Keywords
functional neurosurgery - robotic neurosurgery - surgical error - stereotactic procedures
Introduction
Robotic surgery has made inroads into multiple specialties. Over the past decade,
several common laparoscopic and thoracic procedures can be performed using master-slave
robotic devices. Neurosurgery has been a late adopter of robotics. General-purpose
surgical robots have not found neurosurgical applications. The specialized anatomical
constraints and risks of neurosurgery have been responsible for the delay in research
in this field. Of the various neurosurgical procedures, functional procedures have
caught maximum attention in this field. This is due to the nature and exacting requirements
of these procedures.
An Overview of Robotics
The growth of industrial robotics happened in the late 1960s with the establishment
of dedicated companies that made multi-jointed master-slave systems. The human masters
control the “slave” robotic device and execute the procedure. By this concept, human
surgeons control all aspects of the procedure. The robot facilitates accuracy, task
repeatability with minimal error, and consistency. The multi-jointed robot has been
the traditional model on which most modern surgical robots are designed. A robot generally
can be classified according to several features. They can be classified into serial
and parallel robots based on the organization of elements within the robot. Parallel
robots have greater structural stability and are more accurate than serial robots,
which is of relevance for neurosurgical stereotactic procedures. They can also be
classified based on their number of degrees of freedom or axes. The degrees of freedom
basically indicates the capability of a robot.[1] The joints between these elements can be classified into prismatic (sliding or linear
joint), revolute joints (hinge joint) and screw joints (helical joint). Other types
of joints include cylindrical, spherical planar pair, and Hooke joint. The important
joints are highlighted in the diagrams ([Fig. 1]).
Fig. 1 Example of joints used in robots and the movements associated with each joint.
Prismatic joints allow for one directional motion and are relatively error free. All
these joints have one degree of freedom. Examples of the joints with multiple degrees
of freedom include cylindrical, universal, and spherical (ball and socket) joints.
Based on the joints and the nature of kinematics, robotic devices are said to have
varying degrees of freedom ([Fig. 2]).
Fig. 2 Depiction of degrees of freedom associated with an object. The primary directions
are listed on the left-hand diagram. The rotary movements are depicted in the right-hand
diagram constituting the six degrees of freedom for a given object.
Surgical Robotics
Different surgical robotic systems have differing arms based on the target use. Thus,
robots such as Da Vinci that are used for laparoscopic surgery have actuator arms
designed for laparoscopic use with small gripping end tools based on an “EndoWrist.”
Robots used for orthopaedic procedures, thus having tools built to position and drill
into long bones. In essence, surgical robots have differing designs based on the surgical
requirements. At present there is no single general-purpose surgical robot that can
be used across all surgical specialties. Though these devices are known as surgical
robots, and the procedure-labeled robotic surgery, robots are not used in the complete
surgical procedure. A significant component of the procedure is still performed by
humans. For example, in laparoscopic surgery, human surgeons have to perform peritoneal
insufflation and primary viewing only after which the robot docks with the abdominal
ports. Similarly, the orthopaedic robots are used only after bone exposure is completed.
A surgeon requires specialized training to operate a surgical robot. The surgeon is
thus the master of the surgery and the robot a slave that executes the tasks. The
current surgical robots are no more than action filtering devices with minimal autonomous
function. Thus, they behave as advanced tools that simplify a complex task, for example
steps such as laparoscopic suturing that otherwise have a protracted learning curve.
Thus, the primary role of master-slave robots is in reducing the skill level required
to execute a particular task.
Neurosurgery and Robotics
The primary purpose of using robots in neurosurgery has been to improve targeting
of deep structures. Targeting of deep structures, especially for biopsy, has been
a focus of early robotic devices. The first of these procedures were incidentally
performed using an industrial robot.[2]
[3] Robotic targeting has high precision, and this was the primary motivation for utilizing
them. Surface surgery or surgeries requiring a formal craniotomy requires general
purpose robots. These robots require arms, master-slave control systems, and sophisticated
safety and feedback mechanisms. The first dedicated attempt to build a general-purpose
neurosurgical robot was by Prof. Sutherland and his team at the University of Calgary.[4]
[5] Several other groups notably in Japan and Europe have been working on general-purpose
neurosurgical robots. However, stereotactic robots have more commercial success than
general-purpose neurosurgical robots.
Functional Neurosurgery
A wide range of neurosurgical procedures come under the gamut of functional neurosurgery.
They include procedures for movement disorders such as deep brain stimulation (DBS),
epilepsy surgery including resective and ablative procedure, and procedures for pain
and psychiatric diseases. Functional neurosurgery is characterized by the following
unique features. The targets are functional neuronal tissue. The targets may be on
the surface or deep within the brain substance. Functional procedures are also performed
on spinal cord. Surgical technique requires targeting of these tissues; especially
in the brain, it requires millimetric precision. Procedures for DBS and pain require
placement of biocompatible hardware in precise locations. The important common factors
in such procedures include localization by high-resolution magnetic resonance imaging
(MRI) and high accuracy in reaching anatomical targets. Stereotactic localization
has been the gold standard in localizing small targets within the substance of the
brain. The standard procedure involves imaging with a rigid calibrated metallic frame
fixed to the skull. The frame-based coordinate system ensures that errors are minimized
and the target is reached with high accuracy. Frame-based localization revolutionized
target localization within the brain. Thus, targets could be acquired and localized
without visual confirmation and with a high accuracy. The targets are calculated in
relation to the coordinate system of the frame. Path planning features are available
in most modern image-viewing software. High-resolution 3-Tesla MRI has become standard
of care for obtaining imaging for functional procedures. Improvements in imaging have
been responsible for improved localization of targets such as the subthalamic nucleus.
Image fusion techniques by which MRI and CT (computed tomography) images can be fused
have made significant improvements in reducing procedure time. Electrophysiologic
recording has also improved target localization by adding a physiologic component.
Adverse effects by damage to normal structures have been minimized by electrophysiologic
monitoring during the procedure. In essence, a gradual improvement in several procedural
aspects of functional neurosurgery has been evident.
Target Localization and Reachability
An important component of functional neurosurgery is in reaching targets deep within
the brain. These targets are visualized by high-resolution MRI. Conventionally, the
stereotactic frame-based calculation is used to reach a target within the brain. The
frame provides a volume within which a given target can be localized. Rigid fixation
of the skull to the frame ensures that the volume within the skull that is not visible
can be reached by using mathematical calculation. The co-registration of the frame
and skull volume is done by obtaining high-resolution MRI. Newer techniques such as
image fusion help reduce overall procedure time. The basic premise remains that the
volume within the skull is imaged in relation to the “fixed” frame. The calculated
targets are reached through a set of precision instrumentation mounted on the same
stereotactic frame. The frame is thus a bridge between the intracranial target and
operating tools.
The intracranial targets are small in size and are surrounded by eloquent structures,
damage to which results in significant neurologic deficits. The targeting robot thus
requires the following features. It has to have a small profile to prevent damage
to normal structures surrounding the target. There should be an opening in the skull
such that the device can safely traverse without any change in the path to the target.
The varying consistency of the brain and brain-shift are factors that can cause change
in trajectory. A major issue of concern is that the target is not directly visualized
at any point of the surgery. Thus, the error in reaching the target can only be computed.
Indirect means such as microelectrode recordings are indirect proxies for reducing
error by means of electrical confirmation of the target region. The occurrence of
error in targeting, its identification, and correction form an important application
of robotics in functional neurosurgery. An understanding of what constitutes error
is thus needed.
Error and Its Minimization in Functional Neurosurgery
A complete DBS surgical procedure involves skin incision, dissection, burr-hole placement,
dural incision, electrode array placement and recording, and permanent electrode placement
followed by wound closure. The procedures of frame fixation, image acquisition, fusion,
and registration occur prior to the actual surgery. The time from burr-hole placement
to burr-hole closure after the electrode fixation is crucial and has the maximum risk
of workflow error. The final trajectory from the skin surface to the target tissue
is linear. The end actuator system for this phase of surgery has to be strictly linear
with one direction of freedom. Prismatic or helical joints are ideal joints at the
last set of effectors in contact with the skull. The number of joints in relation
to the skull has to be minimized to reduce errors. The size of the subthalamic nucleus
is approximately 3 mm. Even if a 10% error margin is considered, the device should
be able to reach a given spatial target with an error less than 0.3 mm from the target
tissue in 3D space. Current documentations of error during stereotactic procedures
are in the range of 2 mm.[6]
[7] Newer techniques and developments are expected to reduce the errors further as more
and more automation enters into the workflow.[5]
[8]
[9] Sources of error in robotic stereotactic procedures have been studied in detail.[6]
[10] These include imaging resolution, distortions in image acquisition and the display
system, anatomical changes that may occur between image acquisition and the actual
surgery; registration errors between patient and the imaging; camera, tool, and robot
calibration errors; and kinematic errors. Regarding stereotactic brain procedures,
the access path from the dural entry point to the target is blind. There is no visual
confirmation regarding the accuracy of having reached the target. This is somewhat
mitigated by image guidance systems and microelectrode recordings for DBS procedures.
These techniques in turn have been advancements on the earlier techniques of brain
atlas-based localizations. Even with current (nonrobotic) practices, the issues of
brain-shift, frame errors, and image acquisition issues persist to varying extents.
It is in this context that robots would play a major role in the near future in minimizing
error.
Developing a single robot that can perform skin incision, scalp dissection, and retraction
is a difficult proposition. Benefits of developing tools to this end are not of relevance,
as the chances of error occurring during these steps are minimal. From the point of
burr-hole placement to the electrode implant stage, robotic devices can minimize error.
It is thus imperative that a fully dedicated functional neurosurgery robot should
have multiple components dedicated to diverse tasks such as burr-hole placement, dural
incision, prevention of intradural air entry, and electrode array deployment and implantation.
Some of the tasks may require dedicated separate modules, for example drills, and
haemostatic devices. The other elements such as the electrode array, electrode implantation,
and testing components are best designed coaxial and brought into play at relevant
points of time. The surgical steps after burr-hole placement up to electrode delivery
and implant delivery have to be a seamless robotic procedure.
Robots in Functional Neurosurgery
Of various surgical procedures undertaken in neurosurgery, highest benefits of robotic
intervention can happen in functional neurosurgery. Though a plethora of robotic surgical
programs exist,[8]
[11]
[12]
[13]
[14] commercially available robots are few and far between. A relatively large number
of stereotactic robotic programs have been documented in the literature. However,
conversion of research programs to commercially successful products has been noted
to be very few. The Neuromate Stereotactic robot (Integrated Surgical Systems), Renaissance
system (Mazor Robotics), and ROSA (Medtech) are commercially available and designed
for neurosurgical stereotactic procedures. Neuromate was the first to obtain U.S.
Food and Drug Administration (FDA) approval for stereotactic neurosurgical procedures.
All these systems have been used for functional neurosurgical procedures. The Ronna
G3 (University of Zagreb, Croatia)[7] and iSYS1 (iSYS Medizintechnik)[5] are newer systems for stereotactic applications.
Indian Groups
Surgical robotic research has not achieved institutional status in India. Individual
projects on various aspects of surgical robotics have been conducted with a focus
on assist devices and simulators. Bhabha Atomic Research Centre in collaboration with
Tata Memorial Hospital, Mumbai, have developed a stereotactic robot for neurosurgical
applications.[2] The device has six degrees of freedom and has a parallel kinematic mechanism and
is undergoing clinical testing. The Surgical and Assistive Robotics Laboratory (SARL)
is a joint effort of NIMHANS (National Institute of Mental Health and Neurosciences)
and IIIT (International Institute of Information Technology) Bangalore. Though the
focus of the group is on hyperflexible surgical robots, visualization, and ranging,
linear targeting systems are also in the process of development. Regarding the linear
targeting device, the development philosophy envisages both cranial and spinal applications
encompassing stereotactic and spinal positioning systems. The work on the linear targeting
system is at an early stage, and a few prototype devices have been developed: image
integration and mapping. More and more hospitals in both government and private sectors
have been using commercially available stereotactic robots for neurosurgical procedures.
At the time of this writing, two Indian centers have been listed on the ROSA Web site.
Mazor robots have also been reportedly used in two private sector hospitals for spine
applications.
Discussion
Attempts at using robotic technology for neurosurgical procedures have started nearly
three decades back. However, the number of research programs translating into commercial
products has been few. The cost of a dedicated robotic surgery program is enormous
and is in the range of several million dollars. The cost of the device development
and marketing thus forms a big barrier for most research groups. Unlike in laparoscopic
surgery where the “EndoWrist” of the Da Vinci system had a distinct surgical utility,
current stereotactic robotic neurosurgical devices do not provide a distinct advantage
over a meticulous human surgeon. Studies on outcome differences between robotic and
human neurosurgery have not been conducted. It may be a little too early to conduct
these studies as the technologies, and the current robotic surgical workflows have
not yet matured. The robots developed are linear devices with applications restricted
to image guidance and stereotactic procedures. Thus, two factors determine the popularity
of robotic surgical procedures: cost and significant value addition. Value addition
can be in terms of ease of use, decrease in time of procedure, or an actual reduction
in cost. An ideal functional robot should seamlessly perform a complete functional
neurosurgery procedure with minimal human intervention.[13] One of the major concerns of contemporary DBS procedures is the necessity of performing
surgery awake. The procedure also is known to last several hours. At a conceptual
level, the entire set of surgical tasks involves accessing the target region with
multiple passes of electrodes and recording. It is this part of surgery that a robot
with its inbuilt accuracy and check systems would be able to reduce significantly
and perform consistently with higher accuracy. Simultaneous (bilateral) performance
of the procedure can be achieved with robotic assistance further reducing overall
procedure time. Only when ease of use parameters compared with current nonrobotic
procedures along with a decrease in pricing occur would an inflexion point occur for
robotic neurosurgery. In terms of robotics, technical challenges for functional neurosurgery
are minimal, and in that, no new line of research is required for developing an ideal
robot. However, from the point of functional neurosurgery, significant advances have
to occur over and above the current workflows offered by the robotic systems. The
challenges lie in design and not as much in device development or research. As it
stands today, the elements required for an ideal stereotactic robotic system are already
in place. The coming together of these elements has not occurred till date. The key
goal to be achieved would be in minimizing human intervention during the procedure.
In an ideal system, human presence would only be supervisory following the planning
stage. Only when human intervention is minimized during the procedure will the full
benefits of robotic intervention would become apparent. This is not only in the context
of accuracy but also in minimizing cost per procedure and in ease of conducting the
procedure. These factors in turn would reduce the overall costs. Ultimately, economic
considerations play a major role in adaptation of the novel procedure.
An ideal surgical master-slave robot for functional neurosurgery should have the following
features. The robot should be able to execute all position and spatial targeting related
steps with minimal human intervention. The robot must be able to perceive and correct
for errors at all phases of the operative procedure. The user interface should be
simple enough that no specialized training would be required to operate the robot.
A favorable robotic surgical workflow would require less technical demand on the individual
surgeon. Especially in functional surgical procedures, a well-designed robot would
reduce the skill level required of the surgeon for performing the procedure. More
surgeons can thus undertake the procedure using the robot ultimately benefitting patients
who otherwise have to travel to high-volume centers for a good outcome.
Conclusion
The scope of robotics in functional neurosurgery is immense. As implants for neurologic
diseases evolve, the demand for target-based neurosurgical interventions will increase.
Robots would ensure that high-precision surgery can be performed even in low-volume
centers. Robotic surgery would be the leveler, ensuring widespread availability of
surgical options. As with any emerging technology, cost is a major issue of concern.
However, as more and more research and commercial groups enter into the field and
the market opens out, the only direction the cost can move is down. Ideal robotic
devices would supplement and complement the surgeon's knowledge and reduce the skill
and effort levels required for high-level consistent performance. That ultimately
is the advantage of robotic surgery. Robots are advanced tools. This tool in the hands
of trained surgeons would help in complementing human efforts for the surgical management
of functional neurosurgical disorders.
