Abstract:
Automated surgical procedures, including procedures carried out in conjunction with image-guided surgical navigation systems, are improved using tools and techniques to refine bone modifications. A system-level embodiment of the invention includes a memory for storing information relating to a desired modification of a bone, a bone-modification tool, and tracking apparatus for determining the position and orientation of the bone and the bone-modification tool to ensure that the modification performed by the tool corresponds to the desired modification. In the preferred embodiment, the bone-modification tool is used to refine a previously resected surface by a few millimeters or a few degrees to achieve the desired modification. A method of preparing a bone to receive a prosthetic implant according to the invention includes the steps of using an image-guided surgical navigation system to surface a bone, determining if the surface is optimized for the prosthetic implant, and if not, using a finishing tool in conjunction with the image-guided surgical navigation system to refine the surface. These steps may be repeated as desired to further optimize the surface.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to surgical procedures and, more particularly, to tools and techniques to refine bone modification and other steps encountered during automated surgical procedures. 
     BACKGROUND OF THE INVENTION 
     Computer-automated surgical procedures, variously known as image-guided or navigation-based techniques, are becoming increasing popular as a way of improving the accuracy and throughput of orthopaedic, neuro-surgical and other surgical cases. Companies now engaged in this market segment include Stryker, Medtronic Surgical Navigation Technologies (a unit of Medtronic), BrainLAB, Inc., Radionics, Inc. (a subsidiary of Tyco International), Surgical Navigation Network, Inc. (a division of Cedara Software), and Visualization Technology, Inc. Theses systems assist the surgeon in the placement of instruments, location and depth of bone cuts and placement of implant components during surgery. 
     By way of example, the Stryker Knee Navigation System includes a “smart camera” that provides two-way communication between the instruments, a video monitor, and minimally invasive wireless “pointers” and “trackers” incorporating infrared emitters. A knee-replacement procedure begins with the surgeon inserting two tracking pins, one into the distal femur and one into the proximal tibia. Then a tracking device, positioned to face the camera, is mounted on each of the pins. Using a pointer, the surgeon touches various locations of known anatomy to establish a reference system. The system collects this information, maps it and graphically shows it on the screen. 
     As the surgeon physically manipulates the joint, positional information appears on the computer monitor as two-dimensional graphics in real time. Coupled with images of key anatomical points and areas of bone deficiency and soft tissue, the surgeon is able to make very precise bone resections and then place the prosthesis with great accuracy. As the surgeon prepares to remove bone for placement of the knee implant, the Knee Navigation System also provides data regarding placement of the cutting jig, enabling the surgeon to make real time angular adjustments before the first cut. The system also provides postoperative data once the implant is in place. 
     One of the drawbacks of these and other systems involves the way in which resections are checked for accuracy. Currently, the surgeon uses a position-calibrated “paddle” having a flat surface that is placed against a particular cut. The visualization system then registers the position of the planar system and computes an estimate of accuracy. If the surgeon is “off,” even by a few millimeters, the resection process must be re-entered and tested again. Apparatus and methods of streamlining this process would be welcomed. 
     SUMMARY OF THE INVENTION 
     This invention improves upon automated surgical procedures, including procedures carried out in conjunction with image-guided surgical navigation systems, by providing tools and techniques to refine bone modifications encountered during such procedures. 
     A system-level embodiment of the invention includes a memory for storing information relating to a desired modification of a bone, a bone-modification tool, and tracking apparatus for determining the position and orientation of the bone and the bone-modification tool to ensure that the modification performed by the tool corresponds to the desired modification. 
     In the preferred embodiment, the bone-modification tool is used to refine a previously resected surface by a few millimeters or a few degrees to achieve the desired modification. Alternatively the bone-modification tool may be used to perform an initial shaping, preferably in real time using a display for visualization. 
     Though different techniques may be used, the tracking apparatus includes position-indicating fixtures coupled to bone-modification tool(s) and the bone being modified. The bone-modification tool may assume the form of a milling machine, planer, sander or saw, and the modification may be used to prepare a surface to receive a prosthetic implant, whether joint-related or associated with an osteotomy or trauma fixation. 
     A method of preparing a bone to receive a prosthetic implant according to the invention includes the steps of using an image-guided surgical navigation system to prepare a surface to a bone, determining if the surface is optimized for the prosthetic implant, and if not, using a finishing tool in conjunction with the image-guided surgical navigation system to refine the surface. These steps may be repeated as desired to further optimize the surface. 
     In a preferred embodiment, the tool has a removable cutting portion so it may be exchanged as necessary to provide a sharp cutting surface. The cutting surface may be varied in the case of the “sander” for finer cuts when finishing a surface, or for rougher cuts, as indicated. The tool may cut by means of rotary motion, planar oscillations, or vibration. A planing attachment would, in particular, provide greater degrees of accuracy than a saw, the tip of which may be subject to deflection. The saw could thus be used for initial rough cuts, for example, with the sander or planar being used for corrections. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  SHOWS a typical image-guided surgical navigation system as disclosed in U.S. Pat. No. 6,533,737; 
         FIG. 2  shows a position sensing system including a position sensing device in the form of a video camera (not shown), connected to the computer via conventional connectors and reference clamps and secured respectively to the patient&#39;s femur and tibia; 
         FIG. 3  illustrates a preferred embodiment of the invention in conjunction with knee replacement procedure; 
         FIG. 4  shows an oscillating saw; and 
         FIG. 5  shows sander, planer/milling and rotating saw attachments. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As a foundation for the detailed description of this invention, a typical image-guided surgical navigation system is provided in U.S. Pat. No. 6,533,737, the entire content of which is incorporated herein by reference. The system  10  in  FIG. 1  includes a computer  12  having a memory (not shown), a storage device  14 , and a user interface  15 . The user interface  15  includes input devices  16 , an output device in the form of a display monitor  18 , a surgical tool  20 , and a position sensing system  22 . 
     The storage device  14  is used to store three-dimensional models of the surgical tool  20  and of the anatomical structures, in this case, in the form of a femur  24  and a tibia  26 , (see  FIG. 2 ) on which a surgical procedure is to be performed. The storage device  14  can be directly connected to the computer  12  via local wired or wireless connections, or remotely via a computer network, such as the Internet. The input devices  16  may include a keyboard and mouse, a touch screen or a voice-recognition system, or may be connected to a pointer. The output device  18  may be a video monitor or more advanced image-generation means such as three-dimensional display goggles. The surgical tool  20  can be, for example, an awl, a screwdriver to install, for example, an artificial ligament, or any tool used in surgical procedures. 
     Referring to  FIG. 2 , the position sensing system  22  includes a position sensing device, in the form of a video camera (not shown), connected to the computer  12  via conventional connectors and reference clamps  28  and  30 , secured respectively to the patient&#39;s femur  24  and tibia  26 . The reference clamps  28  and  30  in this case include bent rods  32 ,  34  and reference assemblies  36  and  38 , secured to their respective rods  32  and  34 . Reference assemblies  36  and  38  are of different shapes so that they can be discriminated by the computer  12 . Each of reference clamps  28  and  30  also includes mounting brackets  40  (only one shown) to adequately secure the reference clamps to the tibia  24  and the femur  26 , using small surgical screws  41  (only two shown). Similarly, a reference assembly  42  is secured by welding to the surgical tool  20  via a bent rod  44 . The reference assembly  42  may, alternatively, include a mounting bracket to secure the reference assembly  42  on other surgical tools. 
     The operation of this particular prior art position sensing system will now be described. The camera is used to capture and to transfer to the computer  12  the image of the reference assemblies  36 ,  38  and  42  during the surgical procedure. A registration algorithm, including conventional registration method, is used to convert the real-time image in relative position between each of the reference assemblies  36 ,  38  and  42 . Since the position, shapes and size of each reference assemblies  36 ,  38  and  42  are known to the computer  12 , the relative position of the surgical tool  20  with respect to the anatomical structures  24  and  26  may be calculated. 
     The position sensing system  22  may also include a dedicated processor (not shown) that can determine the relative positions of the reference assemblies  36 ,  38  and  42  and/or the relative positions of the surgical tool  20  and anatomical structures  24  and  26  before sending that information to the computer  12 . Other well known position sensing systems, such as, for example, a magnetic position sensing system, can also be used. In such a system, the camera is advantageously replaced by a magnetic field sensor and the reference assemblies are advantageously replaced by magnetic field emitters. 
     The first step of the method is to provide the computer  12  with three-dimensional models of the tibia  24 , the femur  26  and the surgical tool  20 . These models are transferred from the storage device  14  to the computer memory. The three-dimensional models may be obtained, for example, from two-dimensional slice images of the anatomical structures of interest, using three-dimensional reconstruction systems known to those of skill in the art. The slice images can be obtained, for example, by scanning the anatomical structures with a CT or a MRI scanner. 
     The second step is to calibrate the surgical tools  20  and the reference clamps  28  and  30 . For example, this is accomplished by the computer  12 , by performing transformations, first, from the reference assembly  42  to the tip of the surgical tool  20  and second, by selecting reference points on the three-dimensional models of the anatomical structures  24 ,  26  and by identifying the corresponding points on the anatomical structures  24  and  26 . Of course, other calibration protocols could be used. 
     During the surgical procedure, the position sensing system  22  will first register the positions and orientations of the reference assemblies  36 ,  38  and  42  in the coordinate system of the position sensing system (represented by the axes X,Y and Z in  FIG. 2 ). Then the orientations and positions of the surgical tool  20 , the tibia  24  and the femur  26  are transformed into virtual orientations and position in the reference system of the three-dimensional models. The three-dimensional models of the tool  20  and of the anatomical structures are then reproduced on the display monitor  18  in their new orientations and at their new positions in the computer reference system. 
     The registration process by the position sensing system  22  and the regeneration of the image on the display monitor  18  are performed at a rate sufficient to allow real-time display and interaction with the three-dimensional models. The display is said to be in real-time, since movement of the models is perceived as being continuous, without flicker effect, and synchronized with the movements of the anatomical structures  24 ,  26  and of the surgical tool  20 . 
     The computer  12  is also programmed to allow visualization of the anatomical structures and of the surgical tool as it would be seen from different points of view selected using the input devices  16 . The computer  12  is may further be programmed to display the anatomical structures as translucent (partially transparent) objects, allowing the surgeon to better visualize the interaction between the surgical tool  20  and the anatomical structures. 
     U.S. Pat. No. 6,533,737 goes on to describe the replacement of the anterior cruciate ligament (ACL) of the knee with an artificial ligament using the disclosed system and method. Following the appropriate calibration procedure, the surgeon uses the surgical tool  20 , in the form of an awl, to identify on the patient&#39;s tibia  24  and femur  26  the two points  46  and  48  where he believes he should place the artificial ligament. From those two points, a virtual model of the ligament is created by the computer  12  and displayed on the monitor  18  using stored models of the tibia. The surgeon then flexes the patient&#39;s knee in order to obtain a set of position measurements. As it has been described hereinabove, the positions of the tibia  24  and of the femur  26  will be determined by the computer  12  and modeled on the monitor  18 . 
     According to these positions, the computer  12  will calculate the distance between the two specified points at different flexion angles. A message is then displayed on the monitor  18 , informing the surgeon whether or not the isometry constraint is respected. If the constraint is not within a pre-specified tolerance, the surgeon may change the proposed artificial ligament position and perform another leg flexion to verify isometry. Once a position is found satisfying, the surgeon can use the system  10  to perform the surgical procedure. More specifically, the surgeon can visualize the positions of the two points  46  and  48  on the three-dimensional computer models displayed on the monitor to guide him while drilling the holes that will be used to fix the artificial ligament  50 . 
     With this background information, the reader&#39;s attention is now directed to  FIG. 3 , which illustrates a preferred embodiment of the invention generally at  300 , in conjunction with knee replacement procedure shown generally at  306 . The system includes one or more fixtures such as  310  and  312 , mountable to bones such as distal femur  311  and proximal tibia  313  through plates  314  providing a temporary yet rigid connection. The fixtures  310 ,  312  each include a plurality of devices operative to determine the position and orientation of the bones to which they are attached in multiple dimensions. These devices may be passive, in the sense that they have a particular size, shape or color allowing a visual sensing system such as camera  330  to locate the devices in space, or they may be “active,” in the sense that they may include optical, acoustic and/or electromagnetic transmitters to assist in position/orientation location. It will be appreciated by those of skill that the item  330  may either be a video camera operating in the visible or infrared region of the spectrum, or may be some other type of unit operative to sense. The position and orientation of the device is affixed to fixtures  310 ,  312 . It would further be appreciated that additional sensors such as  331  may be provided for a more complete coverage throughout the surgical field. Furthermore, although this example is based upon knee replacement, the sensors may be adapted for placement on other bones, different types of orthopedic procedures such as total hip replacement, shoulder replacement, other bones within the extremities, and other procedures including osteotomies and trauma fixation. 
     As discussed above, a video camera or other type of sensor  330  includes a field of view which finds and recognizes the devices on fixtures  310 ,  312 , so that the position and orientation of the bones  311 ,  313  may be determined in multiple dimensions. Preferably, the fixtures  310 ,  312  each contain three or more devices, enabling the bones to be identified in three-space. 
     The camera or sensor  330  is interfaced through a computer with storage capabilities  340 , which, in turn, communicates with a display  332  and user interface  350 . The display  332  is operative to generate a model of the bone or bones being operated on, as  311 ′,  313 ′, which may be obtained through any of the techniques discussed hereinabove, including the use of 2-D anatomical slices, 3-dimensional reconstruction systems, CT, MRI scanners, fluoroscopy, or synthesis by anatomic locating sensors. 
     Unique to this invention, a tool  320  is provided, having its own fixture  322  with sufficient devices to determine the position and orientation of the tool  320  in multiple dimensions, preferably three dimensions. In contrast to existing systems, the tool  320  is not just a pointer or resection-sensing “paddle,” but rather, is itself a bone-modification tool. In a preferred embodiment, the tool  320  takes the form of a sander or milling machine, operative to refine a previously made resection  321 , so as to bring that plane into conformance with a desired surface. Given that the physical relationship between the plane of the tool  320  and the devices on fixture  322  are known, the system may, in real time, track the modification of the bone  311 , displaying the results on the screen  332 . Preferably, the system not only shows the surface as it is modified, but also displays a desired goal such as  321 ′ on the screen, enabling the surgeon to see the progress in approaching the desired degree of modification. 
     The invention is not limited to refinements to previously made resections or modifications, but may, itself, be used for primary bone modification, including initial resections or other shaping procedures. In such a case, a tool of the type shown in  FIG. 4  may be provided, in this case an oscillating saw. Again, since the physical relationship between the elements on fixture  402  are known, the plane of the blade  404  may be determined in multiple dimensions and shown on display screen  332  as a particular procedure is performed. 
     In a preferred embodiment, the tool has a removable cutting portion so it may be exchanged as necessary to provide a sharp cutting surface ( FIG. 5 ). The cutting surface may be varied in the case of the “sander” “A” for finer cuts when finishing a surface, or for rougher cuts, as indicated. The tool may cut by means of rotary motion, planar oscillations, or vibration. A planing or milling attachment “B” would, in particular, provide greater degrees of accuracy than a saw, the tip of which may be subject to deflection. The saw “C” could thus be used for initial rough cuts, for example, with the sander or planar being used for corrections.