Abstract:
A threaded rod or pedicle screw for insertion into the pedicle portion of a vertebra and having at its exposed end a second threaded rod rigidly connected in a generally parallel orientation to the longitudinal axis of the pedicle screw. The second rod being offset from the longitudinal axis of the pedicle screw and having a locking pin or arm and retaining nut for affixing a rod to the pedicle screw substantially over and aligned with the longitudinal axis of the pedicle screw.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application derives priority from provisional application 61/137,255 filed on Jul. 29, 2008 which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the invention 
     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. 
     2. Description of the Background 
     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 fixations means or devices, which include bone fastener assemblies, often remain in position for life. 
     Bone fastener assemblies utilized in conjunction with spinal fixation often include a pedicle screw assembly or assemblies that are anchored by threaded engagement into the pedicle of each of the vertebrae that are to be maintained in a desired spatial relationship. One or more longitudinal supports, which may comprise plates or rods that extend generally longitudinally along the spinal column, are connected securely to the pedicle screw assemblies in a manner that allows the vertebrae to be 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 appliance. 
     A variety of means have been utilized to connect the rods or plates 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 rigid screws can be difficult due to 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 are desirable to decrease the possibility that unanticipated and undesired stresses can 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 that minimize the likelihood of the establishment of undesirable stresses. 
     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. 
     SUMMARY OF THE INVENTION 
     Accordingly, there is provided a threaded rod or pedicle screw for insertion into the pedicle portion of a vertebra and having at its exposed end a second threaded rod rigidly connected in a generally parallel orientation to the longitudinal axis of the pedicle screw. The second rod being offset from the longitudinal axis of the pedicle screw and having a locking pin or arm and retaining nut for affixing a rod to the pedicle screw substantially over and aligned with the longitudinal axis of the pedicle screw. 
     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 
       In the drawings: 
         FIG. 1  is a perspective view of an embodiment according to the present invention. 
         FIG. 2  is a side view of an embodiment according to the present invention. 
         FIG. 3  is a exploded view of a embodiment according to the present invention. 
         FIG. 4  is a exploded view of an alternate embodiment according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to  FIGS. 1 and 2 , a preferred embodiment of the present invention comprises a pedicle screw having a first threaded shaft  15  for positive engagement of the vertebra, most commonly in the pedicle region. The first threaded shaft  15  may be tapered to a penetrating point as depicted in  FIG. 2  or may be of constant or stepped diameter. It should be observed that corresponding reference characters indicate corresponding parts throughout the several views of the Figures. Although the drawings represent embodiments of various features and components according to the present invention, the drawings are not necessarily to scale and certain features may be exaggerated in order to better illustrate and explain the present invention. The exemplary embodiment set out herein is illustrative of the invention and is not to be construed as limiting the scope of the invention in any manner. 
     In use, one end of first threaded shaft  15  is inserted into the bone while the other end remains exposed at or near the surface of the vertebra. The inserted end of the threaded shaft  15  may be coated or chemically treated to promoted bone growth and may further be provided with recesses, voids or surface textures into which bone may grow to secure the screw  15  in place. 
     A head  20  is provided at the exposed end of shaft  15 . Head  20  may be at, slightly above, or slightly below the surface of the bone when inserted therein. The head  20  preferably has a hexagonal or other keyed exterior profile for positive engagement with a tool used to rotate the first threaded shaft  15  about its longitudinal axis for insertion into the bone. However, in alternate embodiments the exterior profile of head  20  may be round and a keyed (non-round) recess may be 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 into the bone by insertion of a cooperative tool such as an Allen wrench into the recess, as depicted in  FIG. 3 . 
     Head  20  may have a radius equal to or greater than the maximum radius of the threaded shaft  15 . Head  20  may be symmetric about the longitudinal axis of threaded shaft  15 . Alternately, where head  20  is of insufficient radius to properly offset the second threaded shaft  30 , as described below, head  20  may be provided with an eccentric extension sufficient to support the second threaded shaft  30  in its proper eccentric position. 
     Head  20  is further provided with a rod seat  25  substantially perpendicular to the longitudinal axis of shaft  15  at which a rod  35  or a portion of a plate may be engaged. The term rod or spinal rod as used herein with respect to rod  35  includes non-circular rods as well as comparable portions of spinal plates. Rod seat  25  may be shaped as to form a trough cooperative with the cross section of the rod. Where, for example, a circular rod  35  is employed a trough of similar radius may be formed in rod seat  25  in order to increase the area of contact between the rod  35  and the rod seat  25 . Rod seat  25 , whether or not provided with a troughed shape, may further be provided with a textured surface such as linear or diamond knurling to positively engage rod  35 . 
     The second threaded shaft  30  is affixed to the head  20  eccentrically, joining the top surface of the head  20  offset from the longitudinal axis of the first threaded shaft  15  by a distance “O”, as seen in  FIG. 2 . The second threaded shaft  30  is preferably parallel to the longitudinal axis of the first threaded shaft  15  although the second threaded shaft  30  may be angled in relation to the longitudinal axis of the first threaded shaft and may be provided with means to vary the angle prior to engagement with the rod. Second threaded shaft  30  may be integrally formed with the head  20  or may be joined to the head by any known method. In one embodiment as seen in  FIG. 4 , second threaded shaft  30  includes screw threads at its proximal end for threaded insertion into a corresponding internally threaded bore  21  in the head  20 . Screw threads at the proximal end of shaft  30  in this embodiment are preferably of a smaller radius than the unthreaded medial portion of shaft  30  so as to create a shoulder at the transition, the shoulder abutting the surface of head  20  so as to indicate to the surgeon full engagement of the threaded portion and for support of the shaft  30 . Second threaded shaft  30  may likewise be provided with screw threads at its distal end (as in  FIG. 3 ) for engagement with a retaining nut  40 . Second threaded shaft  30  may be threaded along its entire length (as in  FIG. 2 ), smooth walled, or formed to cooperatively engage the surface of rod circular rod  35  in, for example, a hyperboloid form. Alternately retaining nut  40  may be integrally formed with second threaded shaft  30  (so as to enable engagement of the proximal threaded end with the internally threaded recess in the head  20 ) or may be adapted for threaded engagement with second threaded shaft  30 . 
     The offset distance “O” of  FIG. 2  is a function of the radius of circular rod  35  (R1) where O=R1−R2 and where R2 is the radius of the second threaded shaft  30 . Where said second threaded shaft  30  is removably engaged with the head  20 , as when provided with a threaded proximal end, a variety of second threaded shafts  30  having different radii R2 may be provided to the surgeon at the time implantation so that the surgeon may selectively vary the offset O based on the chosen circular rod  35  diameter or shape. The term “radius” used herein implies that rod  35  is a circular in section however the term is intended to encompass a like dimension of rods having non-circular sections such as elliptical or rectangular. 
     As best seen in  FIG. 3 , a locking arm  45  is provided with a collar  47  and a rod arm  49 . Collar  47  is formed as a circular member with central aperture for receiving the distal end of second threaded shaft  30 . In an alternate embodiment, collar  47  may be “U” shaped in order to receive second threaded shaft  30  in the void of the “U” without completely encircling the second threaded shaft  30 . Rod arm  49  may be integrally joined to collar  49  and extends over rod  35  to retain it under compression force between the retaining nut  40  and rod seat  25 . Rod arm  47  may be formed with two substantially orthogonal lengths merged at an elbow, as depicted in  FIG. 1 , or may be formed curvilinear to conform to the exterior surface of rod  35  as depicted in  FIG. 3 . 
     The distal end of rod arm  49  may be retained in a cooperative recess or aperture in head  20  to more securely retain the locking arm in position. However, it should be observed that such engagement is not required and locking arm  45  may be formed in keeping with the present disclosure without extending to contact head  20  at its distal end. 
     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.