Patent Publication Number: US-2016228193-A1

Title: Method Of Using Augmented Reality For Functional Joint Arthroplasty

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 11/290,039, filed on Nov. 30, 2005, the entire disclosure of which is hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to methods, software and systems to assist in the performance of reconstruction and total and partial replacement surgery for joints. More particularly this invention relates to methods, and software to assist in surgical reconstructive interventions of joints that have kinematic behavior influenced by the soft tissue apparatus surrounding it. Typical examples are surgical procedures of the knee or ankle, such as ACL repair, uni-compartmental or multi-compartmental replacement of the joint surfaces, revision surgery and the like. 
     2. Description of the Background of the Invention 
     Current joint reconstruction and replacement surgery, including ankle, knee, shoulder, or elbow arthroplasty is based in large part on standard methods and guidelines for acceptable performance. In this regard, the positioning of the implants into the joint is based on standard values for orientation relative to the biomechanical axes, such as varus/valgus, or flexion/extension, and range of motion. One surgical goal might be that the artificial components used to achieve the reconstruction of the joint should have a certain alignment relative to the load axes. These standards are based on static load analysis and therefore may not be appropriate to establish optimal joint functionality taking into account life style patterns of the individual undergoing surgery. There have been systems that look at the ipsilateral side to gage parameters for the operative joint. Also, there have been kinematic approaches that attempt to determine appropriate values for varus/valgus, flexion/extension, and range of motion. One reason for the need to properly balance unconstrained joints, like the knee, ankle and elbow, is that these joints are held together by the soft tissue, including the ligaments, that surrounds the joint The proper functioning of the joint is dependent on a combination of the proper resection of the joint to receive the implant, the proper choice of implant sizing and the proper balance of the soft tissue relative to the implants and the resection. Currently, this balancing is done by the surgeon based on experience and rule of thumb guidelines. 
     A computer assisted surgical navigation system normally requires a time consuming setup and registration of the patient&#39;s anatomy with either a pre-operative scan or with a three dimensional model that is constructed from reference points obtained from the patient&#39;s anatomy. Further, prior computer assisted navigation systems have not assisted the surgeon by providing step-by-step procedures to guide the surgeon in making the proper balance between bone cuts, implant size, and soft tissue constraints or balancing. The necessity of additional steps without corresponding added benefits have kept surgeons from using surgical navigation systems for orthopedic surgeries even though the increased accuracy of the surgical navigation systems could improve the end result for the patient. 
     SUMMARY OF THE INVENTION 
     One embodiment of the present comprises a method for performing arthroplasty on a joint using a surgical navigation system. The method includes the steps of locating articular anatomical structures using the surgical navigation system; determining biomechanical properties of the joint; and evaluating the soft tissue envelope properties for the joint. The method also includes the steps of displaying an interactive view of the joint, the soft tissue envelope properties, the biomechanical properties and a chosen implant to enable a surgeon to manipulate simultaneously the soft tissue envelope properties, the biomechanical properties and the chosen implant on the interactive view; preparing the joint to receive the chosen implants; and installing the implants in the prepared joint. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a patient&#39;s knee that has been prepared for knee replacement surgery using components of one embodiment of a surgical navigation system; 
         FIG. 2  is a flow diagram of one embodiment of the present invention; 
         FIG. 3  is a screen shot of a further embodiment showing surveying the biomechanical properties of the knee prior to incisions to open the knee; 
         FIG. 4  is a screen shot of a still further embodiment showing surveying the biomechanical properties of the knee after the initial incisions opening the knee have been made; 
         FIG. 5  is a screen shot showing an additional embodiment displaying the calculation of the internal/external axes by different methods; 
         FIG. 6  is a screen shot of the determination of the natural joint line of a knee made during the open knee survey; 
         FIG. 7  is a screen shot showing a relationship between varus/valgus and flexion over a range of flexion; 
         FIG. 8  is a screen shot of one embodiment of balancing of the knee joint; 
         FIG. 9  is a screen shot of one embodiment of initial implant planning; 
         FIG. 10  is a screen shot similar to  FIG. 9  showing a varus load applied to the joint for implant planning; 
         FIG. 11  is a screen shot of yet another implant balancing embodiment; 
         FIG. 12  is a similar screen shot to  FIG. 11  showing a result using a different set of criteria; 
         FIG. 13  is a screen shot of a still further embodiment of implant planning; 
         FIG. 14  is a screen shot showing one embodiment of an interactive iterative display with the knee in extension; 
         FIG. 15  is a screen shot similar to  FIG. 14  of an interactive iterative display with the knee in flexion; 
         FIG. 16  is a screen shot showing a further embodiment of an interactive iterative display with the knee in extension; 
         FIG. 17  is a screen shot similar to  FIG. 16  of an interactive iterative display with the knee in flexion; 
         FIG. 18  is a flow diagram of a further embodiment of the present invention; 
         FIG. 19  is a screen shot showing yet a further embodiment of the present invention; 
         FIG. 20  is a schematic view of a patent&#39;s ankle that has been prepared for ankle replacement surgery using components of a further embodiment of a surgical navigation system; 
         FIG. 21  is a schematic view of a patent&#39;s elbow that has been prepare for elbow replacement surgery using components of a still further embodiment of a surgical navigation system; and 
         FIG. 22  is an isometric view of one embodiment of an in-situ distraction device useful with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Although the main description that follows illustrates the use of the system and method of the present invention for knee replacement surgery, the present invention can be used to replace or repair any unconstrained joint, such as the ankle, shoulder and the elbow, as illustrated in  FIGS. 20 and 21 . 
     Referring to  FIG. 1 , a patient&#39;s leg  100  is prepared for knee replacement surgery. The leg  100  is bent so that the patient&#39;s upper leg or femur  102  is at an angle of approximately 90 degrees to the patent&#39;s lower leg or tibia  104 . This positioning of the leg  100  places the patient&#39;s knee  106  in position for the procedure. Two tracking devices  108  that can communicate with a camera  110  associated with a computer assisted surgical navigation system  112  are associated with the femur  102  and the tibia  104  such that the tracking devices  108  move with the femur  102  and the tibia  104  respectively. The association can be by direct attachment to the bone or by other association methods as discussed hereinafter. The computer assisted surgical navigation system  112  is one that is well known in the art and will not be further discussed here. Suitable surgical navigation systems are described in U. S. Patent Publication No. 2001/0034350, the disclosure of which is incorporated by reference. A typical navigation system  112  will also include a display device  114 , such as a computer or video monitor. In addition, most navigation systems  112  will also use specialized tools, such as a pointer  116  that has been previously calibrated to work with the navigation system  112 . The calibration of the pointer  116  will enable the navigation system  112  to determine the precise location of a pointer tip  118  by location a series of locator devices  120  such as LED&#39;s located on the pointer  116 . These locator devices are the same type used for the tracking devices  108 . 
     In addition to the femur  102  and the tibia  104 , the knee  106  has a patella  122 . The location of the patella  122  can be determined by using the pointer  116 . Also because the patella  122  is anatomically tied to the location of the tibia  104 , the navigation system  112  can also locate the patella  122  by reference to the location of the tibia  104 . Further, because the patella  122  is constrained, it is often only necessary to locate the patella  122  relative to three degrees of freedom. As is well known, the properties of the patella  122  should be considered during knee replacement surgery and any implant that replicates the patella  122  can have an impact on the post surgical functioning of the knee  106 . 
     One relationship that can be considered in certain embodiments is a joint line between the femur  102  and the tibia  104 , and the relationship of the femoral-tibial joint line to a joint line between the femur  102  and the patella  122 . The joint line is the momentary rotation of the joint in space, in this case the knee  106 . The joint line is different than the functional flexion axis. The functional flexion axis describes the overall flexion of the joint. In any joint are typically distinct points of contact between the bones that comprise the joint. Once the surface of one of these bones has been determined, the system can determine the contact points on one bone and link the contact points of the one bone to the contact points of the second bone at the same position of the joint. In the case of the knee, the two contact points on the first bone are related to each other on a line, the joint line. The two contact points on the second bone are also related on the same line initially. However each line can be viewed individually for each of the bones that comprise the joint. Because one or both bones may be deformed as a result of disease or injury, the optimal joint line relative to the reconstructed joint often will be offset from the initial observed joint line for one or both bones. 
     As set out in  FIG. 2 , a block  200  determines the location of the femur  102  and the tibia  104 . As noted above, this is done by associating tracking devices to the femur  102  and the tibia  104  and then manipulating the respective limb within the view of the camera  110  of the navigation system  112 . Optionally, the block  200  can also determine the location of the patella  112 . 
     Control then passes to a block  202  where the biomechanical properties for the femur  102 , the tibia  104 , the knee  106 , and optionally the patella  122  are determined. Some of the biomechanical properties can be determined before the actual incision is made. For instance, after the tracking device  108  is attached to or associated with the femur  102 , the location of the femoral head can be determined by manipulating the femur  102  to rotate about the hip socket. In addition, an initial functional flexion axis analysis and a range of motion analysis can be performed by manipulating the knee  106  through the entire range of motion of the knee  106 . Also, the location of the lateral malleolus and medial malleolus can be determined using the pointer  116  and touching the pointer tip  118  to the appropriate bony landmarks on the ankle. The navigation system  112  can display a series of screens on the display  114  of the type as shown in  FIG. 3  to guide the surgeon in performing the closed knee evaluation of the knee  106 . The navigation system  112  will track the location of the tracker  108  that is associated with the femur  102  and indicate when sufficient points have been recorded to accurately locate the femoral head and determine the range of motion and the initial functional flexion axis. In addition, the relationship of the patella  122  to the tibia  104  can also be determined at this time. Some of these evaluations on the closed knee  106  can be performed earlier as part of a pre-operative work up and recorded for use during a later surgery. 
     Traditionally knee replacement has had enormous focus around the tibio-femoral joint. Nevertheless the knee is a tri-compartmental joint with the patella gliding in the femoral trochlear groove transmitting quadriceps forces onto the tibia. Failing to re-establish proper patello-femoral kinematics often yields to unsatisfactory pain levels after implantation. One of the most important parameters to take into consideration in order to re-establish proper operation of the quadriceps mechanism is the distance of the patella to the functional flexion axis. If the distance is increased, so called overstuffing of the patella occurs that leads to a limited range of motion and pain. If the distance is decreased, the efficacy of the quadriceps mechanism is compromised and mal-tracking of the patella may occur. Medial-lateral displacement of the patella is in turn influenced by internal-external rotation of the implant. If there is a gross mismatch of the natural track of the patella and the reconstructed trochlear groove of the implant, dislocation and pain may occur. The rotational degrees of freedom of the patella, namely tilt, rotation and flexion, are coupled to the above parameters and are of secondary importance. 
     Tracking only the trajectory of the patella simplifies the needed devices by not having to implement all 6 degrees of freedom. This is important because the patella is relatively small structure and the kinematics of the patella can be altered if a foreign object attached to the patella. Furthermore, the patella has a constant relationship to the tibia. It is attached by a tendon to the proximal spine of the tibia where the forces from the quadriceps mechanism are received and passed on to the foot. This relationship is seldom affected and represents a very faithful landmark intrinsically describing some of kinematic parameters of the knee joint. These parameters are very helpful while re-establishing the function of the knee but are especially helpful in cases where no other references may be available as for example in revision surgery where the tibio-femoral joint has been replaced. 
     After the closed knee biomechanical data has been determined and recorded, the initial incision is made to the knee  106  to expose the distal portion of the femur  102 , the patella, and the proximal portion of the tibia  104 . The surgeon can then determine other landmarks using the pointer  116  as guided by the navigation system  112  through the various screens displayed on display  114 . Typical landmarks include the surface of the lateral and medial condyles, the tibial plateau dishes, and other landmarks. 
     Certain landmarks can be inferred from the digitization of other landmarks. For instance the surfaces of the femoral condyles can be determined by assessing the location of the surface of the tibial plateau throughout the range of motion. Because of the envelope constraints on the knee joint, the surface of the femoral condyles will sweep the tibial plateau as the knee joint is flexed throughout the range of motion and can be computed once these two properties or landmark locations are known. While there can be a lift off situation, this still does not have a significant impact on the computation because the femur can not penetrate into the tibia. Also, the center of the knee can be computed as the most distant point of the groove from the center of the femoral head. 
     In addition through further geometrical analysis of the obtained surfaces fundamental properties can be derived, such as radiuses of curvature or axes of rotation. For example in a knee  106  the internal/external rotation of the femur  102  can be computed by a cone or cylinder fitted to the geometry of the posterior condyles, which are usually unaffected by the disease condition of the knee  106 . The femoral-tibial joint line is constructed by joining the two points of contact of the tibial dishes with the femoral condyles. Because one of the condyles is normally eroded, due to the disease condition, the joint line is affected and reflects the varus or valgus deformation. The varus/valgus deformation has been determined during the closed knee range of motion analysis and the surgical navigation system  112  can compute the current joint line and also the system can propose a restored joint line that will reflect the knee in a repaired state. The restored joint line can be used as a target by the surgeon during the balancing of the knee soft tissue, the implants and the modifications made to the bones to receive the implants. 
     Further to the geometrical analysis of the pairing surfaces of a joint, its functional kinematic data can be analyzed to derive momentary or instantaneous axes of rotation or overall axes of rotation by analyzing portions or all of the instantaneous axes. One of the best known and most widely used techniques is the helical axis computation which is said to describe the home-screw mechanism of unconstrained joints. In the case of the knee, the flexion axis of the patello-femoral joint as well as the tibio-femoral joint can be computed by passively moving the joint throughout its range of motion. The so derived flexion axis of a diseased joint will undoubtedly reflect the diseased kinematics of the joint. Partial correction of some of its degrees of freedom will be necessary before it is used as a guide for the surgical measure. The corrections may be derived by combining information or constraints given by the biomechanical axes of the joint and/or by dynamic load transfer patterns while ranging the joint. Another alternative is by combination of information provided by unaffected degrees of freedom of other pairing surfaces of the joint. In the case of the knee  106 , a possible combination could be perpendicularity of the derived functional flexion axis to the biomechanical axis, translational constraints given by the patello-femoral flexion axis and its internal/external rotation dictated by the conical fit axis of the posterior condyles of the femur. 
     Another method to re-establish normal kinematics of the affected joint is to assess the kinematics of the non-affected ipsilateral side. These parameters can be extracted by the same methods and are expressed preferably in terms of local non-affected anatomical structures to enable its transfer to the affected site after identification of the corresponding anatomical structures. In the case of the knee  106 , a local reference can be established by the intercondylar notch and the anterior cortex of the femur  102 . The identification of these structures can be done intra—but preferably preoperatively with any non-invasive imaging technology. In the case where the functional analysis is not done with the same modality as the one used for the identification of the reference structures the registration of both modalities is necessary. A preferred embodiment uses ultrasound as imaging modality. This coupled with tracking technology relates kinematic information to the underlying reference structures and minimally invasive tracking devices as described in published U.S. Patent application No. 2005/199,250, published Sep. 15, 2005, the disclosure of which is hereby incorporated by reference. 
     The computed kinematics information in form of restored functional axes and joint lines of the joint surfaces may not only be used as a guide for driving the position of prosthetic components but also for establishing an optimum between the kinematics constraints of the individual&#39;s joint and the prosthetic system being used. In a scenario where multiple prosthetic systems or surgical techniques are available the computer system may choose the optimal implant and propose an optimized position to best fit constraints given by the function of joint and those of the implant. A further example of an optimization criterion could be optimal performance for a given activity of the individual that best restore his or her quality of life. 
     A block  204  evaluates the soft tissue surrounding the joint. The soft tissue can be evaluated by further manipulating the knee  106  and also by the use of strain gauges or similar devices. The knee includes four main ligaments that interact with the tibia  104  and the femur  102  to form a stable knee  106 . The tension on these ligaments must be properly balanced to provide stability to the knee  106 . The surgical navigation system  112  can also guide the surgeon through the soft tissue analysis and based on the particular manipulation that is performed record values for the tension of the various ligaments and muscles of the knee  106 . Other methods of acquiring the soft tissue tension values can also be used. 
     Particularly useful are the in-situ devices of the type shown in  FIG. 22  and described hereafter that do not require the modification of the soft tissue joint envelope, such as everting the patella in the case of the knee. Such devices can be balloon systems that can exert distraction forces that simulate normal activity and thus providing a close approximation to the forces experienced during voluntary motion of the joint. These systems require little or no preparation of the site in order to be used and can be introduced through small incisions or through a cannula system such as those used in arthroscopy. Furthermore these systems allow the assessment of function of the joint throughout its entire range of motion as opposed to typically representative stances in flexion and extension. In addition, other devices than those shown in the application can also be used. For instance, the devices shown in U.S. Pat. Nos. 6,702,821, and 6,770,078, the disclosures of which are hereby incorporated by reference, can be used as well. 
     Another benefit of in-situ devices is the ability to iteratively establish or capture the functional parameters, as the functional flexion axis of the joint that exactly describes the actual state of the soft tissue envelope of the joint. Through instant assessment of the effect of a given soft tissue measure are important to precisely drive the desired soft tissue correction. 
     The usage of more sophisticated distraction devices in which force or pressure sensing elements have been incorporated can yield precise information on loading pattern characteristics for the joint throughout range of motion. These in turn can be used to establish a certain soft tissue management strategy in which specific group or bundles of bands are selectively targeted to affect the load or force pattern at a specific flexion or kinematic state of the joint. These devices can transmit wirelessly in a real time fashion the information to the computer system for on the fly analysis. The information can then be displayed numerically or graphically in relationship to the established model of the joint. Using this information the computer system can also deliver the most likely soft tissue management strategy based on e.g. an underlying expert system. 
     The surgical navigation system  112  will also include a database  206  of implant components in digitized form. A block  208  takes the values from the location analysis  200  of the femur  102  and the tibia  104 , the biomechanical properties analysis  202 , the soft tissue evaluation  204 , and the database  206 . Using all these values, as well as other criteria including but not limited to gender, age, race, life style, and the like, the block  208  simultaneously solves for the functional goal and displays the calculated result, including a suggested implant from the database of implants  206 , on an interactive screen on the display  114 . One possible element of the functional goal in one embodiment of the present invention can be the restored joint line. This can be shown on the interactive screen along with other values relative to restoring the joint. Control passes to a block  210  that enables the surgeon to manually adjust the chosen functional goal and other values if necessary to reflect the surgeon&#39;s experience with the procedure. If the surgeon determines that the solution shown by the block  208  is not optimum, control will pass via a NO branch to a block  212  that allows the surgeon to digitally manipulate the joint and possibly change the suggested implant or other parameters as shown on the interactive screen. After the changes are made, the navigation system  112  will recalculate the result and the block  208  will display the updated result. If at this point, the surgeon believes that the proposed solution meets the surgical objective, then control will pass via a YES branch to a block  214  that asks for confirmation and recording of the choice of implant and other parameters. At this point, the surgeon in a block  216  will prepare the joint to match the chosen solution. The navigation system  112  can guide the surgeon through the procedure and make suggestions of modifications necessary to achieve the desired outcome or the surgeon can proceed in a conventional fashion to prepare the joint without the navigation system  112 . After the joint is prepared in the block  216 , the implant is installed in a block  218 . Again, if the surgeon chooses, the navigation system  112  can guide the surgeon through this procedure as well. As will be discussed later, the installing of the implants can also include the use of trial implants that replicate the final implants and allow the surgeon to test the configuration of the joint before the final implants are permanently placed in the joint. 
     The preparation of the joint according to the established goal can be performed manually with the aid of navigation but also with any type of passive, semi-active or active envelope constraining devices, with master-slave manipulators or with autonomous in-situ mounted or external manipulators, including telemanipulators. 
       FIGS. 3 and 4  are two screen shots  300  and  302  that show the surveying of the knee joint set out in the block  202 . As shown, the display  114  will guide the surgeon through the steps needed to survey the knee joint in preparation for the procedure. It also provides checklists  304  and  306  of the parameters that should be determined and the navigation system  112  will also provide additional screens to assist the surgeon in determining the location of the hip center, the range of motion of the knee, the location of the medial malleolus, and the lateral malleolus. In addition, after the knee joint has been opened, the pointer  116  can be used along with the navigation system  112  to determine the knee center, anterior cortex, the center of the tibial plateau, the medial compartment and the lateral compartment. The navigation system  112  can guide the surgeon through the location of these landmarks or just record the location when the surgeon makes a manual location of the landmark using the pointer  116 . 
     Other biomechanical properties and landmarks can be determined by the navigation system  112  by indirect digitization using combinations of the above determined landmarks as noted above. The surface of the femoral condyles can be determined by combining the range of motion analysis with digital location of the tibial plateau. 
     The internal/external rotation of the femur  102  can also be determined by a variety of methods. For instance the internal/external rotation can be derived from the early, 0° to 45°, flexion. There are a number of well known algorithms that can make this calculation including helical axis, residual minimization and other similar geometrical optimization techniques. Alternatively the internal/external rotation can be derived from the shape of the posterior condyles. Normally the shape of the condyles in deep flexion, greater than 90°, are unaffected by a possible disease condition of the joint. The internal/external rotation is determined by fitting a cone or cylinder to the femur  102  as the knee  106  is flexed relative to the femur  102 .  FIG. 5  shows a screen shot  310  that displays the results of the location of the internal/external axes by two different methods as described above and also by an averaging or other weighting of the two results of the internal/external axis of rotation. The tibial internal/external axis of rotation can also be determined in a similar fashion. 
       FIG. 6  shows a screen shot  312  of the determination of the initial joint line. In an upper pane  314 , a representation of the femoral condyles  316  is shown and in a lower pane  318  is shown a representation of the tibial compartments  320 . A pressure zone 321 is where the femoral condyles  316  contact the tibial compartments  320 . A series of contact points  322  represent points of contact on each of the femoral condyles and a similar series of contact points  322   a  represent the points of contact on the tibial compartments. A series of lines  324  are momentary axis of rotation joining pairs of contact points  322  of the femoral condyles  316  with the tibial compartment  320 . For every joint line  324  on the femoral condyles  316  there is a corresponding joint line  324   a . A functional flexion axis  326  for the knee  106  is also shown for reference. A restored joint line can be calculated by the system and used as a surgical goal of the surgeon and the navigation system  112 . The restored joint line is determined by taking the most prominent point on the femoral condyles  316  and taking a line that intersects this most prominent point on the femoral condyles  316  that is perpendicular to the mechanical axis of the femur  102 . 
     In the block  204 , the soft tissue envelope is evaluated. One method of conducting this evaluation is to flex the open knee joint throughout the range of motion while at the same time applying a varus or valgus load to the knee joint. The surgeon will manipulate the knee joint by flexing the knee and press on either the lateral surface to apply a varus load or the medial surface to apply a valgus load.  FIG. 7  is a screen shot  330  plotting loads against the flexion angles. A cursor  332  at 5° valgus and 45° is the current position of the joint. An area  334  in the plot shows the extent of laxity of the joint at particular flexion angles. The surgeon is interested in seeing the amount of laxity of the soft tissue that constrains the knee joint  106 . The amount of laxity will have an impact on the selection of the possible implant as well as the location of the bone cuts that the surgeon will need to make to prepare the joint to receive the implants. Also, this evaluation will suggest to the surgeon the type and amount of any soft tissue releases that will be necessary to balance the knee  106  after the implants are in position. In addition, it will also be possible to determine similar information relative to the restored joint line. 
     As an initial aspect of the step of the block  208 , a screen shot  340  similar to  FIG. 8  will assist the surgeon in the initial balancing of the joint and the initial determination of the location of the cuts that will need to be made to the tibia and to the femur. For most reconstruction surgeries, the amount of varus and valgus deflection will be as close to zero as possible. In a left pane  342 , the knee  106  is shown at 0° flexion. A line  344  is the mechanical axis of the femur. This shows the case where a femoral cut line  346  will be perpendicular to the mechanical axis of the femur. A line  348  shows the mechanical axis of the tibia and a tibial cut line  350  shows the proposed location of the cut to the tibial plateau. This will result in a gap of 24 mm as shown in  FIG. 8 . A right pane  352  shows the joint at 86° flexion. A line  354  shows the cut that will be made to the posterior portion of the femur. This also will provide a 24 mm gap. Typically, the cut lines  346  and  354  will be parallel to each other. This provides an easier solution to the particular implants to be chosen from the database  206 . Also, the gap should be similar during all degrees of flexion of the joint so that the final implants will function as smoothly as possible when the patient is walking and engaging in normal activities appropriate to the patient&#39;s life style after the procedure. 
     The analysis of the soft tissue can be done by manipulation of the knee  106  or it can alternatively be done by making a perpendicular cut to the tibial plateau to provide space for an in-situ, patella in place balancer device. The device can be of any suitable construction so long as the device will enable the surgeon to tense the knee joint  106  against the soft tissue. One suitable balancing device is shown in  FIG. 22  that will be described in more detail below. Because the femur will ride over the balancer device, it is not necessary to make any cuts to the femur at this time. The surgeon can flex the knee joint  106  over the entire range of motion and can establish a functional flexion axis of the knee joint  106 . This provides a time savings as well as adding flexibility to the procedure. The surgeon can also determine the placement of the femoral cuts after the soft tissue has been evaluated. The surgeon will have greater control over the choice of implants and be able to minimize, if desired, the amount of change needed to the soft tissue. Depending on the training of the surgeon, the surgeon may want to respect the soft tissue and make as few changes to the soft tissue as possible. An alternative training is to make more changes to the soft tissue than to the boney structure. There are also schools of thought that provide a combination of the above two approaches. Depending on the approach, the surgeon is in control and can determine the amount of change to the soft tissue or to the bone. 
       FIGS. 9, and 10 , show two screen shots  360  and  362  that have implants  364  and  366  virtually placed in the knee joint  106 . The particular implants  364  and  366  are chosen based on the parameters of the knee joint  106  when combined with the properties of the various implants in the database  206 .  FIG. 9  shows the knee joint  106  without any side load or pressure.  FIG. 10  shows the same knee joint  106  subjected to a 10° varus loading. This shows the ability of the surgeon to modify the choices made prior to committing to a particular joint configuration. The surgeon can also subject the knee  106  to virtual forces as shown in  FIG. 10  and observe the effect of these forces on the knee joint  106  and the proposed implants  364  and  366 .  FIGS. 11 and 12  show screen shots  370  and  372  of an alternative embodiment similar to  FIGS. 9 and 10 .  FIG. 11  shows the proposed implants  364  and  366  that have been selected to restore the joint line.  FIG. 12  shows the proposed implants  364  and  366  that will achieve the boney referencing goal. 
       FIG. 13  shows a screen shot  380  of an alternative embodiment what that does not use a database of components or provides an option for the surgeon when there is no good solution for the implant from the database. In a right pane  382 , the femur  102  is shown with a sizing grid  384 . This shows the placement of various implants relative to a distal  386 , an anterior  384 , and a posterior  388  portion of the femur  102 . After the surgeon manually chooses an implant, the navigation system  112  can display the chosen implant in place in the joint as above. The surgeon then can manipulate the knee  106  as above. 
       FIGS. 14 and 15  show screen shots  400  and  402 . In this embodiment, the balancing is done relative to bony referencing. Both panes  404  and  406  have a series of buttons  408  that can virtually vary any of the values shown. The result of the variation of the value will be immediately shown in both panes  404  and  406 . This will provide the surgeon with greater control over the choices that the navigation system  112  may suggest. In a similar manner,  FIGS. 16 and 17  show screen shots  410  and  412  of a still further embodiment that does the balancing by respecting the restored joint line. 
     As shown in  FIG. 18 , the installation of the implants can include multiple steps. In a block  420 , the navigation system  112  assists the surgeon in the placement of trial implants to the preparation of the joint and the modification to soft tissue tension. It may be necessary in a block  422  to adjust the soft tissue tension by known methods. The navigation system  112  will assist the surgeon by guiding the tools to the proper location for releasing tension of a particular ligament or ligament bundle. When the surgeon is satisfied with the trial implants and the soft tissue, the system proceeds to a block  424  that assists the surgeon in the placement of the final implants. 
       FIG. 19  shows another method of visualizing the interactive nature of the present invention. The diagram shows various curves that are a series of solutions for the optimal positioning of the implant for a particular implant combination. The surgical navigation system  112  displays curves  500  through  510  relative to the flexion and rotation of the knee  106 . The point on the display  512  indicates the current position of the knee joint  106  and of the proposed implants. By manipulation of the knee  106 , the surgeon can choose the combination of implants and soft tissue corrections that produce a desired result or goal. 
     Visualization of the above disclosed information as, flexion axis, joint line, alignment, load distribution through out range of motion, implant size and position, etc. is challenging and can be sometimes overwhelming. An effective and above all ergonomic method to convey this complex context is by virtue of augmented reality techniques where through projection techniques the graphical information can be overlaid onto the anatomical structures being addressed. This results in an intuitive context oriented visualization of the required information. For instance the chosen implant can be superimposed at the correct position directly on the anatomical structure giving the surgeon the opportunity to assess the overall fit and required preparation of an intact joint. 
     The system and methods of the present invention have been described using the knee  106  as an example joint. It should be understood that the knee  106  is the most complicated unconstrained type joint. As such, the methods of the present invention can be applied to other unconstrained joints in the body such as the ankle, shoulder or the elbow. It should also be understood that any type of surgical measure throughout the continuum of care of said joints will profit from the here described methods. The various surgical implants can range from autologous tissue for focal repair to structured load bearing biomaterials to replacement surfaces of inert materials to revision type prosthetic implant. It should also be understood that the methods of the present invention can be accomplished by the surgical navigation system  112  that has software loaded into random-access memory in the form of machine-readable code, such code being executable by an array of logic elements such as a microprocessor or other digital signal processing unit contained within the surgical navigation system  112  or within any standard computer system. 
       FIG. 20  is a schematic view of an ankle  600  being prepared for ankle replacement surgery. The tracking device  108  is associated with the tibia  104 . In addition there is the second tracking devise  108  associated with a talus bone (not shown) of the ankle  600 . As the surgeon manipulates a foot  602 , the ankle  600  will allow the foot to move up and down. The ankle  600  is comprised of the true ankle joint where the tibia  104 , the fibula and the talus come together. Below the talus is the subtalar joint where the talus meets the calcaneus. The true ankle joint allows the foot  602  to move up and down relative to the tibia  104  and the subtalar joint allows the foot  602  to move from side to side. Depending on the procedure, there may also be an optional tracking device  108  associated with the foot  602 . Using the same workflow as above for the knee  106 , the surgeon can perform the appropriate replacement surgery for the ankle  600 . In a similar manner as schematically illustrated in  FIG. 21 , surgery of an elbow  630  can also be performed. The tracking devices  108  are associated with a humerus  632  and an ulna  634  as shown. Again, replacement surgery of the elbow  630  can be performed using the procedures and workflow outlined above. 
       FIG. 22  shows one embodiment of a distracting device  650 . The device  650  has two expendable bladders  652  and  654 . The bladders  652  and  654  can be inflated or deflated to provide pressure for the knee joint  106 . In one embodiment the bladders  652  and  654  are separately controllable such and each blades  652  and  654  can exert a different pressure on that compartment of the knee joint  106 . In other embodiments, the bladders  652  and  654  exert the same pressure on both compartments of the knee joint  106 . The bladders  652  and  654  can be formed from any suitable surgically acceptable flexible material. Examples include plastics as polyethylene terephthalate, polyurethane, polyvinyl chloride, and the like. The bladders  652  and  654  can have a smooth side wall  666  as shown or the side walls  656  can be fluted. The bladders  652  and  654  are mounted on bases  656  and  660  respectively. The bases  658  and  660  can be joined by a hinge  662  or in an alternate embodiment the bases  658  and  660  can be joined so that the bases  658  and  660  cannot move relative to each other. A flexible tube  664  extends between bladders  652  and  654 . In our embodiment where the bladders  652  and  654  exert the same pressure, the tube  664  communicates with the internal of the bladder  652 . In the embodiment where the bladders  652  and  654  exert different pressure, the tube  664  is in communication with an individual air supply not shown. A second tube  666  connects the bladder  652  with the external air supply. The distracting device  650  can be sized such that the distracting device  650  can be inserted into the knee joint  106  using minimal invasive surgical techniques. 
     The computer software can be stored in any convenient format usable by computers that can be found within surgical operating rooms. Often, the software will be made available on media such as CD-ROM, DVD-ROM or similar data storage media. In addition, the software can be made available for download though an Internet connection. 
     Numerous modifications to the present invention will be apparent to those skilled in the art in view of the foregoing description. Accordingly, this description is to be construed as illustrative only and is presented for the purpose of enabling those skilled in the art to make and use the invention and to teach the best mode of carrying out same. The exclusive rights to all modifications which come within the scope of the appended claims are reserved.