Patent Publication Number: US-2015073481-A1

Title: Fixing devices and stabilization systems using said fixing devices

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 12/739,670, filed Apr. 23, 2010, which is a national stage entry of and claims priority from International Application No. PCT/EP2008/064344, filed Oct. 23, 2008, entitled “FIXING DEVICES AND STABILIZATION SYSTEMS USING SAID FIXING DEVICES,” which claims priority from European Patent Application No. EP07301483.9, filed Oct. 23, 2007, entitled “FIXING DEVICES AND STABILIZATION SYSTEMS USING SAID FIXING DEVICES,” the complete disclosures of which are fully incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a fixing device for fixing a rod or similar linking members to a bone, especially a vertebra, and allowing a relative mobility of the linking member with respect to the bone, a stabilization system including at least one fixing device and uses of said fixing device on said stabilization system. 
     BACKGROUND 
     One field of application for the invention is holding bones in a relative position, for example to aid in the healing of breaks or the positioning of bones in the treatment of spinal deformities or spinal degenerative diseases or trauma diseases, or otherwise to correct abnormal curvatures of the spine. Other bone deficiencies and abnormalities may also benefit from embodiments of the present invention. 
     The spine is formed of superposed vertebrae, normally aligned along a vertebral axis, from the lumbar vertebrae to the cervical vertebrae, each having a posterior wall from which projects a spinous process and two lateral edges from the walls of which there project ribs and/or transverse processes and/or lamina. If the spine of a person has abnormal curvature, the vertebrae are typically inclined relative to one another and relative to said vertebral axis. 
     In order to straighten the vertebral column as a remedy for this situation, the lateral edges of the vertebrae on the concave side can be moved away from one another and supported at distances from one another substantially equivalent to the distances between the lateral edges on the other side. Devices known in the art to hold the vertebrae relative to one another include screws that are inserted into the vertebrae or hooks that are inserted along the internal wall of the spinal canal and rods adapted to connect the screws or hooks. 
     When using a hook and rod system, pairs of hooks are generally inserted into each vertebra, one on each side, near the pedicle. The hooks typically have heads that project from the posterior wall of the vertebra, one on each side of the spinous process. The heads can be tulip-shaped and adapted to receive a rod that is immobilized by a nut screwed onto the head and contacting the rod. The heads of the hooks situated on either side of the spinous process can then be connected together and fixed in position by two rods approximately parallel to one another and to the axis of the spine. 
     However, using such hooks can be difficult because their use increases the risk that the physician (or other operative) might contact and potentially damage the spinal cord that extends along the centre of the spinal canal (which can result in paralysis of the patient). 
     Using a screw and rod system reduces this risk, but has other drawbacks. The screws typically have tulip-shaped heads and are inserted in pairs into the pedicles on each side of the spinous process on the posterior wall of the vertebrae. The screws therefore constitute fixing points on the vertebrae for holding the vertebrae in a fixed position relative to one another. However, the screws are inserted into the pedicles of the vertebrae, which in some cases are small or are deteriorated and can be damaged or do not provide sufficient purchase to permanently hold the screw. 
     To solve this problem, the PCT applications WO2004/010881 and WO 2007/036657 in the name of the applicant relate to vertebral fixing devices to fix an end of the connecting member, for example, a rigid rod, to a vertebra. These devices comprise a connecting part or head which can be mechanically fixed to one end of the rod and a conformable elongate ligature for connecting the head to at least one rib and/or transverse process and/or lamina, the head maintaining a traction applied to the ligature. 
     As explained in the above applications, each vertebral fixing device produces a rigid connection between the vertebra and the head of the fixing connecting device and, consequently, between the rod and the vertebra. However, in some situations it is desirable to allow a controlled relative mobility between two or more adjacent vertebrae. 
     To solve this problem, PCT applications WO 2002/07622 and WO 2002/07621 in the name of the applicant relate to flexible linking pieces for stabilizing the spine. 
     The linking piece has the general shape of a rod having two rigid ends, for cooperating with fixing devices secured to two adjacent vertebrae and an intermediate part which is made with an elastically deformable material. 
     Due to the elasticity of the intermediate part of the rod, a relative mobility exists between the two adjacent vertebrae. 
     However, the manufacturing of such elastically deformable linking members is rather expensive and the type of mobility allowed by such linking members is not adapted to all situations. 
     SUMMARY 
     A first object of the present invention is to provide a fixing device for connecting a linking member to a bone, more particularly, a vertebra, and which allows a limited and controlled relative mobility between the vertebra and the linking member. 
     According to this aspect of the invention this aim is achieved by a fixing device for connecting a bone and a linking member comprising:
         a connecting part to be connected to a linking member and having an outer face, a portion of said outer face being in contact with said bone;   a conformable elongate ligature cooperating with said bone, suitable for connecting said connecting part and said bone; and   adjustable locking means fastened to said connecting part to maintain said ligature in a state which allows a controlled relative mobility between said bone and said connecting part.       

     It is to be underlined that due to the fact that the adjustable locking means and/or the ligature are designed to allow a relative mobility between the bone, especially a rib and/or a transverse process of a vertebra, a relative mobility is allowed between the linking member, for example a rod and the bone. As a result, a controlled relative mobility can be obtained between the bone with which the ligature cooperates and another bone to which another portion of the linking member is fastened, even if the linking member is rigid. It is to be emphasized that the traction applied to the ligature by the adjustable locking means maintains the connecting part of the fixing device against a portion of the bone. Due to the shapes of the portions of the connecting part and of the bone, the surface of the portions of these two members in contact is reduced, a controlled relative mobility can be obtained. 
     According to a first improved embodiment, the ligature is made with an elastic conformable material. As a result, the extension capability of the elastic ligature when pressure is applied to it allows an increased relative mobility between the bone and the connecting part of the fixing device. 
     Preferably, the rigidity of the material with which the ligature is made of is within the range of 5 to 300 N/mm. 
     According to a second improved embodiment, which can be combined with the first improved embodiment, the traction maintained in the ligature by the adjustable locking means and initially applied to the ligature, before the operation of the adjustable locking means, allows an increased and controlled relative mobility between the connecting part of the fixing device and the bone. 
     However, the state of the ligature is maintained so that the desired stabilization effect between the bone to which the fixing device is fastened and the other bone to which another portion of the linking member is secured, is obtained despite the relative mobility. 
     According to still another preferred embodiment of the invention, the vertebral fixing device comprises:
         a connecting part having an outer face a portion of which is in contact with said bone and to be connected to a rod;   an elongate conformable ligature adapted to connect together said connecting part; and   adjustable locking means fastened to said connecting part, and adapted to fix simultaneously in position said connecting part relative to said linking member and at least one portion of said ligature relative to said connecting part, so as to allow relative displacement of said linking member and said bone.       

     Preferably, said connecting part includes a passage facing said linking member and said ligature passes through the adjustable locking means to reduce the section of said passage in order to press said ligature against said rod and simultaneously to fix said connecting part and at least one portion of said ligature in position relative to said rod. 
     Still preferably, said connecting part comprises two longitudinal members the first ends of which are connected together so that said members may pivot relative to each other and the middle parts of their two facing faces are adapted to bear on respective opposite sides of said linking member, said adjustable locking means being adapted to drive the second ends of said longitudinal members forcibly towards each other and to fix them in position relative to each other so that said two members form a clamp and grip said linking member, whereby said connecting part can be moved relative to said linking member. 
     According to a further embodiment of the fixing device, it comprises:
         a connecting part presenting first and second sides and suitable for being connected to a rod;   a flexible conformable ligature of elongate shape suitable for connecting together said connecting part and a bone; and   adjustable locking means mounted on said connecting part;   said ligature having two free ends;   said connecting part defining at least one passageway for passing said ligature in such a manner that two distinct strands of said ligature can be engaged in said passageway(s) so that said two ligature strands define a first ligature portion forming a loop that extends from a first side of said connecting part, and second and third ligature portions extending from the other side of said connecting part between respective ones of said ligature strands and said free ends; and   said locking means being distinct from the connecting part and co-operating therewith by screw-fastening, said locking means being capable of taking a first position relative to the connecting part in which the two ligature strands are free in said strand-passing  passageway(s), a second position relative to the connecting part in which the two ligature strands are prevented from moving in translation relative to the connecting part, and intermediate positions in which a coefficient of friction is created between said ligature strands and said connecting part.       

     Preferably, said connecting part of the fixing device defines a single passageway and both ligature strands are engaged in the single passageway. Preferably, according to a first embodiment of the above-defined fixing device, said connecting part comprises two longitudinal elements having first ends that are hinged together, each of said longitudinal elements presenting a recess suitable for receiving a portion of a section of said linking member, a wall of said recess co-operating with the side surface of said linking member to define said passageway for passing said ligature strands, said locking means being mounted at the two second ends of said longitudinal elements. Preferably, according to a second embodiment of the above-defined fixing device, it comprises a part that is generally U-shaped, suitable for receiving said linking member, and more particularly a rod, and having the outer ends of the limbs of the U-shape threaded, and in that the adjustable locking means comprise a tapped ring suitable for co-operating with the thread on the U-shaped part, tightening the ring causing the limbs of the part to be clamped against the linking member and preferably the rod. 
     According to a third embodiment, the connecting part of the fixing device comprises a hollow piece which is substantially cylindrical and provided with an axial passage for receiving a portion of said linking member and two portions of said ligature, said hollow piece comprising a central slot through which a median part of the ligature can pass to define an external loop, two lateral slots to allow the passing through of two free ends of the ligature, whereby said two portions of the ligature are disposed between the wall of said passage and the outer surface of the linking member, and a movable mechanical member forming said locking means to apply a strength on a portion of the outer face of said linking member opposite to said central slot. According to the third embodiment, the mechanical member is preferably a screw cooperating with a threaded opening provided with said hollow piece. A second object of the invention is to provide a stabilization system to stabilize at least two bones, especially two vertebrae so that a relative mobility is allowed between said at least two bones. 
     This aim is achieved thanks to a stabilization system comprising:
         a first fixing device of the type defined previously, fastened to a first bone;   at least a linking member having a first portion secured to said first fixing device and at least a second portion distinct from said first portion; and   at least a second fixing device of any kind fixed to a second bone, said second fixing device being secured to said second portion whereby a relative motion between said at least two bones is allowed.       

     Of course, the first fixing device can be designed in accordance with any previous embodiment described above to allow a relative mobility between the bone to which it is secured and the linking member, and the second fixing device may be also of the type defined above. 
     According to a first embodiment of the stabilization system, the second fixing device includes a screw screwed into the second bone. 
     According to a second embodiment of the stabilization system, the second fixing device is a hook. 
     Preferably, the linking member is a rod. The rod is rigid or the rod can include an intermediate portion which is flexible. 
     According to still another embodiment of the stabilization system, the two fixing devices are of the type defined above, the rod comprises a main portion which is substantially rectilinear and two end fixing portions for cooperating with said fixing devices, said two end portions being substantially parallel one with the other and angled with respect to said main portion of the rod. 
     When the ligatures of the fixing devices cooperate with spinous processes, this shape of the rod allows avoiding a twisting of the ligature. 
     According to a further embodiment of the stabilization system, it comprises at least three fixing devices secured to three bones and at least one linking member including at least three fixing portions, at least two adjacent fixing devices being designed to allow substantially no relative mobility between said corresponding bones and said linking member and said at least third fixing device being of the type previously defined to allow a relative mobility between said linking member and the bone to which said third fixing device is secured. 
     The above described embodiment allows a fixed stabilization between two or more first bones or vertebrae, for example, to treat spinal deformities or spinal degenerative diseases or trauma diseases, and a dynamic stabilization between one extreme of said first bones and at least one second bone or vertebra. 
     A third object of the present invention is the use of a fixing device as described above or of a stabilization system to allow a relative mobility between at least one bone, especially a vertebra and a linking member of a stabilization system and more generally to allow a relative mobility between at least two bones and especially two vertebrae. 
     These, and other aspects of the invention, will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. The following description, while indicating various embodiments of the disclosure and numerous specific details thereof, is given by way of illustration and not of limitation. Many substitutions, modifications, additions or rearrangements may be made within the scope of the disclosure, and the disclosure includes all such substitutions, modifications, additions or rearrangements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a first embodiment of a vertebral fixing device; 
         FIGS. 2A ,  2 B, and  2 C are vertical section views of the fixing device showing the use of said system as shown in  FIG. 1 ; 
         FIG. 3  is a face view showing the  FIG. 1  fixing device put into place on a vertebra; 
         FIG. 4  is a perspective view of a second embodiment of the fixing device, the ligature not being shown; 
         FIG. 5  is an exploded view of the fixing device of  FIG. 4 ; 
         FIG. 6  is a plan view of a portion of the fixing device of  FIG. 1 ; 
         FIG. 6A  is a section view on line AA of  FIG. 6 ; 
         FIG. 7  is a face view of the fixing device of the second embodiment; 
         FIGS. 7A and 7B  are section views on line VII-VII of  FIG. 7  showing two ways in which the flexible ligature can be put into place; 
         FIG. 8  is a perspective view of a third embodiment of the fixing device; 
         FIGS. 9A and 9B  are sectional views of the fixing device shown in  FIG. 8 ; with the locking means non-operating (A) and operating (B); 
         FIG. 10  is an exploded view of the fixing device of  FIG. 8 , the ligature not being shown; 
         FIG. 11  is a partial side view of an embodiment of a tensioning tool; 
         FIG. 12  is a perspective view which shows a first example of stabilization system including two fixing devices as shown in  FIGS. 1 to 3 ; 
         FIG. 13  is a perspective view which shows a second example of stabilization system including one fixing device as shown in  FIGS. 1 to 3  and a fixing screw; 
         FIG. 13A  shows an example of implementation of the stabilization system of  FIG. 13 ; 
         FIGS. 14A and 14B  show two examples of linking rods having a flexible median portion; 
         FIGS. 15 to 18  show different shapes of the linking rod; and 
         FIGS. 19 and 20  show examples of use of stabilization systems comprising more than two fixing devices. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in  FIG. 1 , in the first embodiment, the vertebral fixing device comprises a connecting part  12 , a flexible ligature  14 , and adjustable locking means  16 . The flexible ligature  14  is of elongate shape and is capable of matching the outline of the parts it is to connect together. In this figure, there can also be seen the rod  18  that is to be secured to the vertebra by means of the vertebral fixing device. Other types of linking member may be substituted to the rod such as plate, tube, stiff cables, etc. In the first embodiment, the connecting part  12  is constituted by two longitudinal elements given respective references  20  and  22 , each having a first end  22   a,    20   a  and a second end  22   b,    20   b.    
     As can be seen better in  FIG. 2A , the longitudinal elements  20  and  22  are hinged to each other at their second ends  20   b,    22   b  about a pivot pin  24 . 
     In the described embodiment, the locking means are constituted by a screw  26  having a head  26   a  that is engaged in a bore  28  formed in the first end  22   a  of the longitudinal element  22 . The first end  20   a  of the longitudinal element  20  is pierced by a tapped bore  28  for co-operating with the threaded shank  26   b  of the screw  26 . Each longitudinal element  20 ,  22  has an outside face  20   c,    22   c  and an inside face  20   d,    22   d.  The longitudinal elements  20  and  22  are mounted in such a manner that the inside faces  20   d,    22   d  of the longitudinal elements face each other. The inside faces  20   d,    22   d  of the longitudinal elements  20  and  22  have respective mutually-facing recesses  30  and  32 , each of substantially semi-cylindrical shape. The recesses  30  and  32  define walls  34  and  36  which are ruled surfaces having generator lines parallel to the pivot axis  24 . Finally, slots  38  and  40  cause the bottoms of the recesses  30  and  32  to communicate with the outside faces  20   c  and  22   c  of the longitudinal elements  20  and  22 . As explained in greater detail below, the recesses  30  and  32  are for receiving the rod  18  together with a strand of the ligature  14 , the slots  38  and  40  serving to pass the ligature  14 . 
     With reference to  FIGS. 2A to 2C , there follows an explanation of how the fixing device is used. 
     In  FIG. 2A , there can be seen the longitudinal elements  20  and  22  in the spaced-apart position, a position in which the locking means  16  are naturally not active, the threaded shank  26   b  of the screw  26  not being engaged in the bore  28 . The ligature  14  is engaged in the slots  38  and  40  of the longitudinal elements against one portion of the inside wall  34 ,  36  of the recesses  30  and  32 . The rod  18  is then introduced into the recess  30  of the longitudinal element  20  so that the two strands  42  and  44  of the ligature  14  are disposed between the inside wall of the recesses  30  and  32  and the side face  18   a  of the rod  18 . These two surfaces define a passageway  46  for passing the ligature  14  and having the strands  42  and  44  of the ligature  14  placed therein. 
     As shown better in  FIG. 2B , the strands  42  and  44  of the ligature define a portion of the ligature  14  that forms a loop  48  that extends beyond the outside face  20   c  of the longitudinal element  20 , and also two free portions  50  and  52  that extend beyond the outside face  22   c  of the longitudinal element  22 . When the longitudinal elements  20  and  22  are spaced apart as shown in  FIG. 2B , the ligature  14  can slide freely along the passageway  46 . Once the portion  48  of the ligature  14  forming the loop is placed around the transverse process or a rib or indeed a portion of the posterior arc of a vertebra or other bones, the surgeon engages the threaded shank  26   b  of the screw  26  in the tapped bore  28 , causing the longitudinal element  22  to come progressively closer to the longitudinal element  20 . This approach simultaneously reduces the section of the passageway  46  in which the strands  42  and  44  of the ligature are engaged and simultaneously introduces a certain coefficient of friction between the ligature and respectively the rod  18  and the walls of the recesses  30  and  32 . Nevertheless, it is still possible for the surgeon to extract traction on the free ends  50  and  52  of the ligature  14  until sufficient tension is obtained in the ligature around the vertebral process. This traction is performed with a surgical tool which will be described in detail hereunder. Once the tension in the ligature is sufficient for providing appropriate fastening, the surgeon finishes off tightening the screw  26  in the tapped bore  28 , thus locking the longitudinal elements  20  and  22  together. Simultaneously, it will readily be understood that the strands  42  and  44  of the ligature are pinched between the rod  18  and the wall of the recesses  30  and  32 . 
     In this locking position, the rod  18  is thus secured to the ligature  14  via the connecting part  12 . 
     It will also be understood that because the surgeon exerts traction only on the free ends  50  and  52  of the ligature  14 , there is no risk of jamming between the ligature  14  and the bottom face of the transverse process or of the rib, thus guaranteeing that effective fastening is provided with the transverse process or the rib or indeed a portion of the posterior arc of a vertebra. 
     This is shown in  FIG. 3 , where reference AT identifies the transverse process. 
     In the above description, both of the strands  42  and  44  of the ligature are disposed in the recesses  30  and  32  on the same side of the rod  18 . This disposition serves to obtain an optimum result. Nevertheless, it would not go beyond the invention if the strands  42  and  44  of the ligature  14  were to be placed on opposite sides of the rod  18 . Under such circumstances, it should be considered that the outside face  18   a  of the rod  18  and the inside walls of the recesses  30  and  32  define two passageways, respectively for passing each of the strands  42  and  44  of the ligature  14 . 
       FIG. 3  shows that the external face  20 ′ of the longitudinal element  20  is applied against the outer face of a transverse process AT under the tensioning action of the ligature  14 . More generally, the connecting part of the fixing device can be in contact with a rib, a transverse process or a lamina. 
     In other cases, the external face  20 ′ of the longitudinal element  20  may be applied against other parts of a vertebra or a part of another bone. 
     In any case, the shapes of the outer face of the bone and of the external face of the connecting part  12  are completely different. As a result, the contacting area between the connecting part and the bone is reduced. Moreover, the ligature  14  is flexible and not rigid, even if a significant traction strength has been applied to it. 
     As a result, a relative movement of reduced amplitude is allowed between the bone and the connecting part of the fixing device. Then a relative movement of reduced amplitude is allowed between the bone (e.g. vertebra) on which the fixing device is fastened and another bone secured to another portion of the rod  18 . Consequently, a dynamic stabilization can be obtained. 
     In the above-described embodiment, the ligature  14  is flexible and conformable but not elastic. To increase the amplitude of the relative movement allowed by the fixing device, it is possible to manufacture the ligature  14  in a material having elastic properties with a suitable Young modulus. 
     For the same purpose, it is also possible to control the traction or tension applied by the surgeon to the two free ends of the ligature before the locking part of the fixing device is activated by the surgeon. 
       FIGS. 4 to 7B  show a second embodiment of the fixing device. 
     In these figures, there can be seen the rod  18 , the connecting part now referenced  12 ′, and the flexible ligature  14 . 
     In this embodiment, the connecting part  12 ′ is constituted by a part  50  that is generally U-shaped. The inside wall of this part is constituted by a bottom  52  of substantially semi-cylindrical shape and by two substantially plane portions  54  and  56  that correspond to the two limbs of the part  50 . The width w of the recess  58  formed in the part  50  is substantially equal to the diameter d of the rod  18 . On its outside face  50   a  which is circularly symmetrical about a longitudinal axis of the part  50 , there is provided a thread  60  occupying its upper portion. The thread  60  is located entirely above the rod  18  when it is put into place in the recess  58 . 
     The thread  60  is designed to co-operate with a clamping ring  62  that constitutes the adjustable locking means. This ring has a slightly frusta-conical bore  64  with an inside face  66  that carries tapping  68 . 
     It can thus be understood that when the ring  62  is screwed tight on the threaded portion  60  of the part  50 , it deforms the limbs of the part  50  elastically, thereby pinching and clamping strands of the ligature  14  between the rod  18  and the inside wall(s) of the recess  58 , in a manner explained below. 
     As shown better in  FIGS. 6 and 6A , the part  50  includes in its bottom  70  a passage  72  for passing the ligature  14  in a manner explained below. 
     With reference to  FIGS. 7 ,  7 A, and  7 B, there follows a description of two different ways of putting the flexible ligature  14  into place inside the connecting part  12 ′ in the second embodiment. The side wall of the rod  18  and the inside wall of the recess  58  of the part  50  potentially define two passageways  74  and  76  for passing the middle strands of the flexible ligature  14 . I n the configuration shown in  FIG. 7A , only the passageway  74  is used. Thus, both intermediate strands  42  and  44  of the flexible ligature  14  are disposed in the passage  74 . This disposition presents all of the advantages described with reference to the first embodiment. 
     In the configuration shown in  FIG. 7B , the middle strands  42  and  44  of the flexible ligature  14  are disposed respectively one in each of the passageways  76  and  78 , i.e. on either side of the rod  18 . This configuration likewise presents all of the advantages described with reference to the first embodiment of the device since the free ends  50  and  52  of the ligature  14  are accessible for exerting the desired traction in order to obtain suitable clamping on the spinous process prior to locking the clamping ring  62  on the part  52 . 
     This second embodiment presents the advantage of being simpler in design since it serves in particular to avoid making two longitudinal parts constituting a kind of clamp hinged on the pin  24 . 
     It is to be underlined that this second embodiment allows obtaining the same relative mobility between the connecting part  12 ′ and the bone, especially a vertebra. Indeed, the part  50  of the connecting part  12 ′ has an outer face  50 ′. A limited portion of this outer face  50 ′ is put in contact with the bone by the traction produced by the ligature  14 . Consequently, a relative movement between the connecting part  12 ′ and the bone is allowed irrespective of the strength applied by the ligature. 
     As in the case of the first embodiment, the ligature  14  can be made with an elastic material to increase the relative mobility and the traction initially applied to the ligature can be adapted to the amplitude of the relative movement which is desired. 
     With reference to  FIGS. 8 to 10 , a third embodiment of the fixing device will be described. 
     In this embodiment, the connecting part  12 ″ consists of a cylindrical sleeve tube  80  having a cylindrical central passage  82 . The diameter of the passage  82  is slightly larger than the outer diameter of the rod  18 ′. 
     The wall of the sleeve  80  is provided with a threaded circular opening  84 , two lateral slots  86  and  88  disposed on each side of the circular opening  84 , and a central slot  90  diametrically opposed to the circular opening  84 . A screw  92  forming the locking member  16  cooperates with the threaded circular opening  84 . The passage  82  of the sleeve  80  is adapted to receive the rod  18  and two portions  12 ″ a  and  12 ″ b  of the ligature  14 ″. A median portion  94  of the ligature  14 ″ passes through the central slot  90  to form a loop  96 . The ligature  14 ″ also passes through the lateral slots  86  and  88 . Consequently, the two free ends  14 ″ c  and  14 ″ d  of the ligature  14 ″ extend outside the sleeve  80 . 
     When the rod  18 ′ is inserted within the passage  84 , the portions  14 ″ a  and  14 ″ b  of the ligature  14 ″ are disposed between the outer surface of the rod  18 ′ and the inner face  82 ′ of the sleeve  82 ′. Preferably, the outer face of the rod  18 ′ is provided with a longitudinal flattening  98 . 
     The portion  96  of the ligature  14 ″ is disposed about a bone or, more particularly, about a transverse process of a vertebra, the rod  18 ′ is inserted within the passage  82  of the sleeve  80  and the ligature  14 ″ passes through the slots  86 ,  88  and  90  so that the portions  14 ″ a  and  14 ″ b  of the ligature  14 ″ are disposed between the inner face of the sleeve  80  and the outer face of the rod  18 ′. 
     The surgeon applies the convenient tension to the free ends  14 ″ c  and  14 ″ d  of the ligature  14 ″. When the convenient tension has been applied, preferably by means of a tensioning tool, the surgeon screws the screw  92 . The active end  92   a  of the screw is applied against the flattening  98  of the rod  18 ′. When the screw  92  is fully screwed, the rod  18 ′ is transversally displaced within the passage  82  and the portions  14 ″ a  and  14 ″ b  of the ligature  14 ″ are pinched between the inner face of the sleeve  80  and the outer face of the rod  18 ′. As a result, the rod  18 ′ and the ligature  14 ″ are secured to the connecting part. In an alternative form of the third embodiment of the fixing device, the screw forming the locking member can be replaced by another mechanical system cooperating with the sleeve  80  to apply strength against the rod  18 ′. This mechanical system might be a frustro-conical piece cooperating with a frustro-conical opening provided in the sleeve which replaces the threaded opening. 
     In this embodiment, it is also possible to use a ligature made of an elastic conformable material to improve the relative mobility. In some cases, the surgeon can also adapt the tension applied to the ligature, before locking the ligature, to adapt the relative mobility to a particular situation. 
     It will be understood that in all the embodiments, the locking means are constituted by an element that is distinct from the connecting part and that is removable therefrom. In addition, in both cases, the locking means co-operate with the connecting part by screw engagement. It is thus possible to adjust accurately the dimensions of the ligature-passing passageway(s) as defined by the connecting part and the rod. In an initial stage, the coefficient of friction between the coefficient of the ligature and secondly the rod and the connecting part can be adjusted. In the final stage, very effective clamping of the ligature is obtained between the rod and the locking part. 
     In order to apply to the ligature the convenient tension, the surgeon can use advantageously a tool for tensioning the ligature. An example of such a tool is described in details in the PCT application WO 2007/034112 filed in the name of the applicant. This document should be considered as an integral part of the present description. Thus it is not necessary to describe it in details. 
       FIG. 11  depicts a side view of a portion of one embodiment of tensioning tool  150 , which may be used to apply tension to conformable ligature  14 . As shown in  FIG. 8 , tensioning tool  150  includes tool body  166  for engaging conformable ligature  14 , longitudinal member  160  for advancement in tool body  166 , and distal end not shown for engagement with the connecting part  12  or  12 ′. As shown in  FIG. 8 , tool body  166  includes attachment point  174  (with flange  158 ) for connection to ligature  14 , fixed handle  154 , movable handle  152  for rotation about axis  156 , return spring  162 , catch mechanism  164 , return spring adjustment member  170 , and spring adjustment member  168 . 
     Attachment point  174  can attach first and second ends of conformable ligature  14  to tensioning tool  150 . Distal end of tensioning tool  150  may engage to a portion of connecting part  12  or  12 ′. Fixed handle  154  may be gripped by a surgeon, movable handle  152  may be rotated about axis  156 , such as by squeezing movable handle  152 , to longitudinal member  160  through tool body  166  a selected distance. Advancing longitudinal member  160  to move connecting part  12  away from tool body  166  while maintaining first and second ends of conformable ligature  14  on attachment point  174  applies tension to conformable ligature  14 . 
     As described in above referenced PCT application, the two free ends of the ligature  14  can be interconnected by a fastener element. Then, the ligature forms a second loop which can be engaged on the attachment point  174 . 
     In some embodiments, tool body  166  may include return spring  162 , catch mechanism  164 , and return spring adjustment member  170  for controlling the distance that longitudinal member  160  is allowed to return when movable handle  152  is released. In some embodiments, return spring  162  may bias catch mechanism  164  such that movement is permitted in one direction only. In some embodiments, return spring  162  may bias catch mechanism  164  such that longitudinal member  160  may only move forward through tool body  166 . Advantageously, return spring  162  may ensure that a surgeon does not inadvertently relieve tension from conformable ligature  14 . In other words, tensioning tool  150  may have a default configuration for tensioning conformable ligature  14 . In some embodiments, actuating catch mechanism  164  (such as a surgeon pressing on catch mechanism  164  with a thumb) may change the positioning of catch mechanism  164  such that movement of longitudinal member  160  is permitted in a reverse direction as well. In some embodiments, movement of longitudinal member  160  in a reverse direction may include changing the positioning of catch mechanism  164  in relation to longitudinal member  160  as well as pulling in a reverse direction on grasping member  172 . 
     In some embodiments, tensioning tool  150  may include spring adjustment member  168  for adjusting the compression on a spring (not shown) in body  166 . In some embodiments, rotating spring adjustment member  168  one direction, spring adjustment member  168  may be advanced some distance into body  166  such that a spring may be compressed. In some embodiments, rotating spring adjustment member  168  in the other direction, spring adjustment member  168  may be advanced some distance out of body  166  such that compression forces on the spring may be relieved. By changing the compression forces on the spring, the spring may exert more or less force on longitudinal member  160 , which may affect how much tension can be applied to the ends of conformable ligature  14 . 
     As described in the above referenced PCT application, the tensioning tool can include a dynamometric spring to help the surgeon in applying to the ligature the convenient tension. 
     Referring now to  FIGS. 12 to 20 , several stabilization systems allowing a relative mobility between at least two vertebrae or two bones will be described. All these embodiments comprise at least one fixing device of the types previously described with reference to  FIGS. 1 to 10 , so that a dynamic stabilization of at least a portion of the spine can be achieved. 
     The stabilization system  200  shown in  FIG. 12  comprises two fixing devices  202  and  204  and a linking member consisting in a rod  206  in the particular embodiment presently described. The two fixing devices  202  and  204  are screwed on two distant portions  206   a  and  206   b  of the rod  206 . Each fixing device is secured to a vertebra V 1  and V 2  by means of their respective ligatures  14   1  and  14   2 . 
     As each fixing device allows a controlled relative mobility between the vertebra and the rod  206 , a dynamic stabilization is actually obtained between the two vertebrae V 1  and V 2 . In  FIG. 12 , the fixing devices  202  and  204  are in accordance with the first embodiment. It is clear that fixing devices of the two other types might be used or a combination of two fixing devices of different types. 
       FIG. 13  shows an alternative embodiment of a stabilization system. It comprises a rod  210  (or another linking member) and a first fixing device  212  of the type described above (i.e. fixing device allowing relative mobility). The second fixing device  214  comprises a screw  216  and a connecting head  217  cooperating with a second portion  210   a  of the rod  210 . The second fixing device does not allow any relative mobility between the rod  210  and the vertebra on which the fixing device  214  is crewed. However, since the fixing device  212  allows a relative mobility, a global relative mobility is obtained between the two vertebrae. 
       FIG. 13A  shows the implementation of two stabilization systems as shown in  FIG. 13 . The stabilization systems  220  and  222  are mounted on each side of the spine axis. More precisely, the fixing devices  214  are screwed into the transverse processes of vertebra V 3  and the two fixing devices  212  are secured to vertebra V 4  thanks to ligature  142  which is disposed around the transverse processes of vertebra V 4 . 
     In the preceding description, the linking members are preferably rigid rods. However, in some cases, it can be convenient to use linking rods having a flexible intermediate portion in order to add to the stabilization system flexion capabilities to the relative mobility allowed by the fixing devices. 
       FIGS. 14A and 148  show two flexible linking rods. In  FIG. 14A , the rod  230  comprises two rigid connecting ends  232 ,  234  and an intermediate portion  236  made with an elastic material. In  FIG. 14B , the flexible liking rod  240  comprises two rigid connecting ends  242  and  244  and a flexible intermediate portion  246 . The flexible intermediate portion  246  includes two elongate parallel arms  248  and  250 , the ends of which are connected to the connecting ends  242  and  244 . Since the arms  248  and  250  have a reduced section as compared with the connecting ends, the intermediate portion is actually flexible. 
     As it is better visible in  FIG. 3 , the loop portion  48  of the ligature  14  is twisted due to the relative orientation of the rod  18  of the stabilization system and the orientation of the transverse process AT. The twisting of the ligature can weaken it. To remedy to this possible drawback, it is possible to use rods which are not rectilinear. Broadly speaking, the rod comprises a main portion which is rectilinear and two connecting ends which are angled with respect to the main portion of the rod. 
       FIGS. 15 to 18  illustrate four different possible shapes of the “linking rod”. In  FIG. 15 , the linking rod  260  comprises a main portion  262  which is rectilinear and parallel to the spine axis and two connecting ends  264  and  266  and which are perpendicular to the main portion  262  and extending on the same side of the main portion. Each connecting end cooperates with a fixing device FD 1  and FD 2 . 
     In  FIG. 16 , the linking rod  270  has the shape of a “Z”. The main portion  272  is rectilinear and the two connecting ends  274  and  276  are parallel one to the other and angled with respect to the main portion. The connecting ends  274  and  276  extend respectively in two opposite directions. 
       FIGS. 17 and 18  show further possible shapes of the linking member.  FIG. 17  depicts a linking member having the shape of a rectangular frame  280 . It comprises two parallel connecting portions  282  and  284  cooperating with the fixing devices FD 3  and FD 4 . The two connecting portions are interconnected by two rectilinear portions  286  and  288 , which are parallel to the spine axis.  FIG. 18  shows a linking member  290  having the general shape of an axis “X”. The two parallel connecting portions  292  and  294  cooperate with the fixing devices FD 5  and FD 6 . The ends of the two connecting portions are interconnected by a structure having the shape of an “x” and this structure consists of two rectilinear portions  296  and  298 . 
       FIGS. 19 and 20  illustrate embodiments of the stabilization system comprising more than two fixing devices. 
     In the case of  FIG. 19 , the two stabilization devices  300  and  302  are mounted on each side of the spine axis. Each stabilization system comprises a unique rod  304 ,  306  and four fixing devices  308 ,  310 ,  312 ,  314  and  308 ′,  310 ′,  312 ′, and  314 ′. Each one of the fixing devices is of the type shown in  FIGS. 1 to 10 , i.e. a fixing device which allows a relative mobility. As a result, the stabilization systems  300  and  302  allow dynamic stabilization between the two vertebrae V 10 , V 12 , V 14  and V 16 . 
       FIG. 20  shows a complete stabilization equipment comprising two stabilization systems  310  and  312  mounted on each side of the spine axis. Each stabilization system comprises a unique rod  314 ,  316  and one fixing device  318 ,  318 ′ of the type described above (allowing relative mobility) and three fixing devices  320 ,  322 ,  324  and  320 ′,  322 ′,  324 ′ of the screw type, i.e. which do not allow relative mobility. As a result, vertebrae V 10 , V 12  and V 14  corresponding to screw fixing devices are stabilized without any possibility of relative movement and vertebra V 16  is connected to the three other vertebrae with a capability of dynamic stabilization