A system and method for positioning a tool relative to a patient's bone to facilitate the performance of a surgical bone alteration task. The system comprises a bone immobilization device for supporting the bone in a fixed position with respect to a reference structure, and a robot that includes a base fixed in position with respect to the reference structure. The robot also includes a mounting member, and a manipulator connected between the base and the mounting member and permitting relative movement therebetween. The tool to be positioned by the system is mounted to the mounting member. The mounting member is caused to move relative to the reference structure in response to movement commands, so that the tool can be moved to a desired task position to facilitate performance of the task. The system preferably also includes a template attachable to the mounting member, a feature of the template representing a portion of a task. Preferably, the template is secured to the mounting member, and the template is then manually manipulated such that the template feature is properly oriented with respect to the patient's bone. The template position is then recorded as a reference position that may thereafter be combined with a geometric database defining the task to determine the position of the tool. Particular embodiments for a bone immobilizer, a template and a saw guide are also described, together with a stabilizing device for the robot and a safety device for the robot base.
The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
1. A method for positioning a tool guide relative to a patient's bone to facilitate the performance of a surgical bone alteration task wherein a tool's movement is constrained by the tool guide, the method comprising:
immobilizing the obne by supporting the bone in a fixed posiiton with repsect to a reference structure;
attaching a tool guide to a robot, the robot comprising a base fixed with respect to the reference structure, a mounting member to which the tool guide is attached, and a manipulator connected between the base and the mounting member and permitting relative movement therebetween, and attachment means for securing the tool guide or other device to the mounting member; and
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 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 255 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 percent 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 the 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 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, 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 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 coating 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 grips 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 procedure. Once the bone is immobilized, the operation can proceed. The next step of the present invention is to teach the robot the desired position of the bone alteration task.
Desirable characteristics of a robot used in the present system include that the robot be capable of moving a tool mounted to the robot in six degrees of freedom; that the mounting flange be capable of gripping or adaptable to grip a variety of surgical tools; that the robot has a high repeatability of mounting flange positioning; and that certain safety features be available on, or integratable into, the robot and control system. With reference to FIG. 7, one suitable robot 54 is the PUMA 200 robot available commercially through Unimation. The PUMA 200 robot is relatively small, and thus will fit readily into a surgical area and can be mounted directly onto or adjacent an operating table. The robot is rigidly mounted in relation to the immobilizer device by securing the robot directly to the operating table or to a safety stand 62 which, in turn, is mounted on the operating table.
The robot 54 includes a trunk 142 extending from a base 144, a shoulder 146 connecting the trunk to an upper arm 148, an elbow 150 connecting the upper arm to a lower arm 152, and a wrist 154, attached to the lower arm, from which extends a mounting flange 156. The section of the robot that includes movable parts is referred to as the manipulator. The manipulator includes permanent-magnet DC servomotors for driving the robot movement. Incremental optical encoders for determining manipulator position relative to a fixed point on the base robot, and for determining manipulator velocity, are included in the joints. The encoders convert positional data into electrical signals. Generally, the position of each section of the manipulator is combined to determine the position of the mounting flange or a tool attached to the mounting flange. When the robot is not in use, the wrist rests in nest 157.
With reference to FIG. 8 in conjunction with FIG. 7, the manipulator is capable of moving the mounting flange 56 in six degrees of freedom. The robot rotates about the vertical trunk. The upper arm is raised and lowered by rotation about the shoulder. The lower arm raises and lowers the wrist by rotation around the elbow. Finally, the wrist rotates about three axes defined by the longitudinal axis of the lower arm, the wrist, and the center of the mounting flange.
Attachments to the mounting flange, such as mechanical grippers or electronic sensing devices, are available from the robot manufacturer or from other manufacturers specializing in such attachments. With reference to FIG. 9, the standard mounting flange provided on the PUMA 200 robot has screw holes 158 positioned about the perimeter of the flange. The flange also includes an alignment pin 159 that is used to ensure repeatable positioning of tools on the flange. The present system utilizes a coupler 160 that connects to the mounting flange 156 and couples various tools to the robot wrist. The coupler includes connecting plate 162, coupling block 164, identification component 166, signal port 168, connecting bore 170, and alignment studs 172. The connecting plate alignment bore (not shown) is positioned so as to match with the mounting flange alignment pin 159. The plate is secured to the mounting flange by screws 173 extending through plate bores 174 to screw holes 158. The coupler is fixed on the mounting flange for the duration of the surgery. In this manner, the position of the coupler components are fixed relative to the robot mounting flange.
The coupling block 164 is perpendicular to and offset on the connecting plate. Tools are attached directly to the coupling block during the operation. Each tool has an attachment flange, an example of which is shown in FIG. 10, that mates with the coupling block. The flange 175 includes a thumbscrew 176 that extends through the flange into the connecting bore 170. The flange also includes a pair of bores 178 that mate with the alignment studs 172. Each tool includes a similar attachment flange and is precisely and repeatedly mountable on the coupling block. The position of each tool relative to the position of the mounting flange is thus known when the tool is mounted. This repeatable mounting capability allows the robot to be taught the mounted-tool configuration in a simple manner.
It is standard practice in robotics to direct robot movements in terms of tool positions. This is done by first teaching the robot a tool definition. A tool mounted on the mounting flange is described to the robot as a single point that is offset from and oriented relative to the robot mounting flange. A common point used for defining a tool is the tip of the tool or some similar point remote from the attachment flange. A common robot control function accepts an array of values that defines the tool point relative to the mounting flange. Tool positioning commands can then be utilized. The robot control functions include implicit transformations from a tool position in a coordinate system to a robot mounting flange position. Given a movement command, the robot will move the robot mounting flange so that the tool point is at the commanded position.
The efficiency of the present system is increased by the inclusion of an identification component 166 on the coupling block. This component is used to identify the tool attached to the robot. This identification is then compared to an identification code stored in the control memory that corresponds to the tool required for the task at hand. For example, the identification code for the saw guide will satisfy the tool identification test that is run during the bone-cutting steps of the procedure. The identification component is used as a safeguard against the attachment of an incorrect tool. Additionally, it is a time-saving device in that the robot controller indicates to the surgeon immediately that an improper attachment has taken place so that time is not wasted in identifying a tool attachment error and correcting it during the operation.
In one preferred embodiment, the identification component is comprised of five phototransistors 180 arranged in a pattern along the surface of the coupling block. Each phototransistor is an emitter and receiver pair and is capable of transmitting and receiving an infrared signal. Each phototransistor is connected to the signal port 168 by a wire (not shown) suitable for carrying a signal. Signal port 168 is connected to the controller by input/output cable 58a. Radiation emitted from each phototransistor is detected by the respective receiver if it is reflected back; i.e., if a surface is positioned slightly above the sensor to thereby cause the light to reflect back. If no surface is present, or one lies flush against the sensor, then no reflected light is detected. Thus, the signals sent by the phototransistors to the controller might be interpreted digitally as 1's and 0's for reflected and nonreflected signals, respectively. The output of the sensors is collectively read as a binary word. Thus, 32 different identification codes are represented by the five sensors. The corresponding tool attachment flange surfaces are patterned with slight bores. One pattern corresponds to each tool. On the sample flange 175 the identification pattern 181 will be flush against the phototransistors on the coupling block when the flange is attached. The pattern 181 includes three areas that are slightly bored so as to provide a means for reflecting light emitted from the phototransistor back onto the coupling block. No light will be reflected from the other two phototransistors because the attachment flange will be flush against the phototransistors. Thus, the identification for the tool corresponding to the sample attachment flange is some combination of three 1's and two 0's, the combination depending upon the order in which the signals are read. During the TKA, a continuous signal is passed between the controller and the phototransistors. The return signal is read by the controller at the tool mounting step and the identification code determined by the control program.
With reference to FIG. 11, the robot safety stand 65 includes top plate 182, base plate 183, support legs 184, and spring assemblies 185. The base 144 of the robot is secured to the top plate 182 by screws 186. The base plate in turn is secured to the operating table 50 by screws 187. In this manner, the robot position is stable relative to the operating table.
Although the robot is preferably programmed to move slowly, so that there is little chance of an accident occurring due to the robot striking an object at a high speed, there is still a chance that the robot may encounter a rigid object while it is moving. If such an incident occurs, power to the manipulator should be cut off immediately. In one preferred embodiment, the force of the robot against a rigid object will cause the top plate to tilt away from the base plate. The compliance of the stand prevents the robot from damaging the object and provides an indication to the controller over cable 58b that the power should be automatically shut off. Since the PUMA 200 robot is relatively lightweight, a great deal of force need not be applied by the robot to a rigid object in order for this power-down situation to occur.
As illustrated in FIG. 12, a support leg includes upright 188 and ball bearing 189. The ball bearing is captured between the upper portion of the upright and lower side of the top plate which is slightly indented to receive the ball bearing. Adjacent each upright is a contact switch 190 attached to the lower side of the top plate. The upright and contact switch are made of electrically conductive materials and form a simple ground loop detection circuit. The contact switch is positioned so that when the top plate rests on the ball bearing and upright, the contact switch completes the circuit. Each upright is connected to a wire, not shown, and the wires are bundled into cabee 58b and connected to the robot controller input/output port 60. During periods of robot movement, the controller continuously polls the circuits to check for a break in the continuity between a switch and an upright. When this occurs, power to the robot is immediately shut off.
Each spring assembly 185 includes a spring 193, a casing 194, and a spring mount 195. The spring mount extends downwardly from the lower side of the top plate and includes a securing flange 196 on the unconnected end. The spring wraps around the spring mount and is secured on the mount by securing flange 196. The spring and mount are encased in casing 194 which is hollow and extends upwardly from the upper side of the base plate. When the top plate rests on the support legs, the springs are slightly compressed within the casings, thereby providing a downward force against the top plate and an upward force against the base plate. In this manner, a firm base is established for the robot.
With reference to FIG. 13, the support legs and spring assemblies are positioned between the plates so that the tilting of the top plate in any direction will result in at least one contact switch losing contact with an upright. When the top plate is tilted, the spring mount in the spring assembly, or assemblies, attached to the portion of the top plate tilting away from the base plate will slide upwardly and away from the casing by a compression of the spring. In this manner, the plates continue to be connected by the spring assemblies so that the robot does not topple from the stand.
Referring now to FIG. 14, the robot is controlled by controller 55 that is available through the robot manufacturer. The PUMA 200 controller includes a Digital Equipment Company (DEC) LSI-11 computer 198 which includes a processor, memory, and communications board. The communications board includes robot-communications port 61, input/output port 60, and supervisor port 63. The robot is also provided with a teach pendant, an input device, such as a keyboard, and a display device, such as a terminal display screen, all shown in reference.
In the PUMA 200 controller, the control programs are stored in a Complementary Metal Oxide Semiconductor (CMOS) nonvolatile memory. The robot commands and feedback are routed through the robot-communications port 61. The input/output port 60 is a programmable control output that is actuated by the control programs. A variety of peripherals, such as the safety stand and tool identification device can be attached through the input/output port.
The PUMA controller is provided by the manufacturer with its own operating system/robot control language known as VAL-II (Versatile Assembly Language). Programs are written in VAL-II to control the movement of the robot. The programs include: robot-control programs for directly controlling the robot with motion instructions; process-control programs that run parallel to the robot control programs for monitoring and controlling external processes via lines connected to the input/output port; and system programs for system operation. The programs are stored in the controller memory and processed by the controller. A sampling of the VAL-II commands and functions used by the present system to implement robot movements is listed in TABLE I.
TABLE I
______________________________________
Name Description
______________________________________
MOVE <location> Moves the robot to the
position and orientation
described by "location."
SET <location variable>
Sets the left variable equal to
=<location variable>
the right variable.
HERE <location variable>
Sets the value of a
transformation or precision
point equal to the current
robot location.
TOOL {<compound>}
Sets the value or definition of
the tool transformation equal
to the transformation value
given.
SPEED (<expression>)
Returns one of the speed
values used by the system.
The value is always a
percentage of "normal" speed;
i.e., SPEED 100.
APPRO <location>,
Moves the tool to the position
<distance> and orientation described by
"location," but offset along
the tool Z-axis by the
distance given.
DISTANCE (<compound,
Returns the distance in mm
compound>) between the points defined by
the two specified transformation
values.
NEST Moves the robot into its nest.
______________________________________
There are several methods for controlling the movements of the robot. The robot is equipped by the manufacturer with a teach pendant (not shown) that can be connected to the input/output port 60 of the controller. The teach pendant is a hand-held device for interactively maneuvering the robot. The teach pendant is generally used to maneuver the robot through a series of steps making up a path. The controller records points along the path so that the robot can then repeat the steps under the controller's guidance. A joystick, voice control device, or other movement indicator provide suitable methods for controlling robot movement interactively. These methods are contrasted with the method wherein the controller processes a program written in VAL-II that is stored in the controller memory or passed from a supervisor. As the VAL-II program is run, the robot moves through the programmed steps. The robot can also be placed in a passive mode wherein the manipulator is manually moved; i.e., by an operator actually grasping and moving the manipulator through a recordable path. In this mode, the servomotors are disabled while the encoders remain active. All of the other modes are referred to as active modes. In these modes, both the servo motors and encoders remain active.
The robot is capable of recognizing and operating within two coordinate systems: a world coordinate system and a tool coordinate system. In the world coordinate system, the origin of the system is at the shoulder or some other point of the robot which remains fixed relative to the base. In the present system, the origin is fixed relative to the operating table and to the patient's bone. The robot can be programmed to move an attached tool to any world coordinate position.
In the tool coordinate system, the origin defaults to a point on the robot mounting flange that moves with the flange. The default origin can be replaced by an origin related to, and more useful for, a specific tool attached to the flange. The tool coordinate system moves with the mounting flange as it is moved by the robot. Each tool can be defined by tool data points in the tool coordinate system. As described in greater detail below, the data representing a tool definition is combined with the template reference position and with a geometric database describing the prosthesis to determine the position of the tool, e.g., a saw guide, for each task.
The robot controller has a supervisor port into which a supervisor can be connected. Information describing and aiding in the implementation of the supervisory-communications interface is provided with the controller by the robot manufacturer. The supervisor 56 may include a personal computer (PC) 66 and a display device 67. The PC includes a central processing unit (CPU), input/output ports, and memory. In one preferred embodiment, a PCs Limited 386, an IBM-compatible personal computer manufactured by Dell Computer Corporation, connected to a color-display device, is used as the supervisory system. The PC is connected to the controller's supervisor port 63 through the PC's COM2: serial port via cable 62. Through this connection, programs run on the PC supervise the robot controller. The supervisor provides data as well as control commands and programs in the VAL-II language. Although a supervisor is used, all robot movement commands are preferably routed through the controller to th
