Abstract:
A method, system and apparatus for stabilizing a facet joint are provided herein. The surgical method includes angling a screw from a ventral and a lateral side of the spine toward a dorsal and medial side of the spine, and screwing the screw through a superior facet of an inferior vertebra and into an inferior facet of a superior vertebra. The system includes a screwdriver insertable into a surgical opening, a screw coupled to the screwdriver and a torque transmitter coupled to and rotatable with a head on the screwdriver. The screwdriver apparatus includes a handle and a head projecting at an obtuse angle from the handle. The head in turn is coupled to and rotatable with a torque transmitter. The accompanying facet screw includes a sharp and self-tapping point, an ellipsoidal shaft with progressively spaced threads, and a bulbous screw head. The optional cutter includes a narrow, longitudinal shaft with a two-handle closing mechanism.

Description:
BACKGROUND 
   As the lumbar spine ages, disc degeneration occurs. This degeneration causes a reduction in the vertical height of the disc, and a diminution of its viscoelastic properties. The profile of the spine also changes with age. The swayback curvature of youth becomes the flat-back of old age. As a result, arthritic changes occur in the facet joints due to the increased biomechanical stress on the posterior side of the spine. 
   With a recent increased understanding in the biomechanics of the spine, it is acknowledged that maintenance of the normal curvature of the lumbar spine is preferable. For example, it is now known that the instantaneous center of rotation in the lumbar spine is 7 mm or 8 mm anterior to the posterior edge of the vertebral body, and it is approximately 2 cm anterior to the posterior elements and the facet joints. These joints are arranged in a cross-sectional “J” shape, and are designed to stabilize the spine and transmit the biomechanical forces from one vertebra to another. 
   These joints are an integral part of the stability of the motion segment. The facet joints transmit torsional force, facilitating normal gait. So, when spinal fusions are considered, it is important to re-establish the normal biomechanical arrangement, and to restore the sagittal profile of the spine to obtain optimal results. Arthritic changes in the facet joints following disc degeneration can cause mechanical back paint. If they become excessive, these arthritic changes can cause spinal stenosis. 
   Historically, some of the first attempts to attain spinal fusion, or spinal arthrodesis, utilized a method whereby the facet joints were stabilized by fusion. Fusion of the motion segment entails removal of the cartilage from the facet joint, and then packing bone into this joint to obtain immobility. In so doing, forces were transmitted from one segment to the next by a bony connection rather than by a flexible connection. 
   Another prior art method of stabilizing the facet joint is shown in  FIG. 1 . In this method, a screw was inserted from a dorsal and medial approach, through the facet joint, running from the medial to lateral side of the spine. The screw passed through the inferior facet of the vertebra above, and as it crossed the joint, it penetrated the superior facet of the vertebra below. However, this original technique is flawed in that the surgeon is unable to visualize where the tip of the screw comes to rest. The greatest complication of this technique is that the tip of the screw can end up in the vicinity of the exiting segmental nerve root. Nerve root encroachment can, in turn, produce serious radicular pain, which is the major complication of this original method. Accordingly, a need exists for a method of stabilizing facet joints which improves the safety of the stabilizing screw. 
   SUMMARY 
   The current invention is directed to a method, system, and apparatus for stabilizing a facet joint. One embodiment of a method according to the current invention includes angling a screw from a substantially lateral side of the spine toward a dorsal and medial side of the spine and screwing the screw through a superior facet of an inferior vertebra and into an inferior facet of a superior vertebra. 
   In another embodiment, the screw continues to travel through the inferior facet of the superior vertebra and into a spinous process of the same superior vertebra. 
   In yet another embodiment, the inserting also includes providing a screwdriver coupled to the screw by an attachment end, and the screwing also includes uncoupling the screwdriver to the screw. 
   One embodiment of a system according to the present invention includes a screwdriver insertable into a surgical opening, a screw coupled to the screwdriver and a torque transmitter. The screwdriver in this embodiment may include a head, which is coupled to a handle and the torque transmitter. In this embodiment, the head is rotatable about an axis in response to the torque transmitted by the torque transmitter, which is rotatable about another axis. In such an embodiment the screw rotates with the head about the same axis. 
   In another embodiment, the screw and an attachment end of the head are formed in one piece. In still another embodiment, the screw is grasped by the head. 
   In yet another embodiment, the screwdriver according to the invention includes a head projecting from the handle at an angle between 75 degrees and 120 degrees. 
   In still yet another embodiment, the screwdriver includes a dial located rotatable on the handle and coupled to the torque transmitter to transmit torque from the dial rotation to the head. 
   In still yet another embodiment, the screwdriver includes a stationary section between the handle and the head, where the handle is rotatable and coupled to the torque transmitter to transmit torque to the head. 
   In still yet another embodiment, the torque transmitter is electrically coupled to a motor to transmit torque to the head. 
   In one embodiment of a facet screw according to the invention, the screw includes a screw head portion, a point portion and a shaft portion. In this embodiment, the shaft portion is substantially elliptical in shape and has threads that are spaced from each other at progressively greater distances on the end of the shaft portion closer to the screw head. In one embodiment, the screw can be formed wholly or partially of bioactive or bioabsorbable materials. 
   In one embodiment of a cutter for cutting a facet screw away from a facet screwdriver within a surgical opening according to the invention, the cutter includes a first and second handle rotatably connected and a longitudinal shaft extending from the first handle. In this embodiment, the longitudinal shaft is sufficiently narrow to allow viewing of the screw while both the screwdriver and cutter are within the surgical opening. The longitudinal shaft includes a first and second cutting edge which project at an angle from the shaft and are connected to the first and second handles such that when the handles are squeezed together, the cutting edges move closer to each other. 
   In another embodiment of a cutter, the first and second handles can be substituted with a ratchet mechanism rotatably coupled to the first and second cutting edges, such that when a activating end of the ratchet mechanism is rotated, the cutting edges move closer to each other. 
   In still another embodiment, the cutting edges project substantially orthogonal to the shaft and the first handle. Additionally, in yet another embodiment, at least one of the cutting edges includes carbide. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features and advantages of the present invention will be better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings where: 
       FIG. 1  is a side perspective view of a prior art facet joint fixation system. 
       FIG. 2  is a side perspective view of one embodiment of a system according to the invention. 
       FIG. 3   a  is side view of one embodiment of a screwdriver and a screw system according to the invention. 
       FIG. 3   b  is a side view of another embodiment of a screwdriver according to the invention. 
       FIG. 3   c  is a side view of yet another embodiment of a screwdriver according to the invention. 
       FIG. 3   d  is a side view of another embodiment of a screwdriver according to the invention. 
       FIG. 4   a  is a side view of an embodiment of a screwdriver and facet screw system according to the invention. 
       FIG. 4   b  is a side view of another embodiment of a screwdriver and facet screw system according to the invention. 
       FIG. 4   c  is a side view of another embodiment of a system according to the invention where the facet screw and a screw head of the screwdriver are formed in one piece. 
       FIG. 5  is a side view of an embodiment of a facet screw according to the invention. 
       FIG. 6  is a side view of an embodiment of a cutter according to the invention. 
   

   DETAILED DESCRIPTION 
   The present invention is directed to a method, system, and apparatus for stabilizing a facet joint in a spine.  FIG. 1  shows a prior art facet joint stabilization system. In this system, a screw  10  is inserted from the dorsal and medial sides of a spine  100  into facet joint  12  toward the ventral and lateral sides of the spine  100 . The screw  10  passes through the spinous process  30  to the inferior facet  14  of superior vertebra  18  and into the base of the superior facet  22  of the immediately inferior vertebra  24  pointing towards the transverse process  20 . 
   Nerve  26  exits from the spine  100  through neural foramen  28 . The spinous process  30  largely blocks the view of the exiting nerve  26  from the dorsal side. Because the surgeon&#39;s view of both the tip  32  of the screw  10  and the exiting nerve  26  is blocked, if the screw  10  is angled at even a slightly incorrect angle, the tip  32  of the screw  10  can hit the exiting nerve  26 . 
   In contrast to the system shown in  FIG. 1 ,  FIG. 2  shows one embodiment of the facet joint stabilization system and method of the present invention. In the method shown in  FIG. 2 , an angulated screwdriver  200  and screw  110  are inserted into a posterolateral surgical opening (not shown). The angulated screwdriver  200  then inserts the screw  110  into the base of the superior facet  122  of the inferior vertebra  124 . The angle of insertion is from the slightly ventral and substantially lateral (“inferolateral”) sides of spine  100  towards the dorsal and medial (“dorsomedial”) sides of the spine  100 . The screw  110  then passes through the inferior facet  114  of the immediately superior vertebra  118 . As the screw is inserted, the exiting nerve  126  is in plain sight, so the surgeon is better able to avoid encroaching on the nerve  126  with the screw  110 . 
   Although an angulated screwdriver is shown in  FIG. 2 , it should be understood that any device suitable for inserting screws into a patient as described above may be utilized with the method according to the present invention. 
   One embodiment of the angulated screwdriver  200  is shown in more detail in  FIGS. 3   a - 3   d . In the embodiment shown in  FIG. 3   a , The screwdriver  200  has a handle  202  and a head  204 . For purposes of this disclosure, a “handle” is a graspable shaft, and a “head” is the projecting end portion of the screwdriver. The head  204  can extend at an angle of between 0 and 100 degrees from the axis of the handle  202 . In one embodiment, the head is angled at between 70 and 80 degrees from the axis of the handle  202 , so that the head and the handle form an obtuse angle. In another embodiment, the angle of the head  204  is adjustable. The head  204  in the embodiment shown in  FIG. 3   a  includes an attachment end  205  configured to mate with screw head  220  of screw  110 . The head  204  is rotatable about its angled axis. 
   In the embodiment shown in  FIG. 3   b , the head  304  is coupled to a dial  306  at one end of the handle  302  by a bent axis  308 , such that when the dial  306  is turned relative to the handle  302 , the head  304  axially rotates with the dial  306 . The bent axis  308  can be any material capable of transmitting torque from the dial  306  to the head  304 . In one embodiment, the bent axis  308  is a braided cable, such as is used with a Bowden speedometer. 
   Although a dial is used to generate torque in the head of the embodiment shown in  FIG. 3   b , any suitable mechanism may be used. For example, in the embodiment shown in  FIG. 3   c , a stationary section  410  is coupled between the head  404  and the handle  402 , and the head  404  and handle  402  are coupled along bent axis  408  such that the head  404  rotates about its axis  408  when the handle  402  is rotated about its axis  408 . 
   Further, although any manually moved mechanisms are shown in  FIGS. 3   a - 3   c , in the embodiment shown in  FIG. 3   d , the head  504  is coupled to a motor  505 , which rotates the head  504  about its angled axis  508  and is activated by a switch  512  on the handle  502 . Although several torque transmitting devices are described above, one skilled in the art will recognize other suitable means for rotating the head about its angled axis, such systems are intended to be included in the present invention. 
   One exemplary embodiment of the screwdriver  200  where the screw is coupled to the screwdriver, discussed in relation to  FIG. 3   a , is shown in more detail in  FIGS. 4   a - 4   c . In the embodiment shown in  FIG. 4   a , the head  604  can be secured to the screw head  220  by a basket  605 . The basket  605  includes a circumferential edge  630 , which circumferentially surrounds and holds the screw head  220  securely to the screwdriver  601  through frictional forces or biasing means for example. In this embodiment, once the screw  600  is drilled into the facet joint (not shown), the retaining forces can be overcome by wiggling or popping the basket  605  away from the screw head  220  to allow the screwdriver  601  to be removed from the surgical opening (not shown). 
   In another embodiment shown in  FIG. 4   b , the attachment end  625  of the screwdriver  603  removably engages an opening (not shown) in the screw head  220 , and when the screw  600  is fixed in the facet joint, the attachment end  625  can be snapped away from the screw  600  to allow the screwdriver  603  to be removed from the surgical opening. 
   In the embodiment shown in  FIG. 4   c , the attachment end  655  is formed in one piece with the screw head  660 . After fixation of the screw  670  in the facet joint, the screw shaft  680  can be cut or snapped off from the screw head  660 , or the attachment end  655  can be cut or snapped off from the screw head  660  by a cutter (shown in  FIGS. 6   a  and  6   b ). In one embodiment, the screw can be snapped off with a torque of around 25-30 Newtons. 
   It is also within the scope of the invention to attach the screw to the attachment end by any other suitable means, such as through an adhesive, magnetic coupling, flange, clamp, etc., that is capable of holding the screw head onto the attachment end until it is secured in the facet joint. For example,  FIG. 4   d  shows an embodiment of the screw driver in which a simple biasing clamp is used to secure the screw to the driver. 
   Although any screw can be used with the screwdriver of the present invention, one preferred embodiment is shown in more detail in  FIG. 5 . The screw  700  of this embodiment has a sharp point  710 , which can dig into the bone and minimize skidding. In one embodiment, the screw is around 10-14 mm in length. The sharp point  710  is preferably self-tapping, so the passage through the facet joint does not have to be drilled. The screw  700  has a substantially ellipsoidal shaft  716 , which narrows toward the point  710  and shank  718 . The ellipsoidal shape allows the screw head  714  to compress onto the bone. In one embodiment, the shaft is around 2 mm in diameter. The spacings “s” between the threads  712  of this embodiment become progressively wider toward the screw head  714  of the screw  700  and project at a substantially perpendicular angle outward from the shaft  716 . 
   The screw head  714  in this embodiment is substantially round and bulbous, to allow the screw head  714  to project slightly from the bone. This projection would simplify removal through muscle or cutting of the screw  700 . As this embodiment of the screw  700  is inserted into and compresses the bone, a screwdriver removably fixed to the screw head  714  can be simply pulled away from the screw  700  and removed from the surgical opening, leaving the screw  700  embedded in the bone. In one embodiment, the screw is wholly or partially formed of bioactive or bioabsorbable material. 
   One exemplary embodiment of a cutter, which may optionally be used in conjunction with the screwdriver system shown in  FIG. 4   c , is shown in  FIG. 6 . The cutter  800  includes a first and a second handle  810 ,  812 , and a long, narrow shaft  820  with first and second cutting edges  822  and  824  projecting parallel to each other from its free end. In this embodiment, the cutting edges  822  and  824  project substantially orthogonally from the first handle  810  and the shaft  820 . Although a substantially orthogonal angle is likely more intuitive for a surgeon, any other angle of projection is also within the scope of this invention. 
   In one embodiment, the first and second cutting edges  820  and 822  contain carbide, but one skilled in the art will recognize that any material of sufficient strength to cut the attachment end or the narrow shaft of the screw will also be within the scope of this invention. 
   The first handle  810  is hinged to the second handle  812  and is integral with the shaft  820  and the first cutting edge  822 . The second handle  812  is coupled to the second cutting edge  824  such that when the second handle  812  is squeezed toward the first handle  810 , the second handle  812  pulls the second cutting edge  824  toward the first cutting edge  822 . When the first and second cutting edges  822  and  824  surround the attachment end of the screwdriver or the narrow shank, the attachment end or shank can be cut, allowing the screwdriver to be removed from the surgical opening. 
   A spring  830  can be added to cause the handles  810  and  812  to spring away from each other when the handles  810  and  812  are not squeezed together. Although the embodiment shown in  FIG. 6   a  depicts the spring  830  as two elastic, curved strips, any spring which counters a squeezing force can be used. A grip stabilizer  840  can also be added to the first handle  810  to assist the surgeon in squeezing the handles together. Alternatively, if a surgeon lacks sufficient hand strength to use the previous embodiment, a ratchet closure mechanism  811  can replace the first and second handles  810  and  812  and spring  830  to allow a surgeon to tighten the first and second cutting edges  820  and  822 . 
   Preferably, the shaft  820  is narrow enough to insert into an approximately 1.5″ diameter posterolateral surgical opening, which already contains the screwdriver, and allow enough light into the opening for the surgeon to see around it to the attachment end of the screwdriver or the narrow shank of the screw. The shaft  820  is also preferably long enough to reach the attachment end or the shank. 
   Although specific embodiments are disclosed herein, it is expected that persons skilled in the art can and will design alternate instruments and methods that are within the scope of the following claims either literally or under the Doctrine of Equivalents.