Patent Publication Number: US-2006015104-A1

Title: Bone fixation assembly and method of securement

Description:
CROSS-REFERENCE  
      This application is a divisional application of application Ser. No. 10/815,160, filed Mar. 31, 2004 which is a continuation-in-part application of application Ser. No. 10/731,625, filed Dec. 9, 2003 which is a continuation-in-part application of application Ser. No. 10/615,196, filed Jul. 7, 2003 entitled Spinal Stabilization Implant and Method of Application which is incorporated herein by reference in its entirety and for all purposes. 
    
    
     FIELD OF THE INVENTION  
      The present invention relates generally to spinal fixation systems. More particularly, the present invention pertains to a spinal plate assembly which includes a mechanism for fixably attaching and locking bone fixation screws to the plate at desired angles and for simultaneously locking otherwise adjustable portions of the plate together.  
     BACKGROUND OF THE INVENTION  
      Spinal surgery on the lumbar and thoracic spines have classically been open operations, meaning that the instrumentation used is placed through an incision that exposes all of the spine to be instrumented, as well as a portion of spine above and below the area to be instrumented due to the need for proper visualization. This extensive exposure disrupts a considerable amount of tissue, particularly the lumbar paraspinal musculature which needs to be stripped off the vertebra bones for exposure. This stripping leads to muscle damage directly caused by either electrical cautery or manual cutting or indirectly by interruption of vascular supply to the muscle due to coagulation or cutting of vessels, and caused also by embarrassment of the vascular supply during the course of surgery due to compression by retractors on the muscle which are required to maintain exposure. In addition, spinal implants can impact upon the facet joints of the spine, particularly the upper most pair of pedicle screws, which can cause pain or dysfunction of the involved joint. This is due in part to the fact that the pedicle screw systems are designed to give stability without being made to respect normal anatomy. In other words, the spine is forced to fit the metal, instead of fitting the metal to the spine.  
      The present day surgical approach therefore has added to patient morbidity due to the extent of the surgical exposure, tissue damage done primarily to the posterior longitudinal musculature of the spine during the exposure, blood loss and risk of infection. Large open operations also tend to be the cause of significant postoperative pain and disability. Accordingly, these issues lead to longer hospital stays, higher postoperative complications, such as phlebitis and pneumonia brought on by immobility, and greater consumption of postoperative medications with their resultant side affects. In addition, the paraspinal muscle tissue damage has been implicated in the genesis of postoperative lumbar mechanical dysfunction and stiffness, leading to postoperative pain syndromes or failed back syndrome. Also, interference by metal implants of the normal function of the rostral facet joints has been implicated in the early degeneration of these joints, as well as pain and disability, all which could lead to other more involved surgeries.  
      It is a principal object of the present invention to provide a system, including the spinal implant and a delivery system for applying the implant which allows for minimally invasive placement of the spinal implant, thereby reducing the undesired aforedescribed disadvantages of the prior art surgical procedures.  
      Another object of the present invention is to provide a bone fixation assembly which provides polyaxial locking of the screws to the plate and simultaneously, as required, locking of otherwise adjustable portions of the bone plate together for use in the spinal stabilization application method disclosed in corresponding U.S. application Ser. No. 10/615,196.  
     SUMMARY OF THE INVENTION  
      The bone fixation assembly of the present invention includes a bone plate having through passages for inserting the threaded shafts of fastening screws to secure the plate to underlying bone. The threaded screw shaft is inserted through a bushing located in the through passage of the bone plate and threadably secured into the underlying bone. The bushing is configured and dimensioned whereby it is compressed against the head of the screw with cams which are actuated by rotating the bushing in the through passage of the plate whereby the screw is locked relative to the bone plate. The bushing may also simultaneously be compressed downwardly into a seat in order to clamp separate elements of an otherwise adjustable bone plate together to securely lock them.  
      The head of the bone fixation screw has substantially frusto-spherical shaped side surfaces and the bushing in which the screw head is received has an interior surface which defines a socket bore that extends through upper and lower surfaces of the bushing and is configured and dimensioned for polyaxial rotation of the screw head therein. Exterior surfaces of the bushing are configured and dimensioned for limited axial rotation within the through passage of the fixation device or bone plate. At least one slot is located in the side wall of the bushing for allowing inward compression of the bushing bore against the screw head. A cam mechanism is disposed between the through passage of the plate and the bushing and is configured and dimensioned for inwardly compressing the bushing upon axial rotation of the bushing in the through passage whereby the bore is compressed against the screw head for locking the screw at a desired attitude relative to the fixation device or plate.  
      The bushing socket bore is provided with a substantially frusto-spherical shape with a central longitudinal axis to provide initial polyaxial rotation of the screw head therein. One slot within the bushing may extend from the upper surface of the bushing on through the lower surface of the bushing whereby the bushing is generally C-shaped and may thereby be more readily inwardly compressed with a cam mechanism.  
      In a preferred configuration the through passage of the fixation device is provided with an inverted frusto-conical seat and the exterior surface of the bushing is provided with a mating inverted frusto-conical base configured and dimensioned for seating in this seat. The seat and base are coaxial with the central axis of the bushing and through passage. The cam mechanism may take on different configurations. For example, the cam mechanism may be a threaded engagement of thread cam ramps or the use of other types of cam ramps. For example, the cam mechanism may be comprised of annularly spaced upwardly extending ramp cams on the upper surface of the bushing and inwardly extending overhangs are provided on the through passage above the upper surface of the cams or bushing and this overhang is provided with downwardly facing cam following surfaces that are configured and dimensioned for engaging the ramp cams on the top of the bushing when the bushing is axially rotated in its seat. This rotation causes the bushing to be driven downwardly into its inverted frusto-conical seat by the ramp cams to thereby inwardly compress the bushing bore against the screw head. The cams and cam followers surfaces may also be provided for ridges to prevent back-out of the cams.  
      The bone fixation assembly of the present invention is intended to be used independently or in supplement to the bone fixation assembly and method of application described in the inventor&#39;s related application previously identified. The bone fixation device of this embodiment is adjustable and is provided with a first screw receiving socket element at a distal end of the plate assembly which is configured with a screw shank passage and a screw head seat for attachment to bone with the aid of a bone fixation screw. An elongate arm extends proximally from this first socket element and has an elongate through slot therealong. A second screw receiving socket element is provided and includes the aforedescribed through passage containing the bushing and cam mechanism. This second screw receiving socket element is slidably received over the arm with the socket bore thereof aligned over the slot for receiving the shank of a fixation screw therethrough for attachment to bone. The bushing seat includes portions of the through slot whereby the second socket element is clamped and locked to the arm when the bushing is pressed downwardly into the seat by the cam mechanism. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Other objects and advantages appear hereinafter in the following description and claims. The accompanying drawings show, for the purpose of exemplification, without limiting the invention or appended claims, certain practical embodiments of the present invention wherein:  
       FIG. 1  is a plan view of the bone fixation assembly of the present invention without inclusion of the screw head bushings;  
       FIG. 2  is a view in front elevation and in vertical mid cross section of the bone fixation assembly shown in  FIG. 1  as seen along section line A-A with inclusion of the screw head bushings;  
       FIG. 3  is a top view of the C-shaped compression bushing utilized in the assembly of  FIGS. 1 and 2 ;  
       FIG. 4  is a view in right side elevation of the bushing shown in  FIG. 3 ;  
       FIG. 5  is a view in front elevation of the bushing shown in  FIG. 3 ;  
       FIG. 6  is a view in left side elevation of the bushing shown in  FIG. 3 ;  
       FIGS. 7, 8 ,  9  and  10  are sequential schematic representations illustrating the operation of the locking mechanism for the assembly shown in  FIG. 1  as seen along a mid cross section;  
       FIG. 11  is a top view of an alternative embodiment of the C-shaped compression bushing to be utilized in the assembly of  FIGS. 1 and 2 ;  
       FIG. 12  is a view in front elevation of the bushing shown in  FIG. 11 ; and  
       FIGS. 13, 14  and  15  are sequential schematic representations illustrating the operation of an alternative embodiment of the locking mechanism for the assembly shown in  FIG. 1  as seen along section line B-B and incorporating the bushing shown in  FIGS. 11 and 12 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
      Referring first to  FIGS. 1 and 2 , the bone fixation assembly  10  of the present invention is provided for stabilization of the spine and is an improved modification of the implant plate assembly shown and described in the inventor&#39;s aforementioned copending application for use in the inventive procedure therein described for minimum invasive surgical implantation of a plate assembly for fixation of the spine. The assembly  10  is comprised of two separate portions, a first portion  11  and a second portion  12  which are adjustably assembled together. The first portion  11  includes a first receiving socket element  13  at the distal end  14  of assembly  10 . This first screw receiving socket element  13  is configured with a screw shank through passage  15  for attachment of element  13  to vertebra bone with the aid of a bone fixation screw  23  as seen in  FIG. 2 . The plan view of  FIG. 1  does not include the bone fixation screws and other interior parts which are included in  FIG. 2  in order to provide an exposed view of the screw shank through passage interiors of elements  12  and  13 .  
      First portion  11  further includes an elongate arm  18  extending proximally from the first socket element  13 . Elongate arm  18  is provided with an elongate through slot  20  therealong. The second portion  12  of assembly  10  comprises a second screw receiving socket element which is also configured with a screw shank through passage  22 . Second screw receiving socket element  12  is slidably received over arm  18  with its through passage  22  centered over and aligned over slot  20  for receiving the shank  24  of a fixation screw  23  therethrough for attachment to underlying vertebra bone. The bone fixation or fastening screws  23  have threaded shanks or shafts  24  for insertion through the respective through passages  15  and  22  and they also are provided with heads  25  which have substantially frusto-spherical shaped side surfaces.  
      Bushings  30  are provided for each socket element  12  and  13  to receive the respective screw heads  25 . These bushings have upper surfaces  31  and lower surfaces  32  and a side wall  33 . The detail of these bushings  30  are best illustrated in  FIGS. 3, 4 ,  5  and  6 .  
      The side wall  33  of each bushing  30  is provided with an exterior surface  34  which is configured in dimension for axial rotation within the respective through passages  15  and  22  of screw socket receiving elements  12  and  13 . The interior surface  35  of bushings  30  defines a socket bore that extends through the upper and lower surfaces  31  and  32  and is configured and dimensioned for polyaxial rotation of screw head  25  therein. Plural slots  36  are provided in the side wall  33  for allowing inward compression of bore  35  against screw head  25 . A cam mechanism  37  is disposed between through passages  15  and  22  and bushings  30  and this cam mechanism  37  is configured and dimensioned for inwardly compressing bushing  30  upon axial rotation of each bushing  30  in its respective through passage  15  and  22  whereby the bore  35  of bushing  30  is compressed against its respective screw head  25  received therein for locking the screw  23  at a desired attitude relative to the fixation plate or device  10 . The bushing socket bore  35  has a substantially frusto-spherical shape to compliment the screw heads  25  and has its central longitudinal axis perpendicular to upper and lower surfaces  31  and  32 . Also, one of the slots  36  in the form of slot  38  for bushing  30  extends fully through side wall  33  from the upper surface  31  through the lower surface  32 . This provides a C-shape to bushing  30  and permits greater compression of the bushing.  
      The bottom portion of each through passage  15  and  22  is provided with an inverted frusto-conical seat  39  and the exterior surface  33  of the bushings  30  are provided with a mating inverted frusto-conical base  40  configured and dimensioned for seating respectively in said seats  39 . Seat  39  and base  40  are coaxial with the central axis of the bushing bore  35 .  
      The cam mechanism  37  includes annularly spaced upwardly extending ramp cams  41  on the upper surface  31  of bushing  30  and inwardly extending overhangs  42  on the through passages  15  and  22  which are positioned above the upper surface  31  of cams  30 . Overhangs  42  are provided with downwardly facing cam following surfaces  43  configured and dimensioned for engaging the cam ramps  41  when bushing  30  is axially rotated in either through passage  15  or  22  whereby the bushing  30  is driven downwardly into seat  39  by the ramp cams  41  to thereby inwardly compress bushing bore  35  against a screw head  25 .  
      This cam mechanism  37  further includes radially extending ramp cams  44  on the exterior surface  33  of bushing  30  and these additional ramp cams are dimensioned and configured for also compressing socket bore  35  inwardly when bushing  30  is axially rotated in through passage  15  or  22  due to the manner in which the side walls of through passages  15  and  22  are configured. As illustrated in  FIGS. 3 through 6 , the ramp cams  41  and  44  are provided with ridges to prevent rotary back off of the bushing  30  after it has been secured within respective through passage  15  or  22 .  
      The bushing seat  39  for second socket receiving element  12  includes sloped mating portions  50  of through slot  22  for arm  18  whereby second socket receiving element  12  is firmly clamped to arm  18  when bushing  30  is pressed downwardly into through passage  22  onto seat  39  by the cam mechanism  37 . Bushing  30  not only securely locks screw head  35  at a desired attitude, but simultaneously also securely locks second screw socket receiving element  12  to arm  18  at the position desired. This locking capability is schematically illustrated step by step in  FIGS. 7 through 10 . The schematic illustrations are generally intended to show a cross section through the fixation device  10  of  FIG. 1  as seen along section line B-B. However, for the purposes of simplification of illustration, the exact orientation of the bushings  30  relative to the device  10  is not identical to that illustrated in  FIGS. 1 and 2 .  
       FIG. 7  illustrates the ready position as the parts are initially assembled ready for application. The bushing  30  has been inserted into socket receiving element  12 . This is accomplished at the manufacturing stage by compressing the C-shaped bushing  30  sufficiently that it will pass through upper passage  51  of element  12 . After insertion, bushing  30  is released from compression and the outer edges of upper surface  31  expand radially outward whereby they underlie overhangs  42 . This prevents bushing  30  from accidentally dislodging from element  12 .  
      Note that in this ready position the upper lip diameter d of bushing  30  is slightly less that the diameter of screw head  25  and that the lower lip diameter d′ is less than the diameter screw head  25 . Accordingly, in the second step of the process, screw shank  24  is inserted through the bushing bore  35  and on through passage  22  of element  12  and the head  25  is then forcibly radially expands bushing  30  and the head  25  snaps down into the bushing  30  where it is retained in bushing bore  35 , the diameter d′ being too small for forcible passage of the head  25  therethrough. This step is accomplished by screwing threaded shank  24  of screw  23  into underlying vertebra until head  25  snaps downwardly into bushing  30  as illustrated in  FIG. 8 . To accomplish this, screw  25  is of course rotated clockwise as indicated by the arrow.  
      The next step is then schematically illustrated in  FIG. 9  wherein bushing  30  is rotated counterclockwise as indicated by the arrow at the top of  FIG. 9 . This is accomplished by an outer 8 toothed Phillips&#39; type driver which engages slots  36  and which has a hollow shaft interior whereby it is arranged or coaxially received over a central hex-driver for driving the screws  23 . This combination of screwdrivers is not shown but can be easily visualized and permits the surgeon to retain screw head  25  stationary while rotating the bushing  30  counterclockwise.  
      Due to the cam mechanism  37 , which provides upwardly protruding cam ramps  41  and radially protruding ramp cams  44 , this counterclockwise turn of bushing  30  causes the radially extending ramp cams  44  to compress bushing  30  and corresponding bore  35  inwardly and to thereby firmly engage screw head  25  and continuing counterclockwise turning of bushing  30  also causes bushing  30  to drive downward into seat  39  as further illustrated in  FIG. 10  thereby locking screw head  25  in its trajectory relative to fixation device  10  due to the action of ramp cams  41  acting against follower cam surfaces  43  of overhangs  42 . This securely locks arm  18  relative to socket receiving element  12  and further securely locks screw  23  at the given attitude to the entire device  10 .  
      As is best illustrated in  FIG. 2 , the follower cams  43  of overhangs  42  may be provided with downwardly extending ramp cams as illustrated to compliment the upwardly extending ramp cams  41  of bushings  30 . The follower cam surfaces  41  and also the radially facing cam surfaces  49  of element  12  may be provided with complimentary ridges to prevent rotary back-out of the bushing  30  after it is locked into position.  
      Also, with reference to  FIGS. 1 and 2 , cam surfaces  49  of receiving element  12  are provided with locking recesses  65 . Bushing  30  only requires a one quarter counterclockwise turn to fully compress the bushing against screw head  25 . Accordingly, the recesses  65  are provided just past the point of maximum compression for bushing  30 . Two of these locking recesses are provided on opposite sides of element  12 , one for each radially protruding ramp cam  44 . Once bushing  30  has been fully compressed by the quarter counterclockwise turn, the bushing  30  is allowed very slight expansion whereby the corners of radially extending ramp cams  44  snap into the locking recesses  65 . This prevents the bushing  30  from turning clockwise and releasing itself and it also provides a mechanical feedback to the surgeon that the bushing  30  is fully locked. The incorporation of locking recesses  65  permits the elimination of the requirement of ridges on the ramp cams  41  and  44 . This arrangement also permits the bushing  30  to be turned counterclockwise against maximal torque beyond the quarter turn back to the resting point or starting point of the bushing through another quarter turn which permits release of the bushing  30  and screw head  25 . In this manner, the surgeon may elect to adjust the implant even after the bushing  30  has been locked.  
      The through slot  57  and retainer slot  56  on the proximal end  41  of bone fixation device  10  is provided for coupling the device to an insertion gun as described and illustrated in the inventor&#39;s aforesaid copending application for minimum invasive surgical application of the device of the present invention. For more information in this regard, one should refer to this document and it is accordingly incorporated herein by reference.  
      An alternative embodiment of the cam mechanism  37  is illustrated in  FIGS. 11 through 15 . In this embodiment, the C-shaped bushing  30  is again provided with an inverted frustoconical base portion  33  for mating and seating in the inverted frustoconical seat  39  of through passage  15  in socket element  13 . However, in this embodiment, the cam mechanism  37  is provided in the form of thread cam ramps by male threads  45  on the inverted frustoconical surface  33  of bushing  30  and mating female threads  46  on the inverted frustoconical mating seat of through passage  15 .  
       FIG. 13  illustrates the initial conditions of installation wherein the screw  23  is being inserted into the bore  35  of bushing  30 . Bushing  30  is retained in position in socket element  13  by means of overhangs  42  which overhang annular lip  47  of bushing  30 , thereby preventing back out of bushing  30 .  
      Once screw head  25  is forced downwardly as indicated by the arrow in  FIG. 14 , the C-shaped bushing  30  is spread and permits head  25  to enter and to be confined by the internal bore  35 . The screw head  25  is rotated clockwise by an appropriate screwdriver until the shank portion  24  is fully engaged in underlying bone (not shown).  
      At this point, a special screwdriver is utilized to engage the drive recesses  46  in the top  31  of bushing  30 , as is best illustrated in  FIG. 11 , and bushing  30  is thereby pushed downward and rotated counterclockwise as indicated by the arrow in  FIG. 15 . This causes the threads  45  and  46  of can mechanism  37  to engage and thereby further compresses C-shaped bushing  30  inwardly and downwardly until the protruding annular lip  47  engages under the annular seat  48 , whereby bushing  30  is engaged and prevented from backing out from its threaded engagement. This procedure securely locks the head  25  of screw  23  from further polyaxial rotation within the bore  35  of bushing  30 .