Abstract:
The invention concerns a staiblizing device ( 150 ) designed to link two vertebrae, comprising at least two chambers ( 156, 158, 186, 188 ) arranged proximate to said vertebrae, said chambers containing a shock absorbing fluid. Means ( 192, 194 ) are provided for providing fluid communication with controlled flow between said two chambers, thereby adequately damping the patient&#39;s movements.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The above referenced application is based on PCT Application PCT/FR00/03727, filed on Dec. 28, 2000, having the same inventor, which claims priority from French Patent Application No. 99/16662 which was filed on Dec. 29, 1999 also having the same inventor. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a device and an assembly for intervertebral stabilisation. 
     Such a device is usually intended to replace all or part of an intervertebral disc, when the latter has been destroyed by surgery or disease. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention proposes to produce a stabilising device which ensures a relative movement between the two vertebrae that it connects, which is sufficiently close to the movement allowed by a natural vertebral disc for no major dysfunction to appear at the level of the whole of the vertebral chain. 
     To that end, it has for its object a device for intervertebral stabilisation, designed to link two vertebrae, characterized in that it comprises:
         at least two chambers intended to be arranged in the proximity of said vertebrae, said chambers containing a fluid, and   means for placing said chambers in fluid communication with controlled fluid flow.       

     The invention also has for its object an assembly for intervertebral stabilisation comprising at least two intervertebral stabilisation devices such as defined hereinabove, at least first and second devices being arranged on either side of the principal axis of the vertebral chain. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The invention will be described hereinafter with reference to the accompanying drawings, given solely by way of non-limiting examples and in which: 
         FIG. 1  is a schematic side view, illustrating two adjacent vertebrae between which different stabilising devices according to the invention are intended to be placed. 
         FIG. 2  is a side view, on a larger scale, illustrating a first embodiment of a stabilising device according to the invention. 
         FIG. 3  is a view similar to  FIG. 2 , illustrating a variant of the stabilising device described in this  FIG. 2 . 
         FIG. 4  is a schematic side view, illustrating a second embodiment of a stabilising device according to the invention. 
         FIGS. 5 to 8  are views similar to  FIG. 4 , illustrating variants of the stabilising device described in this  FIG. 4 . 
         FIG. 9  illustrates a variant embodiment of a chamber belonging to a stabilising device according to the invention. 
         FIGS. 10 and 11  are plan views illustrating two stabilising devices in accordance with a third embodiment of the invention. 
         FIG. 12  is a side view illustrating a device for intervertebral stabilisation in accordance with a fourth embodiment of the invention; and 
         FIG. 13  is a plan view, illustrating an assembly for intervertebral stabilisation according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows two vertebrae, upper  2  and lower  2 ′ respectively, intended to be linked via different types of stabilising devices according to the invention, which will be described in the following. Each vertebra comprises a vertebral body  4 ,  4 ′ extended by a pedicle  6 ,  6 ′, an upper articular process  8 ,  8 ′ and a lower articular process  10 ,  10 ′,  12  denotes the intervertebral space,  14 ,  14 ′ the opposite articular surfaces and  16  and  16 ′ the articular capsules. 
     This  FIG. 1  also shows two pedicular screws  18 ,  18 ′ fixed in the corresponding vertebral bodies  4 ,  4 ′. 
       FIG. 2  shows a device for intervertebral stabilisation according to a first embodiment of the invention, which forms an extra-discal member  20 . This latter, which is disposed to the rear of the intervertebral space  12 , is capable of damping a displacement between the vertebrae  2 ,  2 ′. 
     The damping member  20  comprises a rigid cylindrical vessel  22 , made for example of metal, plastics material, or ceramics. This vessel receives, in its internal volume, a piston  24  which is mounted to pivot on the head  19  of the screw, which screw extends through an opening  26  made in the vessel  22 . The lateral walls of the piston are separated from the opposite inner walls of the vessel via two O-rings  28 . Furthermore, the lower end of the vessel  22  is connected to the head  19 ′ of the lower screw  18 ′. 
     The end walls  30  of the vessel  22  define, with the opposite end walls of the piston  24 , two respectively upper ( 32 A) and lower ( 34 A) chambers. These latter are placed in communication by a conduit  36 A, extending in the principal direction of the vessel  22 . This conduit, which is for example made of metal or plastics material, is connected to the vessel  22  by crimping, screwing or clipping. The cross-section of this conduit  36 A is substantially smaller than the cross-section of each chamber  32 A,  34 A. This conduit  26 A presents, for example, a diameter for passage less than 2 mm, advantageously included between 0.2 and 0.9 mm. 
     The end walls  30  of the vessel  22  are in addition provided with stops  38 , made of a supple material such as a polymer. Each chamber is filled with a damping fluid. This fluid comprises at least one liquid, such as water or oil. 
     Functioning of this damping member  20  is as follows: When the patient leans forward in the direction of arrow F, this has the effect of moving the vertebrae  2 ,  2 ′ and therefore the pedicular screws  18 ,  18 ′ away from one another. Consequently, the mobile piston  24 , for separating the chambers, is directed towards the upper wall  30  of the vessel  22  and thus drives the fluid out of the upper chamber  32 A, in the direction of the lower chamber  34 A. This flow, which is produced through conduit  36 A, is materialized by arrow f. 
     It will be appreciated that the speed of displacement of the fluid through the conduit  36 A, which determines the speed of displacement of the piston  24  in the direction of the upper wall  30 ′ is adjustable as a function of the section of the conduit, of the length of the latter and of the viscosity of the fluid employed. The presence of this fluid which must be driven out of the chamber in question in order to allow a given movement of the patient, gives a satisfactory component of damping to this movement. 
     During a movement of great amplitude of the patient, the upper stop  38  makes it possible to limit the afore-mentioned movement, by abutment of the opposite wall of the piston  24  against this stop  38 . The elastic nature of the latter gives an additional component of damping. In a variant, it may be provided that at least one chamber contains a hydrophilic body, such as hydrogel. As the piston continues its stroke, the latter abuts on the hydrophilic body so as progressively to release the fluid which was contained therein. 
     Inversely, when the patient leans backwards in the direction of arrow F′, the piston  24  is directed towards the lower wall  30 ′ of the vessel  22 , with the result that the fluid is driven from this chamber  34 A through the conduit  36 A, in the direction of arrow f′. 
       FIG. 3  shows a variant embodiment of the damping member  20  described in  FIG. 2 . 
     In this embodiment, it employs an additional conduit  36 B linking the two chambers  32 B,  34 B. Moreover, each conduit  36 B,  37  is provided with a corresponding non-return valve  40 ,  42 . Consequently, the conduit  36 B allows only a transfer of fluid from the upper chamber  32 B towards the lower chamber  34 B, in the direction of arrow f, while conduit  37  ensures a movement of the fluid only from the lower chamber  34 B towards the upper chamber  32 B, in the direction of arrow f′. 
     The transverse dimensions of the conduit  36 B for passage, ensuring the flow towards the lower chamber, are advantageously larger than those of the conduit  37  ensuring passage towards the upper chamber  32 . This makes it possible to give a greater component of damping in the direction of intervertebral extension. In other words, the movements of forward bending of the patient are damped to a lesser extent than those of rearward extension. 
       FIG. 4  illustrates a device for intervertebral stabilisation in accordance with a second embodiment, which forms a discal implant  52  intended to be inserted at least partially between the vertebral bodies  4 ,  4 ′ of the vertebrae  2 ,  2 ′ of  FIG. 1 . 
     This implant comprises a bag  54 , made of a deformable but substantially non-extensible material. This bag  54  defines two respectively front ( 56 A) and rear ( 58 A) chambers which are joined by a conduit  60 A, or constriction, placing the internal volume of these two chambers in communication. This conduit  60  presents transverse dimensions substantially smaller than those of chambers  56 A,  58 A. 
     The implant  52  also comprises a coating formed by two shells  62 . Each of the latter, which presents a cross-section substantially in the form of an arc of circle, is made of a rigid material, such as titanium. These shells  62 , which are placed on either side of a median plane of the bag  54 , are intended to come into contact with the vertebral bodies  4 ,  4 ′. They are fastened to the bag  54  for example by gluing. 
     Each chamber  56 A,  58 A is filled with a fluid, similar to that contained in the aforementioned chambers  32 ,  34 . Once the implant  52  is in position, when the patient leans forwards in the direction of arrow F, this has the effect of compressing the front chamber  56 A and therefore of driving the fluid which was initially contained therein, in the direction of the rear chamber  58 A. This flow of fluid, which is effected through conduit  60 , is materialized by arrow f. 
     Inversely, when the patient leans backwards in the direction of arrow F′, there is produced, by a similar phenomenon, a flow of fluid along conduit  60 , materialized by arrow f′. The intensity of the damping thus given during the movements of bending and of extension of the patient, depends on the section of passage and on the length of the conduit  60 , as well as on the physico-chemical characteristics, in particular the viscosity, of the fluid admitted in chambers  56 A,  58 A. 
       FIG. 5  shows a variant embodiment of the implant  52  of  FIG. 4 . In this variant, the two chambers  56 B,  58 B are joined, not by a conduit, but by a porous membrane  64 , constituting an interface between these two chambers. This membrane  64 , which may extend over the whole transverse section of the chambers  56 B.  58 B, is for example made of a textile material, such as DACRON, or of a porous metal. The presence of the pores of this membrane  64  makes it possible to delay, in manner similar to the use of the conduit  60 , the transfer of fluid between the two chambers. The component of damping thus conferred depends in particular on the size and the number of the pores of the membrane  64 . 
       FIG. 6  shows an additional variant embodiment of the implant  52  described in  FIG. 4 . In this  FIG. 6 , the chambers  56 C,  58 C are connected, not only by a first conduit  65 , but also by an additional conduit  66 , parallel to conduit  65 . Each conduit is provided with a respective non-return valve  68 ,  70 . Consequently, the conduit  65  allows a transfer of fluid solely from the front chamber  56 C towards the rear chamber  58 C, while the additional conduit  66  ensures a transfer of fluid solely in the opposite direction. 
     The transverse dimensions of the conduit  65  are advantageously larger than those of the additional conduit  66 , with the result that the fluid is more easily able to flow in the direction of the rear chamber  58 C, which corresponds to the movement of forward bending of the patient. In other words, this forward bending movement is damped to a lesser degree than the inverse movement of rearward extension. 
     As shown in this  FIG. 6 , at least one of the chambers belonging to the implant, in the present case the front chamber  56 C, contains a volume of gas  72  which may for example be air or nitrogen. 
     The presence of this gas  72  is advantageous insofar as it induces an elastic effect during compression of the chamber  56 C. In effect, when the latter is compressed, it is firstly the air  72  which is driven therefrom, with the result that the corresponding movement is damped only to a very small degree. 
     Once the air is driven out, it is then the fluid initially contained in the chamber which flows in conduit  65 , which gives a much greater damping component. Consequently, during movements of low amplitudes, only air flows through the conduit  65 , with the result that these movements are only slightly damped. This is not detrimental as, due to their low amplitude, such movements cannot harm the physical integrity of the patient. On the other hand, when the afore-mentioned movements have a higher amplitude, fluid also flows through the conduit  65 , with the result that such movements are damped to a greater degree. 
       FIG. 7  shows an additional variant of the implant  52 . In this variant, the implant  52  has no bag  52 , but presents end walls  74  hermetically joining the shells  62 . These walls  74  are made of a deformable but substantially non-extensible material. Furthermore, a frustum of cylinder  76 , forming piston, is housed in the internal volume of the implant  52 . This frustum of cylinder, of which the principal axis is transverse with respect to that of the implant  52 , forms a porous system. For example, it contains a labyrinth system comprising small cells linked together by pores, which allows it to capture the fluid so as to provoke a slowing down of the flow of the latter. 
     The movement of the piston from rear to front of the implant  52  is limited, in two opposite directions, by stop means  78 . This piston  76  defines, with the opposite walls  74 , two front ( 56 D) and rear ( 58 D) chambers. 
     During movements of flexion or of extension of the patient, the mobile piston  76 , for separating the chambers, moves from rear to front of the implant  52 . This induces a compression of one of the chambers  56 D,  58 D, with the result that fluid escapes therefrom in the direction of the other chamber, via the pores of the piston  76 . 
       FIG. 8  shows a variant embodiment of  FIG. 7 , in which the piston  76 ′ is rigid and tight. This piston constitutes, as in the embodiment of  FIG. 7 , a pivot for the two shells  62 , which may pivot with respect to each other about the transverse principal axis A of this pivot. The circulation of fluid between chambers  56 E and  58 E, which is not allowed by the piston  76 ′, is ensured via at least one channel  78  made in one of the rigid shells  62 . 
       FIG. 9  illustrates an additional variant of the invention. It shows a chamber  56 F, capable of being used in one of the implants described with reference to the preceding Figures. This chamber  56 F comprises a bag  80 , deformable but non-extensible, of which the open end is fixed, for example by crimping, to a rigid cover  82 , which is mounted to slide in a vessel  84  with closed bottom. The opposite walls of the bag  80  and of the vessel  84  are separated by a lubricant  86 , such as a silicone gel. 
     A conduit  88  traverses the cover  82 , so as to place the internal volume of the chamber  56 F in communication with the outside. As a function of the pressure conditions exerted on the chamber  56 F, fluid escapes therefrom or is admitted therein, with the result that the edges of the cover  82  rise or descend, slidably, along the lateral walls of the vessel  84 . This embodiment is advantageous, insofar as it guarantees a very satisfactory tightness, being given that such tightness is ensured both by the deformable walls of the bag and by the rigid walls of the vessel  84 . 
     As shown in dotted lines in  FIG. 9 , two chambers  56 F may be joined by the conduits  88 , so as to form an extra-discal member, of the same type as that,  20 , of  FIGS. 2 and 3 . These chambers  56 F may also be surrounded by shells, similar to those,  62 . In that case, the conduits  88  extend in the shells and at least one chamber may be surrounded by a helical spring. 
       FIG. 10  illustrates a device for intervertebral stabilisation in accordance with another embodiment, which forms a discal implant  102 . The latter comprises two respectively left ( 104 ) and right ( 106 ) elements, disposed on either side of the principal axis A of the vertebral chain which, when the patient is in standing position, is a vertical axis passing through the median plane P extending from rear to the front of the patient. 
     Each element  104 ,  106  comprises two respectively front ( 108 ,  112 ) and rear ( 110 ,  114 ) chambers. The two front chambers  108 ,  112  are placed in communication by a front conduit  116 , while the two rear chambers  110 ,  114  are connected by a rear conduit  118 . The different chambers contain a fluid similar to one of those described hereinabove, with the result that, as a function of the pressure conditions exerted on these chambers, a fluid communication is established therebetween. 
     The chambers of the same element are separated by means of respective membranes  120 ,  122  which may be tight, or porous like the membrane  64  described hereinabove. Consequently, the two chambers  108 ,  110  and  112 ,  114  of an element in question may possibly be placed in fluid communication. 
     In a variant, the front right chamber  108  may be placed in fluid communication with the rear left chamber  114 , the front left chamber  112  being in that case placed in fluid communication with the rear right chamber  110 . These additional fluid relations may be made additionally to the relations allowed by conduits  116 ,  118 . 
       FIG. 11  shows a discal implant  102 ′ comprising two respectively right ( 124 ) and left ( 126 ) rear chambers, as well as a front chamber  128 , extending over a substantial part of the width of the disc. The rear chambers  124 ,  126  are placed in fluid communication via a conduit  130 . Moreover, each of these chambers  124 ,  126  is placed in fluid communication with the front chamber  126 , via respective conduits  132 ,  134 . 
     The implants  102 ,  102 ′, described hereinabove, induce an additional component of damping, when the patient leans towards the sides, being given that they employ chambers disposed on either side of the axis A. 
       FIG. 12  shows a device for intervertebral stabilisation in accordance with an additional embodiment, which is generally designated by reference  150 . This device comprises a discal implant  152 , intended to be inserted at least partially in the intervertebral space  12 . This implant  152  comprises two respectively front ( 156 ) and rear ( 158 ) chambers, surrounded by two shells  162 , similar to those,  62 , described hereinabove. 
     The device  150  also comprises a damping member  170 , arranged similarly to that,  20 , described previously, namely to the rear of the intervertebral space  12 . This member  170  comprises a rigid vessel  172  inside which is disposed a piston  174  which comprises a head  176 , forming upper end, whose transverse dimensions are similar to those of the vessel. An O-ring  178  is mounted between the opposite walls of the head  176  and of the vessel  172 . 
     The head  176  of the piston extends from a vertical rod  180  which traverses the lower wall  182  of the vessel  172  tightly, with the interposition of an O-ring  184 . The lower end of the rod  180 , opposite the head  176 , is pivotally mounted on the head  19 ′ of the lower screw  18 ′. 
     The head  176  defines respectively upper ( 186 ) and lower ( 188 ) chambers of the vessel  172 . The upper chamber  186  receives a spring  190 , working in compression, which extends vertically between the upper wall of the vessel and the opposite wall of the head  176 . The use of this spring  190  allows the piston  174  to be returned into its low position, which corresponds to a physiologically advantageous lordorsic posture of the patient. 
     The front chamber  156  of the implant  152  is placed in fluid communication with the upper chamber  186  of the member  170 , via a conduit  192 , while the rear chamber  158  is placed in fluid communication with the lower chamber  188  via an additional conduit  194 . Consequently, when the patient leans forward in the direction of arrow F, fluid is driven from the front chamber  156  in the direction of the upper chamber  186 , which contributes to causing the piston  174  to descend in the vessel  172 , opposite the upper screw  18 . This rise induces a displacement of fluid, through the conduit  194 , from the lower chamber  188  towards the rear chamber  158 . This movement of bending is therefore damped by these different flows of fluid 
     The amplitude of such damping may be modulated as a function of the characteristics of the fluid admitted in the different chambers and of the section of passage of the conduits  192 ,  194 . It will be readily appreciated that, during a movement of extension of the patient towards the rear, there are produced both flows of fluid and a movement of the piston  174 , in directions opposite those mentioned above. 
     In a variant, it may be provided to add two additional conduits to each of the conduits  192 ,  194 , the four conduits thus formed being provided with non-return valves ensuring the flow of the fluid in one direction only. It may also be provided that the membrane  196  separating the front ( 156 ) and rear ( 158 ) chambers be porous, so as to allow a flow of fluid between these two chambers. It may also be provided to eliminate this membrane  196 , so as to form a single intradiscal chamber, placed in communciation with the lower chamber  174  by the conduit  192 , the other conduit  194  in that case being eliminated. It may also be provided that the head  176  of the piston  174  be porous, like the one,  76 , described hereinabove. 
     It may also be provided that the two upper ( 186 ) and lower ( 188 ) chambers be placed in fluid communication by a conduit, similar to that,  36 , described hereinabove. These chambers  186 ,  188  may be placed in communication by a plurality of conduits extending in the wall of the vessel  172 . These conduits may be disposed below one another, with the result that the head  176  of the piston, as it moves, successively obturates the openings of these conduits. At the end of movement, there therefore remains, in a chamber in question, a volume of residual fluid which cannot be driven out and constitutes a damping volume. The vessel may comprise a double wall defining an annular housing for reserve of fluid. The vessel may also be partially made of a porous material such as a ceramic. 
     The use of a rod  180  of piston  174  traversing the walls of the vessel  172  is advantageous. In effect, this rod  180  and therefore the piston  174  may be connected to the screw  18 ′ while providing that the latter lie outside the vessel  172 . This is interesting in terms of tightness, insofar as it is necessary to provide solely one sealing member in the vicinity of the opening of the vessel, through which the afore-mentioned rod extends. 
     It is also advantageous to provide a device for intervertebral stabilisation, comprising a discal implant and a damping member, disposed to the rear of the intervertebral space. In effect, the intervertebral implant makes it possible to restore the height of the disc. Moreover, being given that this implant contains a fluid, it is adapted to give the two adjacent vertebral bodies a certain mutual freedom of movement. Furthermore, the extra-discal damping member makes it possible to regulate the movements allowed by the discal implant. 
     As shown in  FIG. 13 , the stabilising device  150  may be disposed offset with respect to the principal axis A, namely the implant  152  and the member  170  are disposed on the same side of the plane P. Such an arrangement makes it possible to overcome asymmetrical collapses of the intervertebral space, viewed from behind. 
     An additional device  150 ′ may be associated with the stabilising device  150 , placed on the other side of the axis A and the plane P, so as to form an assembly for intervertebral stabilisation. This device  150 ′ shown in dashed and dotted lines, may be similar to that,  150 , it being understood that it is possible to give it different dimensions, in particular concerning the height of the implant  152 ′, so as to compensate a slight asymmetrical collapse of the disc, viewed from behind. 
     The invention is not limited to the examples described and shown. In effect, it may be provided to connect two vertebrae which are not adjacent, via a device for intervertebral stabilisation according to the invention. 
     The means for placing in fluid communication described hereinabove control the flow of the fluid as they induce a limitation of the flow of this fluid, during flow thereof between the chambers thus connected. This may be effected by giving these communicating means reduced transverse dimensions, or a considerable length. These means induce a restriction, a slowing down of the fluid and/or a pressure drop of the latter. The flowrate of fluid between the chambers is conserved at a value less than a limiting value, which guarantees that the passage of the fluid from a first chamber towards a second chamber is not too rapid. A certain damping of the movements of the patient&#39;s column is thus created. 
     It is particularly advantageous to provide that two adjacent chambers be separated by a mobile member, such as a piston. In effect, this makes it possible to make extradiscal and/or intradiscal objects which are efficient, take up little room and which are subjected to problems of tightness only to a small degree.