Patent Publication Number: US-7722646-B2

Title: Polyaxial pedicle screw having a rotating locking element

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 10/115,747 entitled “Polyaxial Pedicle Screw Having a Rotating Locking Element”, filed Apr. 3, 2002, now U.S. Pat. No. 6,840,940 which is a continuation of U.S. patent application Ser. No. 09/789,935, filed Feb. 15, 2001, now U.S. Pat. No. 6,451,021. The entire contents of each of these prior applications are hereby expressly incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to screws and coupling element assemblies for use with orthopedic fixation systems. More particularly, the invention relates to a screw and coupling element assembly, for use with an orthopedic rod implantation apparatus, that includes a locking element that simultaneously locks a head of the screw within a seat of the coupling element so that a shaft of the screw is fixed at an angle relative to the coupling element and a rod, of the orthopedic rod implantation apparatus, disposed within a rod-receiving channel of the locking element is fixed relative to the coupling element 
     BACKGROUND OF THE INVENTION 
     The bones and connective tissue of an adult human spinal column consists of more than twenty 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 twenty 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 seven vertebrae. The intermediate twelve bones are the thoracic vertebrae, and connect to the lower spine comprising the five 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. 
     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 pedides  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 dassifications 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 unsuccessful. 
     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 in properly positioning the rod and coupling elements, and the tedious manipulation of the many parts that are used in the prior art to lock the rod, the screw, and the coupling element in position once they are properly positioned. It is not unusual for displacement to occur as these parts are manipulated to lock the elements, which is clinically unacceptable, and repeated attempts at locking the elements in proper position must be made to remedy this displacement. 
     There is, therefore, a need for a screw and coupling element assembly which provides a polyaxial freedom of implantation angulation with respect to rod reception. There is also a need for such an assembly which comprises a reduced number of elements, and which correspondingly provides for expeditious implantation. There is also a need for such an assembly that provides reduced difficulty in locking steps to prevent unwanted displacement of the elements prior to locking. There is also a need for an assembly which is reliable, durable, and provides long term fixation support. 
     SUMMARY OF THE INVENTION 
     The invention provides a screw and coupling element assembly for use with an orthopedic rod implantation apparatus. The assembly includes a securing element such as, for example, a screw, that has a head and a shaft that extends from the head. Preferably, the head has an engagement surface that can be engaged by a screwdriving tool. Preferably, the head also has a curvate proximal portion from which the shaft extends. 
     The screw and coupling element assembly further includes a coupling element and a locking element. The coupling element has a seat within which the head of the screw can be seated such that the shaft of the screw protrudes from the coupling element. The locking element can be mated with the coupling element and thereafter can be selectively moved through a plurality of positions including an unlocked position and a locked position. When the locking element is in the unlocked position, the locking element presents a rod-receiving channel and the head of the screw is movable in the seat of the coupling element, such that the shaft of the screw can be directed in a plurality of angles relative to the coupling element. When the locking element is in the locked position, a rod disposed within the rod-receiving channel is fixed relative to the coupling element, and the head of the screw is immovable in the seat of the coupling element, such that the shaft of the screw is fixed at an angle relative to the coupling element. More particularly, when the locking element is in the locked position, the locking element is compression locked within the coupling element, the head of the screw is compression locked within the seat of the coupling element, and the rod is compression locked within the rod-receiving channel. 
     Preferably, at least one feature on the locking element can be used to move the locking element through the positions when the locking element is disposed within the coupling element Preferably, the feature comprises an engagement surface that can be engaged by a corresponding surface of a tool, such that the tool can engage the engagement surface of the feature and effectively move the locking element. Preferably, the corresponding surface of the tool can engage the engagement surface of the feature despite the presence of the rod in the rod-receiving channel. 
     Preferably, the seat of the coupling element can be defined by a bore in the coupling element and a socket defined by a curvate volume adjacent the bore. The curvate volume corresponds to the curvate proximal portion of the head of the screw such that the curvate proximal portion can be nested in the socket. When the curvate proximal portion of the head of the screw is nested in the socket, the shaft of the screw protrudes from the bore and the curvate proximal portion cannot pass fully through the bore. When the locking element is in the unlocked position, the curvate proximal portion of the head of the screw can rotate and angulate in the socket such that the shaft of the screw can be directed through a range of angles relative to the coupling element. When the locking element is in the locked position, the curvate proximal portion of the head of the screw is immovable in the socket such that the shaft of the screw is fixed at an angle relative to the coupling element 
     Preferably, the locking and unlocking action of the locking element is effected as follows. The head of the screw has a distal portion defined by a compression surface and the locking element has a permissive surface that is presented to the compression surface of the head of the screw when the locking element is in the unlocked position. When presented with the permissive surface of the locking element, the compression surface of the head of the screw is unhindered such that the head of the screw is movable in the seat of the coupling element. The locking element also has a confrontational surface that is presented to the compression surface of the head of the screw when the locking element is in the locked position. The confrontational surface of the locking element is defined by a recess that has a recessed surface corresponding to the compression surface of the head of the screw. Therefore, when the locking element is in the locked position: (1) the locking element is compressed within the coupling element, (2) the compression surface of the head of the screw is compressed by the confrontational surface of the locking element such that the curvate proximal portion of the head of the screw is compressed toward the bore of the coupling element and the head of the screw is immovable in the seat of the coupling element, and (3) the compression surface of the head of the screw seats in the recess of the confrontational surface and thereby is biased against retreat from the recess. The compression surface of the head of the screw can be a curvate surface and the confrontational surface of the locking element can extend from an edge of the permissive surface of the locking element and terminate in the recess of the confrontational surface. The confrontational surface of the locking element preferably becomes increasing confrontational and therefore is gradually presented to the compression surface of the head of the screw as the locking element is moved from the unlocked position to the locked position. 
     Preferably, the rod-receiving channel, presented by the locking element when the locking element is in the unlocked position, is defined by substantially parallel walls. When the locking element is in the locked position, the walls are compressed toward one another such that the rod is compressed within the channel and thereby fixed relative to the coupling element Preferably, the walls are increasingly compressed toward one another as the locking element is moved from the undocked position to the locked position. 
     Preferably, the coupling element has a bore that permits use of the screwdriving tool when the head is seated in the seat, allowing the screwdriving tool to engage the head as described above. Also preferably, the locking element has a bore that permits use of the screwdriving tool when the locking element is disposed within the coupling element in an unlocked position and the head is seated in the seat, allowing the screwdriving tool to engage the head as described above. 
     A use of the invention can be summarized as follows. First, the locking element is disposed within the coupling element. Then, the locking element is rotated within the coupling element until the locking element is in the unlocked position. The top bores of the locking element and the coupling element will then permit the insertion of the screw into the seat of the coupling element. The surgeon can freely rotate and angulate the screw in the socket to direct the shaft in the clinically appropriate direction for secure lodging in a vertebral bone. 
     Once the surgeon directs the shaft of the screw in the clinically appropriate direction, he passes the operative end of the screwdriving tool through the top bores to engage the head of the screw to rotate the screw and drive the shaft of the screw into the vertebral bone. A predrill hole is usually provided in the bone, into which it is desired that the screw be disposed. 
     Typically, the surgeon will repeat the process described above using additional screws, coupling elements, and locking elements, lodging the additional screws into the same vertebral bone or other vertebral bones, depending on the clinically desirable result To that end, the invention encompasses an orthopedic rod implantation apparatus having a plurality of screws and coupling elements of the types described above, and at least one elongate rod. Many implantations will require the use of two rods, however, only one rod or more than two rods may be necessary. The surgeon will, for example, lodge two screws into a first verterbal bone (the screws are positioned laterally adjacent one another in the same bone, one in each pedicle), and lodge two other screws into a second vertebral bone (the screws are positioned laterally adjacent one another in the same bone, one in each pedicle) that may be adjacent the first vertebral bone or may be separated from the first vertebral bone by other vertebral bones that are damaged or unstable. 
     In a healthy spine, each screw in one of the pairs would roughly align with the corresponding screw in the other pair, because the vertebral bones would be vertically aligned. However, in some cases, the screws will not be aligned because the spine is laterally crooked and the surgeon is installing the implant to realign the vertebral bones. Therefore, in such cases, once the screws are in place, and the associated coupling elements and locking elements are in the unlocked position, presenting respective rod-receiving channels, and free to rotate and angulate with respect to the head of the associated screw, the surgeon inserts a rod into one rod-receiving channel, and thereafter into another rod-receiving channel, so that the rod lines up the verterbral bones affected thereby in a clinically desirable manner, urging them, typically, into vertical alignment. The surgeon similarly inserts a second rod into the remaining two rod receiving channels to balance the alignment forces. 
     Once the rods are in the rod receiving channels, the surgeon proceeds to move each locking element into the locked position by rotating the locking element. As the locking element is moved to the locked position, the permissive surface, of the locking element, that is presented to the compression surface of the head of the screw when the locking element is in the unlocked position, is removed and the compression surface is increasingly presented with the confrontational surface. This causes the curvate proximal portion of the head of the screw to be increasingly compressed toward the bore until the head is immovable in the seat of the coupling element. Ultimately, the compression surface seats in the recess and thereby is biased against retreat from the recess. This prevents accidental reverse rotational slippage of the locking element from occurring and moving the locking element to the unlocked position. 
     In addition, as the locking element is moved to the locked position, the rod-receiving channel presented by the locking element when the locking element is in the unlocked position gradually closes. Ultimately, the channel is completely dosed. This prevents the rod from laterally exiting the locking element Further, the walls of the channel are gradually compressed toward one another, until they ultimately secure the rod between them so that the rod is fixed relative to the coupling element. This prevents the rod from axially exiting the locking element. 
     In addition, as the locking element is moved to the locked position, the locking element itself is gradually compressed within the coupling element until it ultimately is secured within the coupling element. More specifically, as the rod seeks to radially force the walls and apart, the outer surface of the locking element seeks to radially push against the inner surface of the coupling element. However, the structural integrity of the coupling element resists the radial force of the locking element, and the locking element is secured in the coupling element thereby. This prevents the locking element from axially exiting the coupling element. 
     In this manner, once the locking element is moved to the locked position, the screw, the coupling element, the locking element, and the rod are all fixed relative to one another and to the bone. When all of the locking elements are so positioned, the implant is installed. After the surgeon moves all of the locking elements in the orthopedic rod implantation apparatus to the locked position, he closes the wounds of the patient and the surgery is complete. 
     Multiple screw and coupling element assemblies are generally necessary to complete the full array of anchoring sites for a rod immobilization system, however, the screw and coupling element assembly of the invention is designed to be compatible with alternative rod systems so that, where necessary, the invention may be employed to rectify the failures of other systems, the implantation of which may have already begun. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top view of a human vertebra. 
         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 . 
         FIGS. 4   a ,  4   b  and  4   c  are side, top, and cutaway side views of a screw suitable for use in the invention. 
         FIGS. 4   d  and  4   e  are side cutaway views of a screwdriving tool suitable for driving the screw of  FIGS. 4   a ,  4   b  and  4   c.    
         FIGS. 5   a ,  5   b  and  5   c  are front, side and top views of a coupling element suitable for use in the invention. 
         FIGS. 6   a ,  6   b ,  6   c ,  6   d  and  6   e  are front, side, front cutaway, side cutaway, and top views of a locking element suitable for use in the invention. 
         FIG. 7  is a front cutaway view of the screw of  FIGS. 4   a - c , the coupling element of  FIGS. 5   a - c , the locking element of  FIGS. 6   a - e , and a rod, with the locking element in an unlocked position. 
         FIG. 8  is front view of the screw of  FIGS. 4   a - c , the coupling element of  FIGS. 5   a - c , the locking element of  FIGS. 6   a - e , and a rod, with the locking element in a locked position. 
         FIG. 9  is a side cutaway view of the screw of  FIGS. 4   a - c , the coupling element of  FIGS. 5   a - c , the locking element of  FIGS. 6   a - e , and the screwdriving tool of  FIGS. 4   d - e.    
         FIGS. 10   a  and  10   b  are front and side cutaway views of a wrenching tool suitable for wrenching hexagonal protrusions of the locking element of  FIGS. 6   a - e.    
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the 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 invention and not as limiting of such broad scope. 
     In an embodiment, the invention provides a screw and coupling element assembly for use with an orthopedic rod implantation apparatus. The assembly includes a securing element such as, for example, a screw, that has a head and a shaft that extends from the head. Other securing elements can include, for example, lamina hooks and sacral blocks. 
     Accordingly,  FIG. 4   a  illustrates a side view of a screw  120  suitable for use in the invention. The screw  120  includes a head  122  and a shaft  126  that extends from the head  122 . 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 screw of the invention. 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, the invention is compatible with a wide variety of shaft designs. 
     Preferably, the head has an engagement surface that can be engaged by a screwdriving tool. In this regard, one of the head and the tool can have a polygonal recess and the other of the head and the tool can have an external structure that fits within the recess. 
     Accordingly, referring again to  FIG. 4   a  and also to  FIGS. 4   b - 4   e , the head  122  of the illustrated screw  120  includes a hexagonal engagement surface  130  that fits within a hexagonal recess  140   a  of a screwdriving tool  140 . The surface  130  defines a receiving locus for the application of a torque for driving the screw  120  into a vertebral bone. Of course, the specific shape of the surface  130  may be chosen to cooperate with any suitable screwdriving tool. For example, the surface  130  may include an engagement surface of another shape, or a recess such as, for example, a slot for receiving a standard screwdriver, a hexagonally shaped hole for receiving an allen wrench, or a threading for a correspondingly threaded post. Preferably, the surface  130  is co-axial with the generally elongate axis of the screw  120  and, most particularly, the shaft  126 . Having the axes of the surface  130  and the shaft  126  co-linear facilitates step of inserting the screw  120  into the bone. 
     Preferably, the head has a curvate proximal portion from which the shaft extends. The curvate proximal portion can be a semi-spherical shape, exhibiting an external contour that is equidistant from a center point of the head. When the head has an engagement surface that can be engaged by a screwdriving tool, it is preferable that the engagement surface does not disrupt the functionality of the curvate proximal portion as described herein. 
     Accordingly, referring also to  FIG. 4   a  again, the head  122  of the illustrated screw  120  includes a hemisphere  122   a  as the curvate proximal portion. The use of the hemisphere  122   a  in this embodiment frees the hexagonal engagement surface  130  to effectively receive the hexagonal recess  140   a  of the screwdriving tool  140 . 
     Further, the head  122  of the illustrated screw  120  is connected to the shaft  126  at a neck  124 . The diameter of the head  122  is equal to the largest diameter of the shaft  126 , and the neck  124  tapers to expose the operative portions of the curvate proximal portion  122   a  of the head  122 . This configuration permits the screw  120  to swing through a variety of angles before its position is fixed, as described below. It should be noted that in other embodiments, the diameter of the shaft  126  can be less than or greater than the diameter of the head  122 , and the neck  124  may be un-tapered or differently tapered. 
     In this embodiment, the screw and coupling element assembly further includes a coupling element and a locking element. The coupling element has a seat within which the head of the screw can be seated such that the shaft of the screw protrudes from the coupling element. The locking element can be mated with the coupling element and thereafter can be selectively moved through a plurality of positions including an unlocked position and a locked position. When the locking element is in the unlocked position, the locking element presents a rod-receiving channel and the head of the screw is movable in the seat of the coupling element, such that the shaft of the screw can be directed in a plurality of angles relative to the coupling element. When the locking element is in the locked position, a rod disposed within the rod-receiving channel is fixed relative to the coupling element, and the head of the screw is immovable in the seat of the coupling element, such that the shaft of the screw is fixed at an angle relative to the coupling element. More particularly, when the locking element is in the locked position, the locking element is compression locked within the coupling element, the head of the screw is compression locked within the seat of the coupling element, and the rod is compression locked within the rod-receiving channel. At least one feature on the locking element can be used to move the locking element through the positions when the locking element is disposed within the coupling element. Preferably, the feature comprises an engagement surface that can be engaged by a corresponding surface of a tool, such that the tool can engage the engagement surface of the feature and effectively move the locking element. Preferably, the corresponding surface of the tool can engage the engagement surface of the feature despite the presence of the rod in the rod-receiving channel. 
     Accordingly,  FIGS. 5   a - c  illustrate a coupling element  150  suitable for use in the invention, in front, top, side, front cutaway, and side cutaway views, respectively.  FIGS. 6   a - e  illustrate a locking element  185  suitable for use in the invention, in front, top, and side views, respectively.  FIG. 7  illustrates a front cutaway view of the screw  120  of  FIG. 4 , the coupling element  150 , and the locking element  185  disposed within the coupling element  150  in an unlocked position.  FIG. 8  illustrates a front view of the screw  120  of  FIG. 4 , the coupling element  150 , and the locking element  185  disposed within the coupling element  150  in a locked position. The coupling element  150  has a substantially cylindrical inner surface  150   c  that accepts a cylindrical outer surface  185   e  of the locking element  185  for coaxial insertion and rotational translation therein so the locking element  185  can be rotated through a plurality of positions. At one extreme of the positional spectrum enjoyed by this locking element  185  is the unlocked position shown in  FIG. 7 . At the other extreme is the locked position shown in  FIG. 8 . In order to permit the locking element  185  to be moved through the positions, the locking element  185  has a feature on each end of the locking element  185 , having a surface defined by a hexagonal protrusion  185   d  extending beyond a respective end of the coupling element  150 . The hexagonal protrusions  185   d  can each be engaged by a corresponding surface of a tool such as, for example, the corresponding hexagonal recess  142   a  of the wrenching tool  142  shown in  FIGS. 10   a  and  10   b . So that the hexagonal recess  142   a  can engage the hexagonal protrusions  185   d  despite the presence of a rod  195  in a rod-receiving channel  190 , the wrenching tool  142  has a slot  142   b  that accommodates the rod  195  as the wrenching tool  142  is rotated to rotate the locking element  185 . 
     Preferably, the seat of the coupling element can be defined by a bore in the coupling element and a socket defined by a curvate volume adjacent the bore. The curvate volume corresponds to the curvate proximal portion of the head of the screw such that the curvate proximal portion can be nested in the socket. When the curvate proximal portion of the head of the screw is nested in the socket, the shaft of the screw protrudes from the bore, the curvate proximal portion cannot pass fully through the bore. When the locking element is in the unlocked position, the curvate proximal portion of the head of the screw can rotate and angulate in the socket such that the shaft of the screw can be directed through a range of angles relative to the coupling element. When the locking element is in the locked position, the curvate proximal portion of the head of the screw is immovable in the socket such that the shaft of the screw is fixed at an angle relative to the coupling element. 
     Accordingly, referring to  FIGS. 5   a - c ,  6   a - e  and  7 - 8 , a seat  150   a  of the illustrated coupling element  150  is defined by a bore  150   b  in the coupling element  150  and a socket defined by a curvate volume  150   c  adjacent the bore  150   b . The curvate volume  150   c  corresponds to the curvate proximal portion  122   a  of the head  122  such that the curvate proximal portion  122   a  can be nested in the socket  150   c . It is understood that the head  122  of the screw  120  is held within the curvate volume  150   c  by the relative size of the curvate proximal portion  122   a  as compared with the bore  150   b . More specifically, the bore  150   b  has a diameter less than the diameter of the curvate proximal portion  122   a . When the curvate proximal portion  122   a  is nested in the socket  150   c , the shaft  126  protrudes from the bore  150   a  and the curvate proximal portion  122   a  cannot pass fully through the bore  150   a . Further when the curvate proximal portion  122   a  is nested in the socket  150   c , and when the locking element  185  is in the unlocked position, the curvate proximal portion  122   a  can rotate and angulate in the socket  150   c  such that the shaft  126  can be directed through a range of angles relative to the coupling element  150 . Further when the curvate proximal portion  122   a  is nested in the socket  150   c , and when the locking element  185  is in the locked position, the curvate proximal portion  122   a  is immovable in the socket  150   c  such that the shaft  126  is fixed at an angle relative to the coupling element  150 . 
     Preferably, the locking and unlocking action of the locking element is effected as follows. The head of the screw has a distal portion defined by a compression surface and the locking element has a permissive surface that is presented to the compression surface of the head of the screw when the locking element is in the unlocked position. When presented with the permissive surface of the locking element, the compression surface of the head of the screw is unhindered such that the head of the screw is movable in the seat of the coupling element. The locking element also has a confrontational surface that is presented to the compression surface of the head of the screw when the locking element is in the locked position. The confrontational surface of the locking element is defined by a recess that has a recessed surface corresponding to the compression surface of the head of the screw. Therefore, when the locking element is in the locked position: (1) the locking element is compressed within the coupling element, (2) the compression surface of the head of the screw is compressed by the confrontational surface of the locking element such that the curvate proximal portion of the head of the screw is compressed toward the bore of the coupling element and the head of the screw is immovable in the seat of the coupling element, and (3) the compression surface of the head of the screw seats in the recess of the confrontational surface and thereby is biased against retreat from the recess. The compression surface of the head of the screw can be a curvate surface and the confrontational surface of the locking element can extend from an edge of the permissive surface of the locking element and terminate in the recess of the confrontational surface. The confrontational surface of the locking element preferably becomes increasing confrontational and therefore is gradually presented to the compression surface of the head of the screw as the locking element is moved from the unlocked position to the locked position. 
     Accordingly, referring to  FIGS. 5   a - c ,  6   a - e  and  7 - 8 , the illustrated head  122  has a distal portion defined by a compression surface  122   b  and the illustrated locking element  185  has a permissive surface  185   a  that is presented to the compression surface  122   b  when the locking element  185  is in the unlocked position. When presented with the permissive surface  185   a , the compression surface  122   b  is unhindered such that the head  122  is movable in the seat  150   a . Further, the illustrated locking element  185  has a confrontational surface  185   b  that is presented to the compression surface  122   b  when the locking element  185  is in the locked position. The confrontational surface  185   b  is defined by a recess  185   c  that has a recessed surface corresponding to the compression surface  122   b . When the illustrated locking element  185  is in the locked position, (1) the locking element  185  is compressed within the coupling element  150 , (2) the compression surface  122   a  is compressed by the confrontational surface  185   b  such that the curvate proximal portion  122   a  of the head  122  is compressed toward the bore  150   b  and the head  122  is immovable in the seat  150   a , and (3) the compression surface  122   b  seats in the recess  185   c  and thereby is biased against retreat from the recess  185   c . The illustrated compression surface  122   a  is a curvate surface and the confrontational surface  185   b  extends from an edge of the permissive surface  185   a  and terminates in the recess  185   c . The confrontational surface  185   b  is tapered to become increasing confrontational as the locking element  185  is moved from the unlocked position to the locked position and therefore is gradually presented to the compression surface  122   a  as the locking element  185  is moved from the unlocked position to the locked position. 
     Preferably, the rod-receiving channel, presented by the locking element when the locking element is in the unlocked position, is defined by substantially parallel walls. When the locking element is in the locked position, the walls are compressed toward one another such that the rod is compressed within the channel and thereby fixed relative to the coupling element. Preferably, the walls are increasingly compressed toward one another as the locking element is moved from the unlocked position to the locked position. 
     Accordingly, referring to  FIGS. 5   a - c ,  6   a - e  and  7 - 8 , the rod-receiving channel  190  that is presented by locking element  185  when the locking element  185  is in the unlocked position has substantially parallel walls  190   a ,  190   b . As the locking element  185  is moved from the unlocked position to the locked position, the walls  190   a ,  190   b  are increasingly compressed toward one another. When the locking element  185  is in the locked position, the walls  190   a ,  190   b  are compressed toward one another such that the rod  195  is compressed within the channel  190  and thereby fixed relative to the coupling element  150 . 
     Preferably, the coupling element has a bore that permits use of the screwdriving tool when the head is seated in the seat, allowing the screwdriving tool to engage the head as described above. Also preferably, the locking element has a bore that permits use of the screwdriving tool when the locking element is disposed within the coupling element in an unlocked position and the head is seated in the seat, allowing the screwdriving tool to engage the head as described above. 
     Accordingly, with reference also to  FIGS. 5   a - c ,  6   a - e ,  7  and  8 ,  FIG. 9  illustrates a side cutaway view of the screw  120 , coupling element  150 , locking element  185 , and screwdriving tool  140 . It can be seen that a top surface of the coupling element  150  has a bore  197  through which the screwdriving tool  140 , which is used to insert the screw  120  into the bone, may access and rotate the screw  120  through the coupling element  150 . It can also be seen that a top surface of the locking element  150  has a bore  199  through which the screwdriving tool  140  may access and rotate the screw  120  through the locking element  185  when the locking element  185  is in the unlocked position. 
     Referring now to  FIGS. 5   a - c ,  6   a - e ,  7 - 9  and  10   a - 10   b , the preferred method of implantation and assembly is described hereinbelow. First, the locking element  185  is disposed within the coupling element  150  so that it can be selectively moved. This can be accomplished by coaxially inserting the locking element  185  into the coupling element  150 , contacting the cylindrical outer surface of the locking element  185  with the substantially cylindrical inner surface  150   c  of the coupling element  150 . 
     Once the locking element  185  is disposed within the coupling element  150  in this manner, the wrenching tool  142  shown in  FIGS. 10   a - 10   b  can be used to engage the hexagonal protrusions  185   d  of the locking element  185  and rotate the locking element  195  within the coupling element  150  until the locking element  185  is in the unlocked position as shown in  FIG. 7 . 
     When the locking element  185  is in the unlocked position, the bore  199  of the locking element and the bore  197  of the coupling element permit the insertion of the screw  120  into the seat  150   a . More particularly, the surgeon can pass the shaft  126  of the screw  120  and the head  122  of the screw through the bores  199  and  197 , but only the shaft  126  will pass through the bore  150   b  of the coupling element  150 . As stated above, the curvate proximal portion  122   a  of the head  122  will not pass through the bore  150   b , but instead will nest in the seat  150   a  in the curvate volume  150   c  of the coupling element  150 . The surgeon is then able to freely rotate and angulate the screw  120  in the socket  150   c  to direct the shaft  126  in the clinically appropriate direction for secure lodging in a vertebral bone. 
     It should be noted that while the insertion of the locking element  185  into the coupling element  150  and the subsequent insertion of the screw  120  through the bores  199  and  197  has been described, it is possible to pre-assemble these elements in this configuration prior to delivery to the surgeon to minimize the effort and time required from the surgeon in preparing the elements for operative use. 
     Once the surgeon directs the shaft  126  of the screw  120  in the clinically appropriate direction, he is able to pass the operative end of the screwdriving tool  140  shown in  FIGS. 4   d  and  4   e  through the bore  199  and the bore  197  to engage the hexagonal engagement surface  130  of the head  122  of the screw  120 . As stated above, the operative end of the screwdriving tool  140  has a hexagonal recess  140   a  for engaging the hexagonal engagement surface  130  and rotating the screw  120  to drive the shaft  126  of the screw  120  into the vertebral bone. A pre-drill hole is usually provided in the bone, into which it is desired that the screw  120  be disposed. The hole may be pre-tapped, or the external threading  128  of the screw  120  may include a self-tapping edge. 
     Typically, the surgeon will repeat the process described above using additional screws, coupling elements, and locking elements, lodging the additional screws into the same vertebral bone or other vertebral bones, depending on the clinically desirable result. To that end, the invention encompasses an orthopedic rod implantation apparatus having a plurality of screws and coupling elements of the types described above, and at least one elongate rod. Many implantations will require the use of two rods, however, only one rod or more than two rods may be necessary. The surgeon will, for example, lodge two screws into a first verterbal bone (the screws are positioned laterally adjacent one another in the same bone, one in each pedicle), and lodge two other screws into a second vertebral bone (the screws are positioned laterally adjacent one another in the same bone, one in each pedicle) that may be adjacent the first vertebral bone or may be separated from the first vertebral bone by other vertebral bones that are damaged or unstable. 
     In a healthy spine, each screw in one of the pairs would roughly align with the corresponding screw in the other pair, because the vertebral bones would be vertically aligned. However, in some cases, the screws will not be aligned because the spine is laterally crooked and the surgeon is installing the implant to realign the vertebral bones. Therefore, in such cases, once the screws are in place, and the associated coupling elements and locking elements are in the unlocked position, presenting respective rod-receiving channels, and free to rotate and angulate with respect to the head of their associated screw, the surgeon inserts a rod into one rod-receiving channel, and thereafter into another rod-receiving channel, so that the rod lines up the verterbral bones affected thereby in a clinically desirable manner, urging them, typically, into vertical alignment The surgeon similarly inserts a second rod into the remaining two rod receiving channels to balance the alignment forces. 
     Once the rods are in the rod receiving channels, the surgeon proceeds to move each locking element  185  into the locked position shown in  FIG. 8 . The surgeon uses the wrenching tool  142  shown in  FIGS. 10   a  and  10   b ; engaging the hexagonal recess  142   a  of the wrenching tool  142  with one of the hexagonal protrusions  185   d  of the locking element and moving the wrenching tool  142  to rotate the hexagonal protrusion  185   d  and thereby rotate the locking element  185 . As the locking element  185  is moved to the locked position, the permissive surface  185   a , of the locking element  185 , that is presented to the compression surface  122   b  of the head  122  when the locking element  185  is in the unlocked position, is removed and the compression surface  122   b  is increasingly presented with the confrontational surface  185   b . This causes the curvate proximal portion  122   a  of the head  122  to be increasingly compressed toward the bore  150   b  until the head  122  is immovable in the seat  150   a . Ultimately, the compression surface  122   b  seats in the recess  185   c  and thereby is biased against retreat from the recess  185 . This prevents accidental reverse rotational slippage of the locking element  185  from occurring and moving the locking element to the unlocked position. The locking element  185  can still be moved back to the unlocked position if the surgeon desires to correct an error or to readjust the components, if the surgeon uses the wrenching tool  142  and applies enough force to overcome the force that seats the compression surface  122   b  in the recess  185   c . 
     In addition, as the locking element  185  is moved to the locked position, the rod-receiving channel  190 , presented by the locking element  185  when the locking element  185  is in the unlocked position, gradually doses. Ultimately, the channel  190  is completely dosed. This prevents the rod  195  from laterally exiting the locking element  185 . Further, the walls  190   a  and  190   b  of the channel  190  are gradually compressed toward one another, until they ultimately secure the rod  195  between them so that the rod  195  is fixed relative to the coupling element  150 . This prevents the rod  195  from axially exiting the locking element  185 . 
     In addition, as the locking element  185  is moved to the locked position, the locking element  185  itself is gradually compressed within the coupling element  150  until it ultimately is secured within the coupling element  150 . More specifically, as the rod  195  seeks to radially force the walls  190   a  and  190   b  apart, the outer surface of the locking element  185  seeks to radially push against the inner surface  150   c  of the coupling element  150 . However, the structural integrity of the coupling element  150  resists the radial force of the locking element  185 , and the locking element  185  is secured in the coupling element  150  thereby. This prevents the locking element  185  from axially exiting the coupling element  150 . 
     In this manner, once the locking element  185  is moved to the locked position, the screw  120 , the coupling element  150 , the locking element  185 , and the rod  195  are all fixed relative to one another and to the bone. When all of the locking elements  185  are so positioned, the implant is installed. After the surgeon moves all of the locking elements in the orthopedic rod implantation apparatus to the locked position, he doses the wounds of the patient and the surgery is complete. 
     While there has been described and illustrated embodiments of a screw and coupling element assembly, for use with an orthopedic 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 invention. The invention shall, therefore, be limited solely by the scope of the claims appended hereto.