Abstract:
A spinal implant is provided for retaining spinal elements in a desired spatial relationship. It includes a tensioning member positionable along the spinal column of a patient to match the contour thereof. A pair of buttress members are located each on one of the ends of the tensioning member, at least one of the buttress members being slidable on the tensioning member so that the distance between the buttress members may be adjusted.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Technical Field  
         [0002]     The present invention relates to apparatus and a method for retaining vertebrae of a spinal column of a patient in a desired spatial relationship. The invention will be described with particular reference to use as an anterior cervical spinal implant although it will be appreciated that the invention has wider application as will appear from the description that follows.  
         [0003]     Cervical spinal implants are used to maintain a desired spatial relationship of cervical vertebrae in patients suffering from a number of conditions such as degenerative disc disease, trauma including fractures, tumours, deformity or failed previous fusions.  
         [0004]     2. Description of the Prior Art  
         [0005]     The usual procedure for decompressing and stabilising cervical spine over several segments involves the introduction of a long bone graft, harvested either from iliac crest or fibula, and stabilised with a rigid elongate perforated plate, screwed to the vertebra above and below the graft.  
         [0006]     Whilst this usual procedure has provided satisfactory results in the past, there are numerous problems associated with it. First, it is difficult to introduce the bone graft into the vertebral defect under sufficient tension for the bone graft to be retained in a stable fashion. Secondly, the implanted plates sometimes work loose because the screws at either end are subjected to pull-out forces akin to those of a nail gripped by a claw hammer. Thirdly, because the screws are driven in from directly anteriorly, the vertical extent of the operative exposure is greater than the vertical extent of the vertebral defect, increasing the time, complexity and hazard of the procedure. Finally, the harvesting of the bone graft from iliac crest or leg is in itself a traumatic procedure and greatly adds to the patient&#39;s postoperative pain and disability, and prolongs rehabilitation.  
         [0007]     It is an object of the invention to provide a spinal implant which provides stability to the spine, without the need for harvesting of a large bone graft from another part of the patient&#39;s body.  
       SUMMARY OF THE INVENTION  
       [0008]     According to the invention there is provided an apparatus to retain in a desired spatial relationship, spaced apart portions of elements such as spinal elements, the apparatus including 
        a pair of spaced apart buttress members,     a tensioning member adjustably connected with at least one of said buttress members, and     retaining means to hold said tensioning member in a fixed position in relation to said buttress members, whereby said buttress members may be connected to the spaced apart portions of the elements, said tensioning member placed in relation to said buttress members such that said spaced apart portions are in a desired relative position and said retaining means is used to hold the apparatus in that position.        
 
         [0012]     In one embodiment of the invention there is provided apparatus for use in retaining spinal elements in desired spatial relationship, said apparatus comprising: 
        a tensioning member positionable along the spinal column and having a first end, a second end and a longitudinal axis extending therebetween;     first and second buttress members located on said first and second ends of said tensioning member, each buttress member including an abutment surface, and wherein at least one of said buttress members includes a passage to receive one of said first or second ends of said tensioning member wherein said tensioning member is slideably moveable in said passage, and retaining means to secure said tensioning member and said buttress in fixed relative position; and     wherein a line coaxial with the axis of said tensioning member at said first and second ends and extending from said tensioning member beyond said first and second ends intersects with said abutment surface.        
 
         [0016]     The tensioning member may be a rod which is circular in transverse section as is commonly used in the art although other shapes or arrangements of tensioning member may be equally suitable. For example, the tensioning member may be square, rectangular “C” or “I” shaped in transverse section. The axis or lengthwise shape of the tensioning member may be straight or curved or arcuate. More preferably it is straight at each end but includes an arcuate section in a medial region. The tensioning member may be made of metal such as stainless steel or titanium alloy for example, severable to the desired length and may be deformable longitudinally to a shape to suit the cervical curvature of the patient. Those skilled will appreciate that the material of which the tensioning member is made should be deformable to a shape that will be retained under the conditions in which it is used. Thus, the shape of the arcuate section of the tensioning member may be chosen to achieve the desired cervical curvature when the apparatus is implanted. The exact configuration of the axis and length of the tensioning member may be determined and adopted intraoperatively.  
         [0017]     Preferably, the axis of the tensioning member in the regions where its ends insert into the passages are substantially perpendicular to the plane of each abutment surface. In this arrangement load applied to the abutment surfaces of the buttresses or distension force applied by the outward facing abutment surfaces is borne substantially along the axis of the tensioning member.  
         [0018]     The buttress may be of any suitable form including a disc shaped or cylindrical member to provide a buttress surface. At least one of the buttress members includes a longitudinally extending recess for receiving an end of the tensioning member and include a laterally extending recess in which a grubscrew may be inserted to hold the end of the tensioning member. Other fastening means of adjustably fastening the tensioning member to the buttress member may be provided as will be appreciated by those skilled in the art.  
         [0019]     The buttress members may be provided additionally with holding means if desired. These may take the form of one or more integral spikes to assist in holding the buttress to the vertebral contact surface. In addition, the abutment surface may include means to facilitate osseous integration therewith. In one embodiment the abutment surface may include a porous coating comprising one or more layers of spherical particles. In another embodiment the abutment surface may include a recessed region into which a bone growth promoting compound is applied.  
         [0020]     Preferably, each buttress includes a passage which opens from the body away from the abutment surface and into which an end of the tensioning member is inserted. Preferably the passage terminates in said body proximate said abutment surface, so that the end of the tensioning member does not pass through or project from the buttress beyond the abutment surface, i.e. passages are blind passages and thus the ends of the tensioning member are contained within the passages.  
         [0021]     Means are provided to secure the tensioning member and each buttress in fixed relative position. In a preferred embodiment this comprises a screw in the body of the buttress which can be tightened against part of the tensioning member which is inserted into the passage so that the tensioning member is pressed against the wall of the passage and is held in frictional engagement therewith. Alternatively, other methods of selectively securing the tensioning member and each buttress in fixed relative position may be provided.  
         [0022]     In another embodiment of the invention, the tensioning member is integrally molded with one buttress member or otherwise fixed thereto.  
         [0023]     In another aspect of the present invention there is provided a buttress for use in conjunction with a tensioning member in a spinal column retaining apparatus for retaining spinal elements in a desired spatial relationship, the buttress comprising: 
        a body having a first end with an abutment surface, and a passage in said body opening at a second end and extending through said body towards said abutment surface, wherein said passage is adapted to receive an end of a tensioning member; and     means to secure said buttress with a tensioning member in said passage.        
 
         [0026]     Preferably the buttress will include one or more of the preferred features previously described.  
         [0027]     In yet another embodiment of the invention there is provided a method for retaining vertebra of a spinal column in a desired spatial relationship, the method comprising: 
        (a) excising portions of upper and lower vertebrae to form contact surfaces;     (b) providing a spinal implant as herein described.     (c) installing said implant between said upper and lower vertebrae by placing said abutment surfaces in contact with said contact surfaces.     (d) distracting said spinal column to adopt the desired spatial relationship of the vertebrae.     (e) sliding at least one of said buttresses apart along said tensioning member such that said abutment surfaces maintain contact with said contact surfaces.     (f) applying said retaining means to secure said buttresses to said tensioning member, to maintain said vertebrae in desired spatial relationship.       
 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]     The foregoing and other features of the present invention will become more apparent to one skilled in the art upon reading the following description with reference to the accompanying drawings, wherein:  
         [0035]      FIG. 1  is a lateral view of a cervical spine with a spinal implant of the prior art.  
         [0036]      FIG. 2  is a side view of an apparatus constructed in accordance with the present invention.  
         [0037]      FIG. 3  is a top plan view of a typical buttress of the present invention, being the upper buttress shown in  FIG. 2 .  
         [0038]      FIG. 4  is a side elevation of the buttress shown in  FIG. 3 .  
         [0039]      FIG. 5  is a transverse section of the buttress of  FIG. 4  along the line AA.  
         [0040]      FIG. 6  is a transverse section of the buttress of  FIG. 4  along the line BB.  
         [0041]      FIG. 7  is a schematic drawing of a portion of spinal column showing excision of vertebrae in preparation for implantation of apparatus of the present invention.  
         [0042]      FIG. 8  is a schematic drawing of the portion of spinal column of  FIG. 7  with an implant of  FIG. 2  in position.  
         [0043]      FIG. 9  is a side elevation of a cervical spine with a spinal implant of  FIG. 2  installed to retain vertebrae in a desired spatial relationship. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0044]     The embodiments of the invention shown are given for illustrative purposes only and should not be taken as limiting the scope of the present invention.  
         [0045]      FIG. 1  shows typical prior art apparatus used in cervical spinal fusion surgery. Implant  1  consists of a plate  2  which is fixed to cervical vertebrae C3 to C6 by bone screws  3  and  4  respectively.  
         [0046]     In  FIG. 2 , spinal implant  10  consists of a rod  12 , and an upper buttress  14  and a lower buttress  16 . Rod  12  has a first end  18  which is inserted into a bore in upper buttress  14  and a second end  20  which is inserted into a bore in lower buttress  16 .  
         [0047]     The required length of rod  12  may be determined by the surgeon during surgery after measurement of the cervical vertebrae and calculation of the length required to keep the vertebrae in a desired spatial arrangement. The rod  12  may be cut to the desired length and bent using equipment known in the art.  
         [0048]     Each buttress  14  and  16  respectively comprises a body  22  and  24  each having an abutment surface  26  and  28 . Abutment surfaces  26  and  28  include spikes shown typically as  30   a ,  30   b ,  30   c  and  30   d  which are spaced apart and extend outwardly from the abutment surfaces  26  and  28 . In the embodiment shown the spikes  30   a  to  30   d  are substantially conical and terminate in points  31   a  to  31   d . In use the spike are pressed into surfaces exposed by excision of bone from the upper and lower vertebrae to which the implant is to be attached. The spikes  30   a  to  30   d  assist to hold the buttresses  14  and  16  in position against the vertebrae.  
         [0049]     Abutment surfaces  26  and  28  may provide bonding surfaces  32  (shown more clearly in  FIG. 3 ) adapted to promote bony fusion of the vertebrae to the bonding surfaces  32 . It should be noted that the numbering used in reference to  FIGS. 3, 4 ,  5  and  6  applies equally to lower buttress  16 . The bonding surface  32  is preferably a porous coating formed from one or more layer of spherical particles or beads such as titanium which are fused to or otherwise fixed to the abutment surfaces  26  and  28  in a manner known in the art. The coating is firmly adhered to the buttress so that it resists removal by abrasion and such that the bonding strength between the coating and the buttress is greater than the bond strength between the coating and bone to which it grafts.  
         [0050]     Alternatively, the bonding surfaces may be etched or roughened. In another embodiment (not shown) at least a portion of the abutment surface may include a recess into which a bone growth promoting compound or putty is applied so that osseous integration of the buttress to the vertebra which it abuts is enhanced. It will be appreciated that any other form of surface treatment that promotes grafting of bone to the abutment surfaces may be suitable.  
         [0051]     Buttresses  14  and  16  have a body  22  and  24  defining a bore  34  (see  FIG. 6 ) into which an end  18  of rod  12  may be inserted. Bore  34  has an opening  36  at the opposite end of the body  22  to the abutment surface  26 . The bore  34  is a blind bore with bore end  46  so that the end  18  of rod  12  inserted into the bore  34  does not protrude from the abutment end of the body  22 . As shown in  FIG. 5 , the bore  34  is preferably elliptical or oval shaped in transverse section when the rod  12  is substantially circular in transverse section. Bore  34  has a first radius  37  which is greater than the radius of rod  12  so that buttress  14  can easily be moved axially along rod  12  when rod  12  is inserted in bore  34 . Second radius  39  is approximately equal to or slightly less than the radius of rod  12 . The elliptical shape of bore  34  in conjunction with rod  12  may be of compatible configuration to that used in the ISOLA spinal implant system. ISOLA is a trade mark of Acromed Corporation, although it may equally be designed to be compatible with other spinal implant hardware systems.  
         [0052]     Body  22  also includes a threaded lateral bore  38  which extends from a side  40  of body  22  and intersects with bore  34 . A grubscrew  42  as shown in  FIG. 1  may be inserted into threaded bore  38  and screwed until it bears against rod  12  and presses it against surface  44 . Until grubscrew  42  has been tightened against rod  12 , the rod  12  and buttress  14  are able to slide relative to each other until the tip (not shown) of rod  12  abuts bore end  46  of buttress  14 . In this arrangement rod  12  is able to move in the portion of the bore  38  defined by first radius  37 . When grubscrew  42  is tightened to the desired torque, rod  12  is forced into the portion of bore  38  defined by second radius  39  and is thus securely clamped so that the rod and buttress  14  are maintained in fixed position relative to each other.  
         [0053]     The length  41  of the bore  34  between the edge of threaded bore  38  and bore end  46  provides the degree to which buttress  14  can be distracted from rod  12 , the importance of which will become apparent from the description below. Body  22  also includes a window opening  43  which is preferably positioned adjacent threaded bore  38  and on the same of body  22 , although it may be positioned elsewhere on body  22  in certain embodiments. The window opening may consist of a bore which extends from side  40  through body  22  and opens in to bore  34  between bore end  46  and the edge of threaded bore  38 . Window  43  allows the surgeon to see the degree to which rod  12  has been retracted along bore  34  by viewing the end of rod  12  in bore  34 .  
         [0054]     The spinal implant  10  may be installed on the spinal column of a patient in the following manner shown with schematic representations in  FIG. 7  and  FIG. 8 . This installation procedure is described for fixation of the anterior cervical spine. First, the upper vertebra  48  and lower vertebra  50  between which the spinal implant is to extend are selected and portions  52   a  and  52   b  (shown in ghost lines) of the vertebrae between those two vertebrae  48  and  50  are excised using surgical techniques known in the art. Portions  54   a  and  54   b  of vertebrae  48  and  50  are also excised to form contact surfaces  56   a  and  56   b  which are substantially planar, and the planes of surfaces  56   a  and  56   b  generally face each other.  
         [0055]     The distance  58  between prepared vertebral surfaces  56   a  and  56   b  are measured for example, using callipers while no distracting force is applied to the patient&#39;s spinal column. The desired length of rod  12  is then calculated such that the length  60  of the spinal implant  10  when the tips of rod  12  are fully inserted into bores of buttresses  14  and  16  and abut the bore ends  46  of the buttresses i.e. the buttress is in a fully compressed arrangement, is substantially equal to or a few millimetres slightly greater than distance  58 . Rod  12  is then cut to the calculated length from a stock length of rod as will be known in the art. Rod  12  may be bent with tools known in the art to a curvature required to achieve the necessary spatial relationship of the vertebrae.  
         [0056]     Grubscrews  42  and  62  are inserted into the threaded bore  38  of each buttress  14  and  16  respectively, but are not screwed in to such an extent that they will protrude into bore  34 . Buttresses  14  and  16  are then slid over the ends  18  and  20  of rod  12  until the ends of rod  12  come into contact with bore ends  46 , i.e. the implant is assembled into its fully compressed arrangement. Screws  42  and  62  may be tightened slightly against rod  12  although only so much as to hold the assembled implant loosely together so it does not disassemble when being handled by the surgeon.  
         [0057]     The implant is then lowered into the trench created by the surgeon with grubscrews  42  and  62  accessible for tightening. A slight distracting force may be applied to the patient&#39;s spine to increase the distance  58  so that the implant can be positioned between surfaces  56   a  and  56   b . Points  30   a ,  30   b ,  30   c  and  30   d  are brought into contact with prepared vertebral surfaces  56   a  and  56   b  and may be pressed slightly into vertebrae  48  and  50  at those surfaces. Optionally, each buttress  14  and  16  may be hammered slightly along the axis of rod  12  so that point  30   a  to  30   d  embed in the vertebrae  48  and  50  below surfaces  56   a  and  56   b . The slight distracting force can then be released.  
         [0058]     A full distracting force is then applied to the patient&#39;s spinal column which increases the distance  58  between surfaces  56   a  and  56   b  once more, but this time to the extent required by the surgeon to adopt the desired spatial relationship of the patient&#39;s spinal column. Buttresses  14  and  16  are maintained in contact with distracted against surfaces  56   a  and  56   b . This distraction causes rod  12  to retract along bores  34  but preferably not to such an extent that the end of rod  12  passes threaded bore  38 . The surgeon can view the extent to which rod  12  has retracted along bore  34  through window  43 . Grubscrew  42  is then tightened to the required torque to ensure that the rod  12  and buttress  14  is firmly secured together and the procedure is repeated for screws  62  and buttress  16 . The distraction force is then released from the patient&#39;s spinal column so that the spinal implant  10  retains the spinal elements in the desired spatial relationship. Spikes  30   a  to  30   d  may further imbed to a small extent once muscular tension of the patient&#39;s neck returns although this will not be to such a degree as to alter the spatial relationship to any significant degree.  
         [0059]     It has been found that by use of the present invention, a patient can be mobilized the day following surgery without the need for external orthoses. Osseous integration with the bonding surface takes place over time but, if care is taken, is not necessary for the immediate proper functioning of the implant.  
         [0060]     This compares with a substantial period of immobilisation using currently available apparatus.  
         [0061]     The components described above may be made from any desired materials known in the art which exhibit the required strength, longevity for permanent implantation and which do not cause the body to establish a rejection reaction. For example, the rod may be made from stainless steel or other suitable implantable metal alloys. The buttresses may be made from implant grades of titanium alloys.  
         [0062]     The invention has been shown to provide a significant advance in surgical procedure. By use of the spinal implant in accordance with the invention, the need to harvest a large bone graft from another part of the patient&#39;s body is avoided. The vertical extent of the surgical field in the neck need not be significantly greater than the length of the vertebral defect. There is a greater ability to customise the implant during surgery. It has been found that by use of the invention, recovery time is reduced while a high degree of permanent strength and rigidity is provided.  
         [0063]     It will be appreciated that various changes, modifications and alterations in the teaching of the invention described herein may be contemplated by those skilled in the art without departing from the ambit of the present invention as defined in the claims hereafter.