Abstract:
A multiaxial connection osteosynthesis system for the spine having a bone anchor, a connector and a connecting member. The bone anchor may be implanted into a vertebral body with a connector being attached to a head of the bone anchor. The connector includes a seat for receiving the connecting member. As the connecting member is locked into position it causes walls of the connector to inwardly deform and lock the head of the bone anchor relative to the connector.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a multiaxial connection osteosynthesis system, in particular for the spine. 
   In the context of surgery on the vertebral column, and in particular when using a posterior approach route, the surgeon is often confronted with the problems of misalignment of the anchor members and interference between anchor members on two adjacent vertebrae. This problem increases the operating time or prevents the surgeon from carrying out the desired procedure under the best possible conditions. The cause of this problem is that the coupling member located between the member connecting the various vertebrae and the anchor member implanted in the vertebra is rigidly connected to the anchor member. 
   Reference WO 97/02786 discloses a multiaxial connection between the members. The anchor member has a hemispherical head. The coupling member includes, in addition to means for receiving the member connecting the various vertebrae together, a split chamber in its bottom part adapted to receive the hemispherical head. The external surface of the coupling member includes a cone that is flared toward the bottom end. A clamping collar can be threaded over the top portion of the coupling member to bear on the conical portion. Before tightening, the coupling member is free to rotate relative to the anchor member. Locking in position is effected when tightening the member connecting the vertebrae together. The member bears on the clamping collar, while at the same time closes the receiving chamber on the hemispherical head of the anchor member. 
   The complexity of the positional locking mechanism, because of its large number of components, obliges the surgeon to carry out supplementary manipulations, and consequently increases the operating time. 
   An object of the present invention is to provide a multiaxial connecting device that is simple to use and requires only a small number of operations to complete assembly. 
   SUMMARY OF THE INVENTION 
   To achieve this object, the present invention provides a multiaxial connection osteosynthesis system, in particular for the spine, including a bone anchor member having a head, a connecting member, a connector having a first housing with a recess adapted to receive the head and a second housing adapted to receive the connecting member. The system further includes clamping means, i.e. a locking member such as, for example, an axially moveable screw, for clamping the connecting member in the second housing. The connector is arranged so that when the clamping means load the connecting member in the second housing, the connecting member loads ridges of the connector directly to reduce the dimension of the recess of the first housing and immobilize the head therein. 
   Thus the first deformable housing is closed onto the head. This reduces the number of components and therefore simplifies the use of the device. 
   The first housing may advantageously includes a chamber with a concave surface. The concave surface chamber is advantageously complementary to all or part of the convex surface of the head. The first housing advantageously includes a cavity adapted to receive part of the head. 
   The connector includes a slot in the first housing. Thus the first housing can have a more pronounced elastic deformability while inserting the head and while locking the device into position. 
   The head may advantageously include a spherical part. 
   The first housing advantageously includes a U-shaped opening having an axis and two branches at a distance from and facing each other. The slot is located preferably perpendicular to the axis of the U-shaped opening. The branches of the U-shaped opening may include have a screwthread. The clamping means may include a locking member adapted to engage between the branches of the U-shaped opening. The clamping means or locking element may additionally include a flange adapted to fit around the branches of the U-shaped opening. Accordingly, when clamping the nut, the flange prevents spreading of the branches of the U-shaped opening. The screwthread may have a face substantially perpendicular to the axis of the thread and oriented in a penetration direction of the clamping means on the connector. 
   Accordingly, during clamping, the face substantially perpendicular to the axis of the thread of the locking member comes into contact with the face of the branches of the U-shaped opening. The reaction force generated in this way is substantially parallel to the axis of the thread and this minimizes the radial reaction force. Otherwise the radial reaction force would spread apart the branches of the U-shaped opening. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features and advantages of the present invention will become more apparent in the course of the following description of three preferred embodiments, which description is given by way of non-limiting examples. In the accompanying drawings: 
       FIG. 1  shows a partial perspective side view of a first embodiment of the present invention; 
       FIG. 2  shows an exploded perspective view of the system shown in  FIG. 1 ; 
       FIG. 3  shows a cross-sectional view of the system shown in  FIG. 1 ; 
       FIG. 4  is a top perspective view of the first embodiment of the present invention; 
       FIG. 5  is a bottom perspective view of the connector shown in  FIG. 4 ; 
       FIG. 6  is a cross-sectional view of the connector shown in  FIG. 4 ; 
       FIG. 6   a  is a diagrammatic sectional view of the Christmas tree thread of the connector shown in  FIG. 4 ; 
       FIG. 7  is a bottom perspective view of a second embodiment of the present invention; and 
       FIG. 8  is a bottom perspective view of a third embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   A first embodiment of the present invention is described with reference to  FIGS. 1 to 6   a.    
   The osteosynthesis system includes at least two vertebral anchor members  2 , connecting means  4 , such as a rod, extending between the anchor member  2  and the other anchor member (not shown) of the osteosynthesis system, a connector  6  adapted to be fitted to the member  2 , and clamping means, i.e. a locking member  8  adapted to cooperate with the connector  6 . 
   The vertebral anchor member  2 , which in the preferred embodiment takes the form of a bone screw, may have a circular section cylindrical threaded body  10  carrying a bone thread (not shown). The preferred member  2  also may have a head  12  with a convex surface  14 . The convex surface  14  has a surface of revolution whose axis coincides with the axis of the screw. The convex surface has a spherical lateral portion  18 , a flat top portion  22 , perpendicular to the axis of the screw, and a connecting fillet  20  between them, as shown in  FIG. 3 . The head  12  includes cavity  24  for operating the screw  2 , here in the form of a hexagonal socket  24 . 
   As shown in  FIG. 2 , the locking member  8  may be generally screw-shaped and have a thread  26  adapted to cooperate with the thread  28  on the connector  6 , described hereinafter. The latter also includes socket  30  in the form of a hexagonal socket  30 . 
   The connector  6  provides a coupling member between the bone screw  2  and the connecting rod  4 . The general shape of the coupling member is that of a circular section cylinder. As shown in  FIG. 6 , it has a top portion  6   a , for connecting to the rod  4  and a bottom portion  6   b  for connecting to the bone screw  2 . 
   As shown in  FIGS. 4 and 5 , the top portion  6   a  of the coupling member  6  includes a U-shaped opening  32  which has an axis  42  and is delimited by two facing branches  34  at a distance from each other. The two branches  34  constitute two sectors of a common circular section cylinder forming the coupling member  6 . The cylindrical external faces  36  of the branches  34  are coaxial and smooth. They can include grooves  38 , for example, in the form of a notch on each of the branches  34 . The grooves  38  have a flat bottom parallel to the axis  42  of the U-shaped opening  32 . The grooves  38  are symmetrical mirror images of each other with respect to the median plane of the U-shaped opening  32 . The cylindrical inside faces  28  of the branches are coaxial and threaded. Their thread is adapted to cooperate with that of the locking member  26 . 
   The two threads  26  and  28  are complementary and are referred to as “Christmas tree” threads. As shown in  FIG. 6 , the thread  28  has a first face  80  substantially perpendicular to the axis of the thread. A second face  81  substantially parallel to the axis of the thread delimits the bottom of the thread. A third face  83  opposite the first face  80  and can be inclined in the direction of the axis relative to the direction of the first face  80 . Lastly, a fourth face  82  located substantially parallel to the axis of the thread, delimits the crest of the thread. The thread  26  is substantially complementary to the thread  28 . The thread  26  has a first face  90  parallel to and complementary to the face  80 , a second face  91  substantially parallel to the axis of the thread delimiting the crest of the thread, a third face  93  substantially parallel to and complementary to the face  83 , and a fourth face  92  substantially parallel to the axis of the thread delimiting the bottom of the thread. During clamping, the face  90  of the thread  28  comes into contact with the face  80  of the thread  26 . The reaction force Fr generated in this way has a normal component Fn parallel to the axis of the threads and a transverse component Fs in the radial direction. The transverse component Fs may cause the branches  34  to spread during tightening of the locking member  8 , but the particular shape of the threads  26 ,  28  previously referred to reduces the transverse component Fs and therefore reduces the spreading of the branches  34  during clamping. 
   In a plane perpendicular to the axis  42 , the bottom  40  of the U-shaped opening is semicircular to receive the connecting rod  4 , with a diameter equivalent to that of the connecting rod  4 . On the other hand, the bottom  40  has a concavity in the median plane of the U-shaped opening  32  with a center of curvature situated on the same side as the portion  6   a  of the connector  6 . Finally, the bottom  40  has ends  44  parallel to the axis  42 , providing a seat to receive the connecting rod  4 . 
   The bottom portion  6   b  of the connector  6  includes a housing  46  opening onto the bottom face  48  at an insertion orifice  50  and opening onto the bottom  40  of the U-shaped opening  32  at a communication orifice  52 . The circular section insertion orifice  50  allows the head  12  of the bone screw  2  to be inserted into the housing  46 . The housing  46  has a part-spherical bottom chamber  54  adapted to receive the head  12  of the bone screw  2 . The shape of the bottom chamber  54  is complementary to the spherical lateral portion  18  of the convex surface  14  of the head  12 . A slot  56  crosses the bottom part  6   b  diametrally. The slot  56  is perpendicular to the axis  42  of the U-shaped opening  32 . It extends downward as far as the bottom face  48  of the connector  6  and upward until it opens onto the bottom  40  of the U-shaped opening  32 . Thus the bottom part  6   b  is divided into two subparts  58  and  60  which are symmetrical mirror images of each other with respect to the median plane of the slot  56 . The slot  56  facilitates clipping the head  12  into the coupling member  6  by facilitating deformation of the receiving space  46 . 
   Prior to the procedure, each connector  6  is premounted by clipping it onto the head  12  of the bone screw  2 . The bone screw is implanted in the patient using an instrument inserted into the cavity  24  via the communication orifice  52 . When the screw  2  has been implanted, the connector  6  is free to rotate relative to the screw  2 . The connector and the screw are connected together by a ball-and-socket connection formed by the head  12  and the bottom chamber  54 . The rod  4  is fitted into the U-shaped opening  32  so that it rests on the ends  44  of the bottom  40 . The locking member  8  is then engaged between the branches  34  of the opening  32  with the threads  26  and  28  interengaged with each other. The locking member  8  is then pressed against the rod  4  and the surgeon tightens the locking member  8 . The locking member  8  bears on the rod  4 . The rod  4  bears on the ends  44  of the bottom  40 , which is then deformed because of the presence of the slot  56  perpendicular to the axis  42  of the opening  32 , which is closed up as the two subparts  58  and  60  move toward each other. Consequently, the receiving space  46  and the chamber  54  are closed up onto the head  12  of the bone screw  2 , locking the assembly in position, rigidly immobilizing the connector on the head of the bone screw. 
   In the second embodiment, shown in  FIG. 7 , the modifications relative to the first embodiment relate to the bottom part of the connector  106 . The housing  146  no longer includes a spherical chamber, but instead is in the shape of a circular cylinder with two flats  102  and  104 . The radius of the circular portion  108  can be equivalent to the radius of the head  12  of the bone screw  2 . The two flats  102  and  104  are parallel to the slot  56 . A circular section hole  110  perpendicular to the flats  102  and  104  passes completely and diametrally through the bottom part in the area of the flats and perpendicularly thereto. The diameter of the hole  110  is substantially equivalent to the width of the flats  102  and  104 . 
   When the connector  106  is clipped to the head  12  of the bone screw  2 , the head  12  locates in the holes  110 , leaving free the ball-and-socket connection formed in this way. 
   The use of this embodiment is identical to that of the first embodiment during clamping, the deformation of the receiving space  146  by the movement toward each other of the two subparts  158  and  160  obliging the two flats  102  and  104  to move toward each other, thereby immobilizing the head  12  of the bone screw  2  in the holes  110 . 
   In the third embodiment, shown in  FIG. 8 , the modifications relative to the previous embodiment relate to the bottom part of the connector  206 . The housing  246  and the slot  256  are coincident, the slot  256  being the same width as the housing  246 . The width L of the slot  256  is less than the diameter of the head  12  of the bone screw  2 . As in the preceding embodiment, a circular section hole  210  passes completely through the bottom part of the connector  206  in the area of the flats and perpendicularly thereto, diametrally and perpendicularly to the walls of the slot  256 . The diameter of the hole  210  is less than that of the spherical part  18  of the convex surface  14  of the head  12 . 
   The clipping is effected in the same manner: the head  12  is accommodated in the holes  210 , leaving free the ball-and-socket connection formed in this way. 
   The use of this embodiment is identical to that of the second embodiment. The locking in position is similar. 
   Of course, many modifications can be made to the invention without departing from the scope of the invention. 
   The anchor members can be hooks. The clamping means can include a ring or a flange adapted to be threaded over the branches of the U-shaped opening instead of or in addition to the locking member. In the latter case, the flange or ring can be connected to the locking member. 
   The thread cooperating with that of the branches can be on the flank, the branches being threaded on their outside face. 
   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.