Abstract:
An implant device designed for replacing a vertebral body of the spine. The implant device contains at least one moving member so as to vary the distance between a first and second member. Most preferably this movement is achieved by rotating at least one cam about an axis. The cam or cams are shaped so as to comprise an irregular polygon thus allowing the height of the implant device to be varied at discrete positions. A slidable telescopic engagement device within the implant device affords the added advantage of simple adjustment and maneuverability.

Description:
BACKGROUND OF THE INVENTION 
     Various implants of this type are already known, and conventionally they comprise two elements for bearing against the overlying and underlying vertebral plates, and means for verifying the distance between said bearing elements so as to match the implant to the height required as a function of the morphology of the patient&#39;s spine. 
     The means for varying the height of the implant can comprise a rotary member, together with screw-and-nut type means. 
     A major drawback of that known implant lies in that the rotary member must be turned about an axis that is substantially parallel to the axis of the spinal column. This can make it extremely difficult to bring the implant to the desired height since the tool used for turning said member, given the surrounding environment of the patient, can be turned through only a small fraction of a complete turn, so it must be maneuvered many times in order to reach the desired height. This is made worse by the fact that the thread must necessarily be fine so as to generate the axial force required for separation purposes without running the risk of producing slip between the bearing elements and the over-and underlying vertebral plates. 
     The present invention seeks mainly to mitigate that drawback, and to propose an implant for replacing a vertebral body, which implant is simpler to handle when being put into place when performing separation, and which is also provided with excellent stability against the axial compression forces to which it will be exposed. 
     SUMMARY OF THE INVENTION 
     Thus, the present invention provides an implant specifically for replacing a vertebral body in surgery of the spine, the implant comprising first and second bearing elements for bearing against under-and overlying vertebral plates, retaining means for retaining the two bearing elements one above the other, and at least one moving member suitable for varying the distance between said bearing elements, the implant being characterized in that the retaining means comprise at least one slideway means provided between the two bearing elements, and in that the or each moving member is constituted by a cam having discrete positions suitable for being rotated about an axis that is essentially horizontal and essentially parallel to the sagittal plane. 
     Preferred but non-limiting features of the implant of the invention are as follows: 
     the or each discrete-position cam has, in cross-section, an outline in the form of an irregular polygon; 
     the or each cam possesses a plurality of pairs of faces, the faces in each pair being mutually parallel, and each pair of faces being spaced apart at a distance different from the spacing between the other pairs of faces; 
     the cam has three pairs of faces; 
     each bearing element possesses a bearing plate having a groove formed in one face thereof, and at least some of the pairs of faces are dimensioned in such a manner as to be of a length in the circumferential direction of the cam that is very slightly shorter than the width of each groove; 
     the faces of at least one pair of faces of the cam are of a length, in the circumferential direction of the cam, which is matched to the width of the grooves in the bearing plates by means of respective setbacks provided in transitions between said faces and adjacent faces; 
     the or each cam is mounted to move in rotation and in translation on the or each slideway means; 
     the or each slideway comprises a hollow slideway-forming upright formed from one element of the implant, and a slider-forming element formed from another element of the implant, and the slideway and the slider have elongate openings passing through them to pass a cam-mounting shaft; 
     the cam-mounting shaft is defined by a screw passing through a central passage of the cam and through said elongate openings, and engaged in a nut; 
     one of the elongate openings of the slideway is a notch suitable for preventing the nut from rotating; 
     the bearing elements have teeth on their outside surfaces for engaging in the vertebrae lying over and under the implant; and 
     the implant comprises two bearing elements and a central element, two cams operating respectively between the top bearing element and the central element and between the bottom bearing element and the central element, and two slideway-forming means operating respectively between the top bearing element and the central element and between the bottom bearing element and the central element. 
     Other aspects, objects, and advantages of the present invention will appear better on reading the following detailed description of a preferred embodiment thereof, given by way of example and made with reference to the accompanying drawings, in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 and 2 are exploded perspective views of an implant of the invention seen from two different directions; 
     FIGS. 3 and 4 are perspective views of an implant of the invention in the assembled state, seen from two different directions; 
     FIG. 5 is an elevation view of the side of the assembled implant of FIGS. 3 and 4; 
     FIG. 6 is an elevation view of the face of the assembled implant of FIGS. 3 to  5 ; 
     FIG. 7 is a plan view of the assembled implant of FIGS. 3 to  6 ; and 
     FIG. 8 is an exploded perspective view of a variant embodiment of an implant. 
    
    
     DETAILED DESCRIPTION 
     With reference to FIGS. 1 to  7 , there is shown an implant for replacing a vertebral body, the implant comprising seven elements, namely: a central element  10 ; two bearing elements  20   a  and  20   b , respectively constituting a top bearing element and a bottom bearing element; two elements forming cams having discrete positions and respectively referenced  30   a  and  30   b , one operating between the element  10  and the element  20   a , and the other operating between the element  10  and the element  20   b ; and finally two screws  40   a  and  40   b  suitable for co-operating with two nuts  50   a  and  50   b  to hold the assembled implant firmly in the required position. 
     The central element  10  comprises an intermediate plate  11  that is generally in the form of a horizontal disk. From this disk there extend upwards and downwards respectively a first slideway-forming hollow cylindrical upright  13   a , and a second slideway-forming hollow cylindrical upright  13   b  situated vertically in line with the upright  13   a . The section of the slideways  13   a  and  13   b  is oval in shape. Each slideway extends from the vicinity of one edge of the disk  11  and has a respective oblong through slot  15   a ,  15   b  in its inside face, and has a respective vertically elongate notch  14   a ,  14   b  in its outside face, where the notch is slightly wider than the slot and opens out to the free end of the associated slideway. 
     In addition, each slideway  13   a ,  13   b  has a small respective setback  16   a ,  16   b  in its inside face, with the oblong slot  15   a  or  15   b  passing through the bottom wall of the corresponding setback. 
     Finally, the central element  10  has a shallow groove  12   a  of predetermined width in the top face of the disk  11 , which groove extends essentially along a diameter of the disk away from the slideway  13   a  as far as the opposite edge of the disk. An identical and symmetrical groove  12   b  is formed in the bottom face of the disk  11 . 
     The top element  20   a  of the implant has a top plate  21  from which there extends downwards a solid slider  23  which is integral with the plate and whose section is slightly smaller than that of the inside section of the slideway  13   a , said slider  23  having a slot  24  passing through it of a size that is close to the size of the slot  15   a  in the element  10 . 
     A series of teeth  26  is formed on the top surface of the plate  21  for reasons explained below, which teeth are more particularly of pyramid shape. Finally, the bottom face of the plate  21  has a groove  22  formed therein of shape and size that are preferably identical to the shape and size of the groove  12   a  in the central disk  11  of the element  10 . 
     In this case, the bottom element  20   b  is completely identical to the top element  20   a , after merely being turned upside-down through 180°. 
     The element forming a discrete-position cam referenced  30   a  for acting between the elements  10  and  20   a  has an irregular and generally hexagonal outline in cross-section. More precisely, the element  30   a  possesses: 
     two mutually parallel opposite large faces  31  and  31 ′ that are spaced apart by an orthogonal distance d 1  (see FIG.  6 ); 
     two mutually parallel opposite faces  32  and  32 ′ of intermediate size that are spaced apart by an orthogonal distance d 2  greater than d 1 ; and 
     two mutually parallel opposite small faces  33  and  33 ′ that are spaced apart by an orthogonal distance d 3  greater than d 2 . 
     It will also be observed that the transitions between the faces  31  and  33  and between the faces  31 ′ and  33 ′ are slightly rounded. 
     It will also be observed that the transitions between the faces  32  and  33  and between the faces  32 ′ and  33 ′ are provided with respective setbacks  321  and  321 ′ whose bottoms are parallel to the respective faces  32  and  32 ′ and of depth substantially equal to the depth of the grooves  12   a ,  12   b , and  22  formed respectively in each of the faces of the central disk  11  of the element  10  and in the inside faces of the plates  21  of the elements  20   a  and  20   b.    
     In addition, the lengths (i.e. dimensions in the circumferential direction) of the various faces of the element  30   a  are as follows: 
     the large faces  31  and  31 ′ are of length greater than the width of the grooves  12   a ,  12   b , and  22 ; 
     by appropriately selecting the width of the setbacks  321  and  321 ′, the middle faces  32  and  32 ′ are of effective length that is very slightly shorter than the width of the grooves  12   a ,  12   b , and  22 ; and 
     the small faces  33  and  33 ′ are likewise of an effective length that is very slightly smaller than the width of the grooves  12   a ,  12   b , and  22 . 
     The element  30   a  also possesses a central through bore  34  which terminates at one of the side faces of the element in an enlarged portion  35  of hexagonal section, forming a socket for receiving a tool as explained below. 
     In this case, the cam-forming element  30   b  for operating between the elements  10  and  20   b  is completely identical to the element  30   a.    
     Finally, the implant of the invention has two identical screws  40   a  and  40   b  each possessing a head  41  provided with a socket  43  for a driving tool, and a threaded shank  42 , and also has two nuts  50   a  and  50   b  of square outline with rounded corners, each having a tapped bore  51  passing therethrough complementary to the thread of the screws. 
     At this point, it should be observed that: 
     the cross-section of the sockets  35  is larger than that of the heads  41  of the screws  40   a  and  40   b , while the depth of said sockets  35  is perceptibly greater than the axial dimension of said heads  41 ; 
     the outside diameter of the threaded shanks  42  is slightly smaller than the diameter of the bores  34  through the elements  30   a  and  30   b ; and 
     the side of each nut  50   a  and  50   b  is selected to be slightly smaller than the width of the notches  14   a  and  14   b  formed in the uprights  13   a  and  13   b  of the element  10 ; 
     There follows a description of how the implant of the invention is assembled, and then of the various steps implemented by the surgeon while putting the implant into place. 
     Firstly, the implant is assembled by inserting the sliders  23  of the elements  20   a  and  20   b  in the respective slideways  13   a  and  13   b  of the element  10 . Thereafter the elements  30   a  and  30   b  are prepositioned between the plate  21  of the element  20   a  and the plate  11 , or between the plate  21  of the element  20   b  and the plate  11 , such that the respective bores  34  thereof are in register with the respective slots  15   a  and  15   b . Then the screws  40   a  and  40   b  are engaged in the respective passages  35  and  34 , in the slots  15   a  and  15   b , and in the notches  14   a  and  14   b  where they are screwed into the respective nuts  50   a  and  50   b.    
     Prior to the implant being put into place, the cam-forming elements  30   a  and  30   b  are initially set to their minimum height, i.e. they have their large faces  31  and  31 ′ bearing against the facing faces of the respective plates, projecting on either side of the grooves  12   a ,  12   b , and  22  so as to provide a stable bearing force. 
     At this stage, the screws  40   a  and  40   b  are not tightened, thereby allowing the elements  30   a  and  30   b  to be rotated subsequently. 
     In this minimum-height configuration, the implant is put into place by the surgeon between the vertebral plates of the over-and underlying vertebrae, and if necessary the surgeon performs a small amount of separation to make this possible. 
     Once the implant has been put into place, a tool is engaged into the sockets  35  of the elements  30   a  and  30   b  so as to enable rotation to be imparted thereto, thereby bringing each of these elements: 
     either into an intermediate position in which the surfaces  32  and  32 ′ bear against the respective plates, and more precisely against the bottoms of the grooves  22  and  12   a  (or  22  and  12   b  ); or else 
     in a position of maximum height in which the surfaces  33  and  33 ′ bear against the respective plates, and more precisely, in this case also, against the bottoms of the grooves  22  and  12   a  (or  22  and  12   b  ). 
     Since a large amount of torque can be exerted very easily via the posterior access to the elements  30   a  and  30   b , this rotation makes it possible to perform controlled separation between the under-and overlying vertebral plates. The stability of the implant during this separation operation is guaranteed by the teeth  26  of the top and bottom plates  21  which bite into the over-and underlying vertebral plates under the action of the axial force that results from the drive from the cams. 
     At this point, it will be observed that the presence of two cam-forming elements each having three discrete positions makes it possible to obtain a satisfactory variety of heights for the implant, which heights are given respectively by the following combinations of cam heights: 
     d 1  and d 1   
     d 1  and d 2   
     d 1  and d 3   
     d 2  and d 2   
     d 2  and d 3   
     d 3  and d 3   
     At this point, it will be observed that by using two elements  30   a  and  30   b  of different shapes, the number of combinations can be further increased. 
     It will also be observed that the surfaces  32  &amp;  32 ′ or  33  &amp;  33 ′ of a cam  30   a  or  30   b  being held in the grooves  12   a  &amp;  22  or  12   b  &amp;  22  also contributes to the stability of the implant by guaranteeing stability for the angular position of the cam, even in the presence of large compression forces being exerted by the over-and underlying vertebrae, after they have been separated. 
     The stability of the cam in its position of least height (when the surfaces  31  and  31 ′ are the bearing surfaces) is guaranteed because these surfaces are long. 
     After the implant has been adjusted to the desired height, the screws  40   a  and  40   b  are tightened by means of an appropriate tool to confirm the fixing of the various elements in the required position, the nuts  50   a  and  50   b  being blocked against any rotation within the notches  14   a  and  14   b.    
     FIG. 8 shows a variant embodiment of the invention which can be used when it is desired to obtain an implant of small height. 
     It differs from the embodiment of FIGS. 1 to  7  in that only one cam-forming element  30  is provided associated with two vertebral thrust elements  100  and  200 . The element  100  has a bottom plate  11  provided with teeth  26  on its bottom face and a slideway  13 , and with a groove  12  in its top face. The element  200  possesses a top plate  21  provided with teeth  26  on its top face and with a slider  23 , and with a groove (not shown) in its bottom face. The cam  30 , the screw  40 , and the nut  50  are identical to the corresponding elements in the preceding embodiment. 
     Naturally, the present invention is not limited to the embodiments described and shown, and the person skilled in the art will be able to apply any variant or modification within the spirit of the invention.