Patent Publication Number: US-8118839-B2

Title: Interspinous implant

Description:
This application claims the benefit of French Application No. 06/09738 filed Nov. 8, 2006 and U.S. Application No. 60/857,589 filed Nov. 8, 2006. 
     The present invention concerns an interspinous implant. 
     It is known, through the document WO 99/42051 in the name of the applicant, how to produce an elastically deformable interspinous implant able to damp the posterior movement of the vertebrae during extension of the spinal column. This implant is substantially in the shape of an H, that is to say it comprises a pair of projecting top arms and a pair of projecting bottom arms, delimiting respective top and bottom recesses able to receive the spinal processes of the two vertebrae; it is formed from a block of deformable viscoelastic material, in particular silicone, possibly contained in an envelope of woven material, in particular polyester. 
     This implant gives satisfaction in practice, constituting an alternative to the other methods of surgical treatment of vertebrae, which are more invasive, in particular the methods of interpedicular osteosynthesis consisting of fitting a rigid material with pedicular screws and connecting rods, joining the treated vertebrae. 
     It appears however that, in certain cases, in particular in the case of strong patients or those having a local compensatory hyperlordosis, the existing implant has insufficient damping capacity. In such patients fitting the largest implant existing in the range of implants available is not a satisfactory solution since there is a risk of producing an excessively great distraction, leading to excessive pressure, in particular anterior, on the intervertebral disc. 
     Moreover, it appears that the holding of the implant in position may not always be perfectly guaranteed, in particular during longitudinal torsion movements of the spinal column or lateral bending movements thereof. This risk is partially attenuated by means of ligaments engaged through the interspinous middle part of the implant and around the spinal processes, which makes the fitting of the implant more invasive. 
     The document WO 2006/106246 describes an implant comprising two distinct rigid pieces, and means for the mutual association of these pieces in rotation, forming a swivel. The existence of such rigid pieces on such an implant is not desirable because of the risks that this may present in the event of wear and/or movement of the implant. This implant is not affected by the aforementioned technical problems posed on an implant consisting of a block of deformable material, namely the problem of insufficient damping capacity in certain cases and the problem of a holding in position that is not perfectly guaranteed. 
     The document US 2005/203512 describes an interspinous implant comprising an interspinous central part made from high molecular weight polyethylene or PEEK, that is to say rigid materials, and projecting arms made from shape-memory metal. In particular, the projecting arms situated on the same side are formed by the same blade made from shape-memory metal, passing through the interspinous part of the implant. 
     The same comments as above can be made concerning the implant according to this prior document. 
     The present invention aims to remedy the aforementioned problems posed with an implant formed by a block of deformable material. 
     The implant concerned is, as known per se, suitable for being engaged between the spinal processes of two vertebrae and is formed by a block of elastically deformable material. 
     According to the invention, the said block is subdivided into a central part made from a first elastically deformable material and two lateral parts made from a second elastically deformable material, more rigid than said first material, said lateral parts being situated on the two opposite lateral sides of the central part, that is to say on the sides intended to be situated longitudinally on each side of the spinal processes after implantation, and being connected to said central part. 
     The inventor in fact found that the insufficient damping capacity of the existing implant resulted, in certain clinical anatomical situations, from a lateral creep of the material of the implant under the effect of said compression forces. In the implant according to the invention, said lateral parts made from a more rigid material oppose this risk of lateral creep of the material constituting said central part of the block. 
     Preferably, the two lateral parts made from a more rigid material are connected to each other through the interspinous middle portion of the implant, by a connecting part. 
     This connecting part makes it possible, in addition to the inherent rigidity of said lateral parts made from more rigid material, to perfectly oppose the aforementioned lateral creep of said central part of the implant. 
     Said connecting part is preferably produced from the same material as the two lateral parts made from more rigid material. 
     Said connecting part can be tubular, in order to allow where necessary the passage of one or more ligaments through the implant, which provide additional holding of the implant with respect to the vertebrae. The connecting part can also be solid in transverse section, then increasing the degree of damping of the interspinous part of the implant. 
     The implant can have a parallelepipedal or cylindrical shape, in which case the resistance to lateral creep of the central part of the implant is obtained by connecting said lateral parts made from more rigid material through the middle portion of the implant. Preferably, however, the implant according to the invention has an H shape, that is to say comprises a pair of projecting top arms and a pair of projecting bottom arms, delimiting respective top and bottom recesses able to receive the spinal processes of the two vertebrae, and said lateral parts made from a more rigid material extend at least over part of the height of at least one of said projecting branches. 
     Said lateral parts made from a more rigid material have in this case the advantage of reinforcing the rigidity of one or more of said projecting arms and therefore increasing the stability and consequently increasing the degree of holding in position of the implant after fitting, in particular in the case of twisting movements of the vertical column or lateral bending thereof. The inventor found that the aforementioned risk of movement of the implant according to the prior art resulted from insufficient rigidity of the projecting arms of this implant, in particular in the case of significant lateral creep of the material of the implant. Said lateral parts made from a more rigid material according to the invention make it possible to solve this problem. 
     These lateral parts made from a more rigid material preferably extend over the whole of the height of said projecting arms. The rigidity of said second material constituting these lateral parts can then be sufficient to ensure the aforementioned lateral restraint of the central part of the implant with regard to the creep of the material constituting said block. These same lateral parts can also be connected to each other through the interspinous middle portion of the implant, by a connecting part. 
     Said lateral parts made from a more rigid material preferably closely bonded to the elastically deformable material forming the central part of the implant, over the whole of the assembly interface of each lateral part with this central part. This connection over the whole of this interface helps to oppose the lateral creep of the material of the central part given that the sliding of one assembly face with respect to the other is prevented. 
     The implant can, in this case, be obtained by moulding said lateral parts made from a more rigid material on said central part or, when there is a connection of said lateral parts made from more rigid material with each other at the interspinous middle part, by moulding said central part on said lateral parts made from more rigid material and on the part forming said connection. 
     When the implant has an H shape, at least one of said lateral parts made from more rigid material can have a reduced thickness at the interspinous middle portion of the implant, so as to allow bending of the corresponding top and bottom projecting arms one towards the other, in order to make possible lateral insertion of the implant in the interspinous space. 
     Such a possibility of lateral insertion is advantageous given that it preserves the integrity of the ligamentary system better than a posterior insertion. 
     Said first material can in particular be a viscoelastic material. This material may have variable rigidity, increasing as the degree of compression undergone by this material increases. 
     Said first material can in particular be a silicone, with a hardness for example of 65 Shore A. 
     Said second material has a higher hardness than that of the first material. It may for example be a silicone with a hardness of around 85 shore A. 
     The implant can be contained in a sheath made from textile material, in particular polyester. 
    
    
     
       The invention will be clearly understood and other characteristics and advantages thereof will emerge with reference to the accompany schematic drawing showing, by way of non-limitative examples, several possible embodiments of the implant that it concerns. 
         FIG. 1  is a view in section of it along the line I-I in  FIG. 2 , according to a first embodiment; 
         FIG. 2  is a view in section of it along the line II-II in  FIG. 1 ; 
         FIG. 3  is a view of it similar to  FIG. 1 , according to a second embodiment; 
         FIG. 4  is a view of it according to this second embodiment, in an implantation configuration; 
         FIG. 5  is a view of it similar to  FIG. 1 , according to a third embodiment, and 
         FIG. 6  is a view of it similar to  FIG. 1 , according to a fourth embodiment. 
     
    
    
     For simplification, the parts or elements of one embodiment that are found again in an identical or similar manner in another embodiment will be identified by the same numerical references and will not be described again. 
       FIGS. 1 and 2  show an interspinous implant  1  suitable for being inserted between the spinal processes of two vertebrae so as to be able to damp the posterior movement of these vertebrae during extension of the spinal column. The implant  1  is substantially in an H shape, that is to say comprises a middle part  2  intended to be placed between said spinal processes, a pair of projecting top arms  3  delimiting a top recess  4 , and a pair of projecting bottom arms  5  delimiting a bottom recess  6 , these recesses  4  and  6  being able to receive the spinal processes of the two vertebrae. 
     As shown by  FIG. 2 , the arms  3  and  5  have truncated anterior top edges  3   a ,  5   a , respectively inclined and rounded, so that the implant  1  can be fitted at the base of the spinal processes of the vertebrae, as close as possible to the lamellae. 
     The implant  1  also comprises a conduit  7  formed through the part  2 , the ends of which are splayed. This conduit  7  serves if necessary for fitting ligaments engaged through it and around the spinal processes of the two vertebrae. 
     The implant  1  can comprise an envelope made from woven material, in particular polyester, which covers it entirely. 
     As shown, the implant  1  is formed from parts made from different materials, namely:
         a central part  10  made from a first elastically deformable material, for example a silicone with a hardness of 65 Shore A, and   two lateral parts  11  made from a second elastically deformable material, more rigid than said first material, situated on the two opposite lateral sides of the central part  10 , that is to say on the sides intended to be situated longitudinally on each side of the spinal processes after implantation, these lateral parts  11  being connected to the central part  10 .       

     The lateral parts  11  extend over the whole of the height of the implant  1 ; they have, in the embodiment shown in  FIG. 1 , a thickness varying little but slightly greater at the free ends of the arms  3  and  5 , and are closely bonded to the central part  10  over the whole of the assembly interface with this central part. They can in particular be made from silicone with a higher Shore A hardness than that of the material of the central part  10 , for example around 85 Shore A, and be moulded onto this central part  10 . 
     These more rigid lateral parts  11  oppose a risk of lateral creep of the material constituting the central part  10  under the effect of high compression forces liable to be exerted on the part  2  by the spinal processes and therefore preserve sufficient damping capacity for this part  2 . 
     The lateral parts  11  also have the result of reinforcing the rigidity of the arms  3  and  5  and therefore increasing the degree of holding of the implant  1  in position after fitting, in particular in the case of twisting movements of the vertebral column or lateral bending thereof, and particularly in the case of significant compression of the part  2 . 
       FIG. 3  shows that the lateral parts  11  can have a portion  11   a  of reduced thickness at the interspinous middle portion of the implant  1 , so as to allow, as shown by  FIG. 4 , bending towards each other of the arms  3  and  5  situated on the same side of the implant  1 , in order to make possible a lateral insertion of the implant in the interspinous space. The arms  3  and  5  can be held in this flexed position by means of a holding link  15 . 
       FIG. 5  shows that the lateral parts  11  can also be connected to each other through the part  2  of the implant  1 . The latter then comprises an element consisting of the two parts  11  and the middle connecting part  12 , on which the central part  10  is overmoulded. 
     In this case, the connecting part  12  is tubular, in order to allow where necessary the passage of one or more ligaments through it affording additional holding of the implant  1  with respect to the vertebrae. 
       FIG. 6  shows that the connecting part  12  can also be solid in transverse section, then increasing the degree of damping of the interspinous part of the implant  1 . 
     As is clear from the above, the invention provides an interspinous implant formed by a block of elastically deformable material, having in all cases sufficient damping capacity, in particular in the case of strong patients or those having local compensatory hyperlordosis, and the holding in place of which is reinforced, which may make unnecessary the fitting of ligaments around the spinal processes of the vertebrae treated. 
     It goes without saying that the invention is not limited to the embodiment described above by way of example but extends to all embodiments covered by the accompanying claims.