Patent Publication Number: US-2007123912-A1

Title: Surgical navigation systems and processes for unicompartmental knee arthroplasty

Description:
RELATED APPLICATION DATA  
      This document is a continuation application of U.S. patent application Ser. No. 10/963,862, entitled “Surgical Navigation Systems and Processes for Unicompartmental Knee Arthroplasty” and filed Oct. 13, 2004, which is a continuation-in-part application U.S. patent application Ser. No. 10/084,278, entitled “Surgical Navigation Systems and Processes for Unicompartmental Knee Arthroplasty,” filed Feb. 27, 2002 and now issued as U.S. Pat. No. 6,827,723, which claims the benefit of U.S. Provisional Patent Application No. 60/271,818, filed Feb. 27, 2001 and entitled “Image Guided System for Arthoplasty,” and U.S. Provisional Patent Application No. 60/355,899, filed on Feb. 11, 2002 and entitled “Surgical Navigation Systems and Processes,” all of which are hereby incorporated in their entirety by this reference. 
    
    
     FIELD OF THE INVENTION  
      This invention generally relates to unicompartmental knee arthroplasty surgical operations using systems and processes for tracking anatomy, implements, instrumentation, trial implants, implant components and virtual constructs or references, and rendering images and data related to them. Anatomical structures and such items may be attached to or otherwise associated with fiducial functionality, and constructs may be registered in position using fiducial functionality whose position and orientation can be sensed and tracked by systems and according to processes of the present invention in three dimensions in order to perform unicompartmental knee arthroplasty. Such structures, items and constructs can be rendered onscreen properly positioned and oriented relative to each other using associated image files, data files, image input, other sensory input, based on the tracking. Such systems and processes, among other things, allow surgeons to navigate and perform unicompartmental knee arthroplasty using images that reveal interior portions of the body combined with computer generated or transmitted images that show surgical implements, instruments, trials, implants, and/or other devices located and oriented properly relative to the body part. Such systems and processes allow, among other things, more accurate and effective resection of bone, placement and assessment of trial implants and joint performance, and placement and assessment of performance of actual implants and joint performance.  
     BACKGROUND  
      Knee arthroplasty is a surgical procedure in which the articular surfaces of the femur, tibia and patella are cut away and replaced by metal and/or plastic prosthetic components. The goals of knee arthroplasty include resurfacing the bones in the knee joint and repositioning the joint center on the mechanical axis of the leg. Knee arthroplasty is generally recommended for patients with severe knee pain and disability caused by damage to cartilage from rheumatoid arthritis, osteoarthritis or trauma. It can be highly successful in relieving pain and restoring joint function.  
      More than 95% of knee arthroplasties performed in the United States are tricompartmental knee arthroplasties (“TKA”), which involves the replacement of all the articular surfaces of the knee joint. TKA is performed when arthritis or trauma has affected two or more of the three compartments of the knee: medial compartment (toward the body&#39;s central axis), lateral compartment (away from the body&#39;s central axis), and patello-femoral compartment (toward the front of the knee).  
      The remaining knee arthroplasties are unicompartmental knee arthroplasties (“UKA”). UKA involves the replacement of the articular surfaces of only one knee compartment, usually the medial compartment. UKA is an attractive surgical treatment for patients with arthritis in only one compartment and with a healthy patella.  
      UKA has several advantages over TKA. UKA allows the preservation of both cruciate ligaments, while the anterior cruciate ligament is usually removed in TKA. Preservation of the ligaments provides greater stability to the joint after surgery. UKA also allows for preservation of more bone stock at the joint, which will be beneficial if revision components must be placed. Finally, UKA is less invasive than TKA because UKA requires smaller resections and components.  
      In spite of these advantages, there continue to be problems in UKA performance. A leading cause of wear and revision in prosthetics such as knee implants, hip implants and shoulder implants is less than optimum implant alignment. In a UKA, for example, current instrument design for resection of bone limits the alignment of the femoral and tibial resections to average values for varus/valgus flexion/extension, and external/internal rotation. Additionally, surgeons often use visual landmarks or “rules of thumb” for alignment which can be misleading due to anatomical variability. Intramedullary referencing instruments also violate the femoral and tibial canal. This intrusion increases the risk of fat embolism and unnecessary blood loss in the patient. Surgeons also rely on instrumentation to predict the appropriate implant size for the femur and tibia instead of the ability to intraoperatively template the appropriate size of the implants for optimal performance. Another challenge for surgeons is soft tissue or ligament balancing after the bone resections have been made. Releasing some of the soft tissue points can change the balance of the knee; however, the multiple options can be confusing for many surgeons. Although much of the bone stock remains after UKA, if a revision is necessary, many of the visual landmarks are no longer present, making alignment and restoration of the joint line difficult.  
     SUMMARY  
      The present invention is applicable not only for knee repair, reconstruction or replacement surgery, but also repair, reconstruction or replacement surgery in connection with any other joint of the body as well as any other surgical or other operation where it is useful to track position and orientation of body parts, non-body components and/or virtual references such as rotational axes, and to display and output data regarding positioning and orientation of them relative to each other for use in navigation and performance of the operation.  
      Systems and processes according to one embodiment of the present invention use position and/or orientation tracking sensors such as infrared sensors acting stereoscopically or otherwise to track positions of body parts, surgery-related items such as implements, instrumentation, trial prosthetics, prosthetic components, and virtual constructs or references such as rotational axes which have been calculated and stored based on designation of bone landmarks. Processing capability such as any desired form of computer functionality, whether standalone, networked, or otherwise, takes into account the position and orientation information as to various items in the position sensing field (which may correspond generally or specifically to all or portions or more than all of the surgical field) based on sensed position and orientation of their associated fiducials or based on stored position and/or orientation information. The processing functionality correlates this position and orientation information for each object with stored information regarding the items, such as a computerized fluoroscopic imaged file of a femur or tibia, a wire frame data file for rendering a representation of an instrumentation component, trial prosthesis or actual prosthesis, or a computer generated file relating to a rotational axis or other virtual construct or reference. The processing functionality then displays position and orientation of these objects on a screen or monitor, or otherwise. Thus, systems and processes according to one embodiment of the invention can display and otherwise output useful data relating to predicted or actual position and orientation of body parts, surgically related items, implants, and virtual constructs for use in navigation, assessment, and otherwise performing surgery or other operations.  
      As one example, images such as fluoroscopy images showing internal aspects of the femur and tibia can be displayed on the monitor in combination with actual or predicted shape, position and orientation of surgical implements, instrumentation components, trial implants, actual prosthetic components, and rotational axes in order to allow the surgeon to properly position and assess performance of various aspects of the knee joint being repaired, reconstructed or replaced. The surgeon may navigate tools, instrumentation, trial prostheses, actual prostheses and other items relative to the femur and tibia in order to perform UKA&#39;s more accurately, efficiently, and with better alignment and stability.  
      Systems and processes according to the present invention can also use the position tracking information and, if desired, data relating to shape and configuration of surgical related items and virtual constructs or references in order to produce numerical data which may be used with or without graphic imaging to perform tasks such as planning proper positioning and sizing of implants, visualizing resection planes or reamer cutting tracks based on sensed position of the cutting block, reamer, or other surgical instrument or item, assessing performance of trial prosthetics statically and throughout a range of motion, appropriately modifying tissue such as ligaments to improve such performance and similarly assessing performance of actual prosthetic components which have been placed in the patient for alignment and stability.  
      Systems and processes according to the present invention can also generate data based on position tracking and, if desired, other information to provide cues on screen, aurally or as otherwise desired to assist in the surgery such as suggesting certain bone modification steps or measures which may be taken to release certain ligaments or portions of them based on performance of components as sensed by systems and processes according to the present invention.  
      According to a preferred embodiment of systems and processes according to the present invention, at least the following steps are involved:  
      1. Obtain appropriate images such as fluoroscopy images of appropriate body parts such as femur and tibia, the imager being tracked in position via an associated fiducial whose position and orientation is tracked by position/orientation sensors such as stereoscopic infrared (active or passive) sensors according to the present invention.  
      2. Locating and registering body structure such as designating points on the femur and tibia using a probe associated with a fiducial in order to provide the processing functionality information relating to the body part such as rotational axes.  
      3. Navigating and positioning surgical instrumentation associated with a fiducialin order to modify bone, at least partially using images generated by the processing functionality corresponding to what is being tracked and/or has been tracked, and/or is predicted by the system, and thereby resecting bone effectively, efficiently and accurately.  
      4. Navigating and positioning trial components such as femoral components and tibial components, some or all of which may be installed using impactors with a fiducial and, if desired, at the appropriate time discontinuing tracking the position and orientation of the trial component using the impactor fiducial and starting to track that position and orientation using the body part fiducial on which the component is installed.  
      5. Assessing alignment and stability of the trial components and joint, both statically and dynamically as desired, using images of the body parts in combination with images of the trial components while conducting appropriate rotation, anterior-posterior drawer and flexion/extension tests and automatically storing and calculating results to present data or information which allows the surgeon to assess alignment and stability.  
      6. Releasing tissue such as ligaments if necessary and adjusting trial components as desired for acceptable alignment and stability.  
      7. Installing implant components whose positions may be tracked at first via fiducials associated with impactors for the components and then tracked via fiducials on the body parts in which the components are installed.  
      8. Assessing alignment and stability of the implant components and joint by use of some or all tests mentioned above and/or other tests as desired, releasing tissue if desired, adjusting if desired, and otherwise verifying acceptable alignment, stability and performance of the prosthesis, both statically and dynamically.  
      This process, or processes including it or some of it may be used in any total or partial joint repair, reconstruction or replacement, including knees, hips, shoulders, elbows, ankles and any other desired joint in the body.  
      Systems and processes according to the present invention represent significant improvement over other previous systems and processes. For instance, systems which use CT and MRI data generally require the placement of reference frames pre-operatively which can lead to infection at the pin site. The resulting 3D images must then be registered, or calibrated, to the patient anatomy intraoperatively. Current registration methods are less accurate than the fluoroscopic system. These imaging modalities are also more expensive. Some “imageless” systems, or non-imaging systems, require digitizing a large number of points to define the complex anatomical geometries of the knee at each desired site. This can be very time intensive resulting in longer operating room time. Other imageless systems determine the mechanical axis of the knee by performing an intraoperative kinematic motion to determine the center of rotation at the hip, knee, and ankle. This requires placement of reference frames at the iliac crest of the pelvis and in or on the ankle. This calculation is also time consuming at the system must find multiple points in different planes in order to find the center of rotation. This is also problematic in patients with pathologic conditions. Ligaments and soft tissues in the arthritic patient are not normal and thus will give a center of rotation that is not desirable for normal knees. Robotic systems require expensive CT or MRI scans and also require pre-operative placement of reference frames, usually the day before surgery. These systems are also much slower, almost doubling operating room time and expense.  
      None of these systems can effectively track femoral and/or tibial trials during a range of motion and calculate the relative positions of the articular surfaces, among other things. Also, none of them currently make suggestions on ligament balancing, display ligament balancing techniques, or surgical techniques. Additionally, none of these systems currently track the patella.  
      An object of certain aspects of the present invention is to use computer processing functionality in combination with imaging and position and/or orientation tracking sensors to present to the surgeon during surgical operations visual and data information useful to navigate, track and/or position implements, instrumentation, trial components, prosthetic components and other items and virtual constructs relative to the human body in order to improve performance of a repaired, replaced or reconstructed knee joint.  
      Another object of certain aspects of the present invention is to use computer processing functionality in combination with imaging and position and/or orientation tracking sensors to present to the surgeon during surgical operations visual and data information useful to assess performance of a knee and certain items positioned therein, including components such as trial components and prosthetic components, for stability, alignment and other factors, and to adjust tissue and body and non-body structure in order to improve such performance of a repaired, reconstructed or replaced knee joint.  
      Another object of certain aspects of the present invention is to use computer processing functionality in combination with imaging and position and/or orientation tracking sensors to present to the surgeon during surgical operations visual and data information useful to show any or all of predicted position and movement of implements, instrumentation, trial components, prosthetic components and other items and virtual constructs relative to the human body in order to select appropriate components, resect bone accurately, effectively and efficiently, and thereby improve performance of a repaired, replaced or reconstructed knee joint.  
      Other objects, features and advantages of the present invention are apparent with respect to the remainder of this document. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic view of a particular embodiment of systems and processes according to the present invention.  
       FIG. 2  is a view of a knee prepared for surgery, including a femur and a tibia to which fiducials according to one embodiment of the present invention have been attached.  
       FIG. 3  is a view of a portion of a leg prepared for surgery according to the present invention with a C-arm for obtaining fluoroscopic images associated with a fiducial according to one embodiment of the present invention.  
       FIG. 4  is a fluoroscopic image of free space rendered on a monitor according to one embodiment of the present invention.  
       FIG. 5  is a fluoroscopic image of femoral head obtained and rendered according one embodiment of the present invention.  
       FIG. 6  is a fluoroscopic image of a knee obtained and rendered according to one embodiment of the present invention.  
       FIG. 7  is a fluoroscopic image of a tibia distal end obtained and rendered according to one embodiment of the present invention.  
       FIG. 8  is a fluoroscopic image of a lateral view of a knee obtained and rendered according to one embodiment of the present invention.  
       FIG. 9  is a fluoroscopic image of a lateral view of a knee obtained and rendered according to one embodiment of the present invention.  
       FIG. 10  is a fluoroscopic image of a lateral view of a tibia distal end obtained and rendered according to one embodiment of the present invention.  
       FIG. 11  shows a probe according to one embodiment of the present invention being used to register a surgically related component for tracking according to one embodiment of the present invention.  
       FIG. 12  shows a probe according to one embodiment of the present invention being used to register a cutting block for tracking according to one embodiment of the present invention.  
       FIG. 13  shows a probe according to one embodiment of the present invention being used to register a tibial cutting block for tracking according to one embodiment of the present invention.  
       FIG. 14  shows a probe according to one embodiment of the present invention being used to register an alignment guide for tracking according to one embodiment of the present invention.  
       FIG. 15  shows a probe according to one embodiment of the present invention being used to designate landmarks on bone structure for tracking according one embodiment of the present invention.  
       FIG. 16  is another view of a probe according to one embodiment of the present invention being used to designate landmarks on bone structure for tracking according one embodiment of the present invention.  
       FIG. 17  is another view of a probe according to one embodiment of the present invention being used to designate landmarks on bone structure for tracking according one embodiment of the present invention.  
       FIG. 18  is a screen face produced according to one embodiment of the present invention during designation of landmarks to determine a femoral mechanical axis.  
       FIG. 19  is a view produced according to one embodiment of the present invention during designation of landmarks to determine a tibial mechanical axis.  
       FIG. 20  is a screen face produced according to one embodiment of the present invention during designation of landmarks to determine an epicondylar axis.  
       FIG. 21  is a screen face produced according to one embodiment of the present invention during designation of landmarks to determine an anterior-posterior axis.  
       FIG. 22  is a screen face produced according to one embodiment of the present invention during designation of landmarks to determine a posterior condylar axis.  
       FIG. 23  is a screen face according to one embodiment of the present invention which presents graphic indicia which may be employed to help determine reference locations within bone structure.  
       FIG. 24  is a screen face according to one embodiment of the present invention showing mechanical and other axes which have been established according to one embodiment of the present invention.  
       FIG. 25  is another screen face according to one embodiment of the present invention showing mechanical and other axes which have been established according to one embodiment of the present invention.  
       FIG. 26  is another screen face according to one embodiment of the present invention showing mechanical and other axes which have been established according to one embodiment of the present invention.  
       FIG. 27  shows navigation and placement of an extramedullary rod according to one embodiment of the present invention.  
       FIG. 28  is another view showing navigation and placement of an extramedullary rod according to one embodiment of the present invention.  
       FIG. 29  is a screen face produced according to one embodiment of the present invention which assists in navigation and/or placement of an extramedullary rod.  
       FIG. 30  is another view of a screen face produced according to one embodiment of the present invention which assists in navigation and/or placement of an extramedullary rod.  
       FIG. 31  is a view which shows navigation and placement of an alignment guide according to one embodiment of the present invention.  
       FIG. 32  is another view which shows navigation and placement of an alignment guide according to one embodiment of the present invention.  
       FIG. 33  is a view showing placement of an alignment guide according to one embodiment of the present invention.  
       FIG. 34  is another view showing placement of a cutting block according to one embodiment of the present invention.  
       FIG. 35  is a view showing navigation and placement of the cutting block of  FIG. 45 .  
       FIG. 36  is another view showing navigation and placement of a cutting block according to one embodiment of the present invention.  
       FIG. 37  is a view showing navigation and placement of a tibial cutting block according to one embodiment of the present invention.  
       FIG. 38  is a view showing the UKA femoral and tibial implant components.  
       FIG. 39  is a view showing the UKA femoral and tibial implant components attached at the knee joint.  
       FIG. 40  is a schematic view of a of a particular embodiment of systems and processes according to the present invention employing modular fiducials.  
       FIG. 41  is a schematic view of a screen face according to embodiments of the present invention showing the edge of a resection plane virtual construct.  
       FIG. 42  is a schematic view of a screen face according to embodiments of the present invention showing a cutting track virtual construct. 
    
    
     DETAILED DESCRIPTION  
      Systems and processes according to a preferred embodiment of the present invention use computer capacity, including standalone and/or networked, to store data regarding spatial aspects of surgically related items and virtual constructs or references including body parts, implements, instrumentation, trial components, prosthetic components and rotational axes of body parts. Any or all of these may be physically or virtually connected to or incorporate any desired form of mark, structure, component, or other fiducial or reference device or technique which allows position and/or orientation of the item to which it is attached to be sensed and tracked, preferably in three dimensions of translation and three degrees of rotation as well as in time if desired.  
      In a preferred embodiment, orientation of the elements on a particular fiducial varies from one fiducial to the next so that sensors according to the present invention may distinguish between various components to which the fiducials are attached in order to correlate for display and other purposes data files or images of the components. In a preferred embodiment of the present invention, some fiducials use reflective elements and some use active elements, both of which may be tracked by preferably two, sometimes more infrared sensors whose output may be processed in concert to geometrically calculate position and orientation of the item to which the fiducial is attached. In some preferred embodiments, fiducials are only temporarily attached to the body part, surgical instrument or other item. In still other preferred embodiments of the present invention, the fiducials are modular, allowing the surgeon or other user to position individual reflective elements on the body part, surgical instrument or other item such that the fiducial is positioned for maximum visibility by the sensors.  FIG. 40  shows schematically the use of modular fiducials  200  on a body part, item and instrument. Exemplary fiducials useable in various embodiments of the present invention are also disclosed in United States Patent Applications U.S. Ser. No. 10/679,158, entitled “Surgical Positioners” and filed Oct. 3, 2003, U.S. Ser. No. 10/689,103, entitled “Surgical Navigation System Component Fault Interfaces and Related Processes” and filed Oct. 20, 2003, and U.S. Ser. No. 10/897,857, entitled “Surgical Navigation System Component Fault Interfaces and Related Processes” and filed Jul. 23, 2004, all of which are herein expressly incorporated by this reference.  
      Position/orientation tracking sensors and fiducials need not be confined to the infrared spectrum. Any electromagnetic, electrostatic, light, sound, radio frequency or other desired technique may be used. Alternatively, each item such as a surgical implement, instrumentation component, trial component, implant component or other device may contain its own “active” fiducial such as a microchip with appropriate field sensing or position/orientation sensing functionality and communications link such as spread spectrum RF link, in order to report position and orientation of the item. Such active fiducials, or hybrid active/passive fiducials such as transponders can be implanted in the body parts or in any of the surgically related devices mentioned above, or conveniently located at their surface or otherwise as desired. Fiducials may also take the form of conventional structures such as a screw driven into a bone, or any other three dimensional item attached to another item, position and orientation of such three dimensional item able to be tracked in order to track position and orientation of body parts and surgically related items. Hybrid fiducials may be partly passive, partly active such as inductive components or transponders which respond with a certain signal or data set when queried by sensors according to the present invention.  
      Systems and processes according to a preferred embodiment of the present invention employ a computer to calculate and store reference axes of body components such as in a UKA, for example, the mechanical axis of the femur and tibia. From these axes such systems track the position of the instrumentation and osteotomy guides so that bone resections will locate the implant position optimally, usually aligned with the mechanical axis. Furthermore, during trial reduction of the knee, the systems provide feedback on the balancing of the ligaments in a range of motion and under varus/valgus, anterior/posterior and rotary stresses and can suggest or at least provide more accurate information than in the past about which ligaments the surgeon should release in order to obtain correct balancing, alignment and stability. Systems and processes according to the present invention can also suggest modifications to implant size, positioning, and other techniques to achieve optimal kinematics. Systems and processes according to the present invention can also include databases of information regarding tasks such as ligament balancing, in order to provide suggestions to the surgeon based on performance of test results as automatically calculated by such systems and processes.  
       FIG. 1  is a schematic view showing one embodiment of a system according to the present invention and one version of a setting according to the present invention in which surgery on a knee, in this case a Unicompartmental Knee Arthroplasty, may be performed. Systems and processes according to the present invention can track various body parts such as tibia  10  and femur  12  to which fiducials of the sort described above or any other sort may be implanted, attached, or otherwise associated physically, virtually, or otherwise. In the embodiment shown in  FIG. 1 , fiducials  14  are structural frames some of which contain reflective elements, some of which contain LED active elements, some of which can contain both, for tracking using stereoscopic infrared sensors suitable, at least operating in concert, for sensing, storing, processing and/or outputting data relating to (“tracking”) position and orientation of fiducials  14  and thus components such as  10  and  12  to which they are attached or otherwise associated. Position sensor  16 , as mentioned above, may be any sort of sensor functionality for sensing position and orientation of fiducials  14  and therefore items with which they are associated, according to whatever desired electrical, magnetic, electromagnetic, sound, physical, radio frequency, or other active or passive technique. In the preferred embodiment, position sensor  16  is a pair of infrared sensors disposed on the order of a meter, sometimes more, sometimes less, apart and whose output can be processed in concert to provide position and orientation information regarding fiducials  14 .  
      In the embodiment shown in  FIG. 1 , computing functionality  18  can include processing functionality, memory functionality, input/output functionality whether on a standalone or distributed basis, via any desired standard, architecture, interface and/or network topology. In this embodiment, computing functionality  18  is connected to a monitor on which graphics and data may be presented to the surgeon during surgery. The screen preferably has a tactile interface so that the surgeon may point and click on screen for tactile screen input in addition to or instead of, if desired, keyboard and mouse conventional interfaces. Additionally, a foot pedal  20  or other convenient interface may be coupled to functionality  18  as can any other wireless or wireline interface to allow the surgeon, nurse or other desired user to control or direct functionality  18  in order to, among other things, capture position/orientation information when certain components are oriented or aligned properly. Items  22  such as trial components, instrumentation components may be tracked in position and orientation relative to body parts  10  and  12  using fiducials  14 .  
      Computing functionality  18  can process, store and output on monitor  24  and otherwise various forms of data which correspond in whole or part to body parts  10  and  12  and other components for item  22 . For example, in the embodiment shown in  FIG. 1 , body parts  10  and  12  are shown in cross-section or at least various internal aspects of them such as bone canals and surface structure are shown using fluoroscopic images. These images are obtained using a C-arm attached to a fiducial  14 . The body parts, for example, tibia  10  and femur  12 , also have fiducials attached. When the fluoroscopy images are obtained using the C-arm with fiducial  14 , a position/orientation sensor  16  “sees” and tracks the position of the fluoroscopy head as well as the positions and orientations of the tibia  10  and femur  12 . The computer stores the fluoroscopic images with this position/orientation information, thus correlating position and orientation of the fluoroscopic image relative to the relevant body part or parts. Thus, when the tibia  10  and corresponding fiducial  14  move, the computer automatically and correspondingly senses the new position of tibia  10  in space and can correspondingly move implements, instruments, references, trials and/or implants on the monitor  24  relative to the image of tibia  10 . Similarly, the image of the body part can be moved, both the body part and such items may be moved, or the on screen image otherwise presented to suit the preferences of the surgeon or others and carry out the imaging that is desired. Similarly, when an item  22 , such as a cutting block, reamer, drill, saw, extramedullary rod, intramedullar rod, or any other type of item or instrument, that is being tracked moves, its image moves on monitor  24  so that the monitor shows the item  22  in proper position and orientation on monitor  24  relative to the femur  12 : The item  22  can thus appear on the monitor  24  in proper or improper alignment with respect to the mechanical axis and other features of the femur  12 , as if the surgeon were able to see into the body in order to navigate and position rod  22  properly.  
      The computer functionality  18  can also store data relating to configuration, size and other properties of items  22  such as implements, instrumentation, trial components, implant components and other items used in surgery. When those are introduced into the field of position/orientation sensor  16 , computer functionality  18  can generate and display overlain or in combination with the fluoroscopic images of the body parts  10  and  12 , computer generated images of implements, instrumentation components, trial components, implant components and other items  22  for navigation, positioning, assessment and other uses.  
      Computer functionality  18  may also store and output virtual construct data based on the sensed position and orientation of items in the surgical field, such as surgical instruments. For example, as shown in  FIG. 41 , monitor  24  may output a resection plane  202  that corresponds to the resection plane defined by a cutting guide whose position and orientation is being tracked by sensors  16 . In other embodiments, such as in the embodiment shown in  FIG. 42 , monitor  24  may output a cutting track  204  based on the sensed position and orientation of a reamer. Other virtual constructs may also be output on monitor  24 , and can be displayed with or without the relevant surgical instrument, based on the sensed position and orientation of any surgical instrument or other item in the surgical field to assist the surgeon or other user to plan some or all of the stages of the surgical procedure.  
      In some preferred embodiments of the present invention, computer functionality may output on monitor  24  the projected position and orientation of an implant component or components based on the sensed position and orientation of one or more surgical instruments associated with fiducials. For example, the system may track the position and orientation of a cutting block as it is navigated with respect to a portion of a body part that will be resected. Computer functionality  18  may calculate and output on monitor  24  the projected placement of the implant in the body part based on the sensed position and orientation of the cutting block. If the surgeon or other user is dissatisfied with the projected placement of the implant, the surgeon may then reposition the cutting block to evaluate the effect on projected implant position and orientation.  
      Additionally, computer functionality  18  can track any point in the position/orientation sensor  16  field such as by using a designator or a probe  26 . The probe also can contain or be attached to a fiducial  14 . The surgeon, nurse, or other user touches the tip of probe  26  to a point such as a landmark on bone structure and actuates the foot pedal  20  or otherwise instructs the computer  18  to note the landmark position. The position/orientation sensor  16  “sees” the position and orientation of fiducial  14  “knows” where the tip of probe  26  is relative to that fiducial  14  and thus calculates and stores, and can display on monitor  24  whenever desired and in whatever form or fashion or color, the point or other position designated by probe  26  when the foot pedal  20  is hit or other command is given. Thus, probe  26  can be used to designate landmarks on bone structure in order to allow the computer  18  to store and track, relative to movement of the bone fiducial  14 , virtual or logical information such as mechanical axis  28 , medial laterial axis  30  and anterior/posterior axis  32  of femur  12 , tibia  10  and other body parts in addition to any other virtual or actual construct or reference.  
      Systems and processes according to an embodiment of the present invention such as the subject of  FIGS. 2-36 , can use the so-called FluoroNAV system and software provided by Medtronic Sofamor Danek Technologies. Such systems or aspects of them are disclosed in U.S. Pat. Nos. 5,383,454; 5,871,445; 6,146,390; 6,165,81; 6,235,038 and 6,236,875, and related (under 35 U.S.C. Section 119 and/or 120) patents, which are all incorporated herein by this reference. Any other desired systems can be used as mentioned above for imaging, storage of data, tracking of body parts and items and for other purposes.  
      The FluoroNav system requires the use of reference frame type fiducials  14  which have four and in some cases five elements tracked by infrared sensors for position/orientation of the fiducials and thus of the body part, implement, instrumentation, trial component, implant component, or other device or structure being tracked. Such systems also use at least one probe  26  which the surgeon can use to select, designate, register, or otherwise make known to the system a point or points on the anatomy or other locations by placing the probe as appropriate and signaling or commanding the computer to note the location of, for instance, the tip of the probe. The FluoroNav system also tracks position and orientation of a C-arm used to obtain fluoroscopic images of body parts to which fiducials have been attached for capturing and storage of fluoroscopic images keyed to position/orientation information as tracked by the sensors  16 . Thus, the monitor  24  can render fluoroscopic images of bones in combination with computer generated images of virtual constructs and references together with implements, instrumentation components, trial components, implant components and other items used in connection with surgery for navigation, resection of bone, assessment and other purposes.  
       FIGS. 2-39  are various views associated with Unicompartmental Knee Arthroplasty surgery processes according to one particular embodiment and version of the present invention being carried out with the FluoroNav system referred to above.  FIG. 2  shows a human knee in the surgical field, as well as the corresponding femur and tibia to which fiducials  14  have been rigidly attached in accordance with this embodiment of the invention. Attachment of fiducials  14  preferably is accomplished using structure that withstands vibration of surgical saws and other phenomenon which occur during surgery without allowing any substantial movement of fiducial  14  relative to body part being tracked by the system.  
       FIG. 3  shows fluoroscopy images being obtained of the body parts with fiducials  14  attached. The fiducial  14  on the fluoroscopy head in this embodiment is a cylindrically shaped cage which contains LEDs or “active” emitters for tracking by the sensors  16 . Fiducials  14  attached to tibia  10  and femur  12  can also be seen. The fiducial  14  attached to the femur  12  uses LEDs instead of reflective spheres and is thus active, fed power by the wire seen extending into the bottom of the image.  
       FIGS. 4-10  are fluoroscopic images shown on monitor  24  obtained with position and/or orientation information received by, noted and stored within computer  18 .  FIG. 4  is an open field with no body part image, but which shows the optical indicia which may be used to normalize the image obtained using a spherical fluoroscopy wave front with the substantially flat surface of the monitor  24 .  FIG. 5  shows an image of the femur  12  head. This image is taken in order to allow the surgeon to designate the center of rotation of the femoral head for purposes of establishing the mechanical axis and other relevant constructs relating to of the femur according to which the prosthetic components will ultimately be positioned. Such center of rotation can be established by articulating the femur within the acetabulum or a prosthesis to capture a number of samples of position and orientation information and thus in turn to allow the computer to calculate the average center of rotation. The center of rotation can be established by using the probe and designating a number of points on the femoral head and thus allowing the computer to calculate the geometrical center or a center which corresponds to the geometry of points collected. Additionally, graphical representations such as controllably sized circles displayed on the monitor can be fitted by the surgeon to the shape of the femoral head on planar images using tactile input on screen to designate the centers according to that graphic, such as are represented by the computer as intersection of axes of the circles. Other techniques for determining, calculating or establishing points or constructs in space, whether or not corresponding to bone structure, can be used in accordance with the present invention.  
       FIG. 5  shows a fluoroscopic image of the femoral head while  FIG. 6  shows an anterior/posterior view of the knee which can be used to designate landmarks and establish axes or constructs such as the mechanical axis or other rotational axes.  FIG. 7  shows the distal end of the tibia and  FIG. 8  shows a lateral view of the knee.  FIG. 9  shows another lateral view of the knee while  FIG. 10  shows a lateral view of the distal end of the tibia.  
      Registration of Surgically Related Items  
       FIGS. 11-14  show designation or registration of items  22  which will be used in surgery. Registration simply means, however it is accomplished, ensuring that the computer knows which body part, item or construct corresponds to which fiducial or fiducials, and how the position and orientation of the body part, item or construct is related to the position and orientation of its corresponding fiducial or a fiducial attached to an impactor or other component which is in turn attached to an item. Such registration or designation can be done before or after registering bone or body parts as discussed with respect to  FIGS. 4-10 .  FIG. 11  shows a technician designating with probe  26  an item  22  such as an instrument component to which fiducial  14  is attached. The sensor  16  “sees” the position and orientation of the fiducial  14  attached to the item  22  and also the position and orientation of the fiducial  14  attached to the probe  26  whose tip is touching a landmark on the item  22 . The technician designates onscreen or otherwise the identification of the item and then activates the foot pedal or otherwise instructs the computer to correlate the data corresponding to such identification, such as data needed to represent a particular cutting block component for a particular knee implant product, with the particularly shaped fiducial  14  attached to the component  22 . The computer has then stored identification, position and orientation information relating to the fiducial for component  22  correlated with the data such as configuration and shape data for the item  22  so that upon registration, when sensor  16  tracks the item  22  fiducial  14  in the infrared field, monitor  24  can show the cutting block component  22  moving and turning, and properly positioned and oriented relative to the body part which is also being tracked.  FIGS. 12-14  show similar registration for other instrumentation components  22 .  
      Registration of Anatomy and Constructs  
      Similarly, the mechanical axis and other axes or constructs of body parts  10  and  12  can also be “registered” for tracking by the system. Again, the system has employed a fluoroscope to obtain images of the femoral head, knee and ankle of the sort shown in  FIGS. 4-10 . The system correlates such images with the position and orientation of the C-arm and the patient anatomy in real time as discussed above with the use of fiducials  14  placed on the body parts before image acquisition and which remain in position during the surgical procedure. Using these images and/or the probe, the surgeon can select and register in the computer  18  the center of the femoral head and ankle in orthogonal views, usually anterior/posterior and lateral, on a touch screen. The surgeon uses the probe to select any desired anatomical landmarks or references at the operative site of the knee or on the skin or surgical draping over the skin, as on the ankle. These points are registered in three dimensional space by the system and are tracked relative to the fiducials on the patient anatomy which are preferably placed intraoperatively.  FIG. 15  shows the surgeon using probe  26  to designate or register landmarks on the condylar portion of femur  12  using probe  26  in order to feed to the computer  18  the position of one point needed to determine, store, and display the epicondylar axis. (See  FIG. 20  which shows the epicondylar axis and the anterior-posterior plane and for lateral plane.) Although registering points using actual bone structure such as in  FIG. 15  is one preferred way to establish the axis, a cloud of points approach by which the probe  26  is used to designate multiple points on the surface of the bone structure can be employed, as can moving the body part and tracking movement to establish a center of rotation as discussed above. Once the center of rotation for the femoral head and the condylar component have been registered, the computer is able to calculate, store, and render, and otherwise use data for, the mechanical axis of the femur  12 .  FIG. 17  once again shows the probe  26  being used to designate points on the condylar component of the femur  12 .  
       FIG. 18  shows the onscreen images being obtained when the surgeon registers certain points on the bone surface using the probe  26  in order to establish the femoral mechanical axis. The tibial mechanical axis is then established by designating points to determine the centers of the proximal and distal ends of the tibia so that the mechanical axis can be calculated, stored, and subsequently used by the computer  18 .  FIG. 20  shows designated points for determining the epicondylar axis, both in the anterior/posterior and lateral planes while  FIG. 21  shows such determination of the anterior-posterior axis as rendered onscreen. The posterior condylar axis is also determined by designating points or as otherwise desired, as rendered on the computer generated geometric images overlain or displayed in combination with the fluoroscopic images, all of which are keyed to fiducials  14  being tracked by sensors  16 .  
       FIG. 23  shows an adjustable circle graphic which can be generated and presented in combination with orthogonal fluoroscopic images of the femoral head, and tracked by the computer  18  when the surgeon moves it on screen in order to establish the centers of the femoral head in both the anterior-posterior and lateral planes.  
       FIG. 24  is an onscreen image showing the anterior-posterior axis, epicondylar axis and posterior condylar axis from points which have been designated as described above. These constructs are generated by the computer  18  and presented on monitor  24  in combination with the fluoroscopic images of the femur  12 , correctly positioned and oriented relative thereto as tracked by the system. In the fluoroscopic/computer generated image combination shown at left bottom of  FIG. 24 , a “sawbones” knee as shown in certain drawings above which contains radio opaque materials is represented fluoroscopically and tracked using sensor  16  while the computer generates and displays the mechanical axis of the femur  12  which runs generally horizontally. The epicondylar axis runs generally vertically, and the anterior/posterior axis runs generally diagonally. The image at bottom right shows similar information in a lateral view. Here, the anterior-posterior axis runs generally horizontally while the epicondylar axis runs generally diagonally, and the mechanical axis generally vertically.  
       FIG. 24 , as is the case with a number of screen presentations generated and presented by the system of  FIGS. 4-39 , also shows at center a list of landmarks to be registered in order to generate relevant axes and constructs useful in navigation, positioning and assessment during surgery. Textural cues may also be presented which suggest to the surgeon next steps in the process of registering landmarks and establishing relevant axes. Such instructions may be generated as the computer  18  tracks, from one step to the next, registration of items  22  and bone locations as well as other measures being taken by the surgeon during the surgical operation.  
       FIG. 25  shows mechanical, lateral, anterior-posterior axes for the tibia according to points are registered by the surgeon.  
       FIG. 26  is another onscreen image showing the axes for the femur  12 .  
      Any desired axes or other constructs can be created, tracked and displayed, in order to model and generate images and data showing any desired static or kinematic function of the knee for any purposes related to a UKA.  
      Modifying Bone  
      After the mechanical axis and other rotation axes and constructs relating to the femur and tibia are established, instrumentation can be properly oriented to resect or modify bone in order to fit trial components and implant components properly according to the embodiment of the invention shown in  FIGS. 4-39 . Instrumentation such as, for instance, cutting blocks, to which fiducials  14  are mounted, can be employed. The system can then track instrumentation as the surgeon manipulates it for optimum positioning. In other words, the surgeon can “navigate” the instrumentation for optimum positioning using the system and the monitor. In this manner, instrumentation may be positioned according to the system of this embodiment in order to align the ostetomies to the mechanical and rotational axes or reference axes on an extramedullary rod that does not violate the canal, on an intramedullary rod, or on any other type of rod. The touchscreen  24  can then also display the instrument such as the cutting block and/or the implant relative to the instrument and the rod during this process, in order, among other things, properly to select size of implant and perhaps implant type. As the instrument moves, the varus/valgus, flexion/extension and internal/external rotation of the relative component position can be calculated and shown with respect to the referenced axes; in the preferred embodiment, this can be done at a rate of six cycles per second or faster. The instrument position is then fixed in the computer and physically and the bone resections are made.  
       FIG. 27  shows orientation of an extramedullary rod to which a fiducial  14  is attached via impactor  22 . The surgeon views the screen  24  which has an image as shown in  FIG. 29  of the rod overlain on or in combination with the femur  12  fluoroscopic image as the two are actually positioned and oriented relative to one another in space. The surgeon then navigates the rod into place preferably along the mechanical axis of the femur and drives it home with appropriate mallet or other device. The present invention thus avoids the need to bore a hole in the metaphysis of the femur and place a reamer or other rod into the medullary canal which can cause fat embolism, hemorrhaging, infection and other untoward and undesired effects.  
       FIG. 28  also shows the extramedullary rod being located.  FIG. 29  shows fluoroscopic images, both anterior-posterior and lateral, with axes, and with a computer generated and tracked image of the rod superposed or in combination with the fluoroscopic images of the femur and tibia.  FIG. 30  shows the rod superimposed on the femoral fluoroscopic image similar to what is shown in  FIG. 29 .  
       FIG. 29  also shows other information relevant to the surgeon such as the name of the component being overlain on the femur image, suggestions or instructions at the lower left, and angle of the rod in varus/valgus and extension relative to the axes. Any or all of this information can be used to navigate and position the rod relative to the femur. At a point in time during or after placement of the rod, its tracking may be “handed off” from the impactor fiducial  14  to the femur fiducal  14  as discussed below.  
      Once the extramedullary rod, intramedullary rod, or any other type of rod has been placed, instrumentation can be positioned as tracked in position and orientation by sensor  16  and displayed on screen face  24 . Thus, a cutting block of the sort used to establish the condylar anterior cut, with its fiducial  14  attached, is introduced into the field and positioned on the rod. Because the cutting block corresponds to a particular implant product and can be adjusted and designated on screen to correspond to a particular implant size of that product, the computer  18  can generate and display a graphic of the cutting block and the femoral component overlain on the fluoroscopic image. The surgeon can thus navigate and position the cutting block on screen using not only images of the cutting block on the bone, but also images of the corresponding femoral component which will be ultimately installed. The surgeon can thus adjust the positioning of the physical cutting block component, and secure it to the rod in order to resect the anterior of the condylar portion of the femur in order to optimally fit and position the ultimate femoral component being shown on the screen. Other cutting blocks and other resections may be positioned and made similarly on the condylar component.  
      In a similar fashion, instrumentation may be navigated and positioned on the proximal portion of the tibia  10  and as tracked by sensor  16  and on screen by images of the cutting block and the implant component.  
       FIGS. 33-37  show instrumentation being positioned relative to femur  12  as tracked by the system for resection of the condylar component in order to receive a particular size of implant component. Various cutting blocks and their attached fiducials can be seen in these views.  
      Navigation, Placement and Assessment of Trials and Implants  
      Once resection and modification of bone has been accomplished, implant trials can then be installed and tracked by the system in a manner similar to navigating and positioning the instrumentation, as displayed on the screen  24 . Thus, a femoral component trial, a tibial plateau trial, and a bearing plate trial may be placed as navigated on screen using computer generated overlays corresponding to the trials.  
      During the trial installation process, and also during the implant component installation process, instrument positioning process or at any other desired point in surgical or other operations according to the present invention, the system can transition or segue from tracking a component according to a first fiducial to tracking the component according to a second fiducial. Thus, the trial femoral component is mounted on an impactor to which is attached a fiducial  14 . The trial component is installed and positioned using the impactor. The computer  18  “knows” the position and orientation of the trial relative to the fiducial on the impactor (such as by prior registration of the component attached to the impactor) so that it can generate and display the image of the femoral component trial on screen  24  overlaid on the fluoroscopic image of the condylar component. At any desired point in time, before, during or after the trial component is properly placed on the condylar component of the femur to align with mechanical axis and according to proper orientation relative to other axes, the system can be instructed by foot pedal or otherwise to begin tracking the position of the trial component using the fiducial attached to the femur rather than the one attached to the impactor. According to the preferred embodiment, the sensor  16  “sees” at this point in time both the fiducials on the impactor and the femur  12  so that it already “knows” the position and orientation of the trial component relative to the fiducial on the impactor and is thus able to calculate and store for later use the position and orientation of the trial component relative to the femur  12  fiducial. Once this “handoff” happens, the impactor can be removed and the trial component tracked with the femur fiducial  14  as part of or moving in concert with the femur  12 . Similar handoff procedures may be used in any other instance as desired in accordance with the present invention.  
      Alternatively, the tibial trial can be placed on the proximal tibia and then registered using the probe  26 . Probe  26  is used to designate preferably at least three features on the tibial trial of known coordinates, such as bone spike holes. As the probe is placed onto each feature, the system is prompted to save that coordinate position so that the system can match the tibial trial&#39;s feature&#39;s coordinates to the saved coordinates. The system then tracks the tibial trial relative to the tibial anatomical reference frame.  
      Once the trial components are installed, the surgeon can assess alignment and stability of the components and the joint. During such assessment, in trial reduction, the computer can display on monitor  24  the relative motion between the trial components to allow the surgeon to make soft tissue releases and changes in order to improve the kinematics of the knee. The system can also apply rules and/or intelligence to make suggestions based on the information such as what soft tissue releases to make if the surgeon desires. The system can also display how the soft tissue releases are to be made.  
      During this assessment, the surgeon may conduct certain assessment processes such as external/internal rotation or rotary laxity testing, varus/valgus tests, and anterior-posterior drawer at 0 and 90 degrees and mid range. Thus, in the AP drawer test, the surgeon can position the tibia at the first location and press the foot pedal. He then positions the tibia at the second location and once again presses the foot pedal so that the computer has registered and stored two locations in order to calculate and display the drawer and whether it is acceptable for the patient and the product involved. If not, the computer can apply rules in order to generate and display suggestions for releasing ligaments or other tissue, or using other component sizes or types. Once the proper tissue releases have been made, if necessary, and alignment and stability are acceptable as noted quantitatively on screen about all axes, the trial components may be removed and actual components navigated, installed, and assessed in performance in a manner similar to that in which the trial components were navigated, installed, and assessed.  
      At the end of the case, all alignment information can be saved for the patient file. This is of great assistance to the surgeon due to the fact that the outcome of implant positioning can be seen before any resectioning has been done on the bone. The system is also capable of tracking the patella and resulting placement of cutting guides and the patellar trial position. The system then tracks alignment of the patella with the patellar femoral groove and will give feedback on issues, such as, patellar tilt.  
      The tracking and image information provided by systems and processes according to the present invention facilitate telemedical techniques, because they provide useful images for distribution to distant geographic locations where expert surgical or medical specialists may collaborate during surgery. Thus, systems and processes according to the present invention can be used in connection with computing functionality  18  which is networked or otherwise in communication with computing functionality in other locations, whether by PSTN, information exchange infrastructures such as packet switched networks including the Internet, or as otherwise desire. Such remote imaging may occur on computers, wireless devices, videoconferencing devices or in any other mode or on any other platform which is now or may in the future be capable of rending images or parts of them produced in accordance with the present invention. Parallel communication links such as switched or unswitched telephone call connections may also accompany or form part of such telemedical techniques. Distant databases such as online catalogs of implant suppliers or prosthetics buyers or distributors may form part of or be networked with functionality  18  to give the surgeon in real time access to additional options for implants which could be procured and used during the surgical operation.