Abstract:
The invention concerns an implant ( 2 ) for replacing a vertebra at least-partially, consisting of two parts ( 4, 6 ) adapted to be mutually connected while enabling the adjustment of the implant total dimension (L), each part having an invariable dimension (m, f) homologous with the implant total dimension (L). The parts ( 4, 6 ) form a screw-nut connection with each other.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to implants for replacing at least part of a vertebra, for example after ablation of the vertebra. 
     The document EP-0 567 424-A1 discloses an implant of this kind comprising an intermediate body and two bearing parts adapted to bear against the vertebral plates of vertebrae adjacent the space left by a vertebra that has been removed. Each bearing part is assembled to one end of the intermediate body by a screw connection so that rotation of each bearing part relative to the body varies the total length of the implant. However, it takes a relatively long time to assemble the various components of the implant. What is more, given the number of parts capable of relative movement, adjusting the length of the implant is relatively complicated and takes a long time, which increases the duration of the surgery. Finally, manufacturing the implant entails defining a large number of accurate surfaces enabling relative movement of the parts. Manufacture is long and costly. 
     U.S. Pat. No. 5,723,013 relates to an implant for replacing a vertebra that is made up of two implant parts sliding one within the other. The two parts are in mutual contact through teeth enabling the length of the implant to be increased by distraction of the two parts. The length cannot be reduced, however. The length of the implant can be adjusted simply and quickly. However, fine adjustment of the length of the implant is not possible. 
     An object of the invention is to provide an implant that is quick to install during surgery and that enables fine adjustment of its length. 
     To achieve the above object, the invention provides an implant for replacing at least part of a vertebra, the implant having two parts adapted to be joined together and enabling a total dimension of the implant to be adjusted, each part having a fixed dimension homologous to the total dimension of the implant, characterized in that the parts form a screw connection with each other. 
     Accordingly, during surgery, the total dimension of the implant is adjusted by moving only the two parts of the implant relative to each other. Adjustment is therefore simple and fast. Similarly, assembling the mobile parts of the implant before or during the operation is simple and fast. What is more, the number of surfaces enabling relative movement of the parts is reduced. Because the surfaces concerned are very accurate surfaces, fabrication of the implant is easy and its cost is low. The screw connection enables fine adjustment of the length of the implant. 
     At least one of the parts is advantageously in one piece. 
     This further reduces the number of parts to be assembled. 
     At least one of the parts is advantageously in more than one piece. 
     This facilitates obtaining some shapes of the part concerned. 
     Each part advantageously has at least one lateral opening and the openings can be superposed to receive a fixing member. 
     This facilitates superposing the openings, in particular when the two parts are relatively mobile by virtue of a screw connection. 
     SUMMARY OF THE INVENTION 
     At least one of the openings is advantageously elongate. 
     The elongate opening is advantageously rectilinear and parallel to a direction of measuring the total dimension of the implant. 
     One part advantageously has an elongate opening and the other part advantageously has at least one circular opening. 
     One part is advantageously a female part adapted to receive the other part and including a body and a flange which can be moved relative to the body to immobilize the other part by wedging it. 
     Accordingly, the wall of at least one of the two parts does not necessarily have to have an orifice to receive a member for fixing the two parts together. The wall of each part can therefore be apertured as much as may be required to show the implant clearly on X-rays and to favor the growth of bone with a view to its osteointegration. 
     The flange is advantageously mobile by virtue of elastic deformation of the female part. 
     The flange and the body advantageously each have a conduit to receive a member positioning the flange relative to the body. 
     The conduits are advantageously parallel to a direction in which the other part is received into the female part. 
     The flange advantageously comprises an uninterrupted collar. 
     The collar is advantageously in a plane perpendicular to a direction in which the other part is received into the female part. 
     At least one of the parts advantageously has a toothed end forming an end of the implant. 
     Other features and advantages of the invention will become apparent in the course of the following description of two preferred embodiments of the invention, which description is given by way of non-limiting example only. In the accompanying drawings: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 and 2 are perspective views of a first embodiment of an implant according to the invention respectively before and after assembly; 
     FIG. 3 is a side view of one variant of the first embodiment; 
     FIG. 4 is a perspective view of a second embodiment of an implant according to the invention before assembly; and 
     FIGS. 5 and 6 are two side views of the implant shown in FIG. 4 after assembly. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 1 and 2, in a first embodiment of the invention the implant  2  has two parts  4 ,  6 . 
     Each part  4 ,  6  includes a cylindrical tubular one-piece body  8 ,  10  that has an axis  9 . The body  8 , also referred to as the male body, is adapted to penetrate into the body  10 , also referred to as the female body, in a direction parallel to the axis  9 . The male body  8  is threaded externally and the female body  10  is threaded internally to cooperate with the male body and provide a screw connection. A side wall of the male body  8  has identical rectilinear elongate openings or slots  12  of constant width that are parallel to each other and to the axis  9 . Each extends more than half the length of the body  8  in a direction parallel to the axis  9 . They are distributed all around that axis. A side wall of the female body  10  has a series of circular fixing openings or slots  14  that are identical to each other and lie in a common plane perpendicular to the axis  9  and in the vicinity of a proximal edge of the female body through which the male body  8  penetrates into the female body  10 . The circular openings  14  are threaded. The diameter of the circular openings  14  is equal to the width of the elongate openings  12 . The female part  6  has a fixing screw  16  adapted to cooperate with the circular openings  14  to provide a screw connection. 
     The female body has an end wall including circular openings  18  at a distal edge of the female body that is opposite the proximal edge in the axial direction  9 . The distal edge of the female body has teeth  19  extending away from the proximal edge. The wall of the female body  10  has other circular openings  18  which are not threaded between the distal edge and the fixing openings  14 . 
     The wall of the male body  8  has an internal thread in the vicinity of a distal edge opposite the proximal edge adapted to penetrate into the female body. The male part  4  includes a cap  22  comprising a threaded cylindrical wall for fixing it by means of a screw connection to the threaded distal edge of the male body. The cap  22  has an end wall perpendicular to the axis  9  and including circular openings  18  and teeth  19  directed away from the male body  8 . The threads of the cap  22  and of the distal edge of the male body  8  are just long enough to rigidly fix the cap  22  onto the male body  8  in an axial abutting relationship so that the cap can be separated from the body  8  by very slightly rotating it about the axis  9 , for example by rotating it through one or two turns. When the cap  22  is not abutted on the distal edge it is connected to the body  8  with play. The various positions of the cap  22  relative to the body  8  when their threads are in mesh do not significantly change the length of the male part  4  along the axis  9  because the threads have a very small inclination to the axis  9 . The male and female parts have respective fixed lengths m and f parallel to the axis  9 . 
     To assemble the implant  2 , the cap  22  is fixed to the body  8  to constitute the male part  4 . The male part  4  is then inserted in the female part  6  with their respective threads meshing. Both threads are very long to provide a wide choice as to the length over which the male part  4  penetrates into the female part  6 . Because of the screw connection, relative rotation of the male and female parts adjusts the total length L of the implant in the direction parallel to the axis  9 . The length L corresponds to the distance between the two vertebral plates between which the implant is to be installed. When the length L suited to the intervertebral space to be occupied is obtained, the screw  16  is inserted in one of the fixing openings  14  in the female body  6  which coincides with an elongate opening  12  in the male body  4 . If there is no such coincidence, all that is required to bring about such coincidence is to turn the two parts relative to each other by a very small fraction of one turn, thanks to the elongate shape of the openings  12 . The screw  16  is inserted as far as the corresponding elongate opening  12 , which prevents subsequent relative rotation of the two parts. Finally, the screw  16  is tightened until its head bears against the female body  6 . The adjustment of the distance L and the fixing of the screw  16  are carried out at least in part with the implant  2  in situ, occupying the space left by the vertebra that has been partly or totally removed. The distal edges of the male and female parts then bear against the respective vertebral plates of two vertebrae adjacent the latter space. The teeth  19  ensure a good grip of the implant  2  on the plates and facilitate osteointegration of the implant. All the openings  12 ,  14 ,  18  of the implant facilitate osteosynthesis for the purpose of osteointegration. 
     In the FIG. 3 variant, the distal edges carrying the teeth are in planes inclined to the plane perpendicular to the axis  9  to allow for the inclined configuration of the vertebral plates of some vertebrae. 
     Referring to FIGS. 4 to  6 , in the second embodiment, in which the reference numbers of corresponding components are increased by  100 , the two parts  104 ,  106  of the implant provide a male-female coupling with a screw connection, as previously. Each distal edge and the teeth it carries are now in one piece with the corresponding body. The male part  104  is in one piece. The male part  104  and the female part  106  have no end walls and the ends of the implant associated with the distal edges are open. 
     The proximal edge of the female part  6  has a slot  130  in a plane perpendicular to the axis  109  and in the shape of a circular arc subtending an angle about the axis greater than 180°, for example equal to 200°. The slot  130  therefore delimits a flange  132  carrying the proximal edge and forming an uninterrupted circular collar which can move relative to the remainder of the body by elastic deformation of a junction part  133  connecting the remainder of the body to the flange. On either side of the slot  130 , and opposite the junction part, the flange and the body have respective facing lobes  134  projecting from the outside face of the female body  106 . The lobes  134  have respective conduits with a common axis  136  parallel to the axis  109 . The female part includes a screw  116  adapted to be inserted through the flange  132  into the two conduits to engage with a thread of the conduit in the body  110 , a head of the screw abutting on the lobe of the flange. 
     The lateral walls of the male and female bodies have triangular openings  138  that extend from one of the corresponding proximal and distal edges to the other. The triangular openings  138  on each male and female part are alternately inverted relative the axis  109  to define between them branches  140  connecting the distal edge to the proximal edge, both of which are circular and uninterrupted. These very large openings  138  ensure that the implant  102  is highly visible in X-rays and encourage osteointegration. 
     The length L of the implant is chosen by relative rotation of the two parts  104 ,  106 , as previously. When the desired length L is reached, the screw  116  is tightened to move the flange  132  towards the body  106  by virtue of elastic deformation of the junction part  133 . Because of the screw connection between the flange  132  and the male body  104  and the screw connection between the male body  104  and the female body  106 , this movement over a very short distance achieves rigid wedging of the male and female parts relative to each other. Alternatively, the fixing by the screw  116  could be such that the wedging effect is achieved by movement of the flange  132  away from the female body  106 . 
     The implant  2 ,  102  according to the invention enables a bone graft to be fitted between two vertebral plates when total or partial corporectomy and ablation of the overlying or underlying intervertebral discs have been carried out. Once adjusted to the size of the space to be filled, by choosing its length L, the implant  2 ,  102  is filled with bone, generally taken from the patient. This achieves a graft and braces the spinal column.