Abstract:
A bone anchor is driven into the pedicle portion of the vertebral body until a shoulder protrusion within the proximal aspect of the anchor abuts the bone surface and prevents further anchor travel into the bone. A feature within the distal aspect of the anchor is actuated producing the emergence of a distal shoulder protrusion. The latter directly abuts the distal aspect of the pedicle at the pedicle/vertebral body interface. Using this method, the anchor captures the pedicle portion of bone and contains it between the proximal and distal shoulder abutments.

Description:
REFERENCE TO PRIORITY DOCUMENT 
     This application claims priority of U.S. Provisional Patent Application Ser. No. 60/834,644, filed Aug. 1, 2006. Priority of the aforementioned filing date is hereby claimed and the disclosure of the Provisional Patent Applications is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     The present disclosure is related to orthopedic devices implanted between skeletal segments. The implanted devices are used to adjust and maintain the spatial relationship(s) of adjacent bones. Depending on the implant design, the motion between the skeletal segments may be returned to normal, increased, modified, limited or completely immobilized. 
     Whether from degenerative disease, traumatic disruption, infection or neoplastic invasion, alterations in the anatomical relationships between the spinal vertebras can cause significant pain, deformity and disability. Spinal disease is a major health problem in the industrialized world and the surgical treatment of spinal pathology is an evolving discipline. The traditional surgical treatment of abnormal vertebral alignment and aberrant motion is the complete immobilization and bony fusion of the involved spinal segment. More recently, preservation of vertebral motion during the treatment of the spinal pathology has been the preferred strategy and many surgical techniques have been formulated to accomplish this treatment objective. 
     Regardless of whether the vertebral motion is abolished or preserved, many surgeons employ implantable orthopedic devices that adjust, align, support and/or maintain the spatial relationship(s) of the adjacent vertebral bones. The effectiveness of theses devices is vitally dependant on the adequacy of their fixation onto the underlying bone. Inadequate device fixation will effectively uncouple the device from the vertebral column and marginalize the beneficiary effects of the implant. Further, poorly anchored devices may damage the attached bone by fracturing and/or avulsing bone fragments at the attachment sites. 
     Screw fixation into the pedicle portion of the vertebral body has emerged as the most common method of device fixation onto the vertebral column. However, it is known that repeated loading and unloading of these screws will lead to screw loosening and eventual pull-out. Implantable devices that promote spinal fusion must bear load for the few months needed to produce bone graft maturation and solid vertebral fusion. In contrast, devices that preserve vertebral motion must bear the cyclical load of movement for the remainder of the patient&#39;s life. With the change in treatment strategy towards motion preservation, the integrity of the bone/device interface and the durability of the device fixation sites are emerging as major determinants of implant&#39;s functional life span. 
     SUMMARY 
     In attempt to improve screw fixation onto the vertebral bodies, a number of devices have been developed. Despite these improvements, there is still significant need for improved devices and methods for screw fixation onto the vertebral column. This need will increase further as surgeons widen the application of the motion preservation procedures. This application discloses novel implant designs and methods of use. The illustrated embodiments provide superior anchor fixation onto the vertebral bones and significantly increase the resistance to anchor pull-out. 
     In an embodiment, a bone anchor is driven into the pedicle portion of the vertebral body until a shoulder protrusion within the proximal aspect of the anchor abuts the bone surface and prevents further anchor travel into the bone. A feature within the distal aspect of the anchor is actuated producing the emergence of a distal shoulder protrusion. The latter directly abuts the distal aspect of the pedicle at the pedicle/vertebral body interface. Using this method, the anchor captures the pedicle portion of bone and contains it between the proximal and distal shoulder abutments. 
     In another embodiment, the bone anchor is driven into the pedicle portion of the vertebral body and the distal aspect of the anchor is actuated producing the emergence of a distal shoulder protrusion. The anchor is gently backed-out of the vertebral bone until the distal shoulder protrusion is snugly lodged against the distal aspect of the pedicle at the pedicle/vertebral body interface. The anchor may be used to fixate orthopedic implants at this point. Alternatively, a proximal shoulder protrusion could be added and lodged against the bone surface at the anchor insertion site. This feature captures the pedicle portion of bone between two abutment surfaces and increase the anchor&#39;s fixation power and pull-out&#39;resistance. 
     Other features and advantages will be apparent from the following description of various embodiments, which illustrate, by way of example, the principles of the disclosed devices and methods. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows two intact vertebral bodies V 1  and V 2 . 
         FIG. 1B  shows the same vertebral bodies V 1  and V 2  after surgical resection of the lamina. 
         FIG. 1C  shows a perspective view of a pedicle screw system in an assembled state. 
         FIG. 1D  shows a perspective view of the pedicle screw system in an exploded state. 
         FIG. 2  shows a shank system of a pedicle screw assembly partially inserted into a pedicle segment of a vertebral body. 
         FIG. 3  shows the shank system fully advanced into the bone such that a shoulder has moved toward the outer bone surface. 
         FIG. 4  shows a screw head and receiver being loaded onto the shank system. 
         FIG. 5  shows the screw head and receiver being coupled onto the shank system. 
         FIG. 6  shows the screw head and receiver fully coupled onto the shank system. 
         FIG. 7  shows a screw assembly that has a shank system of increased length. 
         FIG. 8  shows another embodiment of the pedicle screw system. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1A  shows two intact vertebral bodies. For clarity of illustration, the vertebral bodies are represented schematically and those skilled in the art will appreciate that actual vertebral bodies include anatomical details not shown in  FIG. 1A . For clarity of illustration, certain anatomical details, such as the patient&#39;s skin, are not shown in at least some of the figures. The vertebral arch is comprised of two pedicles, the short stout processes that extend from the sides of the vertebral body and two laminae, the broad flat plates that project from the pedicles and join in a triangle to form a hollow archway (the foramen).  FIG. 1B  illustrates the same vertebral bodies after surgical resection of the lamina. The negative effects of laminectomy can be countered by the reconstruction of the lamina. 
       FIG. 1C  shows a perspective view of a pedicle screw assembly in an assembled state.  FIG. 1D  shows a perspective view of the pedicle screw assembly in an exploded state. The pedicle screw assembly employs a pedicle locking technique that provides powerful screw immobilization and reduces the possibility of loosening and movement with repeated loading. The pedicle screw assembly includes a includes a multi-piece shank system  201  comprising an inner shank member  210  that is slidably disposed within an outer shank member  215 , as described in more detail below. The screw assembly  205  further includes a screw head member  405  and a receiver member  410  that collectively couple to the shank system, as described more fully below. 
     With reference to  FIG. 2 , the pedicle screw assembly  205  is partially inserted into a pedicle segment of the vertebral body. The pedicle screw assembly  205  employs a pedicle locking technique that provides powerful screw immobilization and reduces the possibility of loosening and movement with repeated loading. In this regard, after full insertion or deployment into the bone, the screw assembly  205  is configured to lock onto the outer and inner aspect of the pedicle so as to trap the pedicle, as described more fully below. 
     With reference to  FIG. 2 , the screw assembly  205  includes a multi-piece shank system comprising the inner shank member  210  that is slidably disposed within the outer shank member  215 . The inner shank member  210  has a sharpened distal tip for penetrating the bone and also has a threaded outer surface along a distal region for screwing into bone. A proximal region  220  of the inner shank member  210  has a reduced radial size that slidably fits within the outer shank member  215 . 
     The outer shank member  215  is slidably disposed over the proximal region  220  of the inner shank member  210 . The outer shank member  215  is deformable and is configured to expand radially outward in response to advancement of the shank system into the bone, as described more fully below. A widened shoulder  225  is located at a proximal end of the outer shank member  215 . The shoulder  225  has a convex outer surface that engages a locking member  235  that is washer-like. That is, the locking member  235  is ring-shaped and sized to fit around the shoulder  225  of the outer shank member  215 . With advancement of the screw assembly  205  into the bone, the locking member  235  conforms to the outer surface of the bone as described in detail below. It should be appreciated that the outer surface of the shoulder  225  need not be convex, but can have other shapes that gradually widen moving away from the shank portion of the outer member  215 . 
     The shank system can be rotated such that the threaded engagement between the threads and the bone causes the shank system to advance into the bone.  FIG. 3  shows the shank system fully advanced into the bone such that the distal end of the inner shank member  210  has advanced deeper into the bone and the shoulder  225  has moved toward the outer bone surface  305 . As the shoulder  225  moves toward the outer bone surface  305 , the locking member  235  is urged in a proximal direction (as represented by arrow P in  FIG. 3 ) relative to the convex surface of the shoulder  225 . The gradually-widening configuration of the convex outer surface causes the locking member  235  to wedge between the shoulder  225  and the outer bone surface  305 . The locking member  235  automatically adjusts its position to conform to the outer surface  305  of the bone as the shank system is advanced. This creates a locking engagement between the shank system (at the location of the shoulder  225  and locking member  235 ) and the outer surface  305  of the pedicle. 
     As mentioned, the screw assembly  205  further includes a screw head member  405  and a receiver member  410  that collectively couple to the shank system.  FIG. 4  shows the screw head member  405  and the receiver member  410  ready for coupling to the shank system. The head member  405  is adapted to removably couple to a protrusion  415  on the inner shank member  210 . The protrusion  415  mates with a bore  420  in the head member  405 , such as in a threaded male-female engagement or in any other mating engagement. The head member  405  is removably mounted in the receiver  410 , as described below. The receiver  405  can include means, such as slots, adapted to receive an elongate stabilizer, or interconnecting member, such as a rod. It should be appreciated that the structure and type of engagement between the receiver  410  and the head member  405  can vary. For example, the engagement between the head member  405  and the receiver  410  can be a poly-axial or a mono-axial type engagement. 
     In an embodiment, there is a dynamic engagement between the head member  405  and the receiver member  410 . In such an embodiment, the head member  405  is positioned within a multi-piece inner housing member  430  in which the head can rotate in a ball and socket manner. The inner housing member  430  can be immobilized relative to the receiver  410  to fixedly attach the head member  405  (and the attached shank system) to the housing. However, the head member  430  can rotate within the inner housing member  430  to permit some movement between the screw and the receiver  410 . In addition, the head member  405  can be completely immobilized within the inner housing  430 . 
     A space  435  is located within the inner housing member  430 . The space  435  can contain a material or structure that resists movement of the head member  405  relative to the inner aspect of the inner housing members  430 . The material or structure within the space  435  can be, for example, an elastic material(s), fluids, spring device(s), magnets or any other appropriate materials/devices that will resist movement of the head member  405  relative to the inner aspect of the inner housing members  430 . When the screw head is moved out of a predetermined position in the inner housing members  430 , the material/device within space  435  will apply a force to the head member  405  and resist any bone screw movement away from a neutral position. With movement, the assembly would return the screw and the attached bone to the neutral position once a deflecting force has dissipated. Further, before locking the assembly with a locking nut  610  ( FIG. 6 ), the surgeon can freely adjust the orientation of the shank system relative to receiver  410  without influencing the assembly&#39;s neutral position or pre-loading the screw and bone construct. 
       FIG. 5  shows the head member  405  and the receiver  410  being coupled onto the shank system. As mentioned, the head member  405  can couple to the inner shank member  210  by threading the protrusion  415  into the bore  420  in the head member  405 . The head member  405  is rotated about the protrusion  415  to cause the head member  405  (and attached receiver  410 ) to advance distally relative to the shank system. The distal advancement of the head member  405  over the protrusion  415  causes the outer shank member  215  to deform such that the outer shank member  215  expands radially outward. 
       FIG. 6  shows the head member  405  and the receiver  410  fully coupled onto the shank system. The advancement of the head member  405  onto the protrusion  405  of the inner shank member  210  has caused a portion  605  of the outer shank member  215  to expand radially outward relative to the inner shank member  210 . Thus, the portion  605  is forced against the inner aspect of the pedicle. In this way, the pedicle is captured and locked between the locking member  235  and the expanded portion  605  of the outer shank member  215 . An interconnecting rod  610  can be coupled to the receiver  410  and secured thereto using a locking nut  615 . 
     It should be appreciated that the configuration of the screw assembly can be varied. In an embodiment, the shank system has an increased length that permits increased anchoring.  FIG. 7  shows a similar screw assembly that has a shank system that is of greater length than the previously-described embodiment. The longer assembly permits anchor of the distal end of the shank into the anterior cortical surface of the vertebral body. Such an arrangement provides increased contact strength between screw and bone. Alternatively, the screw may be driven in a more superior trajectory so as to capture the superior cortical surface of the vertebral body.  FIG. 8  shows another embodiment of the screw assembly. In this embodiment, the expanded portion expands outward in the opposite direction with respect to the previous embodiment. 
     The disclosed anchors may be at least partially made of bone or a bone graft substitutes. In addition, any device and any of its components can be made of any biologically adaptable or compatible materials. Materials considered acceptable for biological implantation are well known and include, but are not limited to, stainless steel, titanium, tantalum, combination metallic alloys, various plastics, resins, ceramics, biologically absorbable materials and the like. Any components may be also coated/made with osteo-conductive (such as deminerized bone matrix, hydroxyapatite, and the like) and/or osteo-inductive (such as Transforming Growth Factor “TGF-B,” Platelet-Derived Growth Factor “PDGF,” Bone-Morphogenic Protein “BMP,” and the like) bio-active materials that promote bone formation. Further, any surface may be made with a porous ingrowth surface (such as titanium wire mesh, plasma-sprayed titanium, tantalum, porous CoCr, and the like), provided with a bioactive coating, made using tantalum, and/or helical rosette carbon nanotubes (or other carbon nanotube-based coating) in order to promote bone in-growth or establish a mineralized connection between the bone and the implant, and reduce the likelihood of implant loosening. In addition, the system or any of its components can also be entirely or partially made of a shape memory material or other deformable material. 
     Although embodiments of various methods and devices are described herein in detail with reference to certain versions, it should be appreciated that other versions, embodiments, methods of use, and combinations thereof are also possible. Therefore the spirit and scope of the appended claims should not be limited to the description of the embodiments contained herein.