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Claims  |
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The embodiments of the invention in which an exclusive property or
privilege is claimed are defined as follows:
1. A system for positioning a tool relative to a patient's bone to
facilitate the performance of a surgical bone alteration task, the system
comprising:
bone immobilization means for supporting the bone in a fixed position with
respect to a reference structure;
a robot comprising a base fixed in position with respect to the reference
structure, a mounting member, a manipulator connected between said base
and said mounting member and permitting relative movement therebetween,
and attachment means for securing the tool to said mounting member; and
means for causing said mounting member to move relative to the reference
structure in response to movement commands, whereby the tool can be moved
to a desired task position to facilitate performance of said task.
2. A system as claimed in claim 1, further including task control means for
controlling and monitoring the operation of said robot, said task control
means comprising memory means for storing data and control programs, and
control-processing means for processing said control programs to generate
said movement commands.
3. A system as claimed in claim 2, further comprising a template attachable
to said mounting member, and means for recording a reference position of
said template when said template is positioned such that a feature of said
template is in a desired position relative to the bone, said reference
position being recorded in a world coordinate system that is fixed with
respect to the reference structure.
4. A system as claimed in claim 3, wherein said data in said memory means
includes a geometric database comprised of data representative of the
geometric relationships relevant to performance of the task, and wherein
the task control means includes a tool-positioning program for determining
the task position in said world coordinate system by integrating said
geometric database and said reference position, and for generating
movement commands for directing the mounting member to a position such
that the tool secured to the mounting member is in the task position.
5. A system as claimed in claim 4, wherein the tool includes a tool
identification pattern and wherein said attachment means includes an
identification device positioned so as to be adjacent said identification
pattern when the tool is secured to said mounting member, said
identification device including means for ascertaining said tool
identification pattern and generating signals indicative thereof, said
task control means including means for identifying the tool attached to
said mounting member by actuating said identification device and
interpreting said identification signals.
6. A system as claimed claim 5, wherein said identification device includes
a plurality of LED photodetector pairs, said photodetector pairs being
capable of transmitting and receiving radiation signals, and said tool
identification pattern is a plurality of bored and unbored areas such that
when the tool is mounted on said mounting member and said photodetectors
are actuated, a reflected signal will be detected by said photodectors
when they are adjacent a bored area and will not be detected when said
photodetectors are adjacent an unbored area, whereby said identification
signals are indicative of reflected and nonreflected signals.
7. A system as claimed in claim 4, wherein said robot base includes a
safety stand that tilts the robot if the robot encounters a rigid object
while it is moving, and that generates a safety signal when said stand
tilts, whereby said task control means responds to said safety signal by
preventing further movement of the robot.
8. A system as claimed in claim 3, wherein said task control means further
comprises operator interface means for displaying system information and
receiving data and operator control commands, wherein said robot includes
position-sensing means for generating positional signals indicative of the
position of said mounting member in the world coordinate system, and means
for operating in a passive mode in which the mounting member may be moved
manually by an operator, and wherein the task control means first sets
said robot to the passive mode and then, upon receiving an operator
command indicating that said template has been positioned in a desired
position, records said reference position and sets said robot to an active
mode in which the mounting member moves in response to said movement
commands.
9. A system as claimed in claim 2, further comprising a template attachable
to said mounting member, a feature of said template representing a portion
of a prosthesis, and means for recording a reference position of said
template when said template is positioned such that said feature is in a
desired position relative to the bone, the reference position being
recorded in a world coordinate system that is fixed with respect to the
reference structure.
10. A system as claimed in claim 9, wherein said data in said memory means
includes a prosthesis database comprised of data representative of the
geometric relationships defining one or more interior surfaces of the
prosthesis, and wherein said task control means includes a
tool-positioning program for determining the task position in the world
coordinate system by integrating said prosthesis database and said
reference position, and for generating movement commands for directing the
mounting member to a position such that the tool secured to the mounting
member is in the task position.
11. A system as claimed in claim 10, wherein the tool includes a tool
identification pattern and wherein said attachment means includes an
identification device positioned so as to be adjacent said identification
pattern when the tool is secured to said mounting member, said
identification device being suitable for ascertaining said tool
identification pattern and generating signals indicative thereof, said
task control means including means for identifying the tool attached to
said mounting member by actuating said identification device and
interpreting said identification signals.
12. A system as claimed claim 11, wherein said identification device
includes a plurality of LED photodetector pairs, said photodetector pairs
being capable of transmitting and receiving radiation signals, and said
tool identification pattern is a plurality of bored and unbored areas such
that when the tool is mounted on said mounting member and said
photodetectors are actuated, a reflected signal will be detected by said
photodetectors when they are adjacent a bored area and will not be
detected when said photodetectors are adjacent an unbored area, whereby
said identification signals are indicative of reflected and nonreflected
signals.
13. A system as claimed in claim 10, wherein said base includes a safety
stand that tilts the robot if the robot encounters a rigid object while it
is moving and generates a safety signal when said stand tilts, and wherein
said task control means includes means for responding to said safety
signal by shutting off power to said robot.
14. A system as claimed in claim 10, further includes stabilizing means for
providing a rigid link between said mounting member and the reference
structure in addition to the link provided by said manipulator.
15. A system as claimed in claim 14, wherein said stabilizing means
provides a detectable link with said manipulator, and said task control
means includes means for detecting said link.
16. A system for determining a reference position of a template relative to
a patient's bone to facilitate the performance of a surgical bone
alteration task, the system comprising:
bone immobilization means for supporting the bone in a fixed position with
respect to a reference structure;
a robot comprising a base fixed in position with respect to the reference
structure, a mounting member, a manipulator connected between said base
and said mounting member and permitting relative movement therebetween,
attachment means for securing the template to said mounting member,
position-sensing means for generating positional signals indicative of the
position of said mounting member in a world coordinate system that is
fixed with respect to the reference structure; and
means for operating said robot in a passive mode in which the mounting
member may be moved manually by an operator, whereby the template can be
moved to the reference position.
17. A system as claimed in claim 16, further comprising task control means
for controlling and monitoring the operation of said robot, said task
control means comprising memory means for storing data and control
programs, whereby the reference is recorded by said task control means. |
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Claims |
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Description |
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FIELD OF THE INVENTION
The present invention relates to apparatus and methods applicable in
surgical situations in which the precise positioning of a tool in relation
to a patient is an integral part of the surgery. More particularly, the
present invention utilizes a programmable robot in a system for
determining a reference position for certain surgical tasks, precisely
determining the position for a specific tool relative to the reference
position, positioning the tool, and rigidly holding the tool to aid the
surgeon in a more efficient and accurate completion of the task.
BACKGROUND OF THE INVENTION
During orthopedic surgery, it is often the case that surgeons are required
to make surgical alterations to bone. Such alterations include but are not
limited to making cuts in bone, drilling holes in bone, and affixing a
plate, screw, nail, or prosthesis to bone.
When making such alterations, it is desirable that the alteration be
realized in a manner which precisely conforms to the operative plan of the
surgeon. Among the aspects of a surgical alteration which require careful
control are: (1) the alignment of the cut, hole, plate, screw, nail, or
prosthesis with respect to the anatomy of the patient; (2) if there is
more than one bone cut and/or hole, the relative alignment of the cuts and
holes with respect to each other; and (3) if a plate, screw, nail, or
prosthesis is to be fixed to bone, the alignment of the cuts and/or holes
with respect to the plate, screw, nail or prosthesis.
Current surgical techniques utilize limited mechanical means to assist the
surgeon in making bone alterations. However, existing techniques do not
suffice to ensure that perfect or nearly perfect alterations can be
achieved routinely. Where practical, it is desirable to enhance the
surgeon's decision-making process by providing accurate solutions to
purely geometric problems posed by surgery, while leaving final
positioning decisions up to the surgeon. When the surgeon is provided with
accurate geometric solutions, the quality of the overall subjective
evaluation should be improved.
An example of a surgical procedure that requires accurate geometric
solutions, as well as the evaluation of specific patient physiological
characteristics, is total knee arthroplasty (TKA), which is a total knee
reconstruction surgery. The anatomic knee is a remarkable mechanism.
Contrary to first impression, it is not a simple hinge. Rather, the femur
and tibia move relative to each other with a complex mixture of rolling
and sliding motions. The stability of the joint comes entirely from soft
tissue structures, not from bone geometry. The major stabilizing ligaments
are the medial and lateral collateral ligaments, and the anterior and
posterior cruciate ligaments.
In total knee arthroplasty, the distal femur and the proximal tibia are
resected and are replaced by prosthetic components made of metal and
plastic. The most successful designs are unconstrained prostheses that
closely mimic the natural anatomy of the knee. Like the anatomic knee,
unconstrained designs allow the femur and tibia to roll and slide relative
to each other. They depend on the natural ligamentous structures of the
knee to stabilize the reconstructed joint.
Total knee reconstruction surgery is conceptually simple. The knee is
flexed, the patella moved to one side to give access to the joint, and the
degenerated surfaces of the femur and tibia are cut away. The bone cuts
are made to fit femoral and tibial prosthetic components, which are
available in a wide variety of sizes and styles. These are generally
cemented into place, using polymethyl methacrylate (PMMA). One new
technique uses no cement. Rather, bone grows into a porous backing on the
prosthetic component. This is termed porous-ingrowth fixation.
Each year, approximately 100,000 people undergo a TKA. TKAs are often
performed in people whose knees have become so painful, because of
progressive arthritic changes, that they are unable to rise from a chair,
walk, or climb stairs. For these people, total knee arthroplasty can
provide a return to near-normal, pain-free life.
A great deal of developmental technology has gone into perfecting the femur
prostheses used in TKAs. However, the technology for positioning the
prosthesis properly on the femur has not similarly advanced. Ideally, the
bone cuts should be (1) an exact press-fit to the components, and (2) in
proper alignment with respect to bones and soft tissues. Failure to
achieve these goals will result in poor knee mechanics and/or loosening of
the components, leading eventually to failure of the reconstruction.
At present, the surgical instrumentation used in total knee arthroplasty
consists of hand-held saws which are guided by simple cutting blocks and
mechanical jig systems. There is abundant evidence in the literature that
these tools do not suffice to do a good job. First, most prosthetic
components are not put in with perfect alignment, and misalignment of
three to five degrees or more is not uncommon. Second, prosthetic
components do not fit perfectly on the bone, and there are inadvertent
gaps between the cut surface of the bone and the prosthesis. Third, there
is a learning curve associated with arthroplasty technique. The first
fifty knees a surgeon does are not as good as subsequent knees.
The primary goals of the surgeon during total knee arthroplasty are: proper
alignment of the reconstructed knee, stability of the reconstructed knee,
and press-fit of the components to the bone. With respect to alignment,
the knee should neither be knock-kneed or bowlegged, to ensure that the
medial and lateral sides of the components bear equal loads. Asymmetric
loading leads to early failure. In addition, the ligaments of the knee
should provide stability at all angles of flexion, as they do in the
anatomic knee. If the ligaments are too tight, they will restrict the
motion of the knee. If they are too loose, the knee will "give way" during
use.
Finally, if a prosthetic component is even slightly loose, then each step
will "rock" the component against the bone. The bone soon gives way, and
the reconstruction fails. Ideally, the prosthesis is a press-fit to the
cut bone at the time of surgery. This minimizes micro-scale rocking
motions. Press-fit is especially important for a porous ingrowth
prosthesis, since even a one-millimeter gap between prosthesis and bone is
too large for the ingrowth process to bridge.
These goals are simple to state, but difficult to achieve in the operating
room. To understand the problems, consideration should be given to all the
ways malalignment can occur. There are three different ways a component
can be malaligned in orientation. These correspond to rotations of the
component away from the desired orientation along the internal/external,
varus/valgus, and flexion/extension axes. Similarly, there are three
different ways to malposition a component by translation along an axis.
These correspond to translations along the medial/lateral,
proximal/distal, and anterior/posterior axes.
Thus, to achieve good alignment and good ligament balance, surgeons must
mentally manipulate three translational and three orientational variables
for each of the femoral and tibial components, or twelve spatial variables
in all. Margins for error are small. Repositioning the prosthetic
component by even one millimeter has an appreciable effect on the
stability of the knee. Moreover, each knee presents its own special
problems. It is frequently the case that the knee has a preexisting
deformity which must be taken into account.
In addition, the surgeon must also take care that the bone surfaces are
press-fit to the component. This involves five cut planes and two drill
holes for a typical femoral component, and one cut plane and two drill
holes for a typical tibial component, for a total of ten separate cutting
operations. In each case, imprecision of one millimeter or less can have
significant consequences, especially for porous-ingrowth prostheses.
It is a remarkable fact that present-day surgical instruments for total
knee arthroplasty could have been manufactured in the nineteenth century.
The essential features of present-day instrumentation systems are their
reliance on hand-held oscillating saws to make bone cuts, and mechanical
jigs with slots and cutting blocks to help align the cuts. Considerable
ingenuity has been applied to optimizing instrumentation systems of this
type, and there are dozens of variations on the market. Nonetheless, poor
alignment and inaccurate cuts are common problems when using these
mechanical instrumentation systems.
Poor alignment occurs when femoral and tibial cutting jigs are not properly
aligned with respect to the hip, the ankle, and the stabilizing soft
tissues of the knee. This can happen because the surgeon is mislead by the
anatomic landmarks used by a given system, because the landmarks are
concealed by fat and muscle, because preoperative deformities exist, or
because the jig has shifted slightly during the procedure. The best test
of alignment is flexion of the newly reconstructed knee. Unfortunately, by
the time such a test can be made, the bone cuts have been made, and it is
too late to change the alignment of the components.
Inaccurate cuts occur when the various cuts and drill holes do not
precisely mate with the surfaces of the prosthetic components, possibly as
the result of errors which accumulate during placement and removal of the
various cutting blocks. Also, there is inherent inaccuracy associated with
a flexible, oscillating saw blade resting on a cutting block or in a slot.
The blade tends to "sky" when it encounters a dense section of bone. This
tendency is resisted by canting the hand-held saw in a downward direction.
There is ample evidence in the published literature that the present state
of total knee arthroplasty is not satisfactory. Cameron H.U., Hunter G.A.
in: Failure in Total Knee Arthroplasty, 170 Clinical Orthopaedics and
Related Research: pp. 141, 146, 1982, noted, "[t]he results of knee
arthroplasty range from an acceptable 5.4% failure rate at five years to
an abysmal 70% failure rate at three years. Failure rates of this
magnitude indicate that many revisions are being performed." Bryan R.S.,
Rand M.J. in: Revision Total Knee Arthroplasty, 170 Clinical Orthopaedics
and Related Research: pp. 116-122, 1982, state that, "[p]roper component
alignment is of critical importance" and that "[f]ailure to obtain
appropriate component orientation, axial alignment, and soft tissue
balance predisposes implants to loosening and failure." Hood R.W., Vannie
M., and Install J.N., as noted in, The Correction of Knee Alignment in 255
Consecutive Total Condylar Knee Replacements, 160 Clinical Orthopaedics
and Related Research: pp. 94-105, 1981, found in a series of 225 knees
that, "[e]leven percent of the knees in this series were outside the
alignment limits selected. This may reflect extremes of body habitus but,
more importantly, indicates that deficiencies in instrumentation still
remain." Hvid I., Nielsen S. in: Total Condylar Knee Arthroplasty, 55 Acta
Orthop Scand 55: pp. 160-165, 1984, found in a study of 138 knees that
although "the aim was to place the tibial component at right angles to the
tibial axis," only "53 per cent were within four degrees of tilt in any
direction." Some of their components were eight degrees or more out of
alignment. In summary, there is ample evidence that with existing
instrumentation surgeons cannot obtain good alignment routinely in total
knee arthroplasty.
As is evident from the less-than-satisfactory clinical results, the theory
and practice of jig-assisted knee surgery are two different things. In
practice, total knee arthroplasty is largely a seat-of-the-pants
procedure. Surgeons recruit every pair of eyes in the operating room to
judge how a contemplated cut "looks" from a variety of angles. Equally
important is a steady and practiced hand on the cutting saw, and a sound
understanding of the biomechanics of the knee joint.
The conventional TKA requires that the surgeon attempt to achieve exact
physiologically correct relationships and to make geometrically exact cuts
with inexact methods. As discussed above, both the position and quality of
the cuts and bores greatly affect the success of the operation. While the
background of a TKA has been described, numerous types of surgeries
present the same problem of integrating geometric analysis with a
subjective evaluation of physiological factors. Examples of such surgeries
are osteotomies and ligament repairs. In the majority of these operations,
certain mechanical devices, such as the jig systems described above, have
been developed to aid in the operation. The exactness of these mechanical
devices varies and, thus, so do the efficiencies resulting from their use.
However, most surgical procedures that are not solely based on subjective
medical decisions will suffer from some inaccuracies based on the fact
that surgeons have a limited capacity for making independent exact
geometric calculations and carrying out tasks based on those calculations.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a system and method for
facilitating the performance of a surgical bone alternation task by
accurately positioning a tool relative to the patient's bone. The
illustrative example of a total knee arthroplasty (TKA) of a femur is
used.
In one preferred embodiment, a system according to the present invention
comprises bone immobilization means and a robot. The bone immobilization
means supports the patient's bone in a fixed position with respect to a
reference structure. The robot comprises a base, a mounting member and a
manipulator. The base is fixed in position with respect to the reference
structure. The manipulator connects the mounting member to the base so as
to permit relative movement between the mounting member and the base. The
robot also includes attachment means for securing a tool to the mounting
member. Finally, the system includes means for causing the mounting member
to move relative to the reference structure in response to movement
commands, so that the tool can be moved to a position to facilitate
performance of the task. The movement commands are preferably provided by
task control means that includes memory means for storing data and control
programs, and control processing means for processing the control programs
to generate the movement commands.
Preferably the system also includes a template attachable to the mounting
member. The template may be positioned such that a predetermined feature
of the template is in a desired position relative to the bone. For a TKA
procedure, the template feature may represent a surface of a prothesis to
be mounted on the patient's bone. With the template in the desired
position, the reference position of the template is recorded in a "world"
coordinate system that is fixed with respect to the reference structure.
The reference position may therefore be combined with a geometric database
that includes data representing the geometric relationships relevant to
the performance of the task, to generate movement commands that cause the
robot to move surgical tools into desired tool positions during subsequent
stages of the operation. Preferably, the reference position is determined
by placing the robot in a passive mode in which the mounting member may be
moved manually by an operator. The operator can then mount the template to
the mounting member, move the template and mounting member such that the
template is properly positioned with respect to the bone, and then cause
the system to record the reference position. The robot can then be
returned to an active mode in which the mounting member moves in response
to movement commands.
The present invention further provides a prosthesis template for aiding in
the determination of the desired position and orientation of a prosthesis
relative to a bone. The prosthesis has an exterior surface that simulates
the exterior surface of the bone and an interior surface comprised of one
or more relatively planar surfaces to which the prepared bone must
conform. The prosthesis template has a functional interior surface defined
by at least three contour lines. This functional interior surface
corresponds to the exterior surface of the prosthesis so that when the
template is positioned near the bone it provides a means for evaluating
the position and orientation of the prosthesis exterior surface relative
to the bone.
In accordance with additional aspects of this invention, the prosthesis
template includes cut guide marks on the template. The cut guide marks lie
in the various planes that correspond to the interior surfaces of the
prosthesis, and thus correspond to the bone cuts that must be made in
order to prepare the bone for the prosthesis. The relationship between the
cut guide marks and the functional interior surface of the template
correspond to the relationship between the interior surface and the
exterior surface of the prosthesis. Thus, when the template is positioned
near the bone, it provides a means for evaluating the position and
orientation of the prosthesis interior and exterior surfaces relative to
the bone.
In accordance with additional aspects of this invention, a stabilizing
device is provided for creating a rigid link between the mounting member
and the reference structure, in addition to the link provided by the
manipulator. Any compliance of the end of the saw guide is thus prevented
so that, for example, the inner surface of the guide is held rigidly
within the cut plane throughout the cutting task. The stabilizing device
permits use of a smaller and more compact robot in the surgical system.
In accordance with other aspects of this invention, the stabilizing device
is incorporated into a safety feature of the task control means. When the
mounting member, tool, stabilizing device, and the article to which the
stabilizing device is attached are made of electrically conductive
materials, the attachment of the mounting member to the stabilizing device
produces a simple circuit. The task control means detects when the circuit
is complete and will not allow manipulator movement during that time.
Thus, the robot will never inadvertently move when the mounting member is
stabilized.
In accordance with still further aspects of this invention, the template
and tools used in the system include a tool identification pattern that
uniquely identifies each tool. An identification device is included in the
attachment means so that when the template or tool is mounted, the
identification pattern can be read and transferred to the task control
means. The identification is then compared to the identification for the
template or tool that is appropriate for the task. An error message is
generated and displayed if the incorrect template or tool is attached.
In accordance with still other aspects of this invention, the robot base
includes a tiltable safety stand, including a means of communicating to
the task control means the status of the stand. The safety stand is
configured so that if the robot encounters a rigid object while it is
moving, the stand will tilt away from the object, thereby preventing the
continued force against the object. When the stand tilts, a safety signal
is generated that is received by the task control means and is indicative
of the need to shut off the power to the robot.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing aspects and many of the attendant advantages of this
invention will become more readily appreciated as the same becomes better
understood by reference to the following detailed description, when taken
in conjunction with the accompanying drawings, wherein:
FIG. 1 is an isometric view of a prosthesis and a bone, with the end of the
bone prepared to receive the prosthesis;
FIG. 2 is a side view of a bone with a prosthesis press fit thereon, with
the bone partially cut away to show the prosthesis anchoring stud;
FIG. 3 is a pictorial view of the system of the present invention including
a patient positioned on an operating table;
FIG. 4 is an isometric view of the bone immobilization device of the
present invention with a representative femur suspended by the device;
FIG. 5 is a side view of the bone immobilization device with the frame and
fixation components shown adjusted to a raised position relative to the
base of the device;
FIG. 6 is an exploded view of one fixation component of the bone
immobilization device;
FIG. 7 is an isometric view of a robot used in the system illustrated in
FIG. 3;
FIG. 8 is an isometric view of the robot illustrated in FIG. 7 showing the
movement capabilities of the robot;
FIG. 9 is an isometric view of the wrist and the mounting flange of the
robot with the tool-coupling device of the present invention exploded away
from the mounting flange;
FIG. 10 is an isometric view of the wrist and coupling device illustrated
in FIG. 9 with a sample tool attachment flange shown exploded away from
the coupling device;
FIG. 11 is an isometric view of the robot safety stand used in the system
illustrated in FIG. 3;
FIG. 12 is a side view of the robot safety stand with portions of the upper
plate and one spring assembly cut away to show the compliance features of
the stand;
FIG. 13 is a top view of the top plate of the robot safety stand to show
the configuration of the upright supports and the spring assemblies;
FIG. 14 is a block diagram of the robot and peripherals, controller, and
supervisor of the system of the present invention;
FIG. 15 is an isometric view of a use of the prosthesis template of the
present invention attached to the robot and positioned near an immobilized
bone;
FIG. 16 is an isometric view of the template illustrated in FIG. 15;
FIG. 17 is a side view of the template attached to the robot and positioned
near the end of an uncut bone;
FIG. 18 is a top view of the template with the top portion cut away to show
the relationship between the horizontal plate and an uncut bone;
FIG. 19 is an isometric view of a use of the saw guide of the present
invention positioned near the immobilized bone;
FIG. 20 is an exploded view of the saw guide;
FIG. 21 is a top view of the saw guide with a saw blade positioned between
the guide plates;
FIG. 22 is a front view of the saw guide;
FIG. 23 is a side sectional view of the saw guide illustrated in FIG. 21
with a section taken along line 23 and a saw blade positioned between the
guide plates;
FIG. 24 is an isometric view of a use of the drill guide of the present
invention positioned near the immobilized bone;
FIG. 25 is a flow diagram of the method of the present invention for
determining the desired position and orientation of a prosthesis relative
to a bone;
FIG. 26 is a flow diagram of the method of the present invention for
determining the position and orientation of a saw guide relative to the
desired position of the prosthesis; and
FIG. 27 is a flow diagram of the method of the present invention for
determining the position and orientation of a drill guide relative to the
desired position of the prosthesis.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention is directed to a system and method for aiding in a
surgical procedure that includes the task of determining the precise
position and orientation of a tool relative to a reference structure or
point. That determination may be in part a geometrical solution and in
part a subjective solution based on the physiology of the patient. The
system will be described in terms of a total knee arthroplasty (TKA),
which is a representative procedure having the above-described
characteristics. For illustration, the replacement of the distal end of a
femur will be described. The below-described system and method are
applicable to similar surgical procedures. It is to be understood that
throughout the present application, the use of the term "position"
describes both point and orientation information. This is accepted
terminology in the area of robotics.
In one preferred embodiment, the present invention provides a two-step
method and related apparatus for aiding in surgical procedures. The first
step includes the determination of the desired position of a surgical task
relative to a bone. In the TKA, the surgical task includes replacing a
portion of the bone with a prosthesis. The surgeon is provided with a
template having a feature that represents a portion of the prosthesis,
such as the prosthesis exterior surface. The surgeon positions the
template such that the template feature is in the desired position of the
corresponding prosthesis portion, and then causes the template position
relative to a fixed point to be recorded. The second step, or bone
preparation step, includes determining the position of a tool, e.g., a saw
guide or drill guide, relative to the desired prosthesis position. This
step also includes actually positioning the tool. A tool position is
determined by combining the position of the template with geometric
information defining the task; i.e., the prosthesis' characteristics
defining the manner in which the bone must be prepared. For example, by
determining the position of the anterior cut from the position of the
template, the position of the saw blade is also determined; i.e., the
blade must be held by the saw guide in the plane of the cut near the bone.
This method provides a clear distinction between the
prosthesis-positioning step and the bone-preparation step.
With reference to FIGS. 1 and 2, a prosthesis 10 is used to replace the end
of a bone 12 when the bone is damaged or diseased in some way, or is
malaligned within the knee joint. The bone 12 may be a femur with the
prosthesis 10 fitted onto the distal end 14. Other identifiable portions
of the bone are the anterior side 16, the posterior side 18, the condyles
20 and 21, and the notch margin 22. The exterior surface 26 of the
prosthesis simulates the distal end of a normal femur, including the
condyles and the notch margin.
With respect to gross alignment, the femoral position relative to the knee
joint is important. The translational degrees of freedom of the femur are
the distal-proximal, anterior-posterior, and medial-lateral directions.
Rotations about these axes are referred to as axial, varus-valgus, and
flexion-extension, respectively. Femoral prosthesis gross alignment errors
include: (1) distal-proximal positioning error, which causes excessive
tightness or laxity in the tendons of the knee when the knee is extended;
(2) anterior-posterior positioning error, which causes misalignment of the
mechanical axes of the femur and tibia; (3) flexion-extension rotation of
the prosthesis, which results in excessive flexion or extension of the
joint; and (4) varus-valgus rotation of the components, resulting in a
knock-kneed or bow-legged effect or the tibial and femoral components
meeting in shear. Because there is no exact femur model to follow, and the
natural distal end of the femur may not provide a good model, the correct
gross alignment of the prosthesis is highly dependent upon the surgeon's
subjective evaluation of the knee.
With respect to local fit, the preparation of the femur for the prosthesis
is dictated by the configuration of the interior surface 28 of the
prosthesis. The geometric relationships that define the bone preparation
tasks are the same as the geometric relationships making up the interior
surface. The interior surface of the prosthesis is made up of anterior 30,
posterior 32, and distal 34 planar surfaces, and chamfers 36 and 37, which
are slightly curved. Additionally, two anchoring studs 38 and 39 extend
normally from the distal surface 34. In order to prepare the bone, planar
cuts are made on the femur that correspond to the interior surfaces of the
prosthesis. These cuts result in anterior 40, posterior 42, distal 44, and
chamfer 46 and 47 planar surfaces on the femur. The chamfer cuts 46 and 47
can be single-cut planes that provide a relatively tight fit with the
curved surfaces 36 and 37 of the prosthesis. Alternatively, multiple
chamfer cuts can be made to produce more rounded cut surfaces. Also, stud
holes 48 and 49 are drilled to receive the anchoring studs 38 and 39,
respectively.
With reference to FIG. 2, after bone preparation, the prosthesis is
press-fit onto the femur. The cut surfaces of the bone contact the
interior surfaces of the prosthesis.
For each manufacturer, bone type, and size, the configuration of the
prosthesis can be determined by taking simple physical measurements. The
present system integrates these known geometric relationships between the
interior surfaces of the prosthesis with the subjective determination of
the surgeon as to the desired gross alignment of the prosthesis.
With reference to FIG. 3, one preferred embodiment of the system of the
present invention utilizes an operating table 50, a bone immobilization
device 52, a robot 54, a robot controller 55, and a robot supervisor 56.
The patient is positioned so that the femur is supported and rigidly
secured within the bone immobilizer. In practice, proximal femur
displacement is prevented by placing sandbags or a secure belt over the
hips of the patient. The immobilizer is attached to the operating table by
the immobilizer base, not shown. Thus, throughout the TKA, the femur
position is fixed in relation to the operating table 50.
The robot is rigidly attached to the operating table 50 by robot safety
stand 65. The operating table thus provides a reference structure for the
positional relationship between the femur and the robot. In a preferred
embodiment, a tool attached to a robot mounting flange that extends from
the robot manipulator can be moved relative to the base, in any of the six
degrees of freedom. With this system configuration, a tool connected to
the mounting flange can be accurately positioned about the immobilized
femur. The robot includes position-sensing means for generating signals
indicative of the position of the mounting flange relative to a world
coordinate system fixed with respect to the robot base.
The robot controller 55 directly controls and monitors the movement of the
robot. The robot and its peripherals are connected to the controller by
input/output cables 58a and 58b, and communications cable 59. The
input/output cables 58 and communications cable 59 are connected to
input/output port 60 and communications port 61, respectively. Movement
commands are generated by the controller and sent to the robot via
communications cable 59. Mounting flange position signals are received
from the robot over communications cable 59, and processed by the
controller. Monitoring and control of robot peripherals, su | | |
