Abstract:
A spinal implant has a bone plate with a bone contacting surface and an opposite upwardly facing surface and at least one opening with a groove formed around an inner circumference of the opening. The groove has an axially spaced upper and lower surface. A bone-anchor or bone screw is capable of being accommodated in the opening between the groove and the bone contacting surface. A split-ring is mounted in the groove. The split-ring is sized to expand upon insertion of the anchor element into the opening and to come into direct contact with a head of the bone anchor to hold the bone anchor in the opening. The split-ring has an upper surface and a lower surface for respectively engaging the upper and lower surface of said groove, wherein the split-ring has a variable width so as to optimize its flexibility.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a continuation of U.S. patent application Ser. No. 12/985,733, filed on Jan. 6, 2011, which is a continuation of U.S. patent application Ser. No. 11/060,171, filed on Feb. 17, 2005, now U.S. Pat. No. 7,887,547, which is a continuation of U.S. patent application Ser. No. 10/331,212, filed on Dec. 30, 2002, now abandoned, which is a divisional of U.S. patent application Ser. No. 09/665,530 filed on Sep. 19, 2000, now U.S. Pat. No. 6,602,255, the disclosures of which are incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to osteosynthesis devices for the spinal column, the devices comprising a plate and a mechanism for locking a bone screw or anchoring member in position. This divisional application relates to tools used to remove a bone screw after it has been locked in the plate. 
         [0003]    U.S. Pat. No. 5,876,402 relates to an osteosynthesis plate comprising through-holes of conical shape capable of housing a bone screw with a completely spherical head to which is clipped, so as to form a ball-joint connection. A split coupling element of conical exterior shape complementing that of the hole is provided. A circlip reduces the aperture of the through-hole. A similar clip and groove arrangement is shown in U.S. Pat. Nos. 5,879,389 and 6,102,952. 
         [0004]    In U.S. Pat. No. 5,876,402, the bone screw is placed in the coupling element prior to insertion in the plate. Upon insertion, the split coupling element opens up the circlip. This circlip closes up again once the coupling element has passed through. The coupling element is thus held captive in the through-hole. Final clamping of the anchoring member in position is achieved by the frictional wedging of the coupling element in the bottom of the cone. 
         [0005]    In such a system, the number of parts makes the clamping-in position of the anchoring members weak. In addition, the clamping does not occur at the instant when the circlip closes up again after the passage of the coupling element. This leads to the risk of the assembly becoming unclamped, which is prejudicial to the patient. 
         [0006]    FIGS. 5 and 6 of U.S. Pat. Nos. 5,879,389 and 6,102,952 show a split-ring for installation in a groove after the bone screw or anchor has been installed in the bone. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    One of the objects of the present invention is to provide a spinal implant which is easier to fit while at the same time being reliable. 
         [0008]    With a view to achieving this objective, the present invention envisages an implant, particularly for the spinal column, comprising a joining member such as a plate exhibiting openings or orifices, bone-anchoring members such as bone screws capable of being accommodated in the orifices and at least one split ring capable of holding the members in the orifices. The split ring can come into direct contact with the anchoring member or members to hold the member or members in the orifices. 
         [0009]    Thus, the number of parts involved in the locking is reduced and this locking can be made more reliable. Advantageously, the joining member comprises a plate and the orifices comprise an opening with a spherical seat. 
         [0010]    Preferably, each anchoring member or bone screw comprises a complementary spherical part capable of coming into contact with the spherical seat. Thus, the surgeon has, at his disposal, freedom to orient the anchoring member angularly with respect to the joining member or plate, thus allowing him to optimize the anchorage. 
         [0011]    Advantageously, the anchoring members or bone screws comprise driving means such as a drive socket. 
         [0012]    In one embodiment the split ring is preferably common to at least two orifices and includes a driving means, which driving means comprise openings. In another embodiment, the split ring is specific to each orifice in the plate. 
         [0013]    Advantageously, the split ring has a variable cross-section so as to optimize its flexibility. Thus, the ring will deform more readily when introducing the head into the orifice. The amount of time taken and the number of operations required during surgical intervention will be reduced. 
         [0014]    The bone plate, screw and ring may be supplied as part of a screw locking system for bone plates to be used by a surgeon. The bone plate has at least one opening therein, and normally a plurality of openings, for receiving a bone screw or bone anchor. The openings extend along an axis from a top surface to a bottom bone contacting surface of the plate. Each opening has an upper region with a first diameter with a groove formed therein having a depth defined by a diameter greater than the first diameter. The plate has a lower region including a seat for the bone screw. The bone screw has a head with a maximum diameter which is smaller than the first diameter, thereby allowing the screw head to pass through that region of the opening. 
         [0015]    An expandable ring is provided which is pre-mounted in the groove and having, when relaxed and unexpanded, an external diameter greater than the first diameter, but smaller than the groove diameter. The expandable ring has an internal diameter when relaxed and unexpanded, smaller than both the first and the head diameters. The expandable ring is capable of expanding into the groove so that the internal diameter expands to be larger than or equal to the screw head diameter while, at the same time, the external diameter is less than or equal to the groove diameter. 
         [0016]    With this geometry, the split-ring can be pre-mounted in the groove and the screw can be inserted, shank first into the bone plate from the upper non-bone contacting surface and, upon engagement between the head of the screw and the split-ring, the split-ring expands into the groove, allowing the head to pass therethrough. Once the screw head has passed through this split-ring, it contracts under its natural spring tension. When the ring relaxes to its unexpanded state, it prevents the bone screw from backing out of the plate by the engagement of an undersurface of the split-ring and an upwardly facing surface on the bone screw. 
         [0017]    The openings in the lower portion of the bone plate have a part spherical seat portion located between the groove and the bottom bone contacting surface of the plate with an opening in the bottom plate surface to allow the shank of the bone screw to pass through. The screw head has a corresponding part spherical surface extending from the shank of the screw towards the upwardly facing surface of the screw. Upon insertion of the screw through the plate, the screw head engages the part spherical seat on the bone plate. At that point the screw head is below the split-ring groove. The bone screw shank can be threaded in any well known fashion and may include an axial groove to enable the screw to be self-boring and self-tapping. The bone screw may include an internal bore extending along the longitudinal axis of the screw which includes threads for engaging a pull out tool should removal of the screw be necessary. 
         [0018]    In order to enhance the locking system&#39;s ability to prevent the screw from backing out of the bone plate, both the groove and split-ring have complementary inclined surfaces extending towards the upper surface of the bone plate upon moving towards the center of the opening in the radial direction. The engagement of the surfaces in combination with a force exerted by the screw on the bottom surface of the split-ring causes the internal diameter of the ring to decrease with increasing force from below. This insures the bone screw cannot back out of the opening. 
         [0019]    In order to make the insertion of the bone screw easier, it is provided with an inclined surface complementary to an inclined surface on the internal bore of the split-ring, which inclined surfaces increase in diameter upon moving in a direction from the bottom surface of the plate towards the upper surface of the plate and radially outwardly of the opening central axis. Thus, when the screw head inclined surface engages the complementary inclined surface on the internal diameter of the split-ring, forces are generated which expand the split-ring into the groove. In order to increase the flexibility of the split-ring, at least one cutout and preferably three or more cutouts are spaced around the external diameter of the ring, resulting in a variable cross-section. This allows the ring to have more flexibility in expanding than if the external diameter of the ring were constant. In order to better prevent the egress of the bone screw from the plate, the surface of the split-ring facing towards the bottom of the plate is flat and extends generally perpendicularly to the central axis through each opening. The bone screw has a complimentary upwardly facing generally flat or slightly inclined surface. 
         [0020]    The location of the groove in the plate is such that when the head of the screw fully engages the spherical seat in the plate, the upwardly facing surface is located below the bottom surface of the split-ring. In order to allow the bone screw to rotate from side to side once seated, an angular cutout of 0° to 20° can be provided at the bottom surface of the plate, thereby making the opening on the bottom surface oblong in at least one direction. This allows the longitudinal axis of the screw head and shank to be rotated between 0° and 20° with respect to the central axis of the opening. 
         [0021]    The material for the split-ring must be flexible and be compatible with the body and it has been found that the titanium alloy disclosed in U.S. Pat. Nos. 4,857,269 and 4,952,236, which have modulus of elasticity not exceeding 100 GPa, is acceptable. Polymeric materials such as ultra-high molecular weight polyethylene are also acceptable. 
         [0022]    The joining member or plate may be curved to match the anatomical curvatures. Thus, the implant curved to best suit the anatomy and natural curvature of the spinal column in the case of a spinal application. Of course, the plate may be used in fracture fixation, as a tibial baseplate, as a hip side plate or any application where bone plates and screws are used. For these uses, a larger screw than that described herein is necessary. The screw locking system can be scaled up from that described herein so that any size screw can be utilized in a smaller locking system. 
         [0023]    Also envisaged is a method for implanting the implant involving accessing the spinal column via an anterior route, fitting the implant, preparing the anchorage, fitting the anchorage members, locking the implant and the head of the anchoring members with respect to the joining member, and closing up the access route. 
         [0024]    These and other objects and advantages of the present invention will become apparent from the following description of the accompanying drawings. It is to be understood that the drawings are to be used for the purposes of illustration only and not as a definition of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    Other features and advantages of the invention will become more apparent on reading the description which follows of the preferred embodiments which are given by way of non-limiting examples. 
           [0026]      FIG. 1  is a perspective view of a first embodiment of the invention; 
           [0027]      FIG. 2  is an exploded perspective view of the first embodiment; 
           [0028]      FIG. 3  is a cross-sectional view along lines III-III of the first embodiment of  FIG. 1 ; 
           [0029]      FIG. 4   a  is a plan view from above of the first embodiment; 
           [0030]      FIG. 4   b  is an elevation view of the first embodiment shown in  FIG. 4   a;    
           [0031]      FIG. 4   c  is a front view of the first embodiment shown in  FIG. 4   a;    
           [0032]      FIG. 5  is a perspective view of a second embodiment of the invention; 
           [0033]      FIG. 6  is an exploded perspective view of the second embodiment; 
           [0034]      FIG. 7  is a partial view in section on the plane VII-VII of the second embodiment; 
           [0035]      FIG. 7   a  is a cross-sectional view of a bone screw or anchor of the present invention; 
           [0036]      FIG. 7   b  is a cross-sectional view of a single orifice used in the second embodiment of the invention without the screw and split-ring along line VII-VII of  FIG. 6 ; 
           [0037]      FIG. 7   c  is a plan view of the split-ring of the second embodiment of the present invention; 
           [0038]      FIG. 7   d  is a cross-sectional view of the split-ring of  FIG. 7   c  along lines A-A; 
           [0039]      FIG. 8   a  is a plan view of the second embodiment of the invention; 
           [0040]      FIG. 8   b  is an elevation view of the second embodiment shown in  FIG. 8   a;    
           [0041]      FIG. 8   c  is a front view of the second embodiment shown in  FIG. 8   a;    
           [0042]      FIG. 9  is a perspective view of a third embodiment of the invention; 
           [0043]      FIG. 10  is a plan view of a screw driver for driving the bone screws of  FIG. 7   a  from the orifice of  FIG. 7   b;    
           [0044]      FIG. 10   a  is an end view of the screw driver shown in  FIG. 10 ; 
           [0045]      FIG. 11  is a plan view of an extractor tube for extracting the anchor or bone screw from the plate after implantation; 
           [0046]      FIG. 11   a  is an end view of the extractor shown in  FIG. 11 ; 
           [0047]      FIG. 11   b  is an enlarged detail of the drive and of the extractor shown in  FIG. 11 ; and 
           [0048]      FIG. 12  is a plan view of a threaded extraction shaft designed to engage the screw and pull it axially out of the hole, should it be impossible to unscrew the threaded shank. 
       
    
    
     DETAILED DESCRIPTION 
       [0049]    With reference to  FIGS. 1 to 4   c , there is shown the implant according to the first embodiment comprises a plate  1 , bone screws  5  and circlips or split-rings  4 . The plate  1  is a bone plate such as an anterior cervical plate or any other plate designed to be held on bone by bone screws. Plate  1  may join two bone parts or stabilize a fracture or may sit on a resected bone surface such as on a tibial plateau. 
         [0050]    In the preferred embodiment, plate  1  is formed of a body  11  ending in two ends  12  which have a width slightly greater than that of a mid-zone of body  11 . Each of the ends  12  comprises a pair of openings or orifices  2  which pass through the entire thickness of plate  1 . The four openings are arranged geometrically as at four corners of a rectangle. Each of the openings  2  has a first, upper, cylindrical part  23  which continues in the form of a spherical central part  21  and ends in a second, lower, cylindrical part  22 , the diameter of which is smaller than that of the first cylindrical part  23 . The spherical intermediate part  21  allows the angle of the bone screw  5  that is to be accommodated in the opening  2  to be chosen. 
         [0051]    Plate  1  preferably comprises two blind holes  3  which have a circular opening and a recess  31 . The two blind holes  3  are arranged on the longitudinal mid-segment of the rectangle, near the respective pairs of corners. Recess  31  is such that it protrudes into the pair of openings  2  to which it is adjacent, thus creating an open slot  32  in each opening  2  of the pair. This slot  32  is made in such a way that it is located in the first cylindrical part  23  of the openings  2 . 
         [0052]    Plate  1  has a first curvature  13  in its longitudinal plane, as depicted in  FIG. 4   c . This curvature  13  allows plate  1  to follow the natural lordosis of the section of spine for which the plate  1  depicted in  FIGS. 1 to 4   c , is intended. In addition, the plate  1  has a second curvature  14  in its transverse plane as depicted in  FIG. 4   b . This curvature  14  allows plate  1  to match as closely as possible the shape of the body of the vertebra to which it is connected. 
         [0053]    Each recess  31  is capable of housing a circlip  4 . The circlip  4  is in the form of a circular ring  43  split at  42 . The circlip or split-ring  4  comprises driving means  41  which, in this embodiment, are lugs projecting towards the inside of the ring. Each lug may be shaped to receive the tips of a pair of needle nose pliers (See  FIG. 9 ). 
         [0054]    Once in place in the recess  31 , with the circlip  4  in the position of rest, i.e. in the open position, it protrudes into the pair of openings  2  adjacent to it through the slot  32  of each opening  2 . It thus closes up the opening  2  slightly. 
         [0055]    The bone screw  5  is the preferred anchoring member in the embodiment which allows the plate  1  to be connected to the bodies of the vertebrae which are fitted with the present invention. The preferred screw  5  has a head  57  surmounting a cylindrical part or shank with a thread  51  suited for bone, comprising a self-tapping means  55  at its distal end. These tapping means allow the screw to better penetrate the bone when being driven. The head  57  comprises a drive  52  which, in this instance, is embodied by a hexagonal socket. In addition, the head  57  comprises a slightly conical part  53  which is continued in the form of a part  56  forming a rim extending towards the outside of the screw  5  and inclined slightly with respect to a plane perpendicular to the axis A of the screw  5 . 
         [0056]    Finally, the head  57  of the screw  5  ends in a spherical male part  54  which complements the female intermediate part  21  of the opening  2  and which meets the threaded cylindrical part or shank  51 . These complementing forms allow the bone screw  5  to be set at a chosen angle with respect to plate  1 . The anchoring of plate  1  can thus be optimized by the surgeon during the operation. 
         [0057]    Preferably, the implant of the present invention shown in  FIGS. 1-4  is supplied to the surgeon with the two circlips  4  installed in recesses  31  of plate  1 . If the plate is an anterior cervical plate, it is preferably implanted by an anterior access route and by uncovering the vertebral bodies that will be fitted. The surgeon positions the plate  1  then pierces pilot holes through each pair of openings where he wishes to have an anchorage. He then engages a bone screw in each pilot hole. He screws these in until the part  54  of their head  5  comes into contact with the part of the ring  44  of the circlip  4  that projects through the orifice  32 . At this point, there are two possible options: 
         [0058]    1. The surgeon closes up the circlip  4  by bringing the two lugs  41  closer together using pliers and then, holding the circlip closed, he screws the two bone screws  5  in until the complementary spherical parts  21  and  54  come into contact, then, releasing the circlip which returns to the open position over the rim  56 ; 
         [0059]    2. The surgeon continues to screw in the anchoring member  5 , the spherical part  54  pushing the ring  44  into the slot  32  through a ramp effect and thus forming its passage, and the ring will open again automatically once the rim  56  has passed by, and the complementary spherical parts  21  and  54  will be in contact. 
         [0060]    Locking is provided by contact between the complementary spherical parts  21  and  54  and by the re-opening of the circlip  4  above the rim  56 . The second role of the rim  56  is to limit the possibilities of angular orientation. This prevents the screw from coming out of the vertebral body or from coming into contact with its counterpart fitted in the other opening  2  forming the pair. In both instances, the plate would be poorly anchored or even not anchored to the vertebral body at all. Thus, having introduced each screw into the orifice via its distal end, the circlip prevents the screw from backing out of the orifice. 
         [0061]    In the event of an adjustment, the surgeon can easily withdraw the plate  1  simply by unscrewing the bone screws  5  after having closed up the circlips  4  by moving their lugs  41  closer together, thus uncovering the aperture of the orifice  2 . 
         [0062]    In a second embodiment illustrated by  FIGS. 5 to 8   c , cervical plate  1  is preferably still formed of a body  111  ending at two ends  112  which are slightly wider than the body  111 . Each of the ends  112  still has a pair of openings  102  which pass right through the entire thickness of the plate  101 . Each opening  102  has a first part  123  which is cylindrical, then a spherical intermediate part  121 . Preferably, the orifice or opening  102  has a part  122  in the form of an angular cutout in the lengthwise direction of the plate  101 . Preferably, the cutout allows the screw to pivot an angle B, preferably from 0° to 20°, in the lengthwise direction about axis  164 , preferably the width of the cutout  122  is slightly less than its length. A circular recess or groove  131  is formed in the cylindrical part  123  of each opening  102 . As in the previous embodiment, when used as an anterior cervical plate, the plate  101  has a curvature  13  in its longitudinal plane and a curvature  14  in its transverse plane. The roles of these curvatures are the same as in the previous embodiment. 
         [0063]    The recess  131  is able to accommodate a circlip or split-ring  104 . As before, the circlip  104  is in the form of a circular ring  143 , split at  142 . The preferred circlip or split-ring  104  in this instance has tabs  141  and cutouts  149  distributed uniformly around the entire circumference of the ring  143 . Preferably, there are at least  3  of these tabs. They make it possible to be sure that the circlip will not escape from the groove or recess  131 , while leaving thinner parts of the ring  143  to allow better flexibility when deforming or expanding the circlip as will be discussed hereinbelow. Of course it is possible to make the ring thinner or use other means to achieve flexibility in the ring. For example, one or two tabs could be used if the cutouts in the ring are sized sufficiently to produce the required flexibility. The circlip  104  comprises expansion chamfer or ramp  144  in the form of an inlet chamfer located on the interior side  145  of the ring  143 . 
         [0064]    The bone screw  105  differs from that of the previous embodiment in that the drive  152  which is in the form of a cross is extended in one embodiment by a blind bore  158  coaxial with the axis A of the screw  105 . This allows the use of a screwdriver with a flat or cruciform blade extended by a small cylindrical protrusion that complements the blind bore  158 . Thus, when tightening or loosening, the screwdriver cannot slip to injure nearby living tissues or irreversibly deform the circlip  104 , as this would compromise locking. 
         [0065]    The preferred head  157  has a part  153  which is generally conical and which slightly bows outwardly which is continued radially outward by a part  156  forming an upwardly facing rim surface extending towards the outside of the screw  105  and which preferably is slightly inclined with respect to a plane perpendicular to the axis A of the screw  105 . 
         [0066]    Finally, a part spherical portion  154  that complements the intermediate part  121  of the opening  102  allows the outer edge of the portion  154  to meet the threaded cylindrical part or shank  151 , which is threaded with a bone-screw thread. The purpose of this complementing nature is to allow the angle of the screw  105  to be chosen with respect to the plate  101  in order to optimize anchorage. 
         [0067]    As in the previous embodiment, the implant is supplied to the surgeon with the four split-rings or circlips  104  installed in the four recesses  131  in the plate  101 . As before, the surgeon, having made his access route, then positions the plate  101  and pierces the pilot holes through the pairs of openings  102  where he wishes to anchor, completely screws in the bone screws  105 . At the end of tightening, the spherical part  154  will come into contact with the chamfer  144  of the circlip  104  and then, through a bearing action, open up the latter to make its passage towards the spherical intermediate part  121  of the orifice  102 . The circlip  104  will close back up again automatically once the rim  156  has passed, and the complementary spherical parts  121  and  154  will be in contact. 
         [0068]    Performing these two operations makes sure that the screw  105  is locked in the plate  101 . As before, the second role of the upwardly facing rim  156  is to limit the possibilities of angular adjustment. This prevents the screw from coming out of the vertebral body or its threaded shank  151  from coming into contact with its counterpart fitted in the other orifice  102  forming the pair. In both instances the plate would be poorly anchored or not anchored to the vertebral body at all. In the event of an adjustment, the surgeon can easily withdraw the plate  101  simply by unscrewing the bone screws  105  after having opened up circlip  104  as will be discussed below. 
         [0069]    A preferred bone screw of the second embodiment of the present invention is shown in greater detail in  FIG. 7   a . In the preferred embodiment, the blind bore  158  of screw  105  is threaded for a portion  160  located below drive  152  towards the tip  161  of the screw. The function of the threaded portion will be described in greater detail below. 
         [0070]    It should be noted that the preferred screw  105  has a nominal thread diameter of about 4 mm with the outer diameter of the upwardly facing surface rim  156  being about 5 mm. If desired, the leading end or tip  161  of the screw shank  151  may include a groove or other structure for allowing the bone screw to be self-drilling and self-tapping. In this situation, no pilot hole need be drilled by the surgeon. 
         [0071]    Referring to  FIG. 7   b , there is shown the preferred opening  102  in the bone plate. The recess or groove  131  which accommodates spring clip  104  has an upwardly and inwardly inclined surface  133  which, in the preferred embodiment, extends at an angle of about 20° with respect to the bottom surface  135  of groove  131 . In the preferred embodiment, the bottom surface  135  of groove  131  extends along a plane perpendicular to the axis  164  of the opening  102 . The upper inclined surface  133  is spaced from surface  135  by surface  137  which, in the preferred embodiment, is about 0.3 mm. The maximum diameter to surface  137  of groove  131  is, in the preferred embodiment, about 6.9 mm. Spherical seat  121  for screw head  157  extends from adjacent the bottom bone contacting surface of the plate to surface  135 . In the preferred embodiment, the spherical surface has a radius of 2.67 mm. Consequently, the part spherical portion  154  of the screw has a similar radius. As can be seen in  FIG. 7   b , the opening  102  may have an angular cutout along a portion of surface  122  adjacent the bottom plate surface to allow the shank  151  of the screw to extend in at least one direction at an angle B of approximately 0° to 20° and preferably 10° with respect to the axis  164 . Thus, when viewed from the bottom, the opening would appear to be oblong in at least one direction. Of course, the angular cutout can be enlarged to permit angulation in a plurality of directions. 
         [0072]    Referring to  FIG. 7   c , there is shown a preferred split-ring or circlip  104  which includes five tabs  141  distributed uniformly around the circumference of the ring  143 . In the preferred embodiment, the ring has an internal diameter  145  of approximately 4.5 mm and a maximum external diameter  147  of preferably 6.2 mm. The difference between the external diameter  147  and the groove diameter  137  is preferably about 0.7 mm. This allows the internal diameter to expand to accommodate the screw head. The preferred cutouts have a depth of approximately 0.4 mm so that the external diameter  149  at each cutout is approximately 5.4 mm. The preferred split  142  is 0.26 mm in width when the split-ring is in its relaxed, i.e. unexpanded condition. The above dimensions are given for illustration only and larger screws, openings and split-rings may be used in other applications. 
         [0073]    Referring to  FIG. 7   d , there is shown a cross-section of the split-ring shown in  FIG. 7   c  along lines A-A. The split-ring has a bottom surface  190  oriented to engage the bottom surface  135  of groove  131 . The cross-section has an inclined upper surface  192  for engaging surface  133  which is upwardly inclined on moving towards the center the split-ring. Preferably, the incline is at an angle of about 20° with respect to bottom surface  190 . The surface forming internal diameter  145  is in two sections, the first is surface  194  which is generally parallel to axis  164  of opening  102  and the second is surface  144  which is angled radially outwardly towards surface  192  also at preferably 20° with respect to surface  194  (and the axis  164 ). Surfaces  192  and  144  are preferably connected by a radius  198  rather than a sharp corner. The preferred split-ring has an overall height from the surface  190  to the top of radius  198  of approximately 0.52 mm and the distance along surface  196  between surface  190  and  192  is about 0.29 mm. 
         [0074]    The preferred cross-section allows spring-clip  104  to be assembled within groove  131  by the plate manufacturer and shipped to the user in a pre-assembled condition. It is especially important that the clip  104  have a sufficient number of cutout areas to render it sufficiently flexible for insertion into the inserting recess or groove  131  prior to shipping to the end user. It is also necessary to use a relatively flexible material for the ring, which material has a modulus less than 100 GPa. Such a titanium material is found in U.S. Pat. Nos. 4,857,269 and 4,952,236. If these titanium alloys are utilized for the split-ring, it has been found that advantageous to make the joining member or plate and anchoring or bone screw out of the same material, although such is not absolutely necessary. In addition, polymeric materials can be used for the split-ring. In the preferred embodiment, the split ring  104  has no means for enabling its removal from the groove after assembly. Thus, it is not possible for the surgeon to remove the ring from the plate. 
         [0075]    Another advantageous feature of the split-ring is the preferably 20° incline of the top surface  192  which engages with complementary groove surface  133 . This is advantageous because forces generated from the backing out of the screw  105  against the bottom surface  190  of split-ring  104  tend to keep the inner diameter  145  from expanding. In addition, only a small annular inter-engagement between the bottom surface  190  of ring  104  and the upwardly facing surface  156  is necessary to prevent screw  105  from backing out of hole  2  in plate  1 . In the preferred embodiment, this annular overlap is at least 0.07 mm and preferably between 0.07 mm on a radius and 0.11 mm. 
         [0076]    In a third embodiment illustrated in  FIG. 9 , the device  201  differs from a first embodiment only in the shape of the blind holes  203  and of the circlip  204  that can be housed in the recess  231 . The shape of the holes  203  has a semicircular base  237  continued by two straight surfaces  236  which converge towards each other and are connected at their other end of the side of the associated orifices  2  by a semicircular vertex  235  of smaller radius than the base  237 . This shape facilities the fitting of the circlip  204 . The latter is very similar to the one in the first embodiment, except for the lugs  241  which have holes  243  to take the jaws of a driving instrument. Installation with this embodiment is identical to that of the first embodiment. 
         [0077]    The circlip  104  may have a constant cross-section. 
         [0078]    The bone screws may be monoaxial: they cannot be oriented with respect to the plate. 
         [0079]    It can thus be seen that, in the embodiments of  FIGS. 1 and 9 , one and the same circlip locks two anchoring screws. 
         [0080]    In all these embodiments, each circlip collaborates by direct contact with the screw to prevent it from coming out of the opening, without it being necessary to provide a part acting as an intermediate between the circlip and the screw. 
         [0081]    Referring to  FIG. 10 , there is shown a plan view of the preferred screwdriver  300  for driving screw  105 . The screwdriver  300  includes a handle  302 , a shaft  304  and a drive head  306 . Referring to  FIG. 10   a , there is shown an end view of drive head  306  showing a cruciform drive having a pair of mutually perpendicular blades  308 . Blades  308  engage drive  152  on screw  105 . In the preferred embodiment, the depth of the cruciform slot forming drive  152  is about 2 mm and the depth of the drive blades  308  is somewhat less and the width of the four slots forming drive  152  are about 1 mm with the width of the blades  308  being slightly less. This geometry ensures excellent engagement between the blades on the driver  300  and the drive  152 . 
         [0082]    Referring to  FIGS. 11 through 11   b,  there is shown a tool provided to remove the screw  105  after it has been fully inserted into bone and blocked from backing out by split-ring  104 . Referring to  FIG. 11  there is shown an extraction tool  400  having a handle  402  and a tubular drive shaft  404 , including a drive tip  406 . Handle  402  is also tubular having a cavity  408  open to an end  410  of handle  402 . In the preferred embodiment, the cavity  408  is circular with a diameter of about 8 mm. Inner end  412  of cavity  408  is open to a cannulation  414  which extends the length of shaft  404  and through tip  406 . In the preferred embodiment, this cannulation is circular with a diameter of about 2 mm. The function of cannulation  414  is described below. 
         [0083]    Referring to  FIG. 11   b , there is shown an enlarged view of drive tip  406  of extraction tube  400  which, like driver head  306  previously described, includes a cruciform blade having cross members  416  similar to blades  308 . However, the outer diameter of tip  406  is equal to the outer diameter of surface  156  on screw  105 . Tip  406  includes an inwardly chamfered portion  418  which allows tip  406  to engage the inner diameter  145  of the split-ring and expand it sufficiently to allow the screw to be unthreaded or pulled back out through inner diameter  145  by counter-rotation of screw  105  with extractor  400 . Once the maximum diameter of upwardly facing surface  156  passes through the split-ring, it springs inwardly along surface  154  of screw  105 . 
         [0084]    It has been found that in some instances, the bone deteriorates so that it is impossible to generate a screw removal force by the counter-rotation of screw  105  with extractor  400 . In this instance, referring to  FIG. 12 , there is shown an extraction tool  500  designed to fit within the cavity  408  and cannulation  414  of extraction tool  400 . Extraction tool  500  includes an upper portion  502 , a shaft portion  504 , a threaded tip  506  and an enlarged portion  508 . The threaded tip includes threads matching threads  160  in screw  105 . In the preferred embodiment, the thread is 1.6 mm in diameter. Thus, when the bone screw  105  cannot be removed merely by the counter-rotation of screw  105  with extraction tool  400 , extraction shaft  504  is inserted through the cannulation  414  and out tip  406  thereof and into threaded engagement with threads  160  of bone screw  105 . All the surgeon must then do is pull on portion  508  of the extraction tool  500  which pulls screw  105  out of the bone. 
         [0085]    Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.