Microdrive for use in stereotactic surgery

The present invention is directed to microdrive apparatus useful in human stereotactic surgery. Such apparatus permits safe and accurate placement of a surgical instrument, such as a cannula, into a portion of the central nervous system, e.g. the brain and spinal cord, of a patient by simple mechanical operation.

BACKGROUND OF THE INVENTION 
Stereotactic surgery is that branch of neurosurgery that uses a special 
device to direct a surgical instrument such as a cannula, electrode or 
other type of probe or device, with great accuracy, to a target within the 
central nervous system, particularly the brain or spinal cord, of a 
patient. The target is located and identified by one of a number of 
techniques. Sometimes the target can be visualized on computed tomography 
(CT) or magnetic resonance imaging (MRI). Other times, the position of the 
target must be determined by its relationship to an anatomic structure 
that can be seen on scanning using conventional radiographs or by 
ventriculography. Today, three-dimensional imaging techniques such as CT 
and MRI are the most frequently used techniques to locate and identify 
targets within the central nervous system. 
Stereotactic surgery can only be carried out using a special apparatus 
which enables the surgeon to guide surgical instruments to identified 
targets within the brain, spinal cord, or other part of the central 
nervous system. There are several such devices in use today ranging from 
custom designed devices to commercially available devices. Most 
stereotactic devices have several things in common. The devices are 
attached securely to the patient, generally the head, the exact 
three-dimensional spatial relationship between the target and the device 
is determined after visualization of the target by an x-ray or imaging 
technique, and the device has a probe or instrument holder that can be 
adjusted to advance an instrument from various directions to the target 
with great accuracy. A particularly accurate instrument holder is known as 
a microdrive which is used by a surgeon to accurately advance a surgical 
instrument to a predetermined target in the brain. 
The commonly used and/or known stereotactic apparati are the Leksell 
apparatus, the Riechert-Mundinger Apparatus, the Todd-Wells apparatus and 
the Brown-Roberts-Wells apparatus. The Leksell apparatus is a 
target-centered device utilizing a 19 cm radius arc and traveler 
arrangement to guide a surgical instrument to the target. The traveler 
arrangement may be a microdrive. The entire apparatus is generally 
attached to the patient's head using a metal frame. 
The Ricchert-Mundinger apparatus is a polar coordinate device with two 
movable arcs allowing motion of the probe holder such as a microdrive in 
three dimensions. Once aligned with target point-entry point vector, the 
probe length need only be determined for the proper distance to reach the 
target point. 
Like the Leksell apparatus, the Todd-Wells apparatus is a target-centered 
device in which the target point is placed at the center of an adjustable 
radial coordinate system. The Todd-Wells apparatus differs from the 
Leksell apparatus in that it is the head which is moved to align the 
target point, not the frame. 
The Brown-Robert-Wells apparatus was the first device created for use with 
CT. The apparatus established the concept of frame-centered sterotoxy, 
operating in such a manner so that the target need not be moved to the 
center of some arc. This was possible by making use of the 
three-dimensional information in the CT images to establish a 
three-dimensional vector of entry point to target point. The frame is then 
adjusted by moving four rotational settings so that the probe holder 
aligns with the entry-target vector. 
A variety of instruments may be used with any of these or other 
stereotactic devices including electrodes, cannulas, biopsy instruments, 
catheters and the like. 
A particularly preferred type of probe or instrument holder is a microdrive 
which attaches to and forms a part of the stereotactic apparatus to 
provide the capability of accurately guiding and directing a surgical 
instrument to a target. A microdrive, as opposed to other stereotactic 
instrument holders is an instrument holder which includes means to 
accomplish a controlled advancement of a medical instrument. 
In use, a 0 point or reference point is determined by an appropriate 
technique such as CT or MRI and the target point within the brain is 
calculated in reference to the 0 point. Then, the stereotactic head 
assembly is adjusted and the instrument holder or microdrive is set with 
reference to the 0 point. A surgical instrument is then ready to be 
advanced into the brain. 
Many microdrive devices have been developed for use in laboratory animals. 
For example, Bland et al, "A Direct-Drive, Non-Rotating version of Ranck's 
Microdrive," Physiology & Behavior, Vol. 24, pp. 395-397 (1990) describes 
a direct drive, non-rotating microdrive for the implantation of 
microelectrodes for the measurement of extra cellular unit potentials in 
freely moving animals. Bland's microdrive utilizes a headed stainless 
steel screw to move an electrode into a desired location within the 
animal's brain. This microdrive has only a very limited advancement range 
and is not adapted for stereotactic frames suitable for human use. 
Radionics (Burlington, Mass.) is the only supplier of current commercially 
available microdrives suitable for use in human subjects. The 
Radionics.RTM. microdrive utilizes a gear assembly to achieve depth 
placement. There are inherent drawbacks to such an approach. First, 
actuating the gear mechanism causes undesirable vibration along the 
instrument, which could result in vibration of the implement being lowered 
into the burr hole; e.g., a cannula. This can result in undesirable tissue 
damage. In addition, the mechanical engagement of the gear teeth may cause 
some debris to flake off the instrument, potentially contaminating the 
operation site. Further, some surgeons have found that the instrument 
tends to "free fall," which could cause sudden, uncontrolled movement of 
the cannula into the brain of the patient. 
U.S. Pat. Nos. 5,004,457 and 5,006,122 disclose tissue implantation systems 
which utilize a conventional instrument holder available from David Kopf 
Instruments (Tujanga, Calif.). 
Accordingly, there is a need for a microdrive which will provide easy 
setting of zero points with reference to a patient's scalp, skull, or 
dura, as the case may be. In addition, there is a need for a microdrive 
which can also capture a pusher which is often used to hold an implant in 
position while a cannula or other instrument is being removed from a 
target. 
It is an object of the present invention to develop a microdrive that 
overcomes the disadvantages of the prior art microdrives and which is 
useful with a variety of stereotactic devices used for human surgery. 
DISCLOSURE OF THE INVENTION 
The present invention is directed to a novel microdrive apparatus. As used 
herein the term "microdrive apparatus" means an apparatus useful with a 
stereotactic assembly or equivalent apparatus to hold and direct a 
surgical instrument into a target which is a portion of the central 
nervous system of a patient. Preferably, the stereotactic assembly is a 
head ring and the target is in the brain. The microdrive apparatus 
generally comprises a base member, an elongated guide member fixedly 
attached to the base member, an instrument holding member slidably 
attached to the guide member having a means for holding a surgical 
instrument, a stop member slidably attached to the guide member and 
disposed between the base member and the instrument holding member. The 
stop member has a measuring member containing measuring indicia thereon 
fixedly attached thereto. The instrument holding member is attached to the 
guide member in a manner that prevents its movement and the movement of a 
surgical instrument attached thereto in any direction except along the 
longitudinal axis of the guide member. This is an important feature of the 
present invention since movement in other directions, even slight 
movement, could have detrimental to the surgery being performed. The 
instrument holding member is preferably designed for use with a specific 
instrument; e.g., a cannula. The cannula advanced by the microdrive is 
most preferably used as an implantation device for a NeuroCRIB.RTM. device 
available from CytoTherapeutic Inc. The instrument holding member is 
interchangeable with other such members, depending on the specific 
instrument to be used with it. 
The microdrive apparatus may be used with most of the variety of 
stereotactic assemblies used in stereotactic surgery. In use, the 
microdrive is attached to the stereotactic assembly via its base and 
particularly the adapter portion of the base. Depending upon the 
stereotactic assembly being used, the adapter portion of the base may 
either have to be designed to facilitate attachment or an adapter sleeve 
must be attached thereto to allow for attachment. The use of 
interchangeable instrument holding members enables the microdrive of the 
present invention to be used with any of the surgical instruments used in 
stereotactic surgery. In any case, the general operation of the microdrive 
apparatus remains the same. It facilitates the insertion of surgical 
instruments into the central nervous system, e.g. the brain or spinal 
cord, with accuracy, safety and by means of a simple mechanical operation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The microdrive apparatus of the present invention will now be described 
with reference to the drawings. The embodiment shown in FIGS. 1-6 
describes one apparatus according to the present invention which is 
particularly useful in inserting a cannula into the brain, through which 
cannula an implant may be inserted. While this embodiment describes a 
microdrive which is particularly suited for cannula insertion into the 
brain, it should be understood that the basic structure, design and 
operation of the apparatus is the same for whatever surgical instrument is 
being inserted and whatever instrument holding member is being used. 
Referring to FIG. 1, a microdrive 10 of the present invention comprises a 
base member 12 having an adapter base portion 14 and a base stop portion 
16. As shown, the two portions of the base member are integral therewith. 
However, they may be separate members fixedly attached to each other. The 
base member 12 has a channel which extends therethrough so that a surgical 
instrument which is attached to the instrument holder can pass 
therethrough in use. The channel is best shown in FIGS. 2-4. The base 
member as shown in FIG. 1 is designed for use with the Radionics.RTM. BRW 
head frame assembly. For use with other head assemblies, other adapter 
bases (the shape and size of which are readily determinable from 
measurements of the mounting block of the particular stereotactic frame 
being used) or an adapter sleeve may be used. Attached to the base member 
12 is a guide member shown generally as 18. The guide member in this 
embodiment is composed of two elongated cylindrical rods 20 and 22 which 
are fixedly attached to the base member 12. 
Stop member 24 containing a screw clamp 25 is slidably attached to guide 
member 18. The slidable attachment is achieved by means of two channels 
which pass through the stop member through which the rods 20 and 22 pass. 
As a result, the stop member is able to slide up and down along the 
longitudinal axis of the guide member. The stop member 24 may be 
temporarily fixedly attached to the guide member 18 by tightening the 
screw 26 against rod 22. Fixedly attached to the stop member 24 is a 
measuring member 28, which contains measuring indicia 30 thereon. Such 
measuring member is generally in the form of a ruler with an appropriate 
scale and measuring indicia corresponding thereto. Preferably, the scale 
is a conventional metric scale. In use, as the stop member slides up and 
down the guide member 18, the measuring member follows. 
Instrument holding member 32 is likewise slidably attached to the guide 
member 18. Like the stop member 24, the instrument holding member 32 has 
two channels through which the rods 20 and 22 pass. This arrangement 
permits movement of the instrument holding member along the longitudinal 
axis of the guide member 18, but prevents movement in any other direction. 
Again, like the stop member 24, the instrument holding member may be 
temporarily fixedly attached to the guide member 18 by tightening screw 34 
against rod 22. The instrument holding member 32 contains a notch 36 for 
holding a cannula which may be held in place by a screw 38. As shown, the 
measuring member 28 is disposed so that it may slide through a notch 40 in 
instrument holding member 32. This permits unrestricted movement of the 
stop member 24 relative to the instrument holding member 32. 
As best shown in FIG. 6, the channels in each of the stop member 24 and the 
instrument holding member 32 preferably contain means to maintain 
sufficient pressure to hold the members 24 and 32 in place until 
intentionally changed. Suitable such means include bushings 24a, 24b, 32a, 
and 32b, although other means (not shown) such as leaf springs, jack 
screws, or captured sliders may be used. Preferably the means are bushings 
which are generally made from or at least coated with a low friction 
material such as Teflon.RTM. or Rulon.RTM. material. The bushings or other 
means allow the stop member and the instrument holding member to ride 
smoothly along the rods 20 and 22, which are in contact with the bushings, 
in response to external pressure (surgeon's manipulations) while 
substantially holding the members 24 and 32 in place when no pressure is 
being applied. This keeps the members 24 and 32 from suddenly crashing 
down and causing the instrument to "free fall". 
While not necessary for the operation of the microdrive of the present 
invention, this embodiment shows a second instrument holding member 42 
which is fixedly attached to the distal end of the guide member 18. Like 
instrument holding member 32, the second instrument holding member 42 has 
a notch 44 which permits the measuring member 28 to slide therethrough. 
The second instrument holding member also has a notch 46 with a screw 48 
for holding an instrument. 
In general use a cannula will be inserted into the first instrument holding 
member 32 with an obturator in place inside the cannula. The obturator 
serves to part tissue, keep the cannula from scoring tissue, and keep the 
cannula substantially rigid during insertion. The obturator may or may not 
be imageable. The second instrument holding member 42 can then hold a 
pusher with an implant mounted on the end or another instrument (not shown 
in the drawings). After the obturator has been removed from the cannula, 
an implant or other instrument may be fed into the cannula and the top of 
the pusher may be locked into the second instrument holder. Alternatively, 
the implant or other instrument can simply be inserted into the cannula 
without use of the holding member 42. 
All of the elements of the microdrive should be made of materials which are 
sterilizable by common methods, including both gas and autoclave means. 
The materials are rigid and preferably light weight. Thus, for example, 
the rods 20 and 22 which comprise the guide member 18 can be made of such 
as graphite reinforced composite or a metal such as stainless steel. The 
base member 12, the stop member 24, and the instrument holding members 32 
and 42 can be made of aluminum, titanium, stainless steel, or high 
temperature resistant plastics. 
In other embodiments according to the present invention the instrument 
holding member 32 is interchanged with other instrument holding members 
with slightly different means suitable for holding surgical instruments 
other than cannulas. Such surgical instruments include electrodes, biopsy 
instruments, etc. 
The operation of the microdrive apparatus of the present invention can best 
be described with reference to FIGS. 2-4. While the discussion will be 
with reference to cannula insertion into a brain, it should be understood 
that the operation of the apparatus which holds another surgical 
instrument for insertion into other portions of the central nervous system 
is substantially the same. 
As previously discussed in the Background of the Invention section, the 
target point and a 0 point are established by any of a number of 
techniques, such as CT and MRI. Once a 0 point is established relative to 
the target point within the brain, the necessary adjustments are made with 
the stereotactic head assembly and the necessary hole or holes are drilled 
through the patients skull down to the dura. Although not shown, the 
microdrive is attached to a head assembly at this point in the surgical 
procedure and oriented in the appropriate direction to facilitate surgical 
instrument placement in the brain. 
As shown in FIG. 2, the instrument holding member 32 holds a cannula 50 
having a stem 51 in place. Within the cannula is an obturator having a top 
portion 57 which extends to the tip of the cannula 56 with the obturator's 
tip 57 extending just beyond the tip of the cannula 56. The cannula 
generally includes a conventional luer style taper lock 55 to maintain the 
obturator in place. The stop member 24 and the instrument holding member 
are then held together and adjusted on the guide member 18 to a position 
so that the obturator tip 57 is at the 0 point, e.g. the skin of the scalp 
of the patient. Once the two members are adjusted to the 0 point position, 
both members are temporarily held in place by tightening screws 26 and 34 
against the rod 26. Then, as shown in FIG. 3, the screw on the stop member 
is loosened and it is moved downward or towards the base a predetermined 
distance equal to the pre-measured distance to the target from point 0. 
This distance is easily determined using the measuring member 28 and the 
scale printed thereon. In the embodiment shown in FIG. 4, this distance is 
identified as 5 mm, although the drawings are not to scale. Once the stop 
member 24 reaches this position, it is again held in place by tightening 
screw 26. Then, as best shown in FIG. 4, the screw 38 is loosened and the 
instrument holding member is moved towards the base member 12 until it is 
stopped by stop member 24. In turn, the cannula with obturator therein is 
inserted into the brain the premeasured distance (in this case 5 mm) to 
the target point. Screw 34 is then tightened preventing movement in any 
direction of the instrument holding member 32 or cannula 50. Now that the 
cannula tip is at the target point, the obturator may be removed and other 
surgical devices may be lowered through the cannula into the brain, such 
as an implant device, and held in place by means of the second instrument 
holding member 42 and screw 46. 
Whether a cannula or some other instrument is used, the general operation 
of the microdrive is the same excepting the need and use of the second 
instrument holding member 42 which is only needed when a second surgical 
instrument is used. An example of an alternative second instrument is a 
single action biopsy forceps such as that designed for use with the 
Leksell Stereotactic Instrument. 
A particularly suitable implantation therapy system and method for so doing 
are disclosed in copending application U.S. Ser. No. 07/998,368, Aebischer 
et al, entitled "Implantable Therapy Systems and Methods," filed Dec. 30, 
1992, (Attorney Docket No. CTE-018), the subject matter of which is 
incorporated herein by reference. 
EXAMPLE 
Closed, cannula based, stereotactic implantation technique for placement of 
encapsulated cellular grafts intracranially in the lateral ventricle in a 
human 
Immediately before an implantation procedure, a patient was fitted with a 
stereotactic head ring assembly and localizer ring (or image 
localization/marker device) suitable for guided cannula placement within 
the lateral ventricles using local anesthesia (local infiltration with 
generally 1% lidocaine). The Radioinics.RTM. BRW frame was used. (The 
Radionics.RTM. CRW, Leksell.RTM. and functionally similar devices are also 
appropriate.) A computed tomography (CT) scan had been performed and was 
used to define one or more target sites and stereotactic coordinates for 
the implant(s). In general, implantation cannula trajectory and implant 
site are chosen with the following considerations: (1) avoiding the 
frontal sinuses; (2) avoiding the choroid plexus; and (3) allowing 
straight, undistorted positioning capsule lengths 2.5, 3.75, or 5.0 cm). A 
target site must be selected that will allow a length of the internal end 
of the cannula, i.e. at least the length of the membrane portion of the 
desired capsule, to lie within an acceptable, CSF filled space within the 
ventricle. The zero reference point for determining cannula insertion 
depth was the surface of the skin, as seen on the CT scan, and the target 
site was defined as the intended target of the internal tip (opening) of 
the inserting cannula. Two implant devices may be placed in one patient at 
a single procedure by placing one implant in each lateral ventricle. 
Future implantation sites may target the third ventricle, the aqueduct, 
and/or solid brain structures. The crent stereotactic guidance technique 
used CT imaging for reference, however MRI, stereotactic atlas 
coordinates, ultrasound or other guidance methods may also be appropriate. 
Following completion of data gathering for stereotactic placement of the 
implant(s), the patient was transferred to the operating room for the 
implantation procedure. 
After establishing IV access and administering prophylactic antibiotics 
(currently, cefazolin sodium, 1 gram IV), the patient was positioned on 
the operating table in the semi-supine/seated position with the 
stereotactic head ring assembly secured to the table. The operative field 
was sterilely prepared and draped exposing the intended implantation 
site(s) (generally located in the paramedian, frontal region) and allowing 
for sterile placement and removal of the stereotactic arc 
system/manipulator to the frame base. 
Local infiltration with 1.0% lidocaine was used for anesthesia of the skin 
and deeper scalp structures down to the periostium, and a 4-8 cm skin 
incision is made down to the skull at the calculated entry site(s) for the 
stereotactically guided insertion cannula (generally in the frontal 
region, in the parasagital plane 3 cm to the right or left of the midline) 
using electrocautery for homeostasis. A twist drill guided by the 
stereotactic arc system was then used to create a burr hole (generally 4 
mm diameter) down to the level of the dura. The dura is sharply 
penetrated, and the insertion cannula/obturator assembly 
(CytoTherapeutics, Inc. implantation kit, specific for ventricular 
insertion) is mounted into the stereotaxic microdrive (CytoTherapeutics, 
Inc. custom fabrication, mounted in the guide block of the transverse arch 
slide of the arc system for use) and directed into the burr hole. Blood 
from the wound was excluded from the burr hole by applying the microdrive 
guide tube (CytoTherapeutics, Inc. custom fabrication) directly against 
the rim of the burr hole. The insertion cannula/obturator assembly were 
advanced manually to the preset depth stop on the microdrive, leaving the 
tip of the cannula at the target site. The obturator was then carefully 
withdrawn from the insertion cannula, taking care not to deflect the 
cannula with the top of the obturator. Appropriate position of the tip of 
the cannula within the ventricle was confirmed by a meniscus of 
cerebrospinal fluid (CSF) rising up within the clear insertion cannula 
after removal of the obturator. Samples of CSF were taken for 
preimplantation catacholamine, enkephalin, glucose, and protein levels and 
cell counts. 
The encapsulated adrenal chromaffin cell graft (CytoTherapeutics, Inc. 
Cerecrib.RTM. R) was provided in a sterile, double envelope container, 
bathed in transport medium, and fully assembled including a tubular 
silicone tether. Prior to implantation through the insertion cannula and 
into the ventricle, the capsule was transferred to the insertion kit tray 
(CytoTherapeutics, Inc. implantation kit) where it was positioned in a 
location that allows the capsule to be maintained in transport medium 
while it is grossly examined for damage or major defects, and while the 
silicone tether was trimmed, adjusting its length to the pusher and 
removing the hemaclip that plugs its external stainless steel pusher 
(CytoTherapeutics, Inc. implantation kit, specific for insertion cannula 
and membrane capsule length) by inserting the small diameter wire portion 
of the pusher into the full length of the tether to stiffen the tether for 
passage through the cannula. 
The Cerecrib capsule was handled completely by the silicone tether and the 
handle of the pusher as the membrane portion of the device was carefully 
introduced into the cannula. The capsule was advanced until the tip of the 
cannula positioned in the subarachnoid space (but not extending beyond the 
tip of the cannula positioned in the subarachnoid space (but not extending 
beyond the tip of the cannula). This placement was achieved by 
premeasuring the cannula and the capsule-tether-pusher assembly, and it 
assures that the membrane portion of the capsule is protected by the 
cannula for the entire time that it is being advanced into position. After 
the capsule was positioned manually within the cannula, the pusher is 
locked into position in the microdrive and used to hold the capsule in 
position in the ventricle (without advancing or withdrawing) while the 
cannula is completely withdrawn from over the capsule and pusher. The 
pusher was then removed from the capsule by sliding its wire portion out 
of the silicone tether. Using this method the final placement of the 
capsule was such that the entire membrane portion of the device lies 
entirely within an appropriate, CSF-containing region of the ventricle. 
The membrane capsule was anchored at its external end by a length of 
silicone tether that runs (generally) through a portion of the frontal 
lobe before it exits through the dura and the skull, leaving generally 
5-10 cm of free tether material that was available for securing the 
device. The free end of the tether was then anchored to the outer table of 
the skull adjacent to the burr hole using a standard, maxillo-facial 
miniplate and screws and completely covered with a 2 or 3 layer closure. 
The patient was then transferred to the neurosurgical recovery area and 
followed for 12 hours postoperatively for potential hemorrhage 
complications with no special restrictions. Antibiotic prophylaxis was 
also continued for 24 hours following the implantation procedure.