Abstract:
A multi-piece disc replacement implant for replacing a disc removed by a discectomy including an upper plate member, a lower plate member, and an intermediate resilient member providing movement between te two plate members replicating the natural movement of the spine. The plate members are rigid and have orthogonal sidewalls forming an enclosure. The resilient member is an elastic solid or a multi-chamber balloon structure of fluid-filled sacks that collectively define a non-uniform shape such as an oblate spheroid, or a helically coiled string of beads. Such an implant is capable of supporting the compressive and cyclic loads required of a natural disc. The upper and lower plate members are cooperatively formed to selectively limit the allowable range of motion in any given direction and a provided with protrusions to be received in one or more channels cooperatively formed in the vertebrae and secured in place by a bone screw.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a continuation of co-pending U.S. patent application Ser. No. 12/653,086 for a “Bone Anchoring Member With Clamp Mechanism” filed Dec. 8, 2009 from which priority is derived and which is incorporated herein by reference. U.S. patent application Ser. No. 12/653,086 is itself a continuation-in-part of U.S. patent application Ser. No. 12/462,127 for “Bone Anchoring Member” filed Jul. 29, 2009 which is also incorporated herein by reference and which derives priority from U.S. provisional application 61/137,255 filed on Jul. 29, 2008 which is further incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the invention 
         [0003]    The present invention relates generally to devices and methods for treating spinal disorders and more specifically to a fixation assembly for retaining vertebrate endoskeletal members in a desired fixed spatial relationship. 
         [0004]    2. Description of the Background 
         [0005]    A variety of devices are known for the fixation of endoskeletal members or bones in humans and animals. Fixation of bones may be temporary in order to allow for normal healing, as with a break in the long bones of the limbs, or permanent in order to provide support and alignment of the skeletal members. The latter is often the case with respect to the vertebrae of a spinal column where support and spatial fixation are necessary due to injury or disease. Even where vertebral or spinal healing occurs after fixation, the means and devices of fixation often remain in position for life. Such fixations means and devices generally include hardware such as bone fasteners, plates, rods and connectors. 
         [0006]    Bone fasteners utilized in conjunction with spinal fixation often include a pedicle screw or screws that are anchored by threaded engagement into the pedicle of each of the vertebrae that are to be maintained in a desired spatial relationship along with one or more connector assembly or assemblies for engaging implanted support rods or plates with the screw. One or more longitudinal support rods generally extending longitudinally along the spinal column is connected securely to the pedicle screw by the assembly in a manner that allows the vertebrae to be secured and maintained in a desired alignment. In order to achieve the desired stability, the bone fasteners must be attached securely to the vertebrae and connected firmly to the rod and/or one another. 
         [0007]    A variety of means have been utilized to connect the rod to the pedicle screws or other bone fasteners. The secure placement and alignment of a rod between pedicle screws or the placement of a plate linking two or more firmly implanted screws can be complicated by spinal geometry and the angular orientation of the screws and exacerbated by any deformity of the spinal column. Precise alignment with all of the bone fasteners and secure connections is desirable to decrease the possibility that unanticipated and undesired stresses will cause the bone bodies or vertebrae to fracture or the screws to loosen over time. It is, therefore important that bone fastener assemblies be provided and implanted so as to minimize the likelihood of the establishment of undesirable stresses. 
         [0008]    Thus, it is an object of the present invention to securely place, retain and align a rod or plate with a pedicle screw or screws over the primary longitudinal axis of the pedicle screw in order to minimize the likelihood of the establishment of undesirable stresses and to provide for ease of installation. 
         [0009]    It is further an object of the present invention to provide a bone anchor that is durable and easy to implant yet which provided the surgeon with the versatility to adapt to a wide variety of spinal conditions. 
       SUMMARY OF THE INVENTION 
       [0010]    Accordingly, there is provided a pedicle screw for insertion into the pedicle portion of a vertebra and having at its exposed end a threaded rod connected in a generally parallel orientation to the longitudinal axis of the pedicle screw but laterally offset from the longitudinal axis of the pedicle screw. The threaded rod may be secured directly to the screw but is preferable provided with a collar that is captured by an enlarged screw head to seat, orient and retain the threaded rod in position. A locking arm and retaining nut are provided over the threaded rod for affixing a spinal rod to the pedicle screw substantially over and aligned with the longitudinal axis of the screw. A yoke may be provided in the collar also to receive and secure the rod. An alternate cap and clamping elements enable the incorporation of cross links between multiple screws and/or rods. 
         [0011]    The foregoing objects, features and attendant benefits of this invention will, in part, be pointed out with particularity and will become more readily appreciated as the same become better understood by reference to the following detailed description of a preferred embodiment and certain modifications thereof when taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    In the drawings: 
           [0013]      FIG. 1  is a perspective view of an embodiment according to the present invention. 
           [0014]      FIG. 2  is a front elevation view of an embodiment according to the present invention after removal of the stem. 
           [0015]      FIG. 3  is a front perspective view of an embodiment according to the present invention after removal of the stem. 
           [0016]      FIG. 4  is a rear perspective view of an embodiment according to the present invention after removal of the stem. 
           [0017]      FIG. 5  is an elevation view of an embodiment according to the present invention. 
           [0018]      FIG. 6  is a section through the elevation of  FIG. 5 . 
           [0019]      FIG. 7  is a partial detail view of the section of  FIG. 6 . 
           [0020]      FIG. 8A  is a perspective view of the cap. 
           [0021]      FIG. 8B  is side view of the cap. 
           [0022]      FIG. 8  is a bottom view of the cap. 
           [0023]      FIG. 9A  is perspective view of the cup. 
           [0024]      FIG. 9B  is partial section view of the cap. 
           [0025]      FIG. 10A  is perspective view of the screw. 
           [0026]      FIG. 10B  is top view of the screw. 
           [0027]      FIG. 10C  is an elevation of the screw. 
           [0028]      FIG. 11A  is top view of the yoke. 
           [0029]      FIG. 11B  is perspective view of the yoke. 
           [0030]      FIG. 11C  is side view of the yoke. 
           [0031]      FIG. 11D  is a section view of the yoke at cut A-A of  FIG. 11C . 
           [0032]      FIG. 12  is a perspective view of an alternate embodiment according to the present invention. 
           [0033]      FIG. 13  is a section view of an alternate embodiment according to the present invention. 
           [0034]      FIG. 14  is a side view of an alternate embodiment according to the present invention. 
           [0035]      FIG. 15  is a perspective view of an element of the alternate embodiment of  FIG. 16 . 
           [0036]      FIG. 16  is a perspective view of an alternate embodiment according to the present invention. 
           [0037]      FIG. 17  is a perspective view of an alternate embodiment according to the present invention. 
           [0038]      FIG. 18  is a perspective view of an alternate embodiment according to the present invention. 
           [0039]      FIG. 19  is a perspective view of the nut. 
           [0040]      FIG. 20  is a transparent side view of the nut. 
           [0041]      FIG. 21A  is perspective view of an alternate embodiment of the present invention. 
           [0042]      FIG. 21B  is a top view of an alternate cap according to the present invention. 
           [0043]      FIG. 21C  is a side view of an alternate cup according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0044]    With reference to  FIGS. 1-6 , a preferred embodiment of the present invention  1  comprises a pedicle screw  10  for insertion into the pedicle portion of a vertebra and having at its exposed end a threaded rod or stem  30  connected in a generally parallel orientation to, but laterally offset from, the longitudinal axis of the pedicle screw  10 . It is observed that for purposes of this application the longitudinal axis of the pedicle screw  10  is defined as the vertical axis and references to the vertical are made with respect to this axis. The figures provided herewith generally depict the pedicle screw on the vertical axis although it is, of course, understood that the axis of the pedicle screw is unlikely to be vertical one implanted in the vertebra of a patient. Thus, the stem  30  is substantially vertical when connected to the pedicle screw  10 . The stem  30  is preferable provided as part of a cup assembly  29  that is captured by an enlarged head of screw  10  and may include a yoke  26  for receiving spinal rod  35 . A cap  45  and nut  40  are provided over stem  30  securing the spinal rod  35  in the yoke  26  and compressing and securing the entire assembly. A distal portion of stem  30  may be remove after nut  40  has been secured in place as seen in  FIG. 2 . 
         [0045]    With specific reference to  FIGS. 10A-10C , screw  10  is provided with a threaded shaft  15  for engagement with a bore hole prepared in the pedicle region of the vertebra. Threaded shaft  15  may be of any known bone screw design and is depicted in a preferred form which is tapered to a self tapping point for insertion into the prepared hole. A generally spherical head  20  is provided at the exposed end of shaft  15 . Head  20  has a diameter greater than the major diameter of the threaded shaft  15  as seen in  FIG. 10C  and is preferably round in plan view ( FIG. 10B ) but may be provided with a series of side cut features  21  for positive engagement with other elements of the device further described below. Side cut features  21  are preferably evenly spaced radially about the longitudinal axis of the shaft  15  as extended through the head  20 . Three such side cut features  21  are depicted in a preferred embodiment as seen in  FIG. 10B  but a greater or smaller number may be employed. As depicted in  FIG. 10A , a keyed (non-round) recess  23  is provided in the top surface of the head  20  coaxial with the longitudinal axis of the threaded shaft  15  in order to facilitate rotational insertion of the screw  10  into the bone by insertion of a cooperative tool such as a hexalobe or Allen wrench into the recess. 
         [0046]    With reference to  FIGS. 9A and 9B , a cup  29  having a threaded stem  30  affixed to a collar  31  is provided. Prior to rotational insertion of the screw  10  into the bone as described above the shaft  15  of the screw  10  is inserted through the generally circular central bore  80  of the collar  31  from above, as seen in  FIG. 6 . The screw  10  is advanced through the central bore until the screw head  20  enters the central bore. The diameter of the screw head  20  is small enough to enter the central bore  80  from above but is too large to pass through the tapered lower exit opening, as seen in  FIG. 7 . The lower exit opening is provided with an annular tapered lip  59  having a maximum diameter large enough to receive the head  20  but tapered to a minimum diameter smaller than the diameter of the head  20  such that the cup is captured on the threaded shaft  15  with the cup contacting the screw head only at the annular tapered lip, as seen in  FIG. 6  as well as in  FIG. 7 . So captured, the longitudinal axis of the stem (the stem axis) is vertical although it may deviate from the vertical by up to  30 degrees at the discretion of the surgeon. The taper angle α l  of lip  59  is preferably from 18° to 22° from the stem axis where it to engages the screw head  20  to induce deformation at the side cut features  21  under applied compressive force of the nut  40  on stem  30 , as further described below. 
         [0047]    A series of counter bores  27  radially oriented through the walls of the collar  31  into the central bore  80  are preferably provided to engage a rocker arm or reducer tool as an aid in seating the spinal rod  35  during implantation. Counter bores  20  may also act as points of engagement for additional elements of one or more alternate embodiments of the present invention as further described below. Stem  30  is provided extending upward from the collar  31  of cup  29  so as to be parallel to the central axis though the bore  80  and is threaded on its external surface, the threading preferably being #12-28 (¼-20) ACME having rounded cornices. Stem  30  is provided with a break off feature partially along its length to facilitate removal of the distal end of the stem after implantation. The break-off feature is preferably a circumferential score  32  at the desired point of removal although any known means of facilitating break-off may be employed. Stem  30  is also preferably provided with opposing flat sides  33  to facilitate positive orientation of the cap  45  as described below. 
         [0048]    With reference to  FIGS. 11A-11D , after the screw and cup subassembly are joined a yoke  26  is provided within the central bore  80  of the collar for seating and engaging the spinal rod  35 . Yoke  26  may be inserted after the screw/cup subassembly is implanted but is preferably preassembled, a hole  63  being provided to permit insertion of a tool into keyed recess  23  of head  20  to facilitate rotational insertion of the shaft  15  into the bone. A protrusion or lip  58  (see  FIG. 9B ) inside the central bore  80  of the cup  29  is provided to retain the yoke after preassembly. Yoke  26  is provided with an upper surface contoured for engagement with the spinal rod  35  and a lower surface contoured for engagement with the head  20  of screw  10 . The upper surface contour T ( FIG. 11D ) is generally concave on an axis perpendicular to the stem to receive and engage a spinal rod  35  having a circular cross section. However the shape of the concave surface may be altered to accommodate an alternate rod cross section under the principals disclosed here. More specifically, circular rod  35  engages yoke  26  at the tapered edges  64  of the semi-circular trough T, as seen in  FIG. 7 . The upper edges  64  are provided with a taper having an angle, α3, from 36° to 40° ( FIG. 11D ) from the stem axis, again to facilitate deformation of the assembly under compression of nut  40  which enhances the strength of the completed assembly. Yoke  26  may be keyed for positive orientation within the central bore  80  and ensure alignment of the trough T with the desired axis of spinal rod  35 . Similarly, the bottom surface of the yoke  26  is provided with a cup shaped recess C ( FIG. 11D ) in which to receive the rounded top of generally spherical head  20 . The rim of the cup recess C is provided with a conical taper preferably having an angle, α 2 , from 36° to 40° from the stem axis to engage the upper surface of head  20  ( FIG. 7 ) under the described applied compressive force of the nut  40 . The tapered and other contact surfaces described may be provided with a surface texture such as knurling to enhance engagement. 
         [0049]    With reference to  FIGS. 8A-8C , once the screw/cup subassembly is implanted with the yoke  26  in position, the spinal rod  35  is positioned within the trough T. As noted, and if necessary, a rocker arm, reducer tool or similar may engage the lateral bores  27  to position the rod  35  in the trough T. Once positioned the rod  35  is retained by a cap  45  having a collar  47  and a rod arm  49 . The rod arm  49  is preferably formed at its lower surface with a concave trough shaped recess T′ ( FIG. 8B ), complimentary to trough T in the yoke  26 , in which to receive and engage the top surface spinal rod  35 . The rim of the trough T′ of the rod arm  49  may also be provided with a linear taper preferably from 36° to 40° from the stem axis to engage the upper surface of rod  35  or may be provided with a profile that generally matches the upper surface of the cross section of the rod  35 . 
         [0050]    The collar  47  of the cap  45  is generally circular with central aperture  48  for receiving the distal end of stem  30 . Central aperture  48  is sized to receive stem  30  with little play and is provided with opposing flat sides  51  for cooperative engagement with the flat features  33  of the stem  30 . The trough T′ of rod arm  49  is generally aligned with trough T of yoke  26  and the longitudinal axis of the spinal rod  35  when the flat features  33  and  51  of stem  30  and aperture  48 , respectively, are cooperatively engaged. The upper surface of the collar  47  of the cap  45  is preferably formed with a recess in which to receive at least a portion of nut  40  so as to reduce the overall projection of the device from the bone surface. 
         [0051]    With reference to  FIGS. 5 ,  6 ,  19  and  20 , a flange type nut  40  is threaded on to stem  30  and advanced to engage cap  45  and secure the rod  35  between the yoke  26  and the cap  45 . Nut  45  is preferably provided with non-standard #12-28 (¼-20) ACME internal threading having rounded cornices or fillet radiuses on the internal and external thread form edges for engaging threaded stem  30 , although any threading formed for complimentary engagement with the threading of the stem will suffice albeit with reduced assembly strength. 
         [0052]    As described, prior to implantation the screw  10 , cup  29  and yoke  26  are preferably preassembled. After surgically reaching the implantation site a borehole is prepared into the pedicle region of the subject vertebra and the shaft  15  inserted. A tool such as a hexalobe driver inserted into the drive recess  23  in the screw head  20  via the yoke hole  63  to rotationally advance the shaft into the borehole until the lower surface of the collar  31  of cup  29  approaches the surface of the bone but is not constrained by contact with the surface. Stem  30  is generally parallel to the vertical axis and/or normal to the bone surface although the orientation of the cup  29  and stem  30  may be adjusted at this point as necessary to achieve the desired alignment. The spinal rod  35  is positioned in line with the trough of the upper side of the yoke  26  and a cap  45  is slid onto stem  30  such that the flat alignment surfaces  33  and  51  orient the trough of the rod arm  49  over the rod  35 . Nut  40  is threaded onto stem  30  and rotated to draw the cap  45  down the stem and bring the rod  35  with it until rod  35  contacts the trough of yoke  26 . The extended stem  30  permits the surgeon to use the action of nut  40  to draw the rod  35  into the yoke  26 . In certain situations it may be necessary to use additional tools to gain leverage on the rod  35  in order to seat it in the yoke. Counter bores  27  in collar  31  provide a point of engagement for such tools 
         [0053]    Once the rod  35  has been brought to the yoke  26  by cap  45  and the cup adjusted to the surgeon&#39;s satisfaction, nut  45  is further rotated to compress the assembly. The compressive force of the nut  45  generates hoop stresses in the conical taper of the annular lip  59  in the collar  31  of the cup  29  about the screw head  20  causing the stresses at the side cut features  21  of head  20  to exceed the material yield strength and the annular lip to deform. The result is a passive lock in the form of an indent (screw head) and detent (cup). Similar hoop stresses are generated in the conical taper C of the underside of yoke  26  resulting in a similar passive lock in the form of an indent (screw head) and detent (yoke). As a result the pedicle screw  10 , cup  29  and yoke  26  are mechanically secured together as a single unit. The taper of the trough of the upper surface of the yoke  26  (and possibly cap  29 ) deforms in a similar manner on the rod  35  to retain the rod in position although deformation occurs in to a considerably lesser degree between these components. Once nut  45  is tightened to fully compress and seat the components as described the distal portion of stem  30  beyond the break off feature  32  may be snapped off such that the new distal end stem  30  is generally flush with the upper surface of nut  40 . 
         [0054]    It may be desirable in some cases for a surgeon to cross-link a pair of pedicle screw assemblies according to the present invention implanted in left and right pedicles. An alternate embodiment of the present invention is depicted in  FIGS. 12-18  to provide for such a situation, the alternate embodiment being substantially similar to recited preferred embodiment with the exception of an alternate to cap  45  and additional elements to facilitate attachment of a cross-link connector  180 . Alternate cap  145  is provided to replace cap  45  and is similarly provided with a collar  147  and a rod arm  149 . Collar  147  is formed as a circular member with central aperture  148  (not shown) for receiving the distal end of stem  30 . Central aperture  148  is sized to receive stem  30  and is provided with opposing flat sides  151  (not shown) for cooperative engagement with the flat features  33  of stem  30  such that the rod arm  149  is automatically oriented with respect to the upper surface of yoke  26  to receive and retain the rod  35 . The upper surface of the collar portion  147  of the cap  145  is again preferably formed with a recess to receive at least a portion of nut  40  so as to reduce the overall projection of the device from the bone surface. Rod arm  149  is preferably formed at its lower surface with a trough shaped recess in which to receive and engage the top surface spinal rod  35 . The rim of the trough of rod arm  149  may also provided with a linear taper preferably from 36° to 40° as measured from the stem angle. 
         [0055]    The distal portion  190  of the alternate rod arm  149  is elongated as compared to the previously described rod arm  49  and extends the past spinal rod  35 , as seen in  FIG. 14 . The top surface of the distal portion  190  is formed with a ball socket  160  having a semi-spherical bottom for receiving a ball end  181  of cross-link  180 , as seen in  FIG. 13 . Ball socket  160  is preferably formed along a centerline of cap  145  (as viewed from above) in substantial alignment with the longitudinal axes of stem  33  and shaft  15  (see  FIG. 12 ). A lateral channel is provided into the ball socket  160  to permit the cross-link  180  to enter. The lateral channel widens as it leaves the ball socket  160  to permit articulation of the cross-link  180  up to 25° in any direction from the centerline. 
         [0056]    The ball socket  160  is flanked on either side by through-holes  170 . A U-clamp  175  is provided such that one distal arm  173  of the U extends through each through-hole  170 . The inner surface of each arm  173  is cooperatively threaded to receive an externally threaded nut  185  between them. Externally threaded nut  185  is alternately described as a set screw, blind screw, or grub screw. A locking plate  176  is provided over the distal arms  173  and beneath nut  185 , the lower surface of which is formed to engage the ball  181  of the cross-link  180  and secure it in the socket  160  under force of the nut  185 . The U-clamp  175  is further provided with a pin  176  on one side of its proximal end at the base of the U, as seen in  FIG. 13 . The pin  176  engages lateral bore  27  of the cup  29  (see  FIG. 9   a ) to secure the U-clamp to the assembly. As seen in  FIG. 14 , the arms  173  of the U-clamp are angled back towards the stem  30  so as to maintain pin  176  in lateral bore  27  as nut  185  is tightened. The U-clamp angle, α 4 , is preferably between 20° and 35° (see  FIG. 14 ). The through holes  170  are cooperatively angled. As seen in  FIGS. 15 and 16 , locking plate  176  may alternately be formed to enclose nut  185  and to permit distal arms  173  to pass through as the nut is advanced. 
         [0057]    As stated, the described alternate embodiment is utilized when it is desirable in the view of the surgeon to cross-link a pair of pedicle screw assemblies according to the present invention implanted on either side of the spine. The cross-link is preferably one or more rods  180  or cables having a ball end  181 . Where multiple rods are utilized to join to pedicle screw assemblies due to spinal anatomy or deformity the rods must be securely and rigidly joined.  FIG. 18  depicts an additional element of the present invention for joining cross-link rods  180  utilizing a U-clamp  275  substantially similar to U-clamp  175 . 
         [0058]    As above, U-clamp  275  is formed such that the inner surface of each distal arm  273  is cooperatively threaded to receive an externally threaded nut  285  between them. An upper locking plate  276  is provided on its upper side with a central void  260  for rotatably retaining nut  285 . Void  260  flanked by through holes  270  for slideably receiving distal arms  273 , the through holes intersecting the central void  260  such that the threaded surfaces of arms  273  are engaged by nut  285 . The lower surface of the upper locking plate  276  is contour to cooperatively engage the surface of a cross-link  180 . A medial locking plate  376  is similarly provided with through holes  370  for slideably receiving the distal arms  273 . The upper and lower surfaces of the locking plate  376  are provided with a detent  310  between through holes  370  for engaging the surface of a first and second cross link above and below the medial locking plate  370 , respectively. Detents  310  is preferably a linear protrusion having a tapered or triangular cross section. When assembled as depicted in  FIG. 18  with the cross links  180  positioned as desired by the surgeon, nut  285  is rotated to advance the upper locking plate  276  down the distal arms  273 . The upper and lower cross links  180  are thus compressed between the upper locking plate  276  and the lower curve of the U-clamp causing the detents  310  of the intermediate locking plate  376  to engage and deform the surface of the cross links creating an indent, mechanically securing them together as a single unit. The result is a passive lock in the form of an indent (cross link) and detent (intermediate locking plate) that securely joins the cross links and by extension the left and right pedicel screws of the present invention. 
         [0059]    With reference to  FIGS. 21A-21C , yet another alternate embodiment is provided. The embodiment of  FIGS. 21  provides for additional stability of the cap in the absence of a cross through a cap  245  and cup  129  of alternate design. All other elements are substantially similar to those described with respect to the embodiment of  FIG. 1 . Cap  245  is provided with a hooked end  375  extending from the distal portion of rod arm  249 . Hooked end  375  descend to and engages an integrally formed lip  230  on the collar of the cup  229 . The engagement of the hooked end  375  and lip  230  may be along a straight line substantially parallel to the rod  35  in order to minimize the stresses internal stresses or may be in the form of an arc about a vertical axis which provides greater ease of installation but higher localized stress points. The addition of hooked end  375  and lip  230  provides additional stability to cap  245  allowing a reduction and smoothing of the profile of rod arm  249 . 
         [0060]    It should be understood that the disclosure may be used with a variety of pedicle screw designs and sizes. It should also be understood that the disclosure may be constructed of a variety of suitable surgical grade materials including stainless steel and titanium as well as composite materials having suitable strength and corrosion resistance properties should such materials be approved for surgical implantation. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains.