Abstract:
A modular anchoring device including a threaded shank for anchoring within a vertebra, an intermediate element and a head portion configured to receive and secure a rigid or non-rigid stabilization rod. The threaded shank, the intermediate element and the head portion of the anchoring device are cannulated to permit percutaneous implantation of the device. The intermediate portion is designed to be removable from the threaded shank portion subsequent to implantation of the anchoring device to enable substitution of another intermediate element having different dynamic characteristics.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of U.S. Non-Provisional application Ser. No. 12/202,802, entitled “Modular Pedicle Screw System”, and was filed on Sep. 2, 2008, the entire contents of which are hereby expressly incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to dynamic spinal stabilization systems. The invention provides a modular pedicle screw attached to the vertebrae to anchor the stabilization system. 
       BACKGROUND OF THE INVENTION 
       [0003]    The spine is comprised of an intricate system of bones and assorted tissues that support the body and provides protection of the central nervous system including the spinal cord and associated nerves. Within the spinal column are stacked a plurality of vertebrae separated from one another by an intervertebral disc that dampens and cushions the compressive forces exerted upon the spinal column. Located behind the series of alternating vertebrae and discs is the vertebral canal which contains the spinal cord and other associated nerves. 
         [0004]    There are more than twenty vertebrae within the spinal column and they are categorized into one of four classifications: cervical, thoracic, lumbar or sacral. The upper seven vertebrae, including the first seven extending downward from the base of the skull are referred to as the cervical vertebrae. The next twelve extending downward from the cervical vertebrae are known as the thoracic vertebrae. Extending downwardly from the thoracic vertebrae are the five lumbar vertebrae. At the base of the spinal column is the sacral bone which also includes the coccyx. The structural and functional relationship of the vertebrae, discs, muscles, ligaments and nerves enables a healthy normal spinal column to move and articulate freely almost without limitation. 
         [0005]    The spinal column is comprised of the vertebral body, the pedicle, the spinous process, the transverse process, the facet, the laminar arch, and the vertebral canal. The vertebral body is the generally cylindrically shaped weight bearing structure of the vertebra. The spinous process extends from the rear portion of the vertebra and the transverse processes extend from each side of each vertebra. Both the spinous process and the transverse process connect muscle tissue and ligaments to the spine. The vertebral canal is formed between the vertebral body and the lamina and houses the spinal cord therein. The pedicle is connected to the vertebral body and supports the lamina. 
         [0006]    The spinal column may be subject to numerous abnormalities and disorders which can be caused by trauma, disease, or genetic defect such as ruptured or slipped discs, degenerative disc disease, fractured vertebrae as so forth. Such defects can result in conditions causing extreme pain and reduced or abnormal nerve function. These spinal abnormalities can potentially cause damage to the nervous system and in particular the spinal cord and likewise impair the normal freedom of motion of the spinal column. 
         [0007]    It is not uncommon to treat such abnormalities surgically by spinal fusion wherein one or more vertebral bodies are fused together. However, spinal fusion may limit the spinal cord&#39;s range of motion in rotation and lateral bending. In addition, spinal fusion may increase the stress placed upon non fused adjacent vertebral bodies thereby diminishing their structural integrity. Moreover, the fusion device or material may become dislodged and move away from the area of implantation. 
         [0008]    A wide variety of approaches have been in use to achieve spinal fusion by implanting artificial devices in or on the spinal column to result in immobilization. One approach utilizes an anterior implant where the implant is located on the anterior, or front portion, of the vertebral body. An anterior stabilization can include full or partial disc replacement by a rigid spacer that is approximately the size of the disc that has been removed. A different approach involves the utilization of a posterior implant. Posterior implants include rods that are attached to either the lamina or transverse process by hooks or by pedicle screws. Other posterior implants allow for flexible or dynamic stabilization using pedicle screws connected by rigid or flexible rod member. Prior art posterior pedicle screw based stabilization systems create forces that are often transferred to the anchored pedicle screws. Patients having a relatively brittle bone structure cannot withstand the magnitude of these forces without resulting in the failure of the anchoring system. 
       DESCRIPTION OF THE PRIOR ART 
       [0009]    One example of a dynamic anchoring device is disclosed in US Patent Application Publication 2004/0025289 by Biedermann et al. The device includes an element for anchoring in a bone or vertebra and a head connected to the shank, a receiving part for receiving the head, and a pressure element acting on the head, wherein the pressure element is resilient so that upon a movement of the element from a first angular position of the shank relative to said receiving part into a second angular position the pressure element exerts a return force onto the head to urge the element towards the first angular position. 
         [0010]    Another example of a dynamic anchoring device is disclosed in US Patent Application Publication 2005/014823 to Boyd et al. The dynamic stabilization system disclosed therein includes bone anchors having a flexible portion between the bone engaging and head portions of the anchor. 
         [0011]    U.S. Patent Application Publication 2005/0216003 to Biedermann et al discloses a bone anchoring element such as a screw. The screw has a shaft and a first head. A second head is elastically connected to the first head. The second head is arranged in the receiving member such that the second head can pivot or swivel. The second head is fixed in the resting member in an angular resting position. The screw is deflectable from the angular head position relative to the second head. The second head is elastically connected to the first head such that a restoring force returns the screw to the angular resting position. The resting angular position of the shaft relative to the receiving part is adjustable. 
         [0012]    U.S. Patent Application Publication 2006/0129147 to Biedermann et al discloses a stabilization device for bones or vertebrae that comprises a substantially cylindrical elastic element. The elastic element has a first end and a second end opposite to the first end. An elastic section extends between the first end and the second end. The elastic section includes at least first and second helical coils. The first and second helical coils are arranged coaxially so that the first helical coil extends at least in a portion between the second helical coil. The elastic element may form, for example, a portion of a rod, bone anchoring element, or plate. 
         [0013]    U.S. Patent Application Publication 2007/0055236 to Hudgins et al discloses an apparatus and method for stabilizing the facet joints of the spine. The facet implant may be in the form of a screw or other anchor with the intermediate portion in the form of a polyaxial head, a cord a spring, etc. 
         [0014]    Another device for the dynamic fixation of impaired spinal column segments in disclosed in U.S. Published Patent Application 2007/0233087 to Schlapfer. The device includes an intermediate element for a detachable, lockable, ball joint like connection having an outer wall concentric with the longitudinal axis and an inner wall forming a coaxial cavity. Either the outer wall or the inner wall comprises one of two contact zones that form the ball joint like connection. The intermediate element is at least partly made of a super elastic material. 
         [0015]    U.S. Published Patent Application 2008/0021465 to Shadduck et al discloses a spine implant device for fusion or dynamic stabilization of a spine segment that includes a fixation device with a shaft portion for engaging bone and a proximal end for coupling to a rod that allows for limited flexing of the proximal end relative to the shaft portion. 
         [0016]    A further example of a dynamic spinal stabilization system is disclosed in US Published Patent Application 2008/0071273 to Hawkes et al. Disclosed is a system for stabilizing at least one spinal motion segment that includes a fastener having an anchoring portion and a coupling portion and a longitudinal support member couple to the fastener wherein a portion of the system is formed from a super-elastic material. 
       SUMMARY OF THE INVENTION 
       [0017]    The present invention relates to a spinal stabilization system that provides for dynamic stabilization using a modular screw in conjunction with a rigid or non-rigid rod that permits load transfer at the pedicle screw rod interface as opposed to the dynamic rod per se. The screw has an elastic segment interposed between a threaded portion of the screw and the screw head portion, also referred to as a “tulip”. The amount or degree of motion can be varied based on the rigidity or flexibility of the elastic material as well as the length and diameter of the elastic material. The pedicle screw is designed to be used in a percutaneous dynamic spinal stabilization system. The screw can be used in a single or multi-level construct in combination with a titanium, PEEK or Nitinol rod. The dynamic screw design enables percutaneous delivery of the stabilization system although the dynamic system can be used in an open application as well. 
         [0018]    The dynamic spinal stabilization system includes a dynamic modular pedicle screw system which in turn preserves motion in the posterior column of the human spine. The dynamic screws can be used in conjunction with a rigid or non-rigid rod. The dynamic pedicle screw used with a rigid rod will allow for the load transfer to occur at the screw/rod interface as opposed to a non-physiologic load transferred through a dynamic rod alone. Alternatively, the modular pedicle screw can include a rigid segment interposed between a threaded portion of the screw and the screw head portion, also referred to as a “tulip”. 
         [0019]    Accordingly, it is a primary objective of the instant invention to provide a semi dynamic spinal stabilization system that allows for variable customization of the elastic member thereby increasing the ability to specifically address a greater number of pathologies. 
         [0020]    It is a further objective of the instant invention to provide absorption of the dynamic force transmission within the anchoring screw and not at the bone-screw interface. 
         [0021]    It is yet another objective of the instant invention to provide a modular pedicle screw that is designed to be used in a percutaneous dynamic stabilization system. 
         [0022]    It is a still further objective of the invention to provide a kit of modular anchoring devices for a dynamic spinal stabilization system. The anchoring device is a three part design including a threaded rigid shank, an intermediate component that is an elastic polymer or rigid material, and a rigid multi-axial tulip. The kit would include a plurality of threaded shanks of varying sizes, a plurality of intermediate portions of varying geometries and rigidities, and a plurality of tulip heads. 
         [0023]    It is a further object of the invention to provide an intermediate component that is designed to be removable from the threaded shank portion subsequent to implantation of the pedicle screw should the pathology change thereby necessitating a change in the flexibility of the dynamic system. The ability to change the dynamism of the stabilization system without removing the threaded shank portion allows the surgeon to maintain the original bone purchase in the patient which facilitates the procedure, the healing process and improves the potential for long term success. 
         [0024]    Another distinct objective of the system is to provide a more comprehensive yet less invasive method to address more complex spine cases, i.e. spinal deformity cases. Currently, dynamic systems are limited in their applicability and mostly ruled out for use in more complex spine cases. One reason may be due to the limited ability to manipulate the individual spine segments in order to obtain the overall correction/objective. This reinforces a current perception that a more invasive technique is always required. This system may not be applicable in all complex cases however it will be a minimally invasive/percutaneous dynamic screw option for surgeons to consider. 
         [0025]    Other objectives and advantages of this invention will become apparent from the following description taken in conjunction with any accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention. Any drawings contained herein constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0026]      FIG. 1  is a perspective disassembled view of the dynamic modular pedicle screw. 
           [0027]      FIG. 2  is a perspective view of the dynamic spine stabilization system showing a multi level construction utilizing a pair of dynamic pedicle screws and a stabilization rod. 
           [0028]      FIG. 3A  is a top view of the elastic intermediate member. 
           [0029]      FIG. 3B  is a side view of the elastic intermediate member. 
           [0030]      FIG. 3C  is a sectional perspective view of the elastic intermediate. 
           [0031]      FIG. 4A  is a top view of a second embodiment for the elastic intermediate member. 
           [0032]      FIG. 4B  is a side view of the second embodiment for the elastic intermediate member. 
           [0033]      FIG. 4C  is a sectional perspective view of the second embodiment for the elastic intermediate member. 
           [0034]      FIGS. 5A and 5B  show various configurations for the elastic portion and their relative dynamic properties. 
           [0035]      FIGS. 6A and 6B  show a third and fourth embodiment for the elastic intermediate member. 
           [0036]      FIG. 7A  is an exploded side view of the lower coupling the intermediate member and the threaded shank. 
           [0037]      FIG. 7B  is a top view of the lower coupling member of the intermediate member. 
           [0038]      FIG. 7C  is a top of view of the threaded shank portion. 
           [0039]      FIG. 8A  is a side view of the upper coupling member and the tulip head. 
           [0040]      FIG. 8B  is a top view of the tulip head component. 
           [0041]      FIG. 8C  is a top view of the upper coupling member of the intermediate element. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0042]      FIG. 1  shows a dissembled view of the modular dynamic pedicle screw  1 . Screw  1  includes a threaded shank portion  2  having a one end that tapers into a point  4  at one end and has an opposite end  6  that includes a coupling element  8 . Coupling element  8  includes internal female threads  10 . The pedicle screw  1  has a channel  20  through the entire length of the pedicle screw, including the tulip head  12 , the intermediate component  14 , and the threaded shank portion  2 . This channel  20  allows the pedicle screw  1  to be maneuvered on a Kirschner wire  22 , also know as a K-wire. In practice the K-wire is positioned within the patient using fluoroscopy, or other imaging techniques, so as to provide precise positioning of the pedicle screw  1 . Once the components are securely positioned the K-wire can be easily removed through the channel  20  which is open at the end of the threaded shank portion and extends through the uppermost portion of the head portion or tulip  12 . The threaded shank  2  is externally threaded. The threads  3  can be fenestrated or partially fenestrated. Fenestrated threads are particularly appropriate for osteoporotic patients or patients who require greater assurance of increased pedicle screw purchase based on bone quality. The threaded shank  2  of the pedicle  1  are appropriately sized in relation to the patient&#39;s pathology and can be formed in different lengths and external threaded diameters. 
         [0043]    The head or tulip portion  12  of the pedicle screw  1  includes upwardly extending cylindrical wall  15  wherein grooves  17  are positioned in diametrically opposed relationship. These opposing grooves  18  allow for top loading of either a rigid or non rigid rod  30  into the tulip. The tulip may be fixed or multi axial. The inner portion of the cylindrical wall accepts a threaded lock screw  32  to secure the rod  30  to the pedicle screw  1 . The tulip design can accept tulip extension towers, attached to tulip portion  12 , which will facilitate the percutaneous passing of the rod  30  through multiple screws based upon the number of spinal segments involved in the overall dynamic spinal stabilization system. The tulip extensions allow for external control of the tulip head during the rod delivery process. The screw extensions that are attached to the tulip portions remain in place until the percutaneous delivery and placement of the rod  30  has been achieved and threaded lock screws  32  have been finally tightened. In addition, the pedicle screw  1  is also configured to receive a shank extension tower. The screw extension tower is a completely rigid device that extends dorsally through the skin incision. This feature enables three dimensional manipulation of the spine segment. Once the rigid manipulation of the segment is complete the screw extension tower is removed and the dynamic member is fully functional. The tulip design allows for top loading of the rod  30  delivered under direct visualization as is possible when the surgery is performed under open conditions. A coupling element  21  having a cylindrical wall with external threads for engagement with the intermediate member  14  is attached to tulip portion  12  with a ball and socket arrangement  24 . 
         [0044]    The intermediate portion  14  of the dynamic pedicle screw includes an elastic portion  40 , an upper coupling member  16  and a lower coupling member  18 . As shown in  FIGS. 3A-3C , portion  40  is generally cylindrical in shape and includes a passageway  42  concentric with the longitudinal axis of the cylindrical body. The portion  40  is formed from elastic motion preserving dynamic material which allows for the requisite degree of motion and is capable of standing the mechanical loads associated with the human spine. This provides intraoperative flexibility for the surgeon to choose or customize the construct to address the patient&#39;s specific pathology. The portion  40  is available in varying levels, ranges and modes of dynamism, such as dynamic, motion preserving, non-fusion and rigid. Dynamism can be adjusted based on the type of material used, for example Nitinol or polycarbonate, the length of the cylinder, the diameter and or wall thickness of the cylinder or any combination of the above variables (as shown in  FIGS. 5A and 5B ). Embedded within the wall of cylindrical portion  40  is a jacket  44  made from a polyester material, or the like, which extends outwardly from each end of the cylinder  40 , as shown in  FIGS. 3A through 3C . A second embodiment, shown in  FIGS. 4A through 4C  utilizes a polyester, or the like, jacket that surrounds the outer surfaces of cylindrical member  40  and extends outwardly from each end of the cylinder  40 . Extending portions  48  of the jacket extend into tabs formed in the upper and lower coupling members,  16  and  18  respectively, to complete the assembly of the intermediate portion  14 . Upper coupling member  16  includes a cylindrical wall having an externally threaded surface. Upper coupling member  16  is threadably connected to tulip coupling member  21 . Likewise, lower coupling member  18  includes a cylindrical wall having an externally threaded surface. Lower coupling member  18  is screwed on to coupling member  8  positioned on the threaded shank portion  2 . As an alternative, cylindrical member  40  can be bonded, glued or molded directly on to the upper and lower coupling members,  16  and  18  respectively, without the utilization of a jacket. 
         [0045]    The intermediate portion can also be rigid allowing for rigid fixation. In order to assemble a rigid modular screw a non-elastic intermediate portion  14  is coupled to the threaded shank portion  2  and the tulip head portion  12 . In this instance, cylindrical member  40  can be made from the same material as the threaded shank  2  or the tulip head  12  or some other rigid compatible material. The non-elastic cylindrical member  40  can be threaded into upper and lower coupling members or otherwise suitably affixed thereto. 
         [0046]      FIGS. 6A and 6B  illustrate a third and fourth embodiment for the intermediate member  54 . As shown in  FIG. 6A  intermediate member  54  includes an upper coupling member  56  that includes a threaded portion  57  which is sized and configured to threadably connect to tulip coupling member  21 . Upper coupling member  54  is generally cylindrical in shape. It includes an upper cylindrical portion  51  adjacent the threaded portion  57  having a first diameter. Depending downward from the upper cylindrical portion is a post like cylindrical portion  53  having a center coincident with the upper cylindrical portion diameter  51 . Depending downward from the post like cylindrical portion  53  is an interengaging cylindrical portion  55  whose center is coincident with both the upper cylindrical portion  56  and the post like cylindrical portion  53 . The diameter of the interengaging cylindrical portion  55  is greater that the post like cylindrical portion  53  but less than the upper cylindrical portion  51 . Intermediate member  54  also includes a lower coupling member  58  having a threaded portion that is sized and configured to threadably engage threads  10  on coupling member  8 . The lower coupling member  58  has a lower cylindrical portion  57  having a diameter substantially the same size as the upper cylindrical portion  51  of the upper coupling member  56 . Extending upwardly from the lower cylindrical member is a hollow cylindrical wall  70 . The upper portion of the hollow cylindrical wall terminates in an annular flange  72  that extends radially inward to form a cylindrical cavity having a reduced diameter aperture. The diameter of the aperture is sufficiently large to allow the interengaging cylindrical portion  55  to pass there through when introduced at an appropriate angle. Once the upper and lower coupling members are properly positioned, with the interengagement cylinder  55  of the upper coupling member  56  located within the cylindrical cavity of the lower coupling member  58 , a synthetic material  50 , such as a polycarbonate urethane, is injected into the space formed between the upper and lower coupling members. The modulus of elasticity of the injection molded material  50  is variable and can provide a range of stiffness from rigid to flexible. Likewise, the lengths and diameters of the upper and lower coupling members can be changed to allow for varying amounts of synthetic material  50  to be injected between the two members. By varying the length, diameter, or wall thickness of synthetic material  50  the degree of elasticity of intermediate member  54  can be varied. The synthetic material can be appropriately color coded, and or otherwise marked with indicia, to provide a visual indication of the elasticity of the injection molded material. The surfaces of the upper and lower coupling elements are properly surface treated prior to injection of the synthetic material to provide an optimum amount of adhesion between the synthetic material and the upper and lower coupling members. The assembled intermediate member  54 , including the upper and lower coupling members and the synthetic material  50  is designed to handle a torque in the range of 80 to 120 inch pounds of force. In addition the intermediate member provides five degrees of motion, including flexion/extension and is capable of handling force in the order of 250 to 400 newtons. The upper and lower coupling members  56  and  58  are made from titanium or any other suitable biocompatible material, either metallic or synthetic. All surface edges of the upper and lower coupling members are rounded to remove sharp surface edges from the intermediate member. 
         [0047]    The embodiment shown in  FIG. 6B  is similar to that shown in  FIG. 6A  except that in this embodiment upper flanges  62  and lower flanges  64  are substituted for the threaded portions  57  and  59  respectively. Flanges  62  and  64  include two or more spaced flange segments ( 62 A, 62 B and  64 A,  64 B) that cooperate with complimentary recesses and grooves formed on the tulip coupling member  21  and the threaded shank member  2 . 
         [0048]      FIG. 7A  illustrates the lower coupling member  18  and the threaded shank  2  with its coupling member  8 . Annular threads  10  on coupling  8  mate with external threads on lower coupling member  18 .  FIG. 7B  is a top view of lower coupling member  18  showing a socket  19  that includes a portion of channel  20 . Socket  19  is designed to operatively engage an insertion or removal tool which can be inserted through the intermediate portion  14  via channel  20 . Should it be necessary to change the dynamic characteristics of the spinal support system the surgeon would remove the rod  30  from the head  12  by first removing threaded lock screw  32 . Following removal of the rod  30  the head portion  12  would be unthreaded from the intermediate portion  14  using an appropriate tool. Thereafter, a tool would be inserted through the channel  20  in the intermediate member  14  to engage socket  19  formed in lower coupling member  18 . Upon rotation of the tool the lower coupling  18  of the intermediate member will be unthreaded from the second coupling  8  formed on the threaded shank portion  2 . The intermediate portion can then be removed from the patient. A new intermediate portion  14  can then be positioned over the existing threaded shank portion  2 . Thereafter a tool would be inserted through channel  20  of the intermediate member  14  and engage socket  19  formed in the lower coupling member  8 . Upon rotation of the tool the lower coupling  18  of the intermediate member  14  will be threaded into the second coupling formed on the treaded shank  2 . The head portion  12  can then be threaded onto the intermediate portion  14  and the rod  30  can be affixed thereto by locking screw  32 . The ability to change the dynamism of the stabilization system without removing the threaded shank portion allows the surgeon to maintain the original bone purchase in the patient which facilitates the procedure, the healing process and improves the potential for long term success.  FIG. 7C  is a top view of the threaded shank  2  with channel  20  and coupling threads  10 . 
         [0049]      FIG. 8A  is a side view showing tulip head member  12  with cylindrical side walls  15  and groove  17 . A coupling element  21  having a cylindrical wall with external threads for engagement with the intermediate member  14  is attached to tulip portion  12  with a ball and socket arrangement  24 .  FIG. 8B  is a side view of tulip head member  12 .  FIG. 8C  is a top view of the upper coupling member  16 . 
         [0050]    The rod  30  connects multiple screws based upon the number of segments involved in the overall construct. The rod can be of any compatible material (PEEK, Titanium, Nitinol, etc). This also increases the versatility of the system allowing for more control in defining the rigidity or dynamism of the overall construct. The rod  30  used in conjunction with the dynamic pedicle screw system can be either rigid or non rigid. 
         [0051]    Various types and sizes of the components, namely the intermediate members, the threaded shanks, the tulip heads and rods, etc. are individually wrapped and terminally sterilized. They are brought to the operating room as a kit and individually selected by the surgeon based on the case presented to them by the patient. Once the sterilized package is opened the device contained therein is either used or discarded. The component can not be resterilized. 
         [0052]    All patents and publications mentioned in this specification are indicative of the levels of those skilled in the art to which the invention pertains. All patents and publications are herein incorporated by reference to the same extent as if each individual publication was specifically and individually indicated to be incorporated by reference. 
         [0053]    It is to be understood that while a certain form of the invention is illustrated, it is not to be limited to the specific form or arrangement herein described and shown. It will be apparent to those skilled in the art that various changes may be made without departing from the scope of the invention and the invention is not to be considered limited to what is shown and described in the specification and any drawings/figures included herein. 
         [0054]    One skilled in the art will readily appreciate that the present invention is well adapted to carry out the objectives and obtain the ends and advantages mentioned, as well as those inherent therein. The embodiments, methods, procedures and techniques described herein are presently representative of the preferred embodiments, are intended to be exemplary and are not intended as limitations on the scope. Changes therein and other uses will occur to those skilled in the art which are encompassed within the spirit of the invention and are defined by the scope of the appended claims. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. Indeed, various modifications of the described modes for carrying out the invention which are obvious to those skilled in the art are intended to be within the scope of the following claims.