for the purposes of promoting an understanding of the principles of the invention , reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same . it will nevertheless be understood that no limitation of the scope of the invention is thereby intended , such alterations and further modifications in the illustrated device , and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates . referring now to fig1 initial steps of the surgical technique contemplated by the present invention are illustrated . in particular , the invention contemplates anterior fixation of several vertebral segments , identified as vertebra vertebrae v 1 - v 4 and their adjacent discs d 1 - d 4 . this anterior instrumentation could be used , for instance , to correct a lumbar scoliosis condition followed by fusion of the affected vertebral levels . initially , the lateral aspects of the vertebral bodies are revealed by way of a thoracoabdominal exposure . a discectomy can be performed on each of the disc spaces d 1 - d 4 , since these motion segments will be eliminated by the permanent stabilization implants . using calipers , the distance of the cortices of the intermediate vertebral bodies is determined to assess the length of the bone screw to be placed laterally into the vertebra . in this instance , the intermediate vertebra v 2 is instrumented with a variable angle screw 30 . a preferred variable angle screw is described in detail in u . s . pat . no . 5 , 261 , 909 to dr . chester sutterlin , et al ., which is owned by the assignee of the present invention . the disclosure of this &# 39 ; 909 patent is incorporated herein by reference as it pertains to the description of the variable angle screw 30 and its engagement to a spinal rod . for the purposes of the present disclosure , the variable angle screw 30 includes a bone engaging portion 31 or threaded shank which is sized to preferably span the distance between the cortices on the lateral sides of the vertebral body v 2 . the screw 30 includes a head portion 33 which defines a slot 35 for receiving an eyebolt body therethrough . the head portion 33 also includes a plurality of radial splines 37 formed thereon for interdigitation with a corresponding washer to be discussed below . in the illustrated embodiment , only four vertebral levels are instrumented . consequently , the top and bottom most disc spaces d 1 and d 4 are instrumented with a hollow screw that is configured to enhance the degree of fixation to the adjacent vertebrae . in particular , the hollow screw includes a first portion in the manner of a cylindrical implant 10 . preferably this implant 10 is a fusion device that permits bone ingrowth for fusion and permanent fixation . in accordance with the preferred embodiment of the present invention , this cylindrical implant 10 is a threaded spinal implant as described in u . s . pat . no . 5 , 015 , 247 of dr . gary michelson . the disclosure of this implant in the &# 39 ; 247 patent is incorporated herein by reference . other fusion devices are contemplated , such as the interbody fusion device of dr . kozak described in u . s . pat . no . 5 , 397 , 364 , which disclosure is incorporated herein by reference . this kozak device , and other fusion devices , can be modified for lateral introduction into the intradiscal space for implementation in the present inventive surgical technique . for purposes of the present invention , the cylindrical implant 10 defines a hollow interior chamber 12 which can be filled with bone graft material or bone chips . a plurality of apertures 14 are defined through the implant to communicate with the hollow interior to permit tissue ingrowth . a threaded end cap 15 encloses an open end of the implant to allow the implant to be filled and retain bone chips in the hollow interior chamber 12 prior to implantation . the exterior surface of the implant 10 includes bone engaging threads 16 formed thereon which are configured to be screw - threaded into the end plates of the adjacent vertebrae . the cylindrical implant 10 has a rear surface 18 which is exposed laterally when the implant 10 is threaded into the disc space d 1 . this rear surface 18 defines a threaded opening 20 therethrough . further details of the construction of this preferred cylindrical implant can be gathered from the &# 39 ; 247 patent . the cylindrical implant 10 can be implanted between the two pairs of adjacent vertebrae in the manner disclosed in the &# 39 ; 247 patent , which technique is described at cols . 9 - 10 and is incorporated by reference . once the site has been prepared in the disc space , an insertion rod 23 having a threaded post 25 can be engaged to the threaded opening 20 in the rear surface 18 of the implant . a guide member 27 concentrically surrounds the insertion rod , with the rod specifically passing through bore 29 in the guide member . the insertion rod 23 and guide member 27 are used to drive each of the cylindrical implants 10 into their respective intradiscal spaces d 1 and d 4 . again , further details of the manner of implantation of the hollow implant 10 are disclosed more fully in the &# 39 ; 247 patent . it is understood that other similar intradiscal implants could be utilized , providing that the implant includes a threaded opening at its rear surface , such as the threaded opening 20 of rear surface 18 , or some equivalent thereto . for example , the kozak interbody fusion device of the &# 39 ; 364 patent incorporated above can be modified to include a similar threaded opening at a lateral face of the device . referring now to fig2 the instrumented segments are seen with the hollow implants 10 fully disposed within the disc spaces d 1 and d 4 . likewise , the variable angle screw 30 is fully inserted into the vehicle body v 2 . preferably , the screw 30 sits with the screw head 33 flush against the lateral surface of the vertebra . at the opposite anchoring ends of the construct , at disc spaces d 1 and d 4 , an attachment head 40 is engaged to each cylindrical implant 10 . specifically , the attachment head 40 includes a base 42 from which projects a threaded stem 44 . the threaded stem 44 is adapted to engage the threaded opening 20 in the rear surface 18 of the cylindrical implant 10 . the attachment head 40 includes a head portion 46 which is substantially similar to the head portion 33 of the variable angle screw 30 described above . in particular , the head portion 46 includes an eyebolt slot 47 and is equipped with radial splines 48 on one surface of the head portion . each of the cylindrical implants 10 receive a corresponding attachment head 40 . preferably , the head portions 46 of the attachment heads 40 and the head portion 33 of the variable angle screw 30 are maintained in a straight line from the cephalad to the caudal ends of the instrumented segments . with the surgical approach of the present invention , it is not necessary to use specific derotation or screw placement techniques to produce a lordosis in the instrumented segments . instead , as provided in the next steps of the procedure depicted in fig3 a rod 50 is contoured at a curvature c to produce the lumbar lordosis or thoracic kyphosis in the sagittal plane . generally , the rod 50 will be contoured to conform to the scoliotic curvature with the rod arranged on the concave side of the curvature . once the rod has been contoured , it is attached to each of the head portions 33 and 46 by way of a splined washer 52 and connection member , preferably in the form of an eyebolt assembly . as shown in fig3 the eyebolt assembly includes a body 43 which extends both through the splined washer 52 and through the slots 35 and 47 in the respective variable angle screw head portions 33 and attachment head portions 46 . these details of the eyebolt construct are shown more clearly in fig4 . it is also recognized that this same eyebolt attachment assembly is described in the &# 39 ; 909 patent , which has been incorporated herein by reference . referring to fig4 it is seen that the attachment of the head portion 46 of the attachment head 40 is achieved with the splined face of the head portion facing towards the rod 50 . the splined washer 52 is disposed between the rod and the splined face of the attachment head 40 , more particularly with its corresponding splines interdigitating with the splines 48 . as discussed more fully in the &# 39 ; 909 patent , the interdigitating splines allow the rod 50 to be oriented at a variety of angles in the sagittal plane relative to the attachment head 40 . while fig1 - 3 depict the fixation screws as being generally parallel to each other for clarity , in reality the head portions 33 and 46 will be oriented at different angles in the frontal plane , depending upon the corrected spinal curvature . the eyebolt assembly itself includes an eyebolt body 53 which includes an aperture for receiving the spinal rod 50 . a portion of the eyebolt body 53 projects through the splined washer 52 . extending from the body is an threaded eyebolt stem 54 which receives a nut 55 . tightening the nut 55 on the stem 54 clamps the entire constant together , namely the rod 50 , the splined washer 52 and the attachment head 40 . a similar eyebolt arrangement is used to engage the variable angle screw 30 to the spinal rod 50 . at this step of the procedure , each of the eyebolts is tightened enough to seat the rod and the screw heads , while still allowing the rod to rotate within the apertures of the eyebolt assemblies . a hex end 51 is provided on the rod 50 so that the rod can be rotated while still attached to the variable angle screw 30 and the hollow screw 10 . rolling or rotating the rod in this manner translates the predefined scoliotic curvature into the sagittal plane to produce the requisite lordosis at the instrumented vertebral segments . once the rod has been rolled to its proper position to produce the lordotic curvature , segmental decompression or compression can be applied to further correct any deformity in the frontal plane . it has been found that instrumenting the vertebral bodies from this anterior approach typically results in a more complete and reliable decompression of the spinal canal than with other approaches , such as a posterior instrumentation . once the appropriate disc heights have been restored , along with the proper curvature in the front and sagittal planes has been achieved , the eyebolts are tightened to firmly clamp the rod 50 to each of the fixation elements attached to the spine . in accordance with the preferred embodiment , the spinal rod is offset from the longitudinal axis of the cylindrical implant ( i . e .— the axis extending through the threaded opening 20 ). preferably , the spinal rod is positioned posteriorly to the head portion to which the rod is attached . this placement allows the eyebolt stem and nut to face anteriorly toward the abdominal cavity and away from ore sensitive regions . ( it should be noted that for clarity the head portions depicted in fig1 - 3 have been shown rotated 180 ° from this preferred orientation .) preferably , each of the hollow implants 10 is filled with morcellized bone graft material , such as autograft . moreover , at the surgeon &# 39 ; s option , the remaining disc space can also be filled with morcellized bone material . one object of using the hollow apertured cylindrical implant 10 and the bone graft material is to induce fusion across the intradiscal spaces . once fusion occurs , the cylindrical implants 10 which provide the anchor at the opposite ends of the construct will be more rigidly secured to the spine than other known bone screw constructs . it is believed that the use of the fusion implant 10 at the intermediate vertebral segments is not essential , since the loads are not as harsh in the intermediate portions than at the ends of the instrumented vertebral segments . in specific embodiments , the variable angle screw 30 can be 5 . 5 , 6 . 5 or 7 . 5 mm cancellous screw . the cylindrical implants 10 can have an outer threaded diameter of between 10 mm - 24 mm . further , ¼ or { fraction ( 3 / 16 )} inch diameter rods can be used , together with appropriately sized eyebolt assemblies . it is of course understood that the length of the threaded cancellous portion of the variable angle screw , as well as the diameter of the cylindrical implants 10 is determined by the geometry of the particular instrumented vertebral segments . it is contemplated that the present inventive surgical technique can be used at all levels of the spine with appropriately sized implants . it has been found that even with the best preoperative planning some revision of the fixation construct may be required . in addition , failure of the construct before fusion is complete may also necessitate a complete or partial revision of the implant . in some instances , the revision entails cutting away the spinal rod 50 and unthreading each of the bone screws engaged into the vertebrae . with the present invention anterolateral approach and instrumentation , a more refined revision technique and instrumentation is available . using the present construct , removal of the rod is made extremely simple and reduced simply to the step of loosening the eyebolts attached to each of the head portions at the various vertebral segments . the variable angle screw 30 is readily removed by unthreading the screw from the vertebral body v 2 . at the cephalad and caudal ends of the construct , the attachment heads 40 are unscrewed from their respective cylindrical implants 10 . to this point , the revision procedure is relatively simple . however , difficulties can arise in removing the cylindrical implants 10 from their respective intradiscal spaces d 1 and d 4 . removal of the implants is particularly more difficult if bone ingrowth has occurred between the vertebral end plates and the hollow anterior 12 of the implant . removal of these fusion implants 10 is one important feature of the novel revision technique of the present invention and the associated revision instruments 60 . the instruments 60 include an alignment rod 62 which has a flared end 63 with a threaded post 64 projecting therefrom . the threaded post 64 is configured to engage the threaded opening 20 in the rear surface 18 of the cylindrical implant 10 . in the revision procedure , the alignment rod 62 is engaged to the cylindrical implant 10 by way of the threaded post 64 and threaded opening 20 . with the alignment rod 62 so positioned , a trephine 70 is advanced over the rod . the trephine 70 includes a tubular cutting end 71 which defines a plurality of jagged cutting teeth 72 . preferably , the cutting teeth 72 are of known configurations for cutting through cortical bone as the cutting end 71 is rotated in the proper direction . the cutting end 71 further defines a plurality of depth markings or rings 74 which can be used to determine the depth of the trephine 70 in the intradiscal space . the trephine 70 includes a rotation collet 76 at the proximal end of the trephine which is concentrically disposed around the tubular body of the cutting end 71 . the proximal end of the trephine also defines a driving boss 78 which is configured to receive a conventional driving tool or suitable wrench for rotating the cutting end . finally , the nearmost end of the trephine 70 includes an alignment rod clamp 80 which is used to clamp the proximal end of the alignment rod 62 concentrically received within the trephine . referring to fig6 it can be seen that the alignment rod 62 is engaged with the cylindrical implant 10 disposed in the disc space d 4 . the trephine 70 is advanced over the alignment rod 62 until the cutting edge 72 contacts the vertebrae bodies v 3 and v 4 . rotation of the trephine causes the cutting teeth 72 to bore out a core 84 of the bone material surrounding the cylindrical implant 10 . the core 84 can then be removed together with the implant 10 by first withdrawing the trephine 70 and then pulling the alignment rod to remove the entire assembly . in fig7 the cylindrical implant 10 is seen supported by the alignment rod 62 as the entire revision instrumentation 60 is withdrawn from the surgical site . the depth markings 74 can be used to either limit or determine the depth of insertion of the trephine . in its depth limiting function , the depth of the distal end of the cylindrical implant is already known and is correlated to a particular one of the depth markings . the trephine is then advanced into the vertebrae until the appropriate depth marking is aligned with the vertebral body . the markings 74 can also be used interactively when the insertion of the trephine is monitored under indirect vision . once it is seen that the cutting edge 72 of the trephine has passed the end of the cylindrical implant , the appropriate depth marking can be noted . once the implant has been extracted , the noted depth marking is then used to determine the proper size of bone dowel to be extracted from another location to be inserted back into the intradiscal space . once the core 84 of bone and the cylindrical implant 10 has been excised from the disc space , an exactly fitted plug or dowel can be reinserted into the remaining bore . the trephine 70 can be used to core out bone from the patient , such as at the iliac crest , which can then be reinserted using the same trephine . once the trephine is removed , the adjacent vertebrae will collapse around the newly inserted bone plug . this bone dowel will retain the disc height and again permit fusion at the revision site . moreover , once fusion occurs , the site of the bone plug may be strong enough to support new spinal implants . the present revision technique carries a significant advantage in that it allows use of hollow cylindrical fusion implants at any position along the construct . it has been found that the use of fusion implants enhances the rigidity and strength of the fixation . this same increased rigidity and strength makes ordinary surgical revisions difficult or impossible to do without causing serious damage to what may be an otherwise healthy vertebral body . use of the trephine 70 in accordance with the present technique eliminates this risk and permits easy and quick removal of the threaded cylindrical implant , even where fusion has occurred . while the invention has been illustrated and described in detail in the drawings and foregoing description , the same is to be considered as illustrative and not restrictive in character , it being understood that only the preferred embodiments have been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected . for example , in the preferred embodiment of the anterior instrumentation technique a hollow cylindrical fusion implant is used to support the attachment heads for engagement to a spinal rod . however , other intradiscal implants of various configurations may be used provided that the implants permit bone ingrowth . further , while a spinal rod is disclosed , other longitudinal fixation elements , such as spinal plates , may be utilized in performing this inventive anterior technique . with respect to the revision technique disclosed herein , the preferred embodiment of the procedure is performed in connection with the disclosed anterior instrumentation . however , the same revision procedure can be used to remove other fusion implants in other settings , with appropriate modification to the trephine to accommodate variations in implant configuration . this inventive technique has particular application , however , in removing fusion implants that engage the endplates or cortical bone of the vertebral body .
a system and method for anterior fixation of the spine utilizes a cylindrical implant engaged in the intradiscal space at the cephalad and caudal ends of the construct . the implants are cylindrical fusion devices filled with bone material to promote bone ingrowth and fusion of the disc space . an attachment member is connected to each of the fusion devices and bone screws having similar attachment members are engaged in the vertebral bodies of the intermediate vertebrae . a spinal rod is connected to each of the attachment members using an eyebolt assembly . in a further inventive method , a revision of the construct is achieved by removing the fusion devices . each fusion device is engaged by an elongated guide member over which a cylindrical trephine is advanced . the trephine has an inner diameter larger than the diameter of the fusion implant and includes cutting teeth for extracting a core of bone material around the fusion implant . the trephine and guide member are removed along with the bone core containing the fusion implant . the trephine is also used to extract a bone dowel from a solid bone mass to be inserted into the space left by the removed bone core .