Anterior spinal instrumentation and method for implantation and revision

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 (10) filled with bone material to promote bone ingrowth and fusion of the disc space. An attachment member (40) is connected to each of the fusion devices (10) and a spinal rod (50) is connected to each of the attachment members using an eyebolt assembly (53, 54, 55). 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 (62) over which a cylindrical trephine (70) is advanced. The trephine (70) is used to extract a core (84) of bone material around the fusion implant, while the guide member (62) is used to remove the bone core (84) containing the fusion implant (10). In another aspect of the invention, a removal insert (90, 90') is provided that engages an implanted fusion device (10). The removal insert (90, 90') can be used to guide the trephine (70) around the fusion device, and is connected to a removal tool (100) once the bone core is created. The removal tool (100) includes a shaft (101) attached to the removal insert (90, 90'), and a slap hammer (104) slidably mounted on the shaft.

BACKGROUND OF THE INVENTION 
The present invention concerns a spinal instrumentation system utilizing 
elongated members extending along the length of the spine and attached to 
multiple vertebrae by fixation elements, such as bone screws. In 
particular, the invention concerns anterior instrumentation, together with 
a surgical technique for implanting the instrumentation. The invention 
also contemplates a surgical revision technique for this spinal 
instrumentation. 
Historically, correction of spinal disorders and treatment of spinal 
injuries was approached posteriorly, or namely from the back of the 
patient. Initially, the anterior approach to spinal instrumentation, that 
is from the front and side of the patient, was not favored, due to the 
unfamiliarity of this approach to spinal surgeons and due to the fear of 
severe complications, such as neurovascular injury or compromise of the 
spinal cord. However, in the face of some reported difficulties in 
addressing correction of thoracolumbar and lumbar scoliotic curvatures 
from a posterior approach, surgeons sought anterior forms of correction 
and stabilization. One such technique was developed by Dwyer in Australia 
during the 1960's in which a staple-screw construct was applied to the 
convex side of the scoliotic curvature. The screws were connected by a 
cable and correction was obtained by applying compressive forces at each 
instrumented level. The anterior spinal compression produced tensile 
forces within the cable which in turn generated a corrective bending 
moment at each of the vertebral levels. 
On the heels of Dwyer's success, other anterior instrumentation followed. 
Further, surgeons began to recognize that certain spinal treatments were 
best approached anteriorly, rather than posteriorly. Anterior approaches 
give direct access to the intervertebral disc space for anterior release 
and interbody fusion. Presently, common indications for anterior 
instrumentation include: lumbar scoliosis with deficient posterior 
elements; thoracolumbar curves with extreme lordosis; paralytic 
thoracolumbar scoliosis requiring both an anterior and a posterior fusion; 
thoracolumbar spine trauma, such as burst fractures; and degenerative 
conditions of the vertebral body. In the case of burst fractures, it is 
known that neurocompression occurs from the anterior direction. Further, 
anterior debridement of fracture fragments is frequently believed to be a 
more effective means to decompress the spinal canal, as opposed to known 
posterior techniques. 
Since the initial Dwyer instrumentation, many anterior plate and rod 
systems have been developed, such as the systems of Dunn, 
Kostuik-Harrington, Zielke and Kaneda. Many of these systems permit 
dynamic distraction of the vertebrae followed by direct compression of 
implanted bone graft contained within the resected disc space and after 
decompression of the neural elements. 
Many of these anterior systems can lead to complications. Some of the more 
prominent problems that have occurred involve failure of the fixation 
components, and an often high incidence of loss of reduction or 
correction. Many of the difficulties in this respect can be traced to the 
vertebrae instrumented at the end of the construct where the loads on the 
instrumentation are the greatest. In some cases, bicortical purchase of 
vertebral body screws has been found to assure a more solid fixation at 
the ends of the construct and to protect against dislodgement of the 
screws. There does, however, still remain a need for an anterior 
instrumentation that can provide adequate correction of spinal deformities 
and that can be easily implanted. In addition, the system must ensure a 
strong fixation that will not deteriorate over time resulting in a loss of 
correction. 
In some cases, it has been found that revision surgery is necessary, even 
when following the best possible surgical implantation of the 
instrumentation. Frequent indications for revision of spinal 
instrumentation include extension of existing instrumentation, and 
replacement of failed implants. In the cases involving early spinal 
implants, revision required cutting away the spinal implants. As implant 
design became more sophisticated, capabilities were developed for revision 
surgery that was relatively safe to the patient and non-destructive to 
implants, particularly those implants that were intended to be retained. 
In systems using bone screws, revision surgeries can significantly 
compromise the vertebral body. In addition, in certain anterior approaches 
where stronger fixation is essential, revision procedures to replace 
failed components may necessarily compromise the new construct. 
In view of these difficulties, there is a need for a spinal fixation system 
that is readily suited for revision surgery. Specifically, the system must 
be suitable for the addition or removal of components by way of revision 
without sacrificing either an existing construct or eliminating the 
possibility of implanting a new, more stable construct. In addition, there 
is a need for revision techniques that permit complete removal of a 
construct once fusion has occurred, again without compromising the spine 
or the implants. 
SUMMARY OF THE INVENTION 
To address these needs, the present invention contemplates a method for 
anterior fixation of the spine commencing with a thoracoabdominal exposure 
of the spinal segments to be instrumented. In the preferred embodiment, 
fusion devices are implanted within the cephalad and caudal disc spaces 
after a total discectomy. The fusion devices are configured to contain 
bone growth material to promote bone ingrowth and consequently fusion of 
the instrumented disc spaces. These cephalad and caudal fusion devices 
serve as anchors to ensure a stable and solid anterior construct. 
In accordance with the method, bone screws are engaged within the vertebral 
bodies of the intermediate vertebrae. Preferably, the bone screws are 
variable angle screws having a cancellous threaded portion that is long 
enough to engage both lateral cortices of the respective vertebrae. An 
attachment member is provided having a head portion configured 
substantially similar to the head portion of the variable angle screw. In 
the preferred embodiment, eyebolt assemblies are used to attach and fix 
the head portions of the variable angle screws and the attachment members 
to an elongated spinal rod. The rod is positioned posteriorly of the head 
portions and is offset from the longitudinal axis of the fusion implant. 
It has been found that this anterior system and surgical technique 
provides a more reliable and complete decompression of the spinal canal. 
The use of the fusion devices as anchors at the ends of the construct 
renders this anterior instrumentation a viable alternative to address 
spinal conditions previously reserved for treatment from a posterior 
approach. 
The system in accordance with one embodiment utilizes a threaded 
cylindrical fusion implant that is placed between or threaded into the 
endplates of the adjacent vertebrae. The implant can be filled with 
morcellized autologous bone to promote fusion through the implant and 
between the vertebral endplates. One end of the fusion implant is 
laterally disposed and accessible for placement of an attachment member. 
In one embodiment, the exposed end has a threaded opening for receiving a 
threaded post of the attachment member. In accordance with one feature of 
the present invention, the attachment member also includes a head portion 
that is configured substantially similar to the head of the bone screw 
engaged in the intermediate vertebrae. The head portion defines a slot for 
receiving the body of an eyebolt assembly, and includes a pattern of 
radial serrations. The head portions of the fusion implant attachment 
members and the bone screws are engaged to the spinal rod by a variable 
angle connection member comprising an eyebolt, nut and splined washer. The 
attachment member provides a significant benefit in that it adapts a known 
fusion implant to appear and behave as a bone screw with enhanced rigidity 
and fixation. 
In another aspect of the invention, a surgical technique for revision of 
the spinal instrumentation is contemplated. More specifically, the 
revision technique is applied to removal of a cylindrical fusion implant 
fused in an intradiscal space. The revision technique relies upon a guide 
member connected to the fusion implant in situ. A cylindrical trephine is 
concentrically disposed over the guide member for advancement to the 
implant site. The trephine has an inner diameter slightly larger than the 
outer diameter of the fusion implant. 
With the fusion implant held by the guide member, and the member guiding 
advancement of the trephine, the trephine is rotated so that its cutting 
teeth progressively cut around the fusion implant and into the vertebral 
bone. Once the cutting teeth of the trephine is advanced past the end of 
the fusion implant, the trephine is withdrawn, leaving an excised core of 
bone surrounding the fusion implant. The guide member is used to finally 
extract the fusion device and its surrounding bone core. This defect can 
then be exactly filled with an appropriately sized bone plug or dowel. 
Preferably, the same trephine is used to obtain the bone plug from a solid 
bone mass, such as the iliac crest. 
In a further aspect of the invention, the revision technique is augmented 
by the use of a removal insert. The removal insert engages the fusion 
implant in situ. The insert is configured to act as a guide for the 
trephine that is used to excise the core of bone surrounding the fusion 
implant. In accordance with this aspect of the invention, the insert is 
engaged to the implant and held in place while the trephine performs the 
bone excision. After the core of bone surrounding the fusion implant has 
been created, the trephine is removed and the removal insert is engaged to 
a shaft carrying a slap hammer slidable along the shaft. The slap hammer 
is used to repeatedly contact a handle at the opposite end of the shaft 
from the fusion device to facilitate removal of the fusion device and 
surrounding bone plug. This instrument and technique can be useful in 
instances where the bone plug created by the trephine is difficult to 
manually remove. 
It is one object of the present invention to present a novel method for 
anterior instrumentation of the spine. A further object resides in 
features of the anterior technique and the implant system itself that 
provides a rigid and secure fixation of the spine, especially at the 
cephalad and caudal extremes of the system. 
Another object is to provide a system that can capitalize on benefits of an 
intervertebral fusion device serving as an anchor to the system. Other 
objects and the significant benefits of the present invention will become 
apparent on consideration of the following written description and the 
accompanying figures.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
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 FIG. 1, 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.sub.1 -V.sub.4 and their adjacent discs 
D.sub.1 -D.sub.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.sub.1 -D.sub.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.sub.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 '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.sub.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.sub.1 and D.sub.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 '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.sub.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 '247 patent. 
The cylindrical implant 10 can be implanted between the two pairs of 
adjacent vertebrae in the manner disclosed in the '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.sub.1 and D.sub.4. Again, further details of the 
manner of implantation of the hollow implant 10 are disclosed more fully 
in the '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 '364 Patent incorporated above can be modified to 
include a similar threaded opening at a lateral face of the device. 
Referring now to FIG. 2, the instrumented segments are seen with the hollow 
implants 10 fully disposed within the disc spaces D.sub.1 and D.sub.4. 
Likewise, the variable angle screw 30 is fully inserted into the vertebral 
body V.sub.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.sub.1 and 
D.sub.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 receives 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 FIG. 3, 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 FIG. 3, 
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 FIG. 4. 
It is also recognized that this same eyebolt attachment assembly is 
described in the '909 Patent, which has been incorporated herein by 
reference. 
Referring to FIG. 4, 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 toward 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 '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 
FIGS. 1-3 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 construct 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 screws 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 frontal 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 portions to which the rod is attached. This 
placement allows the eyebolt stem and nut to face anteriorly toward the 
abdominal cavity and away from more sensitive regions. (It should be noted 
that for clarity the head portions depicted in FIGS. 1-3 have been shown 
rotated 180.degree. 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'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, 1/4 or 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 inventive 
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.sub.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.sub.1 and D.sub.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 FIG. 6, it can be seen that the alignment rod 62 is engaged 
with the cylindrical implant 10 disposed in the disc space D.sub.4. The 
trephine 70 is advanced over the alignment rod 62 until the cutting edge 
72 contacts the vertebral bodies V.sub.3 and V.sub.4. Rotation of the 
trephine causes the cutting teeth 72 to bore out a core 84 of 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 FIG. 
7, 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. 
In a further embodiment of the invention, a removal insert 90 is provided 
as shown in FIGS. 8-9. The insert is engaged to the fusion implant 10 to 
facilitate removal of the implant and the bone plug generated by the 
trephine 70. In accordance with the preferred embodiment, the removal 
insert 90 includes a body 91 and an engagement portion 92 that is 
configured to engage the hollow screw or fusion implant 10. In one 
specific embodiment, the engagement portion 92 includes a threaded post 93 
that is adapted to engage the threaded opening 20 of the implant 10. 
Again, in accordance with the preferred embodiment, the threaded post 93 
is arranged so that the removal insert 90, and particularly the body 91, 
contacts the end face of the implant 10 to provide a firm connection 
between the two components. 
The removal insert 90 is also provided with a frusto conical portion 94 
integral with the body 91. This frusto conical portion 94 acts as a guide 
for the trephine 70 during the step of the revision technique in which a 
core of bone is excised surrounding the implant 10. In accordance with a 
preferred technique using the removal insert 90, the insert is engaged to 
a fusion implant, such as implant 10, by way of the threaded post 93. The 
trephine 70 can then be guided along the frusto conical portion 94 of the 
insert 90 until it contacts the vertebrae V.sub.3 and V.sub.4. The 
trephine 70 is operated in the manner described above, namely being 
rotationally driven into the bone to generate a core of bone 84 
immediately surrounding the fusion implant 10. 
Once the core of bone 84 has been created, the trephine 70 can be removed 
and a removal tool 100 utilized as shown in FIG. 11. The removal tool 100 
includes an elongated shaft 101 terminating in a threaded post 102 at one 
end and a handle 103 at an opposite end. The threaded post 102 is 
configured to engage a threaded bore 95 in the frusto conical portion 94 
of the removal insert 90. The removal tool 100 further includes a slap 
hammer 104 that is slidably disposed along the elongated shaft 101. The 
slap hammer 104 is utilized by driving the hammer upward in the direction 
of the arrow S toward the handle 103. When the slap hammer 104 strikes the 
contact surface 105 of the handle 103, it imparts a removal force along 
the shaft 101, through the removal insert 90 and to the fusion device 10 
and its surrounding bone core. In the revision surgery, a few taps from 
the slap hammer 104 will progressively dislodge or unseat the core of bone 
84 and the fusion implant 10 from the disc space D4 and adjacent 
vertebrae. In one specific embodiment, the removal tool 100 is provided 
with a stop 106 that is engaged to the elongated shaft 101. The stop 106 
restricts movement of the slap hammer 104 toward the removal tool 90, or 
more particularly the surgical site. The stop 106 can be integral with the 
shaft 101 or it can be a component that is added separately, such as in 
the form of an O-ring. That is, threaded onto the shaft 101 after the slap 
hammer 104 has been mounted on the removal tool 100. 
The removal insert and associated technique can be applied to remove hollow 
screws or fusion devices of alternative design. One such alternative 
design is shown in FIG. 10 in which a removal insert 90' is engaged to a 
modified fusion implant 10'. The modified implant 10' includes a rear 
portion 18' defining an opening 20' therethrough. The implant 10' includes 
an interior recess 21' adjacent the opening 20'. To accommodate this 
modified implant, the removal insert 90' includes a body 91' and an 
engagement portion 92' that is configured to engage the opening 20' of the 
fusion implant 10'. The insert 90' also includes a frusto conical portion 
94' and threaded bore 95' similar to the embodiment 90 in FIG. 8. In this 
respect, the alternative removal insert 90' is adapted to be engaged by 
the removal tool 100 shown in FIG. 11. 
In this alternative embodiment, the removal insert 90', and particularly 
the engagement portion 92', defines a mating ring 96'. The mating ring 96' 
is sized to be received within the opening 20' of the fusion implant 10'. 
The mating ring 96' also preferably includes an engagement tab 97' that is 
configured to engage the interior recess 21' of the implant 10'. In this 
manner, the engagement portion 92' is solidly engaged to the implant 10'. 
The mating ring 96' and engagement tab 97' can be in the form of a 
continuous annular feature, or may also be in the form of a plurality of 
arms extending from the body 91'. In either case, the mating ring 96' must 
be sufficiently resilient to allow the engagement tabs 97' to be pushed 
through the opening 20' until they snap outward into the interior recess 
21' of the fusion cage 10'. Once the removal insert 90' is engaged to the 
modified fusion cage 10', the remaining steps of the revision technique 
can be performed in the manner described above. 
In the illustrated embodiments, the frusto-conical portions 94, 94' and the 
engagement portions 92, 92' are integral with the body 91. Alternatively, 
these portions can be separately attached to the body. For example, the 
threaded post 93 could itself be threaded into a bore at the bottom of the 
body, as could the mating ring 96', thereby providing a degree of 
interchangeability to the removal insert 90, 90'. 
The present invention further contemplates other means for engaging the 
removal tool to the removal insert. In the illustrated embodiment, a 
threaded post 102 engages a threaded bore 95, 95' in the removal inserts 
90, 90'. Other engagments can be utilized, such as resilient prongs or a 
press-fit, with appropriate mating features defined on the removal insert. 
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. It is understood that these same apparatus and 
techniques can have application for the removal and revision of other 
devices disposed within bone other than in the spine. 
Further in accordance with the several aspects of the invention, and 
particularly of the revision technique and associated instrumentation, the 
removal insert can be configured to engage a variety of hollow screws or 
fusion devices. It is of course understood that only two specific 
embodiments of the fusion implant has been discussed above. The same 
inventive techniques can be adapted for removal and revision of 
non-cylindrical devices. It is also understood that the engagement portion 
of the removal inserts can be modified according to the engagement 
features of the fusion implant. It may be contemplated, for example, that 
only a friction fit is required between the engagement portion of the 
removal insert and the end of the fusion implant. 
The removal insert, and particularly its body, can also assume various 
configurations depending upon the nature of the trephine to be used in the 
revision technique. Likewise, the engagement between the removal insert 
and the shaft of the removal tool can assume a variety of forms other than 
the mating threads of the specific depicted embodiment. It is of course 
understood that the engagement between the shaft and the removal insert 
must be sufficiently strong so that the use of the slap hammer does not 
cause the removal tool to disengage from the removal insert. Further, the 
slap hammer can assume a variety of configurations other than the 
elongated cylindrical form depicted in FIG. 11. For example, the slap 
hammer can simply be a disc having a circumferential size and shape that 
can be readily grasped and manually operated. 
Furthermore, the removal tool 100 can be directly engaged to the device 
disposed within bone, such as device 10, after the core of bone has been 
cut by the trephine. In this instance, the removal of the core of bone and 
device embedded within the core proceeds in the manner described above. In 
particular, repeated striking of the slap hammer 104 against the contact 
surface 105 will extract the core of bone. However, the use of the removal 
insert 90, 90' stabilizes the device and the removal procedure. 
In the preferred method, the trephine is first removed after the core of 
bone has been cut, and then the device and bone core are extracted. In an 
alternative embodiment, the trephine can be removed together with the 
removal tool, fusion device and surrounding bone core. This approach is 
particularly useful when the fusion device is removed using the alignment 
rod 62.