Patent Application
20110152941
The present invention relates to an extra-discal assembly for intervertebral stabilization for arthrodesis.
The invention lies in the field of arthrodesis, i.e. bone fusion between at least two adjacent vertebrae. It is recalled that arthrodesis sets out to allow only micromovements between the vertebrae, and also to damp vibration. Such micromovements enable patients, who can once again adopt an upright posture after the operation, to adapt their equilibrium as well as possible before the bone graft takes. These micromovements also make it possible, after bone fusion has occurred, for the assembly in accordance with the invention to avoid opposing plastic adaptations, which are one of the fundamental characteristics of variation in the vertebral column during the lifetime of the patient.
In typical manner, an extra-discal assembly for arthrodesis in the meaning of the invention allows movement between two vertebrae to have an amplitude, in side view, that is equal to no more than about 10% of the natural physiological amplitude. In other words, if a natural maximum amplitude in pivoting between two given adjacent vertebrae exists with a value of 10°, then the stabilization assembly in accordance with the invention is suitable for allowing those two vertebrae to move but through no more than 1°.
It should be observed that it is fundamental to make a distinction between firstly a prosthesis designed to recreate intervertebral movement, and secondly an assembly of the invention having the sole purpose of obtaining bone fusion between two vertebrae. In this respect, it should be recalled that there exist substantial structural and functional differences between a prosthesis and an osteosynthesis device, such that a prosthesis cannot allow bone fusion to take place and, in similar manner, an arthrodesis assembly cannot perform a prosthetic function.
The stabilization assembly in accordance with the invention is for connecting together at least two adjacent vertebrae while generally being placed on one side only of the vertebral column, i.e. on the left or the right. This stabilization element is implanted in an extra-discal manner, i.e. it may be situated behind, or alternatively in front of, the vertebral space.
A stabilization assembly is already known, such as that made available by the supplier Medtronic under the trade reference Agile. That assembly comprises two rigid members suitable for co-operating with two pedicular screws implanted in two adjacent vertebrae. A damping buffer is also provided that is fitted against the end plates of the rigid members, in particular by hot vulcanization. Under such conditions, the buffer may not only be compressed, while remaining secured to those two members, it may also be stretched, because of the presence of the bonding.
That Agile stabilization assembly also includes centering means for preventing the two rigid members from moving off axis, in particular during flexion movements. For this purpose, use is made of a cable that is secured to one of the two rigid members, that passes through the damping buffer, and that extends into the inside volume of the facing hollow rigid member.
That known solution nevertheless presents certain drawbacks. In particular, the Agile stabilization assembly tends to work much too much in flexion, thereby reducing its effectiveness and its lifetime. It should also be observed that that assembly, which was initially designed as a prosthesis for accompanying vertebral movement, was transformed into an osteosythesis device as a result of administrative constraints in the United States. Such a transformation was implemented without structurally modifying the assembly so as to restrict its movement, thereby showing that that assembly cannot obtain arthrodesis since it allows movements to take place to much too great an extent.
Stabilization assemblies are also known that are sold under the references Dynesis and Nflex. Those posterior dynamic stabilization systems make use of the resilient mechanical properties of elastomer buffers to limit the mobility of pedicular screws during intervertebral movement. Under such conditions, the movement of each screw is the result of deforming one of those buffers in compression, for flexion if the buffer is on the outside of the space between the screws, or for extension if the buffer is situated between the screws.
The main drawback of those known devices lies in the absence of articulation between the screw and the buffer. As a result, when the screw is caused to pivot in the context of a flexion-extension movement, it causes the buffer to perform bending work. Under such conditions, the buffer stressed in that way in turn causes the screw to pivot, and that does not lie within physiologically natural intervertebral movement.
Furthermore, if it is desired to approach physiologically natural movement by means of those two devices, it is necessary for the stiffness of the buffers to increase very considerably. Under such conditions, those devices approximate to rigid systems, thereby giving rise to consequences concerning stresses on the implanted screws, and also giving rise to insufficient absorption of impacts and vibration. The buffers thus behave more like abutments than like dampers, with any residual movement that is obtained resulting solely from the flexibility of the system.
That said, the invention seeks to remedy those various drawbacks. To this end, the invention provides an extra-discal assembly for intervertebral stabilization for arthrodesis, the assembly comprising:
According to other characteristics:
The invention is described below with reference to the accompanying drawings given purely by way of non-limiting example, and in which:
The extra-discal assembly in accordance with the invention as shown in
Thus, the screw may be implanted in the vertebral body, either laterally, or anteriorly, or in the vertebral body through the pedicle. In general, it is possible to provide for any insertion that ensures that the screw is secured stably relative to the vertebra. It is then implanted in the vertebra by a screw thread and it causes a stud to project out from the vertebra suitable for co-operating with a jaw, as described below. The stud may also be supported by a mechanical member other than a thread, e.g. such as a staple or hooks placed on the vertebral body and/or intervertebral bone plates.
The stabilization assembly in accordance with the invention further includes two slideways 30 and 40 extending substantially along the axis A interconnecting the two screws 10 and 20, once they have been implanted in vertebrae. Each slideway is constituted by a corresponding rod 30, 40 that may be rigid, or that may present the ability to deform a little, in the flexion direction. In any event, if it can deform, its ability to deform is controlled. When seen from the side, and regardless of whether or not it is rigid, each rod may be straight, or it may present a little curvature, so as to match the curvature between vertebrae, where appropriate.
Each screw 10 or 20 is associated with two jaws, one of which is stationary and the other of which is mounted to slide relative to the two rods 30 and 40. The two stationary jaws are referenced 50 and 60, and the two movable jaws are referenced 70 and 80.
All four jaws present the same structure, i.e. each of them comprises two tabs 51, 61, 71, 81 for attaching to the corresponding rod. The tabs 51 and 61 of the stationary jaws are provided with respective screws 52, 62 that enable them to be secured relative to the two rods 30 and 40.
Furthermore, the tabs in each facing pair are connected together via respective connection branches 53, 63, 73, and 83. Each connection branch is curved when seen from above, i.e. about the axis of the corresponding screw, so as to make articulated contact with the shank of a corresponding screw. Seen from above, each screw presents a radius of curvature corresponding to its diameter, whereas each curved branch presents a radius of curvature that is greater than the diameter of the screw, thereby making this articulation possible. Furthermore, as shown in
Each branch may be rigid, or it may be capable of deforming, at least in some places, under the effect of stresses of magnitude significantly greater than gravity. In contrast, all of the branches present their own shape, i.e. they are of a shape that does not vary under the effect of gravity, nor indeed under the effect of other stresses of analogous magnitude.
It should also be observed that the two movable jaws 70 and 80 are adjacent, i.e. they are disposed on the facing sides of the two shanks. In contrast, the two stationary jaws are further apart from each other, i.e. they are presented against the opposite faces of those two shanks 12 and 22.
Two springs 74 and 84 are interposed between the two facing movable jaws 70 and 80. These springs tend to urge each of the movable jaws 70 or 80 towards the stationary jaw 50 or 60 that is associated therewith.
It should be observed that the greater the extent to which these springs are stiff and/or prestressed, the more they tend to oppose any movement of the movable jaws. The distinction between these respective concepts of stiffness and of prestress is explained in greater detail below. In the context of the present invention, i.e. arthrodesis, the springs used are thus of considerable stiffness and/or prestress, so as to allow only micromovements, and vibration damping, as defined above.
Each shank 12 or 22 is circular in cross-section. In addition, each branch is also circular, likewise in cross-section. Finally, as mentioned above, the branches present a curved profile, i.e. the two facing branches define a shape that is more or less oval, with a radius of curvature that is greater than the radius of the circular shank.
Under such conditions, each shank 12 or 22 co-operates with each branch 53, 73, 63, or 83 to define articulation substantially about a point, as represented by the points P in
There follows a description of the operation of the above-described stabilization assembly. It is assumed that the two screws 10 and 20 are pedicular screws and that the top portions of these screws, as seen in
The jaws are placed first around the screws 10 and 20. In the present example, it is assumed that the stabilization assembly in accordance with the invention is to form a stay. Under such conditions, and in the absence of stress, the free ends of the two screws are spaced apart by a distance that is greater than the distance between the screws once they are associated with the jaws.
In other words, for assembly purposes, the two screws need to be moved manually towards each other, e.g. by means of a tool that is not shown. Thereafter the two jaws together with the two rods are moved axially towards the screws so as to insert the two shanks 12 and 22 through the two eyelets as defined by the jaws. Thereafter the external action exerted by the tool is released so that the shanks 12 and 22 come to bear against the stationary jaws 50 and 60.
If the patient seeks to exert intervertebral flexion, i.e. to lean forwards, then the two screws 10 and 20 tend to move apart from each other along the axis A. However, this is made impossible since the two shanks 12 and 22 then come into abutment against the stationary jaws 50 and 60. The stabilization assembly of the invention thus substantially prevents any intervertebral flexion movement.
In contrast, during intervertebral extension, the screws 10 and 20 tend to move towards each other along the axis A. Unlike flexion, this relative movement of the screws is possible insofar as they then push the movable jaws 70 and 80 towards each other against the springs 74 and 84. The movable jaws consequently tend to slide along the rods 30 and 40.
This is shown more particularly in
In this variant embodiment, the jaw 60′ is no longer stationary, like the jaw 60 in the first embodiment, but on the contrary it is free to slide along the rods 30 and 40. This sliding takes place against two additional springs 75 and 85 interposed between the tabs 61′ of the jaw 60′ and abutments 31 and 41 that are mounted in stationary manner on the slideways 30 and 40.
The jaws and screws forming part of the assembly in accordance with the second embodiment of the invention are mounted in a manner analogous to that described with reference to the first embodiment. Once the shanks 12 and 22 have been inserted in the eyelets defined by the various jaws, the shanks bear against the respective jaws 50 and 60′.
Given that the jaw 60′ is now movable, equilibrium is established between the forces exerted respectively by the screws 20, the “inner” springs 74 and 84, and the “outer” springs 75 and 85. More precisely, the screw 20 exerts a force tending to push it away the first screw 10, i.e. a reaction against the action of the surgeon tending to move them towards each other. In addition, the springs 74 and 84 also tend to push the screw 20 away from the screw 10. In contrast, the springs 75 and 85 tend to urge the screw 20 towards the first screw 10.
In this second embodiment, intervertebral flexion is now possible. Thus, when the patient leans forwards, the branch 53 of the stationary jaw holds the screw 10 stationary, while the screw 20 pushes back the branch 63′ of the movable jaw 60′ against the two springs 75 and 85. Simultaneously, the springs 74 and 84 tend to push back the branch 83 of the jaws 80 towards the shank 22 of the screw 20, as shown in
In order to explain further the way in which this intervertebral flexion takes place,
The outer springs 75 and 85 are prestressed, i.e. there is initially a zone I in which the patient exerts a prior force in order to overcome the prestress. In other words, so long as the patient does not exert a threshold force, referenced F0, the patient does not manage to “overcome” the prestress, and therefore achieves no intervertebral flexion, i.e. the value of the angle α remains zero.
Thereafter, when the patient exerts a force greater than the threshold force F0, the value of the flexion angle increases linearly with the force exerted by the patient, following the stiffness characteristic of the spring (zone II). This angle α then increases up to a value referenced αmax, which corresponds to a value Fmax, i.e. the maximum physiological force that the patient can apply. In the context of arthrodesis, to which the present invention applies, this value α is small, as explained above.
This type of curve thus serves to explain the distinction that exists between the concepts of stiffness and of prestress. Thus, if there is no prestress, the force-displacement curve would correspond to a straight line segment extending from the origin. In other words, the slightest force exerted by the patient would tend to push away the screw against the springs 75 and 85. This would not be very advantageous insofar as such a situation would lead to overall instability of the system.
In contrast, the higher the prestress, the greater the value of the threshold force F0 before any actual movement of the patient occurs. On the graph, dashed lines show an embodiment in which the springs 75 and 85 present the same stiffness, but in association with greater prestress. The threshold force F0, is then higher, and the maximum angle αmax, is smaller.
Furthermore, if the springs 75 and 85 present stiffness that is different, that has an influence on the slope of the straight line extending from the point F0. Thus, for the same prestress, a chain-dotted line shows an arrangement in which the springs 75 and 85 are stiffer. In other words, the maximum angle αmax″ that the patient can reach by exerting the maximum physiological force if smaller than the maximum angle αmax.
It should be observed that it is possible to adjust the value of the prestress by a small modification to the embodiment of
During intervertebral extension, co-operation of the branches 53 and 73 together with the screw 10 takes place in a manner analogous to that shown in
This third embodiment differs from the above-described embodiment in that the jaw 50′ is now movable, and no longer stationary as is the jaw 50 in
The screws 10 and 20 in this embodiment are mounted in the eyelets defined by the various jaws in a manner analogous to that described above with reference to the second embodiment. Once insertion achieved, two force equilibriums are established, firstly between the screw 10, the springs 74 and 84, and also the springs 76 and 86, and secondly between the screw 20, the springs 74 and 84, and the springs 75 and 85. These force equilibriums are analogous to the equilibrium described with reference to the second embodiment between the screw 20 and the inner and outer springs.
During intervertebral flexion, operation is symmetrical, i.e. the co-operation of the two screws with the four jaws takes place in a manner analogous to that described with reference to the right-hand side of
Furthermore, during extension, operation is likewise symmetrical, i.e. the co-operation between the two screws and the four jaws takes place in a manner analogous to that described on the right in
This embodiment of
Furthermore, in the embodiment of
Furthermore, and advantageously, the distance between the slideways 130 and 140 along an axis perpendicular to their main axis, is close to the cross-section of the screw. This enables the slideways to perform a guide function for the screws, so as to keep them well positioned for continuous co-operation with the studs and the curved ends of the slideway. In the above-described embodiments, the screws present cylinders of constant section. However, in
In the embodiment of
Like the embodiments of
Under such circumstances, the flexible body is associated with a stiffness value, and with a prestress, as are the springs. Furthermore, it is possible to surround the flexible body with a rigid chamber, thereby putting a limit on deformation. Under such conditions, the force-displacement curve no longer has two zones as described above, but three zones as shown in
Firstly there are the two zones I and II analogous to those of
In the above examples, stabilization assemblies are shown that connect together only two stages of vertebrae. Nevertheless, provision may be made in accordance with the invention to connect together at least three stages of vertebrae. For this purpose, two main variants may be envisaged as described below.
Thus, firstly it is possible to associate a plurality of extra-discal assemblies analogous to either of the above-described assemblies, as shown in
Furthermore, and in advantageous manner, provision is made to articulate the two assemblies I and II relative to each other. For this purpose, it is possible to associate a “bead” type spherical separator member P therewith. This member may in particular be in accordance with one of those described and claimed in French patent application 07/59227 filed in the name of the same Applicant on Nov. 22, 2007, the content of which is incorporated herein by reference. It should also be observed that in
As an alternative, it is possible to envisage connecting together at least three stages of vertebrae via a single stabilization assembly in accordance with the invention, as shown in
The embodiment of
Other variant embodiments (not shown) may be envisaged starting from the arrangement of
In the embodiment of
There follows a description of a method of putting the assembly of
Firstly the various pedicular screws 210, 215, and 220 are put into place by placing a tapering end on each of them, e.g. an end of conical shape. Furthermore, the various extrusions are assembled on the slideways. Because of their resilient nature and because of the absence of screws, it should be observed that the various facing studs come mutually into contact.
Then, the assembly formed by the slideways and the extrusions is fitted onto the screws having their pointed ends. These ends are then inserted between the resilient extrusions, and they then push them back so as to create prestress. Finally, once the assembly is in the position shown in
In this embodiment, the bead P serves to prevent the jaws from sliding along the shanks of the screws, away from the vertebral bodies. Advantageously, an additional bead is provided in the vicinity of the vertebral bodies, so as to prevent sliding towards them. Under such circumstances, the beads are inserted along the shanks of the screws, before putting the slideways and the extrusions into place.
There can be seen an extrusion 365 comprising a flexible cylindrical body 374 between rigid endpieces 370 and 380. Four slideways are also provided, comprising two first slideways 3301 and 3302 forming part of a bent metal wire 330 that forms a loop 350 for passing a first pedicular screw 310. The other two slideways 3401 and 3402 forming part of the other wire 340 that is likewise curved form a loop 360 for passing the other pedicular screw 320. Each of these loops defines a jaw in the meaning of the invention.
Each screw presents a middle zone of smaller diameter, like an hourglass. This middle zone co-operates in articulated manner both with a corresponding jaw 350 or 360, and with a facing stud 373 or 383, as in the above-described embodiments.
The first wire 330 is slidably mounted in orifices 370′ formed in the first endpiece 370 adjacent to the loop 350 and then also extends, likewise in slidable manner, in openings 374′ formed in the flexible body. Finally, the ends of this wire are fastened to the opposite endpiece 380, by any appropriate means. In a variant, provision can be made for these ends to pass through the opposite endpiece in slidable manner and also to be provided with abutment means serving to retain the endpiece. In other words, the endpiece is constrained to move with the screw 310 in translation in the direction of compressing the flexible body, i.e. when the two endpieces move towards each other.
In analogous manner, the second wire 340 extends slidably successively through orifices 380′ formed in the endpiece 380 that is adjacent to the loop 360, and then through additional openings 374″ formed in the flexible body. As mentioned above with reference to the first wire, the second wire may either be fastened to the endpiece 370 opposite from the loop 360, or it may be slidably mounted relative thereto, being associated with abutment means.
In the event of intervertebral flexion, as shown in
Furthermore, during intervertebral extension (not shown in the figures), each loop tends to move away from the pedicular screw with which it was originally in contact. This movement is accompanied by corresponding compression being applied to the flexible body, which therefore once more performs a damping function and tends to return the assembly to its initial, rest position.
As can be seen from the above, not only intervertebral extension, but also flexion leads to the flexible body being compressed, such that it acts as a damper in both cases. There can thus be seen to be differences between this embodiment of
It can be seen that the more flexible the body the greater the movement it allows between the two vertebrae. Thus, with arthrodesis, it is appropriate to select a flexible body that is relatively rigid, so as to limit intervertebral movement. It is also possible to make provision for associating the flexible body with a deformation limit. Thus, by way of example, the flexible body 374 may be surrounded by means of a rigid cylindrical chamber 390 that is bonded to one or other of the endpieces, as shown in
With reference to
As an alternative, the flexible body 374 may be surrounded by means of at least one rigid ring 392 (
In both of the preceding embodiments, either with the chamber 390 or with the ring 392, deformation of the flexible body 374 is limited, thereby limiting the relative movement between the two pedicular screws. Nevertheless, other means may be provided for limiting this stroke. In non-limiting manner (not shown), mention may be made for example of any appropriate abutment means serving to stop the stroke of the slideways relative to the rigid endpieces.
The implementation of the device shown in
Thereafter, when the patient leans either forwards or backwards, as explained above, this leads in both situations to the flexible body being compressed, thereby opposing such movement. In addition, the presence of the chamber puts a limit on the movement, as represented by the asymptote A corresponding to an intervertebral angle of inclination αmax. The curve plotting force as a function of movement is thus limited between firstly a “relative” origin as constituted by the points F0 and α0, and secondly the asymptote A.
In the embodiment of
In the various embodiments described above, the extra-discal assembly in accordance with the invention has at least two vertebral screws. Under such conditions, the various screws are generally implanted on one side of the vertebral column, at a distance from the middle vertebral axis thereof, with reference to the patient standing. It is also possible to provide for implanting two sets of vertebral screws, on both sides of this middle axis, with each set of screws then forming part of a corresponding extra-discal assembly.
Nevertheless, in a variant, provision may be made to use not vertebral screws, but rather rods, each forming part of a connection element, e.g. connecting together two pedicular screws at the same vertebral stage. Under such conditions, the extra-discal assembly in accordance with the invention has at least two such rods placed one above the other substantially along the middle vertebral axis, together with jaws, slideways, and return means, as in the above-described embodiments.
| Number | Date | Country | Kind |
|---|---|---|---|
| 08 55574 | Aug 2008 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR09/00880 | 7/17/2009 | WO | 00 | 2/11/2011 |
