Patent Document

BACKGROUND OF THE INVENTION 
   The invention relates to assemblies for osteosynthesis of the spine, and in particular to the tools used while fixing such assemblies. 
   Numerous assemblies are known for the osteosynthesis of the spine, for example the assembly disclosed in document EP-0885 598. Such assemblies generally include anchor members in the form of screws or hooks that are connected to one another by one or more link elements in the form of rods or plates. The function of such assemblies is to rectify the shape of a deformed vertebral column or to enable osteosynthesis to take place in one or more fractured vertebrae. For this purpose, the anchor members are fixed rigidly to the vertebrae. 
   Each such member must be placed in a very precise position. For this purpose, tools are known that enable the member to be held temporarily so as to assemble it rigidly to the other parts in a desired position. Nevertheless, most such tools do not give satisfaction, either because they do not hold the member in satisfactory manner or because their size makes it necessary to release the member in order to be able to fix it to the other elements of the assembly. In particular, such tools do not give satisfaction when it is necessary to hold an anchor member whose head includes at least one branch. 
   BRIEF SUMMARY OF THE INVENTION 
   An object of the invention is to provide a tool enabling the head of an anchor member that includes at least one branch to be held appropriately even while said member is being fixed to other parts of the assembly. 
   To this end, the invention provides a system for osteosynthesis of the spine, the system comprising: 
   a head of anchor member, the head including at least one branch; and 
   a tool for fixing the head to an assembly for osteosynthesis of the spine, 
   the tool being suitable for holding the head by applying lateral thrust on at least one lateral edge of the branch and by applying additional thrust on the head in the opposite direction to the lateral thrust. 
   Thus, the thrust of at least one of the lateral edges makes it possible to apply the additional thrust other than on the face of the branch that is to come into contact with the link element. The additional thrust may, for example, be applied to the tip of the branch or to the other face of the branch. The head can thus be assembled to other elements of the assembly while it is being held by the tool. In addition, the tool presents two or three thrust points on the head, thereby ensuring that the head is held properly by the tool. 
   The system of the invention may also present at least any one of the following characteristics: 
   the tool is suitable for applying two lateral thrusts on the respective lateral edges of the branch; 
   the tool comprises a one-piece arm suitable for applying the lateral thrust and the additional thrust simultaneously; 
   the tool and the lateral edges are shaped with complementary shapes to prevent the tool sliding upwards and/or downwards relative to the head. 
   the tool presents two portions in relief for applying thrust against the edges; 
   the lateral edges present two cavities against which the tool is suitable for applying thrust; 
   the cavities are open towards the tip of the head; 
   the additional thrust is applied to a top edge of the branch; 
   the contact between the tool and the top edge takes place along a line or over an area; 
   the tool includes an elongate element extending transversely relative to a general direction of the tool, whereby the tool is capable of applying thrust to the top edge; 
   the tool includes two fingers supporting the elongate element, the system being arranged in such a manner that the elongate element can be placed in a slot of the head without the fingers extending into the slot; 
   the fingers extend from one end of an arm of the tool; 
   the tool has an arm suitable for applying the additional thrust; 
   the additional thrust is applied against a face of the branch; 
   the tool and the face are shaped with complementary shapes to prevent the tool sliding upwards or downwards relative to the head; 
   the branch is a first branch, and the head includes a second branch extending facing the first branch; 
   the each lateral thrust is applied in a direction away from the second branch; 
   the tool is suitable for holding the branch while leaving access to a slot formed between the two branches; 
   the tool comprises two arms that are hinged to each other; 
   the system includes an anchor suitable for being anchored in the spine, the head being integral with said anchor member; 
   the system comprises an anchor member suitable for being anchored in the spine, the head being suitable for fixing to the anchor member in different angular positions of the head relative to the anchor member; and 
   an assembly for osteosynthesis of the spine comprising an anchor member including a head, the head having a branch presenting at least one lateral edge, and the each edge presenting a blind cavity cut into the edge. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other characteristics and advantages of the invention appear further from the following description of a preferred embodiment and of variants given as non-limiting examples. In the accompanying drawings: 
       FIG. 1  is a perspective view of an anchor member head of a system constituting the preferred embodiment of the invention; 
       FIG. 2  is a view on a larger scale of the anchor member with its head, in a variant embodiment; 
       FIG. 3  is a fragmentary perspective view of a tool of the system constituting the preferred embodiment of the invention; 
       FIGS. 4 to 7  are four perspective views showing four steps in the co-operation between the tool of  FIG. 3  with the head of  FIG. 1  while the anchor member is being assembled with other elements of the osteosynthesis assembly; and 
       FIGS. 8 and 9  are views analogous to  FIG. 1  showing variant embodiments of the head. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A preferred embodiment of the system for osteosynthesis of the spine is described below. This system comprises a tool as shown in  FIG. 3  and an assembly for osteosynthesis of the spine. The assembly comprises various parts, including mutually identical anchor members of the type shown in  FIGS. 1 and 2 . 
   Each anchor member comprises a threaded shank and a head. In the embodiment of  FIG. 1 , the head forms a part of an anchor member of conventional polyaxial type. In such a member, the head  6  can be mounted on the shank of the body so as to occupy different angular positions relative thereto and it can be locked in a selected angular position. The head  6  has two branches  8  extending at a distance from each other and facing each other. The two branches give the head a U-shape defining between them a slot  10 . Overall, the member is in the form of a tuning fork identical to the variant shown in  FIG. 2 . The two branches  8  are in the form of cylindrical sectors geometrically defined by a common cylinder that is coaxial with the shank  4 . The two branches are symmetrical to each other about a longitudinal midplane constituting the plane of symmetry of the member. The member also presents another longitudinal midplane of symmetry perpendicular to the first. 
   Each branch  8  presents an inside face  12  facing the other branch and an outside face  14  facing away from the other branch. Each branch presents an essentially plane tip  16  in the form of a circular arc, and two lateral edges  18  each constituting a junction between the inside and outside faces of the branch. These characteristics of the anchor member  2  are themselves known and are not described in greater detail herein. Reference can be made in particular to European patent document No. 0 885 598. 
   With reference in particular to  FIG. 3 , the tool  20  of the system of the invention comprises two arms  22  and  24  each being of generally elongate and substantially rectilinear shape. The two arms are hinged to each other about an axis  26  perpendicular to the longitudinal direction of the arms. Along said axis  26 , the arm  22  is wider than the other arm  24 . The arm  22  is referred to herein as the “first” arm. The first arm  22  is in the form of a fork over a major fraction of its length extending from a proximal end of the arm (not shown) to a zone of the arm that lies between the axis  26  and the distal end  28  of the arm. This distal end  28  is also in the form of a fork, but over a length that is much shorter than the other fork. This fork thus defines two fingers  30  which together form the distal end  28 . The second arm  24  passes through the larger fork of the first arm level with the axis  26 . 
   The head  6  and the tool  22  are configured so that the tool is suitable for holding the head, and in particular in rigid manner. The means enabling it to hold the head in this way are described below. 
   On the head  6 , each edge  18  presents a chamfer extending from the tip of the edge to a low portion of the head where it extends in continuity with the corresponding edge of the other branch. Each pair of edges thus forms a U-shape. The chamfer is oriented substantially towards the outside of the head, essentially in a direction parallel to an axis  32  along which a link rod  34  is to extend in the slot defined between the branches and as shown in  FIG. 7 . 
   The head  6  presents second chamfers  34  extending to the junction of each edge  18  with the inside faces  12  of the branches. This second chamfer  34  is also generally U-shaped. It faces essentially towards the inside of the head. In its bottom portion it is essentially in the form of a cylinder about the axis  32 . 
   Each lateral edge  18  of each branch presents a blind cavity  36  formed both in the edge  18  and in the second chamfer  34 , cutting into the ridge formed at the junction thereof. Each cavity  36  presents a plane end face  38  which constitutes the largest face of the cavity. The cavity opens out laterally relative to the edge  18  and the second chamfer  34 . In contrast, it is closed upwards and downwards by residual portions of the ridge. The face  38  is parallel to the axis of the cylinder forming the branches. It slopes relative to the axis  32  and is oriented towards the outside of the head. It is oriented essentially in the opposite direction to the portion of the branch that carries it. The head  6  thus presents four cavities  36 , two in each branch  8 . 
   The fingers  30  present two plane faces  40  extending parallel to each other, facing each other, and spaced apart from each other. Each finger includes a portion in relief  42  projecting from the face  40  towards the other finger. This portion in relief is contiguous with a front edge of the finger  30 . Each portion in relief  42  presents in particular a plane face  44  parallel to a longitudinal direction of the tool and oriented towards the other finger while sloping rearwards, i.e. towards the second arm  24  when the arms are in the position shown in  FIG. 3  where the distal ends of the two arms are adjacent to each other. 
   The cavities  36  and the portions in relief  42  are shaped and positioned in such a manner that the first arm  22  can be placed relative to the head  6  so that the faces  44  and the portions in relief  42  come into contact with the faces  38  of the two cavities  36  respectively in either one of the two branches. In this position, as shown in particular in  FIGS. 5 ,  6 , and  7 , each of the portions in relief  42  applies lateral thrust  43  on the associated cavity  36  in the direction opposite to the other branch (only one direction of applied thrust is shown in  FIG. 4 ). The resultant  45  of these two applied thrusts forms thrust oriented away from the other branch. 
   Each portion in relief  42  also presents a top face  46  of plane shape that slopes relative to the vertical direction, forming a bevel on said portion in relief. 
   When the first arm  22  is thrust against the edges  18  in the manner explained above, the faces  46  of the portions in relief are thrust against the top faces  47  of the respective cavities  36 . The corresponding lateral thrust  49  is likewise oriented away from the other branch, but is oriented above all in an upward direction towards the tip of the branch. In this case also, the resultant  51  of these two thrusts forms thrust oriented away from the other branch and upwards. 
   With reference to  FIG. 3 , the tool  20  includes an elongate transverse element  50  implemented in this example as a rod. The rod  50  extends parallel to the pivot axis  26  from one of the fingers  30  to the other finger across the fork that the fingers define. 
   The rod  50  is shaped and positioned in such a manner that when the portions in relief  42  are thrust into the cavities  36  as explained above, the rod  50  thrusts against the tip  16  of the same branch as shown in particular in  FIGS. 5 and 7 . The thrust  58  from the rod  50  on said tip is oriented towards the bottom of the head parallel to the longitudinal direction of the anchor member as shown in  FIG. 5 . Given the cylindrical shape of the outside face of the rod and the essentially plane shape of the tip  16 , contact between these two elements takes place along a line. In the presence of a transverse element  50  of rectangular or square right section, this contact would take place over an area. 
   If the resultant thrust  51  of the thrust  49  of the faces  46  on the faces  47  of the cavities is considered as a first thrust and if the thrust  58  of the rod  50  on the tip  16  is considered as a second thrust, it can be seen that these two thrusts are oriented in opposite directions, even if these two directions are not rigorously collinear. Nevertheless, when the tool holds the head by means of these thrusts only, account needs to be taken of the action of gravity on the head. In the position of  FIG. 5  it can be seen that the geometrical line formed by the two faces  46  is very close to the rod  50  while the center of gravity of the head is offset therefrom. Consequently, if it is considered that the head can be pivoted about the line formed by the two faces  46 , the weight of the head tends to turn it so as to press the tip  16  against the rod  50 , thus leading to a stable position for the head held by the tool. Although in this position the head is not rigidly held by the tool, it can be manipulated without difficulty by means of the tool, in particular in order to be moved. 
   The outside face  14  of each branch  8  presents a cavity  58  whose end wall is formed by a face  60  extending parallel to the axis  32  and to the axis of the cylinder. The face  60  is generally rectangular in shape. It opens out laterally in the outside face  14  on both sides. Nevertheless, it is closed upwards and downwards by the top and bottom edges of the cavity. 
   The distal end of the second branch  24  is oriented towards the distal end  28  of the first branch and is terminated by a plane face  62  of rectangular shape substantially identical to the shape of the faces  60 . The face  62  extends towards the faces  44  of the portions in relief  42 . 
   The cavities  58  and the face  62  are formed and positioned in such a manner that when the first arm  22  is thrust against either one of the branches by means of the portions in relief  42  and by means of the rod  50 , as explained above, then the second arm  24  can be driven so as to cause its distal end to penetrate into the cavity  58  of the same branch, thus thrusting the face  62  against the face  60 . The thrust  63  of the second arm on the branch, as shown in  FIG. 6 , is thus oriented towards the other branch in a direction opposite from the thrust  45  delivered by the first arm by means of the portions in relief  42 . This time, if the two thrusts  43  imparted by the portions in relief  42  in the cavities  36  are considered as constituting a first thrust  45  and the thrust imparted by the face  62  in the cavity  60  is considered as constituting a second thrust  63 , it can be seen that these two thrusts are oriented in opposite directions and deliver a kind of pinching effect on the branch by means of the tool. This pinching enables the operator to hold the head rigidly by means of the tool. For this purpose, the cavities  58  are preferably at the same height as the cavities  36 . Retention is made stronger by the operator clamping the two proximal ends of the arms towards each other. This rigid retention of the head by means of the tool takes place via only one of the two branches of the head. This can be done via either one of the two branches. 
   It should also be observed that retention of the head in the tool solely by means of the portions in relief  42  and the rod  50  likewise takes place via the first arm  22  on its own. 
   The configuration of the end of the first arm  22  is such that the rod  50  can be placed in the slot  10  as shown in  FIG. 4  without the fingers  30  extending into the slot. In this situation, the fingers then extend vertically relative to the edges  18 . When the head is supported movably on the shank of the member, this position of the tool within the head enables the head to be turned about its own axis by pressing the rod against the branches. Similarly, when the head is integral with the shank, this position enables the member to be turned about its longitudinal axis. 
     FIGS. 4 to 6  show various different relative positions of the head and the tool that can appear during surgery to install the osteosynthesis assembly. 
   As described above,  FIG. 4  shows the disposition of the rod in the slot  10  of the head which makes it possible, by turning the tool about its own longitudinal axis, to cause the head of the member to turn about its own axis until it reaches a desired position. 
     FIG. 5  shows the head being held by means of the first arm  22  on its own via the portions in relief  42  and the rod  50 . The second arm  24  is then at a distance from the head. In practice, it is possible to go directly from the position of  FIG. 4  to the position of  FIG. 5 . 
   It will be observed that in the position of  FIG. 4 , the distal end  28  of the first arm  22  automatically centers the tool relative to the head prior to passing into the position of  FIG. 5 . 
   Thereafter, by bringing the distal end of the second arm  24  against the branch that is already held, the head can be held rigidly by the tool as shown in  FIG. 6 . The head can then be held firmly. 
   It will be observed that the tool leaves the slot  10  between the branches completely free, and also the access thereto free so as to make it possible to install a link member such as a rod  34  in the slot. 
   With reference to  FIG. 7 , the following step could be installing a locking element  66 . In this case the locking element is in the form of a ring having an outside thread for co-operating with a thread formed in the inside face  12  of the branches so that the ring can clamp the rod  34  against the bottom of the slot  10 . The arrangement of the anchor member, the rod, and the ring is conventional. The ring presents a hexagonal socket in its center for tightening purposes. The tool allows the head to be held in position possibly until after the ring has been tightened and the anchor member has been locked completely relative to the rod. The operator can then release the hold of the tool on the head. It will naturally be understood that the tool is not intended to remain on the assembly after surgery, since the tool does not form part of the osteosynthesis assembly proper. 
   The tool in this embodiment of the invention thus enables the head to be held in two different ways, firstly by means of the arm  22  on its own and secondly rigidly by means of both arms. As can be seen, in each position for holding the head in the tool, contact takes place via at least three points. 
     FIG. 8  shows a variant embodiment of this invention. In this variant, the system is identical to that described above except that the cavities  136  formed in the edges  18  of the head are upwardly open towards the tips of the branches. It follows that the distal end of the first arm  22  can be separated from the branch by sliding upwards. In contrast, the head can no longer be held on the first arm  22  by means of the rod  50  without also making use of the second arm  24 . 
   Another variant embodiment is shown in  FIG. 9 . In this embodiment, the end faces of the cavities  236  are cylindrical in shape, with the axes of the cylinders being parallel to the longitudinal direction of the anchor member. Again the cavities are upwardly open. 
   When the head is integral with the shank as in the variant of the anchor member shown in  FIG. 2 , the chamfer  34  can be omitted. 
   Naturally, numerous modifications can be applied to the invention without going beyond the ambit thereof. 
   While the head is being held rigidly by the branch being pinched between the two arms, provision can be made for the first arm  22  to apply lateral thrust  43  or  49  in only one cavity  36 , providing the cavity and the face  58  are suitably configured to ensure that the two applied thrusts are in substantially opposite directions. For example the faces  60 ,  62  could be given a shape that is not plane (e.g. spherical) so that they assemble together with male/female interfitting. 
   A tool could be provided that has a transverse element  50  independently of the other characteristics of the invention.

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