Abstract:
An improved set screw with a break off head for use in an osteosynthesis apparatus. The set screw comprising a head or stem preferably having a hexagonal external cross-section, a lower portion having a thread extending around an outer surface thereof and a shank connecting the head to the threaded lower portion. An outer diameter of the shank is equal to a minor diameter of the threaded lower portion, i.e. the outer diameter of the threaded lower portion at the base of the thread. A bore extends into the set screw from an upper surface thereof. The bore has a first bore section and a second bore section wherein the second bore section has a reduced diameter relative to the first bore section and is separated from the first bore section by an internal shoulder. The internal shoulder extends into the bore below an upper end of the thread. The first bore section is sized relative to the shank to result in shearing of the head from the lower portion upon application of a pre-selected torque on the head relative to the lower portion. The stepped down bore configuration also facilitates use of an easy out tool for removing the threaded lower portion, if necessary, from an implant in which it has been secured. The bore may include a plurality of bore sections of increasingly smaller diameter to further facilitate use of an easy out tool.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. patent application, Ser. No. 08/778,765, now U.S. Pat. No. 6,004,349, filed Jan. 6, 1997 and entitled, “Set Screw for Use with Osteosynthesis Apparatus”, which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to improvements in set screws for use with apparatus for correcting orthopedic deformities and, in particular, for use in spinal osteosynthesis. 
     Surgically implanted spinal osteosynthesis apparatus often includes rods which are secured along at least a portion of the spine by a system of hooks, bone screws including sacral screws and pedicle screws and transverse connectors for the purpose of stabilizing and adjusting spinal alignment. In a very basic apparatus of this type, the hooks and bone screws include a spinal rod bore extending through a ring or body or head of the hook or screw. The screws are screwed into the pedicle portion of the vertebra at desired locations and a spinal rod is then extended through the spinal rod bore in each bone screw. 
     Where the bone screw has a rod receiving ring and the rod is to be fixed in position in the ring, a set screw is inserted in a threaded bore extending through a wall of the ring, so as to engage the rod, and is then tightened to fix the translational and rotational relationship of the rod within the ring. The rods may then be bent or shaped to maintain an adjacent portion of the spine in a desired configuration, to provide support to the spine and to exert desired corrective or stabilizing forces on the spine. 
     A slightly more complicated system uses transverse connectors in association with the bone screws to secure the spinal rods. The transverse connectors include an arm and a head. The head has a spinal rod bore extending therethrough. The arm of the connector is inserted through the spinal rod bore in the pedicle screw then the spinal rod may be inserted through the spinal rod bore in the transverse connectors. A threaded bore extends through the head of the connector perpendicular to the axis of the spinal rod bore. Once the rod is inserted through the bore in the transverse connectors the set screws are inserted through the threaded bores and tightened to fix the relative position of the rod within the spinal rod bore and set screws are inserted in the threaded bores and tightened to fix the position of the transverse connector with respect to the pedicle screws. 
     The pedicle screws and transverse connectors may be of the closed type as discussed above or of an open end type wherein the head of the screw or connector generally incorporates a U-shaped groove. Several types of open end type bone screws have been previously used. One type of such screw is shown in the U.S. Pat. No. 5,005,562 of Cotrel. The device in the Cotrel patent has threaded interior surfaces on the two upright branches that form the rod receiving channel therebetween and which receive a threaded set screw having a rod engaging point and outer ring. The set screw in Cotrel is tightened against the rod by advancing the set screw along the threads. However, this system has limitations. In particular, the ability of the set screw of Cotrel to grip and hold the rod is heavily dependent on the torque applied to the set screw during installation. Unfortunately, the torque is limited because too much torque will cause the branches to spread, thereby allowing the set screw to loosen and the implant to fail. Such failure can also occur when forces are applied to the implant during use, such as at time of muscular stress or during accidents when the back is jolted. To try to overcome this problem associated with the Cotrel device, the implant branches and set screw are increased in size to add strength and/or a retention ring is placed around the outside of the branches to reduce the likelihood of expansion. However, the strengthening adds substantial bulk to an implant and a ring adds bulk and complexity to the implant. In implants it is important to try to reduce bulk rather than add to it, as it is desirable for the implants to be as low profile as possible. 
     Rather than have a pair of branches joined only by a set screw or by a set screw and an exterior ring, a cap has been proposed which mates with the branches on opposite sides of the cap to prevent the branches from expanding radially outward upon application of torque to the set screw. The cap also closes off the open end of the bone screw after the rod is placed in the groove in the bone screw. The set screw is then inserted in a threaded bore in the cap and tightened to fix the position of the transverse connector with respect to a respective bone screw. A substantial torque can then be applied to the set screw while held in the surrounding threads of the cap without expanding the bone screw branches. 
     Various implants such as hooks, pedicle screws and transverse connectors used in the present invention may be of the closed type, as discussed above, or of an open end type, such as described above, wherein the head of the hook screw or connector generally incorporates a U-shaped groove or slot, an upper end of which may be closed off by a cap after a rod is placed in the open end so as to complete the rod bore. A threaded screw bore for the set screw typically extends through the cap. 
     The efficacy of the set screw is critical to the overall performance and efficiency of the osteosynthesis apparatus. The set screw must firmly secure the spinal rod or the arm of transverse connectors to prevent rotational or translational movement of the rod or arm after installation. Due to the nature of use of the set screw, it is important that the set screw be relatively small yet constructed to receive sufficiently high torque to firmly set the set screw and hold the rod. The set screw must also be easily manipulated to permit relatively rapid insertion and tightening during surgical procedures. It is also preferable that after insertion, no portion of the set screw extends beyond the threaded bore into which the set screw is inserted. The remaining portion of the set screw should be removable to facilitate disassembling of the osteosynthesis apparatus at any time. It is desirable that the set screw take advantage of physical penetration into the rod so as to improve the strength of the connection to resist axial movement of the rod relative to the set screw over that provided only by abutting friction. 
     Set screws have been previously developed with break-off heads or stems which break off after the set screw is inserted through a threaded bore and tightened to a preselected torque. Preferably, no portion of the set screw that remains after the head or stem breaks off extends above or beyond an outer edge of the threaded bore. However, prior art set screws normally have undesirable burrs that are left after the head breaks off that must be removed, thereby making the procedure more difficult or alternatively such burrs may lead to irritation of the patient, if not removed. Often, after installation, a set screw must be removed to reposition a rod or fix a broken apparatus. Prior art set screws have been difficult to remove after the head or stem is broken off. Consequently, it is desirable to have a set screw that can be comparatively easily removed even without a head. 
     It is also desirable to have a set screw that has an axially aligned tip that penetrates relatively deeply into a rod for preventing movement along or around the rod of an associated implant once tightened, but also includes structure that helps prevent rocking or translational movement of the set screw relative to the point of penetration. Rocking or movement of the screw relative to the location of penetration weakens the grip provided by the tip in the rod and the prevention of such movement substantially strengthens the juncture of the screw and the rod. The set screw tip, such as a point can only penetrate deeply into the rod if sufficient torque can be applied to the set screw to do so. In general greater torque is available due to greater bulk or due to special construction that allows greater strength without adding bulk. The latter is preferred in implants. 
     In general, there is still a need for an improved set screw which is quite strong in size, reliable in securing an osteosynthesis apparatus in place without burrs or high profile, is easily removable and is relatively small yet easily manipulable to facilitate its insertion and removal. 
     SUMMARY OF THE INVENTION 
     The present invention comprises an improved set screw for use in an osteosynthesis apparatus. The set screw is adapted for use in securing a rod or elongate member in a bore of a ring or head of an implant or within a channel in an open headed implant from translational or rotational motion. The ring or ringlike structure formed by an open head with a closure cap is of the type formed in the head of a hook, the head of a bone screw, the head of a connector secured to the bone screw or other type of implant to which a rod is secured. The rod is of the type including spinal rods or the rod portion of a connector which may be round, square or otherwise shaped in cross-section and which has an elongate axis. 
     A threaded set screw bore extends generally radially through a wall of the ring or head where the implant has a closed head or through a threaded bore in a cap used in conjunction with an “open” head, so as to normally be aligned such that a central axis of the set screw intersects with the elongate axis of the rod or member receiving bore associated with the ring or head of the closed hook, screw or connector. In some instances the axis of the set screw will be perpendicular to an axis of the rod; but in some use the axis of the set screw, while intersecting with the axis of the rod will be non-perpendicular thereto. 
     The set screw has a head or stem preferably having a hexagonal external cross-section, and a lower portion having a threaded outer surface. A tip is centrally formed on a lower surface of the set screw so as to be coaxially aligned with the axis of the set screw. A peripheral break notch preferably is formed between the head and the lower threaded portion of the set screw to facilitate breaking and separation of the two portions. A cylindrical bore preferably is formed in the set screw and extends through the head or stem and partially, but not completely, through the lower threaded portion. 
     The set screw is especially effective in conjunction with open headed implants such as bone screws. The set screw is utilized in conjunction with a cap having opposed slots which mate and lock with opposed slots on respective branches of the implant to prevent spreading or separation of the branches once the cap is in position on the head. The set screw is threadably received in an entirely surrounding bore in the cap so as to stabilize the threaded portion of the set screw while torque is applied to the set screw and as the set screw tip or point drives or penetrates into the rod. In a preferred embodiment the set screw includes a tip and a ring having a sharp lower edge that encircles the point and penetrates into the rod to further stabilize the resulting structure. 
     In use, after the rod is positioned in the ring or ringlike structure, the set screw is tightened or advanced in the set screw bore by a socket type wrench or other suitable driver such that the tip or point engages and bites or penetrates into the outer surface of the rod, while biasing an opposite side of the rod against a side wall of the ring so as to fix the position of the rod relative to the ring, that is, to prevent translational or rotational movement of the rod relative to the ring. Preferably, further tightening of the set screw causes the head or stem to shear off along the peripheral notch preferably without burrs and at a preselected and desired consistent torque that is sufficiently high to allow for considerable penetration of the tip or point into the rod and such that the axis of the set screw generally intersects the elongate axis of the rod or member. 
     In the break off head set screw, the cylindrical internal bore in the set screw includes an upper bore section and a lower bore section. The upper bore section generally extends coaxial with the head or stem of the set screw and the lower bore section extends partially through the lower threaded portion of the set screw. The upper bore section is adapted to facilitate removal of the set screw head or stem once it is sheared off. 
     In the break off head set screw, the set screw is adapted for use with a socket type wrench having a male member or projection extending centrally in the wrench socket that mates with the set screw internal bore. The projection has an outwardly extending biasing element thereon. The projection is sized for insertion into at least the upper bore section when the head or stem of the set screw is positioned in the socket. The biasing element biases against the internal wall of the head defining the upper section of the cylindrical bore to help grip the head. The socket type wrench applies torque to the head until a preselected torque is achieved at which time the notch directs the location or point of breakage and the head breaks from the remainder of the screw without leaving substantial burrs or the like extending above the surface of the ring. 
     In the break off head set screw, after sufficient torque is applied and the head or stem of the set screw is sheared off, the lower bore section is adapted to receive an easy out type tool to permit removal of the set screw lower threaded portion, if necessary. The lower bore section in some embodiments is of a smaller diameter than the upper bore section. A partial reverse starter thread, of at least one half turn, is in some instances formed inside a section of the internal wall of the bore of the set screw defining the lower bore section near an upper end thereof. The reverse thread facilitates gripping and thus starting the easy out type tool to allow the easy out type tool to be used to remove the lower portion of the screw after the head or stem has been broken off. In certain embodiments the side wall of the set screw is threaded which increases breakoff torque and which in some instances provides sufficient wall thickness for an easyout to obtain purchase in the remaining wall without the starter thread. 
     As noted above in a particular embodiment of the invention, the set screw is used in conjunction with a cap utilized to close an open ring or body surrounding a rod or other elongate member. Caps of this type have a pair of curved ears or slot followers that slideably are received in slots in opposite sides of branches forming the remainder of the ring at an opening to be filled by the cap. The ears slideably lock with the slots so as to prevent radially outward separation of the branches when torque is applied to the set screw or other forces are applied to the implant. 
     The cap has a central threaded bore to receive a set screw such that the axis of the set screw is positioned to intersect the longitudinal axis of a rod or other member surrounded by the ring. Normally, the slots and ears are aligned such that the ears of the cap can easily slide into the slots from one side, but are tapered so that the ears are trapped by the slots on the opposite side and effectively form a stop to limit movement or prevent removal of the cap from the remainder of the ring from the opposite side. The cap also has a front edge that may be rotated relatively toward the rod when the cap is pushed such that the ears thereof are as deep as possible into the slots of the remainder of the ring. 
     When the ears are positioned as deeply as possible in the slots and the set screw is tightened against the rod or member, the set screw tip or point penetrates into the rod and the front edge of the cap also engages and wedges against the surface of the rod. The penetration of the tip and wedging of the edge are partially opposed such that, if forces try to move the rod along the axis thereof relative to the ring after the set screw is tightened in a first direction, then such movement is opposed especially by the tip of the set screw, and if forces try to move the rod in the opposite direction such movement is opposed especially by the edge of the cap wedging more tightly and then biting into the surface of the rod. 
     In a second embodiment a ring having a lower sharp edge encircles the axial point of the screw and penetrates into the rod during use to help prevent movement of the set screw relative to the rod and to thereby help stabilize a set screw that is aligned perpendicularly relative to the major axis of the rod. 
     OBJECTS AND ADVANTAGES OF THE INVENTION 
     The objects and advantages of the invention include: providing a locking mechanism with set screw for use in an osteosynthesis apparatus for securing a rod or elongate member from rotational and translational movement within a bore of a securement ring or body; providing such a set screw which is relatively small, yet which can be readily manipulated; providing such a set screw which includes a head or stem which breaks off during tightening at a preselected torque after the set screw has been tightened down; to provide such a set screw which includes a peripheral break inducing and directing notch on an outer surface of the screw between the head or stem of the screw and a lower threaded portion thereof; to provide such a set screw which includes a tip or point for biting or penetrating into the rod to be secured by the set screw; to provide such a set screw incorporating means for facilitating removal of the head of the set screw after it has been broken off; to provide such a set screw which incorporates means for facilitating removal of the lower threaded portion of the set screw when desired; to provide such a set screw having a cylindrical bore extending partially therethrough; to provide such a set screw having an upper bore section extending through the head or stem of the set screw and a lower bore section extending partially through the lower threaded portion of the set screw; to provide such a set screw to be used in cooperation with a cap for completing a ring such that the cap prevents separation of opposite branches of an implant forming a portion of the ring; to provide such a set screw and cap combination wherein the point of the set screw penetrates into the rod and especially resists movement of the rod relative to the ring in a first direction and wherein the cap is rotated to have an edge that is urged to wedge and in some instances to bite into the rod under load by tightening the set screw such that the edge resists movement of the rod relative to the ring in a second direction; to provide such a set screw having a ring with a lower sharpened edge encircling an axially aligned point of the set screw and which during usage penetrates the surface of the rod that is also penetrated by the point so as to stabilize and help prevent movement of the screw relative to the rod once the screw is tightened; and to provide such a set screw which is relatively simple to manufacture and particularly well suited for its intended uses thereof. 
     Other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention. 
     The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a set screw in accordance with the present invention. 
     FIG. 2 is a front elevational view of the set screw of the present invention; 
     FIG. 3 is a cross-sectional view, of the set screw, taken along line  3 — 3  of FIG.  2 . 
     FIG. 4 is a cross sectional view of the set screw, taken generally along line  4 — 4  of FIG.  3 . 
     FIG. 5 is a front elevational view on a reduced scale of the set screw showing a lower threaded portion of the set screw engaging a spinal rod secured within a spinal rod bore in a bone screw and showing a head or stem of the set screw after being broken off. 
     FIG. 6 is a side elevational view on a reduced scale of a bone screw secured within a vertebra and with portions broken away to show a lower threaded portion of the set screw of the present invention secured within the bone screw. 
     FIG. 7 is a fragmentary front elevational view of the set screw shown secured within a socket wrench with portions broken away to show detail. 
     FIG. 8 is a bottom plan view of the socket wrench, as shown in FIG. 7, without a set screw secured therein. 
     FIG. 9 is a front elevational view similar to FIG. 5 showing use of an easy out type tool to remove a lower threaded portion of the set screw of the present invention from a bone screw. 
     FIG. 10 is a fragmentary side elevational view of the set screw utilized in conjunction with an open ring hook and a cap for the hook showing a ring of the hook completed by the cap and the hook secured to a rod. 
     FIG. 11 is a fragmentary cross-sectional view of the set screw, hook and cap, taken along line  11 — 11  of FIG.  10 . 
     FIG. 12 is a fragmentary cross-sectional view of the screw, hook and cap, taken along line  12 — 12  of FIG.  10 . 
     FIG. 13 is a fragmentary side elevational view of the set screw, hook and cap, showing the cap complete but with a portion of the ring broken away to illustrate details. 
     FIG. 14 is a fragmentary side elevational view of the set screw, hook and cap, showing portions of the ring and cap broken away to illustrate position of the set screw and cap engaging the rod after the set screw is tightened to a torque just prior to a head of the set screw breaking therefrom. 
     FIG. 15 is a side elevational view of a modified set screw in accordance with the present invention. 
     FIG. 16 is a cross-sectional view of the modified set screw, taken along line  16 — 16  of FIG.  15 . 
     FIG. 17 is a bottom plan view of the modified set screw. 
     FIG. 18 is a side elevational view of the modified set screw partially securing a rod in an implant extension with portions broken away to show detail thereof. 
     FIG. 19 is a cross-sectional view of the modified set screw and extension, showing internal detail of the set screw prior to removal of a head thereof. 
     FIG. 20 is a front elevational view of the modified set screw and extension after removal of the head of the set screw with portions broken away to show detail thereof. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which may be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. 
     Referring to the drawings in more detail, the reference numeral  1  generally refers to a set screw for use in osteosynthesis apparatus and in particular for use in spinal osteosynthesis apparatus  2 . As shown in FIGS. 5 and 6, the set screw  1  is adapted for use in securing a rod  5  of the apparatus  2  relative to a rod receiving bore  6  of a head or ring  7 , from translational or rotational motion. The ring  7  is of the type formed in the head of a bone screw  10  or the head of a connector or bone hook (not shown) secured to the bone screw  10 . In the field of spinal osteosynthesis, the bone screws  10  are often referred to as sacral screws or pedicle screws. The rod  5  may be of the type including spinal rods or the arm or rod portion of a connector. The illustrated rod  5  is round; however, it is foreseen that the rod could be square to help prevent rotation in a similarly shaped bore in the bone screw  10 , or have a cross section of almost any shape. A threaded set screw receiving bore  11  extends through the ring  7  perpendicular to the axis of the rod receiving bore  6  and extends radially relative to the ring  7  for the closed hooks, screws and connectors. For open hooks, screws and connectors the angle of point of penetration on the rod may vary with respect to the axis of the rod and to the design for a closing cap thereof. 
     The set screw  1 , as is shown in FIGS. 1 through 4, comprises a head or stem  20 , of hexagonal external cross-section and round internal cross section, and a lower portion  22 , having a threaded outer circumferential surface  23 . The head  20  is relatively elongated to facilitate manipulation of the set screw  1 . A tip, illustrated as a point  28 , is formed on a lower surface  29  of the set screw  1  centrally thereof so as to extend outward along a central axis of rotation of the set screw  1 . The point  28  forms a point receiving notch, depression, or indentation  30  in the rod  5 . A peripheral break inducing notch  32  is formed between the head  20  and the lower threaded portion  22  of the set screw  1  on an outer surface  33  of the set screw  1 . The notch is positioned and sized to initiate breakage along the radially innermost portion thereof at this level and at a preselected torque without forming substantial burrs on the resulting upper surface of the set screw lower portion  22 . 
     As best seen in FIG. 3, a cylindrical bore or projection receiving bore  35 , comprising an upper bore section  36  and a lower bore section  37  is formed in the set screw  1  and extends through the head  20  and partially through the set screw lower threaded portion  22 . The upper bore section  36  generally extends coaxial with the head  20  of the screw  1  and the lower bore section  37  extends partially through the lower threaded portion  22  of the screw  1 . The lower bore section  37  is of slightly smaller diameter than the upper bore section  36 . A reverse thread  40 , of preferably at least one half turn, is formed along an internal wall  41  of the set screw  1  defining the lower bore section  37  near an upper end  42  thereof, see FIG.  3 . 
     A drive slot  46  is located at a top end  47  of the set screw head  20 . The slot  46  is a rectangular notch extending downward in FIG. 3 from the top end  47  with portions on diagonally opposite sides of the screw  1 . The set screw  1  is preferably driven by a hexagonal socket type wrench  49 , partially shown in FIG.  7 . The slot  46  can receive mating parts of the wrench  49 ; however, the drive slot  46  is adapted to also receive a set screw holder type tool for starting the set screw  1  into the threaded set screw bore  11  in some applications. 
     In use, the set screw  1  may be inserted in the set screw receiving bore  11  in the ring  7  after the bone screw  10  is inserted into a bone  50  of a patient and after a rod  5  is inserted through the rod receiving bore  6 . To secure the rod  5  in position, thereby preventing further rotational or translational movement of the rod  5  with respect to the rod receiving bore  6 , the set screw  1  is further driven through the set screw receiving bore  11  until the point  28  engages and bites into the rod  5  at depression  30 . Further driving or tightening of the set screw  1  produces a preselected torque on the head  20  and causes the head  20  of the set screw  1  to shear off along the radially innermost portion of the peripheral notch  32 , as shown in FIG.  5 . The bore  11  and the penetration of the point  28  into the depression  30  stabilizes the set screw  1  relative to the rod  5 , so that the set screw  1  is able to secure the rod  5  and prevent relative movement of the rod  5  with respect to the bone screw  10  even under substantial load. 
     The lower threaded portion  22  of the set screw  1  is preferably sized such that after the head  20  is sheared off, an upper surface  55  of the set screw lower portion  22  is generally flush with an upper edge or upper surface  57  of the ring  7  such that no portion of the set screw lower portion  22  extends beyond the upper surface  57  of the ring  7 . Further, after the head  20  is sheared off, the upper surface  55  of the set screw lower portion  22  is generally free from burrs or jagged edges. 
     The set screw  1  may also be pre-loaded into the bone screw  10 , or related structure, prior to insertion into the patient. In particular, the set screw  1  may be manually inserted in the threaded set screw receiving bore  11  of a bone screw  20  or a connector before insertion in a patient and rotated a sufficient number of turns such that the set screw  1  is secured in the set screw receiving bore  11 , but such that the point  28  does not extend substantially into the rod receiving bore  6 . The bone screw  10 , with the set screw  1  secured thereto, may then be secured into the appropriate bone  50  of a patient. After a rod  5  is inserted through the rod receiving bore  6  of the bone screw  10 . Thereafter, the set screw  1  is tightened, as discussed above. 
     The upper bore section  36  of the set screw  1  is adapted to facilitate removal of the set screw head  20  once it is sheared off from the lower threaded portion  22 . The set screw is adapted for use with the socket type torque wrench  49 , as shown in FIGS. 7 and 8, having a hexagonal socket  64  and a male member or projection  65  extending centrally in the socket. The projection  65  includes a resilient biasing member  66  circumferentially secured thereon. The projection  65  is sized for insertion into at least the upper bore section  36  when the set screw head  20  is positioned in the socket  64 . The resilient biasing member  66  biases against an internal wall  70  of the head  20  defining the upper bore section  36  to grip the head  20 . 
     The internal wall  70  has a chamfer  71  at the top end  47  of the set screw head  20  to facilitate insertion of the projection  65  into the projection receiving bore  35  in part by facilitating compression of the resilient biasing member  66 . The resilient biasing member  66 , as shown in FIGS. 7 and 8, generally comprises a split washer type spring, however it is foreseen that the biasing element  66  may be of a wide range of configurations and structures. Further other retention means for releasably securing the set screw  1  to the projection  65  may be utilized including a rubber washer, magnetic coupling means, and various structure producing an interference fit between the projection  65  and the projection receiving bore  35 . 
     The projection  65  may include a pair of drive projections or tabs  74  extending laterally from opposite sides of the projection  65  and adapted to mate with the drive slot  46  extending across the top end  47  of the set screw head  20  to permit an installing surgeon to drive or rotate the set screw  1  by the projection  65 . 
     After the head  20  has been sheared off from the set screw lower threaded portion  22 , the lower bore section  37  is adapted to receive an easy out type tool  75  to permit removal of the set screw lower portion  22  when necessary and as is shown in FIG.  9 . The reverse thread  40  facilitates starting the easy out type tool  75  by allowing the tool  75  to get an initial grasp after which it would be expected to cut further into the lower bore  37 . 
     The bone screws  10  and related connectors (not shown) discussed above are of a closed end variety in that the ring  7  is of one piece construction. The set screws  1  of the present invention are also adapted for use with bone screws and connectors of the open end variety (not shown). In the open end variety, the ring  7  includes a generally U-shaped groove opening along an upper end of the head or ring  7 . A saddle or cap is securable to the head  7  to close off the groove and form the rod receiving bore  6 . The set screw receiving bore  11  may be formed in the cap or another part of the head  7 . 
     Shown in FIGS. 10 through 14 is a modified embodiment of apparatus of the invention utilizing the set screw  1  described above. 
     In particular, FIGS. 10 through 14 show an elongate implant first member, here a rod  100  having a central elongate axis designated by the reference numeral A. The screw  1  is as described above and has a central axis B. The axes A and B intersect subsequent to assembly, as is shown in FIGS. 10 through 14, although at an angle that is not a right angle. 
     The set screw  1  is utilized in conjunction with a second member, here an implant hook  102 , having a hook body or head  103  and a hook finger portion  104 . The head  103  is somewhat U-shaped forming a central partial bore  105  surrounded by a partial ring  106 . When the apparatus is assembled, the rod  100  is cradled by the bore  105  and the partial ring  106 . 
     On opposite sides of the facing ends of the partial ring are curved or U-shaped slots  108  and  109 . The slots  108  and  109  are not parallel but slightly converge to the left in the view seen in FIG.  10 . 
     A cap  115  is located so as to complete the ring initiated by the partial ring  106 . The cap  115  has a pair of slot followers or ears  116  and  117  that are sized and shaped to be received in the slots  108  and  109  respectively. The ears  116  and  117  are parallel and do not converge and are curved to conform to the curve of the slots  108  and  109 . The lower ends  119  and  120  of each side of the cap  115  are curved and are received by simultaneously curved shoulders  122  and  123  on opposite sides with the partial ring  106 . The cap  115  is united with the partial ring  106  by inserting the ears  116  and  117  of the cap  115  into the slots  108  and  109  of the partial ring  106  from the right side as seen in FIG.  10 . The cap  115  is then partially rotated to the left (again as viewed in FIG. 10) by allowing the cap lower ends  119  and  120  to slide on the shoulders  122  and  123  respectively until the convergence of the slots  116  and  117  binds with the ears  108  and  109  so as to limit further relative movement of the cap to the left in FIG.  10  and operably function as a stop. This bound position is seen in FIGS. 10 through 14. It is foreseen that a stop can be provided by other structure and function within the concept of the invention. When the cap  115  is located so as to complete the partial ring  106 , the cap  115  prevents lateral separation of opposite branches forming the partial ring  106  when torque or lateral forces are applied thereto. 
     The cap  115  has a rear edge  130  and a front edge  131 . When the cap  115  is in or near the bound position, the front edge  131  engages and partially penetrates into the rod  100 , as is seen in FIG. 14, as the set screw  1  is tightened. This penetration forms a notch  135  into the rod  100 . 
     The cap  115  has a centrally located threaded bore  137  that receives the threaded lower portion  22  of the set screw  1 , as is seen in FIGS. 10 through 14. 
     As the set screw  1  is advanced in the cap bore  132  the set screw tip or point  28  advances toward and eventually engages the rod  100 . The set screw  1  is torqued until the point  28  penetrates the rod  100  so as to produce a point receiving depression or notch  139  in the rod  100 . In this manner both the point  28  and the cap front edge  130  penetrate the rod  100 . The set screw  1  also biases the rod  100  against a side wall  140  of the partial ring  106 . The surrounding nature of the cap bore  132  relative to the set screw  1 , as seen in FIG. 14, in conjunction with the penetration of the point  28  into the rod  100  and the engagement of the cap edge  131  with the rod  100  forms a very stable configuration that substantially resists movement of the rod  100  relative to the implant hook  102  even when substantial forces are applied through use or accident to cause relative movement. 
     As seen in FIG. 14, if forces urge the rod  100  to the left relative to the partial ring  106 , then the cap front edge  130  especially resists relative movement, and if forces urge the rod  100  to the right relative to the partial ring  106 , then the set screw point  28  especially resists relative movement. As has been discussed before the upper portion  20  of the set screw is subsequently removed by application of additional rotational force thereto until a preselected torque is achieved. In this manner the hook  102  is securely held to the rod  100  with relative good stability. 
     Shown in FIGS. 15 through 20 is a modified embodiment of a set screw in accordance with the present invention and generally represented by the reference numeral  200 . The set screw  200  is shown in use with an implant extension  201  to secure a rod  202  in the extension  201 . 
     The parts of the set screw  200  are in many ways quite similar to the parts of the set screw  1  except for size of parts relative to each other and except as noted below. Consequently, the set screw  200  will not be described in detail, but rather reference is made to the description of set screw  1  for detail not described here. The set screw  200  includes a head  205  with a central and axial bore  206 , a lower portion  207  with an exterior thread  208  and a lower axially aligned point  209 . 
     The main difference between the screw  1  and the screw  200  is the inclusion of a ring  215  positioned to encircle the point  209 . The ring  215  extends  360  degrees around and is radially spaced from the point  209 . A lower edge  216  of the ring  215  is sharpened and adapted to cut into the rod  202  when urged thereagainst. The point  209  preferably extends axially outward and downward further than the ring  215  so as to penetrate deeper into the rod  202  during use. 
     The illustrated extension  201  is a conventional extension having an elongate rod shaped member  220  fixedly attached to a ring member  221  having a central bore  222 . The bore  222  is aligned perpendicularly with respect to a major axis of the rod shaped member  220 . 
     The ring member  220  is two piece and includes a V-shaped portion  224  with a pair of arms or branches  225  and  226  aligned on opposite sides of the bore  222  and a closure cap  227 . The cap  227  has a threaded bore  228  adapted to threadably receive the set screw  200 , as seen in FIGS. 18 to  20 . 
     The branches  225  and  226  each include facing and inwardly directed flange like structures  235  which form slots  236 . Opposite sides of the cap  227  include slot followers  237  which are slideably received in respective slots  236  when the set screw  200  is in a non tightened state. The slot followers  237  are shown in the slots  236  in FIGS. 18,  19  and  20 . 
     In use the set screw  200  is threaded into the receiving bore  223 , such that the point  209  engages a rod  202  received in a bore  222  of the ring member  221 . As the set screw  200  is tightened the point  209  penetrates the rod  202  and forms an indentation  230  in the rod  202 , see FIG.  19 . Preferably, the point  209  penetrates substantially into the rod  206 . As the point  209  continues to penetrate the rod  206 , the edge  216  of the ring  215  engages and then also penetrates the rod  202  so as to form a groove  231 , although preferably not as deeply as the point  209 . Torque is then further applied to the set screw  200  until a desired predetermined torque is applied to the set screw  200  at which time the head  205  breaks from the remainder of the screw  200  leaving the lower body  207  in the bore  228  and the point  209  penetrated into the rod  202 . The ring  215  also partially penetrates into the rod  202 . 
     When the set screw  200  is fully tightened, the cap  227  is biased away from the rod  202 . This causes the slot followers  237  to snugly and tightly fit against the respective slots  236  into which they are received. This in turn secures and locks the cap  227  in position to complete a ring with the V-shaped portion  224 . This also secures the branches  225  and  226  so as to prevent the branches from separating radially outward from each other so as to loosen the set screw  200  or the connection of the rod  202  to the connector  201  when stress is placed upon the implant due to strain or accident as well as when torque is applied to the set screw  200  during installation. The combination of the cap bore  228  holding the set screw acting in conjunction with the point penetration into the rod  202  by the point  209  and the penetration of the ring  215  into the rod  202  at least at two axially spaced locations on the rod  202  on opposite sides of the point  209 , substantially stabilizes the set screw  200  relative to the rod  202  and greatly resist axial or rotational movement of rod  202  relative to the connector  201  once the set screw  200  has been fully torqued, as in FIG.  20 . 
     The set screw body  207  can be removed from the bore  223  in the manner described for removing the set screw lower portion  22  from the position shown in FIG.  5 . 
     It is foreseen that while a hook and connector have been described and illustrated in certain embodiments of the invention in conjunction with a cap, that the apparatus and method of joining an open ring that is completed with a cap with an elongate member such as a round rod, can be utilized with other devices using similar structure such as bone screws or connectors and that a first member having a non-round cross-section, such as a square cross-section, could be used. 
     It is noted that while the set screws of the present invention may be used in conjunction with knurled rod, knurlling causes the rod to be weakened and fail more easily. Therefore, it is normally preferable to use the set screws of the invention with smooth surface rod. The set screws of the present invention are especially effective in penetrating into and preventing relative motion between the set screw and smooth rod. In addition the set screws of the present invention can be applied with a relatively high torque because the bore in which the set screw is received is closed and completely surrounds the set screw so that it does not spread during torquing and such that the set screws of the present invention can relatively deeply penetrate into rod, especially smooth rod, and hold securely against relative movement while stabilizing the screw with respect to the rod, even when the screw is positioned in the closure cap of an open ended implant. The set screws of the present invention may also be relatively small, for example 5.5 mm. in diameter, and still provide a strong and stable positional stabilization of an associated implant relative to a rod received in the implant. 
     It is further noted that the stabilization system of the present invention may in some instances utilize a ring with a lower cutting edge to penetrate into a rod on diagonally opposed locations relative to the ring without including a point. Consequently, the set screw may have a tip that has a point and/or has a ring that in each case penetrates into the rod and that functions with the bore that surrounds the set screw to stabilize the structure. 
     It is also foreseen that in some specialized uses of the set screw that the set screw will be configured to incorporate a stabilizing structure, but that the head will not be removable, that is, broken from the remainder of the set screw upon application of torque. In such instances the head will normally be solid without an interior bore, but such a head could also include an interior bore for receiving a tool for guidance or control during installation. 
     It is to be understood that while certain forms of the present invention have been illustrated and described herein, it is not to be limited to the specific forms or arrangement of parts described and shown.