Abstract:
The invention relates to a distractible intervertebral implant having a first assembly ( 10 ) constituted by a first prosthetic plate ( 16 ) and a first spacer element ( 18 ), the prosthetic plate being secured to the first spacer element; a second assembly ( 12 ) constituted by a second prosthetic plate ( 22 ) and by a hollow second spacer element ( 24 ), having a first end secured to the second prosthetic plate and the first spacer element being suitable for sliding in the second spacer element; and means ( 14 ) for preventing the spacer element from moving in translation or in rotation relative to each other, the means having a clamping collar ( 28 ) surrounding a portion of the outside surface of the second spacer element, and a lock screw ( 30 ) for tightening the collar onto the second spacer element.

Description:
RELATED APPLICATION 
     This application claims priority to French Patent Application No. 0652544, filed Jun. 20, 2006, which application is assigned to the assignee of the present invention and the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a distractible intervertebral implant. 
     BACKGROUND OF THE INVENTION 
     So-called “replacement” intervertebral implants serve to replace one or two vertebral bodies together with their associated disks after one or two vertebrae have been removed in part. This type of implant is thus disposed between the vertebral bodies of two vertebrae. At each of its ends, the implant itself has a respective prosthetic plate for anchoring in an adjacent vertebral body, these vertebral plates usually being fitted with portions in relief firstly to provide effective anchoring of the prosthetic plates in the vertebral plates, and also to damage the plates in such a manner as to encourage bone growth within the implant itself. 
     Such a replacement implant is said to be “distractible” if the distance between the two prosthetic plates can be adjusted or adapted as a function of the spacing it is desired to achieve between the two vertebrae. 
     There exist numerous distractible intervertebral implants and they can be classified in two major categories. In the first category, there are distractible implants, which themselves include a mechanism enabling the distance between the prosthetic plates to be modified and adjusted. Those implants present the drawback of being relatively complex because of the presence of the mechanism for distracting the prosthetic plates. 
     In the second category, there are intervertebral implants that enable the distance between the prosthetic plates to be modified with the help of an external surgical instrument. Under such circumstances, the implant needs to have a mechanism that enables the distraction between the two plates to be maintained after the surgical distraction instrument has been removed. 
     In this second category, distractible intervertebral implants are known in which the prosthetic plates are mounted respectively at the ends of two cylinders that are themselves mounted telescopically. The two telescopic cylinders are prevented from moving relative to each other by known solutions using one or more cone-point screws. 
     It will be understood that such a system for interconnecting the cylinders on which the prosthetic plate are mounted present the drawback of not allowing the distance between the prosthetic plates to be adjusted continuously, since it is necessary to provide blind holes in the inside cylinder for receiving the cone-point screws, which are themselves mounted in the outside cylinder. 
     An object of the present invention is to provide a replacement intervertebral implant that is distractible in continuous manner, while presenting a structure that is mechanically simple. 
     SUMMARY OF THE INVENTION 
     To achieve this object of the invention, the implant includes a first assembly constituted by a first prosthetic plate and a first spacer element having at least a portion of its outside surface that is cylindrical, the prosthetic plate being secured to the first spacer element; a second assembly constituted by a second prosthetic plate and by a hollow second spacer element having at least a portion of its inside surface that is cylindrical, the first end of the second spacer element being secured to the second prosthetic plate and the first spacer element being suitable for sliding in the second spacer element; and means for preventing the first spacer element from moving in translation or in rotation relative to the second spacer element, the means having a clamping collar surrounding a portion of the outside surface of the second spacer element, and a lock screw for tightening the collar onto the second spacer element in such a manner that the elastic deformation of the second spacer element holds the two spacer elements together, thereby enabling the distance between the two prosthetic plates to be adjusted. 
     It will be understood that by means of the characteristics of the invention, once the surgeon has adjusted the distance between the two prosthetic plates using the external distraction instrument, it suffices for the surgeon to act on the screw associated with the clamping collar to ensure that the two spacer elements cannot move relative to each other. This is achieved by the elastic deformation produced by tightening the collar against the outside face of the second spacer element. It will be understood that such a fastening technique enables the distance between the prosthetic plates to be adjusted continuously. 
     Preferably, each prosthetic plate is removable from the spacer element that is associated therewith. 
     It will be understood that it is thus possible with a given pair of spacer elements to mount distinct prosthetic plates that are adapted to the particular anatomic situation. 
     Also preferably, each prosthetic plate presents an anchor face for anchoring in a vertebral plate, each prosthetic plate including an anchor element projecting from the anchor face. 
     Also preferably, each anchor element is removable from the prosthetic plate with which it is associated, and also preferably the anchor element can occupy a plurality of angular positions relative to the prosthetic plate that is associated therewith. 
     This disposition makes it possible to adapt the direction of the anchor element to the surgical technique used for putting the intervertebral implant into place. 
     In a preferred embodiment, the implant further comprises a tubular distance piece suitable for being interposed between a prosthetic plate and the first end of the spacer element that is associated therewith, in order to increase its length. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other characteristics and advantages of the invention appear better on reading the following description of a plurality of embodiments of the invention given as non-limiting examples. The description refers to the accompanying figures, in which: 
         FIG. 1  is an elevation view of the vertebral implant; 
         FIG. 2  is a horizontal section view on line II-II of  FIG. 1 ; 
         FIG. 3  is a horizontal section view on III-III of  FIG. 1 ; 
         FIG. 4  is a vertical section view on line IV-IV of  FIG. 1 ; 
         FIG. 5  is a rear elevation view of the distractible implant; 
         FIG. 6  is a section view on line VI-VI of  FIG. 5 ; 
         FIG. 7  is a view from beneath of the implant shown in  FIG. 5 ; and 
         FIG. 8  is a view of the implant assembly from the side. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As can be seen in  FIG. 1 , the distractible vertebral implant is constituted by a first or bottom assembly  10 , by a second or top assembly  12 , and by means  14  for securing the assemblies  10  and  12  to each other, and constituted in the embodiment shown by a clamping collar  14 . 
     In the embodiment shown in  FIG. 1 , the first assembly  10  is constituted by a bottom prosthetic plate  16 , a bottom spacer element  18 , and a distance piece  20 . Similarly, the second or top assembly  12  is constituted by a top prosthetic plate  22 , a top spacer element  24 , and a top distance piece  26 . The fastener means  14  are constituted by a clamping collar  28  and a clamping screw  30 . 
     In principle, the first spacer element or bottom spacer element  18  is mounted telescopically in the top spacer element  24 . With the help of an external distracter instrument engaged in notches  25  and  27  formed at the ends of the spacer elements  18  and  24 , this enables the surgeon to adjust the distance between the bottom prosthetic plate  16  and the top prosthetic plate  22  by causing the spacer element  18  to slide in the spacer element  24 . When the desired spacing is reached, the surgeon can act on the clamping screw  30  to clamp the top spacer element  42  on the bottom spacer element  18  by elastically deforming the top spacer element  24 . This prevents the bottom assembly  10  from moving relative to the top assembly  12 . It can be understood that by preventing relative movement in this way, the distance between the two prosthetic plates can be adjusted in fully continuous manner. 
     The top prosthetic plate  22  has an anchor face  22   a  for anchoring in the vertebral plate, and in its other face it has a fastener skirt  22   b . The fastener skirt  22   b  of the prosthetic plate  22  is engaged in an enlarged portion  26   a  of the distance piece  26 . 
     The bottom prosthetic plate  16  is mounted in the bottom distance piece  20  in the same manner. 
     The prosthetic plate  16  has pairs of notches such as  40  that are diametrically opposite, as can be seen in  FIG. 6 . In each pair of notches, it is possible to engage a support part  42 . In the central portion  42   a  of the support part, there is inserted an anchor member  44  that projects from the anchor face  16   a  of the prosthetic plate  16 . The anchor member  44  is preferably in the form of a triangular fin, as can be seen more clearly in  FIG. 8 . Because of the presence of the pairs of notches  40 , it is possible to give the anchor fin  44  a desired angular direction. The anchor support part  42  having its ends engaged in the notches  40  of the skirt  16   b  is held on the axial direction of the implant by the bottom of the notch  40   a  and by a shoulder  46  formed in the inside face of the bottom distance piece  20 . 
     The top prosthetic plate  22  is also fitted with a fin-shaped anchor member  44  that is mounted in the same manner as the fin of the bottom prosthetic plate  16  and that presents the same facilities for being oriented angularly. 
     The top distance piece  26  has a fastener skirt  26   b  having exactly the same shape as the fastener skirt  22   b  of the prosthetic plate. This skirt  26   b  is engaged in a portion  24   a  of enlarged diameter provided at the top end of the top spacer element  24 . The top spacer element  24  is generally in the form of a cylinder presenting a cylindrical inside face  45 . A groove  51  is formed in its outside face  50  for receiving the clamping collar  28 . In order to prevent the clamping collar  28  with its screw  30  leading to a zone of greater thickness,  FIG. 2  shows that the spacer element  24  level with the clamping collar presents an opening  52  in a portion of its periphery, which opening performs a function that is explained below. 
     As shown in  FIG. 4 , the bottom end  18   a  of the spacer element  18 , which is secured to the distance piece  20 , is cylindrical in shape. In contrast, in its main portion, as can be seen in  FIG. 2 , this spacer element  18  comprises both a cylindrical portion  18   b  and a portion  18   c  having a larger radius of curvature in register with the opening  52  formed in the top spacer element  24 . Thus, the clamping collar  28  surrounds the outside face of the element  24  and penetrates into it through the opening  52 . The clamping collar  28  is thus pressed in part against the outside face of the top spacer element  24  and in part against the larger radius of curvature portion  18   c  of the spacer element  18 . Since the outside face of the top spacer  24  has no relief, the direct contact between the collar and the portion of the top spacer does not disturb the continuous adjustment of the distance between the two prosthetic plates. In addition, and preferably, the top spacer element  24  has at least one opening  63  for inserting additional graft material into the internal cavity of the implant. The clamping collar  28  has two ends  28   a  and  28   b . In the end  28   a , there is formed an orifice  60  which is smooth, while in the end  28   b , there is formed a tapped orifice  62 . Thus, with the clamping screw  30 , which is of the hollow hexagonal cylindrical type, for example, it is possible to move the ends of the clamping collar  28  towards each other, thereby elastically deforming the top spacer element  24 , which is thus pressed against the outside face of the bottom spacer element  18 , the clamping collar acting in part through the opening  52  against the outside face of the spacer element  18 . To encourage elastic deformation spacer element  24 , it may advantageously be provided with a longitudinal slot  64  that extends as far as its bottom end. 
     To improve the way the two spacer elements  18  and  24  are held stationary relative to each other under the action of the clamping collar, the inside face  45  of the spacer element  24  and the outside face  47  of the spacer element  18  are treated so as to increase their roughness coefficients. 
     Preferably, the fastening of the prosthetic plates  16  and  22  on the distance pieces  20  and  26 , and the fastening of the distance pieces  20  and  26  on the spacer elements  18  and  24  is performed by snap-fastening. As shown in  FIG. 3 , in order to secure the distance piece  20  with the bottom element  18   a  of the spacer element  18 , a snap-fastening pin  66  presents one end that is secured to the bottom end  18   a  of the spacer element  18  and its other end penetrates into a notch  68  formed in the top edge of the distance piece  20 . The same applies to the other elements for securing to one another. 
     In the example described, each assembly  10  or  12  is constituted by a prosthetic plate, a distance piece, and a spacer element. The distance pieces  20  and  26  are naturally used only when it is desired to achieve a considerable distance between the prosthetic plates  16  and  22 . Under all other circumstances, the distance pieces are not interposed between the prosthetic plates and the spacer elements. The prosthetic plates are then mounted directly to the ends of the spacer elements  18  and  24  by snap-fastening. 
     In the description above, the prosthetic plates  16  and  22  are removable from the distance pieces  20  and  26 , and thus from the spacer elements  18  and  24 . This disposition makes it possible with a single set of spacer elements  18 ,  24  to put different kinds of prosthetic plates  16  and  22  into place, depending on the morphological characteristics of the patient, for example. Nevertheless, it would naturally not go beyond the ambit of the invention for the prosthetic plates to be permanently secured to the ends of the spacer elements  18  and  24 . 
     It is also clear that the particular shapes given to the bottom spacer element  18 , of section that is not circular, presents the advantage of enabling the clamping screw  30  to be inset so as to avoid it projecting out from the implant. Nevertheless, it would not go beyond the ambit of the invention if the bottom spacer element  18  were likewise cylindrical in shape.