Abstract:
A anti-splay and rod securing member for use with vertebral implants of the hook, monoaxial and polyaxial types, in which a pair of arcuate upstanding internally threaded posts define a transverse opening for securing a rod to immobilize bone segments comprises an integrally formed cap and set screw with the cap having opposed curved side walls extending around the posts and flat end walls, the set screw being positioned within the posts and free to rotate within the cap so that rotation of the set screw will advance the cap and set screw in unison along the implant posts to clamp the rod and implant together. The implant may be provided with opposed catch basins for releasably securing the distal end of a rod persuasion instrument.

Description:
FIELD OF THE INVENTION 
   The present invention relates to the medical field commonly referred to as Osteosynthesis, i.e., the fusion between segments of the spine and more particularly to an implant system for use with a rod for immobilizing the segments during the fusion process. 
   BACKGROUND OF THE INVENTION 
   Osteosynthesis is achieved by immobilizing separate bone segments and in particular vertebral segments on either side of a failed or damaged disc. When trying to achieve osteosynthesis and specifically fusion between different segments of the spine, one has to provide some type of immobilization. There are various prior art systems which try to achieve this purpose. The different systems involve placement of implants in the form of hooks secured against the bone or pedicle screws threaded into the bone. The implants are then secured to each other by stabilizing rods or plates. 
   A typical prior art hook implant system, illustrated in  FIG. 1 , includes a hook implant  10  having a bottom section  10   a  in the form of a hook shaped finger to be mounted on a bone segment and an upper rod receiving section  10   b  having a pair of internally threaded posts  10   c  extending upwardly from a generally unshaped rod receiving cradle or chamber  10   d . The posts define a transverse opening  10   e  for receiving a stabilizing rod  14 . A plug  16 , e.g., a set screw, having external threads complementary to the internal threads in the posts, completes the system. The set screw, when tightened, clamps the rod and the hook implant together. The term “hook implant”, as used herein, refers to the type of hook  10  illustrated in  FIG. 1 . 
   Another type of typical prior art implant for use with a stabilizing rod, commonly referred to as a monoaxial pedicle screw, is illustrated in  FIG. 2 . The implant  18  consists of a bottom section, in the form of a threaded shaft  18   a , joined to an upper rod receiving section  18   b  formed by a pair of opposed internally threaded posts or branches  18   c  extending upwardly from a cradle  18   d . A threaded plug, e.g., a set screw  16 , when threaded into the posts, clamps the implant and rod together. The term monoaxial pedicle screw as used herein refers to the type of implant illustrated in  FIG. 2 . The monoaxial pedicle screw implant system may include an anti-splay ring  20  (shown in dashed lines) extending around the posts and supporting a flat or curved plate  20   a  below the set screw so that the plate is forced against the rod by the advancing screw. See, for example, U.S. Pat. Nos. 5,154,719 and 5,385,583. 
   A prior art implant, initially providing several degrees of freedom between the rod receiving portion of the implant (upper section) and the portion (bottom section) secured to the bone, is commonly referred to as a polyaxial pedicle screw. An implant system employing a polyaxial screw is illustrated in  FIGS. 3   a  and  3   b  in which the threaded shaft  22   a  of the bone screw  20  is inserted through a longitudinal opening in a cage or housing  24  with the enlarged head  22   b  of the screw resting against a reduced cross-sectional area  24   c  of the housing. The housing, like the upper section of the monoaxial housing, has opposed upstanding internally threaded posts  24   c  which define a transverse opening or slot  24   d  for receiving the rod. A set screw  16 , when tightened, forces the rod against the head of the screw to clamp the rod, housing and bone screw together. Prior to tightening the set screw the housing may be pivoted relative to the head of the bone screw. See U.S. Pat. No. 6,261,287 and  FIGS. 15 and 16 . A more advanced polyaxial screw system is shown in  FIG. 4  and described in U.S. Pat. No. 6,565,567 in which a pressure washer  26  (initially vertically moveable relative to the housing  28 ) is placed between the rod and the head  22   b  of the bone screw  22 . The &#39;567 implant system also includes a cap  30  with openings  32   a  which slide over the posts  28   a  and a lower lateral connector  32   b  interposed between the bottom of the set screw  16  and the rod  14 . 
   The term polyaxial pedicle screw as used herein refers to implants of the types shown in  FIGS. 3   a ,  3   b  and  FIG. 4  minus the set screw and cap. 
   The caps or rings which extend around the internally threaded posts in certain of the above implants serve the purpose of limiting or preventing the outward deflection or splaying of the posts as the set screw is tightened. However, the use of such a cap or ring along with a separate set screw requires two separate steps on the part of the surgeon, i.e., first inserting the cap over the posts and second placing the set screw between posts so that the threads are properly aligned. The step of aligning the set screw with the threaded posts is a problem whether or not a separate cap is used. Since the threads in the posts are interrupted it is difficult, particularly in light of the limited view in the surgical opening for a surgeon to ensure that the set screw is not canted with respect to the opening in the posts, resulting in cross threading and likely a failed installation. 
   There have been several attempts, by way of patent disclosures, to overcome these problems. See, for example, U.S. Pat. Nos. 5,261,912 (“&#39;912 patent”), 5,667,508 (“&#39;508 patent”), 5,752,957 (“&#39;957 patent”) and 6,440,137 (“&#39;137 patent”). All of such patents teach the use of a unitary locking cap having a male threaded plug or set screw (adapted to be received in the threaded posts) with the cap forming a concentric rim joined to the top of the plug. While this arrangement solves the multiple component problem it presents another problem of raising the vertical profile of the installed implant. The extra space required to accommodate the top of the clamp, which necessarily overlies the top of the implant posts, may result in considerable discomfort to a tall, thin patient particularly if the implant is installed in the thorax region. It is preferable to keep the vertical profile of the implant as short as possible. 
   In addition, the above unitary cap/screw designs do not entirely solve the screw/threaded posts alignment problem. Such arrangements may tend to allow the cap/screw to pivot or toggle slightly along the openings between the implant posts. 
   A feature of the present invention is directed towards a solution to such problems by providing an integrally formed cap and set screw with the set screw free to rotate within the cap and the cap/set screw defining means for aligning the set screw with the implant posts. Preferably, the cap defines diametrically opposed openings which slide over the posts with the ends of the openings cooperating with the posts&#39; edges to guide the cap/set screw into alignment with the longitudinal axis of the implant. 
   In addition to the separate component and alignment problems, conventional implants systems suffer another problem. It is necessary to seat the stabilization rod between the implant posts. This seating step is frequently difficult due to the rigidity of the rod and the vertebra into which the implant has been installed, particularly where the vertebrae is to be moved toward the rod. 
   To aid the surgeon in accomplishing this task, implants are typically provided with opposed generally cylindrical recesses or slots in the upper section, generally below the rod receiving cradle. A rod persuasion instrument or persuader, in the form of an elongated carriage member, has spaced arms at its distal end which are arranged to slide into the slots or arms provided with inwardly extending flexible fingers arranged to snap into the recesses to releasably fix the distal end relative to the implant. A shaft slidably mounted in the carriage member and actuable from the proximal end (i.e., located exterior of the patient&#39;s body) allows the surgeon to push the rod into the receiving chamber in the implant and retain it in that position while the set screw or other clamping means is threaded into the implant posts to lock the rod and implant together. See, for example, U.S. Pat. No. 6,660,006 (“&#39;006 patent”) and U.S. Patent Publication No. US2004/0147936 A1 (“&#39;936 publication). The arms of the &#39;936 device are offset and slide into opposed slots in the upper section below the cradle. This persuader design requires that adjacent implants be at least as far apart as the transverse/offset arms at the distal end of the instrument thereby limiting its use to adjacent implants with the required spacing. In addition, it is difficult to align and slide the offset arms into the slots. 
   The implant grasping arms at the distal end of the &#39;006 persuader, enclosed within a sleeve or cannula, are flexed outwardly to allow the inwardly projecting fingers to clear the sides of an implant. The arms then snap inwardly to allow the fingers to snap into the recesses at the implant base. The distal end of the cannula is then moved forwardly to retain the fingers within the implant recesses. This arrangement, like the &#39;936 device, also limits the persuader&#39;s use on closely spaced adjacent implants. In addition, the securing set screw must be passed through the length of cannula before entering the threaded posts of the implant. 
   A feature of the present invention, whether of the hook, monoaxial or polyaxial pedicle screw type, overcomes such shortcomings by providing opposed flat recessed sides along the base of the upper section of the implant, aligned with the posts, with the recesses terminating at their upper ends in concave, e.g., outwardly projecting, shoulders. 
   This arrangement permits the use of a simple persuader which (a) accommodates a close spacing between adjacent implants, (b) eliminates the need for a releasably attachable arm/finger mechanism and (c) is user friendly in terms of placing the distal end of the persuader within the recesses. 
   Another feature of the present invention, applicable to hook or monoaxial implants, is the design of the cradle to accommodate stabilizing rods of different diameters by providing the cradle with different radii. 
   With respect to polyaxial screws, a feature of the present invention is to provide a pressure washer with the rod receiving cradle having a slightly smaller radius than the radius of the rod so that the upper edges of the cradle are internally forced outwardly by the rod to create an interference fit. The pressure washer is also preferably made of a material such as a CoCvMo alloy, which has a hardness value about 15% to 22% greater than the hardness value of titanium. The increased hardness of the washer provides a better locking interface between the underside spherical sector of the washer and the screw head thereby requiring less force to secure the rod, polyaxial housing and the pedicle screw together. It is noted that the &#39;567 patent indicates that heat treating titanium may increase the hardness value by about 10% to 20%. However, tests have shown heat treatment actually increases the hardness value by only one or two points on the Rockwell C hardness scale or about 5% or less. 
   SUMMARY OF THE INVENTION 
   An improved anti-splay and rod securing member for use with vertebral implants of the hook, monoaxial and polyaxial types, in which a pair of arcuate upstanding internally threaded posts define a transverse opening for securing a rod to immobilize bone segments comprises (a) a cap having opposed curved side walls adapted to extend around the posts and preferably substantially flat end walls extending between the side walls, with the cap and posts having cooperating means to coaxially align the cap with the posts and (b) a set screw mounted in the cap to be positioned within the posts and having a top wrench engaging surface, a bottom and external threads arranged to engage the post threads. The cap and set screw are formed as an integral unit with the set screw being free to rotate within the cap so that rotation of the set screw will advance the cap and set screw in unison along the implant posts. A lower portion of the cap or set screw is arranged to engage the rod to clamp the rod and implant together. 
   Preferably the cap includes at least one top and bottom strut extending between the end walls between which the set screw is encased. The bottom strut may be in the form of a concave rod-engaging surface, e.g., semi-cylindrical, saddle or u-shaped, so that when the set screw is advanced along the posts the lower strut engages the top of the rod. Alternatively, the lower strut may extend between one or both side walls leaving a portion of the bottom of the set screw free to engage the top of the rod. 
   In another embodiment, the upper rod holding section of an implant, whether of the hook, monoaxial or polyaxial pedicle screw type, may be provided with a flat opposed recessed sides, adjacent the bottom of the posts, with the recesses terminating at their upper ends in concave outwardly projecting shoulders. This arrangement, sometimes referred to herein as a catch basin, accommodates the use of a rod persuader having a fixed forked distal end, i.e., parallel arms, with inwardly projecting stubs adapted to slide under the shoulders to releasably secure the persuader distal end to the installed implant. 
   A rod persuasion instrument, for use with the above recessed sided implant, includes an elongated carriage member extending along a longitudinal axis with a proximal and distal end. A pair of spaced parallel arms are preferably offset at an acute angle to the longitudinal axis at the distal end with each arm having an inwardly projecting stub or button on the end thereof. The stubs are arranged to slid along the flat recessed sides and abut the concave shoulders to releasably fix the persuader distal end relative to the implant. A pusher bar is slidably mounted in the carriage member with a distal end having a concave surface for engaging the upper surface of the rod. The pusher bar is manually actuable via a rachet arrangement from the proximal end of the persuader to allow a surgeon to force the rod between the implant posts. 
   The construction, operation and features of the disclosed invention may best be understood by reference to the following description taken in conjunction with the appended drawings where like components are generally given the same reference numerals in the several figures. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a prior art hook implant and securing set screw; 
       FIG. 2  is a perspective unassembled view of a prior art monoaxial screw implant with a securing set screw and an antisplaying ring (in dotted lines); 
       FIGS. 3   a  and  3   b  are cross-sectional and side elevational views, respectively, of a prior art polyaxial pedicle screw implant and securing set screw; 
       FIG. 4  is an unassembled perspective view of a prior art polyaxial pedicle screw with a securing set screw, an antisplaying cap and a pressure washer; 
       FIGS. 5   a  and  5   b  are perspective views of a partially complete cap and a set screw in an unassembled condition, in accordance with the present invention; 
       FIGS. 6   a ,  6   b  and  6   c  are top perspective, cross-sectional, and bottom perspective views, respectively, of an assembled cap/set screw in accordance with this invention for use with implants having upstanding internally threaded posts; 
       FIG. 7  is a perspective view of the top section of a generic implant adapted to receive the integral cap/set screw of the present invention; 
       FIGS. 8   a  and  8   b  are cross-sectional views of the cap/set screw of  FIG. 6  being installed on a polyaxial pedicle screw implant, illustrating how the openings between the bottom strut and the cap inner wall aid in guiding the set screw into the post threads; 
       FIGS. 9   a ,  9   b  and  9   c  illustrate, in simplified perspective views, the installation of the cap/set screw of  FIG. 6  on the upper section of a monoaxial pedicle screw; 
       FIGS. 10   a  and  10   b  are cross-sectional and bottom views, respectively, of a modified cap/set screw; 
       FIGS. 11   a  and  11   b  are cross-sectional views of the modified cap/set screw design of  FIGS. 10   a  and  10   b  being installed on the housing of a polyaxial pedicle screw implant; 
       FIGS. 12   a  and  12   b  are side elevational and top plan views of a rod persuasion instrument or persuader; 
       FIGS. 12   c  and  12   d  are top plan and side elevational views of a rod pusher bar forming a part of the persuader; 
       FIGS. 12   e  and  12   f  are side elevational (partially in section) and top plan views of the tip of the persuader for engaging the catch basin of an implant; 
       FIGS. 13   a ,  13   b  and  13   c  are diagrammatic views showing the use of the rod persuader of  FIG. 12  to seat a stabilizing rod in the implant rod receiving opening; and 
       FIG. 14  is a cross-sectional view of the rod receiving opening of a hook or monoaxial pedicle screw implant in accordance with the features of the invention for accommodating stabilizing rods of different diameters. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   A partially completed cap  38  and a set screw  40  are illustrated in  FIGS. 5   a  and  5   b . The cap is manufactured from a high strength bio-compatible material such as titanium, e.g., by machining, a solid piece to provide opposed curved side walls  38   a  which are arranged to extend around and follow the outside contour of a generic implant&#39;s upstanding posts  36   a  ( FIG. 6 ) leaving an open top. The implant posts have free edges  36   b  which are aligned parallel to the implant&#39;s longitudinal axis X I -X I . It is to be noted that the bottom section  36   c  of the generic implant of  FIG. 7  may be of the hook, monoaxial or polyaxial type. 
   Flat end walls  38   b  of the cap extend between the side walls, as shown, and along with the side walls are arranged to encircle the implant posts. The inner surfaces  38   c  of the end walls are contoured to match the outer circumference of the set screw with a slight clearance, e.g., 0.0005 to 0.010″ to allow the set screw  40  to turn freely or with a slight resistance while aligning the set screw with the longitudinal axis Xc-Xc of the cap ( FIG. 6   b ). A split bottom strut  38   d  extends across the bottom center section of the end walls leaving diametrically opposed openings  38   e  between the strut and the inner surface  38   g  of the side walls which serve to guide the cap onto the implant posts as will be explained. Each segment of the bottom strut  38   d  extends downwardly and outwardly along a radius generally following the contour of the rod  14 . End walls  38   b  are formed with an upwardly concave surface  38   f  which, like the split strut, generally follow the contour of the rod. 
   The set screw  40  (with external threads  40   a  matching the post threads  36   b ) may then be inserted inside the cap where it is automatically aligned with the cap axis Xc which axis will be coincident with the longitudinal axis X I -X I  ( FIG. 7 ) of the implant when the cap/set screw is inserted over the posts  36   a . A top strut  38   h  ( FIG. 6   a ) is then positioned across to the top of the end walls and welded thereto, for example, along lines  38   i . The top strut provides diametrically opposed cutout openings (also  38   e ) aligned with the bottom openings. The cut out openings have a length lc ( FIG. 6   a ). 
   The set screw  40 , with its top surface  40   b  underlying the strut  38   h  and its bottom surface  40   c  overlying the split strut  38   d , is thus positioned or encased within the cap while remaining free to rotate therein. The set screw is provided with a wrench engaging surface  40   d , e.g., a hexagonal socket compatible with an allen wrench, accessible through a central aperture  38   j  and the top strut  38   g.    
   The diameter dc of the inner curved surfaces  38   g  of the cap ( FIG. 5   a ) is slightly greater, e.g., 0.0005″ to 0.010″ or 0.020″ than the outside diameter dp 1  of the curved implant posts ( FIG. 7 ) while the perimeter of the inside surfaces  38   g  are sized to be slightly larger, i.e., 0.001″ to 0.010″ or 0.020″ than the perimeter of the outside surface of the posts. By the same token the length (lc) of each cutout or opening  38   e  ( FIG. 6   a ) is just slightly greater than the width Wp of the posts ( FIG. 7 ), e.g., 0.001 to 0.010″ or 0.020″. The upper limit for the above clearances is not to be considered limiting and in any event such clearances will depend to some extent on the size of the implant. This arrangement insures that when the cutouts formed by the side walls inner surfaces  38   g  and the strut at the bottom portion of the cap are slid over the implant posts, with the post edges cooperating with the ends of the cutouts, the set screw will be aligned with the posts threads thereby inhibiting any cross threading. 
   It is to be noted that while the end walls are illustrated as flat, they may be curved, e.g., matching the curvature of the side walls, with openings  38   e  and the posts edges or other cooperating means between the cap and posts (with or without a bottom strut) serving to prevent rotation of the cap with the set screw while guiding the cap/set screw into a coaxial alignment with the implant posts. Such cooperating means may include ribs extending inwardly from the cap inner walls which engage the posts edges or a tongue and groove arrangement between the cap side walls and the posts. The preferred embodiments are as shown in  FIGS. 6 and 10  (to be described) with flat end walls and a bottom strut. 
   The manner in which the cap openings  38   e  and the implant posts, including the free edges thereof, serve to align the screw with the longitudinal axis of the implant is illustrated in  FIGS. 8   a  and  8   b  in which the implant is in the form of a polyaxial pedicle screw with a housing  42  having internally threaded posts  42   a  terminating in top circular sector surfaces  42   b . The head  22   b  of the bone screw  22  is positioned between a reduced cross section  42   c  of the housing and a pressure washer  44 .  FIG. 8   a  illustrates the initial engagement of the set screw threads with the posts threads after the lower cap openings  38   e  in cooperation with the post edges, adjacent the top surfaces  42   b , have aligned the cap/set screw with respect to the housing longitudinal axis.  FIG. 8   b  illustrates the set screw and cap in the installed position with the bottom surface  40   c  of the set screw forced against the rod  14  which in turn forces the washer against the pedicle screw head to lock the assembly together. 
     FIGS. 9   a ,  9   b  and  9   c  illustrates the installation of the integrally formed cap/set screw  38  on a monoaxial pedicle screw  43  with upstanding internally threaded posts  43   a .  FIG. 9   a  shows the top of the implant posts entering the lower cap openings  38   e  to coaxially align the cap/set screw with the longitudinal axis X I -X I . of the implant.  FIG. 9   b  shows the partial installment and  FIG. 9   c  (like  FIG. 8   b ) illustrates the cap/set screw in its final position. 
   It is to be noted that while  FIGS. 8 and 9  illustrate the placement of the integrally formed cap/set screw over the posts of a polyaxial and monoaxial screw implant the integrally formed cap/set screw of the invention may be inserted over the posts of a hook implant. 
   As noted earlier, the preferred pressure washer  44  has a hardness value of about 10% to 25% (and preferably 15% to 22%) greater than the hardness value of the pedicle screw head. It has been found that a washer made of a cobalt chromium molybdenum alloy (CoCrMo) has a Rockwell C hardness value of around 43 as compared with a Rockwell C hardness value of around 35 for titanium. The increased hardness of the CoCrMo alloy serves to cause the ridges  44   a  of the underside of the pressure washer to bite into the pedicle screw head and ensure a superior locking arrangement. The washer has a partial cylindrical surface  44   b  on its upperside for engaging the rod. Preferably the radius of the surface  44   b  is slightly smaller than the radius of the rod to allow the sides of the washer creating the upper surface to be slightly spread apart by the rod creating a clamping action between the washer and rod. The washer is provided with diametrically opposed short vertical slots  44   c , illustrated as being aligned with center of the posts. Pins  45 , inserted through the side walls of the posts, extend into the slots to allow a small vertical movement of the washer while maintaining the surface  44   b  aligned along the transverse rod receiving opening. 
   An important feature of the present invention is a rod persuasion instrument catch basin which is formed by opposed flat recessed sides  42   d  on the polyaxial housing side walls ( FIG. 8 ) or  43   d  on the upper section of the monoaxial implant ( FIG. 9 ) with the recessed sides terminating at their upper ends in concave outwardly projecting shoulders  42   e  ( FIG. 8 ) or  43   e  ( FIG. 9 ) adjacent the bottom of the posts. The depth of the shoulders need to be sufficiently deep, e.g., about 0.4 mm to 3 mm and preferably about 2 mm, to serve as a catch for inwardly projecting stubs or buttons on a persuasion instrument described in connection with  FIGS. 13   a  and  13   b . For example, the outside diameter of the posts dp 1  ( FIG. 7 ) may range from 0.012″ to 0.55″ depending upon the anticipated diameter of the stabilization rods. A typical outside diameter of 0.4528 with an inside diameter dp 2  ( FIG. 7 ) of 0.2600 will readily accommodate rods having a diameter of 0.216″ to 0.250″. In any event, the width W CB  ( FIG. 8   b ) of the catch basin, i.e, recessed sides, will normally be the thinnest portion of the polyaxial housing or upper section of a hook or monoaxial pedicle screw implant. It is to be noted that while the catch basin is shown on the housing of a polyaxial pedicle screw implant ( FIG. 8 ) and on a monoaxial pedicle screw implant ( FIG. 9 ) it is equally applicable for use on the upper section of a hook implant. 
   An alternative embodiment of an integrally formed cap  38 ′/set screw  40 , in accordance with this invention, is illustrated in  FIGS. 10   a  and  10   b  with the top of the cap/set screw being the same as that shown in  FIG. 6   a . The difference resides in the bottom strut  38 ′ d  ( FIG. 11   b ) having a semi-cylindrical, saddle or u-shaped centrally disposed lower surface  38 ′ l  (generally following the contour of the rod) which extends between the end walls  38 ′ b  (marked  38   b  on  FIG. 6   a ). The upper surface  38 ′ m  of the lower strut  38 ′ d  is substantially flat. As is illustrated in  FIGS. 10   a  and  10   b  the diametrically opposed openings  38 ′ e  between the lower strut  38 ′ d  and the inner surfaces  38   g  of the cap side walls  38 ′ a  serve to guide and align the cap/set screw onto implant posts. 
     FIGS. 11   a  and  11   b  illustrate the use of the cap/set screw of  FIG. 10  with the polyaxial pedicle screw housing  42  of  FIG. 8 . As is illustrated in  FIG. 11   b , the lower surface  38 ′ l  of the bottom strut  38 ′ d  (instead of the bottom of the set screw) is forced against the top of the rod  14  to lock the implant, rod and bone screw  22  together. The uninterrupted bottom strut  38 ′ d  provides a smoother surface against which the set screw rotates and also provides a greater downward force per axial rotation. This is beneficial in locking the polyaxial pedicle screw to the rod. See  FIG. 11   b . This additional downward force, while advantageous for polyaxial systems, is not needed for the monoaxial screws or hooks and thus the set screw is allowed to contact the rod directly reducing the overall height of the top of the monoaxial screw or hook. 
   A rod persuasion instrument or persuader  48  (made, for example of stainless steel), for use with the catch basin described above, is illustrated in  FIGS. 12   a - 12   g . The persuader includes a carriage, in the form of an elongated member  48   a , extending along a longitudinal axis Xp-Xp with branched parallel arms  48   b  having a short horizontal section  48   c  extending from the distal end  48   g  of the elongated member and an offset section  48   d  extending at an acute angle θ to the longitudinal axis as is shown in  FIGS. 12   a  and  12   g . The carriage further includes an integrally formed handle  48   e . Inwardly extending short (e.g., 1 mm in length) stubs or buttons  48   f  ( FIG. 12   f ) are provided at the terminal ends of the offset arms  48   d  with the distance between the stubs being slightly greater, e.g., 0.001″ to 0.003″, than the width W BC  ( FIG. 8   b ) of the catch basin on the implant upper section or housing. The catch basin engaging stubs are formed with convex semi-cylindrical surfaces  48   g  facing the proximal end of the carriage, as is best shown in  FIG. 12   e . Spaced parallel rod pusher guides  48   h  are formed along inner portions of the horizontal section  48   c.    
   A rod pusher bar  50  ( FIGS. 12   c - 12   d ) is slidably mounted in the elongated member of the carriage via lateral tongues  50   a  which slide into cooperating grooves in the carriage (not shown). The distal end of the pusher bar rides between carriage guides  48   h  and is formed with a concave end  50   b , e.g., sector of a circle, which is arranged to engage a stabilizer rod when stubs  48   f  are seated in the catch basin of an implant and the pusher bar is advanced toward the rod via a pivotally mounted actuating trigger  52  in a conventional manner. A base  54  is pivotally mounted to the end of the handle  48   e  and has a serrated upper side  54   a  which is biased against the free end of the trigger. A spring  56  biases the trigger away from the handle. The surgeon may advance the pusher bar by pulling the trigger toward the handle and it will remain in the deployed position until the base  54  is manually rotated away from the trigger to withdraw the bar to its stored position as will be obvious to those skilled in the art. 
   It is to be noted that the arms  48   b  may extend to the distal end of the carriage eliminating the need for the horizontal section  48   c . Also, the angle θ may vary within the range of 0° to about 45°, but preferably is within the range of about 15° to 30°. It has been found that an angle θ of about 25° functions quite well. 
     FIGS. 13   a - 13   c  illustrate, in diagrammatic form, the use of the persuader of  FIGS. 12   a - 12   f  to force a stabilizing rod  14  within the rod receiving opening between posts of a polyaxial pedicle screw implant.  FIG. 13   a  illustrates a polyaxial pedicle screw installed into an underlying bone segment with the housing free to be aligned with the anticipated position of a stabilizing rod in a conventional manner.  FIG. 13   b  illustrates one of the persuader offset arms  48   b  extending around the lower portion of one side wall  42   a  of the housing  42  with one of the catch basin engaging stubs  48   f  engaging the concave shoulder  42   e  to fix the distal end of the persuader relative to the housing. The other stub would be in engagement with the concave shoulder on the other side of the housing. The rod engaging end of the pusher bar has been moved forwardly to seat the rod between the housing posts.  FIG. 13   c  shows the cap/set screw installed in the housing with the persuader still in place. 
     FIG. 14  illustrates another aspect of the invention for the rod receiving portion (i.e., cradle) of a monoaxial pedicle screw or hook implant. The rod receiving portion  56   a  of the monoaxial pedicle screw  56  shown in  FIG. 14  is sized to accept a spinal rod of two different diameters of rods, e.g., 0.216″ and 0.250″ diameters. The lower portion of the cradle is provided with a smaller radius r 2 , e.g., 0.1080″ to capture the smaller rod while the upper portion of the cradle is provided with a larger radius r 1 , e.g., 0.130″ to capture the larger rod. 
   There has thus been described a novel integrally formed cap/set screw for use with vertebral implants having upstanding internally threaded posts along with additional implant features enhancing the reliability and user friendliness of the implant system. The details and parameters of the present invention may be altered in numerous ways without departing from the spirit and scope of the invention as defined in the appended claims. Therefore, it is intended that the drawing be interpreted as illustrative and not in any way viewed as being a limitation on the invention.