Patent Publication Number: US-2012046698-A1

Title: Methods and devices for spinal fusion

Description:
FIELD OF THE INVENTION 
     The present invention relates to methods and devices for use in spinal fusion and, more particularly, to bone fasteners and methods of implantation thereof. 
     BACKGROUND OF THE INVENTION 
     There are many situations in which bones or bone fragments are fused, including fractures, joint degeneration, abnormal bone growth, infection and the like. For example, circumstances requiring spinal fusion include degenerative disc disease, spinal disc herniation, discogenic pain, spinal tumors, vertebral fractures, scoliosis, kyphosis, spondylolisthesis, spondylosis, Posterior Rami Syndrome, other degenerative spinal diseases, and other conditions that result in instability of the spine. In many situations, bone screws are used in orthopedic surgery to stabilize the bones or to affix stabilizing equipment thereto. 
     For instance, during spinal fusion, pedicle screws are often implanted to correct, or to aid in correcting, the spinal instability. For example, pedicle screws may be implanted to assist spinal fusion by holding unstable bone structures together to immobilize a portion of the spine. The pedicle screws are typically implanted to affix stabilizing equipment such as rods or plates to the spine. 
     Typically, the pedicle screw is inserted directly into the patient&#39;s bone to form a screw purchase. However, in some patient populations, the pedicle screws loosen and fail after installation, particularly for patients with osteoporosis or where the pedicle screw was installed to replace a previously failed pedicle screw that already stripped the screw hole. Therefore, solutions have been developed to repair and/or discourage these screw failures. 
     One known solution for improving the screw purchase is to increase the size of the pedicle screw. However, in many instances, the size and/or shape of the patient&#39;s pedicle may limit the size of the pedicle screw, thereby making it impossible to increase the size thereof. 
     A second known solution for discouraging failure of the screw purchase is to provide a geometric feature on the pedicle screw that interacts with the bone, such as an expanding screw tip. However, these geometric features may compromise implant strength and may be difficult to revise in the future, if necessary. 
     A third known solution for discouraging screw purchase failure is to augment the screw purchase with PMMA (polymethyl methacrylate). However, the stiffness of PMMA does not match that of the surrounding bone, which can lead to additional stress at the screw purchase. Additionally, the time before the PMMA sets, during which it may be implanted and/or worked, is relatively small, providing a surgeon with a very limited window in which the augmented pedicle screw must be inserted. In the event of a screw purchase failure or implantation error, PMMA is also difficult to rework, due to its rigidity and brittleness. Additionally, PMMA is not inherently adhesive to either bone or metal and, therefore, may not form a long lasting bone-PMMA or PMMA-screw interface. The act of screwing the pedicle screw into the patient&#39;s bone may also scrape the PMMA off of the threads of the pedicle screw, thereby further compromising the screw purchase. 
     A fourth solution for discouraging failure of the screw purchase is to augment the screw purchase with mineral cement such as calcium phosphate. However, mineral cements have low tensile strength and may resorb before adequate bone ingrowth is achieved around the pedicle screw. Additionally, mineral cement, like PMMA, is not inherently adhesive to either bone or metal and, therefore, may not form a long lasting interface. 
     Accordingly, there is a need for pedicle fasteners and methods and systems of implantation thereof that provide more workability and easier implantation. Additionally, there is a need for a pedicle fastener implantation system that allows for augmentation of pedicle fasteners with adhesive that provides adhesion with the pedicle fasteners and the surrounding bone and that approximates the strength and stiffness of natural bone to provide mechanically stable augmentation of the pedicle fasteners. Although the present invention will be described in terms of pedicle fasteners for simplicity, it should be understood by those skilled in the art that the methods and devices of the present invention are equally applicable to other bone fasteners of varying shapes and sizes. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a pedicle fastener for implantation during a spinal fusion procedure includes a head and shank. The shank is connectable to the head and extends outwardly therefrom. The shank may include a shielding feature formed therein for accommodating an adhesive augmenting the pedicle fastener for improving implantation strength and stiffness thereof. The adhesive may be a biocompatible polymeric material and, preferably, is biocompatible polyurethane. In some embodiments, the adhesive and the pedicle fastener may be provided together as a spinal fusion kit. 
     According to the present invention, the shielding feature may take a variety of forms including annular grooves, longitudinal or spiral flutes, slots, channels, threads and/or combinations thereof. The shank may also include a reverse taper at its tip for centering the pedicle fastener. In some embodiments, attachable features may be provided for, or pre-attached to, the pedicle fastener for controlling adhesive flow of the adhesive and/or for filling the pedicle hole. For instance, the pedicle fastener may include a restrictive disc attached to the shank. The pedicle fastener may include a surface coating formed on at least a portion of the shank for improving adhesion thereto. In some embodiments, the pedicle fastener may include a mesh sleeve surrounding the shank for controlling adhesive flow. 
     According to some embodiments of the present invention, the pedicle fastener may also include a longitudinal cannula extending at least partially through the head or shank. The longitudinal cannula may be adapted to allow cleaning and/or drying of the implant site after the pedicle fastener has been inserted therein. The longitudinal cannula may also allow injection of the adhesive through the pedicle fastener into the implant site after the pedicle fastener has been inserted therein. In some embodiments, the pedicle fastener may also include fenestrations connecting the longitudinal cannula with the shielding features to deliver adhesive thereto. 
     According to the present invention, the pedicle fastener may include a heating feature for applying a heat source to the pedicle fastener to accelerate polymerization. In some embodiments, the heating feature may be the longitudinal cannula, which may accept a heating element therein for accelerating polymerization of the adhesive. In other embodiments, the heating feature may include a removable extension extending from the head of the pedicle fastener. In some embodiments, the pedicle fastener may be provided with frozen adhesive provided thereon such that the frozen adhesive thaws and polymerizes after implantation. In some embodiments, the frozen adhesive may thaw and polymerize due to natural body heat, while in other embodiments, polymerization of the frozen adhesive may advantageously be accelerated using the heating features discussed above. 
     According to the present invention, a method for implanting a pedicle fastener includes forming a hole in a pedicle having a diameter sufficient to surround the shank of the pedicle fastener, applying an adhesive to the hole and sliding the shank of the pedicle fastener into the hole. The method may also include forming the adhesive by mixing a prepolymer component, a polyol component and a filler material. 
     According to some embodiments of the present invention, the adhesive may be deposited on the shank of the pedicle fastener such that the adhesive is applied to the hole when the shank is slid therein. When depositing the adhesive on the shank, it may be desirable to apply the adhesive to at least one shielding feature formed in the shank so that the adhesive is not scraped off of the shank upon insertion into the hole. 
     According to the present invention, the adhesive may also be applied to the hole by injecting the adhesive through a syringe. In some embodiments, the adhesive may be injected through a longitudinal cannula formed in the pedicle fastener after the pedicle fastener has been inserted into the pedicle hole. 
     According to some embodiments of the present invention, the method may also include heating the pedicle fastener to accelerate polymerization of the adhesive. In some embodiments, an electric heating element may be connected to the head of the pedicle fastener. In other embodiments, a heating element may be inserted into a longitudinal cannula extending through at least a portion of the pedicle fastener. In some embodiments, the pedicle fastener may be heated under a heat lamp or may be preheated prior to insertion into the pedicle hole. 
     These and other objects, features and advantages of the present invention will become apparent in light of the following detailed description of non-limiting embodiments, with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side perspective view of a pedicle fastener according to an embodiment of the present invention; 
         FIG. 2  is a perspective view of an embodiment for forming an adhesive for augmenting the pedicle fastener of  FIG. 1 ; 
         FIG. 3  is a perspective view of a kit for implanting the pedicle fastener of  FIG. 1 ; 
         FIG. 4  is a cross-sectional view of the pedicle fastener of  FIG. 1  immediately after implantation; 
         FIG. 5  is a cross-sectional view of the implanted pedicle faster of  FIG. 4  after curing of the adhesive; 
         FIG. 6  is a cross-sectional view of adhesive being applied to a bone hole according to another embodiment of the present invention; 
         FIG. 7  is a cross-sectional view of the bone hole of  FIG. 6  with a pedicle fastener inserted therein; 
         FIG. 8  is a cross-sectional view of an implanted pedicle fastener according to another embodiment of the present invention; 
         FIG. 9  is a side perspective view of a pedicle fastener according to a further embodiment of the present invention; 
         FIG. 10  is a side perspective view of a pedicle fastener according to a further embodiment of the present invention; 
         FIG. 11  is a side perspective view of a pedicle fastener according to a further embodiment of the present invention; 
         FIG. 12  is a cross-sectional view of the pedicle fastener of  FIG. 11 ; 
         FIG. 13  is a cross-sectional view of adhesive being applied to a pedicle fastener according to another embodiment of the present invention; 
         FIG. 14  is a side perspective view of the pedicle fastener of  FIG. 1  according to yet another embodiment of the present invention; 
         FIG. 15  is a cross-sectional view of a further embodiment of a pedicle fastener according to the present invention; 
         FIG. 16  is a partial cross-sectional view of another embodiment of a pedicle fastener according to the present invention; 
         FIG. 17  is a side perspective view of the pedicle fastener of  FIG. 1  according to yet another embodiment of the present invention; 
         FIG. 18  is a cross-sectional view of a further embodiment of a pedicle fastener according to the present invention; 
         FIG. 19  is a cross-sectional view of another embodiment of a pedicle fastener according to the present invention; 
         FIG. 20  is a cross-sectional view of yet another embodiment of a pedicle fastener according to the present invention; 
         FIG. 21  is a cross-sectional view of an embodiment for accelerated polymerization of adhesive according to the present invention; and 
         FIG. 22  is a cross-sectional view of another embodiment for accelerated polymerization of adhesive according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     Referring to  FIG. 1 , a pedicle fastener  10  includes a head  12  and a shank  14  extending outwardly from the head  12  to a shank tip  16 . The head  12  may be similar to known pedicle screw heads, including at least one suitable connection feature  13  for rods, plates or the like. The shank  14  has at least one shielding feature  18  formed therein. For example, the shielding features  18  may be a plurality of annular grooves  20  formed in an outer surface  22  of the shank  14  and extending around a circumference thereof. The annular grooves  20  are separated by ridges  26  also extending around the circumference of the shank  14 . In some embodiments, the annular grooves  20  and the ridges  26  may include smooth transitions therebetween, providing the outer surface  22  with an undulating sinusoidal-type profile. 
     Although shown as a single unit, in some embodiments, the pedicle fastener  10  may be part of a spinal fusion device (not shown) including the pedicle fastener  10 , a rod (not shown) and a locking member (not shown). Like known pedicle screws, the head  12  and the shank  14  may be swivelable or polyaxial and, in some embodiments, may be able to be connected and disconnected from one another, depending upon the intended application. 
     The annular grooves  20  and ridges  26  allow the shank  14  to be augmented by an adhesive  28  to improve fixation of the pedicle fastener  10 . In particular, the adhesive  28  may be applied to the shank  14  within the annular grooves  20 , where it will be protected during installation by the ridges  26 , as will be discussed in greater detail below. 
     The adhesive  28  is preferably a reactive biocompatible polymeric material, which has adhesive characteristics. Additionally, the adhesive  28  is preferably osteoconductive when used in medical procedures. An example of one suitable reactive biocompatible polymeric material for the adhesive  28  is the KRYPTONITE™ bone cement product, available from DOCTORS RESEARCH GROUP, INC. of Southbury, Conn., which is described in U.S. patent application Ser. No. 11/089,489, which is hereby incorporated by reference in its entirety. 
     Referring to  FIG. 2 , in some embodiments, the adhesive  28  may be formed by combining a prepolymer component  30  and a polyol component  32 , along with an optional filler material  34 , and permitting the combination to react to form the adhesive  28 . 
     The prepolymer component  30  for forming the adhesive  28  includes prepolymer molecules formed by reacting diisocyanate with polyol. The prepolymer component  30  may be a true prepolymer, formed with a two to one ratio of diisocyanate to polyol, or the prepolymer component  30  may be a quasi-prepolymer, formed with a ratio of diisocyanate to polyol in excess of two to one. As will be understood by those skilled in the art, a broad variety of diisocyanates and polyols may be suitable for use in the prepolymer component  30  and the adhesive  28  of the present invention. Both aromatic and aliphatic diisocyanates may be used to form the prepolymer component  30  of the present invention. The polyol used to form the prepolymer component  30  may be the same as or different than the polyol of the polyol component  32 . Additionally, the polyol used to form the prepolymer component  30  may be a blend of different polyols to achieve desired properties. Various polyols suitable for synthesis with the diisocyanate will be discussed in greater detail below. 
     The polyol component  32  for forming the adhesive  28  may include naturally occurring polyols and biocompatible, synthetic polyols, and mixtures thereof to achieve desired properties in the adhesive  28 . The polyol component  32  preferably also includes a catalyst for controlling and/or reducing the time required for polymerization of the adhesive  28 . Additionally, the polyol component  32  may include water, which is known to react with diisocyanate to produce carbon dioxide. Thus, the water may be provided to react with the diisocyanate to generate a sufficient amount of carbon dioxide to impart a degree of porosity to the adhesive  28 . Alternatively, rather than including water in the polyol component  32 , moisture from the atmosphere or moisture included in the optional filler material  34  may impart the degree of porosity to the adhesive  28 . Additionally, in instances where moisture is provided from the atmosphere or within the optional filler material  34 , it may be desirable to dry the polyols to provide improved control over the amount of carbon dioxide produced and, therefore, the degree of porosity imparted to the adhesive  28 . 
     The optional filler material  34  for forming the adhesive  28  may include, but is not limited to, calcium carbonate, bone (e.g., demineralized bone, allograft bone, and/or autogenous bone), calcium phosphate, calcium pyrophosphate, hydroxyapatite, poly methyl methacrylate, glass-ionomer, calcium sulfate, tricalcium phosphate (e.g., beta tricalcium phosphate), or any combination thereof, or the like. In certain embodiments, the filler material  34  may be chosen so as to impart a desired degree of porosity to the adhesive  28 . For example, the filler material  34  may include water for reacting with the diisocyanate of the prepolymer component  30  to generate carbon dioxide and impart the porosity to the adhesive  28 . The filler material  34  may also be present in the adhesive  28  in an amount sufficient to modify the adhesive&#39;s mechanical properties (e.g., compressive strength, compressive modulus, Young&#39;s Modulus of Elasticity, flexural strength, and the like). The filler material  34  may also comprise calcium carbonate and, in certain of these embodiments, the filler material  34  may comprise calcium carbonate in an amount sufficient to provide free calcium to a body of a mammal and enhance osteoconductivity. In some embodiments, the filler material  34  may be a material that allows visualization under radiographic imaging, such as barium sulfate, so that the condition of the adhesive  28  may be determined in situ through non-invasive imaging procedures. 
     Although the adhesive  28  may be formed with a variety of compositions to achieve desired properties, preferably, the adhesive  28  is a biocompatible polyurethane material, wherein the prepolymer component  30  includes aromatic pMDI diiscyanates synthesized with polyols derived from castor oil. The polyol component  32  preferably also includes polyols derived from castor oil and a small percentage of catalyst. The optional filler material  34  is preferably calcium carbonate powder, at a concentration of thirty percent (30%) by weight, with approximately ninety percent (90%) of the powdered particles being less than ten microns (10 μm) in diameter. 
     The adhesive  28  is initially prepared in a liquid state when the prepolymer component  30 , polyol component  32  and optional filler material  34  are combined. The adhesive  28  is chemically adhesive in this liquid state. As the adhesive  28  cures, it passes through a taffy-like state, in which the adhesive  28  is still chemically adhesive and is also easily malleable and may be shaped and sculpted. The biocompatible polymeric material then passes into a putty-like state in which the material&#39;s adhesive properties are reduced and the material is easily malleable and may be shaped and sculpted. The adhesive  28  then cures into a final solid state. As the adhesive  28  cures, it becomes more viscous and less adhesive, i.e., the adhesive  28  loses tackiness, which, as used herein, is defined as the ability of the adhesive  28  to be slightly adhesive or gummy to the touch or to adhere to a dry surgical instrument, for example, a freer. Additionally, the adhesive  28  expands as it cures from the liquid state to the final solid state. Since the adhesive properties of the adhesive  28  are greatest when the material is in the liquid or taffy-like state, the adhesive  28  is preferably in either the liquid state or the taffy-like state when used to augment the pedicle fastener  10 , shown in FIG. 
     Referring to  FIG. 3 , a kit  36  for implanting pedicle fasteners  10  includes at least one pedicle fastener  10 , as well as the prepolymer component  30  and polyol component  32  for forming the adhesive  28 . The kit  36  may also include the optional filler material  34  for forming the adhesive  28 , if desired. The prepolymer component  30 , polyol component  32  and optional filler material  34  are each held in a suitable container such as syringes  38 , canister  40  or the like. 
     Referring to  FIG. 4 , in operation, during spinal fusion a surgeon first prepares a hole  42  in bone  44 . For example, the surgeon may drill hole  42  or, in the case of a revision surgery, the hole  42  may be preexisting and may simply be cleaned or resized by the surgeon. Preferably, the hole  42  has a diameter that is as large as or larger than a diameter of the ridges  26  of the shank  14 . Preparation of the hole  42  preferably also includes cleaning and or drying the bone  44  forming the inner surface of the hole  42  to achieve a clean dry surface with which the adhesive  28  may bond. For instance, in some embodiments, a brush (not shown), such as a test tube brush, may be inserted into the hole  42  to clean the hole of debris. Additionally, or in the alternative, the hole  42  may be cleaned with a cleaning fluid, for example, by rinsing with a liquid (e.g. sterile saline) or by drying the bone  44  of the hole  42  with a gas such as compressed air or carbon dioxide through a commercial lavage system. In some embodiments, cleaning of the hole  42  may be achieved by applying a hemostatic agent to the hole  42 , for example, by applying the hemostatic agent on a brush used to clean the hole  42 . 
     As seen in  FIG. 2 , the surgeon may mix the prepolymer component  30 , polyol component  32  and optional filler material  34  from the kit  36 , shown in  FIG. 3 , to form the adhesive  28 . Referring back to  FIG. 4 , the surgeon may then apply the adhesive  28  to the shank  14  and, in particular, to the annular grooves  20  of the shank  14 . Preferably, the adhesive  28  is substantially viscous when applied to the shank  14 . The surgeon may then install the pedicle fastener  10  by sliding the shank  14  into the hole  42 . While the shank  14  is slid into the hole  42 , the adhesive  28  is shielded within the annular grooves  20  by the ridges  26  and, therefore, is not scraped out of the annular grooves  20  and off of the shank  14  by the surface of the hole  44 . The pedicle fastener  10  is then maintained in position within the hole  42  while the adhesive  28  cures. 
     Referring to  FIG. 5 , as the adhesive  28  cures, it expands outward from the annular grooves  20  and into the hole  42 , contacting the bone  44  and expanding into the porous structure of the bone  44 . Once the adhesive  28  has cured, it provides strong chemical adhesion between the pedicle fastener  10  and the bone  44  due to the adhesive characteristics of the adhesive  28 . Additionally, the adhesive  28  provides strong mechanical adhesion since the adhesive  28  cures at least partially within the annular grooves  20  and also expands at least partially into the porous structure of the bone  44  and cures therein. Thus, even though the pedicle fastener  10  is not threadedly engaged with the bone  44 , the pedicle fastener  10  has a strong mechanical bond therewith. Additionally, the adhesive provides a chemical bond between the pedicle fastener  10  and the bone  44  since the adhesive  28  was not scraped out of the annular grooves  20  by the bone  44  due to a threaded engagement. 
     In addition to the chemical and mechanical bonds provided by the adhesive  28 , the adhesive  28  may also be formulated to provide strength and stiffness similar to that of bone  44 , thereby reducing stress concentrations at the spinal fusion site. Additionally, the adhesive  28  may provide an osteoconductive structure to promote ingrowth of bone  44  into the adhesive  28  and hole  42 , thereby providing for improved patient recovery. 
     Referring to  FIG. 6 , in some embodiments of the present invention, after preparation of the hole  42  as discussed above, the hole  42  may be filled with adhesive  28  prior to insertion of the pedicle fastener  10 , shown in  FIG. 1 , therein. For example, the adhesive  28  may be injected into the hole  42  through syringe  38  or the like. Once the adhesive  28  has been injected into the hole  42 , the pedicle fastener  10  may be inserted therein. The shank  14  of the pedicle fastener  10  may have adhesive  28  applied thereto, for example, within the annular grooves  20  as discussed above or may be inserted without any adhesive  28  applied thereto, since adhesive  28  is already within the hole  42 . 
     Referring to  FIG. 7 , as the pedicle fastener  10  is slid into the hole  42 , the shank  14  displaces the adhesive  28  up and around the shank  14  within the hole  42 . The pedicle fastener  10  is then maintained in position within the hole  42  while the adhesive  28  cures. As the adhesive  28  cures, it expands, filling the annular grooves  20  and at least partially invading the porous structure of the bone  44 . As discussed above, once the adhesive  28  has cured, it provides strong chemical adhesion between the pedicle fastener  10  and the bone  44  due to the adhesive characteristics of the adhesive  28 . Additionally, the adhesive  28  provides strong mechanical adhesion since the adhesive  28  cures at least partially within the annular grooves  20  and also expands at least partially into the porous structure of the bone  44  and cures therein. 
     Referring to  FIG. 8 , wherein like numerals represent like elements, in some embodiments, particularly those where the adhesive  128  is applied to the hole  142  prior to insertion of the pedicle fastener  110 , the outer surface  122  of the shank  114  may be substantially smooth, rather than having shielding features  18 , shown in  FIG. 1 , formed therein. The shank  114  may have a circular cross-section to substantially conform to the shape of the hole  142  or the shape may have an acircular cross-section, e.g. an elliptical or oval cross-section, to provide space within the hole  142  between the bone  144  and the shank  114  for the adhesive  128  be displaced and expand. Acircular cross-sections may also be beneficial in that they may provide greater mechanical strength and/or stiffness in some planes than other planes, thereby allowing for optimization of both the mechanical properties and the adhesive potential of the pedicle fastener  110 . Additionally, pedicle fasteners  110  with acircular cross sections may also be beneficial since pedicle bones are typically acircular in cross section, such that a corresponding pedicle fastener geometry may provide a superior fit. Pedicle fasteners  110  with acircular cross sections may be particularly advantageous for use with pedicles devoid of cancellous bone, i.e. through osteoporosis or iatrogenic causes, since the pedicle fasteners  110  may be implanted within the voids formed by the absence of the cancellous bone and may be better shaped to fill said voids than other pedicle fasteners. 
     Since the adhesive  128  is applied to the hole  142  rather than the shank  114  prior to insertion of the shank  114  into the hole  142 , the shielding features  18 , shown in  FIG. 1 , are not necessary to protect the adhesive  128  from being scraped off of the shank  114  during insertion. The smooth outer surface  122  may more easily allow the adhesive  128  to be displaced upward around the shank  114  when the pedicle fastener  110  is implanted and may provide a broad contact surface with which the adhesive  128  may chemically bond. Although the total surface area of the shank  114  with the smooth outer surface  122  will be less than the shank  14  having shielding features  18 , shown in  FIG. 1 , the realized adhesive  128  to outer surface  122  contact area may be greater with the smooth outer surface  122 , since some of the annular grooves  20 , shown in  FIG. 1 , may not be completely filled by the displaced adhesive  28 , shown in  FIG. 1 , when the pedicle fastener  10 , shown in  FIG. 1 , is implanted. Thus, the pedicle fastener  110  with the smooth outer surface  122  may provide for increased chemical adhesion with the adhesive  128 . 
     Referring to  FIG. 9 , in some embodiments, the pedicle fastener  210  may include shielding features  218  that are flutes  246  extending longitudinally along at least a portion of the shank  214 . In operation, the pedicle fastener  210  is preferably implanted after adhesive  28 , shown in  FIG. 6 , has been injected into the hole  42 , shown in  FIG. 6 . In particular, as the shank  214  is slid into the hole  42 , shown in  FIG. 6 , the flutes  246  will facilitate displacement of the adhesive  28 , shown in  FIG. 6 , upward around the outer surface  22  of the shank  214 . Additionally, as the adhesive  28 , shown in  FIG. 6 , cures, the flutes  246  may provide an avenue for expansion of the adhesive  28 , shown in  FIG. 6 . The flutes  246  also provide the outer surface  222  of the shank  214  with an increased surface area with which the adhesive  28 , shown in  FIG. 6 , may bond. Although the various embodiments of shielding features  218  have been described separately herein for simplicity, it should be understood by those skilled in the art that the embodiments may be combined to achieve desired adhesive flow and bonding characteristics. For example, the pedicle fastener  210  may include both flutes  246  and annular grooves  20 , shown in  FIG. 1 , interconnected therewith to provide the outer surface  222  of the pedicle fastener  210  with a further increased surface area, while still providing flutes  246  for improved adhesive flow therethrough. 
     Referring to  FIG. 10 , in another embodiment of the present invention, the pedicle fastener  310  may be provided with shielding features  318  that include spiral flutes  348 , having both a longitudinal component and a circumferential component. As with the embodiments discussed above, the pedicle fastener  310  provides an increased surface area with which the adhesive may bond while still providing a path for adhesive displacement and expansion along the shank  314 . Due to the longitudinal and circumferential components of the spiral flutes  248 , this embodiment may be particularly beneficial where the pedicle fastener  310  is not only pushing into the hole  42 , shown in  FIG. 6 , but also rotated slightly while being inserted. 
     Referring to  FIGS. 11 and 12 , in some embodiments, the pedicle fastener  410  may include shank  414  bounding an inner longitudinal channel  450  that is substantially coaxial with the shank  414 , similar to a rolled pin. A longitudinal slot  452  passes through the shank  414  and is in communication with the longitudinal channel  450 . The longitudinal slot  452  preferably extends substantially the entire length of the shank  414 . This embodiment further increases the surface area of the shank  414  with which the adhesive  28 , shown in  FIG. 6 , can bond, since the adhesive  28 , shown in  FIG. 6 , may fill and bond with the longitudinal channel  450  in addition to the outer surface  422 . 
     Referring back to  FIG. 6 , thus far, the present invention has been described as having adhesive  28  applied to at least one of the shank  14  or the hole  42  prior to insertion of the pedicle fastener  10 . However, referring to  FIG. 13 , in some embodiments, the pedicle fastener  510  may be inserted into the hole  542  prior to application of the adhesive  528 . The pedicle fastener  510  includes head  512  and shank  514 , which are substantially similar to those discussed above. The outer surface  522  of the shank  514  may include shielding features  518 , such as the annular grooves  520  as shown, or may be substantially smooth as discussed above. The pedicle fastener  510  also includes an internal longitudinal cannula  554  extending through the head  512  and at least partially through the shank  514 . Connected to the longitudinal cannula  554  are a plurality of fenestrations  556  that extend outwardly through the outer surface  522 . Preferably, the fenestrations  556  extend into the shielding features  518 , if present. 
     In operation, the surgeon may push the pedicle fastener  510  into the hole  542  in substantially the same manner discussed above. In some embodiments, the surgeon may then spray a cleaning fluid, which may be a liquid (e.g. sterile saline) or a gas (e.g. compressed air or carbon dioxide from a lavage system), into the longitudinal cannula  554  through the head  512 . The cleaning fluid passes through the fenestrations  556  and is expelled into the hole  542 , thereby cleaning and/or drying the bone  544  prior to injection of the adhesive  528 . Alternatively, the surgeon may simply prepare and clean the hole  542  prior to insertion of the pedicle fastener  510  therein as discussed above. Using a syringe  538  or similar injection tool, the surgeon may then inject the adhesive  528  through the head  512  and into the longitudinal cannula  554 . The adhesive  528  passes through the fenestrations  556 , into the annular grooves  520  and into contact with the bone  544 . The adhesive  528  is then allowed to cure to form a bond between the pedicle fastener  510  and the bone  544  as discussed above. 
     Although described as having the adhesive  528  injected into the longitudinal cannula  554  after inserting the pedicle fastener  510  into the hole  542 , in some embodiments, the adhesive  528  may first be injected into the hole  542  as discussed above prior to insertion of the pedicle fastener  510 . Then, once the pedicle fastener  510  has been inserted into the hole  542 , a vacuum may be connected to the longitudinal cannula  554  to draw adhesive against the shank  514  and into the fenestrations  556  to provide improved adhesion therewith. Additionally, although described in connection with the embodiment having annular grooves  520 , the longitudinal cannula  554  and fenestrations  556 , or variations thereof, may be implemented in the other embodiments discussed herein. For example, referring back to  FIG. 11 , the longitudinal cannula  454  may extend through the head  412  and be in communication with the longitudinal channel  450  extending through the shank  414 , thereby allowing adhesive  528 , shown in  FIG. 13 , to be injected into the longitudinal channel  450  after insertion of the pedicle fastener  410  into hole  542 , shown in  FIG. 13 . 
     Referring to  FIG. 14 , in another embodiment of the present invention, the pedicle fastener  10  may include a surface coating  58  to further improve adhesion between the pedicle fastener  10  and the adhesive  28 , shown in  FIG. 6 . The surface coating  58  may be, for example, formed from the same biocompatible polymeric material as the adhesive  28 , shown in  FIG. 6 . Preferably, once cured on the shank  14 , the surface coating  58  is machined to remove any nonporous skim coat that could inhibit a strong bond with the adhesive  28 . In some embodiments, the surface coating  58  may be hydrophobic such that it forces water and blood away from the surface to allow better wetting of the liquid adhesive  28  on the pedicle fastener  10 . Although shown with annular grooves  20 , the surface coating  58  may be applied to the various other shielding features  18  discussed herein or may be applied to the substantially smooth shank  114 , shown in  FIG. 8 . 
     Referring to  FIG. 15 , in some embodiments, the shank tip  616  may include a reverse taper  660  such that a diameter D Taper  of the distal end  662  of the reverse taper  660  is greater than the diameter of the shank  614 . The larger diameter D Taper  acts to center the fastener within the hole  642  and provides a substantially uniform gap between the shank  614  and the bone  644  to be filled by adhesive  628 . In some embodiments, the reverse taper  660  may not extend around the entire circumference of the shank tip  616 , but rather may include three or more reverse taper features radiating from the shank  614  to form an effective (circumscribed) diameter D Taper  that still centers the fastener within the hole  642 . This embodiment may be particularly beneficial, as the space between the reverse taper features may allow for the passage of adhesive  628  therebetween. In some embodiments, the shank tip  616  with the reverse taper  660  may also include a self-drilling tip  664  at the distal end  662  of the reverse taper  660  to allow insertion of the pedicle fastener  610  without additional instrumentation. 
     Referring to  FIG. 16 , in another embodiment of the present invention, the pedicle fastener  110  may have restrictive discs  166  connected thereto at two different longitudinal locations along the shank  114 . Preferably, one restrictive disc  166  is connected proximate to the shank tip  116  and the second restrictive disc  166  is connected proximate to the head  112 . The restrictive discs  166  are formed to be substantially the same diameter as the hole  142 . Thus, the restrictive discs  166  surround the outer surface  122  of the shank  114 , trapping the adhesive  128  therebetween. Locating one of the restrictive discs  166  proximate to the shank tip  116  may be particularly advantageous as it resists adhesive flow out of the higher density pedicle region and into the vertebral body. In some embodiments, the restrictive discs  166  may have one or more secondary holes  168  to allow a measured amount of adhesive  128  to flow past the restrictive discs  166 , for example to allow a predetermined amount of pressure relief as the adhesive  128  expands during polymerization, while still preventing excessive flow of the adhesive  128 . The restrictive discs  166  may be provided pre-attached to the pedicle fastener  110  or may me provided as part of the kit  36 , shown in  FIG. 3 , to be attached during implantation. Although shown in connection with the embodiment having a smooth outer surface  122 , the restrictive discs  166  may be applied to the other various embodiments discussed herein. 
     Referring to  FIG. 17 , in another embodiment of the present invention, a mesh sleeve  70  is provided for accommodating at least a portion of the shank  14  of the pedicle fastener  10 . The mesh sleeve has a generally cylindrical shape extending longitudinally from a first end  72  to a second end  74 . The first end  72  has a first opening  76  formed therein, while the second end  74  may be either open or closed as desired. The mesh sleeve  70  is preferably formed from metal and is relatively rigid. However, in other embodiments, a more pliable mesh sleeve  70  may be formed from a polymer or the like. The mesh sleeve  70  may be provided as part of the kit  36 , shown in  FIG. 3 . 
     In operation, the mesh sleeve  70  is pressed into the hole  42 , shown in  FIG. 6 , preferably after preparation of the hole  42 , shown in  FIG. 6 , as discussed above. The mesh structure of the mesh sleeve  70  may advantageously provide for a slight expansion in the longitudinal direction, while simultaneously providing a slight contraction in the diameter of the mesh sleeve  70 , which may ease insertion of the mesh sleeve  70  into the hole  42 , shown in  FIG. 6 . Alternatively, if a greater contraction in the diameter is necessary, the mesh sleeve  70  may include a longitudinal slit (not shown) extending from the first end  72  to the second end  74  to allow the mesh sleeve  70  to curl upon itself to reduce the diameter. Once the mesh sleeve  70  has been inserted in the hole  42 , shown in  FIG. 6 , adhesive  28 , also shown in  FIG. 6 , may be injected into the mesh sleeve  70  through the first opening  76 . The pedicle fastener  10  may then be pushed into the mesh sleeve  70  through the first opening  76 . The pedicle fastener  10  displaces the adhesive  28 , shown in  FIG. 6 , into and through the mesh sleeve  70 , where the adhesive  28 , shown in  FIG. 6 , interdigitates with the surrounding bone  44 , shown in  FIG. 6 . The mesh structure of the mesh sleeve  70  holds the adhesive  28 , shown in  FIG. 6 , in place, inhibiting it from inadvertently flowing down into the hole  42 , shown in  FIG. 6 , in the pedicle. Additionally, migration of the adhesive  28 , shown in  FIG. 6 , may be further inhibited if the second end  74  of the mesh sleeve is closed as discussed above. 
     The mesh structure of the mesh sleeve  70  may be selected to have a mesh size, i.e. the size of the openings of the mesh structure, and mesh pattern, i.e. shape and/or style of the mesh structure, to manage and/or control migration of the adhesive  28 , shown in  FIG. 6 , through the mesh sleeve  70 . For instance, in some embodiments, the mesh structure may include approximately square openings with a mesh size having approximate dimensions of less than two millimeters (2 mm) by two millimeters (2 mm). More preferably, the mesh size has approximate dimensions of less than one millimeter (1 mm) by one millimeter (1 mm). 
     Although the mesh sleeve  70  has been discussed as being provided separately from the pedicle fastener  10 , in some embodiments, the mesh sleeve  70  may be connected to the pedicle fastener  10 , for example by welding of the like. Similar to the embodiments discussed above, in the pre-connected embodiment of the mesh sleeve  70 , the adhesive  28 , shown in  FIG. 6 , may be injected in the hole  42 , shown in  FIG. 6 , prior to simultaneous insertion of the mesh sleeve  70  and the pedicle fastener  10 . Alternatively, the adhesive  28 , shown in  FIG. 6 , may be applied directly to the mesh sleeve  70 , preferably being pressed into the mesh structure, and then the mesh sleeve  70  and the pedicle fastener  10  may be simultaneously inserted into the hole  42 , shown in  FIG. 6 . 
     Referring to  FIG. 18 , in another embodiment of the present invention, an insert  778  may be provided for filling the hole  742 , particularly if the hole  742  is relatively large. The insert  778  may have a pre-threaded female thread pattern  780  formed therein for accommodating the pedicle fastener  710 . In this embodiment, the pedicle fastener  710  may be a pedicle screw having shielding features  716  in the form of a thread pattern  782  that matches the female thread pattern  780  of the insert  778 . In other embodiments, rather than the pre-threaded female thread pattern  780 , the insert  778  may include a pilot hole (not shown) that the surgeon may tap to a desired size to match the thread pattern  782  of the pedicle fastener  710  or that the surgeon may install a self tapping pedicle fastener  710  within. In yet other embodiments, the insert  778  may not include any hole feature, but rather may allow the surgeon to both drill and tap the insert  778  to provide a hole of desired size and location on the insert. In some embodiments, the exterior surface of the insert  778  may itself be threaded to provide enhanced mechanical fixation within the hole  742 . Preferably, the insert  778  is formed from substantially the same biocompatible polymeric material as that used to form the adhesive  728 . 
     In operation, after preparation of the hole  742 , the adhesive  728  may be applied to either the insert  778  or the hole  742  and then the insert  778  may be pushed into the hole  742 . The adhesive  728  is then allowed to cure, bonding the insert  778  to the hole  742 . The pedicle fastener  710  can then be threaded into the female thread pattern  780  or pilot hole (not shown) to affix a plate  783  or similar instrumentation thereto. The insert  778  advantageously allows a small pedicle fastener  710  to be screwed into a large bone hole  742 . This may be particularly beneficial when the pedicle fastener  710  must pass through a plate  783  that limits the length of the shank  714 . The insert  778  is also advantageous since it fills a majority of the hole  742  with a rigid volume that does not need to cure, whereas filling a similar hole  742  with only adhesive  728  would require significantly more curing time. 
     Referring to  FIG. 19 , in another embodiment of the present invention, the pedicle fastener  810  may include a shank  814  with a first longitudinal portion  884  having a thread pattern  882  and a second longitudinal portion  885  that is threadless. The pedicle fastener  810  may be advantageously applied to hole  842  having a first region  886  of good quality bone  844  and a second region  887  of poor quality bone  844 , for example osteoporotic bone. In operation, the hole  842  may be prepared as discussed above and then adhesive  828  may be applied to the hole  842  and/or the shank  814  also as discussed above. The shank  814  may then be inserted into the hole  842 , with the first longitudinal threaded portion  884  located in and threadedly engaged with the good quality bone  844  of the first region  886 . The second longitudinal threadless region  885  may be located in the second region  887  where the bone  844  is of poor quality. Thus, the mechanical connection between the good quality bone  844  and the thread pattern  882  as well as the adhesive  828  secures and stabilizes the pedicle fastener  810  in the first region  886 , while the pedicle fastener is secured in the second region  887  of poor quality bone  844  by the adhesive  828 . As should be understood by those skilled in the art, the second longitudinal portion  885  of the shank  814  may be smooth or may include the various shielding features  18 , shown in  FIG. 1 , discussed above. Additionally, as should also be understood by those skilled in the art, the location of the first longitudinal threaded portion  884  on the shank  814  may be varied to correspond to the first region  886  of good quality bone  844  and, in some embodiments, the shank  814  may include multiple longitudinal threaded portions  884  and/or multiple longitudinal threadless portions  885 . 
     Referring back to  FIG. 1 , the embodiments discussed above have all included applying adhesive  28  to the pedicle fastener  10  or to the hole  42 , shown in  FIG. 6 , substantially immediately prior to insertion of the shank  14  of the pedicle fastener  10  into the hole  42 . However, referring to  FIG. 20 , in some embodiments of the present invention, the pedicle fastener  910  may be provided with frozen adhesive  928  coating the shank  914 . In these embodiments, the adhesive  928  may be formed and applied to the shank  914  in substantially the same manner as discussed above. The pedicle fastener  910  may then be maintained at a decreased temperature to freeze the adhesive  928  to the shank and to prevent polymerization thereof. The pedicle fastener  910  may be stored in a sterile environment at the decreased temperature until needed for use in a spinal fusion procedure. 
     In operation, the pedicle fastener  910  with adhesive  928  frozen thereto may be pushed into the hole  42 , shown in  FIG. 4 , preferably after preparation thereof, as discussed above. The patient&#39;s natural body temperature will elevate the temperature of the adhesive  928 , which will cause the adhesive  928  to thaw within the hole  42 , shown in  FIG. 4 , and polymerize therein forming the adhesive bond between the bone  44 , shown in  FIG. 4 , and the shank  914 . This embodiment may be particularly beneficial since it eliminates the necessity to prepare the adhesive  928  during the spinal fusion procedure. Although shown in connection with the pedicle fastener  910  having annular grooves  920 , the frozen adhesive  928  may be equally applicable to the various other embodiments discussed herein. For example, in the embodiment having the mesh sleeve  70 , shown in  FIG. 17 , connected to the shank  14 , shown in  FIG. 17 , the adhesive  928  could be applied onto and into the mesh sleeve  70 , shown in  FIG. 17 , and frozen therein for future use. 
     Although pre-application of the adhesive  928  has been described in connection with freezing the adhesive  928  to inhibit polymerization thereof, other methods for inhibiting polymerization of the adhesive  928  and pre-applying the adhesive to the shank  914  of the pedicle fastener  910  are also within the scope of the present invention. For example, in some embodiments, moisture curing adhesive  928 , such as a one component moisture curing polyurethane, may be pre-applied to the shank  914  of the pedicle fastener  910  and then the pedicle fastener  910  may be packaged and/or stored in a moisture free environment until insertion into the hole  42 , shown in  FIG. 4  of the bone  44 , shown in  FIG. 4 . Once inserted into the hole  42 , shown in  FIG. 4 , moisture in the bone  44 , shown in  FIG. 4 , would then activate the moisture curing adhesive  928  and allow the adhesive  928  to fully cure, thereby securing the pedicle fastener  910  within the bone  44 , shown in  FIG. 4 . 
     In some embodiments of the present invention, it may be desirable to accelerate polymerization of the adhesive  28 , shown in  FIG. 6 , most particularly in embodiments implementing the frozen adhesive  928  discussed above. Referring to  FIG. 21 , the pedicle fastener  10  may be provided with a removable heating extension  88  that connects thereto, for example, through connection feature  13  in substantially the same manner as a stabilizing rod or the like. The removable heating extension  88  is conductive, such that when heated, it conducts the heat to the pedicle fastener  10 , which, in turn, heats up. The elevated temperature of the pedicle fastener  10  increases the temperature of the adhesive  10 , which accelerates polymerization thereof. For example, an increase in temperature of the adhesive  28  of approximately ten degrees Celsius (10° C.) approximately doubles the polymerization rate of the adhesive  28 . Once polymerization of the adhesive  28  is complete, the removable extension  88  may be removed from the head  12  of the pedicle fastener  10  and the pedicle fastener  10  may be attached to a stabilization rod or the like, as desired. In some embodiments, the removable heating extension  88  may be a break-away feature formed to be integral with the head  12  of the pedicle fastener  10 . In these embodiments, once heating is completed, the removable heating extension  88  may simply be broken off of the head  12 . 
     Referring to  FIG. 22 , in another embodiment, accelerated polymerization may be provided by a heating instrument  1089  having an electric heating element  1090  connected thereto. The heating instrument may include a handle  1092 , an activation switch  1094  and a temperature gauge  1096 . In some embodiments, the heating instrument  1089  may also include a temperature sensor that monitors and limits the peak temperature within the pedicle fastener  1010  to prevent excessive heating, which could damage surrounding tissue. The heating element  1090  may be long, slender and adapted to fit within the longitudinal cannula  1054  of the pedicle fastener  1010 . Preferably, various heating elements  1090  having varying diameters may be connectable to the heating instrument  1089  to provide functionality with different pedicle fasteners  1010  having different sized longitudinal cannulas  1054  formed therein. These heating elements may be formed from nichrome or the like. In some embodiments, at least a portion of the heating element  1090  may be adapted to simply rest in the stabilization rod connection feature  1013  of the head  1012  or may be adapted to be accommodated over or around a portion of the head  1012 . 
     In operation, once the pedicle fastener  1010  has been inserted into the hole  1042 , as discussed above, the heating element  1090  of the heating instrument  1089  may be inserted into the longitudinal cannula  1054  or may be brought into contact with the connection feature  1013  or a portion of the head  1012 . The surgeon may then activate the heating instrument using the activation switch  1094 . As the temperature of the heating element  1090  increases, heat is transferred through the shank  1014  and into the adhesive  1028 . As discussed above, the increase in temperature of the adhesive  1028  increases the polymerization rate thereof. Once polymerization of the adhesive  1028  is complete, the surgeon may deactivate the activation switch  1094  removed the heating element  1090  from the pedicle fastener  1010 . 
     In addition to the electrical and/or conductive heating discussed above for accelerating polymerization of the adhesive  1028 , various other forms of heating, i.e. radiant, chemical, frequency or other forms of conductive heating, may be used to heat the pedicle fastener  1010  and accelerate polymerization of the adhesive  1028 . For example, in some embodiments, a heat lamp may be position to direct radiant heat toward the spinal fusion site and the pedicle fastener  1010 . In some embodiments, the temperature of the pedicle fastener  1010  may be elevated prior to implantation, for example, in an oven, in hot water or the like, such that when inserted into the adhesive  1028 , the heat stored in the pedicle fastener  1010  accelerates polymerization of the adhesive  1028 . This embodiment may be particularly beneficial since it does not require a heat source that connects to the pedicle fastener  1010 . 
     The present invention advantageously provides various pedicle fasteners  10 ,  110 ,  210 ,  310 ,  410 ,  510 ,  610 ,  710 ,  810 ,  910  and  1010  and methods for implantation thereof that allow augmentation with adhesive  28 ,  128 ,  528 ,  628 ,  728 ,  828 ,  928  and  1028  without removal of the adhesive  28 ,  128 ,  528 ,  628 ,  728 ,  828 ,  928  and  1028  from the shank  14 ,  114 ,  214 ,  314 ,  414 ,  514 ,  614 ,  714 ,  814 ,  914  and  1014  of the pedicle fastener  10 ,  110 ,  210 ,  310 ,  410 ,  510 ,  610 ,  710 ,  810 ,  910  and  1010  during insertion. The shank  14 ,  114 ,  214 ,  314 ,  414 ,  514 ,  614 ,  714 ,  814 ,  914  and  1014  of the pedicle fastener  10 ,  110 ,  210 ,  310 ,  410 ,  510 ,  610 ,  710 ,  810 ,  910  and  1010  may advantageously be provided with various adhesive shielding features  18 ,  218 ,  318  and  518  and attachments to allow adhesive migration and expansion, where desired, while preventing unwanted migration and expansion. Additionally, the shank  14 ,  114 ,  214 ,  314 ,  414 ,  514 ,  614 ,  714 ,  814 ,  914  and  1014  may be connectable to known pedicle fastener heads  12 ,  412 ,  512  and  612  to provide interchangeability thereof. 
     Another advantage of the present invention is that it provides pedicle fasteners  10 ,  110 ,  210 ,  310 ,  410 ,  510 ,  610 ,  710 ,  810 ,  910  and  1010  having non-polymerized adhesive  28 ,  128 ,  528 ,  628 ,  728 ,  828 ,  928  and  1028  frozen thereto for direct implantation in the pedicle hole  42 ,  142 ,  542 ,  642 ,  742 ,  842  and  1042 . This advantageously eliminates the need to form the adhesive  28 ,  128 ,  528 ,  628 ,  728 ,  828 ,  928  and  1028  during the spinal fusion procedure, which may reduce the complexity of the procedure as well as the operating room time for performing the spinal fusion procedure. 
     A further advantage of the present invention is that it provides methods and devices for accelerating polymerization of the implanted adhesive  28 ,  128 ,  528 ,  628 ,  728 ,  828 ,  928  and  1028 , which can reduce operating room time for performing spinal fusion procedures. Additionally, the methods and devices for accelerated polymerization may be particularly beneficial for accelerating polymerization in connection with pedicle fasteners  10 ,  110 ,  210 ,  310 ,  410 ,  510 ,  610 ,  710 ,  810 ,  910  and  1010  having frozen adhesive  28 ,  128 ,  528 ,  628 ,  728 ,  828 ,  928  and  1028  deposited thereon. 
     A further advantage of the present invention is that the pedicle fastener  10 ,  110 ,  210 ,  310 ,  410 ,  510 ,  610 ,  710 ,  810 ,  910  and  1010  is compatible with known stabilization instrumentation, meaning that no customization is required for spinal fusion procedures. 
     Although this invention has been shown and described with respect to the detailed embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail thereof may be made without departing from the spirit and the scope of the invention. For example, although the pedicle fasteners have been described for providing stabilization after spinal fusion surgery, the pedicle fasteners may also be configured for other orthopedic applications such as fusion of critical defects in other bones.