Abstract:
A device for spinal stabilization includes bone anchors, a metal cord, and a spacer. The materials of the cord and spacer are chosen to allow the physician to customize the stiffness of the stabilization device based on a particular patient&#39;s needs. Each bone anchor has a clamping mechanism for securing the cord to the bone anchor. In an assembled and implanted state of the device, the spacer is positioned between two neighboring bone anchors, thereby impeding the motion of the bone anchors toward each other; the cord is clamped to the bone anchors, thereby impeding the motion of the bone anchors away from each other. By increasing or decreasing the tension in the cord during implantation, the physician can create a stabilization device that is either relatively stiff or relatively flexible to accommodate the specific needs of the patient.

Description:
FIELD  
       [0001]     The invention relates to spinal fixation systems and devices. More particularly, the invention relates to an improved fixation device having bone anchors and interconnecting components.  
       BACKGROUND  
       [0002]     Traditional spinal stabilization systems include the type which has multiple bone anchors for attaching to the respective vertebrae, and a linking member secured to the bone anchors to prevent or limit the motion between the vertebrae. Anchors typically include pedicle screws and/or hooks. Linking members typically include rods and/or plates. Alternatively, the linking member may be a substantially flexible cord, which permits relative motion between the stabilized vertebrae, and a resilient spacer between each pair of linked bone anchors. One exemplary system is made of braided Polyethylene Terephthalate (PET; available under the Sulene™ brand from Sulzer Orthopedics, Ltd., Baar, Switzerland). An exemplary spacer is made of Polycarbonate Urethane (PCU, also available under the Sulene™ brand name from Sulzer Orthopedics, Ltd.).  
         [0003]     In the prior exemplary systems, the spacer generally operates to resist, without completely preventing, motion of the bone anchors toward each other, while the cord operates to resist motion of the bone anchors away from each other. Because the spacer and cord are generally more flexible than rigid metal rods or plates, systems employing a cord and/or spacer arrangement generally permit movement between vertebrae and thus provide a more dynamic stabilization system than those employing rigid linking members. Additionally, the interconnections between the anchors, spacers, and cords may permit relative motions that are meant to be prevented with rigid metal systems, which also provide a more dynamic stabilization system than those employing rigid linking members.  
         [0004]     Each of the foregoing types of systems may have advantages and disadvantages in a given patient condition. Because of the diverse patient conditions requiring treatment, however, there is a need in the art for a spinal stabilization system that combines the advantages of rigid and dynamic stabilization systems, and permits the physician to modify the rigidity of the system based on the needs of the patient.  
       SUMMARY  
       [0005]     The invention disclosed herein is aimed at providing a method and apparatus for providing improved spinal treatment to individual patients. According to one embodiment, the present invention is an implantable orthopedic stabilization device comprising a cord, at least one pair of bone anchors, wherein each bone anchor includes a bone attachment portion adapted to engage a vertebra, and a head portion attached to the bone attachment portion and including a cord receiving portion and a clamping mechanism adapted to secure the cord to the bone anchor, and a substantially incompressible spacer having a channel sized to receive the cord therethrough. The spacer is adapted to be positioned between and maintain a predetermined spacing between the head portions of the at least one pair of bone anchors. The spacer further includes an end surface configured such that the end surface can articulate along a face of at least one of the head portions of the at least one pair of bone anchors.  
         [0006]     In another embodiment, the present invention is a method of stabilizing a portion of a spinal column of a patient comprising implanting first and second bone anchors into respective vertebrae with a substantially incompressible spacer positioned between head portions of the bone anchors, wherein each head portion has a cord receiving portion, and wherein the spacer comprises a channel adapted to receive a cord therethrough. Next, the method includes passing a cord through the cord receiving portions and the channel, and then securing the cord to at least one of the bone anchors. Next, the method includes applying a tensile load to the cord such that the head portions exert a compressive force on the spacer with a face of each of the head portions bearing upon one of a first and second end surface of the spacer. The step of applying the tensile load further includes setting the tensile load such that the compressive force permits a desired amount of articulation of the faces of the head portions along the respective end surfaces of the spacer.  
         [0007]     While the invention is amenable to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and are described in detail below. The intention, however, is not to limit the invention to the particular embodiments described. On the contrary, the invention is intended to cover all modifications, equivalents, and alternatives falling within the scope of the invention as defined by the appended claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     The objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings in which:  
         [0009]      FIG. 1  is a lateral elevation view of a human spinal column;  
         [0010]      FIG. 2  schematically illustrates a posterior elevation view of the human spinal column with two spinal stabilization devices, according to one embodiment of the present invention, implanted therein;  
         [0011]      FIG. 3  is a perspective view of the spinal stabilization device depicted in  FIG. 2 ;  
         [0012]      FIG. 4  is a cross-sectional schematic view of the spinal stabilization device depicted in  FIG. 3 ;  
         [0013]      FIG. 5  is a perspective view of the spinal stabilization device shown in  FIG. 3 , with one of the pedicle screws tilted relative to its position shown in  FIG. 3 ; and  
         [0014]      FIG. 6  is a posterior elevation view of a human spinal column with two multi-level spinal stabilization devices according to an embodiment of the present invention implanted therein. 
     
    
       [0015]     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.  
       DETAILED DESCRIPTION  
       [0016]      FIG. 1  illustrates a human spinal column  2  including vertebrae  5  belonging to one of a cervical region a, a thoracic region b, a lumbar region c and a sacral region d of the spinal column  2 . Each vertebra  5  includes a superior end plate  6  and an inferior end plate  7 . An intervertebral disc is positioned in an intervertebral space  9  between adjacent vertebrae  5 .  
         [0017]      FIG. 2  is a posterior elevation view of a patient&#39;s spinal column  2  into which a pair of spinal stabilization devices  100  according to the present invention are implanted between vertebrae L 3  and L 4 . As shown in  FIG. 2 , each of the stabilization devices  100  includes a pair of bone anchors, which in this case are pedicle screws  110   a  and  110   b , a cord  120 , and a spacer  130  which, as discussed in more detail below, is substantially non-compressible and substantially rigid. As illustrated, each pedicle screw  110   a ,  110   b  is driven into a respective vertebra  5 . In the illustrated embodiment, the pedicle screw  110   a  is implanted in the L 3  vertebra, and the pedicle screw  110   b  is implanted in the L 4  vertebra. As will be understood by those skilled in the art, however, fixation of other vertebral pairs can be accomplished using the device of the present invention.  
         [0018]      FIG. 3  is a perspective view of the stabilization device  100  according to one embodiment of the present invention. As illustrated in  FIG. 3 , each pedicle screw  110   a  and  110   b  includes a head portion  140  with a top surface  142 , and a threaded shank portion  146 . As will be understood by those skilled in the art, the threaded shank portions  146  are adapted for implantation into a vertebra  5  of the patient. Each head portion  140  includes a threaded set screw  150 .  
         [0019]     As illustrated, the spacer  130  includes end portions  132  and  134 , and is disposed between and in contact with the pedicle screws  110   a  and  110   b . The cord  120  is a cord, cable or wire and is threaded through the spacer  130  and clamped to each pedicle screw  110   a  and  110   b  using the set screws  150 . In other embodiments, other clamping mechanisms may be used besides set screws  150  to secure the cord  120  in the pedicle screws  110   a  and  110   b . When so clamped, the cord  120  serves to substantially prevent displacement of the screw heads  140  of the pedicle screws  110   a  and  110   b  away from each other. The spacer  130  operates to maintain the relative spacing of the pedicle screws  110   a ,  110   b , and to substantially prevent displacement of the screw head  140  of the pedicle screws  110   a  and  110   b  toward each other.  
         [0020]     In the illustrated embodiment, the cord  120  and the spacer  130  each have a generally cylindrical cross-sectional shape, although in other embodiments, they may have different shapes (e.g., rectangular, elliptical).  
         [0021]      FIG. 4  is a cross-sectional view of the spinal stabilization device  100 . As shown in  FIG. 4 , each screw head  140  includes a screw aperture  156  defined by an inner wall  158 . As discussed in more detail below, the aperture  156  operates as a cord receiving portion of the screw head  140 . A threaded hole  170  extends from the screw aperture  156  to the top surface  142 . As further shown, the spacer  130  includes a channel  160  positioned to generally align with the screw apertures  156  in the pedicle screws  110   a  and  110   b  such that the cord  120  can be extended through the screw apertures  156  and the channel  160 . As illustrated, in each of the pedicle screws  110   a ,  110   b , the threads of the sets screws  150  match the threads of the threaded hole  170  such that each set screw  150  can be turned to advance into the screw apertures  156  to clamp the cord  120  against the inner wall  158 .  
         [0022]     As further shown, the screw head  140  of each of the pedicle screws  110   a  and  110   b  may include generally spherical concave faces  180  and  184  disposed approximately 180 degrees apart. Additionally, each of the end portions  132  and  134  of the spacer  130  may include a generally spherical convex face  190  and  194 . As shown, the generally convex faces  190  and  194  are sized and shaped to mate with the generally concave faces  180  and  184 , respectively. In some embodiments (not shown), the faces  180 ,  184  of the screw heads  140  may be generally spherical convex faces, and accordingly, the mating faces  190 ,  194  of the spacer  130  have concave profiles. Alternatively, one of the faces  180  and  184  may be concave, and the other convex, in which case, the respective matching face  190 ,  194  of the spacer  130  will be profiled to match as appropriate.  
         [0023]     The illustrated configuration operates to optimize the contact stress and friction between the end portions  132 ,  134  of the spacer  130  and the pedicle screw heads  140 . Additionally, this embodiment further permits three-dimensional angular displacement of the pedicle screws  110   a  and  110   b  relative to the spacer  130  by allowing the concave faces  180  and  184  to rotatably articulate along the convex faces  190  and  194 , respectively, of the spacer  130 . As further shown, to permit such angular displacement while minimizing wear to the metal cord  120  and preventing kinking of the cord  120  by the ends of the spacer  130 , the channel  160  may flare outward such that the channel  160  is larger near the end portions  132  and  134  than near the middle of the spacer  130 .  
         [0024]     In yet other embodiments (not shown), the faces  180 ,  184 ,  190 , and  194  may be generally flat, thus shaped to permit relative rotation of the spacer  130  relative to the screw heads  140  in one dimension defined by the mating flat faces. In one such embodiment the screw faces  180  and  184  may not be perpendicular to the axis of the shank of the screw  146 , thereby permitting the device  100  to adapt to a patient&#39;s anatomy or pathology.  
         [0025]      FIG. 5  is a perspective view of the stabilization device  100  in which the threaded shank portion  146  of the pedicle screw  110   b  is tilted toward the pedicle screw  110   a . As shown, the longitudinal axis  250  of the pedicle screw  110   b  is shown to have tilted toward the other pedicle screw  110   a  by an angle φ from its original position  260  in which the shank portions  146  of the pedicle screws  110   a  and  110   b  are substantially parallel. As will be apparent to those skilled in the art, tilting of the pedicle screw  110   b  as shown in  FIG. 5  would not be possible if the interface between the spacer  130  and pedicle screw heads  140  were not the convex/concave configuration according to one aspect of the present invention. Tilting of the screw  110   a  relative to the screw  110   b  permits the stabilization device  100 , when implanted, to have the proper relative configuration with respect to anatomic features such as the vertebral end plates  6  and  7 .  
         [0026]     The spacer  130  can be of any suitable material that is substantially rigid and substantially non-compressible. As used in this context, “rigid” means having a stiffness greater than that of the unreconstructed spine, and “non-compressible” means having a compressive strength sufficient to effectively prevent the heads  140  of the pedicle screws  110 ,  110   b  from being displaced toward each other under loads created by the patient&#39;s bodily movements when the device  100  is implanted as shown in  FIG. 2 . Suitable materials include metals or alloys, such as titanium and its alloys, stainless steel, ceramic materials, rigid polymers including polyether-etherketone (PEEK™), and substantially rigid and non-compressible composite materials.  
         [0027]     The cord  120  is generally configured and sized to be substantially resistant to strain or elongation under tensile loads that may be applied by the patient&#39;s bodily movements (e.g., bending and twisting of the spinal column). In one embodiment, the cord  120  may be a biocompatible metal (e.g., titanium and its alloys, stainless steel) wire or cable. Such a cord  120  can be tensioned much more tightly than a polymeric cord, thereby creating a stiffer stabilization system than existing dynamic systems. Thus, by varying tension in the cord  120 , the spinal stabilization device  100  can therefore achieve a wider range of flexibility than a device with a polymeric cord.  
         [0028]     Thus, in the assembled and implanted state of the device  100 , the cord  120  effectively prevents displacement of the heads  140  of the pedicle screws  110   a ,  110   b  away from each other. The substantially incompressible spacer  130  substantially prevents movement of the heads  140  of the pedicle screws  110   a ,  110   b  toward each other.  
         [0029]     In operation, the pedicle screws  110   a  and  110   b  are attached to their respective vertebrae, in the embodiment illustrated in  FIG. 2 , the L 3  and L 4  vertebrae. The cord  120  is threaded though the screw apertures  156  and the spacer aperture  160 , with the spacer  130  sequenced between the pair of pedicle screws  110   a ,  110   b . The cord  120  is then tensioned to a desired amount against the pedicle screws  110   a ,  110   b  so that the pedicle screws  110   a ,  110   b  are biased against and exert a compressive load on the spacer  130  and the concave faces  180 ,  184  bear upon the convex faces  190 ,  194  of the spacer  130 . The physician may vary or customize the stiffness of the stabilization device  100  by adjusting the tension applied to the cord  120 . In general, the greater the tension in the cord  120 , the more frictional resistance will impede articulation of the concave faces  180  and  184  along the convex faces  190  and  194 . Thus, the tension in the cord  120  in the assembled and implanted stabilization device  100  generally determines the overall stiffness of the device.  
         [0030]     For example, the stabilization device  100  may be made substantially rigid by tensioning the cord  120  to a high degree. In such a case, the large frictional forces produced substantially prevent articulation of the convex faces  180  and  184  along the concave faces  190  and  194  of the spacer  130 . Alternatively, the physician may choose to apply a lesser degree of tension to the cord  120 , thus allowing articulation of the convex faces  180 ,  184  along the concave faces  190 ,  194 , which in turn creates a more flexible stabilization device  100 . Cord tension may also be varied from left side to right side. In such case, the cord  120  may be tensioned to a high degree between pedicle screws  110   a  and  110   b  on one side but tensioned to a lesser amount on the other side. This may be advantageous for patients requiring differing amounts of stabilization between left and right sides due to their disease (e.g., deformity or scoliosis). The amount of tension applied can thus be varied based on the particular patient&#39;s needs. In those cases where the tension in the cord  120  is minimal, or just enough to bring the various parts of the stabilization device  100  in contact, there may be very little resistance to articulation. In such cases the stabilization device  100  may predominantly provide the desired spacing between the vertebrae.  
         [0031]     Tensioning of the cord  120  can be done, for example, by tightening one of the set screws  150  to clamp the cord  120  to one of the pedicle screws  110   a  or  110   b  and then tensioning the cord  120  against the other pedicle screw  110   a  or  110   b . The set screw  150  of the second pedicle screw  110   a  or  110   b  is then tightened to clamp the cord  120  to that pedicle screw  110   a  or  110   b . The tension in the cord  120  is thus maintained, and the pedicle screws  110   a ,  110   b  remain biased against the spacer  130 .  
         [0032]      FIG. 6  depicts a posterior elevation view of a human spinal column with two multi-level spinal stabilization devices  300  according to an embodiment of the present invention implanted therein to stabilize the L 3 , L 4  and L 5  vertebrae. As shown in  FIG. 6 , each stabilization device  300  includes three bone anchors, in this case pedicle screws  310   a ,  310   b  and  310   c , rigid spacers  320   a  and  320   b , and a metal cord  330 . In the illustrated embodiment, the pedicle screws  310   a ,  310   b  and  310   c  are implanted in the L 3 , L 4 , and L 5  vertebrae, respectively. As will be apparent to those skilled in the art, the present invention also includes stabilization devices having more than two levels.  
         [0033]     As illustrated, the spacer  320   a  is disposed between the pedicle screws  310   a  and  310   b , and the spacer  320   b  is disposed between the pedicle screws  310   b  and  310   c , and the cord  330  extends through the pedicle screws  310   a ,  310   b ,  310   c  and the spacers  320   a  and  320   b . The pedicle screws  310   a ,  310   b ,  310   c , the spacers  320   a ,  320   b , and the cord  330  may be constructed and configured to operate substantially the same as the corresponding components described above with respect to the single level stabilization device  100 .  
         [0034]     In one embodiment of the multi-level stabilization device  300 , the cord  330  may be clamped to each of the pedicle screws  310   a ,  310   b  and  310   c . Alternatively, the cord  330  may be clamped only to the two distal-most pedicle screws  310   a  and  310   c . In one embodiment, the cord  330  may be tensioned to different degrees for different levels. For example, the segment of the cord  330  between the pedicle screws  310   a  and  310   b  be may be highly tensioned, while the segment of the cord  330  between the pedicle screws  310   b  and  310   c  may be tensioned to a lesser amount. As a result, the stabilization system  300  will be more rigid between the pedicle screws  310   a  and  310   b  than between  310   b  and  310   c . Accordingly, fixation of the L 3  and L 4  vertebrae will be more rigid than fixation of the L 4  and L 5  vertebrae. As discussed above, the use of a metal cable or wire for the cord  330  allows for a much wider range of stiffness of the portion between any pair of pedicle screws as compared to an existing dynamic stabilization system. Thus, in a multi-level system, a stabilization device with a more patient-specific, side-specific, and level-specific stiffness profile can be achieved.  
         [0035]      FIG. 7  is a perspective view of a stabilization device  100  including an exemplary tensioning mechanism  400  according to one embodiment of the present invention. The tensioning mechanism operates to permit the surgeon to apply the desired amount of tension to the cord  120 . As shown in  FIGS. 7-9 , the tensioning mechanism  400  may include a cable tensioning nut  406 , a body  412 , a collet tightening nut  420 , and a collet  516 . (Tensioning mechanism  400  can be utilized for a cable  120  or cord  120  but is described in terms of a cable  120 ). The cable tensioning nut  40  may be that portion of the tensioning mechanism  400  that contacts the head portion  140  of the pedicle screw  110 . The collet tightening nut  420  may be positioned on the opposite side of the body  412  from the cable tensioning nut  406 . The collet  516  may be positioned around the cable  120  and in an interior portion of the body  412 . One end of the collet  516  may be contacted by the collet tightening nut  420 .  
         [0036]     Each of the cable tensioning nut  402 , body  412 , and collet tightening nut  420  may further include flats  432 ,  462 , and  482 , respectively, which are configured to be engaged by a tool, such as a wrench, for holding or turning that portion of the cable tensioning device  400 . Other tool engaging structures may likewise be incorporated in other designs. The collet tightening nut  420  may further include a cable exit hole  426  and the cable tensioning nut  406  may further include a cable entrance hole  436  and a pedicle screw contact surface  442 . The body  412  may further include external threads  466  that cooperate with internal threads on the cable tensioning nut  406  and external threads  490  that are engaged with internal threads on the collet tightening nut  420 . The body  412  may further include a shaped interior hollow portion (also known as a bore, cavity or passage) in a shape that tapers downwardly from the collet tightening nut  420  towards the cable tensioning unit  406 .  
         [0037]     In operation, the cable tensioning device  400  is first slipped over the wire  120  until the pedicle screw contact surface  442  of the cable tensioning nut  406  contacts the head  140  of the pedicle screw  110 . The body  412  is then grasped by a wrench or other tool such that it can be prevented from twisting or moving around the cable  120 . The collet tightening nut  420  may then be grasped and rotated such that the collet  516  is pushed through the internal channel in the body  412  towards the cable tensioning unit  406 . Because of the tapering hollow portion in the body  412  the collet will be pressed, tightened or crimped inwards around the cable  120 . The collet tightening nut  420  may push the collet  516  into the hollow portion of the body  412  far enough to effectively secure the collet  516 , and therefore the body  412 , to the cable  120  in that position. It has been found that once the collet tightening nut  420  is tightened and the collet  516  is secured around the cable  516 , loosening the collet tightening nut  420  does not then allow the body  412  or the cable tensioning device  400  as a whole to slide over the cable  120  again. In alternative embodiments, however, such a releasable system may be realized.  
         [0038]     The body  412  may then be again secured (or may still be secured from before) and the cable tensioning nut  406  may be rotated such that threads on the cable tensioning nut  406  interact with the external threads  466  of the body  412  so as to push the body  412 , collet  516 , and collet tightening nut  420  away from the head  140  of the pedicle screw  110 . This action, in effect, lengthens the cable tensioning device such that, because the body  412  and cable tensioning nut  406  are secured in relation to one section of the cable  120  by action of the collet  516 , the cable  120  is drawn through the head  140  of the pedicle screw  110 . The body  412  is moved by continued rotation of the cable tensioning nut  406  until the desired tension on cable  120  is achieved. The set screw  150  is then tightened in the head  140  of the pedicle screw  110  to secure the cable  120  in the desired position.  
         [0039]     After the set screw  150  is tightened so as to secure the cable  120 , the cable tensioning nut  406  is then rotated in the opposite direction so as to provide some amount of slack in the cable  120  between the cable tensioning device  400  and the head  140  of the pedicle screw  110 . The cable  120  can then be cut between the cable tensioning device  400  and the head  140  so as to trim and remove the excess cable and the cable tensioning device  400 .  
         [0040]     As may be appreciated, in further embodiments various cord tensioning devices may be employed, such as, for example, the use of pliers or other devices to pull the cord to the desired tension.  
         [0041]     The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.