Method and system for positioning surgical robot

A method for transforming the image of a long bone into a system coordinate space, such as robotic system coordinate space, comprises identifying in the image data set directional coordinates representing bone axis and at least one positional coordinate on the bone surface. Corresponding coordinates in the actual bone immobilized in the robotic or other system space are then determined by contacting a probe, such as a probe at the end of a manipulatable arm on a robot, to corresponding locations in the actual bone. The coordinates within the image data set are then registered with the actual coordinates within the immobilized bone to produce a transfer function that can be used to transform the image data set to the coordinate system space.

BACKGROUND OF THE INVENTION 
1. Field 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 of a long bone. 
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 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. 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.TM. presurgical planning workstation being developed by 
Integrated Surgical Systems, Inc., Sacramento, Calif., 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.TM. 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. In particular, the 
ROBODOC.TM. surgical robot system relies 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 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 capable of achieving a high degree of accuracy and precision, the use 
of pins requires an additional surgical procedure for implantation. 
Moreover, the need to implant pins at the distal end of the femur requires 
surgical access to a site which might otherwise be left intact. The need 
to perform the additional procedure increases the time, cost, and patient 
discomfort associated with the total hip replacement procedure to a 
significant extent. 
For these reasons, it would be desirable to provide improved methods in 
robotic systems for performing surgical procedures on long bones, such as 
joint replacement procedures on femurs and other long bones. It would be 
particularly desirable to provide methods and systems for transforming a 
preoperative image of the bone and operative plan to a robotic coordinate 
system without the need to rely on access to a remote portion of the bone 
which need not otherwise be accessed. It would be particularly desirable 
if such procedures could dispense with the use of marker pins altogether, 
although procedures which relied on one or more marker pins at the 
proximal end of the bone which would normally be accessed during the 
replacement surgery would also be beneficial. 
2. Description of the Background Art 
The ORTHODOC.TM. presurgical planning workstation and the ROBODOC.TM. 
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 Robotics & Automation, 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, 
Ill., 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, Mich., 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, Tex., 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, Pa., 
September 1994. 
Other publications which describe image registration in robotic surgical 
and other procedures include the following: (9) Grimson, W. E. L., 
Lozano-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 Assisted Surgery, Volume I, Sessions I-III, Pages 
82-89, Pittsburgh, Pa., Sep. 22-24, 1995. (10) Nolte, L. 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, Pa., Sep. 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, Pa., 
Sep. 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 Assisted Surgery, Volume I, Sessions I-III, Pages 
98-104, Pittsburgh, Pa., Sep. 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 
Robotics and Computer Assisted Surgery, Volume I, Sessions I-III, Pages 
90-97, Pittsburgh, Pa., Sep. 22-24, 1995. (14) Peria, O., 
Fran.cedilla.ois-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 Medical Robotics and Computer Assisted Surgery, Volume II, 
Session IV, Pages 58-62, Pittsburgh, Pa., Sep. 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 International Symposium on Medical 
Robotics and Computer Assisted Surgery, Volume I, Sessions I-III, Pages 
63-68, Pittsburgh, Pa., Sep. 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, Pa., 
Sep. 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, Pa., Sep. 
22-24, 1995. 
A system and method for performing robotically assisted surgery is 
described in U.S. Pat. No. 5,086,401. Computer-assisted imaging and probe 
tracking systems are described in U.S. Pat. No. 5,383,454; U.S. Pat. No. 
5,198,877; and WO 91/07726. 
SUMMARY OF THE INVENTION 
According to the present invention, improved methods, systems and apparatus 
are provided for registering the image of a long 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) 
directional coordinates representing the bone axis and (2) at least one 
positional coordinate on the bone surface. The image data set is obtained 
in a presurgical imaging procedure, such as computerized tomography (CT), 
digital radiography, or the like. Locations representative of the bone 
axis are identified by a user reviewing the image, typically by marking a 
plurality of center points along the medullary canal in the bone image or 
automatically by the system software. In addition, at least one point on 
the bone surface is also marked by the user on the image data set, 
typically a plurality of marks representing a surface region on the bone 
are marked, or automatically identified by the system. The corresponding 
locations in the actual bone are then located while the bone is 
immobilized in the robotic or other system which defines the system 
coordinates. A system controller then transforms the image data set to the 
robotic coordinate system by registering the axial and positional 
coordinates in the image coordinate system with those in the system 
coordinate system. 
The methods, systems, and apparatus of the present invention are 
particularly advantageous since they provide for a very accurate 
registration of the image data set to the actual bone position when 
immobilized in the coordinate system. It has been found that a combination 
of the axial coordinates with the surface coordinate(s) provides for 
registration within a tolerance of less than .+-.2 mm, usually less than 
.+-.1 mm, and preferably less than .+-.0.5 mm. Such tolerances allow even 
cementless positioning of hip joint implants with good initial mechanical 
stability and excellent tissue ingrowth. Moreover, the methods, systems, 
and apparatus of the present invention obviate the need to implant a 
plurality of bone surface markers prior to the actual implant surgery. 
Even when the bone marker is utilized in the upper end of the bone (which 
is an option in the method of the present invention), there is no need to 
implant additional markers at the lower end of the bone, e.g., the distal 
end of the femur in hip replacement surgery. 
In a first specific aspect of the method of the present invention, the long 
bone is immobilized in a workspace of a mechanical manipulator having an 
effector or probe positionable in a coordinate system. The effector is 
translated through the medullary canal of the bone to acquire axial 
coordinates. The probe is also contacted against the surface of the bone 
to acquire at least one surface positional coordinate. The data set 
representing an image of the bone and having predefined axial and 
positional coordinates may then be transformed into the system coordinates 
by registering the corresponding axial and positional coordinates between 
the image and the immobilized bone. Typically, the medullary canal will be 
surgically accessed from one end of the bone and cleared in order to 
receive the probe. The probe is then introduced through the access hole, 
and its distal end is centered at various axial positions along the bone. 
Centering is typically accomplished by inflating a centering balloon near 
the distal end of the probe and force-balancing the probe. The manipulator 
system then records the center positions along the medullary canal as 
defined in system coordinates. Similarly, the probe is contacted against 
one or more surface locations on the bone, typically in the proximal 
calcar region of the femur for hip replacement. The image is then 
registered with analogous positions which have been marked in the image 
data set by the user. Transforming the data set is typically accomplished 
by generating a transform function which can transform the image data set 
into the coordinate system of the manipulator system. 
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 user selects and marks a plurality of coordinates along a center line 
through the image of the medullary canal. For example, this can be done by 
centering elliptical templates over cross-sectional images of the bone to 
define the center points. Additionally, the user will generate a surface 
model representing a region near one end of the bone, typically in the 
proximal calcar region of the femur for hip replacement. These locations 
are then stored within the image data set and relied on for registering 
the image data set with the actual position of the immobilized bone within 
the robotic or other system. Alternatively, the system software can 
determine the centerline and surface coordinates automatically. 
In a second aspect of the method 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 to produce a cavity for receiving a hip 
joint replacement prosthesis. The hip joint replacement prosthesis is then 
implanted in the cavity. 
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 coordinating 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) directional coordinates representing 
the bone axis and (2) at least one positional coordinate on the bone 
surface. 
The present invention still further provides an improved robotic system of 
the type having a manipulatable effector and a positioning probe removably 
attached to the effector. The improvement comprises a probe having an 
expansible element, such as an inflatable cuff, lined centering cone 
(umbrella structure), or the like, for centering the probe within a 
lumenal space and an actuator for expanding the element while the probe is 
positioned within the lumenal space. Such robotic system may be used in 
the image transforming methods described above. 
The present invention still further provides a positioning probe for use 
with a robotic system having a manipulatable effector. The probe comprises 
a rigid probe body having a proximal and a distal end. The connector is 
disposed at the proximal end of the probe and is removably connected 
thereto. An expansible element on the probe body permits the probe to be 
centered within a lumenal space. The positioning probe is particularly 
useful in the image transforming methods described above.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS 
The present invention is intended for registering the image of the long 
bone with the long bone itself immobilized within a system coordinate 
space. Long bones which may be imaged and registered include the femur, 
tibia, humerus, ulna, and radius. Image registration of such long 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. 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 with an immobilized long 
bone without the need to preimplant multiple markers along the bone and/or 
surgically access the bone at multiple 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 work station (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 include 
specific marker points or locations which are subsequently relied on to 
transform the image data set into the system coordinate space, as 
described in detail hereinafter. The marker locations may be identified 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. Alternatively, the 
preoperative planning workstation could be programmed to identify suitable 
marker locations without specific user intervention. In both cases, the 
marker locations 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, 
center point data taken along the medullary canal in the image data set 
are obtained and compared to corresponding center points of the actual 
bone immobilized in the operative space. It will be appreciated that the 
medullary canal of the long bone will, in most cases, not be completely 
straight, and may vary from straightness by as much .+-.15 mm over a 
length of 20 cm. The center point data in both the image data set and the 
system data set will thus be non-linear and will require alignment by 
non-linear techniques, such as regression analysis. Usually, the center 
point data will be aligned by the robotic system at the same time as the 
surface data are aligned. 
Surface positional information will comprise one or more points on the 
exterior surface of the bone, typically near the proximal end so that the 
distal end need not be surgically exposed. Usually, the surface 
information will be a surface model generated from the image data set, and 
the surface model will be aligned with at least one, and preferably at 
least three points determined by the robotic system within the robotic 
field. The surface data in the image data set will be aligned with the 
point(s) by regression analysis. The use of a fixed surface point, 
typically provided by a surgically preimplanted marker, can eliminate the 
need to fit multiple points with a surface model since a single, precisely 
located point will provide a high level of accuracy when combined with 
canal center line data according to the present invention. 
A particular advantage of the present invention is the ability to transform 
the image data set without the need to surgically implant locating markers 
onto the bone. The ability to eliminate the markers derives largely from 
the use of the axial positional information which is obtained without the 
use of markers by the methods described in more detail below. Usually, the 
surface locational information of the bone will also be obtained without 
the use of markers. Optionally, one or more surface markers may be 
attached to the bone and used to provide surface information in 
combination with the axial information obtained without the use of surface 
markers. In particular, it has been found that use of single surface 
marker at one end of the long bone, typically at the head of the femur in 
hip replacement or the bottom of the femur in knee replacement, will 
provide sufficient surface information for performing the transformation 
of the present invention. In the exemplary embodiment, surface information 
is obtained in a plurality of positions over a surface region of the bone 
and the use of implanted markers is eliminated entirely. 
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. Optionally, a single pin may be implanted in the proximal 
femur for determining a surface data point. 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.TM. presurgical planning system cited above. In addition to 
the implant planning and data generation, the user will identify axial and 
surface coordinate positions 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.TM. robotic surgical system. 
As described to this point, the system 10 architecture and the preoperative 
planning work station 12 and robotic operative system 20 are generally 
conventional. In order to practice the present invention, these systems 
must be modified as described hereinafter. 
Before describing the system modifications in detail, however, it is 
necessary to describe the physical characteristics of the femur, a typical 
long bone. Referring now to FIGS. 3 and 4, 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 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. Finally, the medullary canal 
70 runs generally axially through the cortical bone region of the femur, 
as shown in broken line. 
The specific details of the preoperative planning component of the method 
of the present invention will now be described in more detail. After the 
user completes the preoperative plan including selection and positioning 
of a suitable implant prosthesis, the preoperative planning workstation 12 
will produce a plurality of cross-sectional images on the viewing screen. 
The image data will delineate the periphery of the medullary canal of the 
long bone. Typically, from two to eight cross-sections will be produced, 
but in some cases it may be desirable to produce 12, 16, or even more 
images. The axial distances of each cross-section from the selected 
position of the implant tip are known. 
The user then identifies a center point in each of the cross-sectional 
images. Such selection could be done subjectively, i.e., by positioning a 
screen marker visually within the periphery of the medullary canal and 
marking the position in the image data set when it is selected. 
Preferably, the system will produce elliptical templates which the user 
may position and size within each cross-sectional image. In particular, 
the elliptical templates can be rotated about their elliptical center and 
sized in both the major and minor diameters in order to match the 
periphery of the medullary canal as closely as possible. The system can 
then mark and store the center of the ellipse as the center of the 
medullary canal at that cross-sectional location. Alternatively, the 
system could be programmed to generate such cross-sectional information 
automatically, without specific user intervention. In either case, the 
center points are made part of the image data set and transferred to the 
robotic system 20. The image center points will then be aligned with 
actual canal center points (collected as described below) to provide for 
axial alignment of the image and the immobilized bone. 
In addition to obtaining bone canal center data points, as just described, 
at least one surface locational point will be identified within the image 
data set within the preoperative planning workstation 12. This could be 
done in a variety of ways. For example, the user could identify one or 
more specific locations on the exterior surface of the bone and mark them 
for storage within the image data set. Alternatively, a single data point 
could be relied on if a marker had been surgically preimplanted in the 
bone prior to imaging. In that case, the preoperative planning workstation 
could automatically identify the marker without intervention by the user. 
In the exemplary embodiment, the workstation 12 will generate a surface 
model of a portion of the exterior of the bone, usually representing the 
outer cortex and the proximal calcar region. The particular boundaries for 
the region may be determined by the user or may be calculated by the 
workstation 12 based on the implant placement which in turn determines the 
level of which the femoral head will be excised. While it would be 
possible to generate a surface model of the entire bone, it has been found 
that use of a small portion of the proximal calcar region is sufficient to 
provide accurate image registration without excessive computational time. 
The image data set, including the identified positional coordinates, 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.TM. robotic operative system set forth above. The only unique 
aspects of the method of the present invention relate to the acquisition 
of positional information which is to be used for registration with the 
positional information acquired as described above and incorporated into 
the image data set transferred to the robotic operative system 20. As 
described in FIGS. 5-7, the probe 32 as utilized to obtain canal center 
line information through the medullary canal 70. Initially, the surgeon 
performs a femoral head osteotomy and prepares the acetabulum in a 
conventional manner, except that the depth of the osteotomy may be at a 
higher level in order to retain more of the neck which includes the 
surface-model region of the bone. An access hole 80 is opened in the 
trabecular bone and the bone is then installed in the femoral fixator of 
the robotic operative system 20. 
The probe 32 then introduced into the medullary canal 70 by manually 
guiding the probe. The ability to manually guide the probe and cause the 
manipulator arm 28 to follow is well described in the literature 
describing the ROBODOC.TM. robotic operative system. The probe 32 is 
positioned so that an expansible distal end, illustrated as an inflatable 
balloon, is sequentially aligned at least two axially spaced-apart 
positions within the canal corresponding to the axial positions which have 
been cross-sectionally imaged in the preoperative planning session. By 
expansion, the distal end of the probe will be automatically centered, and 
the robot is able to store the position of the probe with reference to the 
robot coordinate system within the robot controller. The balloon is then 
deflated (or other expansible end reduced in size) and moved to the at 
least second position, as shown FIG. 7, where it is reinflated. Typically, 
the probe will be located and centered at each of the cross-sectional 
locations which have been established during the preoperative planning 
procedure. Positional measurement of the probe will occur while the 
balloon is inflated and the robotic system is in a force-control mode that 
will move the probe tip in order to balance forces on the probe as sensed 
by the force sensor 30. The probe will be moved about the bone entry 
point, when the forces are balanced, the user will verify that the probe 
shaft is not contacting the proximal femur. Upon confirmation, the system 
will record the center point of the probe. This procedure is repeated at 
each level at which the center point location is determined. 
Referring now to FIG. 8, the probe 32 is used to collect surface locational 
information in the proximal calcar region 90 of the femur 60 as follows. 
Typically, the probe tip 33 will be engaged against at least one point on 
the anterior surface of the calcar region and one point on the posterior 
surface of the calcar region. The system will assure that the probe tip is 
force-balanced prior to recording the position in the robot controller. 
Optionally, a greater number of points, e.g., from 10 to 15 point, may be 
used in order to increase the accuracy of the transformation function 
which is produced. 
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.TM. 
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 and the canal center 
point and surface positional information, transformation of the image data 
set to the robotic coordinate system can be achieved by conventional 
mathematical techniques. In the exemplary embodiment the canal center 
points are fit to the robotically determined center points by conventional 
regression analysis. Similarly, a plurality of robotically determined 
surface points are fit with the surface model, also by 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.TM. robotic surgical 
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.