Abstract:
An osteosynthesis device, in particular for the spine. The device has an outer spherical surface and a substantially cylindrical inner surface, designed to receive a rod of the device. A ring has two matching parts ( 10 ) each being U-shaped and having a bottom and a pair of branches taking up a fraction of the axial dimension of the bottom. The distance between the branches is about the same as that of the rod diameter, their respective bottoms facing each other enclosing the rod, and their pairs of branches adjacent. The parts jointly define the outer and inner surfaces. The invention is applicable to linkages adjustable at different angles.

Description:
FIELD OF THE INVENTION 
     The present invention relates in general to implants for osteosynthesis, particularly of the spine. 
     It relates more specifically to a new ring of spherical type intended to allow angular orientation between two components of the implant prior to tightening, and to an implant equipped with such a ring. 
     BACKGROUND OF THE INVENTION 
     Document FR-A-2 659 546 discloses a spinal osteosynthesis system which comprises a collection of pedicle screws, each having a spherical seat made in a so-called “forked” head and in which a split ring can be housed. Passing through this ring itself is a rod intended to connect the various screws together, and the ring offers a possibility of angularly orientating the axis of the pedicle screw and the axis of the rod in one or two planes. Once this angular adjustment has been performed, a threaded cap compresses the ring and locks the assembly firmly in position. 
     The known ring is made of one single piece, with a spherical outer face, a cylindrical inner face with symmetry of revolution, and a slot directed radially and extending between the inner and outer faces. 
     This slot gives the ring the required deformability so that when clamped into the seat in the pedicle screw using an appropriate threaded member, it can be compressed firmly against the rod, so that once the angular adjustment has been performed, the assembly is held perfectly immobilized. 
     This known ring does, however, have a limitation: when preparing the implant system prior to or during fitting, it is necessary to slip as many split rings as are needed over the rod from one of its ends. 
     This is because the deformability of the ring which is afforded by its slot is far from sufficient to allow this slot to be parted temporarily to allow the rod to be introduced radially into the cylindrical passage through the ring. Thus, the practitioner attempting to proceed in this manner would inevitably cause breakage or significant damage to the mechanical qualities or the shape of the ring. 
     The use of this kind of known ring is therefore restricted to scenarios in which the rod is accessible from at least one of its ends. 
     SUMMARY OF INVENTION 
     The present invention aims to alleviate this drawback of the state of the art and to propose a new deformable ring which can be fitted onto a rod even when the latter has neither end free for the said ring to be slipped over it. 
     Thus the present invention proposes an angular orientation ring for an osteosynthesis device, particularly for the spine, of the type comprising an essentially spherical outer face and an essentially cylindrical inner face intended to take a rod of the device, the ring being characterized in that it is made in two mating parts, each part being substantially U-shaped with a bottom and at least one pair of branches occupying a fraction of the axial dimension of the bottom, the distance between the two branches of one same pair being close to the diameter of a rod intended to take the ring, and the two parts being designed to be inserted over the rod from its side in such a way that their respective bottoms come to substantially face each other, enclosing the rod, and such that their pairs of branches come to be side by side, the said two parts therefore jointly defining an essentially spherical outer face and an essentially cylindrical inner face. 
     Preferred but non-limiting aspects of the ring according to the invention are as follows: 
     each part has a pair of branches and each of the said branches occupies substantially half of the axial dimension of the ring. 
     each of the said branches has, at its free end, a projecting shape able to be received in an essentially complementary housing provided in the bottom of the other part. 
     the projecting shape of each of the branches of one part is defined by the intersection of a spherical contour of the branch with an end face essentially parallel to the axis of the ring and forming an extension of a semi-cylindrical inner surface defined by the bottom of this same part. 
     the said end face of each branch extends slightly obliquely with respect to the axis of the ring. 
     the two branches are separated from the bottom by two shoulders extending in planes which, with respect to the axis of the ring, have an obliqueness similar to that of the said end faces. 
     the branches of each part are delimited, on a face facing towards the branches of the other part, by a plane perpendicular to the axis of the ring. 
     the minimum distance between the two branches of one same pair is locally slightly shorter than the diameter of a rod intended to take the said ring. 
     The invention also proposes an osteosynthesis device, particularly for the spine, comprising a bone fixation part which has an essentially spherical seat, a ring housed in the said seat, an element for clamping the ring into its seat, and a rod passing through an essentially cylindrical passage formed in the ring, characterized in that the rod forms part of a component comprising, at the two ends of the rod, two parts which are wider than the said rod, and in that the ring is made as defined above, its two parts being fitted onto the rod from the side thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other aspects, objects and advantages of the present invention will emerge more clearly from reading the following detailed description of a preferred embodiment thereof, given by way of example and made with reference to the appended drawing, in which: 
     FIG. 1 is a view along the axis of part of the ring according to the present invention, 
     FIG. 2 is a side view in the direction of arrow II of FIG. 1, 
     FIG. 3 is a side view in the direction of arrow III of FIG. 1, 
     FIG. 4 is a view from above in the direction of arrow IV of FIG. 1, 
     FIG. 5 is a perspective view of the part of the ring of FIGS. 1 to  4 , 
     FIG. 6 is a perspective view of an osteosynthesis rod on which a ring consisting of two ring parts identical to the one illustrated in FIGS. 1 to  5  has been mounted, and 
     FIG. 7 is a view in transverse section illustrating the mounting of a ring part on an osteo-synthesis rod. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference first of all to FIGS. 1 to  5 , there have been depicted a part intended, together with another, in this particular instance identical, part, to form a ring with an essentially spherical outer surface and an essentially cylindrical inner surface. 
     This part  10  has, viewed face-on (FIG.  1 ), the overall shape of a U with a bottom part  11  and two lateral branches  12   a ,  12   b , these three parts being delimited by respective outer surfaces  111 ,  121   a ,  121   b  which belong to a common sphere. 
     The bottom part  11  extends, in the direction of the axis XX (that is to say horizontally in FIGS. 2 and 3) a dimension equal to the axial length of the ring, while the two branches  12   a ,  12   b  are only, in this direction, only about half of this dimension, the bottom and the two branches having a common end face  101  extending perpendicularly to the axis XX of the ring. 
     The transition between the bottom and the branches occurs, in the direction of the axis XX, at two shoulders or cut surfaces  113   a ,  113   b  which, as shown particularly in FIG. 4, extend in slightly re-entrant planes from the outside of the ring towards the inside. 
     On the opposite side to the common edge  101 , the bottom is delimited by an opposite edge  112  extending in a plane perpendicular to the axis XX, while the branches  12   a ,  12   b  are delimited by respective opposite edges  123   a ,  123   b  extending in another plane perpendicular to the axis XX is located substantially mid-way between the planes bounding the bottom part. 
     The bottom part  11  has an inner face  110  which is a portion of a cylinder with symmetry of revolution and which, in the region of the inner faces of the two branches  12   a ,  12   b , is extended to essentially form half a cylinder. 
     In their free-end region, the inner faces of the two branches  12   a ,  12   b  have a cut surface  122   a ,  122   b , respectively, which extends essentially in the continuation of the semi-cylindrical wall  111 , but with a slight obliqueness, namely a re-entrant orientation from the outside inward. This obliqueness is preferably similar to that of the shoulders  113   a ,  113   b.    
     The minimum distance L between the two branches perpendicular to the axis XX (see FIG. 1) is chosen to be slightly shorter than the diameter of a rod on which two parts  10  are to be mounted in order together to form a spherical angular-orientation ring. 
     It will be seen here that this minimum distance L is in the region of the cut surfaces  122   a ,  122   b , while below this restriction, the dimension of the interior space of the part perpendicular to the axis XX is greater than that of the associated rod, by choosing, for the semi-cylindrical wall  110 , a diameter which slightly exceeds the diameter of the rod. 
     By way of example, and for a part  10  made of a material of the titanium alloy type, or material of comparable elasticity, the distance L is preferably about 15% shorter than the diameter of the rod. Furthermore, the diameter of the surface  110  is larger than the diameter of the rod by a sufficient margin that the necessary freedom of sliding can be achieved. 
     Now, with reference to FIGS. 6 and 7, a spherical ring is produced on a cylindrical rod  21  by inserting a first part  10  over this rod, and more specifically by placing this part in such a way that its interior space opens towards the rod and by exerting radial force in the direction of arrow F 1 . 
     The branches  12   a ,  12   b  of the part  10  then temporarily part elastically to allow the rod to enter its interior space; once this has been done, the rod  21  is received, with a small amount of clearance, in its semi-cylindrical cradle defined by the wall  110  of the part. The part  10  can then slide freely along the rod, and held captive thereon in that removal of the part can be achieved only with the use of a thin tool, forming a lever. 
     A second part  10 , identical to the first, is then placed on the rod  21  in the same way, but facing in the opposite direction, or head-to-tail with respect to the part already fitted, and by being subjected to a force in the direction of arrow F 2 , in the opposite direction to arrow F 1 . 
     The positioning of the second part with respect to the first at the instant of this fitting is chosen in such a way that the branches  12   a ,  12   b  of the second part run alongside the branches  12   a ,  12   b  of the first part. 
     It will be noted here that the end part of each branch, defined by the end parts of the walls  121   a ,  122   a ,  123   a  (or of the walls  121   b ,  122   b ,  123   b , respectively) is in the form of a point which, at the end of the fitting of the second part  10 , becomes wedged in a housing of essentially complementary shape defined by, on the one hand, the outer surface of the rod  21  and, on the other hand, by the face  113   a  and by the face  123   a  of the part  10  fitted earlier (or, respectively, by the outer surface of the rod  21  and by the faces  113   b  and  123   b ). By symmetry, the same wedging effect is achieved between the points of the branches  12   a ,  12   b  of the part  10  fitted earlier and the corresponding housings of the newly-fitted part. 
     Thus, the rod  21  makes it possible to prevent translational movement of the two parts  10  in a plane perpendicular to its axis, while the aforementioned fourfold wedging effect ensures that the two parts  10  nest snugly together as shown in FIG. 6, without the possibility of coming apart. 
     In an extremely simple and easy way the two parts  10  thus form a spherical ring which need not be slipped onto the rod from an end, in contrast to what was the case with an ordinary split ring. 
     Thus, the present invention makes it possible for a rod  21  to be fitted with a spherical ring particularly when the rod belongs to an osteosynthesis element, designated as a whole as  20 , where it forms a connection between two broader elements, depicted diagrammatically as  22 ,  23  in FIG.  6 . This osteosynthesis element  20  may, in particular, be a part for connecting with the sacrum. 
     Once the ring has been fitted on the rod  21 , this ring can collaborate with the other components of an implant as described, in particular, in document FR-A-2 659 546, the ring according to the invention replacing the one-piece split ring described in that document. More specifically, the U shape of each part  10  forming the ring according to the invention exhibits the elastic deformability required so that when the ring is compressed by the tightening of the threaded cap of the implant, it firmly locks the rod  21  against any translational movement and against any tilting with respect to the angular orientation given before tightening. Naturally, the invention is just as applicable to other types of implants involving rings. 
     Of course, the present invention is not in any way restricted to the embodiment described hereinabove and depicted in the drawings. 
     In particular, provision may be made for each of the parts  10  to comprise several pairs of branches spaced apart, between which the branches of the other part engage. 
     Furthermore, although the two parts  10  used to form the ring are, in this example, strictly identical, which makes manufacture and handling easier, two differing parts could be envisaged.