Patent Publication Number: US-10772617-B2

Title: Knee flexion and extension gap tensioning and measuring apparatus

Description:
This invention relates generally to medical devices and instruments, and more particularly to a method for applying tension along or across a human knee joint to take measurements to repair, augment, or replace it. 
     BACKGROUND 
     Total knee arthroplasty (“TKA”) is a procedure for treating an injured, diseased, or worn human knee joint. In a TKA, an endoprosthetic joint is implanted, replacing the bearing surfaces of the joint with artificial members. Proper alignment of the joint and substantially equal tension in the soft tissues surrounding the joint are important factors in producing a good surgical outcome. 
     A human knee joint “J” is shown in  FIG. 1-4 . The joint J is prepared for implantation by cutting away portions of the femur “F” and the tibia “T”.  FIGS. 1 and 2  show the joint in extension, with cutting planes for a tibial cut  1  and a distal femoral cut  2 . The tibial cut  1  and the distal formal cut  2  cooperate to define an extension gap “EG”.  FIGS. 3 and 4  show the joint J in flexion, with cutting plane  3  for a posterior cut. The tibial cut  1  and the posterior cut  3  cooperate to define a flexion gap “FG”. 
     A goal of total knee arthroplasty is to obtain symmetric and balanced flexion and extension gaps FG, EG (in other words, two congruent rectangles). These gaps are generally measured in millimeters of separation, are further characterized by a varus or valgus angle measured in degrees, and are measured after the tibia cut, distal femoral cut, and posterior femoral cut have been done (to create flat surfaces from which to measure). It follows that, to achieve this balance, the ligament tension in the lateral and medial ligaments would be substantially equal on each side, and in each position; it also follows that the varus/valgus angle in flexion and extension would be 0°. 
     Some surgeons favor the use of a measured resection technique in which bone landmarks, such as the transepicondylar, the anterior-posterior, or the posterior condylar axes are used to determine proper femoral component rotation and subsequent gap balance. Others favor a “gap balancing technique” in which the femoral component is positioned parallel to the resected proximal tibia with each collateral ligament substantially equally tensioned to obtain a rectangular flexion gap. 
     One problem with prior art balancing techniques is that it is difficult and complex to achieve the proper balance. Current state-of-the-art gap balancing devices do not enable balancing with the patella in-place and are large, overly-complicated devices that work only with their respective knee systems. 
     BRIEF SUMMARY OF THE INVENTION 
     This problem is addressed by a gap tensioner operable to apply a load to a gap between the bones of a joint and measure the resulting gap distance and angle between the bones. 
     According to one aspect of the invention, a knee gap tensioning apparatus includes: a gap tensioner including: a baseplate; a top plate; a linkage interconnecting the baseplate and the top plate and operable to move the gap tensioner between retracted and extended positions; and wherein the top plate is pivotally connected to the linkage so as to be able to freely pivot about a pivot axis. 
     According to another aspect of the invention, a knee gap tensioning apparatus includes: a gap tensioner including: a baseplate; a top plate; a linkage interconnecting the baseplate and the top plate and operable to move the gap tensioner between retracted and extended positions; and wherein the top plate is pivotally connected to the linkage so as to be able to freely pivot about a pivot axis; two or more tracking markers, at least one of which is coupled to the gap tensioner; an electronic receiving device, wherein the receiving device is operable in combination with the tracking markers to determine a position and orientation of each of the tracking markers relative to the electronic receiving device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
         FIG. 1  is a view of the anterior aspect of the human knee joint in extension showing cutting planes for a total knee arthroscopy; 
         FIG. 2  is a view of the lateral aspect of the human knee joint of  FIG. 1 ; 
         FIG. 3  is a view of the anterior aspect of the human knee joint in flexion showing cutting planes for a total knee arthroscopy; 
         FIG. 4  is a view of the lateral aspect of the human knee joint of  FIG. 3 ; 
         FIG. 5  is a perspective view of an exemplary gap tensioner; 
         FIG. 6  is a front elevation view of the gap tensioner of  FIG. 5 , in a retracted position; 
         FIG. 7  is a front elevation view of the gap tensioner of  FIG. 5 , in an extended position; 
         FIG. 8  is a front elevation view of the gap tensioner in  FIG. 5 , in an extended and tilted position; 
         FIG. 9  is an exploded perspective view of the gap tensioner of  FIG. 5 ; 
         FIG. 10  is a front elevation view of the gap tensioner of  FIG. 5  showing an internal cable routing path; 
         FIG. 11  is a perspective view of an alternative embodiment of a gap tensioner; 
         FIG. 12  is a perspective view of a linkage of the gap tensioner shown in  FIG. 11 ; 
         FIG. 13  is a schematic top plan view of the gap tensioner of  FIG. 11 , showing an internal cable routing thereof; 
         FIG. 14  is a front elevation view of an alternative gap tensioner linkage in a retracted position; 
         FIG. 15  is a front elevation view of the alternative gap tensioner linkage of  FIG. 14 , in a partially extended position; 
         FIG. 16  is a front elevation view of the alternative gap tensioner linkage of  FIG. 14 , in an extended position; 
         FIG. 17  is a schematic perspective view of an exemplary gap tensioner actuated by a mechanical screw; 
         FIG. 18  is a schematic perspective view of an exemplary gap tensioner having a linkage incorporating variable-rate springs; 
         FIG. 19  is a schematic perspective view of the gap tensioner of  FIG. 18  with a portion removed to show the linkage in more detail; 
         FIG. 20  is schematic perspective view of an alternative spring for use with the gap tensioner of  FIG. 18 ; 
         FIG. 21  is a graph illustrating the stress-strain properties of the ligaments in a human knee joint; 
         FIG. 22  is a perspective view of an exemplary actuating instrument; 
         FIG. 23  is a perspective view of a gap tensioner coupled to the actuating instrument of  FIG. 22 ; 
         FIG. 24  is a side elevation view of the instrument of  FIG. 22  in a retracted position; 
         FIG. 25  is a side elevation view of the instrument of  FIG. 22  in an extended, unloaded position; 
         FIG. 26  is a side elevation view of the instrument in  FIG. 22  in a tensioned position; 
         FIG. 27  is a perspective view of an exemplary actuating instrument; 
         FIG. 28  is a perspective view of a gap tensioner coupled to the actuating instrument of  FIG. 27 ; 
         FIG. 29  is a side elevation view of the instrument of  FIG. 27  in a retracted position; 
         FIG. 30  is a side elevation view of the instrument of  FIG. 27  in an extended, unloaded position; 
         FIG. 31  is a side elevation view of the instrument in  FIG. 27  in a tensioned position; 
         FIG. 32  is a perspective view of an exemplary actuating instrument; 
         FIG. 33  is a perspective view of a gap tensioner coupled to the actuating instrument of  FIG. 32 ; 
         FIG. 34  is a side elevation view of the instrument of  FIG. 32  in a retracted position; 
         FIG. 35  is a side elevation view of the instrument of  FIG. 32  in an extended, unloaded position; 
         FIG. 36  is a side elevation view of the instrument in  FIG. 32  in a tensioned position; 
         FIG. 37  is a perspective view of an exemplary actuating instrument in a retracted position; 
         FIG. 38  is a perspective view of the actuating instrument of  FIG. 37  in an extended position; 
         FIG. 39  is an enlarged view of a portion of  FIG. 37 ; 
         FIG. 40  is a perspective view of an exemplary actuating instrument coupled to a gap tensioner; 
         FIG. 41  is a cross-sectional view of the actuating instrument of  FIG. 40 ; 
         FIG. 42  is a side elevation view of the actuating instrument of  FIG. 40 : 
         FIG. 43  is a perspective view of an exemplary actuating instrument coupled to a gap tensioner, in combination with a remote display; 
         FIG. 44  is a perspective view of an exemplary actuating instrument coupled to a gap tensioner, in combination with a remote display; 
         FIG. 45  is a perspective view of a human knee joint showing an alternative gap tensioner inserted therein; 
         FIG. 46  is a perspective view of the human knee joint showing another alternative gap tensioner inserted therein; 
         FIG. 47  is view of the interior aspect of the human knee joint having a gap tensioner inserted therein; 
         FIG. 48  is a view of the anterior aspect of a human knee joint illustrating the process of augmenting a ligament thereof; 
         FIG. 49  is a view of the anterior aspect of the human knee joint illustrating the process of releasing a ligament thereof; 
         FIG. 50  is a view of the anterior aspect of the human knee joint having a varus angulation; 
         FIG. 51  is a view of the anterior aspect of the joint of  FIG. 50 , showing correction of the varus angulation through release of the medial collateral ligament; 
         FIG. 52  is a view of the medial aspect of the human knee joint having a gap tensioner inserted therein, in combination with a tensile member used to augment or replace the lateral collateral ligament; 
         FIG. 53  is a view of the anterior aspect of the human knee joint shown in  FIG. 52 ; 
         FIG. 54  is a view of the lateral aspect of the human knee joint of  FIG. 52 ; 
         FIG. 55  is a view of the medial aspect of the human knee joint having a gap tensioner inserted therein, in combination with a tensile member used to augment or replace the medial cruciate ligament; 
         FIG. 56  is a view of the anterior aspect of the human knee joint shown in  FIG. 55 ; 
         FIG. 57  a view of the lateral aspect of the human knee joint of  FIG. 55 ; 
         FIG. 58  is a perspective view of a human knee joint having a knee endoprosthetic implanted, in combination with a tensile member used to replace or augment the medial collateral ligament; 
         FIG. 59  is a view of the anterior aspect of the human knee joint, illustrating a first step in a total knee arthroscopy; 
         FIG. 60  is a view of the lateral aspect of the joint of  FIG. 59 ; 
         FIG. 61  is a view of the anterior aspect of the human knee joint of  FIG. 59 , illustrating a second step in a total knee arthroscopy; 
         FIG. 62  is a view of the lateral aspect of the joint of  FIG. 61 ; 
         FIG. 63  is a view of the anterior aspect of the human knee joint of  FIG. 59 , illustrating a third step in a total knee arthroscopy, with a gap tensioner inserted; 
         FIG. 64  is a view of the lateral aspect of the joint  FIG. 59 ; 
         FIG. 65  is a view of the anterior aspect of the human knee joint of  FIG. 52  in flexion, illustrating a fourth step in a total knee arthroscopy, with the gap tensioner inserted; 
         FIG. 66  is a view of the lateral aspect of the joint of  FIG. 65 ; 
         FIG. 67  is a view of the anterior aspect of the human knee joint of  FIG. 59  in flexion, illustrating a fifth step in a total knee arthroscopy; 
         FIG. 68  is a view of the lateral aspect of the joint of  FIG. 67 ; 
         FIG. 69  is a perspective view of the human knee joint of  FIG. 59 , showing a marking device positioned within the joint; 
         FIG. 70  is another view of the medial aspect of the joint in  FIG. 69 , showing the patella in place; 
         FIG. 71  is a perspective view of the joint in  FIG. 69 , showing the patella in place; 
         FIG. 72  is a perspective view of a cutter guide block; 
         FIG. 73  is a view of an anterior aspect of the human knee joint in flexion, with the cutter guide block of  FIG. 72  in place; 
         FIG. 74  is a view of the medial aspect of the human knee joint having a gap tensioner inserted therein; and 
         FIG. 75  is a perspective view of the human knee joint having a gap sensor inserted therein and tracking markers attached thereto; 
         FIG. 76  is a view of the anterior aspect of a human knee joint illustrating an alternative process of augmenting a ligament thereof; 
         FIG. 77  is a view of the anterior aspect of a human knee joint illustrating an alternative process of augmenting a ligament thereof; 
         FIG. 78  is a view of an anterior aspect of the human leg in extension, with tracking markers superimposed thereon; 
         FIG. 79  is a view of a medial aspect of the human leg of  FIG. 78  in flexion; 
         FIG. 80  is a view of a medial aspect of the human leg of  FIG. 78  in flexion, with a gap tensioner inserted in the knee joint and tracking markers attached thereto; 
         FIG. 81  is a view of an anterior aspect of the human leg of  FIG. 80  in extension; 
         FIG. 82  is a schematic perspective view of a human knee joint having a gap tensioner and trial condyle element inserted therein; and 
         FIG. 83  is a schematic perspective view of a human knee joint having a gap tensioner and trial condyle element inserted therein. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIG. 5  depicts an exemplary gap tensioner  10  (alternatively referred to as a “jack”) which is useful for balancing a gap a human knee joint as part of a total knee arthroscopy. In one aspect, the gap tensioner  10  may be described as having the ability to constrain or fix four degrees of freedom of a knee joint while permitting controlled movement of two degrees of freedom. 
     The gap tensioner  10  comprises a baseplate  12  and a top plate  14  interconnected by a linkage  16 . The linkage  16  and the gap tensioner  10  are movable between a retracted position in which the top plate  14  lies close to or against the baseplate  12 , and an extended position in which the top plate  14  is spaced away from the baseplate  12 . As described in more detail below, a mechanism is provided to actuate the linkage  16  in response to an actuating force in order to separate the baseplate  12  and the top plate  14  in a controllable manner. 
     Solely for purposes of convenient description, the gap tensioner  10  may be described as having a length extending along a lateral direction “L”, a width extending along an axial direction “A”, and a height extending along a vertical direction “H”, wherein the lateral direction, the axial direction, and the vertical direction are three mutually perpendicular directions. 
     The baseplate  12  includes a generally planar tibia interface surface  18 . The baseplate  12  may include pin holes  20  for the purpose of receiving alignment pins (not shown) which would be driven into bone during a surgical procedure. The baseplate  12  includes a gap tensioner coupler  22  having a first interface  24 . In the illustrated example, the first interface  24  is configured as a threaded socket. 
     The top plate  14  includes a generally planar femur interface surface  26 . The top plate  14  is mounted to the linkage  16  in such a manner that it can freely pivot about pivot axis  28 . The pivot axis  28  is parallel to the tibia interface surface  18  and the femur interface surface  26 , and in the illustrated orientation is parallel to the axial direction A. The gap tensioner  10  may be configured to permit use with the patella in place. This may be achieved by a careful selection of its dimensions and physical configuration. More specifically, an overall width of the gap tensioner  10  parallel to direction A in  FIG. 5  may be selected to fit between a patella and either the medial colleratal ligament or the lateral collateral ligament so the device can be inserted into the knee joint. Additionally, the gap tensioner coupler is positioned at or near the distal end of the baseplate  12  and is oriented so that it extends along and protrudes in the axial direction “A”. 
       FIGS. 6-8  illustrate the construction and operation of the gap tensioner  10  in more detail. In this embodiment, the linkage  16  is configured as a pair of links  30  each having a lower end  32  and an upper end  34 . The upper ends  34  are joined together such that they can pivot about the pivot axis  28 , thus forming a “X” or “V” configuration. The lower ends  32  are mounted to the baseplate  12  such that they can slide in the lateral direction. Thus assembled, movement of the lower ends  32  away from each other retracts or lowers the height of the gap tensioner  10 , and movement of the lower ends  32  towards each other extends or raises the height of the gap tensioner  10 . Accordingly, in use, an actuating force may be applied to the lower ends  32  to move them towards each other, thus extending the top plate  14  away from the baseplate  12 . The linkage  16  has predetermined kinematic properties, or stated another way, the ratio of displacement of the top plate  14  to input displacement is known and can be plotted a graph, for the entire range of motion. 
       FIG. 6  illustrates the gap tensioner  10  in the retracted position, and  FIG. 7  illustrates the gap tensioner in the extended position. In all positions, the top plate  14  is free to pivot about the pivot axis  28 .  FIG. 8  shows an example of the top plate  14  in a “tilted” position, or stated another way, with the femur interface surface  26  not parallel to the tibia interface surface  18 . 
       FIG. 9  illustrates a structure of the linkage  16  in more detail. In particular it shows the links  30  as well as a pivot pin  35  which serves to connect the upper ends  34  of the links  30  as well as to join the links  30  in a pivoting connection to the top plate  14 . 
     In this embodiment, the lower end  32  of each link  30  has a roller  36  mounted thereto. The rollers are received in tracks  38  formed in the baseplate  12 . This permits low-friction operation of the linkage  16 . 
     Various means are possible for applying an actuating force to the linkage  16 . In the example shown in  FIGS. 9 and 10 , the linkage  16  is cable-actuated. One of the links  30  incorporates a cable anchor recess  40  which receives a first end of the cable (not seen in  FIG. 9 ). The cable  42  is routed through the interior of the baseplate  12 , exiting the gap tensioner coupler  22 , as seen in  FIG. 10 . Thus routed and assembled, a tensile actuating force applied to the cable  42  will move the lower ends  32  of the links  30  closer together. 
       FIGS. 11-13  illustrate another exemplary gap tensioner  110 . The gap tensioner  110  is similar in overall construction to the gap tensioner  10  described above. Elements of the gap tensioner  110  not explicitly described may be considered to be identical to corresponding elements of the gap tensioner  10 . 
     The gap tensioner  110  includes a baseplate  112 , top plate  114 , and a linkage  116 . In this embodiment, the linkage  116  is configured as a pair of links  130  each having a lower end  132  and an upper end  134 . The lower ends  132  are mounted to the baseplate  112  such that they can slide in the lateral direction. The lower ends  132  of the links  130  are received in sliders  144  which are in turn received in slots or tracks (not visible) and the baseplate  112 . 
       FIG. 13  illustrates cable routing within the baseplate  112 . In this example, two separate cables  142  are provided, one cable being terminated in each of the sliders  144 . 
       FIGS. 14-16  illustrate an alternative linkage  216  comprising links  230 , which may be substituted for the linkages described above. In this linkage  216 , one of the links  230  includes a cam  246  around which a cable  242  is wrapped. The cam  246  is shaped such that as the linkage moves from a retracted position ( FIG. 14 ) towards an extended position ( FIG. 16 ), the force-versus-displacement characteristics of the linkage  216  change. Stated another way, the effective leverage of the cable  242  changes as the linkage  216  moves through its range of motion. For example, the cam may be shaped such that a greater force is required to provide a given deflection as the device moves towards extended position. 
       FIG. 17  illustrates another example of a gap tensioner  310 . The gap tensioner  310  is similar in overall construction to the gap tensioner  110  described above. Elements of the gap tensioner  310  not explicitly described may be considered to be identical to corresponding elements of the gap tensioner  110 . 
     The gap tensioner  310  includes a baseplate  312 , top plate  314 , and a linkage  316 . In this example, the linkage  316  is configured as a pair of links  330  each having a lower end received in a slider  344  which is in turn mounted for sliding movement in the baseplate  312 . A linear actuating element  348  such as the illustrated threaded shaft is mounted in the baseplate configured such that rotating movement of the actuating element  348  causes lateral sliding of the sliders  344 , in turn actuating the linkage  316 . 
       FIGS. 18 and 19  illustrate another exemplary gap tensioner  410 . The gap tensioner  410  is similar in overall construction to the gap tensioner  10  described above. Elements of the gap tensioner  410  not explicitly described may be considered to be identical to corresponding elements of the gap tensioner  10 . 
     The gap tensioner  410  includes a baseplate  412 , top plate  414 , and a linkage  416 . The linkage  416  is configured as a pair of links  430  each having a lower end  432  pivotally connected to the baseplate  412 . Upper ends  434  of the links  430  are pivoted to each other and to the top plate  414 . Each of the links  430  is a telescoping assembly and is provided with one or more springs  450  which are arranged so as to urge the linkage  416  towards an extended position. The springs  450  may be configured to have a variable rate. In one example, the springs  450  and/or the geometry of the associated link  430  may be arranged to have a constant force-displacement characteristic. Stated another way, a force acting in the extension direction may be constant or substantially constant regardless of the position of the top plate  414 . In this example, no actuating force is required to operate the device. To the contrary, the device may be compressed, placed in the working position, and then released to apply a working force. 
       FIG. 20  illustrates an alternative link  431  which may be substituted for the links  430 . This link  431  has a plurality of spring members  433  formed therein and is a single integral, monolithic, or unitary element which serves as both a telescoping link and a spring. 
     As noted above, the gap tensioner  10  is useful for balancing the gap in a human knee joint when performing a total knee arthroscopy. The use of the gap tensioner  10  may be better understood by considering the characteristics of the human knee joint, particularly of the soft tissue (e.g. ligaments).  FIG. 21  is a representative diagram of knee joint gap height versus applied extension load, similar to a stress-strain plot. In  FIG. 21 , the solid line is representative of properties of a perfectly elastic member (e.g. a rubber band). The dashed line is representative of the properties of a hypothetical infinitely rigid member. The dotted line is representative of the properties of a human knee joint ligament. It can be seen that the ligament is quite stiff and exhibits a low elongation to failure. The vertical portion of the dotted line indicates the range of motion where a minimal applied load will take up all available slack in the ligament. The slope of the gap height/load curve then rapidly transitions through the arcuate corner in the dotted line, to a very rigid characteristic. Given these properties, it will be apparent that the application of a relatively small load will ensure that the ligament is at full extension. In one example, an extension load of about 300 N or less may be applied. It will be understood that the chart in  FIG. 21  is general in nature, and that specific ligaments in specific joints may have different magnitudes of slack available, or stated another way, the length of the vertical segment of the dotted line will vary from joint to joint and ligament to ligament. For example, in one patient&#39;s knee joint, all slack may be taken up at a relatively small gap height such as 9.5 mm. In another patient&#39;s knee joint, all slack may be taken up at a relatively larger height such as 20 mm. 
     Numerous instruments may be provided which are suitable for applying actuation loads of this magnitude to the gap tensioner  10 , as well as indicating, measuring, or recording physical properties of the gap tensioner  10  such as position, applied load, and/or tilt position. 
       FIGS. 22-26  illustrate an exemplary actuating instrument  500  for use with the gap tensioner  10 . The actuating instrument  500  includes a barrel  502 . The distal end of the barrel  502  includes an instrument coupler  504  defining a second interface  506  complementary to the first interface  24  of the gap tensioner  10 . In the illustrated example, the second interface  506  is configured as external threads. 
     The proximate end of the barrel  502  is connected to an actuating assembly  508  including a handle  510 , a lever  512 , and actuating linkage  514 , and a load setting mechanism  516 . 
     The actuating instrument  500  is configured to be coupled to the gap tensioner  10  by joining their mutual couplers  22 ,  504 , to receive the cable  42  as described above (not shown), and to apply an actuating load, that is a tensile load, to the cable  42 , thus actuating the gap tensioner  10 . 
     The actuating instrument  500  may include some means for measuring or indicating displacement of the gap tensioner  10 . In the illustrated example, the handle  510  carries a movable pointer  518  which pivots relative to a scale  520 . The pointer  518  is arranged to contact or otherwise be driven by the cable in operation, thus driving pointer movement. The scale  520  may be calibrated to directly indicate the “gap height” (i.e. the distance between the tibial and femoral surfaces  18 ,  26 ) of the gap tensioner  10 . 
     The lever  512  is pivoted to the handle  510  and coupled to the actuating linkage  514  and the load setting mechanism  516 . Operation of the lever  512  causes the linkage  514  to apply tensile force to the cable  42  (not shown). The actuating force is applied through a spring element  522  which is a portion of the load setting mechanism  516 . Preload of the spring element  522  may be set using an adjuster  524  such as the illustrated threaded knob. Accordingly, there is a definite adjustable force-displacement characteristic of the actuating instrument  500 . The actuating linkage  514  has predetermined kinematic properties, or stated another way, the ratio of displacement of the cable  42  to input displacement of the lever  512  is known and can be plotted a graph, for the entire range of motion. The kinematic properties of the actuating linkage  514  can be configured to have a predetermined relationship to the kinematic properties of the linkage  16  of the gap tensioner  10  described above. In one specific example the kinematic properties of the actuating linkage  514  may be configured to have an inverse relationship to the kinematic properties of the linkage  16 . That is, the ratios of input to output displacement for the linkage  514  and the linkage  16  would be inverse to each other for each position in the range of movement. This would result in a 1:1 output/input displacement ratio for the entire mechanical system. This may be referred to as the actuating instrument  500  and the gap tensioner  10  having “inverse kinematics” relative to each other. With such a relationship, the actuating instrument  500  would provide in essence no mechanical advantage. This has the result that a unit deflection of the lever  512  results in a unit deflection of the top plate  14 , and a unit force applied to the lever  512  results in an equal unit force being applied to the gap tensioner  10 . 
     Operation of the actuating instrument  500  is explained in more detail with reference to  FIGS. 24-26 .  FIG. 24  shows the actuating instrument with the lever  512  in a released position.  FIG. 25  shows the lever  512  in an intermediate position in which the cable is displaced and the gap tensioner  10  is partially extended, but no appreciable load is applied to the gap tensioner  10 , other than overcoming friction and other minor forces. This corresponds generally to the vertical segment of the dotted line graph in  FIG. 21 . 
       FIG. 26  shows the lever  512  in a fully actuated position in which the cable is not displaced any further appreciable amount, but an actuating load (i.e. tension) is applied to the cable. It will be understood that the absolute position of the lever  512  relative to the body  510  when full preload is applied to the gap tensioner  10  will vary depending on the actual gap height. The load setting mechanism  516  allows the actuating load to be accurately displayed and/or controlled. For example, observation or measurement of the displacement of the spring element  522 , with the spring rate being known, gives the force being applied. Alternatively, the adjuster  524  may be used to set the preload on the spring element  522  such that the desired actuating load is required to be applied in order to bring the spring element  522  to a known position, for example a fully compressed position. 
       FIGS. 27-31  illustrate another exemplary actuating instrument  600  for use with the gap tensioner  10 . The actuating instrument  600  includes a barrel  602  with an instrument coupler  604  at its distal end. The proximate end of the barrel  602  is connected to an actuating assembly  608  including a handle  610 , a lever  612 , actuating linkage  614 , and a load setting mechanism  616 . The lever  612  carries a movable pointer  618  which pivots relative to a scale  620  in order to indicate displacement. 
     Operation of the actuating instrument  600  is similar to that of the actuating instrument  500 . As seen in  FIGS. 29-31 , the instrument  600  can be moved from a released position, through an extended but unloaded position, and finally to a fully actuated, loaded position as shown in  FIG. 31 . 
       FIGS. 32-36  illustrate another exemplary actuating instrument  700  for use with the gap tensioner  10 . The actuating instrument  700  includes a barrel  702  with an instrument coupler  704  at its distal end. The proximate end of the barrel  702  is connected to an actuating assembly  708  including a body  710 , a handle  712 , actuating linkage  714 , and a load setting mechanism  716 . The device may include a movable indicator  718  which shows displacement against a scale  720 . The linkage  714  is directly actuated by pressure of the load setting mechanism  716  which is set by the position of the handle  712 . In this example, the handle  712  is a threaded member connected to a threaded rod. 
     The actuating instrument  700  is operated by turning the handle  712 , compressing the spring element  722  of the load setting mechanism  716 , thus applying force to the linkage  714 , which is translated to tension applied to the cable (not shown). Operation of the actuating instrument. As seen in  FIGS. 34-36 , the handle  712  can be rotated to vary compression of the spring element  722 , moving the actuating instrument  700  from a released position, through an extended but unloaded position, and finally to a fully actuated, loaded position as shown in  FIG. 36 . 
       FIGS. 37-39  illustrate another exemplary actuating instrument  800  for use with the gap tensioner  10 . The actuating instrument  800  includes a barrel  802  with an instrument coupler  804  at its distal end. The proximate end of the barrel  802  is connected to an actuating assembly  808  including a handle  810 , a lever  812 , actuating linkage  814 , and a load setting mechanism  816 . Operation of the actuating instrument  800  is similar to that of the actuating instrument  500 . 
       FIGS. 40-41  illustrate another exemplary actuating instrument  900  for use with the gap tensioner  10 . The actuating instrument  900  includes a barrel  902  with an instrument coupler  904  at its distal end. The proximate end of the barrel  902  is connected to a housing  908  including a handle  910 , an operating knob  912 , and a load setting mechanism  916 . 
     The load setting mechanism  916  includes a spring element  922  having a first end  952  configured to be coupled to a cable (not shown) and a second end  954  connected to threaded plug  956 . The threaded plug  956  engages complementary threads of the operating knob  912 . Rotation of the operating knob  912  causes a tensile load to be applied to the spring element  922 . The tensile load is proportional to the displacement of the operating knob  912 . As seen in  FIG. 42 , the housing  908  may be marked with a scale  958  which shows the applied actuating load for measured gap sizes. 
       FIG. 43  illustrates another exemplary actuating instrument  1000  for use with the gap tensioner  10 . The actuating instrument  1000  includes a barrel  1002  with an instrument coupler  1004  at its distal end. The proximate end of the barrel  1002  is connected to an actuating assembly  1008  including a lever  1012 . The interior of the actuating assembly  1008  includes an appropriate mechanism (not shown) such as an actuating linkage similar to those described above. The internal mechanism is operable to apply an actuating load to the gap tensioner  10  in response to movement of the lever  1012 . The actuating instrument  1000  includes an electronic data transmitter, shown schematically at  1060 . The transmitter  1060  may operate over a wired or wireless connection. The actuating instrument  1000  and/or the tensioner  10  are supplied with an appropriate combination of transducers (not shown) to detect physical properties such as force, tilt angle, and/or applied load and generate a signal representative thereof. For example, the tensioner  10  may be provided with sensors operable to detect the magnitude of extension (i.e. “gap height”), the angle of the top plate about the pivot axis (i.e. varus/valgus), and/or the applied force in the extension direction. Nonlimiting examples of suitable transducers include strain gages, load cells, linear variable differential transformers (“LVDT”), rotary variable differential transformers (“RVDT”), or linear or rotary encoders or resolvers. (Alternatively, the gap tensioner  10  may be provided with simple visual scales, not shown, for displacement/gap height and/or tilt angle, or may include a mechanical linkage, not shown, which can transmit movement representative of tilt angle to a mechanical or electronic actuating instrument). The transmitter  1060  is operable to transmit the signal. A remote display  1062  is configured to receive the signal and produce a display  1064  of the transducer data. As one example, the remote display  1062  may be embodied in a conventional portable electronic device such as a “smart phone” or electronic tablet with suitable software programming. 
     In use, the remote display  1062  permits the surgeon to observe the physical properties of the gap tensioner  10  in real time as the actuating instrument  1000  is used to operate the gap tensioner  10 . 
       FIG. 44  illustrates another exemplary actuating instrument  1100  for use with the gap tensioner  10 . The actuating instrument  1100  includes a barrel  1102  with an instrument coupler  1104  at its distal end. The proximate end of the barrel  1102  is connected to an actuating assembly  1118 . The interior of the actuating assembly  1118  includes an appropriate driving mechanism such as an electrically-powered linear actuator  1114 . The driving mechanism  1114  is operable to apply an actuating load to the gap tensioner  10 , through cable  42 . The actuating instrument  1100  includes an electronic data transmitter, shown schematically at  1160 , and may include an appropriate electrical power source such as a battery (not shown). The transmitter  1160  may operate over a wired or wireless connection. The actuating instrument  1110  and/or the gap tensioner  10  are supplied with an appropriate combination of transducers as described above with respect to actuating instrument  1000 , such as force transducer  1116 , to detect one or more physical properties of the gap tensioner  10  and generate a signal representative thereof. The transmitter  1160  is operable to transmit the sensor signal. A remote display  1162  is configured to receive the signal and produce a display  1164  of the transducer data. As one example, the remote display  1162  may be embodied in a conventional portable electronic device such as a “smart phone” or electronic tablet with suitable software programming.  
     In use, the remote display  1162  permits the surgeon to observe the physical properties of the gap tensioner  10  in real time as the actuating instrument  1100  is used to operate the gap tensioner  10  Optionally, the actuating instrument  1100  may incorporate a tracking marker  1161 . It includes one or more tracking points (not individually illustrated) which may be configured as transmitting antennas, radiological markers, or other similar devices. Using an appropriate receiving device, described in more detail below, the position and orientation of the receiving device to the tracking marker  1161  may be determined by receipt and analysis at the receiving device of signals transmitted by the tracking marker  1601 . 
     In the example described above, the gap tensioner  10  is intended to be used for a total knee arthroscopy and is sized and shaped to be inserted into the human knee joint into span the entire gap across both condyles. Other configurations are possible. For example,  FIG. 45  shows an alternative gap tensioner  1210  inserted between the tibia T and femur F of a human knee joint J. It can be seen that one condyle of the tibia has been cut away and that the gap tensioner is sized and configured to be inserted into the gap above the cut-away condyle, from the medial aspect of the joint. 
       FIG. 46  shows another alternative gap tensioner  1310  which has one movable top plate  1314  positioned under one condyle and one fixed block  1315  positioned under the other condyle. 
     The gap tensioner  10  is especially useful for adjusting the soft tissue lateral tension balance of a human knee joint. Referring to  FIG. 47 , a gap tensioner  10  is shown inserted in an extension gap EG between the femur F and the tibia T. The gap tensioner  10  pivots freely about pivot axis  28  as described above. The tilt angle of the top plate  14  may be manipulated by selective augmentation and/or release of the lateral collateral ligament or the medial colleratal ligament.  FIG. 48  illustrates a process of augmentation in which an artificial tensile member  1400  secured with anchors  1402  is passed through the femur F and tibia T, spanning the lateral aspect of the extension gap. The term “anchor” as it relates to element  1402  refers to any device which is effective to secure a tensile member  1400  passing therethrough. Nonlimiting examples of anchors  1402  and include washers, screw plates, ferrules, and swage or crimp anchors. Properly tensioned, this tensile member  1400  replaces or augments tension provided by the natural lateral colleratal ligament. The augmentation shown in  FIG. 48  is trans-osseous, but other forms of augmentation are possible. For example,  FIG. 76  illustrates a process of augmentation in which an artificial tensile member  1401  secured with anchors  1403  spans the lateral aspect of the knee joint J. The tensile member  1401  may have both ends anchored directly to the femur F and tibia T. Alternatively, as seen in  FIG. 77 , the tensile member  1401  may have its ends anchored to the components of a knee arthroplasty  1407 , which components are in turn anchored to the femur F and tibia T. As yet another alternative (not shown), one end of a tensile member may be anchored directly to the femur F or tibia T, with the other end indirectly anchored, for example it may be anchored to an arthroplasty element which is in turn anchored directly to the opposite bone of the joint. As yet another alternative, a tensile member may be anchored directly or indirectly to the fibula.  FIG. 49  illustrates a process of release in which a ligament (in this example the lateral collateral ligament LCL) is partially severed to release tension thereon. Either action, augmentation or release, would change the balance of tension acting on the joint J and thus change the tilt angle (varus or valgus). 
       FIGS. 50 and 51  illustrate how the tilt angle (varus or valgus) may be manipulated.  FIG. 50  shows the joint J having a varus angulation. It can be seen that central axes  1404 ,  1406  of the tibia T and femur F, respectively are not coaxial, but define an oblique angle therebetween.  FIG. 51  shows a result of releasing the MCL, i.e. effectively lengthening it and/or releasing its tension. It can be seen that the central axes  1404 ,  1406  are coaxial. This type of adjustment may use any combination of augmentation and/or release and may be used to correct varus or valgus angulation. 
     Various methods are known for augmentation of the soft tissues. As noted above, one method involves the use of an artificial tensile member such as a suture, cable, or filament, suitably anchored in tension. Examples of this type of device are illustrated in  FIGS. 52-57 . 
       FIGS. 52-54  illustrate a tensile member  1400  fixed by anchors  1402  and routed through the human knee joint J across the lateral aspect of the extension gap e.g. in order to replace or augment the lateral collateral ligament (not shown). 
       FIGS. 55-57  illustrate a tensile member  1400  fixed by anchors  1402  and routed through the human knee joint J across the medial aspect of the extension gap e.g. in order to replace or augment the medial collateral ligament (not shown). 
       FIG. 58  illustrates a human knee joint J having an endoprosthetic  1408  of a known type implanted therein. The endoprosthetic  1408  includes a tibial component  1410  and the femoral component  1412 . The joint J also includes a tensile member  1400  fixed by anchors  1402  and routed through the knee joint J across the medial aspect of the extension gap in order to replace or augment the medial collateral segment (not shown). 
     A method for using the gap tensioner an instrument will now be described with reference to  FIGS. 59-73 . 
     Initially,  FIGS. 59, 60 , a tibial cut (cutting plane labeled  1 ) is made in the tibia T. This may be done using conventional techniques. Ideally, the tibial cut  1  makes a surface that is normal to vertical in the coronal plane and at a slight angle (e.g. 0° to 7°) to vertical in the sagittal plane. 
     In a second step,  FIGS. 61, 62  a distal femoral cut (cutting plane labeled  2 ) is made in the femur F. This may be done using conventional techniques. Ideally the distal femoral cut  2  makes a surface that is normal to a “perfect” anatomical mechanical axis between the center of the knee and the femoral head. 
     In the third step,  FIGS. 63, 64 , the gap tensioner  10  is inserted between the femur F and the tibia T and used to conduct a soft-tissue balancing procedure. The gap tensioner  10  is moved towards an extended position which has the effect of driving the femur F and the tibia T apart from each other, defining the extension gap EG. The gap tensioner  10  is conformable to lateral angulation. That is, the free pivoting of the top plate  14  about the pivot axis  28  permits the knee joint J to take up whatever varus or valgus angulation naturally occurs. 
     The specific varus or valgus angulation will be governed by the relative lengths of the medial collateral ligament and the lateral collateral ligament. The extension of the gap tensioner drives both of these ligaments to their full extension. The preload of the tensioner  10  provides a margin to ensure full extension. 
     Once desired extension, for example full extension. of both ligaments is achieved, the lateral angulation (varus or valgus) can be observed, measured, and/or recorded. Measurement may be by various means. In one example, dimensions and angles may be measured directly using measuring instruments. 
     Once the lateral angulation is determined, the extension gap EG may be balanced. To balance extension gap, soft-tissue is augmented and/or soft-tissue is released, using the procedure described in detail above with reference to  FIGS. 47-51 . In the example shown in  FIGS. 61 and 62 , the knee joint J exhibits valgus angulation (knock-knee) when loaded by the gap tensioner  10 . Correction may be performed by augmenting the medial collateral ligament and/or releasing the lateral collateral ligament. The balance procedure is done until the extension gap EG is uniform (i.e. no varus or valgus angulation). 
     Once the extension gap EG has been balanced, resulting in the condition shown in  FIGS. 63 and 64 , they gap tensioner  10  may be used to establish the flexion cuts. 
     Referring to  FIGS. 65 and 66 , the gap tensioner  10  is placed in the flexion gap FG and tensioned to the same load as used to establish the extension gap EG. This may be carried out with the patella and/or patellar tendon (not shown) in place (i.e., the patella is not required to be everted and may be in the normal anatomical position). 
     Once full extension of both ligaments is achieved, the characteristics of the flexion gap FG (magnitude and angulation) can be observed, measured, and/or recorded. The soft tissue is not altered in this step. It will be understood that the magnitude (height) and/or tilt of the flexion gap FG are likely to be different from the extension gap EG. It will be further understood that is desirable for the flexion gap FG to be parallel and equal to the extension gap EG. In the example shown in  FIGS. 65 and 66 , the knee joint J exhibits valgus angulation (knock-knee) when loaded by the gap tensioner  10 . Furthermore, since the posterior cut has not yet been made, the flexion gap FG is smaller than the final required amount. It can be seen from  FIG. 65  that in order to create a balanced flexion gap FG, the posterior cut must remove the different amounts of material from the two condyles of the femur F. 
     The posterior cut (cutting plane labeled  3 ) is then made so as to create the desired (balanced) flexion gap FG′, as seen in  FIGS. 66 and 67 . The gap tensioner  10  may be utilized to accurately mark the desired cutting plane. Referring to  FIGS. 69-71 , the gap tensioner  10  is left inserted into the flexion gap FG, maintaining tension on the soft tissue while references are marked for the posterior cut. 
     In the example shown in  FIGS. 69-71 , a marking attachment  1500  is provided which includes a body  1502  of a predetermined height extending between first and second ends  1504 ,  1506 . The first end  1504  is provided with means for attachment to the base plate  12  of the gap tensioner  10 . For example, a dovetail joint (not shown) may be used. The second end  1506  is provided with suitable marking implement such as the illustrated two spaced-apart marking tips  1508  positioned along the line parallel to the tibial surface  18  of the base plate  12 . The marking attachment  1500  may further include a handle  1510  to allow for surgical manipulation. 
     The marking attachment  1500  is used by attaching it to the baseplate  12  with the joint J in flexion and then using the marking tips  1508  to strike or impress two indentations which serve as a reference for mounting of a cutter guide block described below. This may be carried out with the patella “P” in place (not everted), as seen in  FIGS. 70 and 71 . It will be understood that the geometry of the marking attachment  1500  ensures that the two indentations lie on a line parallel to the tibial cut in the tibia T. Accordingly, this provides a basis for making a posterior cut which is automatically ensured to be parallel to the tibial cut. 
       FIG. 72  illustrates a suitable cutter guide block  1520  of a known type. The cutter guide block  1520  includes, among other features, a posterior cut guide surface  1524  and at least one pair of space-apart guide holes  1526 . The guide holes  1526  are configured to receive guide pins or screws (not shown) which may be driven into bone to hold the cutter guide block  1520  in position while the posterior cut guide surface  1524  is used to guide the blade of a bone saw (not shown). Fundamentally, in order to produce the desired cut, the posterior cut guide surface  1524  is placed parallel to and coplanar with or slightly spaced away from the posterior cut plane  3 . 
     It will be understood that the guide holes  1526  lie along a line which is parallel to the posterior cut guide surface  1524 , at a known distance from the posterior cut guide surface  1524 . Accordingly, in order to accurately position the cutter guide block  1520 , it is a straightforward matter to select the height of the marking attachment  1500 , and thus the position of the marking tips  1508 , taking into account the distance between the guide holes  1526  and the posterior cut guide surface  1524 , and the desired final height of the flexion gap FG.  FIG. 73  shows the cutter guide block  1520  in-place against the femur F, ready to make the posterior cut  3 . 
     Once a posterior cut  3  is made, the knee joint J as a balanced flexion gap FG which matches the extension gap EG. Subsequently, conventional steps may be carried out to complete the total the arthroscopy, such as making chamfer cuts, trial fitting the endoprosthetic components, and cementing the endoprosthetic components. 
     The devices described above have additional usefulness in related surgical procedures, particularly in determining the proper bone entry points for artificial tensile members used to augment the natural ligaments.  FIG. 74  is a view of the medial aspect of the human knee joint J having a gap tensioner  10  inserted therein. The view shows the joint with three different positions of the femur F superimposed—full extension labeled 0°, an intermediate position labeled 45°, and full flexion labeled 90°. Shown also on the femur F is a curve  1600  which represents the locus of the instantaneous axis of rotation (“IAOR”) of the joint J for each position. The IAOR changes throughout the joint range of motion. This happens because during joint motion the femoral condyle translates in the anterior/posterior directions and the femoral condyle rides against the tibia in a cam motion. 
     For best surgical outcomes, it is preferable to route an artificial tensile member through a hole in the femur F passing through curve  1600 . The exact location of this curve  1600  can be difficult to determine using prior art methods. The apparatus described herein can provide a method for accurately locating this curve to serve as a drilling target. 
     In one example, the location method may be carried out using the instrument  1100  described above. As noted above, the instrument  1100  may include appropriate sensors for determining the extension load, the varus/valgus tilt angle, and the gap height. In order to locate the curve  1600 , the instrument  1100  would be coupled to a gap tensioner  10  inserted into the knee joint J between the tibia T and the femur F, after making the tibial cut but prior to making the distal femoral cut. In one option, a predetermined extension load would be applied by the instrument  1100 . The joint J would then be moved through the range of full extension to full flexion, while using the sensors to determine the gap height and varus/valgus angle in each location within the range of motion. This data may be translated through empirical means to derive the location of the curve  1600 . 
     Optionally, this method may be performed after the tibial and femoral cuts have been made, by providing a trail condyle element of an arthroplasty. For example,  FIG. 82  illustrates a trial condyle element  1800  inserted between the femur F and the gap tensioner  10 . This configuration permits a continuous range of joint motion after the distal femoral cut is made. As another example,  FIG. 83  illustrates a trial condyle element  1802  inserted between the femur F and the gap tensioner  10 . This configuration permits a continuous range of joint motion after the distal femoral and posterior formal cuts are made. 
     In another option, the instrument  1100  will be controlled so as to provide a fixed gap height. The joint J would then be moved to the range of full extension of full flexion, while using the sensors to determine the change in extension load and the varus/valgus angle in each location within the range of motion. This data may be translated through empirical means to derive the location of the curve  1600 . 
     In a related method, described with respect to  FIG. 75 , the instrument  1100  described above may be used with other apparatus not only to derive the location of the curve  1600  but to guide the surgeon to drill a hole in a proper location along the curve. The instrument  1100  would be coupled to the gap tensioner  10  inserted into the knee joint J between the tibia T in the femur F, after making the tibial cut prior to making the distal femoral cut. A tracking marker  1602  would be attached to the tibia T. The tracking marker  1602  is attached to the tibia T in such a way that it has a substantially fixed position and orientation relative to the tibia T. It includes one or more tracking points  1604  which may be configured as transmitting antennas, radiological markers, or other similar devices. Using an appropriate receiving device such as the illustrated instrumented, receiver-equipped cordless drill  1606 , the position and orientation of the cordless drill  1606  relative to the tracking marker  1602  may be determined by receipt and analysis at the cordless drill  1606  of signals transmitted by the tracking marker  1602 . Tracking marker  1602  and appropriate receivers are known within the state-of-the-art. Additionally, a second tracking marker  1608  would be attached to the femur F in such a way that as a substantially fixed position orientation relative to the femur F. Again, the position orientation of the cordless drill  1606  relative to the tracking marker  1608  may be determined. 
     Once the gap tensioner  10 , actuating instrument  1100 , and tracking markers  1602  and  1608  are implanted, the joint J would then be moved to the range of full extension of full flexion, while monitoring the position of tracking markers  1602  and  1608 . The path swept out by the tracking marker  1602  and  1608  is representative of the movement of the condyle of the femur against the gap tensioner  10 . The data representing the path may be translated using empirical means to determine the position of the curve  1600 . 
     Once a position of the curve  1600  is determined, the tracking markers  1602  and  1608  may be used to guide the cordless drill  1606  to drill a hole passing through the curve  1600 , with the drill bit  1610  extending an appropriate angle. This guidance is possible because intercommunication between the cordless drill  1606  and the tracking marker  1602  and  1608  will give the relative to position and orientation of the cordless drill  1606  to those markers. The drilling guidance may be provided in the form of information displayed on the remote display  1162  described above. For this purpose, 2-way data communications may be provided between and among the cordless drills  1606  (or other surgical instrument), the tracking markers  1602  and  1608 , the actuating instrument  1100 , and the remote display  1162 . 
     This method is especially helpful in providing drilling guidance because it provides the benefits of a surgical navigation system, which is typically large, complex, and expensive, using simple inexpensive local relative position information. For example, the absolute position and orientation of the knee joint J is not required to perform the step of moving the joint J through the range of motion and then guiding the cordless drill  1606  to drill a hole at the appropriate location and orientation. 
     In another related method, described with respect to  FIGS. 78-81 , the instrument  1100  described above may be used in combination other apparatus to improve accuracy of alignment of the knee joint J.  FIG. 78  illustrates the knee joint J between the femur F and tibia T, the talus joint “U” between the tibia T and the foot “O”, and the acetabulofemoral joint “B” between the femoral condyle “C” and the pelvis “P”.  FIG. 78  also illustrates an axis  1405  passing through the femoral condyle C and the talus joint U. The ability to measure lateral position of the knee joint J relative to the axis  1405  is helpful in determining angulation (i.e. varus/valgus) of the knee joint J. For purposes of this measurement, the center of the knee joint J and the lateral sense is considered to be the midpoint between the medial and lateral condyles. For example, in general a nominal varus/valgus angulation occurs when the center of the knee joint J is coincident with the axis  1405 . Different patients may exhibit variations in the nominal position. Furthermore, a surgically desirable varus/valgus angulation may be different from nominal this may be done, for example to correct a defect in the patient&#39;s anatomy. 
     Various methods are possible for establishing the location of the axis  1405 . One method involves the use of an atomic old tracking markers or “navigation” systems.  FIG. 76  illustrates schematically a first tracking marker  1700  disposed at the center of the femoral condyle C, a second tracking marker  1702  located at the center of the knee joint J, and a third tracking marker  1704  located at the center of the talus joint U. The tracking markers  1700 ,  1702 ,  1704  are representative of the position of the associated anatomical structures. In practice, the tracking markers may be physical or virtual. For example, surgical navigation systems are commercially available which have the capability of measuring the position of hardware tracking markers and/or generating virtual tracking markers through the use of medical imaging methods. 
     Once the tracking markers  1700 ,  1702 ,  1704  are established, the knee joint J may be placed in flexion as shown in  FIG. 79 . The complete leg may then be moved laterally to the left and the right alternatively, thus generating a position track for each of the tracking markers  1700 ,  1702 ,  1704 . This movement will cause rotation of the leg about the axis  1405  which intersects the first tracking marker  1700  and the third tracking marker  1704 . The axis  1405  may be analytically constructed, for example by observing that the first and third tracking marker  1700 ,  1704  exhibit little to no movement while the second tracking marker  1702  sweeps out an arc. Computation will show the center of this arc lies on the axis  1405 . 
     The function of the tracking markers  1700 ,  1702 ,  1704  may be replaced in whole or in part with local, relative navigation devices such as the tracking markers described above. This is shown in  FIGS. 81 and 81 . In this example configuration, a tracking marker  1608  (described above) is attached to the femur F in such a way that as a substantially fixed position orientation relative to the femur F. 
     Another tracking marker  1708  is coupled to the talus joint U in such a way that as a substantially fixed position orientation relative to the femur F. For example, it may be coupled t the talus joint U using a C-shaped ankle clip or ankle clamp  1710  which may be resilient, spring-loaded, etc. 
     Finally, the gap tensioner  10  is inserted into the knee joint J and coupled to the actuation instrument  1100  having a tracking marker  1161  as described above. Alternatively, the gap tensioner  10  may be provided with a built-in tracking marker  1163 . 
     A receiving device such as remote display  1062  is configured to receive the signals and or otherwise track the positions of the tracking markers  1161 ,  1608 , and  1708  and to store, manipulate, and/or display the position data. 
     Once the gap tensioner  10 , actuating instrument  1100 , and tracking markers  1161 ,  1608 , and  1708  are in place, the leg would then be moved to a flexion position. The complete leg may then be moved laterally to the left and the right alternatively, while monitoring the position of tracking markers  1161 ,  1608 , and  1708 , thus generating a position track for each of the tracking markers  1161 ,  1608 , and  1708 . The axis  1405  may be analytically constructed, for example by observing that the tracking markers  1161 ,  1608  exhibit little to no movement while the tracking marker  1708  sweeps out an arc. Computation will show the center of this arc lies on the axis  1405 . 
     Once a position of the axis  1405  is determined, the leg can be placed back in extension and tracking markers  1602  and  1608  may be used to measure lateral position of the knee joint J relative to the axis  1405 . As noted above, this information is helpful in determining angulation (i.e. varus/valgus) of the knee joint J. 
     The apparatus and method described herein have numerous advantages over prior art apparatus and techniques. 
     The gap tensioner enables patella-in-place gap balancing during total knee arthroplasty. By allowing the patella (and other soft tissue around the knee space) to remain in its anatomical position during the balancing procedure, a more accurate and anatomically relevant gap can be established. 
     Furthermore, due to its non-intrusive nature, the gap tensioner can enable in-situ gap balancing by means of soft tissue releases (to open one side of the gap relative to the other to make it more “rectangular” and less “trapezoidal”) and tension ligament augmentation (to close one side of the gap by tightening or augmenting ligaments to make it more “rectangular” and less “trapezoidal”). 
     The foregoing has described apparatus and methods for knee gap tensioning. All of the features disclosed in this specification, and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. 
     Each feature disclosed in this specification may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. 
     The invention is not restricted to the details of the foregoing embodiment(s). The invention extends, or to any novel one, or any novel combination, of the steps of any method or process so disclosed.