Patent Publication Number: US-11642118-B1

Title: Knee tensioner-balancer and method

Description:
BACKGROUND 
     This invention relates generally to medical devices and instruments, and more particularly to a tensioner-balancer for a knee joint and methods for its use. 
     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  FIGS.  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 femoral cut  2  cooperate to define an extension gap “EG”.  FIGS.  3  and  4    show the joint J in flexion, with a cutting plane  3  shown for a posterior cut. The tibial cut  1  and the posterior cut  3  cooperate to define a flexion gap “FG”. 
       FIG.  5    depicts an exemplary endoprosthesis  10  (i.e., implant) of a known type. The endoprosthesis  10  includes a tibial component  12  and a femoral component  14 . The tibial component  12  is made up of a tibial tray  16  and an insert  18 . The insert  18  has a back surface  20  which abuts the tibial tray  16  and an opposed articular surface  22 . The tray includes a prominent keel  24  protruding in the inferior direction (i.e. down a longitudinal axis of the tibia). The tibial tray  16  may be made from a hard, wear-resistant material such as a biocompatible metal alloy. The insert  18  may be made from a low-friction material such as a biocompatible plastic. 
     The femoral component  14  includes a back surface  28  shaped to abut a surface of the femur F that has been appropriately shaped and an articular surface  30  comprising medial and lateral condyles  32  and  34 , respectively. The femoral component  14  may be made from a hard, wear-resistant material such as a biocompatible metal alloy. 
     The back surface  28  includes multiple faces collectively defining a rough “U” or “J” shape. The back surface  28  includes protruding locator pins  36 . 
     The tibial tray  16  is implanted into the tibia T and the femoral component  14  is implanted into the femur F. The insert  18  is placed into the tibial tray  16 . The articular surface  22  of the insert  18  bears against the articular surface  30  of the femoral component  14 , defining a functional joint. 
     In the illustrated example, the endoprosthesis  10  is of the cruciate-retaining (“CR”) type. It includes a cutout or notch  38  in the posterior aspect of the tibial component  12  which provides a space for the posterior cruciate ligament (“PCL”). 
     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 or have a surgeon-selected relationship, and in each position. 
     One problem with prior art arthroplasty 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 implant systems. 
     BRIEF SUMMARY OF THE INVENTION 
     This problem is addressed by a tensioner-balancer (also referred to as a gap balancer, distractor, or distractor-tensioner) operable to measure characteristics of the joint such as a gap distance, angle between the bones, loads, and/or deflections, and optionally to apply a load to a gap between the bones of a joint (i.e., distract the joint). 
     According to one aspect of the technology described herein, a method is described of evaluating a human knee joint which includes a femur bone, a tibia bone, and ligaments, wherein the ligaments are under anatomical tension to connect the femur and tibia together, creating a load-bearing articulating joint. The method includes: inserting into the knee joint a tensioner-balancer that includes a femoral interface surface, and at least one force sensor; providing an electronic receiving device; moving the knee joint through at least a portion of its range of motion; while moving the knee joint, using the electronic receiving device to collect data from the at least one force sensor; processing the collected force data to produce a digital geometric model of the knee joint, wherein the data includes: a medial spline representing a locus of points of contact of a medial condyle of the femur F with the femoral interface surface, over a range of knee flexion angles; and a lateral spline representing the locus of points of contact of the femur F with the femoral interface surface over a range of knee flexion angles; and storing the digital geometric model for further use. 
     According to another aspect of the technology described herein, an apparatus for evaluating a human knee joint, comprising: a tensioner-balancer, including: a baseplate; a top plate defining a femoral interface surface, wherein the top plate includes a lateral cantilevered pad and a medial cantilevered pad, wherein each cantilevered pad is provided with two or more spaced-apart strain gages at the intersection between the respective cantilevered pad and a stationary portion of the top plate; a distracting mechanism interconnecting the baseplate and the top plate and operable to move the tensioner-balancer between retracted and extended positions; and wherein the top plate is pivotally connected to the distracting mechanism so as to be able to freely pivot about a pivot axis. 
    
    
     
       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 arthroplasty; 
         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 arthroplasty; 
         FIG.  4    is a view of the lateral aspect of the human knee joint of  FIG.  3   ; 
         FIG.  5    is an exploded perspective view of a representative knee endoprosthesis; 
         FIG.  6    is a perspective view of a human knee joint in an extended position, with a tensioner-balancer inserted therein; 
         FIG.  7    is a view of the knee joint and tensioner-balancer of  FIG.  6   , in a flexed position; 
         FIG.  8    is a top plan view of a top plate of the tensioner-balancer of  FIG.  6   ; 
         FIG.  9    is a front elevation view of the top plate of  FIG.  8   ; 
         FIG.  10    is a perspective view of the top plate of  FIG.  8   , in a deflected position; 
         FIG.  11    is a perspective view of the top plate of  FIG.  8   , showing movement of contact points superimposed thereon; 
         FIG.  12    is a perspective view of an alternative top plate configuration of the tensioner-balancer of  FIG.  6   ; 
         FIG.  13    is a perspective top plan view of an alternative top plate configuration of the tensioner-balancer of  FIG.  6   ; 
         FIG.  14    is a perspective view of the human knee joint with a tensioner-balancer inserted therein and coupled to a instrument; 
         FIG.  15    is a perspective view of a portion of a human knee joint with an alternative tensioner-balancer disposed of thereon, in a retracted position; 
         FIG.  16    is a view of the knee joint and tensioner-balancer of  FIG.  15   , in an extended position; 
         FIG.  17    is a perspective view of a human knee joint with a load cell disposed contact with the patella; 
         FIG.  18    is a perspective view of an exemplary robot arm coupled to a tensioner-balancer; 
         FIG.  19    is a perspective view of an exemplary cable saw; 
         FIG.  20    is another perspective view of the cable saw of  FIG.  19   ; 
         FIG.  21    is a perspective view of a section of an exemplary cable for the saw of  FIG.  19   ; 
         FIG.  22    is a perspective view of a section of an alternative cable for the saw of  FIG.  19   ; 
         FIG.  23    is a perspective view of a cable saw and robot arm in proximity to a human knee joint; 
         FIG.  24    is a perspective view of a tensioner-balancer and robot arm inserted into human knee joint, in combination with a spotting apparatus; 
         FIG.  25    is a perspective view of a robot arm mounted to a floor-standing robot; 
         FIG.  26    is a schematic cross-sectional view of a base of the floor-standing robot of  FIG.  25   ; 
         FIG.  27    is a perspective view showing a femur in contact with a tensioner-balancer; 
         FIG.  28    is a perspective view showing plots of collected spline data superimposed on the top plate of a tensioner-balancer; 
         FIG.  29    is another perspective view showing plots of collected spline data superimposed on the top plate of a tensioner-balancer; 
         FIG.  30    is a perspective view of a human knee joint in conjunction with an exploded view of an endoprosthesis; 
         FIG.  31    is a perspective view of a human knee joint in conjunction with a mixed reality display device and an instrumented bone saw; 
         FIG.  32    is a perspective view of a human knee joint in conjunction with an instrumented bone saw coupled to a robot; 
         FIG.  33    is a perspective view of a human knee joint in conjunction with a mixed reality display device and an instrumented drill; 
         FIG.  34    is a perspective view of a human knee joint in conjunction with an instrumented drill coupled to a robot; 
         FIG.  35    is a perspective view of a human knee joint in conjunction with an a robot that is manipulating an endoprosthesis; 
         FIG.  36    is a perspective view of a human knee joint having a trial endoprosthetic device implanted, in conjunction with a tensioner-balancer; 
         FIG.  37    is a perspective view of a posterior aspect of a human knee joint having a posterior cruciate ligament reinforced by artificial tensile member; 
         FIG.  38    is a view of the medial aspect of the human knee joint of  FIG.  37   ; 
         FIG.  39    is a view of the anterior aspect of the human knee joint in combination with an instrument for tensioning an artificial tensile member; 
         FIG.  40    is a diagram showing a tensioner-balancer labeled with data parameters; and 
         FIG.  41    is a diagram showing a knee joint and tensioner-balancer labeled with data parameters. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Now, referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIGS.  6  and  7    depict an exemplary embodiment of a tensioner-balancer  40  (alternatively referred to in various embodiments as a gap balancer, distractor, distractor-tensioner, or jack) which is useful for balancing a gap in a human knee joint as part of a total knee arthroplasty and for other therapeutic procedures. 
     Solely for purposes of convenient description, the tensioner-balancer  40  may be described as having a length extending along a lateral-to-medial 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. These directional terms, and similar terms such as “top”, “bottom”, “upper”, “lower” are used merely for convenience in description and do not require a particular orientation of the structures described thereby. 
     In one aspect, the tensioner-balancer  40  may be described as having the ability to control the movement of one degree of freedom (e.g., translation along H) and measure the movement of a second degree of freedom (rotation about A) while constraining or fixing the remaining four degrees of freedom (translation along A and L; rotation about H and L). 
     The tensioner-balancer  40  comprises a baseplate  42  and a top plate  44  interconnected by a linkage  46 . The linkage  46  and the tensioner-balancer  40  are movable between a retracted position in which the top plate  44  lies close to or against the baseplate  42 , and an extended position in which the top plate  44  is spaced away from the baseplate  42 . As described in more detail below, a means is provided to actuate the linkage  46  in response to an actuating force in order to separate the baseplate  42  and the top plate  44  in a controllable manner. This separation enables it to extend so as to apply a load to a knee joint. While the illustrated tensioner-balancer  40  includes a mechanically-operated linkage  46 , it will be understood that this is just one operative example of a “distracting mechanism” operable to move the tensioner-balancer between retracted and extended positions. It is envisioned that the mechanical linkage could be replaced with other types of mechanical elements, or electrical, pneumatic, or hydraulic devices. 
     The top plate  44  includes a femoral interface surface  48  and is mounted to the linkage  46  in such a manner that it can freely pivot about pivot axis  47  (an axis corresponding to a varus/valgus angulation of the knee). 
     The baseplate  42  includes a tensioner-balancer coupler  50  having a first interface  52 . In the illustrated example, the first interface  52  is configured as a socket. The coupler  50  is interconnected to the linkage such that an actuating force applied to the coupler  50 , such as a torque, actuates the linkage  46 . 
     Optionally, the tensioner-balancer  40  may incorporate means for measuring a force input. For example, the coupler  50  may incorporate a sensor (not shown) such as a strain gage operable to produce a signal representative of the torque applied to the coupler  50 . 
     As a further option, the tensioner-balancer  40  may incorporate a separate measuring linkage (not shown) connected to the top plate and arranged to follow the movement of the top plate  44 . The measuring linkage would be connected to a crank which would be in turn connected to a indicating shaft coaxial to the coupler. The measuring linkage may be arranged such that pivoting movement of the top plate results in rotation of the indicating shaft. The movement of the indicating shaft may be observed visually, or it may be detected by a sensor such as an RVDT or rotary encoder or resolver, which may be part of an instrument described below. This permits measurement of plate angle and/or vertical position. 
     The tensioner-balancer may be supplied with an appropriate combination of transducers to detect physical properties such as force, tilt angle, and/or applied load and generate a signal representative thereof. For example, the tensioner-balancer 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  47  (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. 
       FIGS.  8  and  9    illustrate an exemplary configuration in which the top plate  44  includes grooves  54  which define medial and lateral cantilevered pads  56 A,  56 B respectively. Two or more spaced-apart strain gages  58  are mounted to the top plate  44  in a first left-right row  60 A at the intersection between the medial pad  56 A and the forward portion  62  of the top plate  44 . Two or more spaced-apart strain gages  58  are mounted to the top plate  44  in a second fore-aft row  60 B at the intersection between the lateral pad  56 B and the forward portion  62  of the top plate  44 . 
       FIG.  10    shows the medial and lateral cantilevered pads  56 A,  56 B in a deflected position under load. The magnitude of deflection is greatly exaggerated for illustrative purposes. 
     Referring to  FIG.  8   , when the knee joint is articulated it is possible to identify an instantaneous point of peak contact pressure. There is one such point for each of the condyles. These positions are mapped onto the medial and lateral cantilevered pads  56 A,  56 B and labeled “MC” (standing for “medial load center”) and “LC” (standing for “lateral load center”). 
     Analysis by the inventors has shown that using the depicted configuration, with at least two spaced-apart strain gauges provided for each of the cantilevered pads  56 A,  56 B, it is possible to resolve the position of the load centers MC, LC in two axes. Stated another way, using this hardware, it is possible to identify the instantaneous lateral-medial and anterior-posterior position of the load centers LC, MC. Referring to  FIG.  11   , and as will be described further below, this enables the ability of the tensioner-balancer  40  to track certain relative movements of the femur F. One of these is referred to as “medial pivot” shown by arrow  64  and the other is referred to as “rollback”, shown by arrow  66 . 
     Various physical configurations of the top plate with cantilevered pads are possible with similar functionality. For example,  FIG.  12    illustrates medial and lateral cantilevered pads  56 A′,  56 B′ which are cantilevered along an anterior-posterior axis (as opposed to a lateral-medial axis as shown in  FIGS.  8 - 10   ). As another example,  FIG.  13    illustrates medial and lateral cantilevered pads  56 A″,  56 B″ which are cantilevered along both an anterior-posterior axis and a lateral-medial axis. As another alternative (not separately illustrated), the lateral pad could be cantilevered along one axis and the medial pad could be cantilevered along a different axis. 
       FIG.  14    illustrates an exemplary actuating instrument  70  for use with the tensioner-balancer  40 . The actuating instrument  70  includes a barrel  72  with an instrument coupler  74  at its distal end defining a second interface (hidden in this view) which is complementary to the first interface  52  of the tensioner-balancer  40 . The interior of barrel  72  includes an appropriate internal mechanism to apply torque to the instrument coupler  74  through a shaft  78 , such as a stepper motor  80  with related control electronics including a rotary encoder coupled to a planetary gearset  76  that interconnects the stepper motor  80  and shaft  78 . 
     The internal mechanism is operable to apply an actuating load to the tensioner-balancer  40 . The actuating instrument  70  includes an electronic data transceiver, shown schematically at  82 . The transceiver  82  may operate over a wired or wireless connection. The actuating instrument  70  may be supplied with an appropriate combination of transducers (not shown in  FIG.  14   ) to detect physical properties such as force, tilt angle, and/or applied load and generate a signal representative thereof. For example, the tensioner-balancer  40  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. 
     Displacement of the tensioner-balancer  40  may be derived from the encoder signals, knowing the kinematics of the linkage  46 . The transceiver  82  is operable to transmit the signal. 
     A remote display  84  is configured to receive the signal and produce a display of the transducer data. As one example, the remote display  84  may be embodied in a conventional portable electronic device such as a “smart phone” or electronic tablet with suitable software programming. Optionally, the remote display  84  or other suitable transmitting device may be used to send remote operation commands to the actuating instrument  70 . 
     In use, the remote display  84  permits the surgeon to observe the physical properties of the tensioner-balancer  40  in real time as the actuating instrument  70  is used to operate the tensioner-balancer  40 . 
     Optionally, the actuating instrument  70  may incorporate a tracking marker  86 . The tracking marker  86  is operable such that, using an appropriate receiving device, the position and orientation of the receiving device relative to the tracking marker  86  may be determined by receipt and analysis at the receiving device of signals transmitted by the tracking marker  86 . 
     The function of the tracking marker is to provide six degree of freedom (6-DOF) position information in a local coordinate reference space (i.e., position and orientation in each of three mutually perpendicular axes). Some devices or systems may be able to provide 6DOF position information without requiring line of sight for signals (e.g., electromagnetic spectrum energy). For example, as illustrated, the tracking marker  86  may be configured as an inertial navigation device including one or more accelerometers and gyroscopic elements capable of providing angular rate information and acceleration data in 3D space. 
     In an alternative embodiment which is not illustrated, the tracking marker may include one or more tracking points which may be configured as transmitting antennas, radiological markers, or other similar devices. This type of tracking marker may make use of line of sight transmission of signals to determine position. 
     Tracking markers  86  and appropriate receivers are known within the state-of-the-art. 
     A tracking marker  88  would be attached to the femur F in such a way that it has a substantially fixed position and orientation relative to the femur F. For example, a tracking marker  88  may be attached directly to the femur F. 
     In addition to the femur-mounted tracking marker  88 , at least one additional tracking marker is provided which has a substantially fixed position and orientation relative to the tibia T. Where the actuating instrument  70  is rigidly coupled to the tensioner-balancer  40 , the tibial tracking function may be provided by the tracking marker  86  of the actuating instrument  70 . Alternatively, a tracking marker  90  may be attached directly to the tibia T. 
       FIGS.  15  and  16    show another embodiment of a tensioner-balancer  100 . The tensioner-balancer  100  comprises a body  102  with a tibial interface surface  104  and an opposed femoral interface surface  106 . The tensioner-balancer  100  is generally U-shaped in plan form. It may include a coupler  108  providing electrical, fluid, and/or mechanical connections. 
     Generally, the overall thickness of the tensioner-balancer  100  (i.e., measured in direction H) may be on the order of one or two millimeters. This enables the tensioner-balancer  100  to be inserted into a knee joint J without first having to distract the joint or cut away any tissue. 
     The body  102  may be divided into a plurality of segments  110  which may be hinge elements  112  (e.g., live hinge strips) to allow the segments  110  to flex or pivot relative to each other. Each of the segments  110  may take the form of an expandable hollow chamber which may be inflated by fluid pressure or other means such as discrete electromechanical actuation, for example applying an electrical charge to a superelastic or memory metal.  FIG.  15    shows the segments  110  in a deflated or retracted position.  FIG.  16    shows the segments  110  in an inflated or extended position. The walls forming the segments  110  may be configured as an “accordion” or “corrugated” structure to permit them to selectively expand or collapse into a compact size. 
     An array of tibial force sensors  114  are attached to or integrated into the tibial interface surface  104 . They may be arranged in a pattern such as a grid layout or a radial layout. 
     An array of femoral force sensors  116  are attached to or integrated into the femoral interface surface  106 . They may be arranged in a pattern such as a grid layout or a radial layout. 
     Each of the force sensors  114 ,  116  includes one or more transducers operable to detect an applied force and produce a signal representative of (e.g., proportional to) the applied force and/or pressure. Optionally, each of the force sensors  114 ,  116  may detect and produce a signal representative of (e.g., proportional to) displacement and/or position (e.g., height). Nonlimiting examples of transducers effective to produce a signal include strain gauges, or miniature linear variable differential transformers (LVDT), or piezoelectric transducers. The force sensors are segmented into at least a 2D or two-axis array of sensor elements, e.g., a matrix which is addressable by X, Y reference, radial coordinates, or other suitable position location. The size of the individual sensor elements in the arrays may be selected as required to produce useful and actionable information. 
     The sensor arrays may be connected to an electronic receiving device as described elsewhere herein by a wired or wireless connection. Appropriate processors and software may be provided for interpretation of the signals from the sensor arrays. 
     In addition to collecting force, pressure, and/or displacement data between the femur F and the tibia T, an additional device may be used to collect force, pressure, and/or displacement data between the femur F and the patella P.  FIG.  17    shows a human knee joint J in flexion. A patella force sensor  120  is shown disposed between the patella P and the femur F. The patella force sensor  120  may include one or more individual sensors operable to detect force, pressure, and/or displacement and produce representative signals, as described above with respect to the sensors of the gap balancer embodiments. This data may be transmitted through a flexible cable as shown in  FIG.  15   , or over a wireless connection. 
     The utility of the tensioner-balancer  40  may be extended by various attachments. As an example,  FIG.  18    illustrates a robot arm  130  adapted for use with the tensioner-balancer  40 . The robot arm  130  extends between a proximal end  132  and a distal end  134 . The proximal end includes a mount  136  which permits the robot arm  130  to be coupled to the tensioner-balancer  40  or another suitable object. The robot arm  130  includes a number of arm segments  138  interconnected with actuators  140  of a known type capable of rotating the arm segments  138  to desired positions in response to command signals. In the illustrated example, the robot arm  130  includes actuators sufficient to produce motion about four separate axes labeled  142 ,  144 ,  146 , and  148 , respectively. This motion can be controlled relative to the local coordinate system established by the local tracker connected to the tensioner-balancer baseplate or femur or tibia (not shown). 
     The robot arm  130  is suitable for holding and manipulating various attachments coupled to the distal end  134 , including but not limited to: a saw, a drill, a retractor, a mechanical (i.e. drill, saw) or visual guide or a physical cutting guide for a surgeon, and/or an implant (endoprosthesis). 
       FIGS.  19  and  20    Illustrate a cable saw  150  for use with the robot arm  130 . For example, the cable saw  150  can be used to make a distal femur cut, posterior femur cut, anterior femur cut, patellar surface cut, and/or femoral chamfer cuts. The cable saw  150  includes a beam  152  extending between a first end  154  and a second end  156 . A drive pulley  158  is disposed at the first end  154  and an idler pulley  160  is disposed at the second end  156 . The beam  152  is configured so that it can adjust the distance between the two pulleys and/or apply a selected amount of tension. In the illustrated example the beam  152  comprises two telescoping sections. The beam  152  includes some means for changing the overall length of the telescoped sections, such as an internal threaded adjustment, an internal actuator, or one or more springs (not shown). 
     A cutting cable  162  runs over the pulleys  158 ,  160  in a closed loop. The cutting cable  162  includes one or more cutting elements such as protruding ridges or teeth  164  shown in  FIG.  21    or a continuous spiral ridge  166  shown in  FIG.  22   . Optionally, the cutting cable  162  may incorporate or be coated with an abrasive substance and/or a substance to minimize the increase in temperature associated with cutting. The tension on the cutting cable  162  is sent by adjusting the overall length of the beam  152  as described above. Additionally, the cutting cable may be immersed in a stream of biocompatible cutting fluid to reduce the time and temperature of the machining process (i.e. cold saline) supplied by a targeted nozzle or by dripping from the housing directly onto the wire saw. 
     The drive pulley  158  is coupled to an appropriate rotary driver  168  such as an electric motor or pneumatic motor (shown schematically). The rotary driver  168  is in turn coupled to a mount  170  which permits the cable saw  150  to be coupled to the robot arm  130 . Optionally, a rotary actuator  172  may be disposed between the rotary driver  168  and the mount  170 . The actuator may be a known type capable of rotating to desired positions in response to command signals. This permits the beam  152  to be rotated about an axis  174 . Optionally, a rotary actuator  173  may be disposed between the rotary driver  168  and the beam  152 . This permits the beam  152  to be rotated about an axis  175  parallel to the axis of rotation of the drive pulley  158 . 
     In the example shown in  FIG.  18   , the complete structure consisting of the robot arm  130  and the attached cable saw  150  has mobility about a total of six axes. The assembled and articulated robot arm  130  and cable saw  150  are capable of making different cuts in the knee joint J as shown in  FIG.  23   . 
     Another example of a robot arm attachment is a spotting apparatus  180  seen in  FIG.  24   . This comprises a bar  181  extending from an articulated mount  182  which is in turn coupled to the robot arm  130 . The mount  182  includes an actuator  184  permitting the bar  180  to pivot about a first axis  186 . The bar  181  is formed in two telescoping sections  188 ,  190  which may be extended or retracted using an internal actuator (not shown). The first telescoping section  188  carries a first spotting element  192  such as a rotary center drill, and the second telescoping section  190  carries a second spotting element  194  such as a rotary center drill. Combined movement of the robot arm  130  and the spotting apparatus  180  permits the first and second spotting elements  192 ,  194  to be driven to selected locations relative to the condyles of the femur F. The spotting elements  192 ,  194  may then be used to form identifiable reference features in the femur F, such as small blind center drill holes. These reference features may then be used to provide a fixed position reference on the femur F for further surgical procedures. 
     The robot arm  130  is suitable for being mounted to different objects. As shown in  FIG.  18   , it may be mounted to the baseplate  42  of the tensioner-balancer  40 . The mount is configured such that the robot arm  130  and the actuating instrument  70  may be simultaneously mounted to the tensioner-balancer  40 . 
     Alternatively, as shown in  FIG.  25   , it may be mounted as an end effector to a conventional floor-standing robot  200 . The floor-standing robot  200  may be mobile, for example by being mounted on casters (not shown). Alternatively ( FIG.  26   ) its base  201  may include a peripheral skirt  202 , blower  204 , power source  206 , and control electronics  208  enabling it to selectively float or hover on an air cushion so that it can move or be moved over a floor with low friction. The blower  204  may alternatively be used to provide suction in order to fix the floor-standing robot in one position as desired. 
     The apparatus described above is suitable for various surgical procedures. 
     In one procedure, the tensioner-balancer  40  is used to evaluate the knee and to model the articular surfaces of the knee over its range of motion. 
     More particularly, the locus of points of contact of the femur F and the top plate  44  are modeled as a medial spline and a lateral spline. 
     To carry out this modeling, the tensioner-balancer is inserted between the femur F and the tibia T. As shown in  FIG.  14   , this is accomplished after having first made the tibial plateau cut. However, the tibial plateau cut is not required. 
     The actuating instrument  70  is coupled to the tensioner-balancer  40 . Femoral tracking marker  88  is implanted to the femur F. At least one of tibial tracking marker  90  and instrument tracking marker  86  is placed. 
     The tensioner-balancer  40  is extended to apply a load to the knee joint. 
     While different modes of operation are possible, one exemplary mode is to extend the tensioner-balancer  40  until a predetermined distraction load (also referred to as distraction force) is applied. Feedback control or mechanical spring preload may then be used to maintain this distraction load, while the top plate  44  is permitted to pivot freely. One example of a suitable distraction load is approximately 130 N (30 lb.) to 220 N (50 lb.). As one option, the distraction load may be constant over the knee joint range of motion. As another option, the distraction may be a predetermined variable load, where the distraction load is correlated to knee joint position. As another option, the tensioner-balancer may be used to maintain a constant distraction gap while the knee joint is moved through its range of motion. 
     The knee joint J is then moved through its range of motion from full extension to full flexion while collecting data from the tensioner-balancer  40  and tracking markers  86 ,  88 ,  90 . Specifically, the instantaneous location of the load centers LC and MC are recorded and correlated to the flexion angle of the knee joint (as determined from the tracking marker data). The recorded data is represented by the medial spline “MS” and the lateral spline “LS” as shown in  FIG.  27   .  FIGS.  28  and  29    show the splines superimposed on the top plate of the tensioner-balancer  40 .  FIG.  28    illustrates idealized or nominal shape splines.  FIG.  29    illustrates splines indicative of discontinuities or “notching” which may be found in an actual or pathological knee joint J. The splines may be characterized by two or more points (a Starting point and Terminal point, with zero or more Intermediary points in between), each with a location (defined by cartesian or polar coordinates relative to a fixed reference point defined by tracker on the tensioner-balancer baseplate), a direction, and a first and second derivative. 
     The spline information may be used to select an appropriate endoprosthesis, specifically a femoral component. Multiple femoral components of different sizes and articular surface profiles may be provided, and the one which has the best fit to the splines MS, LS may be selected for implantation. Alternatively, the spline information may be used to generate a profile for manufacture of a patient-specific femoral component. 
     The spline information may be used in conjunction with other information to determine appropriate cutting planes for the femur F. For example, the back surface  28  of the femoral component  14  has a known relationship to the articular surface  30 . The desired final location and orientation of the articular surface  30  is known in relation to the top plate  44  of the tensioner-balancer  40 , which serves as a proxy for the tibial component  12 . The final location of the tibial component  12  is known in relationship to the position of the tibial tracking marker  90 . Finally, the actual orientation and location of the femur F in relation to the other parts of the joint J is known from the information from the femoral tracking marker  88 . Using appropriate computations, the orientation and location of the cutting planes of the femur F can be calculated and referenced to the position of the tensioner-balancer  40 . With reference to  FIGS.  40  and  41   , it will be understood that the tensioner-balance  40  and associated tracking apparatus may be used to collect the following data related to the knee joint: distraction height “Z” of the top plate  44 , tilt angle “A” (i.e., varus-valgus) of the top plate  44 ), medial and lateral distraction heights “ZM”, “ZL” (e.g., derived from the top plate distraction height and top plate tilt angle), the medial and lateral spline data, the position of the contact points of the femur F on the top plate (medial-lateral and anterior-posterior) (MX, MY, LX, LY), the distraction load on the medial and lateral condyles (MD, LD), the knee joint flexion angle “FA”, and the abovementioned 6-DoF position data for each tracking marker (X, Y, Z position and Xr, Yr, Zr rotation). 
     A nominal distal femoral cutting plane  2  ( FIG.  30   ) may be determined by anatomical analysis using known anatomical references and techniques. For example, this plane  2  could be uniformly spaced away from and parallel to the tibial cutting plane  1  (i.e., a nominal cut). Alternatively, this plane  2  could be at an oblique angle to the tibial cutting plane  1 , in one or more planes (i.e., simple or compound tilted cut, potentially usable as a corrective cut). 
     In one method, the cable saw  150  may be coupled to the robot arm  130  which is in turn mounted to the tensioner-balancer as seen in  FIG.  16   . The cutting plane information may then be used to drive the robot arm  130  with attached cable saw  150  to make the cuts in the femur F. 
     In another method, the cable saw  150  may be coupled to the robot arm  130  which is in turn mounted to the floor-standing robot  201  as seen in  FIG.  25   . The cutting plane information may then be used to drive robot arm  130  with attached cable saw  150  to make the cuts in the femur F. 
     Information from the tensioner-balancer  40  and tracking markers may be used with hand-held equipment. Once the cutting planes are determined, the tracking markers  86 ,  88 , or  90  may be used to guide a bone saw  250  equipped with a tracking marker  252  to make the distal femoral cut  2  at appropriate angle and location, as depicted in  FIG.  31   . In this context, the cutting plane (or a portion thereof) defines a computed tool path. This guidance is possible because intercommunication between the bone saw  250  and the associated tracking marker  252  will give the relative position and orientation of the bone saw  250  to that tracking marker. The cutting guidance may be provided in the form of information displayed on the remote display  84  described above. For this purpose, 2-way data communications may be provided between and among the bone saw  250  (or other surgical instrument), the tracking markers  86 ,  88 , or  90 , and the remote display  84 . 
     It should be noted that the bone saw  252  can be guided with reference to only a single tracking marker  88  coupled to the femur F. Alternatively, the cutting guidance (optionally along with other information, such as the virtual future position of the drilled holes and implants used) may be displayed on a body-worn display providing 2D or 3D graphics or providing a holographic heads-up display with an information panel (e.g., a Virtual Reality or augmented reality or mixed reality headset  300 ). Alternatively, the cutting guidance may be provided to a conventional robot  301  ( FIG.  3032    to which the bone saw is mounted. 
     Information from the tensioner-balancer  40  and tracking markers may optionally be used for drilling holes, for example to anchor tensile elements. Referring to  FIG.  32   , once a position of a hole to be drilled is determined, the tracking markers  86 ,  88 , or  90  may be used to guide a cordless drill  254  equipped with a tracking marker  256  to drill a hole, with the drill bit  258  extending an appropriate angle. In this context, the hole to be drilled (or a portion thereof) defines a computed tool path. Guidance along the tool path is possible because intercommunication between the cordless drill  254  and the tracking marker  256  will give the relative position and orientation of the cordless drill  254  to those markers. The drilling guidance may be provided in the form of information displayed on the remote display  84  described above. For this purpose, two-way data communications may be provided between and among the cordless drill  254  (or other surgical instrument), the tracking markers  86 ,  88 , or  90 , the actuating instrument  70 , and the remote display  84 . It should be noted that the drill  254  can be guided with reference to only a single tracking marker  88  coupled to the femur F. Alternatively, the drilling guidance (optionally along with other information, such as the virtual future position of the drilled holes and implants used) may be displayed on a body-worn display providing 2D or 3D graphics or providing a holographic heads-up display with an information panel (e.g., a Virtual Reality or augmented reality or mixed reality headset  300 ). Alternatively, the drilling guidance may be provided to a conventional robot  301  ( FIG.  34   ) to which the bone saw is mounted. 
     Information from the tensioner-balancer  40  and tracking markers may optionally be used for automated placement of components. Referring to  FIG.  35   , the tracking markers  86 ,  88 , or  90  may be used to guide a robot  301  to implant one or more of the endoprosthetic components into the knee joint J, such as the tibial tray  16 , insert  18 , and/or femoral component  14  to which the bone saw is mounted. 
     As seen in  FIG.  36   , the tensioner-balancer  40  may be used with a trial implant (femoral component  14 ) to collect data and evaluate the femoral component  14 . 
     In addition to retaining the patients&#39; PCL in a knee arthroplasty, it may be augmented (reinforced) using one or more artificial tensile members. The term “tensile member” as used herein generally refers to any flexible element capable of transmitting a tensile force. Nonlimiting examples of known types of tensile members include sutures and orthopedic cables. Commercially-available tensile members intended to be implanted in the human body may have a diameter ranging from tens of microns in diameter to multiple millimeters in diameter. Commercially-available tensile members may be made from a variety of materials such as polymers or metal alloys. Nonlimiting examples of suitable materials include absorbable polymers, nylon, ultrahigh molecular weight polyethylene (“UHMWPE”) or polypropylene titanium alloys, or stainless steel alloys. Known physical configurations of tensile members include monofilament, braided, twisted, woven, and wrapped. Optionally, the tensile member may be made from a shape memory material, such as a temperature-responsive or moisture-response material. 
       FIGS.  37  and  38    illustrate a tensile member passing through transosseous passaged formed in bone (e.g., by drilling), fixed by anchors, and routed across the posterior aspect of a human knee joint J. The tensile member replaces or augments or reinforces or tethers the PCL. 
     In the illustrated example, two tensile members are present, referred to as first and second tensile members  440 ,  440 ′ respectively. 
     The first tensile member  440  has a first end  442  secured to the femur F on the outboard side thereof, by a first anchor  444 . (With reference to this example, the terms “inboard” and “outboard” are used to describe locations relative to their distance from the meeting articular surfaces of the joint J. For example, the endoprosthesis  10  would be considered “inboard” of the joint J, while the anchor  444  would be considered “outboard”). The first tensile member  440  passes through a first femoral passage  446  formed in the femur F, exiting the inboard side of the femur F. 
     The second tensile member  440 ′ has a first end  442 ′ secured to the femur F on the outboard side thereof, by a second anchor  448 . The second tensile member  440 ′ passes through a second femoral passage  450  formed in the femur F, exiting the inboard side of the femur F. 
     The first and second tensile members  440 ,  440 ′ span the gap between femur F and tibia T and enter a tibial passage  452  at an inboard side. The first and second tensile members  440 ,  440 ′ pass through the tibial passage  452  at a single entry  453 , exiting the outboard side of the tibia T. Second ends  454 ,  454 ′ of the first and second tensile members  440 ,  442 ′ are secured with a third anchor  456 . 
     The term “anchor” as it relates to elements  444 ,  448 , and  456  refers to any device which is effective to secure a tensile member passing therethrough. Nonlimiting examples of anchors include washers, buttons, flip-anchors, adjustable loop devices, fixed loop devices, interference screw devices, screw plates, ferrules, swages, or crimp anchors. 
     The tensile members  440 ,  440 ′ can be routed through or along the PCL. 
       FIG.  39    illustrates an exemplary insertion instrument  500  which may be used to insert, tension, and activate swage-type anchors. The basic components of the insertion instrument  500  are a body  502 , a stem  504  extending from the body  502  and having an anchor connection mechanism  506  disposed at a distal end thereof, a hollow pushrod  508  extending through the stem  504  and slidably movable between retracted and extended positions, and a driving mechanism  510  for moving the pushrod  508  between retracted and extended positions. The stem  504  and the pushrod  508  may be rigid or flexible. 
     In the illustrated example, the driving mechanism  510  comprises an internal threaded mechanism which is manually operated by a star wheel  512 . 
     A tensioner  514  is part of or connected to the insertion instrument  500 . It has a housing  516 . A shuttle assembly  518  including an adjustment knob  520  and a grooved spool  522  is received inside the housing  516 . A compression spring  524  is captured between the shuttle assembly  518  and the housing  516 . The shuttle assembly  518  can translate forward and aft relative to the housing  516  in response to rotation of the adjustment knob  520 . 
     In use, a first end of a tensile member  440  passes through the hollow interior of tensioner  514  and is secured to the spool  522 . The tension applied to the tensile member  440  may be indicated, for example, by observing the position of the shuttle assembly  518  relative to a calibrated scale  526  on the housing  516 . When a suitable final tension is achieved, the star wheel  512  may be operated to actuate the pushrod  508 , swaging the tensile member  440  and fracturing the breakaway structure of the anchor. In the illustrated example, two separate tensioners  514  are provided, allowing the tension of each of the tensile members to be set independently. 
     In one example procedure where two tensile members are used, a first provisional tension is applied to the first tensile member and a second provisional tension is applied to the second tensile member. The second tensile member may have the same or different tension at the first tensile member. Next, the provisional tensions evaluated to determined if they are suitable. In response to the evaluation, they may be increased or decreased. Finally, the anchor may be swaged to secure the tensile members and finalize the tension. In one example, the tension may be from about 0 N (0 lb.) to about 220 N (50 lb.) 
     The apparatus and method described herein will permit knee arthroplasty with improved patient outcomes with a minimum amount of added equipment and procedures. 
     The apparatus and techniques described herein are also applicable to surgical procedures and arthroplasty on other joints. The apparatus can be used to distract, track, and proceed with corrective actions based on distraction feedback. For example, these techniques may be used on hip or shoulder joints. 
     The foregoing has described apparatus and methods for knee arthroplasty. 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.