Abstract:
A polyaxial orthopedic device for use with rod implant apparatus includes a screw having a curvate head, a two-piece interlocking coupling element which mounts about the curvate head, and a rod receiving cylindrical body member having a tapered socket into which both the screw and the interlocking coupling element are securely nested. The interlocking coupling element includes a socket portion which is slotted and tapered so that when it is radially compressed by being driven downwardly into the tapered socket in the cylindrical body it crush locks to the screw. The securing of the rod in the body member provides the necessary downward force onto the socket portion through a contact force on the top of the cap portion. Prior to the rod being inserted, therefore, the screw head remains polyaxially free with respect to the coupling element and the body. In a preferred embodiment, the cap portion and the socket portion are formed and coupled in such a way that when the cap portion is compressed toward the socket portion, there is an additional inward radial force applied by the cap portion to the socket portion, thereby enhancing the total locking force onto the head of the screw.

Description:
CROSS-REFERENCE TO PRIOR APPLICATION 
     This application is a continuation-in-part of prior application U.S. Ser. No. 08/663,383, entitled “A Polyaxial Pedicle Screw”, filed Jun. 13, 1996, now U.S. Pat. No. 5,669,911 and which, in turn, was a continuation-in-part of Ser. No. 08/421,087, filed Apr. 13,1995, now issued U.S. Pat. No. 5,520,690, entitled “An Anterior Spinal Polyaxial Locking Screw Plate Assembly”. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a polyaxial screw and coupling apparatus for use with orthopedic fixation systems. More particularly, the present invention relates to a screw for insertion into spinal bone, and a coupling element polyaxially mounted thereto, via a two-piece interlocking coupling element having a socket portion and a threaded compression member, for coupling the screw to an orthopedic implantation structure, such as a rod, therein enhancing the efficacy of the implant assembly by providing freedom of angulation among the rod, screw and coupling element. 
     2. Description of the Prior Art 
     The bones and connective tissue of an adult human spinal column consists of more than 20 discrete bones coupled sequentially to one another by a tri-joint complex which consist of an anterior disc and the two posterior facet joints, the anterior discs of adjacent bones being cushioned by cartilage spacers referred to as intervertebral discs. These more than 20 bones are anatomically categorized as being members of one of four classifications: cervical, thoracic, lumbar, or sacral. The cervical portion of the spine, which comprises the top of the spine, up to the base of the skull, includes the first 7 vertebrae. The intermediate 12 bones are the thoracic vertebrae, and connect to the lower spine comprising the 5 lumbar vertebrae. The base of the spine is the sacral bones (including the coccyx). The component bones of the cervical spine are generally smaller than those of the thoracic and lumbar spine. For the purposes of this disclosure, however, the word spine shall refer only to the cervical region. 
     Referring now to FIGS. 1,  2 , and  3 , top, side, and posterior views of a vertebral body, a pair of adjacent vertebral bodies, and a sequence of vertebral bodies are shown, respectively. The spinal cord is housed in the central canal  10 , protected from the posterior side by a shell of bone called the lamina  12 . The lamina  12  includes a rearwardly and downwardly extending portion called the spinous process  16 , and laterally extending structures which are referred to as the transverse processes  14 . The anterior portion of the spine comprises a set of generally cylindrically shaped bones which are stacked one on top of the other. These portions of the vertebrae are referred to as the vertebral bodies  20 , and are each separated from the other by the intervertebral discs  22 . The pedicles  24  comprise bone bridges which couple the anterior vertebral body  20  to the corresponding lamina  12 . 
     The spinal column of bones is highly complex in that it includes over twenty bones coupled to one another, housing and protecting critical elements of the nervous system having innumerable peripheral nerves and circulatory bodies in close proximity. In spite of these complexities, the spine is a highly flexible structure, capable of a high degree of curvature and twist in nearly every direction. Genetic or developmental irregularities, trauma, chronic stress, tumors, and disease, however, can result in spinal pathologies which either limit this range of motion, or which threaten the critical elements of the nervous system housed within the spinal column. A variety of systems have been disclosed in the art which achieve this immobilization by implanting artificial assemblies in or on the spinal column. These assemblies may be classified as anterior, posterior, or lateral implants. As the classifications suggest, lateral and anterior assemblies are coupled to the anterior portion of the spine, which is the sequence of vertebral bodies. Posterior implants generally comprise pairs of rods, which are aligned along the axis which the bones are to be disposed, and which are then attached to the spinal column by either hooks which couple to the lamina or attach to the transverse processes, or by screws which are inserted through the pedicles. 
     “Rod assemblies” generally comprise a plurality of such screws which are implanted through the posterior lateral surfaces of the laminae, through the pedicles, and into their respective vertebral bodies. The screws are provided with upper portions which comprise coupling elements, for receiving and securing an elongate rod therethrough. The rod extends along the axis of the spine, coupling to the plurality of screws via their coupling elements. The rigidity of the rod may be utilized to align the spine in conformance with a more desired shape. 
     It has been identified, however, that a considerable difficulty is associated with inserting screws along a misaligned curvature and simultaneously exactly positioning the coupling elements such that the rod receiving portions thereof are aligned so that the rod can be passed therethrough without distorting the screws. Attempts at achieving proper alignment with fixed screws is understood to require increased operating time, which is known to enhance many complications associated with surgery. Often surgical efforts with such fixed axes devices cannot be achieved, thereby rendering such instrumentation attempts entirely unsucessful. 
     The art contains a variety of attempts at providing instrumentation which permit a limited freedom with respect to angulation of the screw and the coupling element. These teachings, however, are generally complex, inadequately reliable, and lack long-term durability. These considerable drawbacks associated with prior art systems also include difficulty properly positioned the rod and coupling elements, and the tedious manipulation of the many small parts in the operative environment. 
     It is, therefore, the principal object of the present invention to provide a pedicle screw and coupling element assembly which provides a polyaxial freedom of implantation angulation with respect to rod reception. 
     In addition, it is an object of the present invention to provide such an assembly which comprises a reduced number of elements, and which correspondingly provides for expeditious implantation. 
     Accordingly it is also an object of the present invention to provide an assembly which is reliable, durable, and provides long term fixation support. 
     Other objects of the present invention not explicitly stated will be set forth and will be more clearly understood in conjunction with the descriptions of the preferred embodiments disclosed hereafter. 
     SUMMARY OF THE INVENTION 
     The preceding objects of the invention are achieved by the present invention which is a polyaxial locking screw and coupling element for use with rod stabilization and immobilization systems in the spine. More particularly, the polyaxial screw and coupling element assembly of the present invention comprises a bone screw having a head which is curvate in shape, for example semi-spherical, and a two-piece interlocking coupling element mounted thereto. This combination is mounted inside the bottom of an internal channel of a cylindrical body member. 
     More specifically, with respect to the cylindrical body member, the tubular body comprises a rod receiving channel formed in the upper portion thereof, with a threading formed on the remaining upper elements so that a rod securing nut and/or set screw may be threaded thereon once a rod has been placed in the channel. The body further includes an axial bore which includes extends from the rod receiving channel through to the bottom of the cylinder. The portion of the axial bore which is below the channel forms a receiving chamber, the upper portion thereof having a constant diameter, and the lower portion of the chamber being inwardly tapered. The inner surface of the upper portion of the chamber and/or the inner surface of the portion of the axial bore which is above the chamber may further include a threading. 
     The two-piece interlocking coupling element comprises and socket portion and a cap portion. The socket portion is designed with an interior semi-spherical volume, so that it may receive the semi-spherical head of a corresponding bone screw. The interior volume of the socket portion is open at both axial ends thereof. The exterior surface of the socket portion, at the bottom thereof, includes a first set of slots which extend upwardly from the opening so that the interior semi-spherical volume may be expanded or contracted by the application of a radial force. In addition, the exterior surface at the bottom is tapered so that it is narrower at the bottom than at a midpoint. This taper is designed to mate with and nest in the tapered lower portion of the socket portion of the axial bore of the body member. 
     The upper exterior surface of the socket portion comprises a second set of slots, directed axially along the element to the midpoint, such that the upper opening of the socket element may expand and contract in accordance with the application of a radial force thereon. The exterior surface of this upper section of the socket portion is not tapered and is narrower than the widest taper position of the bottom of the socket portion. The upper section, however, does further include an outwardly extending annular lip at the uppermost axial position. This upper section is designed to be inserted into, and joined with, the cap portion of the coupling element. 
     The cap portion has a generally cylindrical shape, having an open bottom. The open bottom is inwardly tapered, forming an inwardly extending annular lip, so that as the upper end of the socket portion is inserted, its upper slots are narrowed. Once axially inserted beyond this taper, the upper section of the socket portion expands outward over the inwardly extending annular lip. The inwardly extending annular lip engages the outwardly extending lip of the socket portion so as to prevent disengagement of the two pieces. The socket portion is then permitted to slide into the cap portion, until the larger diameter of the tapered lower portion of the socket contacts the entrance of the cap portion. 
     The exterior surface of the cap portion may be threaded, so that it may engage a threading of the upper portion of the socket portion and/or the inner surface of the axial bore which is above the socket portion. In addition, the top of the cap includes an opening so that a screw driving tool may directly engage the top of the screw. 
     The assembly of the entire device begins with the joining of the socket portion to the cap portion of the two-piece interlocking coupling element. This is achieved by the slideable interlocking mating of the two elements. Next, the semi-spherical head of the screw is inserted into the socket portion through the lower expandable opening in the taper portion. Once these parts have been assembled the screw and coupling element should be polyaxially rotateable relative to one another. The screw and coupling element are then inserted through the axial bore of the body (which may require the threading the cap portion of the coupling element along the threading on the inner surface of the axial bore and/or the threading of the cap along the threading of the upper portion of the chamber) until the socket portion nests in the tapered lower portion of the axial bore. If the upper portion of the chamber includes a threading it should not extend beyond the point of the initial nesting of the coupling element in the chamber. This is important because the cap portion must be able to move relative to the socket portion. 
     In this initial position, the top of the cap portion should rest above the bottom of the rod receiving channel so that a rod, when placed therein, seats directly onto the top of the cap. This direct contact provides the downward force necessary to compress the coupling element into the chamber so that the socket portion is compressed in the tapered portion and locks to the head of the screw. 
     In a preferred variation of this embodiment, the interior surface of the cap portion includes a slight narrowing taper so that as the cap is compressed downward by the rod, the upper slots of the socket portion are also narrowed, further increasing the crush locking effect on the head of the screw. 
     The implantation of this screw by a surgeon may proceed first by the assembly of the screw into its initial state. The shaft of the screw is then driven into the vertebral bone at the desired angulation. A rod is then introduced into the rod receiving channel, and the body is angulated into the most ideal position for receiving the rod. A nut and/or set screw is then used to secure the rod in the channel, and simultaneously to provide a sufficient downward translational force to cause the socket portion to be driven into the tapered portion of the chamber in the axial bore, and further to cause the cap portion to drive downwardly also (this further compression locking the screw head in the embodiment wherein the sliding of the cap portion toward the socket portion provides an additional compression on the top of the socket portion and therefore onto the head of the screw). 
     In a preferred variation, the locking nut comprises a cap nut which has a central post which is designed to provide additional structural support to the inner walls of the element at the top thereof, as well as providing a central seating pressure point for locking the rod in the channel. In either variation, the locking nut seats against the rod and prevents it from moving translationally, axially and rotationally. 
     Multiple screw assemblies are generally necessary to complete the full array of anchoring sites for the rod immobilization system, however, the screw assembly of the present invention is designed to be compatible with alternative rod systems so that, where necessary, the present invention may be employed to rectify the failures of other systems when the surgery may have already begun. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a top view of a human vertebra, which is representative of the type for which the present invention is useful for coupling thereto a rod apparatus; 
     FIG. 2 is a side view of a pair of adjacent vertebrae of the type shown in FIG. 1; 
     FIG. 3 is a posterior view of a sequence of vertebrae of the type shown in FIGS. 1 and 2; 
     FIG. 4 is a side view of a screw having a curvate head which is an aspect of the present invention; 
     FIG. 5 is a side view of a two-piece interlocking coupling element of present invention; 
     FIG. 6 is a side view of a two-piece interlocling coupling element of present invention mounted around the head of a screw of the type shown in FIG. 4; 
     FIG. 7 is a side cross-sectional view of a cylindrical body having a chamber for receiving the two-piece interlocking coupling element and the screw of the present invention; 
     FIG. 8 is a side cross-sectional view of a top locking nut which is an aspect of the present invention; 
     FIG. 9 is a side cross-sectional view of an embodiment of the present invention in its fully assembled disposition having a rod securely locked therein; and 
     FIG. 10 is a side view of an alternative embodiment of the present invention in its fully assembled disposition having a rod securely locked therein. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     While the present invention will be described more fully hereinafter with reference to the accompanying drawings, in which particular embodiments and methods of implantation are shown, it is to be understood at the outset that persons skilled in the art may modify the invention herein described while achieving the functions and results of this invention. Accordingly, the descriptions which follow are to be understood as illustrative and exemplary of specific structures, aspects and features within the broad scope of the present invention and not as limiting of such broad scope. 
     Referring now to FIG. 4, a side view of the screw portion of the present invention, comprising a curvate head, is shown. The screw  120  comprises a head portion  122 , a neck  124 , and a shaft  126 . In FIG. 4, the shaft  126  is shown as having a tapered shape with a high pitch thread  128 . It shall be understood that a variety of shaft designs are interchangeable with the present design. The specific choice of shaft features, such as thread pitch, shaft diameter to thread diameter ratio, and overall shaft shape, should be made be the physician with respect to the conditions of the individual patient&#39;s bone, however, this invention is compatible with a wide variety of shaft designs. 
     The head portion  122  of the screw  120  comprises a semi-spherical shape, which has a recess  130  in it. It is understood that the semi-spherical shape is a section of a sphere, in the embodiment shown the section is greater in extent than a hemisphere, and it correspondingly exhibits an external contour which is equidistant from a center point of the head. In a preferred embodiment, the major cross-section of the semi-spherical head  122  (as shown in the two dimensional illustration of FIG. 4) includes at least 270 degrees of a circle. 
     The recess  130  defines a receiving locus for the application of a torque for driving the screw  120  into the bone. The specific shape of the recess  122  may be chosen to cooperate with any suitable screw-driving tool. For example, the recess  130  may comprise a slot for a screwdriver, a hexagonally shaped hole for receiving an allen wrench, or most preferably, a threading for a correspondingly threaded post. It is further preferable that the recess  130  be co-axial with the general elongate axis of the screw  120 , and most particularly with respect to the shaft  126 . Having the axes of the recess  130  and the shaft  126  co-linear facilitates step of inserting the screw  120  into the bone. 
     The semi-spherical head portion  122  is connected to the shaft  126  at a neck portion  124 . While it is preferable that the diameter of the shaft  126  be less than the diameter of the semi-spherical head  122 , it is also preferable that the neck  124  of the screw  120  be narrower than the widest portion of the shaft  126 . This preferable dimension permits the screw to swing through a variety of angles while still being securely joined to the locking collar (as set forth more fully with respect to FIGS. 5,  8 - 9 ). 
     Referring now to FIG. 5, the two elements which form the two-piece interlocking coupling element of the present invention are shown in a side cross-section view. Phantom lines show the interior structure of the elements along the diametrical cross section. With specific reference to the socket portion  132 , the coupling element comprises a roughly cylindrical shape having an interior volume  134  in which the semi-spherical head  122  of the screw  120  is disposed. The interior volume  134  is open at the top  136  of the socket portion  132  and at the bottom thereof  138 . The lower section  131  of the socket portion  132  comprises a set of slots  133  which extend vertically from the bottom  138  of the socket portion  132  to a position above the maximum diameter of the semi-spherical interior volume  134 . These slots  133  permit the interior volume to expand and contract in accordance with the application of a radial force thereon. The external surface  135  of the lower section  131  of the socket portion  132  is tapered such that the narrowest part of the lower section  131  is at the bottom  138 . 
     The upper section  139  of the socket portion  132  has a generally constant diameter, which is less than the diameter at the uppermost position  137  of the taper of the lower section  131 . A second set of vertical slots  141  are provided in this upper section  139  so that it may also expand and contract in accordance with radial forces applied thereto. In addition, the uppermost end of this upper section  139  comprises an outwardly extending annular lip  140 . 
     The cap portion  142  of the coupling element comprises an opening  143  in the bottom thereof, having an inwardly tapered entrance surface conformation  144 . As the upper section  139  of the socket portion  132  is inserted into the opening  143  in the cap portion  142 , the taper  144  of the opening  143  provides an inwardly directed force which causes the upper section  139  to contract (causes the slots  141  to narrow). This tapered entrance  144  opens to form an annular lip  145  which is useful for engaging and retaining the annular lip  140  of the upper section  139  of the socket portion  132 . The interior surface  146  of the cap portion has a constant diameter, therein permitting the inserted upper section  139  of the socket portion  132  to slide and rotate relative to the cap portion  142 . 
     The exterior surface of the cap portion  142  comprises a threading  147  which is designed to engage threadings  211  disposed in the axial bore of the rod receiving body member (see FIG.  7 ). In addition, the cap portion  142  comprises an axial hole  148  through which a surgeon may insert a screw driving tool to access the head of the screw which is positioned in the interior volume  134  of the socket portion  132 . 
     More particularly, with respect to the disposition of the head  122  of the screw  120  in the socket portion  132 , and with reference to FIG. 6, a partially assembled screw  120  and coupling element is shown in a side cross-section view. The top  136  of the socket portion  132  is inserted into the opening in the cap portion  142  until the annular lip  140  of the socket  132  seats into the cap  142 . The screw  120  is loosely held within the socket  132 , which is, in turn, loosely retained within the cap  142 . 
     Referring now to FIG. 7, the rod receiving body member  200  of the present invention is shown in a side view, wherein critical features of the interior of the element are shown in phantom. The body member  200 , which comprises a generally cylindrical tubular body having an axial bore  201  extending therethrough, may be conceptually separated into a chamber portion  202  at the bottom of the axial bore  201 , and an upper rod receiving channel portion  204 , each of which shall be described more fully hereinbelow. 
     The upper rod receiving channel portion  204  of the body  200  includes a channel  206  formed therein, having rounded bottom surfaces  207 . The channel  206 , in turn, divides the walls of the cylindrical body of the upper portion  204  into a pair of upwardly extending members  214 a,  214 b. As shown in the embodiment illustrated in FIG. 7, the vertical distance from the top  208  of the channel to the curvate bottom  207  thereof, is larger than the diameter of the rod which is to be provided therein. This distance is necessarily larger than the diameter of the rod (see FIGS. 9 and 10) so that the rod may be fully nested in the channel  206 . In addition, the depth of the bottom curvate surface  207  of the channel is such that the cap portion  142  of the two-piece interlocking coupling element initially seats above the curvate bottom  207  of the body  200 . 
     The upwardly extending members  214 a,  214 b further have, disposed thereon, a threading  216  (which may be provided on the inner and/or outer circumferential surfaces, but which is shown in FIGS. 7,  9  and  10  as being on the inner circumferential surface). This threading  216  is ideally suited for receiving a top locking nut (see FIG.  8 ). 
     Referring now to the lower portion of the body, the chamber portion  202  can further be subdivided into a lower chamber portion  203  which includes an inwardly tapered surface, and an upper chamber portion  205  which has a constant diameter. The inwardly tapered portion  203  defines a nesting volume into which the socket portion  132  may nest. Prior to its being fully driven into this nesting volume, the socket portion  132  and the screw  120  disposed therein may be angulated relative to one another, and the screw  120  may be angulated relative to the body  200 . Once driven fully into the tapered lower chamber portion  203 , however, the taper of the axial bore  201  provides the necessary inwardly directed radial force to cause the socket portion  132  to crush lock to the head  122  of the screw  120 . 
     The force which causes the socket portion  132  to be driven downwardly into the tapered lower chamber portion  203  is provided by the cap portion  142 . More specifically, as stated above, when the initially assembled screw  120  and coupling element combination  132  and  142  (see FIG. 6) is advanced into the bottom of the axial bore  201  of the body  200 , and the socket portion  132  nests in the lower chamber portion  203 , the top of the cap portion  142  is positioned to receive the rod (see FIGS. 9 and 10) directly thereon. The locking of the rod in the channel  206  of the body  200  causes the cap portion  142  to be forced downwardly onto the socket portion  132 , which in turn drives the socket portion  132  into the tapered lower chamber portion  203  and causes it of compression lock to the head  122  of the screw  120 . 
     Referring now to FIG. 8, a top locking nut  185  is shown in side cross-section view. The nut  185  comprises post portion  186  and a flange portion  187 , each of which is rotafionally free, relative to the other. The post portion  186  includes a threading  188  thereon, for engaging and advancing along a threading  216  on the inner surface of the upwardly extending members  214 a,  214 b of the upper portion  204  of the body  200 . The bottom surface  189  of the flange portion  187  (which does not rotate relative to the body as the post portion  186  is rotationally advanced) is intended to seat against the top surface of the rod  250 . 
     Referring now to FIG. 9, in which the fully assembled and body member  200 , screw  120 , coupling element portions  132  and  142 , rod  250  and locking nut  185  are shown in side cross-section views, the implantation of this embodiment is described. First, the screw  120  and the two portions  132  and  142  of the coupling element are assembled into their initial association (see FIG.  6 ). The combination of the screw  120  and the two coupling element portions  132  and  142  are then advanced down the axial bore  201  of the body  200  until the socket portion  132  nests in the lower chamber  203  and the top of the cap portion  142  seats above the bottom  207  of the channel  206 . (This insertion of the subassembly of the screw  120  and coupling element portions  132  and  142  into the axial bore  201  of the body  200  may require the threaded advance of the cap portion  142  along the interior threads  216  of the body.) 
     The shaft of the screw  120  is then inserted and driven downward into the vertebral bone at the desired angle. Once properly positioned, the body  200  is rotated into the ideal rod receiving position. The rod  250  is then inserted into the channel  206  and the top locling nut  185  is threaded onto the threading  216  and compresses the rod  250  to securely lock it in the channel  206 . This downward force of the nut  185  and the rod  250  onto the cap portion  142  causes the cap portion to translate downward thus causing the socket portion  132  to translate downward in the tapered chamber  203  and contract to crush against the head  122  of the screw  120 . The assembly is thereby fully locked in position. 
     Referring to FIG. 10, a variation of the above device is shown in a similar cross-section view. In this embodiment, the inner surface  146 ′ of the cap portion  142  is tapered inwardly in the vertical direction so that the downward translation of the cap portion  142  causes the annular lip  140  of the socket portion  132  to be compressed inwardly. This causes the slots  141  of the upper section  139  of the socket portion  132  to narrow. This may be utilized to further clamp the interior volume  134  against the head  122  of the screw  120 . 
     While there has been described and illustrated embodiments of a polyaxial screw and coupling element assembly for use with posterior spinal rod implantation apparatus, it will be apparent to those skilled in the art that variations and modifications are possible without deviating from the broad spirit and principle of the present invention. The present invention shall, therefore, be limited solely by the scope of the claims appended hereto.