Patent Abstract:
A system and method for reinforcing bone in preparation for screw implantation. One system embodiment comprises a threaded and centrally bored cannula with a perforated distal end, a cannula applicator frictionally fitting within the central bore, a plunger translating within the central bore (the plunger having a internal longitudinal guide wire), bone cement, and a cannulated drill bit. One method embodiment comprises drilling and tapping a hole in a vertebral body, inserting the applicator into the central bore, screwing the cannula into the tapped hole by rotating the applicator, removing the applicator, injecting the bone cement into the central bore, distributing the bone cement out the holes in the distal end of the cannula and into the surrounding bone using the plunger, letting the bone cement harden, and drilling out the plunger using the cannulated drill following the guide wire.

Full Description:
FIELD OF THE INVENTION 
     This invention relates generally to a bone reinforcement process and surgical tool for, and more particularly, the present invention relates to an application device for injecting poly methyl methacrylate into a bone matrix through a canulated element through which a screw may subsequently be inserted. 
     BACKGROUND OF THE INVENTION 
     The bones and connective tissue of an adult human spinal column consists of an upper portion having more than 20 discrete bones, and a lower portion which consists of the sacral bone and the coccygeal bodies. The bones of the upper portion are generally similar in shape, however, they do vary substantially in size in accordance with their individual position along the column and are, therefore, anatomically categorized as being members of one of three classifications: cervical, thoracic, or lumbar. 
     These similarly shaped bones vary in size, but are each similarly coupled to the next by a tri-joint complex. The trijoint complex consists of an anterior disc and the two posterior facet joints, the anterior discs of adjacent bones being cushioned by cartilage spacers referred to as intervertebral discs. The posterior portion of the vertebral bone is coupled to the anterior portion by a pair of bone bridges referred to as pedicles, between which the spinal canal is housed. 
     In its entirety, the spinal column is highly complex in that it houses and protects critical elements of the nervous system which have innumerable peripheral nerves and arterial and veinous bodies in close proximity. In spite of these complexities, the spine is a highly flexible structure, capable of a high degree of curvature and twist through a wide range of motion. 
     Genetic or developmental irregularities, trauma, chronic stress, tumors, and disease, however, can result in spinal pathologies which either limit this range of motion, or which threaten the critical elements of the nervous system housed within the spinal column. A variety of systems have been disclosed in the art which achieve this immobilization by implanting artificial assemblies in or on the spinal column. These assemblies may be classified as anterior, posterior, or lateral implants. As the classification suggests, posterior implants are attached to the back of the spinal column, generally hooking under the lamina and entering into the central canal, attaching to the transverse process, or coupling through the pedicle bone. Lateral and anterior assemblies are coupled to the vertebral bodies. 
     The region of the back which needs to be immobilized, as well as the individual patient&#39;s anatomy, determine the appropriate surgical protocol and implantation assembly. Because the spine is routinely subject to high loads which cycle during movement, primary concerns of physicians performing spinal implantation surgeries focus on screw pull-out and screw failure. Screw pull-out occurs when the cylindrical portion of the bone which surrounds the inserted screw fails. Screw pull-out often an additional danger in that it often leaves the bone into which the screw was implanted completely useless with respect to continued implant support. This is especially true when the patient suffers from osteoporosis. In such patients the bone matter is often much less structurally supportive and lacks the necessary holding strength to prevent macromotion of the screws which may be implanted therein, thus severely limiting the immobilization potential of the assembly. 
     The use of artificial materials, such as bone cements and specific organic bone mimicking compounds such as poly methy methacrylate (PMMA), have been taught in the art as being effective in strengthening the osteoporotic bones to effect better immobilization of the screws. Percutaneous insertion of bone reinforcing agents has been successful in many instances, and is generally known as vertebroplasty. This “closed” use of PMMA and/or bone cement is useful in supporting subsiding bone masses in some instances, but is insufficient in those cases in which pedicle screw support is required. One of the failings of vertebralplasty, however, is that the cured PMMA/bone cement is often so much more dense and hard than the surrounding natural bone material that if subsequent screws need to be inserted, the bone drill is confounded by the difference in material properties. 
     The “open” use of PMMA and/or bone cement has been thought of as an alternative to “closed” use, especially when posterior implants are expected to be utilized. In such an instance, the patient&#39;s posterior spine is exposed and a bone drill is used to bore a hole through the pedicles for the posterior assembly to be implanted. Prior to the screws being implanted, however, the surgeon injects a quantity of PMMA/bone cement into the hole. Subsequently, the screw is inserted into the hole with the uncured cement. As the cement harden around the threads of the screw, however, the screw becomes thoroughly incarcerated in the hole, and is thus irretrievable. This presents a significant problem for potential revision surgery as well as being a cumbersome and time sensative process (as the PMMA/bone cement must not dry before the screw is implanted. 
     It is, therefore, the principal object of the present invention to provide a bone cement injector system for use in spine surgery wherein the surgeon has the ability to assemble the bone cement injectors without the time pressure of inserting the screws exactly after the material has been inserted. 
     It is also an object of the present invention to provide a bone cement injector system for use in spine surgery wherein the surgeon has the ability to insert the pedicle screws into a dried bone cement cavity which will support, but not incarcerate the screw against removal if necessary. 
     Other objects of the present invention not explicitly stated will be set forth and will be more clearly understood in conjunction with the descriptions of the preferred embodiments disclosed hereafter. 
     SUMMARY OF THE INVENTION 
     The preceding objects are achieved by the present invention, which is a system and method for reinforcing bone in preparation for screw implantation. A system of the invention in one embodiment comprises a threaded cannula having a central bore and a perforated distal end, a cannula applicator that is insertable into the central bore and which achieves a friction fit within the central bore, a plunger that is insertable into the central bore and which achieves an intimate fit within the central bore (the plunger having a guide wire passing through its central longitudinal axis), bone cement, and a cannulated drill bit. A method of the invention in one embodiment comprises drilling and tapping a hole in a vertebral body, inserting the applicator into the central bore of the cannula, screwing the cannula into the tapped hole by rotating the applicator, removing the applicator, injecting the bone cement into the central bore, distributing the bone cement out the holes in the distal end of the cannula and into the surrounding bone using the plunger, letting the bone cement harden, and drilling out the plunger using the cannulated drill following the guide wire. Thereafter, the surgeon can re-tap the hole and insert a bone screw into the reinforced vertebral body. 
     More particularly, a cannula of the invention has an elongated cylindrical body with a central bore, the body having a proximal end providing access to the bore (especially access by a cannula applicator, plunger, syringe and drill bit of the present invention, as described in greater detail below), and a distal end that is perforated. The outer surface of the cannula is threaded for engagement with threads of a tapped drill hole and to restrict proximal migration of the bone cement, as described in greater detail below. The cannula should be formed from biocompatible material (e.g., poly methyl methacrylate) inasmuch as it will become incarcerated into the target vertebral body in accordance with the procedures described herein. Preferably, the cannula has a radiodense tip that can be used to aid the surgeon in determining the position of the cannula after the cannula has been placed into the target vertebral body. 
     A cannula applicator of the invention has an elongated cylindrical body and is used to assist the surgeon in threading the cannula into a tapped drill hole and in determining the placement of the cannula in the vertebral body, as described in greater detail below. Accordingly, the applicator is dimensioned so that it can be placed into and removed from the proximal end of the cannula and so that when the applicator is placed into the bore of the cannula, it fits snugly within the bore. The intimate fit enables the applicator to provide structural support for the cannula as the cannula is twisted into the drill hole, and causes the applicator to grip the walls of the bore so that cannula will rotate when the applicator is rotated, so that the cannula will threaded into the drill hole. Preferably, the applicator comprises a radiodense material or is of a radiodense configuration, so that it can be used to determine the position of the cannula as the cannula is threaded into the drill hole. 
     A plunger of the present invention has an elongated cylindrical body that fits tightly within the bore of the cannula so that it can be used to squeeze bone cement out the holes in the distal end of the cannula as described in greater detail below. Preferably, the body is formed from a material that is softer than the biocompatible material from which the body of the cannula is formed. As described in greater detail below, this difference in material facilitates the drilling away of the plunger after it is used to distribute the bone cement. Also preferably, the plunger is formed from biocompatible material (e.g., poly methyl methacrylate), as some of the plunger may remain after most of the plunger has been drilled away, and the remaining portion would become incarcerated in the vertebral body. Also preferably, the body has a central longitudinal axis and an internal guide wire passing through the central longitudinal axis. As described in greater detail below, this guide wire also facilitates the drilling away of the plunger. 
     During use of the invention, upon proper preparation of the target vertebral body or bodies in accordance with known and accepted surgical procedures, the surgeon drills a hole in the target vertebral body. Then, the surgeon threads the hole using a tap in a manner known in the art. The surgeon repeats the above procedure for each hole he wishes to drill. 
     Next, the surgeon inserts into the hole a cannula of the present invention, using an appropriately sized cannula applicator of the present invention. The surgeon inserts the applicator into the proximal end of the cannula and into the bore of the cannula, establishing a tight fit of the applicator against the walls of the bore. Once the applicator is fitted into the bore, the surgeon places the cannula into the tapped hole, and repeatedly turns the applicator to screw the cannula into the hole to the desired position (typically, all the way into the hole). The intimate fit of the applicator in the bore facilitates the rotation of the cannula in response to the rotation of the applicator. The structural integrity of the applicator, in conjunction with the intimate fit of the applicator in the bore, provides structural support for the thin-walled cannula as the cannula is twisted into position. For each drilled hole, the surgeon places a cannula into the drilled hole using an appropriately sized applicator in accordance with the above procedure. The surgeon should leave each applicator in place until it is time to inject the bone cement, as described below. This will keep bleeding to a minimum and will continue to make possible radiographic assessments of the position of each applicator and accordingly each cannula. 
     Once each drilled hole has been fitted with a cannula, the surgeon prepares the appropriate bone cement mixture and loads one or more syringes with the bone cement, in a manner know in the art. Then, for each installed cannula, one at a time, the surgeon removes the applicator, injects an appropriate amount of the bone cement into the bore using the syringe(s), and applies a plunger of the present invention to distribute the bone cement through the holes of the distal end of the cannula. In order to effect this procedure for each cannula, the surgeon first removes the applicator from the cannula by pulling it from the bore. Next, the surgeon prepares the bone cement, loads the syringe(s), and injects the bone cement into the bore. Then, the surgeon inserts an appropriately sized plunger of the present invention into the distal end of the cannula and into the cannula bore, pushing the plunger down the bore so that the bone cement squeezes out the holes at the distal end of the cannula and into the bone surrounding the cannula. For each cannula, the surgeon leaves the plunger in until each plunger has been applied and the bone cement has set in the surrounding bone. The setting of the bone cement in the surrounding bone strengthens the surrounding bone in preparation for the next steps, which involve re-tapping the target vertebral body for a bone screw. 
     Once each plunger has been applied and the distributed bone cement has set, the surgeon drills out each plunger using a drill and cannulated drill bit. The surgeon selects a cannulated drill bit having an appropriate outer diameter, sets the drill bit into the drill, passes the drill bit over the guide wire extending from the plunger, and proceeds to drill into the plunger body, following the guide wire to ensure that primarily the plunger body is being drilled away. As noted above, the preferable softness of the plunger body relative to the cannula body facilitates the drilling away of primarily the plunger body. The surgeon repeats this procedure for each installed cannula. 
     Finally, the surgeon threads each hole that remains after each plunger has been removed, using a tap in a manner known in the art. Once each new hole has been tapped, the surgeon can insert a bone screw of the surgeon&#39;s choice into each hole, and complete the operation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 a ,  1   b  and  1   c  illustrate a cannula of an embodiment of the present invention, with FIG. 1 a  showing a side view of the cannula, FIG. 1 b  showing a cannula applicator of an embodiment of the present invention, and FIG. 1 c  showing a side view of the cannula engaged with the cannula applicator. 
     FIGS. 2 a  and  2   b  illustrate a plunger of an embodiment of the present invention, with FIG. 2 a  showing the plunger alone and FIG. 2 b  showing the cannula of FIG. 1 a  engaged by the plunger. 
     FIGS. 3 a-f  illustrate a method of an embodiment of the present invention, with FIGS. 3 a-b  illustrating the drilling and tapping of a hole in a target vertebral body, FIG. 3 c  illustrating the placement of the cannula of FIG. 1 a  in the hole, FIGS. 3 d-e  illustrating the filling of the cannula bore with bone cement, and the distribution of the bone cement out the holes in the distal end of the cannula using the plunger of FIG. 2 a , and FIG. 3 f  illustrating the drilling out of the plunger body from the cannula bore using the guide wire in the plunger as a guide. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     While the present invention will be described more fully hereinafter with reference to the accompanying drawings, in which particular embodiments and methods of implantation are shown, it is to be understood at the outset that persons skilled in the art may modify the invention herein described while achieving the functions and results of this invention. Accordingly, the descriptions which follow are to be understood as illustrative and exemplary of specific structures, aspects and features within the broad scope of the present invention and not as limiting of such broad scope. Like numbers refer to similar features of like elements throughout. 
     Referring now to FIGS. 1 a ,  1   b  and  1   c , a cannula  100  of the present invention is shown, with FIG. 1 a  showing a side view of the cannula  100 , FIG. 1 b  showing a cannula applicator  118  of the present invention, and FIG. 1 c  showing a side view of the cannula  100  engaged with the cannula applicator  118 . The cannula  100  has an elongated cylindrical body  102  with a central bore  104 , the body  102  having a proximal end  106  providing access to the bore  104  (especially access by a cannula applicator  118 , plunger  200 , syringe  300  and drill bit  318  of the present invention, as illustrated in other figures and as described in greater detail below), and a distal end  108  that is perforated with holes  110  as shown. The outer surface of the cannula  100  is threaded with outer threads  114  for engagement with inner threads of a tapped drill hole and to restrict proximal migration of the bone cement, as described in greater detail below, and for other reasons. The cannula  100  should be formed from biocompatible material (e.g., poly methyl methacrylate) inasmuch as it will become incarcerated into the target vertebral body in accordance with the procedures described herein. 
     For example in the illustrated embodiment, the body  102  of the cannula  100  has a length of 70.0 mm, an inner diameter of 4 mm, and an outer diameter of 6.5 mm. It should be understood that the cannula  100  can have other dimensions without departing from the scope of the present invention. For example, in some applications, a useful outer diameter would be 5.5 mm, 7.5 mm, or any measurement between 5.5 mm and 7.5 mm, and any other measurement less than 5.5 mm, or greater than 7.5 mm, as needed depending on the clinical application for which the invention is used, and the corresponding dimensions of the other instruments used with the cannula  100 . For another example, in some applications, a useful cannula length would be shorter or longer than 70.0 mm, as necessary or desirable depending on the depth of the drill hole. The illustrated embodiment has a cannula  100  with a body length of 70.0 mm, inner diameter of 4.0 mm, and outer diameter of 6.5 mm. An inner diameter of 4.0 mm, while not required, is useful for minimizing resistance to flow of the bone cement, as described in greater detail below. 
     Further preferably, the cannula  100  has a radiodense tip  116  that can be used to aid the surgeon in determining the position of the cannula  100  after the cannula  100  has been placed into the target vertebral body. While any radiodense material or configuration can be used to make the tip  116  radiodense, suitable examples include using metal, wires, beads or barium. Any method know in the art for determining the position of a radiodense mass in a vertebral body can be used to determine the position of the radiodense tip  116  in the target vertebral body. 
     Referring again to FIGS. 1 b  and  1   c , a cannula applicator  118  of the invention is shown, alone in FIG. 1 b  and in FIG. 1 c  engaged with the cannula  100 . The applicator  118  has an elongated cylindrical body and is used to assist the surgeon in threading the cannula  100  into a tapped drill hole (e.g., by providing structural support for the cannula  100  and allowing purchase of the cannula  100  so that the cannula  100  can be twisted into position) and in determining the placement of the cannula  100  in the vertebral body, as described in greater detail below, and for other purposes. Accordingly, the applicator  118  is dimensioned so that it can be placed into and removed from the proximal end  106  of the cannula  100  and so that when the applicator  118  is placed into the bore  104  of the cannula  100 , it fits snugly within the bore  104  as shown. For example in the illustrated embodiment, the diameter of the applicator  118  is 4.0 mm and its length is 100.0 mm. The intimate fit enables the applicator  118  to provide structural support for the cannula  100  as the cannula  100  is twisted into the drill hole, and causes the applicator  118  to grip the walls of the bore  104  so that cannula  100  will rotate when the applicator  118  is rotated, so that the cannula  100  will threaded into the drill hole. 
     Also preferably, the applicator  118  comprises a radiodense material or is of a radiodense configuration, so that it can be used to determine the position of the cannula  100  as the cannula  100  is threaded into the drill hole. While any radiodense material or configuration can be used to make the applicator  118 , suitable examples include using metal or barium. Any method know in the art for determining the position of a radiodense mass in a vertebral body can be used to determine the position of the applicator  118  in the target vertebral body. 
     Referring now to FIGS. 2 a  and  2   b , a plunger  200  of the present invention is shown, with FIG. 2 a  showing the plunger  200  alone and FIG. 2 b  showing the cannula  100  engaged by the plunger  200 . The plunger  200  has an elongated cylindrical body  202  that fits tightly within the bore  104  of the cannula  100  so that it can be used to squeeze bone cement out the holes  110  in the distal end  108  of the cannula  100  as described in greater detail below. For example in the illustrated embodiment, the body  202  has a length of 80.0 mm and a diameter of 4.0 mm. Preferably, the body  202  is formed from a material that is softer than the biocompatible material from which the body  102  of the cannula  100  is formed. As described in greater detail below, this difference in material facilitates the drilling away of the plunger  200  after it is used to distribute the bone cement. Also preferably, the plunger  200  is formed from biocompatible material (e.g., poly methyl methacrylate), as some of the plunger  200  may remain after most of the plunger  200  has been drilled away, and the remaining portion would become incarcerated in the vertebral body. Also preferably, the body  202  has a central longitudinal axis and an internal guide wire  204  (such as, for example, a k wire) or guide rod passing through the central longitudinal axis. As described in greater detail below, this guide wire  204  also facilitates the drilling away of the plunger  202 . 
     A use of the invention will now be described with reference to FIGS. 3 a-f . As illustrated in FIG. 3 a , upon proper preparation of the target vertebral body or bodies in accordance with known and accepted surgical procedures, the surgeon drills a hole  302  in the target vertebral body  300 , typically using drill bits of increasing diameter (e.g., starting with a 2.5 mm diameter bit and ending with a 4.0 mm diameter bit, in preparation for tapping the hole with a 5.25 mm diameter tap). 
     Then, as illustrated in FIG. 3 b , the surgeon threads the hole  302  using a tap in a manner known in the art, establishing threads  308  on the walls of the hole  302 . Preferably, a plurality of taps are provided, so that the surgeon can choose from taps with, for example, 5.25 mm, 6.25 mm or 7.25 mm diameters, depending on the size of cannula that the surgeon is planning to use for a particular patient. (For many applications, the use of a tap that is 0.25 mm diameter smaller than the cannula to be used is preferred.) Typically, during the preparation of the tapped hole  302 , the surgeon will use a probe to determine the proper angulation and depth of the hole  302 . The surgeon repeats the above procedure for each hole  302  he plans to drill. 
     Next, as illustrated in FIG. 3 c , the surgeon inserts into the hole  302  a cannula  100  of the present invention, using an appropriately sized cannula applicator  118  of the present invention. The surgeon inserts the applicator  118  into the proximal end  106  of the cannula  100  and into the bore  104  of the cannula  100 , establishing a tight fit of the applicator  118  against the walls of the bore  104 . It should be noted that the applicator  118  may already be inserted into the bore  104  before the surgeon is provided with the cannula  100 , so that procedural steps to be made by the surgeon can be minimized. Once the applicator  118  is fitted into the bore  104 , the surgeon places the cannula  100  into the tapped hole  302 , and repeatedly turns the applicator  118  to rotate the cannula  100  so that the outer threads  114  of the cannula  100  engage the threads  308  of the hole  302  and the cannula  100  is twisted deeper into the hole  302  to the desired position (typically, all the way into the hole  302 ). The intimate fit of the applicator  118  against the walls of the bore  104  facilitates the rotation of the cannula  100  in response to the rotation of the applicator  118 . The structural integrity of the applicator  118 , in conjunction with the intimate fit of the applicator  118  in the bore  104 , provides structural support for the thin-walled cannula  100  as the cannula  100  is twisted into position. Inasmuch as the applicator  118  is preferably radiodense, the surgeon is able to assess the position of the cannula  100  in a manner known in the art as needed until he is satisfied that the cannula  100  has been placed in the desired position. For each drilled hole  302 , the surgeon places a cannula  100  of the present invention into the drilled hole  302  using an appropriately sized applicator  118  in accordance with the above procedure. The surgeon should leave each applicator  118  in place until it is time to inject the bone cement, as described below. This will keep bleeding to a minimum and will continue to make possible radiographic assessments of the position of each applicator  118  and accordingly each cannula  100 . 
     As illustrated in FIGS. 3 d-e , once each drilled hole  302  has been fitted with a cannula  100  of the present invention, the surgeon prepares the appropriate bone cement mixture  310  and loads one or more syringes  312  with the bone cement  310 , in a manner know in the art. Then, for each installed cannula  100 , one at a time, the surgeon removes the applicator  118 , injects an appropriate amount of the bone cement  310  into the bore  104  using the syringe(s)  312 , and applies a plunger  200  of the present invention to distribute the bone cement  310  through the holes  110  of the distal end  108  of the cannula  100 . FIG. 3 d  illustrates the injection of the bone cement  310  into the cannula bore  104 . FIG. 3 e  illustrates the distribution of the bone cement  310  using the plunger  200 . Typically, the appropriate amount of bone cement will be 1.5 cc to 2.0 cc of cement per hole. In order to effect this procedure for each cannula  100 , the surgeon first removes the applicator  118  from the cannula  100  by pulling it from the bore  104 . The engagement of the threads  114  of the cannula  100  with the threads  308  of the drilled hole  302  prevent the cannula  100  from also being removed. Next, the surgeon prepares the bone cement  310 , loads the syringe(s)  312 , and injects the bone cement  310  into the cannula bore  104 . Then, the surgeon inserts an appropriately sized plunger  200  of the present invention into the proximal end  106  of the cannula  100  and into the cannula bore  104 , pushing the plunger  200  down the bore  104  so that the bone cement  310  squeezes out the holes  110  at the distal end  108  of the cannula  100  and into the bone  314  surrounding the cannula  100 . The engagement of the outer threads  114  of the cannula  100  with the inner threads  308  of the drilled hole  302  limit the migration of bone cement  310  out of the drilled hole  302  during the distribution process. For each cannula  100 , the surgeon leaves the plunger  200  in until each plunger  200  has been applied and the bone cement  310  has set in the surrounding bone  314 . The setting of the bone cement  310  in the surrounding bone  314  strengthens the surrounding bone  314  in preparation for the next steps, which involve re-tapping the target vertebral body for a bone screw. 
     As illustrated in FIG. 3 f , once each plunger  200  has been applied and the distributed bone cement has set, the surgeon drills out each plunger  200  using a drill  316  and cannulated drill bit  318 . The surgeon selects a cannulated drill bit  318  having an appropriate outer diameter (preferably, the outer diameter of the drill bit  318  has the same diameter as the diameter of the cannula bore  104  which in the illustrated embodiment is 4.0 mm), sets the drill bit  318  into the drill  316 , passes the drill bit  318  over the guide wire  204  extending from the plunger  200 , and proceeds to drill into the plunger body  202 , following the guide wire  204  to ensure that only the plunger body  202  material (and in some applications part, e.g., 05.mm, of the cannula  100 ) is being drilled away. As noted above, the preferable softness of the plunger body  202  relative to the cannula body  102  facilitates the drilling away of primarily the plunger body  202 . The surgeon repeats this procedure for each installed cannula  100 . 
     Finally, the surgeon threads each hole that remains after each plunger  200  has been removed, using a tap in a manner known in the art. Typically, a tap having a diameter of 5.5 mm to 7.5 mm will be useful, preferably matching the diameter of the cannula  100  that has been used. A tap suitable for use in the illustrated embodiment would have a diameter of 6.5 mm. Once each new hole has been tapped, the surgeon can insert a bone screw of the surgeon&#39;s choice into each hole, and complete the operation. 
     While there has been described and illustrated specific embodiments of an intervertebral spacer device, it will be apparent to those skilled in the art that variations and modifications are possible without deviating from the broad spirit and principle of the present invention. The invention, therefore, shall not be limited to the specific embodiments discussed herein.

Technology Classification (CPC): 0