METHOD AND SYSTEM FOR REGISTERING THE POSITION OF A SURGICAL SYSTEM WITH A PREOPERATIVE BONE IMAGE

Abstract: A method and system for transforming the image of a bone (60) into a system coordinate space, such as robotic system coordinate space, comprises identifying in the image data set at least two positional coordinates and at least one directional vector. The positional coordinates and directional vector are preferably defined by a pair of fiducial markers (70, 72) which are axially spaced apart on the bone (60). Corresponding positional coordinates and directional vector in the actual bone (60) are determined by contacting a probe (32), such as the probe at the end of a manipulatable arm (28) on a robot (26), against the fiducial markers (70, 72) in the bone (60) while the bone (60) is immobilized in the robotic space. The positional coordinates and directional vector within the image data set are then registered with the actual positional coordinates and directional vector determined by the robot (26) in the robotic coordinates base to produce a transfer function that can be used to transfer the image data to the coordinate system space for performing surgery or other procedures on the bone (60).

Description: METHOD AND SYSTEM FOR REGISTERING THE POSITION OF A SURGICAL SYSTEM WITH A PREOPERATIVE BONE IMAGE This application is a continuation-in-part of Provisional Application No. 60/038,178, filed on February 13, 1997, the full disclosure of which is incorporated herein by reference. BACKGROUND OF THE INVENTION The present invention relates generally to surgical methods and systems. More particularly, the present invention relates to a method and system for registering the position of a robotically manipulated surgical tool with a preoperative image . Robotic systems for assisting in a number of medical procedures have been proposed, including neurosurgical , laparoscopic, and orthopedic procedures. While the details of a particular procedure may vary widely, a number of such procedures rely on first obtaining a preoperative image of the region to be operated on, and subsequently robotically controlling and/or manually manipulating a medical tool based on information in the preoperative image. The procedures are usually surgical but can also be diagnostic. A need thus exists for transforming the preoperative image (usually in the form of a digital data set obtained by conventional imaging techniques) to a coordinate system employed by the robot or other mechanical system. In this way, the robot is able to navigate the surgical tool based on the image data set which is representative of the patient's actual anatomy. Of particular interest to the present invention, robotically assisted total hip replacement surgery is performed by first imaging the femur, typically by computerized tomography (CT) , and producing a digital data set representative of the femur. Selection and positioning of an implant within the femur is then planned at a computer workstation, such as the ORTHODOC™ presurgical planning workstation being developed by Integrated Surgical Systems, Inc., Sacramento, California, assignee of the present application. Once the doctor has planned the implant placement on the workstation, a digital data set including both the image data (patient anatomy) and the planned positioning of the implant is produced. It is then necessary to transfer this data set to a computer-controlled robotic system intended to perform the surgery, such as the ROBODOC™ surgical robot system which is also being developed by Integrated Surgical Systems. Successful hip replacement surgery, particularly when using cementless implants, relies on the highly accurate creation of a cavity within the proximal (upper) end of the femur for receiving the implant. Deviations less than ± 1 mm from the planned cavity placement are desirable. A critical requirement in achieving such accuracy is precise registration between the image data set and the coordinate system of the surgical robot. Image registration within the robotic coordinate system requires correlation between the physical position of the patient body site to be operated on, e.g., the femur in total hip replacement and knee replacement procedures, the digital image set representing the body feature, and the robotic coordinate system. Such correlation may be achieved by registering the image data set with the actual position of the body feature within the robotic coordinate space by physically contacting a probe at the end of a manipulator arm of the robot against certain imaged features on the body part. The information thus obtained by the robot controller can then be used to register the image with the actual body site, e.g., an immobilized femur, within the operative space of the robot. Prior to the present invention, the ROBODOC™ surgical robot system has relied on the surgical implantation of a pair of metallic pins on the distal (lower) end . of the femur and one additional metallic pin in the proximal end of the bone. These pins, usually referred to as fiducial markers, are readily apparent in the CT image of the bone and can thus be relied on to register the bone image with the robotic coordinate space by engaging a probe placed on the manipulator arm against each of the pins. Such registration is described in detail in Taylor et al . (1994) IEEE Trans. Robotics Automat. 10:261-275. While very successful in achieving adequate image registration, the need to implant three fiducials subjects the patient to significant discomfort. In particular, the need to implant two fiducials at the knee is problematic since they generally must be implanted on opposite sides of the knee during surgery. Usually, only one side of the knee is readily accessible, and the need to access both sides of the knees is a significant complicating factor. For these reasons, it would be desirable to reduce the number of pins to be implanted to two rather than three. In particular, it would be desirable to be able to reduce the number of pins to be implanted in the knee during hip replacement surgery to a single pin. Use of a single pin would permit the surgeon to place the fiducial on the side of the knee which will be most accessible during surgery. Moreover, the need to retract tissue to expose the distal-most pin subjects the patient to even more trauma, and it would be desirable to provide marker designs and methods which would avoid the need of exposing at least one of the markers. Various aspects of the present invention will address at least some of the above deficiencies in the prior art. 2. Description of the Background Art The ORTHODOC™ presurgical planning workstation and the ROBODOC™ robotic surgical system are described in a number of references, including the following: (1) Kazanzides, P., Zuhars, J., Mittelstadt, B.D., Taylor, R.H.: "Force Sensing and Control for a Surgical Robot," Proc . IEEE Conference , on Roboti cs & Automa tion, Pages 612-616, Nice, France, May 1992. (2) Kazanzides, P., Zuhars, J., Mittelstadt, B.D., Williamson, B., Cain, P., Smith, F., Rose, L., Mustis, B.: "Architecture of a Surgical Robot," Proc . IEEE Conference . on Systems, Man , and Cybernetics , Chicago, Illinois, Pages 1624-1629, October, 1992. (3) Paul, H.A., Bargar, W.L., Mittelstadt, B., Musits, B., Taylor, R.H., Kazanzides, P., Zuhars, J., Williamson, B., Hanson, W.: "Development of a Surgical Robot For Cementless Total Hip Arthroplasty, " Clinical Orthopaedics , Volume 285, Pages 57 - 66, December 1992. (4) Kazanzides, P., Mittelstadt, B.D., Zuhars, J., Cain, P., Paul, H.A., "Surgical and Industrial Robots: Comparison and Case Study," Proc . International Robots and Vision Automation Conference, Pages 1019-1026, Detroit, Michigan, April 1993. (5) Mittelstadt, B., Kazanzides, P., Zuhars, J., Williamson, B., Pettit, R. , Cain, P., Kloth, D., Rose, L . , Musits, B.: "Development of a surgical robot for cementless total hip replacement," Robotica , Volume 11, Pages 553-560, 1993. (6) Mittelstadt B., Kazanzides, P., Zuhars, J., Cain, P., Williamson, B.: " Robotic surgery: Achieving predictable results in an unpredictable environment," Proc . Sixth International Conference on Advanced Robotics , Pages 367 - 372, Tokyo, November, 1993. (7) Cain, P., Kazanzides, P., Zuhars, J., Mittelstadt, B., Paul, H.: "Safety Considerations in a Surgical Robot," Biomedical Sciences Instrumentation, Volume 29, Pages 291-294, San Antonio, Texas, April 1993. (8) Mittelstadt, B.D., Kazanzides, P., Zuhars, J., Williamson, B., Cain, P., Smith, F. Bargar, W. : "The Evolution of A Surgical Robot From Prototype to Human Clinical Use," in Proc . First International Symposium on Medical Robotics and Computer Assisted Surgery, Volume I, Pages 36-41, Pittsburgh, Pennsylvania, September 1994. Other publications which describe image registration in robotic surgical and other procedures include the following: (9) Grimson, W.E.L., ozano-Perez , T., Wells III, W.M., Ettinger, G.J., White, S.J., Kikinis, R.: "Automated Registration for Enhanced Reality Visualization in Surgery, " Proceedings of the First International Symposium on Medical Robotics and Computer Assi sted Surgery, Volume I, Sessions I- III, Pages 82-89, Pittsburgh, Pennsylvania, September 22-24, 1995. (10) Nolte, .P., Zamorano, L.J., Jiang, Z., Wang, Q., Langlotz, F., Arm, E., Visarius, H.: "A Novel Approach to Computer Assisted Spine Surgery, " Proceedings of the First International Symposium on Medical Robotics and Computer Assisted Surgery, Volume II, Session IV, Pages 323-328, Pittsburgh, Pennsylvania, September 22-24, 1994. (11) Lavallee, S., Sautot, P., Troccaz, J., Cinquin, P., Merloz, P.: "Computer Assisted Spine Surgery: a technique for accurate transpedicular screw fixation using CT data and a 3-D optical localizer, " Proceedings of the First International Symposium on Medical Robotics and Computer Assisted Surgery, Volume II, Session IV, Pages 315-321, Pittsburgh, Pennsylvania, September 22-24, 1994. (12) Potamianos, P., Davies, B.L., Hibberd, R.D. : "Intra-Operative Imaging Guidance For Keyhole Surgery Methodology and Calibration, " Proceedings of the First International Symposium on Medical Robotics and Computer Assis ted Surgery, Volume I, Sessions I-III, Pages 98-104, Pittsburgh, Pennsylvania, September 22-24, 1994. (13) Simon, D.A., Hebert, M. , Kanade, T.: "Techniques for Fast and Accurate Intra-Surgical Registration, " Proceedings of the First International Symposium on Medical Roboti cs and Computer Assisted Surgery, Volume I, Sessions I-III, Pages 90-97, Pittsburgh, Pennsylvania, September 22-24, 1995. (14) Peria, 0., Franςois-Joubert , A., Lavallee, S., Champleboux, G. , Cinquin, P., Grand, S.: "Accurate Registration of SPECT and MR brain images of patients suffering from epilepsy or tumor, " Proceedings of the First International Symposium on Medi cal Robotics and Computer Assisted Surgery, Volume II, Session IV, Pages 58-62, Pittsburgh, Pennsylvania, September 22-24, 1995. (15) Lea, J.T., Watkins, D., Mills, A., Peshkin, M.A. , Kienzle III, T.C., Stulberg, D.S.: "Registration and Immobilization for Robot -Assisted Orthopaedic Surgery, " Proceedings of the First Interna tional Symposium on Medical Robotics and Computer Assisted Surgery, Volume I, Sessions I-III, Pages 63-68, Pittsburgh, Pennsylvania, September 22-24, 1995. (16) Ault, T., Siegel, M.W.: "Frameless Patient Registration Using Ultrasonic Imaging, " Proceedings of the First International Symposium on Medical Robotics and Computer Assisted Surgery, Volume I, Sessions I-III, Pages 74-81, Pittsburgh, Pennsylvania, September 22-24, 1995. (17) Champleboux, G., Lavallee, S., Cinquin, P.: "An Optical Conformer for Radiotherapy Treatment Planning, " Proceedings of the First International Symposium on Medical Robotics and Computer Assisted Surgery, Volume I, Sessions I-III, Pages 69-73, Pittsburgh, Pennsylvania, September 22-24, 1995. A system and method for performing robotically assisted surgery is described in U.S. 5,086,401. Computer- assisted imaging and probe tracking systems are described in U.S. 5,383,454; U.S. 5,198,877; and WO 91/07726. Other patents relating to patient imaging and image registration include 5,590,215; 5,397,329; 5,222,499; 5,211,164; 5,178,164; 5,142,930; 5,119,817; 5,097,839; 5,094,241; 5,016,639; and 4,945,914. Copending application serial no. 08/526,826, assigned to the assignee of the present application, describes a method and system for transforming a bone image into a robotic coordinate system by aligning a robotic probe within the medullary canal of the femur. This application has been published as WO 97/09929. SUMMARY OF THE INVENTION According to the present invention, improved methods, systems and apparatus are provided for registering the image of a bone with the bone itself immobilized in a coordinate system, typically a robotic coordinate system of the type used for performing surgical procedures, such as hip replacement surgery, knee replacement surgery, long bone osteotomies, and the like. The improvement comprises registering an image data set with the robotic or other coordinate system based on a correlation between (1) two positional coordinates axially spaced-apart on or along the bone and (2) a directional vector passing through at least one of the positional coordinates. Preferably, the positional coordinates as well as the directional vector will be obtained from just two surgically implanted fiducial markers, rather than three or more as was often necessary for the prior art. The image data set is usually obtained in a presurgical imaging procedure, such as computerized tomography (CT) , digital radiography, or the like. Locations of image artifacts representative of the positional coordinates, typically resulting from pre-imaging implantation of suitable fiducial markers, are then identified. Such identification may be performed by a user reviewing the image and marking the locations in the image data set or preferably by suitable image analysis software. Similarly, the directional vector passing through at least one of the positional coordinates may also identified manually by a user reviewing the image or automatically marked in the image data set using suitable image analysis software. The corresponding positional locations and directional vector in the actual bone are then located while the bone is immobilized in the robotic or other surgical system which defines the system coordinates. A system controller then transforms the image data set to the robotic coordinate system by registering the positional coordinates and directional vector in the image data set with the actual positions in the robotic or other coordinate system. The methods, systems, and apparatus of the present invention are particularly advantageous since they provide for very accurate registration of the image data set to the actual bone position when immobilized in the coordinate system. It has been found that the methods and systems of the present invention can provide for registration within a tolerance of less than +1 mm, usually less than +0.5 mm. Such close tolerances allow for positioning of hip joint implants with very good initial mechanical stability and excellent tissue ingrowth. Moreover, the methods, systems, and apparatus of the present invention may be implemented with the implantation of only two fiducial markers, rather than three or more fiducial markers as has been generally required in the past. Additionally, the use of certain novel fiducial markers allows for implantation of the distal -most marker at a position proximal to the knee joint. Implantation of markers in the knee joint has been associated with patient discomfort and is thus undesirable. In a first specific aspect of the method of the present invention, the bone is immobilized in a work space of a mechanical manipulator having an effector or probe positionable in a coordinate system. The effector is positioned at at least two external surface positions axially spaced-apart along the bone to acquire two positional coordinates within the manipulator coordinate system. The effector is then positioned at at least one of the external surface positions to acquire a directional vector passing through the positional coordinate at said position. The positional coordinates may be obtained by engaging or contacting the effector at the surface position a single time. The directional vector, in contrast, will require determining an external surface position at least twice, and usually more than twice, in order to obtain sufficient information to define the directional vector. It will be appreciated that at least one of the surface contacts with the effector may provide both the positional coordinate and a portion of the information necessary for determining the directional vector. After the positional coordinates and directional vector are obtained, an image data set will be transformed to the manipulator coordinate system by registering the positional coordinates and directional vector with corresponding features on the bone itself. The external surface positions on the bone and corresponding image features will usually be defined by fiducial markers which are implanted in the bone at the axially spaced-apart positions. The fiducial markers may be conventional metallic pins which may be anchored within the bone, typically by exposing the bone and screwing in the markers in a pre-imaging surgical procedure. Optionally, at least one of the fiducial markers may be a novel pin design having a shaft which extends from the bone to at least near the surface of the skin and which optionally may further comprise an extender which may be attached to and axially aligned with the shaft to provide information on the directional vector. Particular methods for determining the directional vector include defining the plane which is normal to the axes of the fiducial marker. Most simply, this can be accomplished by engaging the effector against at least three non-collinear points on a "head" of the marker pin which extends over the surface of the bone (and optionally over the surface of the skin) . Use of a plane which is normal to the axial direction of the pin is particularly preferred since there is no need to determine the rotational position of the plane relative to the axis. Non-normal planes and/or non-planar surfaces on a fiducial pin could be utilized, however, if the points detected on the surface are observable in the image data set and can be marked by the user. In the broadest sense of the present invention, it will be appreciated that the methods could be performed by determining two fixed points on a single fiducial marker and a third point on a second fiducial marker. For enhanced accuracy, however, it is desirable that the positional and directional coordinates be determined separately and thereafter correlated with the image and manipulator coordinate system in order to precisely and accurately register the image and manipulator coordinates. Another method for determining the directional vector comprises aligning the axis of the effector relative to the directional vector. For example, the effector may be axially aligned with the fiducial marker by positioning within a canal or channel within the marker. A variety of other techniques for physically aligning the effector with the implanted fiducial marker are available, such as using a pin extender as described below. In this way, the directional vector could be determined with a single contact or alignment of the effector with the implanted fiducial marker. The method may further comprise obtaining the image data set. Typically, the image data set is obtained by providing a raw image data set of the bone, typically acquired by any of the imaging techniques described above. The positional coordinates and directional vector are selected and marked in a presurgical planning system (either automatically by system software or manually by a user viewing the image on a screen) which in turn stores the marked positions within the image data set. The stored positions are then registered with the actual positions in system space detected by the manipulator arm. Transforming the image data set into the manipulator coordinate system is typically accomplished by generating a transform function which can transform the image data set into the coordinate system of the manipulator system as the subsequent surgical procedure is performed in real time . In a second aspect of the present invention, hip replacement surgery may be performed by positioning a surgical cutter based on information in a transformed image data set obtained by any of the methods described above. The cutter is positioned according to a preoperative plan, and the cutter is actuated and manipulated to produce a cavity in the femur for receiving a hip joint prosthesis. The hip joint prosthesis is implanted within the cavity in a generally conventional manner. The present invention still further provides an improved robotic system of the type having a manipulable arm which carries a surgical cutter. The system further includes a programmable controller which positions the cutter within a robotic coordinate system. An image data set representing the image of a long bone is transformed to the robotic coordinate system to permit the controller to position the cutter according to a predetermined operative plan. The improvement comprises a system controller which transforms the image data set to the robotic coordinate system by registering (1) two positional coordinates axially spaced-apart along the bone and (2) a directional vector passing through at least one of the positional coordinates. The present invention still further comprises a controller program for robotic system of the type described above. The controller program comprises an instruction set embodied in a tangible medium which registers (1) two positional coordinates axially spaced-apart along the bone and (2) a directional vector passing through at least one of the positional coordinates with corresponding positions and direction within the bone itself. The instruction set may be embodied in any conventional medium of the type used to program controllers and computers, including disks, read-only memory, random access memory, flash memory, tapes, CD ROM, and the like. The present invention still further provides an improved fiducial marker comprising a shaft having an anchor end and a proximal end, and an axial extender which is removably mounted on the marker end of the shaft . The fiducial marker has dimensions which are particularly intended to permit implantation into a preselected target location, e.g. the femur at a location between the knee joint and hip joint. The length of the shaft is preferably sufficient so that the proximal end thereof extends through the tissue overlying the femur and lies immediately beneath the skin of the patient. Thus, the shaft can be surgically implanted in sterile environment with the skin sutured shut after implantation in order to reduce the risk of infection. At the time of robotic or other surgical procedure, when it is necessary to access the shaft to attach the extender, the skin can be opened with a small incision. Preferably, initial implantation of the shaft can be performed using a standard surgical power drill without the need to dissect tissue down to the bone surface. Typically, the shaft will have a length from 75 mm to 125 mm and a diameter in the range from 4 mm to 6 mm. The anchor end will usually have a threaded length in the range from 40 mm to 100 mm. The extender will usually have a length in the range from 50 mm to 75 mm. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 illustrates the architecture of a robotic surgical system according to the present invention. The system includes a presurgical planning component and a surgical component. Fig. 2 illustrates the surgical component of the surgical system of Fig. 1 and includes the surgical robot with its associated controller, tooling, and safety interlocks, a fixator to hold the bone securely to the robot, a bone motion detector, a human-machine interface with online display computer, and a hand-held terminal interfaced to the robot controller. Fig. 3 is a schematic illustration of a human femur. Fig. 4 is a schematic illustration of the use of the robotic probe of the system of Figs. 1 and 2 for determining the positional coordinate and/or directional vector defined by a fiducial marker implanted in the proximal end of a femur. Fig. 5 is a schematic illustration of the use of the probe of the system of Figs. 1 and 2 for determining the positional coordinate and/or directional vector defined by a fiducial marker implanted in the distal end of a femur. Fig. 6 is a schematic illustration of the use of the probe of the system of Figs . 1 and 2 for determining the positional coordinate and directional vector determined by a novel fiducial marker according to the present invention. Fig. 6A illustrates the novel fiducial marker utilized in the method of Fig. 6 in detail. DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention is intended for registering the image of a bone with the bone itself immobilized within a system coordinate space. The invention is useful with a variety of bone structures, including long bones, vertebrae, bones of the skull, pelvis, and the like, being particularly useful with long bones. Long bones which may be imaged and registered include the femur, tibia, humerus, ulna, and radius. Image registration of such bones will be particularly useful in conjunction with robotic surgical procedures, such as joint replacement, with specific procedures including total hip joint replacement, knee joint replacement, long bone osteotomy, and the like. The invention is also useful, however, for image registration in non-robotic surgical and diagnostic procedures, such as frameless stereotactic neurosurgical procedures with real-time navigational systems (optical or mechanical) . Exemplary methods, systems, and apparatus for transforming an image data set of the femur within a robotic system intended for performing total hip replacement surgery are described hereinafter, but such descriptions are not intended to be limiting to the scope of the present invention. The present invention provides methods, systems, apparatus for transforming the image data set of the long bone to a system coordinate space, typically a robotic system intended to perform or assist in any of the procedures described above. The present invention, however, is not limited to such robotic procedures and will be equally useful in manual surgical, diagnostic, and other medical procedures where it is necessary to align a pre-obtained image of a long bone within an actual coordinate space, such as an operative space. Such manual systems and procedures include computer- assisted surgical procedures that employ optical surgical measurement tools, passive electromechanical devices, and the like. In such cases, the use of the present invention is advantageous in that it will provide highly accurate image registration using markers at only two sites on an immobilized long or other bone without the need to pre- implant three or more markers on the bone and/or surgically access the bone at three or more points along its length. The present invention relies on obtaining an image of the bone using a conventional medical imaging technique, such as computerized tomography (CT) , radiography (digitized X-ray images), magnetic resonance imaging (MRI), and the like. Usually, CT and radiographic imaging will be preferred since they provide particularly accurate imaging information of bone material. In all cases, the image will be obtained in or converted to a digital form to produce an image data set which is suitable for digital manipulation using conventional computerized image processing equipment and software. Usually, the image processing equipment will be in the form of specially programmed computers, which are generally referred to as controllers and processors hereinafter. In particular, the present invention will utilize a preoperative planning workstation (computer) for analyzing and manipulating raw image data which is obtained directly from the image itself. The raw image data set will be processed to locate and define two positional coordinates and at least one direction vector which are subsequently relied on to transform the image data set into the system coordinate space, as described in detail hereinafter. The positional coordinates and directional vector will usually appear in the raw image as the result of prior implantation of fiducial markers which have geometries and which are positioned to facilitate user specification. The markers may be identified automatically by the imaging software or directly by the user who views the image on the screen and marks particular locations on the image which are intended for alignment with the actual bone when the bone is immobilized in the system coordinate space. For automatic identification, the preoperative planning workstation could be programmed to identify suitable marker locations without specific user intervention. In both cases, the positional coordinates and directional vector defined by the markers will become part of the image data set which is subsequently transferred to the operative or other system in which the bone is to be immobilized. The present invention relies particularly on obtaining axial and surface positional information on the bone and registering such information between the image data set and the system data set (representing the actual bone) as part of the image transformation process. In particular, only two pins are required to register the actual position of the patient's femur or other bone and the preoperative image data set. The two positional coordinates are axially spaced-apart along the length of the bone and provide five of the six degrees of freedom necessary to register the bone image with the actual bone in the robotic coordinate system. The sixth degree of freedom is provided by a directional vector which passes through one of the positional coordinates. The positional coordinates and directional vector are conveniently defined by a pair of fiducial markers which are implanted into the femur or other long bone so that they appear clearly in the pre-operative image of the bone and remain in the bone for subsequent identification by the operative or manipulator system which is to be employed. The positional coordinates and directional vector will usually be defined in both the pre-operative image and the robotic or other coordinate system by a pair of fiducial markers which are implanted at two axially spaced-apart locations in the long or other bone prior to producing the pre-operative image. The fiducial marker are typically in the forms of pins, screws, or other anchors which can be surgically implanted into the bone and which remain in fixed positions relative to the bone throughout the imaging and operative or other subsequent procedures. It is essential that the fiducial markers be observable under the imaging procedure which is used. For example, in the case of x-rays and CT scanning, the fiducial markers must be radiopaque so that they appear with high contrast in the image which is produced. It is further essential that a surface or other portion of at least one fiducial marker be accessible to the manipulator arm of the robotic system which is being used to perform the patient procedure, such as hip replacement surgery. As described above, an effector or probe on the manipulator arm will be used to contact each of the fiducial markers prior to performing the robotic procedure in order to register the position of the immobilized bone relative to the pre-operative image which is being used to control the surgery. The fiducial markers may be conventional, such as those used in prior robotic total hip replacement surgery as described above, or may be novel and optimized for providing the direction vectors needed in the methods of the present invention, as described in more detail below. Conventional fiducial markers are typically in the form of screws having a threaded shaft with a head which remains on the surface of the bone after the screw is implanted. The screws are typically formed from a biocompatible, radiopaque metal, such as titanium, having a length in the range from 10 mm to 40 mm and a diameter in the range from 3 mm to 6 mm. The head is typically circular with a diameter in the range from 4 mm to 10 mm. Conveniently, a target site may be provided near the center of the head to serve as a single "point" for defining the positional coordinate in both the pre-operative image and the robotic coordinate system. Suitable target sites include a hemispherical depression, groove, detente, or other physical or visual element that facilitates alignment of a probe carried by the manipulator arm. Thus, the target site can be contacted by the probe or effector on the manipulator arm to serve as the precise point of a positional coordinate for image registration. Such conventional fiducial markers can also define the directional vector in both the image and the robotic system. In the image, the directional vector is defined by the axis of the shaft of the screw. In the robotic system, the manipulator effector or probe will usually be contacted against the top (exposed) surface of the screw head which defines a plane normal to the axis of the pin. In this way, the precise direction of the vector which passes through the position coordinate defined by the pin can be determined. With the two positional coordinates and the single directional vector determined, the pre-operative image and the position of the action bone within the robotic coordinate system is precisely and accurately defined. The present invention can further employ improved fiducial markers which permit implantation in the cortical bone (between the hip joint and knee joint) , provide for more accurate determination of the directional vector, and facilitate maintenance of sterility in the operative environment. The implantation of fiducial markers in the knee joint can sometimes cause patient discomfort. It would be desirable to implant the fiducial marker in regions of the cortical bone above the knee joint. The cortical bone, however, is less accessible than the knee joint due to the overlying tissue, typically having a thickness in the range from 3 cm to 8 cm. An improved fiducial marker according to the present invention comprises a shaft having an anchor portion, typically a threaded length which is implanted into the bone, and a proximal shank having a length sufficient to extend over the bone and lie at or just beneath the skin surface, typically having a length in the range from 30 mm to 80 mm. The threaded anchor will typically have a length in the range from 10 mm to 40 mm. Optionally, this fiducial marker may further comprise a removable axial extender which can be placed in an upper surface of the shank portion of the fiducial marker during performance of the surgical procedure. The axial extender will be axially aligned with the axis of the shaft and will be readily accessible by the manipulator effector or probe of the manipular arm to permit detection of the directional vector. The directional vector could be detected by determining the orientation of the plane defined by a proximal surface of the axial extender (in which case the shaft of the extender would not have to be exposed) , but will more usually be determined by contacting two or more points along the length of the axial extender. The points will, of course, preferably be axially aligned on the axial extender in order to precisely determine the directional vector. In a preferred aspect of this novel fiducial marker, the axial extender will be sterile and attached to the shaft in the sterile operative environment immediately prior to the surgical procedure. The length and diameter of the axially extender are not critical, but will usually be within the ranges from 10 mm to 100 mm, and 4 mm to 15 mm, respectively. All portions of the fiducial marker may be formed from the same metals described above for the conventional fiducial markers . In order to enhance accuracy, it is desirable to implant the two fiducial markers as far apart from each other as possible. By increasing the spacing between the positional coordinates defined by the markers, any loss of accuracy from imprecise detection of the positional marker (as indicated by the fiducial) will be lessened. For long bones, one of the two fiducial markers will typically be implanted in the proximal end of the bone, i.e. the greater trochanter area in the case of the femur. The proximal end of the bone will typically be exposed during surgery and access to the conventional fiducial markers will be relatively simple. The second fiducial marker will be implanted distally of the first fiducial marker, often being implanted at or near the distal end of the bone, i.e. the knee joint in the case of the femur. Maximizing the distance between the two fiducial markers is advantageous since it will increase accuracy in registering the image to the robotic coordinate system. The placement of the fiducial marker in the knee, however, is disadvantageous because of patient discomfort, as discussed above. Thus, the fiducial marker may also be implanted in the cortical bone above the knee joint but spaced relatively far from the femoral head. Use of the novel fiducial marker of the present invention is preferred for implantation in the cortical bone. The marker will preferably be implanted at least one-third of the length of the femur or other long bone from the proximal end toward the distal end, preferably being implanted at least two-thirds of said length from the proximal end. An exemplary system 10 capable of implementing the methods of the present invention for hip replacement surgery is illustrated in Fig. 1. The system 10 includes both a presurgical planning workstation 12 and a library of implant designs 14 in the form of CAD models which are available from manufacturers on disks 15. A raw image data set 16, typically CT data, of the bone is obtained and transferred into the presurgical planning workstation 12. The user, typically the treating physician or an assistant working with the treating physician, is able to work at the presurgical planning workstation to select and position a suitable implant design within the patient femur. Details of such presurgical planning are well described in the literature relating to the ORTHODOC™ presurgical planning system cited above. In addition to the implant planning and data generation, the user or system software will identify at least two coordinate positions and one directional vector in the raw image data which are relied to subsequently transform the image data set to the robotic coordinate system, as described in more detail below. The system 10 of the present invention further comprises a robotic operative system 20 which includes a robotic controller 22 (typically a digital processor in the form of a programmable computer), an online display screen 24, and a robot 26. Details of the robotic operating system 20 are shown in Fig. 2. The robot can be any conventional industrial robot having a manipulatable arm 28 preferably having at least 5 axes and capable of high precision placement . A suitable robotic is available from Sankyo Robotics with the model designation SR-5427-ISS. For use in the present invention, a force sensor 30 is mounted at the distal end of arm 28, and an effector in the form of a probe 32 or a surgical cutting tool (not illustrated) may be attached to the force sensor. The robotic system 20 further includes a safety processor 44, and a real time monitoring computer 46, as illustrated in Fig. 1. The force sensor 30, the safety processor 44, the real time monitor 46, and a bone motion monitor 50, each help monitor the position, slippage, and blockage of the effector end of the manipulatable arm 28 while the femur 60 is held in place in a fixator assembly 52. Real time monitoring of these parameters helps assure that the robotic system is operating as planned. Details of these monitoring systems are described in the literature cited above which describes the ROBODOC™ robotic surgical system. As described to this point, the system 10 architecture and the preoperative planning workstation 12 and robotic operative system 20 are generally conventional. In order to practice the present invention, these systems architecture may be modified as described hereinafter. Before describing system modifications in detail, however, it is useful to describe the physical characteristics of the femur, a typical long bone. Referring now to Fig. 3, a femur 60 comprises a head region 62 and a lower region 64. The trabecular bone 65 that is located adjacent the femoral head 62 and the cortical bone 66 is located generally between the two ends of the bone. A neck region 68 is located just below the femoral head above the trabecular bone. ' In a first exemplary method according to the present invention, conventional fiducial pins 70 and 72, such as those, available from Integrated Surgical Systems, Inc., Sacramento, California, will be surgically implanted near the proximal end and distal end of the femur 60, respectively. In particular, the pins 70 and 72 will be implanted directly into the trabecular bone 65 and into the enlarged region of the knee joint, as illustrated. Each of the pins 70 and 72 will be similar, including a shaft portion and a head portion. For example, pin 72 includes head 76 which lies over the surface of the bone and shaft 74 which is implanted inwardly into the bone. The lengths of the pins may vary. It will be appreciated that each of the pins 70 and 72 will define a positional coordinate, typically at the center of their respective head, and that either of the pins could define the single directional vector which is needed by the method of the present invention. Typically, shaft 74 of pin 72 implanted in the knee joint will define the directional vector. In the pre-operative image, the observed axis of the shaft 74 defines the directional vector. In the robotic coordinate system, the direction of the directional vector is determined by contacting the outer surface of the pin head 76 at at least three spaced-apart non-linear locations in order to fix the plane which is normal to the directional vector. Specific computational techniques for calculating the transform function used to register the image with the robotic coordinate system are described in more detail with respect to Fig. 7, below. Before determining both the positional coordinates and the directional vector, the manipulator arm 28 is guided to engage a distal tip of probe 32 against the pin 70 and 72, as illustrated in Fig. 4. Determination of the positional coordinates may be done in a variety of ways. In a first example, the distal tip of probe 32 is contacted against the center of the head of the pin, as specifically illustrated in Fig. 4. Preferably, the probe is contacted with an initial force F maχ which is selected to be above an upper threshold value. The force is then reduced to a value below F min , and the precise position of the manipulator arm 28 determined at each of these force values . The force values are detected by force sensor 30. The position of the manipulator arm 28 is then determined at a number N of intermediate forces to determine the relationship between manipulator arm position and the engagement force. It will be appreciated that the relationship will be affected by a number of variables, including stiffness of the probe 32. The position of the manipulator arm at zero force F 0 can then be calculated using conventional regression analysis. It is the position of the manipulator arm at such zero force which is used for registering the image of the positional coordinate defined by pin 70 with the position of the manipulator arm when located at pin 70. In an alternative protocol, the pins or other fiducials may be localized using a ball probe attached to the manipulable arm of the robot or other digitized tracking system. The ball probe is positioned so that its center point can be precisely determined based on the positional coordinates of the manipulable arm which are typically determined by encoders present on the joints in the arm. Other tracking techniques for the manipulable arm such as visual tracking using a CCD or other known techniques can also be employed. The probe is mounted to a force sensor which detects a threshold contact force with the pin. The pin may have a detente, chamfer, groove, a spherical depression, or other physical location for receiving the ball probe. The method for determining the second positional coordinate defined by pin 72 using the probe 32 is illustrated in Fig. 5. The probe is first contacted at the head 76 of pin 72 to determine the second positional coordinate. The probe may then be used to determine the planar orientation of the head 76 in order to define the directional vector. It will be appreciated that probing of pins 70 and 72, as illustrated in Figs. 4 and 5, requires surgical exposure of the pins. In total hip replacement surgery, the trabecular bone 65 will have to be exposed in any case, so probing of pin 70 is not problematic. Surgical exposure of the knee joint, however, is necessary only for probing of the pin 72. Thus, it would be desirable to provide alternative pin locations and methods for probing the pins in order to determine the second positional coordinate and optionally directional vector according to the methods of the present invention. As illustrated in Figs. 6 and 6A, a novel fiducial marker 80 according to the present invention comprises a shaft 82 and an axial extender 84 which is removably securable to the shaft. The shaft 82 includes a threaded anchor 86 and a shank portion 88 where the threaded anchor is intended to be implanted with the femur or other bone and the shank is intended to extend transcutaneously through tissue overlying the femur or other bone. The fiducial marker 80 is particularly intended for implantation into the cortical bone 66 of a femur, as illustrated in Fig. 6, and the threaded anchor will typically have a length in the range set forth above. The shank portion will typically have a length in the range set forth above, and may extend fully through the layer of tissue T overlying the bone so that a proximal surface 90 is exposed over the tissue (as illustrated in Fig. 6) or may be implanted immediately beneath the surface of the skin (not illustrated) . In either case, axial extender 84 may be attached to the proximal surface 90 after the anchor has been implanted and prior to the imaging and/or operative procedure. The extender 84 has an axis 92 which will be aligned with axis 94 of the shaft 82 when the two are attached to each other. Conveniently, a threaded pin 96 may extend from a distal end of the axial extender 84 and be threadably received within a threaded receptacle 98 formed in the proximal surface 90 of shaft 82. As best observed in Fig. 6, the axial extender 84 is readily engaged by probe 32 of the manipulator arm 28. By contacting the extender 84 at at least two axially aligned points along the extender 84, the directional vector can be determined. A preferred method is to contact both the center point of the proximal surface of the shank 88 and the center point of the proximal surface of the extender 84. Alternatively, the probe 32 may be contacted at any two or more locations along an axial line on the outer surface of extender 84, as shown in broken line in Fig. 6. The positional coordinate may be determined by contacting probe 32 against virtually any location on the axial extender 84. For example, the probe 32 could be contacted against a distal surface 100, typically at the center point, as illustrated in broken line in Fig. 6. After the image of the femur or other long bone is obtained, it is converted to a digital form and transferred into the presurgical planning workstation 12. The user observes the images of the bone, typically as cross-sectional slices, and marks the positions of the positional coordinates and directional vectors by observing the image artifacts of the fiducial markers. Alternatively, of course, the presurgical planning workstation 12 could be programmed to analyze the image data and determine locations of the positional coordinates and directional vector without operator intervention. For the present, however, image evaluation by the operator to determine these positions has been found to be adequate . Automatic determination of the fiducial locations, positional coordinates, and directional vectors can be effected, by conventional image processing thresholding techniques applied to the image data set to produce a binary image set which identifies the three-dimensional coordinates of all pixel elements for the fiducial marker in the three- dimensional space. Based on the identified pixels, the center of mass of the fiducial image can be calculated. Based on the center of mass, a precise positional coordinate (usually the top center point of the fiducial head) can then be determined based on the second moment of inertia. The directional vector through at least one of the positional coordinates can then be calculated based on the eigen values and eigen vectors of the second moment of inertia. In order to obtain accurate values for the vectors, it is important to use fiducials having the lengths set forth above. The image data set, including the identified positional coordinates and directional vector, is then transferred to the robotic operative system 20 as part of a data transfer file 70 including the image information, implant shape data, and implant placement data. Transfer is conveniently accomplished via a transfer tape 71, but could be done using any conventional data transfer methodology. Additionally, the three-dimensional models of the bone and implant (implant files 14) are also transferred to the online display 24 of the robotic system 20 via the tape 71. The robotic operative system 20 is then operated to obtain positional information on the bone when the bone is immobilized within the robotic system. The patient will be prepared for hip replacement surgery in a conventional manner, and will be immobilized within the robotic operative system 20 generally as described in the literature related to the ROBODOC™ robotic operative system set forth above. The unique aspects of the method of the present invention relate to the acquisition of positional information which is to be used for registration, as described above, for incorporation into the image data set transferred to the robotic operative system 20 and subsequent utilization of the image data set by the robotic system. The robot controller 22 now has sufficient information to generate a transformation function which can be used to transform the image data set into robotic coordinates. Thus, the image data set can be used to control the manipulator arm 28 of the robot for performing the desired surgical procedure, e.g., creation of an implant cavity for receiving the prosthetic hip implant, as generally described in the earlier ROBODOC™ publications. The transfer file 72 received from the presurgical planning workstation 12 will include the implant data, canal center point data, surface model of the femoral neck region in a suitable file format, and all other planning information necessary to operate the robotic system 20. Once the robotic system 20 has both the image data set information, transformation of the image data set to the robotic coordinate system can be achieved by conventional mathematical techniques, typically utilizing regression analysis. Once an optimum fit has been calculated, the robotic system 20 generates a transfer function which is used by the robotic system 10 to transform the image data set coordinates to the robotic coordinate system. After the transformation function has been obtained the remainder of the surgical procedure for hip joint replacement can be performed generally as described in the literature relating to the ROBODOC™ robotic surgical system. The transformation function may be calculated as a transformation matrix as follows. First, a pin-based coordinate system is defined based on the measured directional vector and a calculated directional vector between the two positional coordinates. The resulting two vectors are then used to define a plane, and the plane is used to define a third axis normal to the plane. After the plane and normal axis are calculated, the transformation matrix which transforms points in the image coordinate system to the robotic or "pin" coordinate system is calculated by conventional mathematical techniques. In a particular aspect of the present invention, the robotic system will comprise a controller program which transforms the image data set to the robotic coordinate system by registering the positional coordinates and directional vector of the image data set with the corresponding information obtained by the manipulator arm of the robotic system. The robotic system may further comprise a controller program for controller manipulator arm movement to determine specific points in the robotic coordinator system. Although the foregoing invention has been described in some detail by way of illustration and example, for purposes of clarity of understanding, it will be obvious that certain changes and modifications may be practiced within the scope of the appended claims.

Claims: WHAT IS CLAIMED IS: 1. An improved method for transforming a data set representing the image of a long bone to a surgical coordinate system of the type wherein preselected coordinates of the bone image are registered with corresponding coordinates of the actual bone immobilized in the surgical system, wherein the improvement comprises: registering between the robotic coordinate system and the image data set (1) two positional coordinates spaced- apart along the bone and (2) a directional vector passing through at least one of the positional coordinates. 2. An improved method as in claim 1, wherein the long bone is a femur having a proximal end which joins the hip and a distal end which joins the knee. 3. An improved method as in claim 2, wherein a first positional coordinate is located near the proximal end of the bone and a second positional coordinate is located at least one-third of the bone length from the proximal end toward the distal end. 4. An improved method as in claim 3, wherein the second positional coordinate is disposed near the distal end of the bone . 5. An improved method as in claim 2, wherein the directional vector passes through the second positional coordinate . 6. A method for registering an image of a long bone with the bone immobilized in a system having a mechanical manipulator including an effector positionable in a coordinate system, wherein the method comprises : positioning the effector at at least two external surface positions axially spaced-apart along the bone to acquire two positional coordinates; positioning the effector at at least one of the external surface positions to acquire a directional vector passing through the positional coordinate; and transforming an image data set representing an image of the bone to the coordinate system by registering the positional coordinates and directional vector with features in the image corresponding to the positional coordinates and the directional vector. 7. A method as in claim 6, wherein the image features are defined by at least two fiducial markers implanted at said axially spaced-apart positions in the bone, wherein one of the markers has an elongate axis which is positioned to define the directional vector in the image. 8. A method as in claim 7, wherein the effector is contacted against at least once against a preselected point on each fiducial marker to acquire each positional coordinate. 9. A method as in claim 8, wherein the effector is contacted at least twice along the axis of the directional vector marker to acquire the directional vector marker to acquire the directional vector. 10. A method as in claim 8, wherein the effector is contacted at least three times against a planar surface on the directional vector marker to acquire the directional vector. 11. A method as in claim 8, wherein the effector is directionally aligned relative to the elongate axis of directional vector marker to acquire the directional vector. 12. A method as in claim 6, wherein the bone is a femur and a first marker is implanted in a proximal end which joins the hip and a second marker is implanted at least one- third of the bone length toward a distal end of the bone which joins the knee. 13. A method as in claim 12, wherein the distal marker comprises the directional vector marker. 14. A method as in claim 13, wherein the distal marker is implanted in the distal end of the femur. 15. A method as in claim 6, wherein the transforming step comprise generating a transform function which can transform image data into the coordinate system manipulator. 16. A method as in claim 6, further comprising obtaining the image data set . 17. A method as in claim 16, wherein the image data set is obtained by: providing a raw image data set of the bone; and identifying and marking the positional coordinates and the directional vector in the image data set. 18. A method for positioning a movable effector relative to a bone in an operative space, said method comprising: providing an image data set which includes two positional coordinates and a directional vector passing through one of the positional coordinates; immobilizing the bone in the operative space; storing in a system data set positional information of the effector at each of the positional coordinates and relative to the directional vector; comparing the system data set with the positional coordinates and the directional vector in the image data set to generate a transform function; and positioning the effector relative to the bone using the transform function. 19. An improved method as in claim 18, wherein the long bone is a femur having a proximal end which joins the hip and a distal end which joins the knee. 20. An improved method as in claim 19, wherein a first positional coordinate is located near the proximal end of the bone and a second positional coordinate is located at least one-third of the bone length from the proximal end toward the distal end. 21. An improved method as in claim 20, wherein the second positional coordinate is disposed near the distal end of the bone . 22. An improved method as in claim 19 wherein the directional vector passes through the second positional coordinate. 23. A method as in claim 18, further comprising: providing a robotic system comprising (a) a manipulatable arm that carries the effector, (b) a bone fixator which is fixed relative to the manipulator arm and the operative space; and (c) a system controller; wherein the bone is immobilized in the bone fixator. 24. A method as in claim 23, wherein the controller positions the effector by moving the manipulatable arm. 25. A method as in claim 23, wherein the image data set and the system data set are maintained in the system controller. 26. A method for performing hip replacement surgery, said method comprising: positioning a cutter according to the steps of claim 18, wherein the cutter is positioned according to a preoperative plan; actuating the cutter to produce a cavity for receiving a hip joint prosthesis; and implanting the hip joint prosthesis in the cavity. 27. An improved method for performing hip replacement surgery, said method being of the type wherein an image of the femur is used to control a robotic system to machine a cavity in the femoral canal to receive prosthetic implant and wherein an image data set is transformed to a coordinate system of the robotic system by registering preselected coordinates of the image data set with corresponding coordinates of the actual bone immobilized in the robotic system, wherein the improvement comprises: registering between the robotic coordinate system and the image data set (1) two positional coordinates axially spaced-apart along the bone and (2) a directional vector passing through at least one of the positional coordinates. 28. An improved robotic system of the type having a manipulatable arm which carries a surgical cutter and a programmable controller which positions the cutter within a robotic coordinate system, wherein an image data set representing the image of a long bone is transformed to the robotic coordinate system, wherein the improvement comprises: a controller program which transforms the image data set to the robotic coordinate system by registering (1) two positional coordinates axially spaced-apart along the bone and (2) a directional vector passing through at least one of the positional coordinates. 29. A controller program for a surgical system having a manipulatable arm which carries a surgical instrument and a programmable controller which positions the instrument within a surgical coordinate system, wherein an image data set representing the image of a bone is transformed to the surgical coordinate system, wherein the controller program comprises : a instruction set embodied in a tangible medium which registers (1) two positional coordinates axially " spaced-apart on the bone and (2) a directional vector passing through at least one of the positional coordinates with corresponding positions and direction within the bone itself. 30. A fiducial marker comprising: a shaft having an anchor end and a proximal end; and an axial extender which is removably secured to the marker end of the shaft . 31. A fiducial marker as in claim 30, wherein the shaft has a length in the range from 75 mm to 125 mm and a diameter in the range from 4 mm to 6 mm. 32. A fiducial marker as in claim 31, wherein the shaft has a threaded anchor with a length in the range from 40 mm to 70 mm and proximal shank for extending from a bone to a skin surface having a length in the range from 40 mm to 80 mm. 33. A fiducial marker as in claim 32, wherein the extender has a length in the range from 50 mm to 75 mm. 34. A method for providing a fiducial marker in a bone, said method comprising attaching a axial extender to a previous implanted shaft anchored in the bone. 35. A method as in claim 34, wherein the extender is sterile and the shaft is not sterile. 36. A method as in claim 34, wherein the axial extender is axially aligned with the axis of the implanted shaft . 37. A method as in claim 34, further comprising exposing a proximal end of the shaft through tissue before attaching the axial extender thereto. 38. A method as in claim 34, wherein a proximal end of the shaft is disposed over a skin surface.

Inventors: MITTELSTADT BRENT D, MITTELSTADT, BRENT, D.
Applicants: INTEGRATED SURGICAL SYS INC, INTEGRATED SURGICAL SYSTEMS, INC.
Application year: 1998
Main classification: G06K9
IPC codes: A61B19, G06T7