaces 10b, 11 b.

Preferably, the surfaces 10b, 11b are located at the condyles 9' and are in the form of mating surfaces.

Preferably, as described in connection with the other embodiments, the surface 10b is made to reproduce the femoral condyle, and the corresponding surface 11b is produced as a matching contour of the surface 10b according to a spherical movement (which approximates the actual spatial movement of the knee 3).

Alternatively, the load surface 10b is preferably in the form of a portion of a conical surface having a curvilinear directrix with its apex at "O", which approximates a natural condyle.

Advantageously, the load surface 11b produced using the above-described process is also part of a conical surface with a curvilinear directrix with its apex at "O".

In fact, in this embodiment, the relative movement of the parts 10 and 11 is defined by the spherical pair 22 and constrained by the surfaces 25 and 26 so that the device 1 has only one degree of freedom left.

The third and fourth embodiments described above are particularly advantageous when used as a second implant or revision implant.

In some cases, when the primary prosthesis fails or the joint is severely damaged, it is actually necessary to implant a prosthesis with more constraint in a second operation in order to replace the first prosthesis with a second prosthesis.

The invention described above has industrial applications and can be varied and modified in many ways without thereby departing from the scope of the invention. Moreover, all the details of the invention may be substituted by technically equivalent elements.

Claims (20)

1. An orthopedic device for articulating first and second bone segments (4, 5) to form a joint (2), the orthopedic device comprising: a first component (10), said first component (10) being associable with a first bone segment (4); a second component (11), said second component (11) being associable with a second bone segment (5); -connecting means (13, 17, 20, 21, 22, 27, 30, 31) for connecting the first component (10) and the second component (11) in order to connect the first and second components (10, 11) to each other; -constraining means (10a, 11a, 17, 20, 21, 22, 25, 26, 30, 31) for constraining the relative movement between the first component (10) and the second component (11) to cause the first and second components (10, 11) to undergo a spherical relative movement with one degree of freedom, the orthopaedic device being characterized in that: -the first and second parts (10, 11) are virtually or physically connected by a spherical kinematic pair centred at a point (O) which is the centre of the spherical motion approximating the actual relative spatial motion between the first (4) and second (5) bone segments, by means of connecting means (13, 17, 20, 21, 22, 27, 30, 31) and constraining means (10a, 11a, 17, 20, 21, 22, 27, 30, 31), the constraining means (10a, 11a, 17, 20, 21, 22, 25, 26, 30, 31) limiting the spherical kinematic pair to one degree of freedom, further characterized in that: spherical motion is obtained from a model (100) comprising a first part connected to a second part by a spherical kinematic pair (6) and by a first and a second rod (7, 8) for connecting the first and the second part, the spherical kinematic pair (6), the first rod (7) and the second rod (8) defining an instantaneous axis of rotation (R0), the geometric trajectory of the positions occupied by the instantaneous axis of rotation (R0) during motion being a first regular surface (S1) having a curvilinear guideline for the first part (4) and a second regular surface (S2) having a curvilinear guideline for the second part (5), the first and second regular surfaces (S1, S2) having curvilinear guidelines being axial planes of the spherical motion movable with reference to the joint (2).

2. The orthopedic device according to claim 1, characterized in that: the first part (10) has at least one first load surface (10b) and the second part (11) has a second load surface (11b) which matches said first load surface (10b) in a spherical movement which approximates the actual relative spatial movement between the first (4) and second (5) bone segments.

3. The orthopedic device according to claim 2, characterized in that: the first load surface (10b) is located substantially at the condyle (9) of the first bone segment (4).

4. The orthopedic device according to any one of the preceding claims, characterized in that: the first part (10) has a third load surface (10b) at a second condyle (9') of the first bone segment (4), and the second part (11) has a fourth load surface (11b) which matches the third load surface (10b) in a spherical motion which approximates the actual relative spatial motion between the first and second bone segments (4, 5).

5. The orthopedic device according to any one of the preceding claims, characterized in that: the first load surface (10b) replicates a condyle (9) of the first bone segment (4).

6. The orthopedic device according to any one of the preceding claims, characterized in that: the third load surface (10b) replicates a second condyle (9') of the first bone segment (4).

7. The orthopedic device according to claim 2, characterized in that: at least one of the first or third load surfaces (10b) is in the form of a portion of a conical surface having a curvilinear directrix with its apex at a center point (O) of spherical motion approximating actual relative spatial motion between the first bone segment (4) and the second bone segment (5).

8. The orthopedic device according to claim 7, wherein: the load surface (11b) mating with the first or third load surface (10b) is in the form of a portion of a conical surface having a curvilinear directrix with its apex at the center point (O) of a spherical motion approximating the actual relative spatial motion between the first (4) and second (5) bone segments.

9. The orthopedic device according to any one of the preceding claims, characterized in that: the constraint means (10a, 11a, 17, 20, 21, 22, 25, 26, 30, 31) comprise a guide rod (20, 21) operating between the first (10) and second (11) parts.

10. The orthopedic device according to claim 9, wherein: the first and second rods (20, 21) are positioned in the same way as the cruciate ligaments of the knee joint, the first and second bone segments (4, 5) being constituted by a femur (4) and a tibia (5), respectively.

11. The orthopedic device according to claim 10, wherein: the first and second rods (20, 21) are formed by cruciate ligaments.

12. The orthopedic device according to any one of the preceding claims 9 through 11, characterized in that: the guide rods (20, 21) are formed by natural, artificial or biological ligament parts.

13. The orthopedic device according to any one of the preceding claims, characterized in that: the connecting device (13, 17, 20, 21, 22, 27, 30, 31) comprises a spherical pair (17, 22) for connecting the first component (10) to the second component (11), the center of the spherical pair (17, 22) being at the fixing point (O).

14. The orthopedic device according to claim 13, wherein: the first component (10) comprises a spherical portion (19, 24) and the second component (11) comprises a socket (18, 24), the spherical portion (19, 24) being constrained to move within said socket (18, 24).

15. The orthopedic device according to any one of the preceding claims, characterized in that: the first and second parts (10, 11) comprise matching first and second guide surfaces (10a, 11a, 25, 26) in the form of portions of first and second regular surfaces (S1, S2), respectively, said first and second regular surfaces (S1, S2) having curvilinear directrix and constituting instant planes of spherical motion centred at a fixed point (O), said spherical motion approximating the actual relative spatial motion between the first and second bone segments (4, 5), the first and second guide surfaces (10a, 11a, 25, 26, 30, 31) constituting at least in part the constraint means (10a, 11a, 17, 20, 21, 22, 25, 26, 30, 31).

16. The orthopedic device according to claim 15, wherein: the connecting means (13, 17, 20, 21, 22, 27, 30, 31) comprise at least a first and a second flexible element (14, 15, 28, 29) fixed to the first and second parts (10, 11) at the first and second guide surfaces (10a, 11 a).

17. The orthopedic device according to claim 15 or 16, characterized in that: the first and second guide surfaces (10a, 11a) are located substantially at a central portion of the first and second components (10, 11), that is, between the first and second condyles (9, 9') of the bone segment (4).

18. The orthopedic device according to claim 15 or 16, characterized in that: the first and second guide surfaces (10a, 11a) are located at lateral ends of the first and second components (10, 11).

19. The orthopedic device according to any one of the preceding claims, characterized in that: the central point (O) of the spherical movement, which approximates the actual relative spatial movement between the first bone segment (4) and the second bone segment (5), is located in the middle condyle (9) of the femur (4), the first and second bone segments (4, 5) being formed by the femur (4) and the tibia (5), respectively.

20. The orthopedic device according to any one of the preceding claims, characterized in that: the centre point (O) of the spherical movement, which approximates the actual relative spatial movement between the first (4) and second (5) bone segments, is located in a central position with respect to the sagittal median plane of the knee, the first and second bone segments (4, 5) being constituted by the femur (4) and the tibia (5), respectively.