Patent Publication Number: US-2007123904-A1

Title: Distraction instrument and method for distracting an intervertebral site

Description:
BACKGROUND OF THE INVENTION  
      Spinal surgery involves many challenges as the long-term health and mobility of the patient often depends on the surgeon&#39;s technique and precision. One type of spinal surgery involves the removal of the natural disc tissue that is located between adjacent vertebral bodies. Procedures are known in which the natural, damaged disc tissue is replaced with an interbody cage or fusion device, or with a disc prosthesis.  
      The insertion of an article, such as an artificial disc prosthesis, presents the surgeon with several challenges. The adjacent vertebral bodies collapse upon each other once the natural disc tissue is removed. These bodies must be separated to an extent sufficient to enable the placement of the prosthesis. However, if the vertebral bodies are separated, or distracted, to beyond a certain degree, further injury can occur. The disc prosthesis must also be properly positioned between the adjacent vertebral bodies. Malpositioning of the prosthesis can lead to pain, postural problems and/or limited mobility or freedom of movement.  
      Instrumentation that uses linked mechanisms to distract the disc space has been developed. For example, one end of a distracting spreader is threaded and screwed into an internal threaded opening in the spine. Such threaded instruments can provide the surgeon with a mechanical advantage for slowly distracting the disc space by pushing a spreader through a hollow tube. Using this convention, however, leads to a slow distraction that requires the surgeon to turn the instrument many times to obtain the desired distraction. Further, the repeated turning motion can cause the instrument and implant to shift out of alignment during the distraction procedure.  
      Similarly, as disclosed in U.S. Pat. No. 6,755,841, a surgeon may apply an alternative method of distraction by impacting a spreader between two blades or rails of a distraction instrument. Impaction, however, is a less desirable means of distraction because it is less controlled and distraction occurs at a sudden and an increased rate. The result can be undesired overdistraction or damage to the neural tissue. Furthermore, some parallel distraction-type instruments force the surgeon to impact the implant endplates into the disc space without a distraction aid, and distraction of the disc space is only provided for the implantation of the implant core.  
      Greater accuracy and precision are critical for an artificial disc. Artificial discs offer several theoretical benefits over spinal fusion for chronic back pain, including pain reduction and a potential to avoid premature degeneration at adjacent levels of the spine by maintaining normal spinal motion. However, like spinal fusion surgery, surgical techniques and procedures do not always work reliably for artificial disc implantation. Despite existing tools and technologies, there remains a need to provide a device to facilitate the proper and convenient insertion of an object, such as a disc prosthesis, between adjacent vertebral bodies while minimizing the risk of further injury to the patient. There remains a need for improved instrumentation and techniques for disc space preparation and artificial disc implantation.  
     SUMMARY OF THE INVENTION  
      The invention generally is directed to a distraction instrument for distracting an intervertebral site and to a method of implanting an artificial disc or fusion implant into an intervertebral site.  
      One embodiment of the invention is a distraction instrument that includes a drive rod, a driver mechanism coupled to the drive rod, and a pair of arms linked to the driver mechanism. The driver mechanism can include a housing adapted to be coupled to the distraction arms and a gear mechanism disposed in the housing for distracting the intervertebral site. The gear mechanism can include a switch having at least two positions, wherein one position provides for a first linear movement of the drive rod and another position disposes the drive rod in a free-floating position. Optionally, the switch can have a position for a second linear movement of the drive rod such as where the first linear movement and the second linear movement are parallel to each other. The gear mechanism can be a ratcheting gear mechanism that includes a first wheel gear and a second wheel gear, wherein the first gear and the second gear are integrated. The drive rod engages the first wheel gear, and a first straight gear engages the second wheel gear, wherein the straight gear is engaged by the switch. In another embodiment, the ratcheting gear mechanism further includes a third wheel gear integrated with the first wheel gear, and a second straight gear engaging the third wheel gear, wherein the switch engages the first straight gear or the second straight gear. In one embodiment, the actuating mechanism is a lever. The drive rod can further include a plurality of teeth extending along a length of the drive rod and optionally a head at an end of the drive rod. In one embodiment, the head of the drive rod has a height in a range of between about 5.0 mm and about 30 mm. The head can include a pair of diametrically opposed wheels. In another embodiment, an implant holder is coupled to the rod at one end.  
      In a specific embodiment, the distraction instrument of the invention includes a pair of arms, a housing linking the pair of arms, a driver mechanism within the housing, a drive rod engaging the driver mechanism, and an implant holder coupled to the drive rod. Actuation of the gear mechanism causes the implant holder to move to an intervertebral site until the implant holder abuts vertebrae at the intervertebral site, at which point continued actuation of the gear mechanism causes the arms to move away from the intervertebral site, thereby disengaging the distraction instrument from the vertebrae.  
      In another embodiment, the distraction instrument includes a pair of arms, a housing linking the pair of arms, a gear mechanism within the housing, a drive rod engaging the gear mechanism, and a head at one end of the drive rod. Actuation of the gear mechanism causes the head to move the distraction instrument from a reduced position to a distraction position, thereby causing distraction of vertebrae at an intervertebral site into which a portion of the arms distal to the housing have been inserted.  
      A method of implanting an artificial disc or a fusion implant in an intervertebral site, includes the steps of preparing an intervertebral site, actuating a gear mechanism of a distraction instrument to distract the intervertebral site, and inserting at least a core of the artificial disc or a spacer of the fusion implant into the intervertebral site. In one embodiment, the gear mechanism is a ratcheting gear mechanism. The gear mechanism can be coupled to a drive rod and can include a switch having at least two positions, wherein one position provides for a first linear movement of the drive rod and another position that disposes the drive rod in free-floating position. The gear mechanism in this embodiment also includes a first wheel gear and a second wheel gear, wherein the first gear and the second gear are integrated, and wherein the first gear engages the drive rod. Also in this embodiment, a first straight gear engages the second wheel gear, wherein the straight gear is engaged or disengaged by the switch.  
      In one embodiment, the method further includes the step of inserting end plates of an artificial disc or of a fusion implant before distracting the intervertebral site. In this embodiment, the drive rod can include a head, whereby actuating the gear mechanism causes the head to separate the distal ends of arms inserted into the intervertebral site, thereby distracting the intervertebral site.  
      In an alternate embodiment of the method, the drive rod is coupled to an implant holder, wherein the implant holder is guided along a pair of arms of the distraction instrument. In this embodiment, actuation of the gear mechanism directs the drive rod and implant holder in a first direction until the artificial disc or fusion implant is inserted into the intervertebral site.  
      In one embodiment, the method further includes the step of removing the distraction instrument from the intervertebral site. One method of removing the distraction instrument includes actuating the gear mechanism. In one embodiment, actuating the gear mechanism to remove the distraction instrument causes movement of the drive rod in the same direction as during distraction of the intervertebral site. In another embodiment, the distraction instrument is removed by moving the switch to a removal position and repeatedly depressing a lever coupled to the gear mechanism until the distraction instrument moves from a distraction position to a reduced position.  
      The present invention provides many advantages, such as desired distraction without the use of distraction linkage or impaction. Further, the present invention utilizes a gear mechanism to control the amount of distraction to a surgical site. Contrary to some distraction instruments described previously, the gear mechanism of the present invention minimizes impaction of the spine. The use of the gear mechanism of the present invention decreases the amount of required impacts during the procedure, thereby significantly reducing the potential for neural damage.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a perspective view of the lower spine, highlighting a surgically prepared disc space;  
       FIG. 2A  is a perspective view of an embodiment of the distraction instrument of the invention in a retracted position, wherein the distraction instrument includes distraction arms, drive rod and drive mechanism;  
       FIG. 2B  is a perspective view of the embodiment of  FIG. 2A  in a distracted position;  
       FIG. 3A  is a perspective view of a driver mechanism of the embodiment of the invention shown at  FIG. 2A ;  
       FIG. 3B  is a plan view of the embodiment of  FIG. 3A ;  
       FIG. 3C  is a perspective internal view of one embodiment of the driver mechanism of  FIG. 3A ;  
       FIG. 3D  is a plan internal view of the embodiment of  FIG. 3A ;  
       FIG. 4A  is a perspective internal view of another embodiment of the driver mechanism, in a first driving mode;  
       FIG. 4B  is a plan internal view of an embodiment of  FIG. 4A ;  
       FIG. 4C  is a plan internal view of the embodiment of  FIG. 4A  in a removal mode;  
       FIG. 4D  is a plan interval view of the embodiment of  FIG. 4A  in a free-floating mode;  
       FIG. 5A  is a perspective view of another embodiment of a distraction instrument of the present invention in a non-distracted, or reduced, position;  
       FIG. 5B  is a plan view of the distraction instrument of  FIG. 5A ;  
       FIG. 6A  is a perspective view of the distraction instrument of  FIG. 5A  with an artificial disc core sandwiched between two implant endplates of the distraction instrument in a distracted position;  
       FIG. 6B  is a plan view of the embodiment of  FIG. 6A ;  
       FIG. 7A  is a plan view of the distraction arms of the embodiment of  FIGS. 6A and 6B , independent of other distraction tools;  
       FIG. 7B  is a perspective view of the distraction arms of  FIG. 7A ;  
       FIG. 8A  is a perspective view of a v-shaped hinge on the distraction arms of the embodiment of the invention shown in  FIGS. 6A and 6B ;  
       FIG. 8B  is a close-up view of the v-shaped hinge of  FIG. 8A ;  
       FIG. 9A  is a perspective view of an embodiment of a drive rod of the distraction instrument of the invention;  
       FIG. 9B  is a close-up plan view of the head portion of the drive rod of  FIG. 9A ;  
       FIG. 9C  is a close-up perspective view of the head portion of the drive rod of  FIG. 9A ;  
       FIG. 9D  is a plan view of the drive rod of  FIG. 9A ;  
       FIG. 10A  is a perspective view of another embodiment of a drive rod of the distraction instrument of the invention;  
       FIG. 10B  is a plan view of the embodiment of  FIG. 10A ;  
       FIG. 11A  is a perspective view of an embodiment of a distraction mechanism of the present invention at an insertion position;  
       FIG. 11B  is a perspective internal view of the embodiment of  FIG. 11A ;  
       FIG. 11C  is a perspective view of  FIG. 11A  in a retracted position; and  
       FIG. 11D  is a plan view of the embodiment of  FIG. 11A  of the retracted position. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The features and advantages of the invention will be apparent from the following more descriptive explanation of the invention&#39;s preferred embodiments, as illustrated in the accompanying drawings. The same number in different figures represents the same item. The drawings are not necessarily to scale, with emphasis instead being placed upon illustrating the principles of the invention.  
      In general, surgical implantation utilizes an anterior approach. During the surgery, a small incision is made in the abdomen below the umbilicus. Internal organs are carefully moved to the side so that the surgeon can visualize the spine. The surgeon then removes a portion of a disc as shown in  FIG. 1 , a perspective view of the lower region of spine  100 . This region comprises lumbar spine  120 , sacral spine  130 , and coccyx  140 . Lumbar spine  120  is comprised of five (5) vertebrae L5, L4, L3, L2, and L1 (not shown). Intervertebral discs  150  link contiguous vertebra from C2 (not shown) to sacral spine  130 , wherein a single quotation (′) denotes a damaged disc, for example  150 ′.  
      Intervertebral disc  150  is comprised of a gelatinous central portion called the nucleus pulposus (not shown) and surrounded by an outer ligamentous ring called the annulus fibrosus (“annulus”)  160 . The nucleus pulposus is composed of 80-90% water. The solid portion of the nucleus is Type II collagen and non-aggregated proteoglycans. Annulus  160  hydraulically seals the nucleus, and allows intradiscal pressures to rise as the disc is loaded. Annulus  160  has overlapping radial bands which allow torsional stresses to be distributed through the annulus under normal loading without rupture.  
      Annulus  160  interacts with the nucleus. As the nucleus is pressurized, the annular fibers prevent the nucleus from bulging or herniating. The gelatinous nuclear material directs the forces of axial loading outward, and the annular fibers help distribute that force without injury. Damaged disc  150 ′ is prepared to receive the artificial disc by removing a window the width of the artificial disc to be implanted from annulus  160  of damaged disc  150 ′. The nucleus pulposus of disc or intervertebral site  150 ′ is completely removed and ready for distraction procedures.  
      In a first embodiment of the distraction instrument of the present invention, shown in  FIGS. 2A and 2B , distraction instrument  200  includes drive rod  202 , distraction arms  204 ,  206  and driver mechanism  210 . Driver mechanism  210  operates by a gear mechanism, meaning that the incremental actuating movements of the gears propel drive rod  202  towards or away from intervertebral site  150 ′ ( FIG. 1 ). In turn, drive rod  202  moves linearly along an axis.  
       FIG. 2A  shows distraction instrument  200  in a reduced position.  FIG. 2B  shows distraction instrument  200  in a distracted position. In both  FIGS. 2A and 2B , drive rod  202  is coupled to implant holder  220 , which holds artificial disc  230 . Located between arms  204 ,  206  is drive rod  202 . Implant holder  220  is a work piece that is put in motion to transport artificial disc  230  into intervertebral site  150 ′ ( FIG. 1 ). Implant holder  220  is releasably coupled to both artificial disc  230  and drive rod  202 , wherein artificial disc  230  is located at a distal end of implant holder  220  and drive rod  202  is located at a proximal end. Implant holder  220  has guiding feature  221 ,  223  to align with guiding surfaces  205 ,  207  of distraction arms  204 ,  206  as implant holder  220  approaches to thereby insert artificial disc  230  into intervertebral site  150 ′ ( FIG. 1 ). Artificial disc  230  includes end plates  234 ,  236  and core  238 , which allows movement of end plates  234 ,  236  relative to each other, and is removably attached to implant holder  220 . It is to be understood that, rather than an artificial disc, a fusion implant can be surgically implanted by the apparatus and the method of the invention. A fusion implant can include, for example, endplates and a support member to properly space the endplates.  
      The proximal end of distraction arms  204 ,  206  are coupled to driver mechanism  210 . Distraction arms  204 ,  206  are coupled to driver mechanism  210  by a suitable linkage. In one embodiment, shown in  FIGS. 2A and 2B , the linkage includes slotted tabs  208 ,  209  of driver mechanism  210  and clips  211 ,  213  of distraction arms  204 ,  206  which lock onto slotted tabs  208 ,  209 . This linkage provides for limited movement of arms  204 ,  206  along the slots, and limited rotation of arms  204 ,  206  about the point of linkage with driver mechanism  210 . Blades  222 ,  224  are located at the distal ends  237 ,  239  of distraction arms  204 ,  206 . Distal ends  237 ,  239  bear against the boney endplates adjacent to intervertebral site  150 ′ ( FIG. 1 ) when distraction instrument  200  is in use. Outer surfaces  205 ,  207  act as guiding surfaces for implant holder  220  during the insertion of artificial disc  230 . Distal ends of arms  204 ,  206  have stops  237 ,  239  for engaging the anterior surface of intervertebral bodies adjacent to site  150 ′.  
      Drive rod  202  includes rack of teeth  232  along at least part of its length, to engage driver mechanism  210 . Actuation of driver mechanism  210  by depressing lever  254  drives implant holder  220  towards intervertebral site  150 ′ ( FIG. 1 ) until artificial disc  230  is located therein. Intervertebral site  150 ′ is distracted by the force of moving implant holder  220  between distraction arms  204 ,  206  toward blades  222 ,  224 . When artificial disc  230  is positioned at intervertebral site  150 ′, implant holder  220  is stopped by abutment of implant holder stop pairs  225 ,  227  against anterior portions of vertebrae, and no longer moves forward. At this point, the surgeon continues ratcheting movement by repeatedly depressing lever  254 . However, since implant holder stops  225 ,  227  prevent further movement of implant holder  220 , distraction arms  204 ,  206  now move relative to drive rod  202  and implant holder  220  in a direction indicated by arrow  233  and thereby remove blades  222 ,  224  from the disc space. Withdrawal of blades  222 ,  224  from the intervertebral site causes adjacent vertebrae to collapse onto artificial disc  230 . Pressure of the adjacent vertebrae on artificial disc  230  holds artificial disc  230  in place while the surgeon removes distraction instrument  200  from the surgical site, thereby releasing artificial disc  230  from implant holder  220 .  
      In one embodiment, shown in  FIGS. 3A and 3B , housing  240  includes a gear mechanism that is a ratcheting gear mechanism. Referring to  FIGS. 3C and 3D , ratcheting gear mechanism  241  includes a gear system that engages rack of teeth  232 . Gear mechanism  241  includes two integrated wheel gears  244 ,  246  and straight gear  248 . Straight gear  248  is attached pivotally in housing  240  at post  252 . Lever  254  of ratcheting gear mechanism  241  is pivoted at a point  255  near post  252 , whereby directing lever  254  toward the remainder of driver mechanism  210  causes post  252  to move along slot  253  in a direction parallel to a major axis of drive rod  202 . Actuation of lever  254  thereby translates movement to straight gear  248 , which engages wheel gear  246 , rotating it counterclockwise. Because wheel gear  246  is also integrated with wheel gear  244 , wheel gear  244  also turns counterclockwise and engages rack of teeth  232  of drive rod  202  and moves drive rod  202  towards an intervertebral site. When the surgeon repeatedly depresses lever  254 , drive rod  202  is driven incrementally towards the intervertebral site.  
      In a second embodiment, ratcheting gear mechanism  243 , shown in  FIGS. 4A and 4B , includes three wheel gears and two diametrically opposed straight gears  267 ,  268 . The three wheel gears consist of two outer wheel gears  262 ,  266  and inner wheel gear  264 . Each of the outer wheel gears are paired with one of the two straight gears, and only one outer wheel gear-straight gear pair is actuated depending on the mode of driver mechanism  210 .  
      The mode of the ratcheting gear mechanism is controlled by switch  242 . In one embodiment, shown in  FIGS. 3C and 3D , there are two modes of operation: a driving mode and a free-floating mode. In another embodiment, shown in  FIGS. 4A through 4D , the gear mechanism can have three modes of operation: a driving, a removal and a free-floating mode. When switch  242  is set in a driving mode or position, shown in  FIGS. 4A and 4B , actuation of lever  254  engages straight gear  267  with outer wheel gear  266 , whereby inner wheel gear  264  linearly moves drive rod  202  along an axis. Switch  242  can also be set at a removal position, or mode, shown in  FIG. 4C , for removal of drive rod  202 , whereby ratcheting movement reverses the movement of drive rod  202  away from the intervertebral site. In this mode, actuation of lever  254  engages straight gear  268  with outer wheel gear  262 , thereby retracting drive rod  202 . The surgeon can then remove distraction instrument  300  from the surgical site and the patient. The ratcheting gear mechanism of this embodiment also has a free-floating position, or mode, shown in  FIG. 4D , wherein straight gears  267  and  268  disengage the wheel gears, allowing drive rod  202  to move freely.  
      Driver mechanism  210  can be adapted to be integrated with the distraction tools disclosed in U.S. Patent Application No. 2005/0027300, filed Mar. 31, 2004, the entire teachings of which are herein incorporated by reference.  
      In another embodiment of the distraction instrument of the present invention, shown in  FIGS. 5A and 5B , distraction instrument  300  includes drive rod  305 , distraction arms  304 ,  306  and driver mechanism  210 . Distraction arms  304 ,  306  are coupled to driver mechanism  210  and hold end plates  310 ,  312 . Distraction instrument  300  can use the same driver mechanism as distraction instrument  200  of the first embodiment of the present invention.  
      As shown in  FIGS. 6A and 6B , core  311  is sandwiched between end plates  310 ,  312  of implant  308 . It is to be understood, however, that, instead of the core of an artificial disc, a supporting spacer can be sandwiched between endplates of a fusion implant. During implantation, the distal ends of distraction arms  304 ,  306  bear against the anterior faces of vertebral bodies adjacent to site  150 ′ ( FIG. 1 ). The drive rod touches distraction arms  304 ,  306 , along the internal slots of distraction arms  304 ,  306 . When the surgeon repeatedly depresses lever  254 , drive rod  305  is driven incrementally towards intervertebral site  150 ′ ( FIG. 1 ). As drive rod  305  proceeds from one end of the rails toward intervertebral site  150 ′ ( FIG. 1 ), distraction arms  304 ,  306  spread apart. When arms  304 ,  306  are sufficiently apart, artificial disc core  311  is inserted between end plates  310 ,  312 .  
       FIGS. 7A and 7B  show distraction arms  304 ,  306  unattached to any driver mechanism  210 . Distraction arms  304 ,  306  are kept apart by v-shaped hinge  318 . Hinge  318  has a built-in spring mechanism that returns distraction arms  304 ,  306  from a collapsed position, wherein distraction arms are essentially parallel, to a desired angular position, shown in  FIGS. 7A and 7B .  
      As shown in  FIGS. 8A and 8B , hinge  318  is held together by knob  320 . Knob  320  also works as a connecting point in conjunction with bridge  322  to couple driver mechanism  210  to distraction arms  304 ,  306 . Bridge  322 , pivotally mounted on driver mechanism  210 , has a depression (not shown) at its tip. As bridge  322  folds to engage hinge  318 , a depression locks onto knob  320 , whereby driver mechanism  210  and distraction arms  304 ,  206  are together immobilized and coupled.  
      In one embodiment of the drive rod, shown in  FIGS. 9A through 9D , head  330  of drive rod  305  has a pair of diametrically opposed wheels  332 ,  334  that are fitted for and slide along the respective slots (not shown) of arms  304 ,  306  ( FIGS. 8A and 8B ). Inner slot surfaces  309  of arms  304 ,  306  act as guiding surfaces for drive rod  305  during distraction. In another embodiment of the drive rod, shown in  FIGS. 10A and 10B , head  336  of drive rod  307  does not have any wheels. Head  336  has a height (“h” in  FIG. 9B ) from 5.0 mm to 30 mm. Furthermore, as the intervertebral site is distracted by the force of driving drive rod  305  or drive rod  307  between distraction arms  304 ,  306  ( FIG. 8A and 8B ), the height “h” of head  330  or  336  determines how much the intervertebral site is distracted. Both drive rod  305  and drive rod  307  have a rack of teeth which engages driver mechanism  210 .  
       FIGS. 11A through 11D  show another embodiment of the gear mechanism and actuating mechanics of a distraction instrument of the present invention. In this embodiment, and referring to  FIGS. 7A and 7B , endplates of an artificial disc or a fusion implant, attached to distraction arms  304 ,  306 , coupled by hinge  318 , are inserted into an intervertebral site. Drive rod  398  ( FIG. 11A ), having drive rod head  399 , is coupled to driver mechanism  400  at the modular connection  402 . Modular connection  402  is a component of gear mechanism  412  shown in  FIG. 11B . Gear mechanism  412  includes wheel gears  410   a  and  410   b , arms  404   a  and  404   b , and halves  406   a  and  406   b . Halves  406   a  and  406   b  collectively form head  408 .  
      While gear mechanism  412  and the actuating mechanism are in a first position, shown in  FIGS. 11A and 11B , the surgeon impacts head  408  to drive rod head  399  down the length of distraction arms  304 ,  306 , thereby distracting the intervertebral site to provide a space between endplates held by distraction arms  304 ,  306 . The surgeon can then insert an artificial disc core or a fusion implant spacer between the endplates. Once the disc core is in place between the endplates, the activating mechanism is moved from the first, or driving, position, shown in  FIGS. 11A and 11B , to a second, or removal, position, shown in  FIGS. 11C and 11D . Actuation of the driver mechanism from the first to the second position causes wheel gears  410   a  and  410   b  to direct gear mechanism  412  away from the surgical site, thereby retracting drive rod  398 , which in turn allows distraction arms  304 ,  306  to collapse and, consequently, the endplates to rest on either side of the artificial disc core. Distraction arms  304 ,  306  can then be withdrawn from the surgical site by the surgeon, thereby dislodging the endplates from distraction arms  304 ,  306  and completing implantation of the artificial disc.  
      While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.