Patent Document

BACKGROUND 
     This invention relates generally to spinal fixation devices and more specifically relates to a system and associated method for minimally invasive installation of pedicle screws and spinal rods of spinal fixation constructs. 
     The human spinal column is a highly complex system of bones and connected tissues that provide support for the body and protects the delicate spinal cord and nerves. The spinal column includes a series of vertebrae stacked one atop the other. Each vertebral body includes a relatively strong cortical bone portion forming the outside surface of the body and a relatively weak cancellous bone portion forming the center of the body. An inter-vertebral disc is situated between each vertebral body that provides for cushioning and dampening of compressive forces applied to the spinal column. The vertebral canal containing delicate spinal cords and nerves is located just posterior to the vertebral bodies. 
     A variety of types of spinal column disorders may be caused by abnormalities, disease, trauma or the like and result in debilitating pain as well as diminished nerve function in many cases. One known technique to address many such spinal conditions is commonly referred to as spinal fixation. Surgical implants are used for fusing together and/or mechanically immobilizing adjacent vertebrae of the spine. Spinal fixation may also be used to improve the position of the adjacent vertebrae relative to one another so as to alter the overall alignment of the spine. Such techniques have been used effectively to treat many spinal conditions and to relieve pain suffered by the patient. 
     One particular spinal fixation technique includes immobilizing the spine by using orthopedic spine rods which run generally parallel to the spine. Exposing the spine posterially and fastening hooks or bone screws to the pedicles of the appropriate vertebrae accomplish this. The pedicle anchors or screws are generally placed two per vertebrae, one at each pedicle on either side of the spinal column and serve as anchor points for the spine rods. The aligning influence of the rods forces the spine to conform to a more desirable shape. In many cases, the spine rods are bent to achieve the desired curvature of the spinal column. 
     Installation of such spinal fixation constructs conventionally requires a surgeon to prepare a long incision aligned with the spinal column of a patient. The pedicle screws, hooks or other anchors are then inserted into a number of vertebrae after which the spine rod is located with respect to saddles or U-shaped channels attached to the pedicle screws. The spine rod is then bent to match the relative position of the pedicle screw heads. Visualization of the accuracy of the alignment of the spine rod and the screw heads may be difficult because of visual interference from tissue and blood in the surgical field. Conventional surgical methods require a large midline incision and retraction of skin, muscle and other tissue to provide the surgeon with sufficient visualization of the pedicle bone structure. 
     Bending of the spine rod is performed once the screws are placed into the vertebrae and, therefore, visualization of the accuracy of the placement and configuration of the spine rod is very difficult. Improper alignment and inaccurate bending of the spine rod decreases the effectiveness of the fixation construct and often increases the surgical difficulty and time required for the surgery. In combination with the relatively long incision required for the installation of the spine rod, extended surgical procedures and related difficulties or complications are generally recognized as major contributing influences for extended patient recovery and less than optimal spinal fixation results. 
     SUMMARY OF THE INVENTION 
     This invention addresses these and other shortcomings in the prior art. The devices and methods associated with this invention are used to aid in the surgery for vertebral stabilization using pedicle hooks, screws, anchors and fixation rods. 
     According to various embodiments of this invention, pedicle screws are inserted into the target vertebrae of a patient&#39;s spinal column. The pedicle screw may be cannulated for proper positioning and installation. In one aspect, this invention provides an access system which involves a pedicle screw with incorporated extended tabs that form a single solid pedicle screw assembly The surgical procedure associated with this invention involves making small, discrete incisions for the placement of select pedicle screws. The extended tabs of the pedicle screws retract soft tissue, muscle and the like to thereby provide visibility and access to the head of the pedicle screw. Through the extended tabs, instrumentation such as a spine rod, set screw and other required hardware may be delivered to the pedicle screws. 
     In another aspect of this invention, a collar may be coupled around the extended tabs to provide stability to the construct and a platform for additional components of this invention. 
     Once the rod is secured with the set screws, a frangible joint joining the tabs to the pedicle screw assembly is broken off using appropriate instrumentation and the extended tabs may be removed. The extended length of the tabs for each pedicle screw extend beyond the surface of the skin (i.e., percutaneously) thereby retracting soft tissue and muscle enabling the surgeon to install the spinal fixation construct with smaller discrete incisions as opposed to an extended incision. As such, a more minimally invasive surgical procedure can be accomplished with this invention thereby allowing for visualization of the installation components during the surgery and promoting patient recovery post-surgery. Additionally, the extended tabs provide an indication to the surgeon of the configuration of the patient&#39;s vertebrae thereby enabling the surgeon to pre-bend the spine rod prior to insertion into the patient. The spine rod is inserted into the patient in a minimally invasive procedure through slots provided in the tabs and associated collars when coupled with the tabs. 
     This invention is to be distinguished from reduction screws that are often used in correcting spinal problems, particularly reducing various grades of spondylolisthesis (anteriorly dislocated vertebrae). Reduction screws do not typically reach beyond the skin&#39;s surface when installed, whereas this invention is intended to provide minimally invasive access and soft tissue retraction during installation of a spinal construct. The tabs of this invention are intended to extend beyond the skins surface, independently or in conjunction with the telescoping collar. The extended threads on a reduction screw allows for a set screw engagement with the pedicle screw head to provide leverage against the rod to reduce the vertebral dislocation. On the other hand, the extended tab and pedicle screw combination of this invention allows for set screw engagement to fully seat the rod by pushing it into position. 
     As a result of these and other aspects of this invention, increased efficiency and accuracy is provided for installation of a spinal fixation construct in a minimally invasive atmosphere thereby promoting patient recovery and optimum spinal surgery results. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a side elevational and partial cross-sectional view of a spinal fixation construct being surgically implanted in selected vertebrae of a patient&#39;s spine according to one embodiment of this invention; 
         FIG. 2  is top elevational view of the components of a spinal fixation construct as shown in  FIG. 1 ; 
         FIGS. 3A-3D  are sequential views of a pedicle screw construct being assembled together according to one embodiment of this invention for use in a spinal fixation system; 
         FIG. 4A  is a cross-sectional view of a polyaxial head adapted to be used with a pedicle screw according to one embodiment of this invention; 
         FIG. 4B  is a view similar to  FIG. 4A  with extended tabs on the polyaxial head being removed on a frangible joint according to one embodiment of the invention; and 
         FIG. 5  is an alternative embodiment of a spinal fixation construct according to this invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , various embodiments of a minimally invasive spinal fixation construct  10  and associated installation method are shown. The spinal fixation construct  10  includes a number of pedicle screw assemblies  12 , each of which is inserted into selected vertebrae  14  of a patient. The pedicle screw assemblies  12  are joined together in the spinal fixation construct by a spine rod  16 . According to various aspects of this invention, the individual pedicle screw assemblies  12  may be inserted into the patient through discrete and often individual incisions  18  in the patient&#39;s skin  20 . In certain instances, a single incision  18   a  may be available to provide installation of multiple pedicle screw assemblies  12  in adjacent vertebrae  14  as shown in  FIGS. 1 and 2 . The small, discrete incisions  18  provide the opportunity for insertion of a cannulated pedicle screw  22  via a K-wire  24  inserted through the incision  18  to the precise location on the vertebrae  14  for proper installation of the pedicle screw  22 . While cannulated and other pedicle screw assemblies are shown and described herein, one of ordinary skill in the art will appreciate that other types of vertebrae engaging mechanisms can be utilized such as hooks for anchoring the spine rod to the patient&#39;s spinal column. 
     As shown generally in FIGS.  1  and  3 A- 3 D, a pedicle screw assembly  12  according to one embodiment of this invention includes a pedicle screw  22  having a threaded shaft  26  and a distal tip  28  for insertion and stable positioning into the pedicle area of the patient&#39;s vertebrae  14 . The pedicle screw assembly  12  shown herein is a polyaxial pedicle screw in which a polyaxial body  30  mounted opposite from the distal tip  28  of the screw  22  to a screw head  32  provides for a variety of orientations of the polyaxial body  30  relative to longitudinal axis of the screw  22  as is common with many pedicle screw systems. The polyaxial body  30  coupled to the pedicle screw head  32  includes a saddle or U-shaped channel  34  formed between a pair of spaced arms  36  extending upwardly from the body  30 . The polyaxial body  30  is adapted to receive the spine rod  16  in the saddle or U-shaped channel  34  and the spine rod  16  is securely retained by the polyaxial body  30  via a set screw  38  ( FIG. 4B ) threadably received therein as is common with many known pedicle screw systems. 
     One aspect of various embodiments of this invention includes tabs  40  extending upwardly from the polyaxial body  30  as shown in FIGS.  1  and  3 A- 3 D. A pair of generally arcuate channel-shaped tabs  40  is each coupled to one of the arms  36  of the polyaxial body  30  via a reduced thickness frangible joint  42 . Due to the relative position and configuration of the tabs  40  of the pedicle screw assembly  12  according to one embodiment of this invention, a pair of spaced slots  44  is formed between the tabs  40  and each slot  44  is in communication with the U-shaped channel or saddle  34  of the polyaxial body  30 . Advantageously, the tabs  40  are extended to project through the incision  18  such that a distal end of the tabs  40  is located percutaneously above the patient&#39;s skin  20  when the pedicle screw  22  is inserted into the vertebrae  14  as shown in  FIG. 1 . 
     Another aspect of various embodiments of this invention is also shown in FIGS.  1  and  3 A- 3 D as a collar  46  adapted to slidably couple or telescopically mate with the extended tabs  40  on the pedicle screw assembly  12 . The collar  46  in one embodiment includes a generally cylindrical side wall  48  having a circular cross-sectional configuration. The collar  46  includes a tapered distal end  50  as well as a full-length slot  52  adapted to be aligned with one of the slots  44  formed between the tabs  40  of the polyaxial pedicle screw assembly  12 . A distal partial slot  54  is positioned diametrically opposite from the full-length slot  52  and is adapted to be aligned with a corresponding slot  44  between the tabs  40  on the pedicle screw assembly  12 . The collars  46  are adapted to be telescopically mated with the tabs  40  when the pedicle screw assembly  12  is installed as shown generally in  FIGS. 1 and 2 . 
     A proximal flange  56  is formed opposite from the tapered distal end  50  of the collar  46 . The flange  56  may have a generally rectangular configuration and adapted to be juxtaposed on top of the patient&#39;s skin  20  at the associated incision  18 . The collar  46  provides added stability to the construct and a platform for additional devices such as a light attachment (not shown) to increase the visualization of the surgical site. The rectangular or square profile of the flanges  56  on the tops of the telescoping collars  46  assist in avoiding interference between the collars  46  of adjacent levels. In addition, the top surface of the flanges  56  may include a hole, boss or other feature (not shown) that could be used for attachment of an additional instrument for stabilization or alignment of the construct as needed. 
     An alternative embodiment of the pedicle screw assembly  12  according to this invention is shown in  FIG. 5  in which elements of the invention similar to those of the other embodiments are identified with the same reference numerals. The pedicle screw assembly  12  in  FIG. 5  includes a clocking feature that will inhibit rotation of the collar  46  relative to the tabs  40  and thereby hold the orientation of the slots  52 ,  54  in the collar  46  in line with the slots  44  between the tabs  40 . In the embodiment shown in  FIG. 5 , the clocking feature is provided by two pair of ribs  58   b  formed on the outer surface of the tabs  40  and a pair of single ribs  58   a  formed on the inner surface of the collar  46 . Each of the single ribs  58   a  is positioned between one of the pairs of ribs  58   b  on the collar  46  as the collar  46  slides onto the tabs  40  and the ribs  58   b  capture the rib  58   a  there between to inhibit relative rotation between the tabs  40  and the collar  46  to maintain registration of the slots  44 ,  52 ,  54 . The ribs  58   a ,  58   b  are only one embodiment of the clocking feature according to this invention and the clocking feature could be achieved by using cooperating flats on both the screw tabs  40  and telescoping collar  46  to resist relative rotation. Alternatively, the clocking feature may extend only partially along the height of the tabs  40  and/or collar  46  to inhibit relative rotation for some engagement positions of the collar  46  and tabs  40  while permitting relative rotation for other engagement positions. 
     The telescoping collar  46  not only provides stability to the construct, but also extends the length of retraction in cases where the extended tabs  40  alone do not extend beyond the surface of the skin. The telescoping collar  46  provides for adjustability in the depth of retraction. Additionally, rotation of the collar  46  by about go degrees around the extended tabs  40  allows the telescoping collar  46  to fully seat the rod  16  into the polyaxial body  30 . 
     One advantage provided by this invention is shown generally in  FIG. 1 . With the pedicle screws  22  inserted into the appropriate vertebrae  14  and the extended tabs  40  and collars  46  projecting from the screw assemblies  12  percutaneously, the relative position of the polyaxial bodies  30  is projected percutaneously. As such, the surgeon may bend the spine rod  16  to conform to the position of the polyaxial bodies  30  installed on the vertebrae  14  according to the relative position of the tabs  40  and collars  46  positioned percutaneously. The enhanced visualization and access afforded by the percutaneous positioning of the tabs  40  and collars  46  in the general orientation of the associated polyaxial bodies  30  is a significant advantage for the accurate and precise positioning and bending of the spine rod  16 . Moreover, once the spine rod  16  is appropriately configured, it may be inserted through the slots  44 ,  52  and  54  and into the saddles  34  of the polyaxial bodies  30 . Specifically, one end of the spine rod  16  may be initially inserted into a leading pedicle screw assembly  12  through the slots  44 ,  52 , and  54 . This may be accomplished with an incision  18   a  that is adjacent to a pair of pedicle screw assemblies  12  as shown in  FIG. 1 . The access slots  44 ,  52 ,  54  provided by the tabs  40  and the collars  56  provide communication between the various adjacent pedicle screw assemblies  12  so that the spine rod  16  may be inserted into the saddles  34  of the polyaxial bodies  30  in a minimally invasive and less disruptive procedure than previously realized. 
     After the spine rod  16  is inserted through the discrete incisions  18  and the slots  44 ,  52 ,  54  of the pedicle screw assemblies  12  and seated within the saddles  34 , the set screws  38  may be inserted through the collars  46  and tabs  40  to secure the spine rod  16  in place. Once the set screws  38  are installed, the collars  46  may be individually removed upwardly from the pedicle screw assemblies  12  and the tabs  40  broken along the frangible joints  42  as shown in  FIG. 4B  and removed from the surgical site. 
     As a result, a more minimally invasive spinal fixation construct installation procedure is provided by the pedicle screw assemblies, collars and associated devices of this invention without the need for extended incision and associated difficulties. Moreover, increased visualization and minimally invasive disruption are realized with this invention. 
     From the above disclosure of the general principles of this invention and the preceding detailed description of at least one embodiment, those skilled in the art will readily comprehend the various modifications to which this invention is susceptible. Therefore, we desire to be limited only by the scope of the following claims and equivalents thereof.

Technology Category: 1