Abstract:
The intersomatic spine implant comprises a body, at least one anchoring element movable relative to the body to project from a contact face of the body making contact with a vertebra, and at least one cam slidable relative to the body and suitable for displacing the anchoring element relative to the body by the effect of a ramp engaging the anchoring element. The cam and the anchoring element are arranged so that the cam moves the anchoring element in two opposite displacement directions.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to intersomatic spine implants. 
     Document FR-2 727 003 discloses an intersomatic spine implant for putting into the place of a vertebral disk after it has been removed, and comprising a body having two plane faces that come into contact with the adjacent vertebral bodies. It has two housings for receiving anchoring screws, disposed in such a manner that the screws project from respective contact faces so as to be anchored into the adjacent vertebral bodies. Each screw slopes relative to the associated contact face because the head of the screw projects from a side of the body so as to be capable of being driven once the implant has been received between the vertebral bodies. Nevertheless, it is difficult to put the screws into place because of the slope of their axes. Furthermore, the positioning of the screws cannot be improved in order to optimize the quality of the anchoring they provide without making them less accessible. 
     U.S. Pat. No. 5,702,391 discloses an intersomatic implant comprising a body, slidably movable pins in the body for projecting from outside faces of the body, and spherical cams slidably movable in an axial duct of the body. An actuator piece disposed at the mouth of the duct enables thrust to be applied to the cams which move the pins by a ramp effect so that they project and thus anchor the implant in the plates of the associated vertebrae. Such an implant makes it much easier to achieve robust anchoring between the vertebrae. However, the implant is very difficult to remove should that be necessary, which in contrast is not the case with the implant disclosed in above-mentioned FR-2 727 003 since it needs only to have the screws undone to eliminate anchoring between the implant and the plates. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide an implant that is easy to install and remove. 
     To achieve this object, the invention provides an intersomatic spine implant comprising a body, at least one anchoring element movable relative to the body to project from a contact face of the body making contact with a vertebra, and at least one cam slidable relative to the body and suitable for displacing the anchoring element relative to the body by the effect of a ramp engaging the anchoring element, wherein the cam and the anchoring element are arranged so that the cam moves the anchoring element in two opposite displacement directions. 
     Thus, the anchoring element is moved by means of the cam, by taking action on the cam. Since action is no longer taken directly on the anchoring element, constraints associated with accessibility of the anchoring element are to a very large extent eliminated. As a result, the anchoring element is easier to drive into place during surgery. Furthermore, since it is no longer necessary to make the anchoring element directly accessible, its positioning can be modified in a very wide variety of ways so as to ensure that it performs its anchoring function as well as possible. Consequently, the operation of installing the anchoring element is made easier, while also making it possible to improve the quality of anchoring. 
     In addition, since the action of the cam is reversible, it enables the or each anchor element to be actuated so as to go from the extended position to the retracted position. It is then easy to remove the implant. This action of the cam on the anchoring element is positive in the sense that the cam entrains the anchoring element. The action of the cam does not consist solely in leaving the way open for the implant to be capable of penetrating into the body under the effect of external pressure exerted on the anchoring element by the material of the vertebrae. Thus, in particular, it is possible to remove the implant a long time after it has been put into place. 
     The implant of the invention may also present one or more of the following characteristics: 
     the cam has a thread suitable for co-operating by screw engagement with an actuator for driving the cam from outside the body; 
     the actuator is suitable for being mounted to move 
     in rotation relative to the body; 
     the cam is mounted to move sliding relative to the body; 
     the cam has an end providing a face that is undercut relative to a travel direction of the cam so as to enable the cam to be extracted from the body; 
     the implant includes at least two anchoring elements and at least two cams suitable for moving respective anchoring elements; 
     the two cams are arranged so that their threads cooperate with a common actuator; 
     the anchoring element slopes relative to a general plane of the contact face; 
     the implant includes at least two anchoring elements suitable for projecting from the same contact face; 
     the implant includes at least four anchoring elements suitable for projecting from the same contact face and disposed in two rows defining mutually-parallel alignment directions; 
     the body has two contact faces for making contact with respective vertebrae and at least one recess extending between the contact faces; 
     the portion of the anchoring element suitable for projecting from the contact face has faces that are undercut relative to the sliding direction of the element towards the vertebra; and 
     the implant presents at least two anchoring elements and an element-carrier rigidly connected to the anchoring elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other characteristics and advantages of the invention appear further from the following description of five preferred embodiments given as non-limiting examples. In the accompanying drawings: 
     FIGS. 1 and 2 are two perspective views of an implant constituting a first embodiment of the invention shown respectively in the assembled state and in an exploded state; 
     FIGS. 3 and 4 are two section views of the FIG. 1 implant during two respective steps of installation thereof; 
     FIG. 5 is a view analogous to FIG. 4 showing the implant between two vertebrae; 
     FIGS. 6 and 7 are respectively an exploded perspective view and a section view of an implant constituting a second embodiment of the invention; 
     FIG. 8 is a perspective view of an implant constituting a third embodiment of the invention; 
     FIG. 9 is a perspective view of the FIG. 8 implant with the top portion of its body removed; 
     FIGS. 10 and 11 are perspective views showing an implant constituting a fourth embodiment of the invention respectively in the assembled state and in an exploded state; 
     FIG. 12 is a fragmentary axial section view of the cam and the screw of FIG. 11; 
     FIG. 13 is a perspective of an implant showing a fifth embodiment of the invention; 
     FIG. 14 is a section view of the implant on plane XIV—XIV of FIG. 13; 
     FIGS. 15 and 16 are two perspective views of the cam and the element-carrier of the FIG. 13 implant shown respectively in an element-retracted position and an element-extended position; and 
     FIG. 17 is a section view of the cam and the element-carriers on the midplane XVII—XVII FIG.  16 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A first embodiment of the implant is described with reference to FIGS. 1 to  5 . The implant  102  comprises a body  4  that is generally in the form of a rectangular parallelepiped flattened in one direction so as to define two large faces  6 , namely a top face and a bottom face, and four side faces  8 . The edges and the corners of the body are rounded to avoid injuring the tissues of the human body for which the implant is intended. The top and bottom faces  6  are generally plane in shape and they have respective transverse profiles that are sawtoothed or zigzag, the tips of the teeth defining continuous mutually-parallel edges. These edges provide good engagement of the top and bottom faces  6  against the plates of the vertebral bodies of the destination vertebrae  9 , as shown in FIG.  5 . 
     The body  4  has two through recesses  10  extending close to two opposite edges of the body and between the top and bottom faces  6  so as to open out into both of them. Furthermore, each side face  8  has an orifice  12  opening out into one of the recesses  10  halfway between the top and bottom faces  6 . When the implant is put into place, the recesses  10  are filled with graft tissue which can thus grow towards the vertebral plates through the recesses  10  and the orifices  12 . 
     The body  4  has a cylindrical duct  14  so that its axis  16  extends through two opposite corners of the body that are remote from the recesses  10 , and parallel to the general planes of the top and bottom faces  6 . The duct  14  is open at both ends. In the vicinity of a distal one of its ends, it presents a segment  18  of smaller diameter that is threaded. The remainder of the duct  14  constitutes a smooth segment  20  of larger diameter than the segment  18 . 
     The body  4  has channels  22  of cylindrical shape extending from the segment  20  to the top and bottom faces  6 . All of the channels  22  have axes  24  which intersect the axis  16  in the segment  20 , in this case at right angles. There are twelve such channels  22  in this case. They are subdivided into two groups of six channels  22 . In each group, the six channels  22  are parallel to one another, in axial alignment in pairs, and situated in a common plane that includes the axis  16 . The section of FIGS. 3 and 4 lies in one such plane so that all six channels in one of the groups can be seen in these figures. This plane slopes relative to the general planes of the top and bottom faces  6  such that the axes  24  of the channels  22  slope likewise. The slopes are symmetrical and have the same angle for both groups of channels. In each group of six channels  22 , three of the channels open out into the top face  6  and three into the bottom face  6 . Each channel  22  that opens out into the top face  6  is in axial alignment with one of the channels in the same group that opens out into the bottom face. In each group, the three channels that open out into the same face are spaced apart a common pitch. Thus, in each face  6 , channels  22  open out that are distributed in two rows defining mutually-parallel alignment directions, with the channels in each row sloping in opposite directions. 
     Each channel  22  slidably receives an anchoring element constituted in this case by a pin  26  having a smooth cylindrical body presenting a point at an end closer to the face  6  and presenting, at an opposite end, a head having a convex spherical face of radius greater than the radius of the channel  22  and of width greater than the diameter of the associated channel  22 . The head is situated in the segment  20  and thus holds the pin  26  prisoner against being extended fully through the associated face  6 . 
     The implant has a cam  130  that is circularly cylindrical about an axis lying on the axis  16 . At a distal axial end it presents a threaded cylindrical face  134  suitable for entering into screw engagement with the segment  18  of the body. At a proximal axial end  136  it presents a head of diameter greater than the diameter of the segment  20  of the body so as to come into abutment against the outside of the body. The head presents a socket, e.g. a hexagonal socket with six flats, thus enabling the cam  130  to be rotated about its axis by means of a suitable tool such as a key. 
     Between its two ends, the cam  130  presents three broad cylindrical faces  138 , three cylindrical faces  140  that are narrow compared with the broad faces  138 , and three frustoconical faces  142  that slope towards the threaded distal end  134 . These faces alternate and are distributed in three consecutive groups each comprising in the proximal-distal direction: a broad face  138 ; a frustoconical face  142 ; and a narrow face  140 , the frustoconical face  142  providing a transition in level between the other two faces. The lengths of the faces along the axis  16  are identical for each type of face. These lengths are adapted so that when the threaded distal end  134  of the cam engages with only the proximal end of the segment  18 , as shown in FIG. 3, the heads of the pins  26  bear against the narrow faces  140 , with each narrow face  140  being in contact with the heads of four pins  26  whose axes  24  are coplanar in a plane perpendicular to the axis  16 , whereas when the head of the cam  130  is in abutment against the body, as shown in FIG. 4, the heads of the pins  26  bear against the broad faces  138 , with each broad face  138  being in contact with the four above-mentioned pins  26 . The diameter of the narrow faces  140  is such that when in the position shown in FIG. 3, referred to below as the “retracted position”, the points of the pins  26  do not project beyond the associated face  6 , or project so little that they do not significantly impede installation of the implant  102  between the vertebral bodies. The diameter of the broad faces  138  is such that when in the position of FIG. 4, referred to below as the “extended position”, the pins  26  project from the face  6 , e.g. by one-fourth to one-third of their length, and penetrate far enough into the associated vertebral body to prevent the implant being withdrawn. 
     The implant is put into place as follows. After a vertebral disk has been removed, and after the recesses  10  have been filled with graft tissue, as mentioned above, the implant  102  is inserted between the vertebral bodies of the vertebrae  9  associated with the disk that has been removed. The height of the body  4  of the implant corresponds to the thickness of the removed disk. The implant is inserted in such a manner that the threaded segment  18  is substantially in the posterior position. The faces  6  extend facing respective vertebral plates, being parallel thereto and in contact therewith. The implant is inserted while it is in its retracted configuration as shown in FIG.  3 . 
     Once the implant is in position, a key is used to drive the head of the cam  130  which is situated in the anterior position so as to cause the cam to turn about its axis  16 . Given the screw engagement between the distal end  134  of the cam and the segment  18 , the cam thus follows a helical path along its axis  16 . For each group of four pins, the contiguous frustoconical face  142  comes progressively into contact with the heads of the pins and constitutes a ramp which, given its displacement towards the segment  18 , urges the four pins so as to cause them to slide towards the outside of the body. As the pins  26  extend outwards, they penetrate into the vertebral plates and anchor the implant in the vertebrae. The four pins  26  then come to bear against the contiguous broad face  138  and project from the respective faces  6  in the extended configuration. At the end of driving the cam  130 , the head of the cam bears against the body and the distal end  134  of the cam is at the distal end of the segment  18 . 
     In a variant of this first embodiment, the screw engagement between the cam  130  and the body  4  could be replaced by snap-fastening or clipping to prevent the cam from moving relative to the body after the cam has merely been thrust parallel to its axis. The cam is then slidably movable relative to the body. Under such circumstances, its cross-section relative to its axis need not be circular, for example it could be rectangular. 
     A second embodiment of the implant is described with reference to FIGS. 6 and 7. Elements that differ from those of the first embodiment are given numerical references plus  100 . 
     In the implant  202 , the body  4  has substantially the same configuration as in the preceding embodiment, apart from the fact that the smooth larger-diameter segment  20  constitutes the entire length of the cam duct  14 . In this case, the cam  130  is replaced by a set of three cams  230  and a screw  250 . The screw  250  has a drive head  236  forming an abutment against the body, analogous to that of the cam  130 . The screw has a threaded rod  252 . 
     The three cams  230  are identical to one another. Each cam  230  is generally cylindrical in shape. It has a threaded cylindrical duct  253  suitable for co-operating with the rod  252  by screw engagement. Each cam  230  has four slots  254  each associated with a respective specific one of four pins  26  to be actuated by the cam. Each slot  254  extends in a plane that is radial relative to the axis  16  of the cam. Each slot  254  has a shallow, high portion  238 , a low portion  240  which is deep relative to the shallow portion, and an intermediate portion  242  forming a transition in level between the high and low portions. The high portions of the four slots  254  in any one cam are contiguous to the proximal end of the cam. Perpendicularly to the axis  16 , each slot has a profile in the form of an outwardly open circular arc extending over more than a semicircle, of radius that is constant along the slot, with the edges of the circular arc extending outwards level with the intermediate and low portions in the form of two mutually-parallel plane flanks. Each slot is adapted to receive the head of the associated pin  26  in the axial direction via either end, while preventing the head from escaping in the radial direction of the cam. The bottom of each slot  254  constitutes a first ramp for causing the associated pin  26  to slide outwards when the cam slides towards the distal end of the duct  14 . An advantage of this embodiment is that it is reversible. Since the head of each pin  26  is held captive in the associated slot  254 , the edges of the slot constitute a second ramp enabling the pin to be moved back into the retracted position when the cam  254  slides towards the proximal end of the duct. 
     The various parts of the implant are assembled as follows. After the pins  26  have been received in their channels  22  in the body  4 , one of the cams  230  is slid to the central position associated with the pins  26  in the middle of the row. To be able to do this, the cam  230  must be capable of “getting past” the four pins  26  at one of the ends of the rows, e.g. the four pins  26  at the distal end if the cam is inserted via said end. This step is performed by inserting the cam  232  in the distal end of the duct  14  and inserting the pins  26  into respective slots  254 . Since the pins  26  are initially projecting, continued thrust of the cam towards the center of the duct has the effect, given the ramps in the slots, of moving the pins  26  into the retracted configuration. As the cam continues to be thrust in, the distal ends of the pins leave the low portions  240  of the slots. Applying continued thrust to the cams serves to insert the pins in the centers of the rows into the high portions  238  of the slots  254  and finally to bring them into the low portions  240  of the cam. Thereafter, the cam  230  for occupying the distal position is inserted in the same manner. After that the cam  230  for occupying the proximal position is inserted in the same manner via the proximal end. Once all of the pins  26  are in the low portions  240  of the slots, in the retracted configuration, the screw  250  is screwed into the cam  230  so as to be in screw engagement therewith. 
     Once the implant has been installed between the vertebrae, the screw  250  is pushed towards the distal end of the duct  14 , thereby causing the cams  230  to slide in the same direction. By means of the ramps at the bottoms of the slots  254 , the pins are then caused to slide along their respective channels so as to move from the low portions  240  to the high portions and thus reach the extended position. Given the above-described reversibility, it is possible by means of steps that are the converse of those implemented for assembly, to remove the implant from its position between the vertebrae. 
     In the third embodiment, as shown in FIGS. 8 and 9 in which some of the numerical references have  400  added thereto, the body  4  in plane view generally has the shape of half a disk, being defined by a plane side wall  8  for occupying the posterior position and a circular side wall  8  for occupying an anterior position. Each side wall  8  has orifices  12  opening out into the recesses  10  as described above. In this case, the body  4  comprises a top portion  4   a  and a bottom portion  4   b  which meet in a joint plane parallel to the top and bottom faces  6 , with each portion carrying one of said faces  6  and being fixed to the other by means of screws  5 . There are still twelve channels  22 , but they are oriented so that their axes are perpendicular to the top and bottom faces  6 . 
     The cam duct  14  has a section that is generally rectangular in shape perpendicularly to its axis  16  and half of it is defined in each of the portions  4   a  and  4   b  of the body. The cam  430  has a male rectangular section corresponding to the female rectangular section of the duct  14  which receives it. It is suitable for sliding along its axis. The cam is generally in the form of a rectangular parallelepiped. The two longitudinal side faces  460  of the cam  430  have six slots  454  each suitable for receiving respective specific pins  26 . To this end, instead of having a head, each pin has a respective projection that is received in the slot so that each pin is generally L-shaped, with the projections extending towards the other row of pins. Each slot  454  presents two mutually-parallel plane ramps or faces  462  that are perpendicular to the associated side face  460 , and that slope relative to the sliding direction in such a manner that the end of the slot  454  that is further away from the associated face  6  is its end which is further away from the proximal end of the cam. 
     The cam  430  presents a threaded bore passing through it along its axis. The implant  402  has a screw  450  presenting a threaded rod suitable for screw co-operation with the cam  430 . The screw  450  has a groove receiving a collar  466  that is secured to the body  450  that the screw  450  is free to rotate in the body while the cam  430  is free to slide in the body. 
     After the implant  402  has been inserted between the vertebrae  9  with the pins  26  in the retracted position, when the head  436  of the screw is driven, rotation of the screw causes the cam  430  to slide towards the distal end of the duct  14 , which end is closed in this case, and by the effect of the ramps in the faces  462  of the slots oriented towards the associated faces  6 , the pins  26  are caused to slide so as to project and take up the extended position. 
     The operation of the implant  402  is reversible, with the faces  462  of the slots that face away from the associated faces  6  being suitable for moving the pins  26  into the retracted position when the cam  430  slides towards the proximal end of the duct  14 . Specifically, the proximal end associated with the head of the screw  436  opens out into the left-hand portion of the curved side wall  8 . The pins  26  in this case present circumferential grooves  70  in the vicinity of their points on the segments thereof that are designed to project, with the grooves presenting respective faces that are undercut relative to the direction in which the pins slide so as to project, and the grooves improve anchoring of the pins by enabling bone growth to take place in the grooves. 
     FIGS. 10 and 11 show a fourth embodiment with some of the reference numerals having  400  added thereto. In this case there are four pins  26  in each face  6  and they are oriented in the same manner as in the third embodiment. The slots  562  are analogous to those of the third embodiment but they are oriented in the opposite direction so that sliding the cam  530  towards the distal end of the cam duct  14  causes the pins  26  to project. The implant  502  has a screw  550  in screw engagement with the body  4  on the axis of the cam duct and suitable for urging the cam  530  at its proximal end in its duct towards the distal end of the duct. 
     With reference to FIG. 12, the proximal end of the cam has a cutout  572  suitable for slidably receiving the end of the screw  550  so that it can bear against the cam  530 . Between the far wall of the cutout and its edge, the cam  530  presents a slot  574  with a flank  576  that presents an undercut relative to the cam sliding towards the proximal end of the duct. By way of example, this face  576  can be an annular plane perpendicular to the axis  16  of the cam and facing away from the proximal end. 
     In order to remove the implant, the screw  550  is removed and then a tool is inserted into the duct  14  that is suitable for bearing against the undercut face  576  as to catch hold of it and pull the cam towards the proximal end of the duct, thereby causing the pins  26  to be moved into the retracted configuration. This embodiment avoids the need to provide a threaded bore passing through the cam  530 . It thus enables the dimensions of the cam  530  to be reduced and the dimensions of the recesses  10  for receiving graft tissue to be increased. 
     A fifth embodiment of the implant is described with reference to FIGS. 13 to  17 . The implant  602  comprises a body  4  whose plane is general in the shape of a bean whose hilum is in the posterior position, the body being flat in one direction so as to define two large faces  6 , namely a top face and a bottom face, and a peripheral side wall  8 . The top and bottom faces  6  are generally plane in shape with a transverse profile that is sawtoothed or zigzag, the tips of the teeth defining mutually-parallel continuous edges. These teeth provide good engagement between the top and bottom faces  6  and the plates of the vertebral bodies of the vertebrae constituting the destination location. 
     The body  4  has two through recesses  10  extending between the top and bottom faces  6  and opening out into them. When the implant is put into place, the recesses  10  are filled with graft tissue which can thus grow towards the vertebral plates through the recesses  10 . 
     The body  4  has a cylindrical duct  14  whose axis  16  extends between the recesses  10  parallel to the general planes of the top and bottom faces  6  and is separated from the single plane portion of the side face  8  by one of the recesses  10 . The duct  14  opens out at only one of its ends. 
     The body  4  has two channels  22  opening out into each of the top and bottom faces. Each channel  22  is of constant section along an axis perpendicular to the main faces and in section its profile is rectangular with rounded ends. Each channel opens out on one side over its entire height into a respective one of the recesses  10  with which it is contiguous. Furthermore, it opens out sideways on its opposite side in its middle portion into the duct  14 . The two channels  22  extend in register with each other on either side of the duct  14 . The channels  22  thus put the two recesses  10  into communication with the duct  14 . 
     In the vicinity of each of the main faces, each channel  22  receives a pair of anchoring elements each in the form of a pin  26  having a smooth cylindrical body presenting a point at its end closer to the face  6 . Each pin  26  extends against a respective curved edge of the channel so as to slide there against perpendicularly to the main face  6  of the body. 
     The implant includes two pin-carriers or anchoring element-carriers  680  associated with respective main faces  6 . Each pin-carrier  680  is generally in the form of a flat H-shape having two rectangular branches  682  parallel to each other and a middle segment  684  interconnecting the middles of the branches. Each pin-carrier  680  has rigidly fixed thereon all four pins  26  associated with the corresponding main face  6 . The bases of the four pins  26  rest on respective ends of the branches  682 , and all lie on the same side of the pin-carrier. The two pin-carriers  680  extend permanently in register with each other so that their outlines coincide, and regardless of whether the pins  26  are in the extended or retracted position, as shown in FIGS. 15 and 16. The branches  682  extend in respective channels  22  and have the same profile, while the middle segment  684  extends across the duct  14 . 
     The implant has a cam  630  with left and right cylindrical faces  631  that are left and right at the rear, and top and bottom at the front, for the purpose of slidably guiding the cam in the cylindrical duct  14 . The “rear” of the cam is its end closest to the mouth of the duct  14 . For each pin-carrier  680 , the cam  630  has a corresponding slideway  633  and bearing surface  635  that can be seen in particular in FIGS. 14 and 17. The slideway  633  is formed by a very flat duct open to both longitudinal edges and sloping relative to the axis  16 , going towards the corresponding main face when going from the front end towards the middle of the cam. The slideway  633  is closed at its front end and open at its rear end, with the inner face  641  of the slideway extending continuously from the bearing surface  635 . This surface is parallel to the axis  16  and to the associated main face  6 . The middle segment  684  can be moved by thrust against the slideway  633  and the bearing surface  635  as explained below. The sloping slideways  633  and the parallel bearing surfaces  635  give the cam a shape reminiscent of a boat anchor. 
     At its rear end, the cam has a tapped bore  637  whose thread can mesh with that of a suitable tool for driving the cam by pushing it or pulling it. 
     The implant is used as follows, with the pins  26  initially being in the retracted position as shown in FIG.  15 . After a vertebral disk has been removed and after the recesses  10  have been filled with graft tissue as described above, the implant  2  is inserted between the vertebral bodies of the vertebrae associated with the disk that has been removed. The height of the body  4  of the implant corresponds substantially to the thickness of the removed disk. The faces  6  extend in register with the respective vertebral plates, parallel thereto, and in contact therewith. The cam is close to the mouth of the duct  14 , at the rear. 
     The tool is screwed into the bore  637  of the cam and the cam  630  is pushed forwards. The ramp acting on each middle segment  684  via the inner face  641  of the associated slideway  633  causes the pin-carrier  680  together with the four pins  26  to move perpendicularly to the main face  6 . 
     After the pins  26  have been caused to project from the main face  6 , continued thrust on the cam causes the middle segment  684  to bear against the bearing surface  635  as shown in FIG. 14, thus locking the pins  26  in the extended position where the anchor in the vertebral plates. The tool is then unscrewed so as to be separated from the cam. 
     However, if it is desired to remove the implant, the tool is reconnected in the bore  637  of the cam via the duct  14  and then the cam is pulled so as to slide rearwards. The middle segment  684  then follows the bearing surface  635  and, by ramp engagement against the outer face  643  of the slideway  633 , it becomes moved towards the axis  16  towards the inside of the implant, thereby retracting the pins  26  so that they no longer project. The cam thus enables the pins  26  to be driven in both directions, i.e. reversibly. 
     By having all four pins  26  in each group fixed together, it is possible to obtain very accurate guidance for the pins in the channels  22  without needing to provide a cylindrical channel for each pin. In addition, this guidance is obtained using a single two-part ramp surface  641 ,  635 , or  643  for all four pins in any one given displacement direction. 
     At the base of the point forming its tip, each pin  26  has circular grooves  70  forming undercut zones and improving the anchoring of the pin in the vertebral plate. 
     The body  4  is made up of two portions that are assembled together on a joint plane (not shown) parallel to the main faces  6  and including the axis  16 , thereby enabling the cam  630  and the pin-carriers  680  to be inserted in the body. 
     Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.