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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 prothesis 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 prothesis.
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 pressfit 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 handheld 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 total 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 per cent 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 alteration 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.
In another preferred embodiment, the present invention provides a bone immobilization device to be used in a surgical procedure requiring the rigid positioning of a bone throughout the procedure. The bone immobilization device rigidly secures a
bone in relationship to a reference structure such as an operating table. The bone is inserted through a frame and rigidly suspended relative to the frame by fixation means attached to the frame and the exposed bone. The frame is secured relative to
the reference structure by an adjustable support means. The frame can be disassembled into an upper section and a lower section for ease of positioning the bone as well as for removal of the bone in case of an emergency.
In accordance with further aspects of this invention, the fixation means include two coacting gripping components attached to, and extending radially into, the frame. The components include a pointed shaft and a contact element having a serrated
contoured contact surface. The point of the shaft contacts and slightly enters the bone, thereby providing a force against the bone coaxial with the shaft axis. The contact element is adjustably mounted on the shaft and the angle of the contact surface
is adjusted relative to the shaft axis so that the bone surface is contacted by the contact surface. The contact element is secured against the bone to provide a force against the movement of the bone parallel to the shaft axis. In this manner, a
two-point suspension system is provided that minimally contacts the exposed bone and minimally interferes with the area of the bone to be operated on.
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 other aspects of this invention, the template includes rod alignment tabs positioned on the anterior side of the template. A reference rod can be attached to the alignment tabs. In this manner, the rod provides an additional
reference between the position of the template and the longitudinal axis of the bone. Additionally, the template includes mounting means for rigidly securing the template relative to the bone.
An additional object of the present invention is to provide an orthopedic saw guide for confining the blade of a surgical saw to movement in a single plane while allowing translational and rotational movement of the blade within the plane. A
pair of elongated guide plates are secured together at either end to form a partially enclosed space. The distance between the inner surfaces of the guide plates is adjustable, and is preferably adjusted to be slightly greater than the thickness of the
specific blade to be used. Mounting means for rigidly securing he saw guide relative to the bone, so that the plane defined by the space between the inner surfaces corresponds to the cut plane, is provided.
In accordance with still further aspects of this invention, the inner surfaces of the guide plates include guide liners that are comprised of a low-friction material. The liners may be permanently secured to the guide plates or removable and
disposable. The guide plates are curved in the plane of the guide surfaces so that the saw guide can be mounted close to the end of a bone to provide maximum access to the bone with the closest possible positioning of the guide.
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 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 pressfit 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, such as the safety stand, are also carried out by the controller. Such peripherals are connected to the controller by
input/output cables 58 via input/output port 60.
In one preferred embodiment, the robot supervisor 56 supervises the communications between the robot and the controller. The supervisor in the illustrated embodiment includes a personal computer (PC) 66 (shown in reference) and display device
67. The PC houses the robot supervisory programs and the system data. The supervisor may enhance the operation of the system by providing a simplified operator interface. The surgeon can then control the system without having to understand the robot
command language utilized by the controller. The controller is connected to the supervisor by communications cable 62 that extends between the controller's supervisor port 63 and the supervisor's communication port 64 (both shown in reference). The
robot controller and supervisor can be covered with a sterilized shroud during the operation and still be easily manipulated.
As noted above, the TKA is divided into two steps. In the first step, the desired spatial relationship between the prosthesis and the distal end of the femur is established. In one preferred embodiment, this step is accomplished by means of a
prosthesis template that is attached to the robot mounting flange. Generally, the template includes a feature, such as a surface, a bore or a pointing member, that can be used to represent a task position that is relevant to the performance of a bone
alteration task. For replacing the distal end of the femur with a prosthesis, the template feature preferably comprises a surface that corresponds to an outer surface of the prosthesis. When the template feature is placed in a desired position relative
to the desired position of the prosthesis surface, then the template position, termed the reference position, is stored in the system database. In one preferred embodiment, the surgeon manually positions the template near the femur. Once the template
position is correct, the robot arm is locked and the position of the template in the world coordinate system is recorded.
The second step of the TKA comprises the bone alteration tasks of cutting and drilling the femur in preparation for the prosthesis. In these tasks, surface cuts will be made by a surgical saw, and stud bores will be drilled by a surgical drill
bit. The supervisory program combines the reference position with a geometric database to generate coordinate data for each cutting and drilling task. The geometric database describes the planes and axes in which the saw guides and drill guides,
respectively, must be aligned to perform the specified bone alterations. A program is generated to command the robot to move the tool attached to the mounting flange to the proper position so that the tool is in place for the specific task. Once the
tool is positioned, the robot arm holds the tool while the surgeon carries out the sawing or drilling task.
After the bone is prepared, the prosthesis is placed on the bone end. Because of the geometric exactness provided by the robot system, bone cuts and bores are achieved that allow for an accurate press-fit of the prosthesis onto the bone end.
Referring now to FIG. 4, one preferred immobilizer to be used with the present system includes base 68, bone-positioning frame 70, and bone fixation components 72. In one preferred embodiment, a tool-stabilizing device 73 is attached to the
immobilizer. The stabilizing device will be discussed below. Although the full femur is shown, in an actual procedure, only a small portion of the distal end of the femur would be exposed. The tibia, kneecap, ankle, etc., would be beside or below the
femur in the foreground of FIG. 4.
The immobilizer base 68 connects the immobilizer to the operating table 50. The base includes sliding plate 74, base clamp 76, upright 78 (shown in reference) and upright clamp 79. The sliding plate includes bolts 82 and washers 83 to secure
the plate onto the operating table. The bolts extend through channels 84 and threaded table runs 85. The immobilizer position can thus be adjusted relative to the table, both side to side and end to end. The position is secured by tightening the bolts
82 in the runs 85.
The base clamp 76 is hollowed to slidably receive upright 78. The height and rotational position of the immobilizer is adjusted by loosening screws 86 in the base clamp, thereby loosening the grip of the clamp on upright 78. Once the upright is
adjusted to the desired position, the screws are tightened, thereby compressing the base clamp against the upright in secured relationship.
The upright 78 is preferably an integral part of the upright clamp 79. The upright clamp extends vertically as clamp flanges 90. Attachment cylinder 92 extends through and between the clamp flanges. Set screws 93 are positioned in the flanges
normal to the attachment cylinder. The rotational position of the attachment cylinder is secured by tightening the set screws 93 into the flanges and against the attachment cylinder.
The bone-positioning frame 70 is made up of semicircular upper frame 94 and lower frame 96. Lower frame 96 includes frame tab 98, connecting ledges 100, and anchor pins 101 (discussed below). The frame tab 98 extends centrally from the lower
frame. A bore runs through the frame tab and is shown in reference. The frame tab is split from the bore to the bottom of the tab. Bolt 102 connects the lower portions of the split tab and can be tightened to reduce the bore diameter. The frame tab
is coupled to the frame clamp by attachment cylinder 92 which extends through the tab bore. The angle of the frame relative to the base plate is adjustable by rotating the frame over the attachment cylinder. Once the desired frame angle is achieved,
bolt 102 is tightened to secure the tab against the attachment cylinder. Thus, the angle of the frame is adjusted by loosening either the bolt 102 or the pair of set screws 93, to allow the frame or the attachment cylinder, respectively, to rotate.
The upper frame 94 includes projections 106. The upper frame is connected to the lower frame by the securing of projections 106 to the ledges 100 by screws 108. The upper frame is thus readily removable from the lower frame for ease of
positioning the bone within or removing the bone from the immobilizer.
The frame includes a plurality of threaded radial bores 110 spaced along the frame edge. The fixation components 72 of the immobilizer extend through the bores. Once the bone is positioned through the frame, and the knee joint exposed, the
fixation components are tightened to the femur and to the frame edge to hold the femur in place.
It is preferable to grip the exposed bone rather than the skin or other tissue. Since the posterior portion of the knee is not generally widely exposed, the fixation components will typically be situated in the upper portion of the frame so as
to be in a position to contact the exposed anterior portion of the femur. Thus, the majority of the bores 110 are positioned in the upper area of the frame. Alternatively, if other bones are being operated on, it may be preferable to have the fixation
components enter the bone from the posterior side. In such a case, the frame bores 110 are positioned about the lower frame 96. Additionally, if the fixation components are both positioned through the upper frame or both through the lower frame, then
the upper frame can be removed from the lower frame while the bone remains secured to one half of the frame by the fixation components.
As shown in FIG. 5, the bores 110 the through the frame do not lie within the plane of the frame, but are set at an angle. The angling of the bores allows the fixation components to extend back and away from the area of the operation, thereby
allowing maximum access to the bone. In this manner, the end of the femur is approachable by the surgeon from almost any angle and complete surface cuts can be made without repositioning any part of the immobilizer.
The immobilizer provides six degrees of freedom for positioning the bone. Translational freedom is provided by adjusting the sliding plate along the channels 84, and runs 85, and by adjusting the height of upright 78 within base clamp 76.
Rotational adjustments are provided by rotation of the upright 78 within base clamp 76, rotation of the frame 70 about attachment cylinder 92, and rotation of the fixation components about the frame 70; i.e., by repositioning the fixation components
relative to the frame by using a number of bores 110.
With reference to FIG. 6, one preferred fixation component is a coacting grip 116. A pair of grips are adequate to rigidly secure a bone within the bone immobilization device. Each coacting grip 116 includes a threaded shaft 118, a contact
washer 120, a point 126, a washer nut 128, a frame nut 130, screw nut 131, and a screw head 132. The contact washer is preferably wedge shaped and has a serrated contact surface 134. A shaft bore 136 extends through the contact washer, relatively
normal to the contact surface. The bore is oversized so that the angular relationship between the contact surface and the shaft is adjustable by rocking the washer about the shaft. In one preferred embodiment, a variety of contact washers are
available, the washers being differentiated by the contours of their respective contact surfaces. The contact surfaces range from a flat surface to a concave surface. The specific contact washer for each coacting grip is chosen during the operation so
that the contact surface closely conforms to the bone surface against which the washer will be tightened.
The point 126 is attached to the shaft by set screw 138 inserted through bore 139 against the point. Alternatively, the point 126 may be an integral part of the shaft.
In use, the pointed shaft 118 is threaded through a bore 110 in the frame. The portion of the shaft in the interior of the frame is threaded with the washer nut 128 and extended through the contact washer 120. The shaft position is adjusted
until the point 126 slightly penetrates the bone. On the outside of the frame, the frame nut 130 is then screwed onto the shaft and tight against the frame to secure the shaft relative to the frame. The contact washer 120 is then adjusted about the
shaft so that the contact surface 134 contacts as much bone surface as possible. The washer nut 128 is then tightened down to hold the contact element in contact with the bone. The teeth of the serrated contact surface slightly penetrate the bone to
prevent slippage. The screw head, screwed against screw nut 131 and attached by set screws 140 to the shaft, provides a gripping surface to be used in the shaft-adjusting process. The tightening of the two coacting grips generally takes place
simultaneously.
The coacting strips 116 are capable of suspending the femur without support from below. The suspension is achieved by the coacting forces of the shaft points and contact washers. Each shaft point provides a force against the bone that prevents
the bone from moving in directions perpendicular to the axis of the shaft. Each contact washer provides a surface force against the bone that prevents the bone from moving in directions parallel to the shaft. This two-point suspension method reduces
interference with the femur end as well as damage to the bone.
The immobilizer 52 can be used in a variety of operations where it is desirable to rigidly immobilize a bone throughout a pro | | |
