Patent Publication Number: US-2023157808-A1

Title: Reinforced knee method and apparatus

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to medical implants, and more particularly to a method for treating an injured, diseased, or worn human knee joint. 
     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  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 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°. 
     One problem with prior art TKA techniques is that they often irreversibly sacrifice the posterior cruciate ligament (“PCL”). These existing techniques fail to take into account the native knee kinematics controlled by the PCL and offer no way to replicate or control these kinematics post-operatively. Even techniques that intend to preserve the PCL pose a risk of damaging the ligament, in which case there is no means of restoring its strength and function during the arthroplasty procedure. In the prior art, producing a posterior-stabilized knee takes additional time due to the extra steps required to remove the PCL. 
     BRIEF SUMMARY OF THE INVENTION 
     This problem is addressed by a method and apparatus for reinforcing the cruciate ligament structure of a knee. 
     According to one aspect of the technology described herein, an anchor for anchoring one or more tensile members to bone, includes: a housing extending along a central axis between open first and second ends, and having a hollow interior; a collet disposed in the hollow interior of the housing, the collet having a peripheral wall defining a central bore for accepting one or more tensile members therethrough and an exterior surface, wherein the collet is configured to be swaged around and against the one or more tensile members; a sleeve having a peripheral wall defining opposed interior and exterior surfaces, the sleeve disposed in the hollow interior of the housing and positioned generally axially adjacent to the collet, so as to be movable parallel to the central axis between first and second positions; wherein at least one of the exterior surface of the collet and the interior surface of the sleeve is tapered and the sleeve and the collet are arranged such that movement of the sleeve from the first position to the second position will cause the interior surface of the sleeve to bear against the exterior surface of the collet, causing the collet to swage radially inwards around and against the one or more tensile members; and a flange element, wherein the housing is pivotally mounted to the flange element. 
    
    
     
       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 the posterior aspect of a cruciate-retaining (“CR”) knee implant; 
         FIG.  6    shows the knee implant of  FIG.  5    in an assembled condition; 
         FIG.  7    illustrates a short segment of a representative tensile member; 
         FIGS.  8 ,  9 ,  10 , and  11    are schematic views of the anterior, lateral-posterior, medial, and posterior aspects, respectively of a human knee joint, showing a total knee replacement in conjunction with a double strand posterior cruciate ligament reinforcement; 
         FIG.  12    is a schematic view of the posterior-lateral aspect of a human knee joint, showing a total knee replacement in conjunction with a double strand posterior cruciate ligament reinforcement; 
         FIG.  13    is a schematic view of the posterior aspect of a human knee joint, showing a total knee replacement in conjunction with a double strand posterior cruciate ligament reinforcement; 
         FIGS.  14  and  15    are schematic views of the lateral-posterior and posterior aspects, respectively of a human knee joint, showing a total knee replacement in conjunction with a double strand posterior cruciate ligament reinforcement, showing intraosseous articular points; 
         FIGS.  16 ,  17 ,  18 , and  19    are schematic views of the anterior, lateral-posterior, medial, and posterior aspects, respectively of a human knee joint, showing a total knee replacement in conjunction with a double strand posterior cruciate ligament reinforcement; 
         FIGS.  20 - 22    are diagrammatic views of a human knee joint in extended, mid-flexed, and extended positions respectively; 
         FIGS.  23 - 25    are diagrammatic views of a human knee joint in extended, mid-flexed, and extended positions respectively; 
         FIG.  26    is a view of the lateral aspect of a human knee joint, showing a total knee replacement; 
         FIG.  27    is a view of tibia having a tibial tray implanted therein; 
         FIGS.  28 ,  29 , and  30    are schematic views of the anterior, lateral-medial, and posterior aspects, respectively of a human knee joint having a tibial tray implanted thereon, with a drill guide coupled to the tibial tray; 
         FIG.  31    is a view of the drill guide of  FIG.  28    incorporating a drill stop; 
         FIG.  32    is a schematic view of the anterior-lateral aspect of a human knee joint having a tibial tray implanted thereon, with a drill guide coupled to the tibial tray, along with a curved drill; 
         FIG.  33    is a view of the human knee joint of  FIG.  32   , with a curved drill inserted into the drill guide; 
         FIG.  34    is a schematic perspective view of an adjustable drill guide; 
         FIGS.  35 ,  36 , and  37    are schematic views of the anterior, inferior, and posterior aspects, respectively of a human femur, with a drill guide coupled to the femur; 
         FIGS.  38 ,  39 , and  40    are schematic views of the anterior, inferior, and posterior aspects, respectively of a human femur, with an alternative drill guide coupled to the femur; 
         FIGS.  41  and  42    are schematic views of the anterior, and posterior aspects, respectively of a human knee joint having two uni-compartmental protheses implanted therein; 
         FIG.  43    is a front elevation view of an exemplary snap-off anchor; 
         FIG.  44    is a side elevation view of the snap-off anchor of  FIG.  43   ; 
         FIG.  45    is a cross-sectional view of the snap-off anchor of  FIG.  43   ; 
         FIG.  46    is a perspective view of the snap-off anchor of  FIG.  43   , in a snapped-off condition; 
         FIG.  47    is a cross-sectional view of the snap-off anchor of  FIG.  43   ; 
         FIG.  48    is a perspective view of an exemplary T-pivot anchor; 
         FIG.  49    is a cross-sectional view of the T-pivot anchor of  FIG.  47   ; 
         FIG.  50    is a perspective view of the T-pivot anchor of  FIG.  47   , coupled to an insertion instrument; 
         FIG.  51    is another perspective view of the T-pivot anchor of  FIG.  47   ; 
         FIG.  52    is an exploded perspective view of the T-pivot anchor of  FIG.  47   ; 
         FIG.  53    is a top plan view of the T-pivot anchor of  FIG.  42   ; 
         FIG.  54    is a side elevation view of the T-pivot anchor of  FIG.  42   ; 
         FIG.  55    is a perspective view of an anchor implanted into bone; 
         FIG.  56    is a cross-sectional view of the anchor of  FIG.  55    at a first angle; 
         FIG.  57    is a cross-sectional view of the anchor of  FIG.  55    at a second angle; 
         FIG.  58    is a partially-sectioned perspective view of the anchor of  FIG.  55   ; 
         FIG.  59    is another perspective view of the anchor of  FIG.  55   ; 
         FIG.  60    is a side view of an anchor; 
         FIG.  61    is another side view of the anchor of  FIG.  60   ; 
         FIG.  62    is a side view of a threaded anchor; 
         FIG.  63    is a schematic view of the anterior aspect of a human knee joint having a tibial tray implanted thereon, with an insertion instrument being used to tension an anchor; and 
         FIG.  64    is a schematic perspective view of a human knee joint with sleeves protecting tensile members implanted therein. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS.  5  and  6    depict an exemplary endoprosthetic  10  (i.e., implant) of a known type. The endoprosthetic  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  26  shaped to abut a surface of the femur that has been appropriately shaped and an articular surface  28  comprising medial and lateral condyles  30  and  32 , respectively. The femoral component  14  may be made from a hard, wear-resistant material such as a biocompatible metal alloy. 
     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  28  of the femoral component  14 , defining a functional joint. 
     In the illustrated example, the implant  10  is of the cruciate-retaining (“CR”) type. It includes a cutout or notch  34  in the posterior aspect of the tibial component  12  which provides a space for the posterior cruciate ligament (“PCL”). 
     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.  FIG.  7    illustrates a short segment of a representative tensile member  40  having a diameter “D1”. Commercially-available tensile members intended to be implanted in the human body may have a diameter “D  1 ” 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 (“UBMWPE”) 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.  8 - 11    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  40 ,  40 ′ respectively. 
     The first tensile member  40  has a first end  42  secured to the femur F on the outboard side thereof, by a first anchor  44 . (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 implant  10  would be considered “inboard” of the joint J, while the anchor  44  would be considered “outboard”). The first tensile member  40  passes through a first femoral passage  46  formed in the femur F, exiting the inboard side of the femur F. 
     The second tensile member  40 ′ has a first end  42 ′ secured to the femur F on the outboard side thereof, by a second anchor  48 . The second tensile member  40 ′ passes through a second femoral passage  50  formed in the femur F, exiting the inboard side of the femur F. 
     The first and second tensile members  40 ,  40 ′ span the gap between femur F and tibia T and enter a tibial passage  52  at an inboard side. The first and second tensile members  40 ,  40 ′ pass through the tibial passage  52  at a single entry  53 , exiting the outboard side of the tibia T. Second ends  54 ,  54 ′ of the first and second tensile members  40 ,  42 ′ are secured with a third anchor  56 . 
     The term “anchor” as it relates to elements  44 ,  48 , and  56  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. Examples of some particularly useful anchors are described below. 
     As seen in  FIGS.  12  and  13   , the tensile members  40 ,  40 ′ can be routed through or along the PCL. In one example, at least one of the ends of the first and second passages may be positioned inside a “footprint” defined by the portion of the cruciate ligament which contacts the femur. In another example, at least one end of the third passage may be positioned inside a “footprint” defined by the portion of the cruciate ligament which contacts the tibia. As seen in  FIGS.  15  and  16   , one or more of the anchors could be an internal anchor  58 . 
     Analysis by the inventors has shown that the use of two or more tensile members  40  with non-coextensive transosseous routing can be especially helpful in providing a desired alignment and range of motion of the knee joint J. As used herein, the term “non-coextensive transosseous routing” refers to two or more tensile members passing through bone along different routes, i.e. two or more routes which are not coextensive over their entire lengths. Stated another way, the two or more transosseous passages would be either (a) intersecting at one location or (b) spaced-apart from the other transosseous passages and running along parallel or divergent paths. 
     In the example shown in  FIGS.  8 - 11   , the femoral passages  46 ,  50  intersect and have a single exit  60  at the articular surface of the medial condyle. In this example, both tensile members  40 ,  40 ′ pass through a single tibial passage  52 . The tensile members  40 ,  40 ′ may have the same or different properties (e.g., material, diameter) and may have the same or different final implanted tensions. 
     In the example shown in  FIGS.  16 - 19   , the femoral passages  46 ,  50  do not intersect (they may be parallel or non-parallel). The femoral passages  46 ,  50  have individual exits  58 ,  58 ′ at the articular surface of the medial condyle. In this example, both tensile members  40 ,  40 ′ pass through a single tibial passage  52 . The tensile members  40 ,  40 ′ may have the same or different properties (e.g., material, diameter) and may have the same or different final implanted tensions. 
     The exact routing of the femoral and tibial passages may be selected to suit a particular patient and desired correction. This may be determined using the surgeon&#39;s judgment, optionally supplemented by computer simulation of the knee joint. It will be understood that each tensile member  40 ,  40 ′ constrains the joint J in a predictable manner, and that the use of two tensile members  40 ,  40 ′ with non-coextensive routing produces a compound effect on the joint J. 
       FIGS.  20 - 25    illustrate the effect of the non-coextensive routing. It can be seen that the changes in length of the elastic tensile members  40 ,  40 ′ as the joint J moves through extended, mid-flexed, and extended positions are different for the different routings. The dimension “LP” represents the overall length of one of the tensile members, and the dimension “LR” represents the overall length of the other tensile member.  FIGS.  20 - 22    model routing similar to that shown in  FIGS.  8 - 11   , while  FIGS.  23 - 25    model routing similar to that shown in  FIGS.  16 - 19   . 
     The cruciate retaining method described above can be employed for sagittal plane control. As shown in  FIG.  26   , appropriate selection, routing, and tensioning of the tensile members  40 ,  40 ′ can be used to move or constrain the femur F in different translational or rotational directions relative to the tibia, e.g., posterior, anterior, superior, or inferior translational directions; and internal or external rotational directions. 
     The cruciate retaining method described above can be employed for axial plane control. As shown in  FIG.  27   , appropriate selection, routing, and tensioning of the tensile members  10  can be used to move or constrain the femur F in different directions relative to the tibia, e.g., lateral, medial, posterior, or anterior translational directions; and internal or external rotational directions. Furthermore, appropriate selection, routing, and tensioning of the tensile members  10  can be used to control “medial pivot”, i.e., the magnitude to which the lateral condyle of the femur  10  moves and rotates in the anterior or posterior directions as the knee joint J is moved between flexed and extended positions. 
     The tibial passage  52  may be oriented at a substantially acute angle to the surface of the tibia T and may thus be difficult to accurately drill using a manual process.  FIGS.  28 - 30    illustrate a drill guide  70  which may be used to form the tibial passage  52 . The drill guide  70  includes a base plate  72  which is shaped and sized to engage with a tibial tray  16 . A support arm  74  extends away from the base plate  72  in the inferior direction anatomically. A drill bushing  76  is disposed at the far end of the support arm  74 . The drill bushing  76  has an internal bore for receiving a drill bit or other similar boring tool. The drill bushing  76  is positioned and oriented so as to guide the boring tool along a path to create the tibial passage  52 , as shown by the line  78  in the figures. 
       FIG.  31    illustrates a modified version of the drill guide, labeled  70 ′, including a base plate  72 ′, support arm  74 ′, and drill busing  76 ′. The drill guide  70 ′ includes a drill stop  77 ′ extending from the base plate  72 ′. It is positioned and oriented in line with the bore axis of the drill bushing  76 ′. The drill stop  77 ′ is effective to stop the boring tool after it has passed through the tibia T, preventing damage to other anatomical structures. 
       FIGS.  32  and  33    illustrate another drill guide  80  which may be used to form the tibial passage  52 . The drill guide  80  includes a base plate  82  which is shaped and sized to engage with a tibial tray  16 . A support arm  84  extends away from the base plate  82  in the inferior direction anatomically. A drill bushing  86  is disposed at the far end of the support arm  84 . The drill bushing  86  has an internal bore for receiving a drill bit or other similar boring tool. The drill bushing  86  is positioned and oriented so as to guide the boring tool along a path to create the tibial passage  52  as shown by the line  88  in the figures. The drill bushing  86  has a curved bore, in contrast to the straight axial bore of the drill bushing  76  shown in  FIGS.  28 - 30   . 
     The drill guide  80  is used in conjunction with a flexible drill  90 . The flexible drill  90  has a generally stiff non-rotating central support member  92  surrounded by a flexible member  94  (similar to a coil spring). The flexible member  94  terminates in a cutting tip  96  and is driven by a drill motor  98  (shown schematically). 
       FIG.  34    illustrates an adjustable drill guide  100  which may be used to form the tibial passage  52 . The drill guide  100  includes a base plate  102  which is shaped and sized to engage with a tibial tray  16 . A support arm  104  extends away from the base plate  102  in the inferior direction anatomically. A first end  106  of the support arm  104  is connected to the base plate  102  with a first pivoting joint  108 . A drill bushing  110  is disposed at the far end  112  of the support arm  102 , and is connected to the far end  112  of the support arm  102  by a second pivoting joint  114 . The pivoting joints  108 ,  114  are configured to permit the connected elements to be relatively pivoted about one or more axes, and to retain the connected parts in the pivoted positions. For example, they may incorporate a clamp, lock, or friction mechanism (not shown in detail). The drill bushing  110  has an internal bore for receiving a drill bit or other similar boring tool. The drill bushing  110  is positioned and oriented so as to guide the boring tool along a path to create the tibial passage, as shown by the line  116  in the figures. 
       FIGS.  35 - 37    illustrate another drill guide  120  which may be used to form the femoral passages  46 ,  50 . The drill guide  120  includes a base plate  122  which is shaped and sized to engage the distal end of the femur F. For example, it may be shaped complementary to the shape of a femur F that has been prepared for an implant. A support arm  124  extends away from the base plate  122  in the superior direction anatomically. A drill bushing  126  is disposed at the far end of the support arm  124 . The drill bushing  126  has one or more internal bores for receiving a drill bit or other similar boring tool. The drill bushing  126  is positioned and oriented so as to guide the boring tool along a path to create the femoral passages  46 ,  50  as shown by the lines  128 ,  130  in the figures. 
     The drill guide  120  is intended for a approach from the inferior direction.  FIGS.  38 - 40    illustrate a drill guide  120 ′ which may be used to form the femoral passages  46 ,  50  in an approach from the superior direction. The drill guide  120 ′ includes a base plate  122 ′, support arm  124 ′, and a drill bushing  126 ′. The drill bushing  126  is positioned and oriented so as to guide the boring tool along a path to create the femoral passages  46 ,  50  as shown by the lines  128 ,  130 . in contrast to drill guide  120 , the support arm  124 ′ of drill guide  120 ′ extends away from the base plate  122 ′ in the opposite direction. 
     The cruciate-preserving and augmenting and reinforcing techniques described herein are applicable to varying scopes of knee surgery. It may be used in combination with resurfacing of all or part of the knee with artificial or biological materials. It may be used in combination with a knee arthroplasty. The endoprosthesis used in the arthroplasty may be a total knee, using the implant  10  of  FIGS.  5  and  6   , or it may be a bicompartmental arthroplasty, using individual implants  132 ,  134  for the medial and lateral compartments, as seen in  FIGS.  41  and  42   . Alternatively, the arthroplasty may be uni-compartmental. As a further alternative, the cruciate-preserving and augmenting techniques described herein are useful even where resurfacing is not carried out, for example other surgical procedures to restore the function of the knee joint. 
       FIGS.  43 - 47    illustrate an exemplary embodiment of an anchor  200 . The anchor  200  includes three functional elements, namely a housing  202  configured to be implanted into bone, a collet  204  received in the housing  202  and configured to be swaged around and against a tensile member  40  ( FIG.  7   ) without moving axially relative to the housing  202  or tensile member  40 , and a sleeve  206  received in the housing  202  which is capable of moving axially within the housing  202  so as to swage the collet  204 , thus retaining the tensile member  40 . (Some minimal axial movement of the collet  204  not significantly affecting tension may occur during swaging). 
     The housing  202  has a body portion  208  extending along a central axis “A” between first and second ends  210 ,  212 . The body portion  208  is defined by a peripheral wall having opposed interior and exterior surfaces, and defining a hollow interior. In the illustrated example, the body portion  208  is generally cylindrical in shape. The first end  210  has an internal flange  214  which is sized to define a stop against axial motion of the collet  204 . 
     A generally annular flange  216  is located at or near the second end  212  and extends radially outwards from the body portion  208 . The size and shape of the flange  216  may be selected to suit a particular application. In the example illustrated in  FIGS.  43  and  44   , a reference plane “P” passing through the flange  216  is oriented at a compound angle oblique to the central axis A, represented by angles θ 1 ,  02  in two perpendicular planes. It will be understood that the orientation of the flange  216  may be varied to suit a particular application. The anchor  200  may have an overall size which is generally small enough to be implanted inside a human body. In one example the housing  202  may be cylindrical in shape with an outside diameter “D2” of about 3 to 12 mm, and the flange  216  may have an outside diameter “D3” about 5 to 20 mm. 
     The housing  202  includes an extension portion  218  extending away from the second end  212  of the body portion  208 . The extension portion  218  is coupled to the body portion  208  by a breakaway structure  220 . As manufactured and prior to use, the entire housing  202  forms a single unitary, integral, or monolithic structure including the body portion  208 , extension portion  218 , and breakaway structure  220  that provides a “breakaway” or “snap-off” connection between the body portion  208  and the extension portion  218 . 
     The extension portion  218  extends between a first end  222  and a second end  224 . The second end  224  is interconnected to the breakaway structure  220 . The first end  222  may be provided with a mechanical connector for being connected to an insertion instrument which is described in more detail below. In the illustrated example, the first end  222  is provided with a connector  226  in the form of screw threads. As described in more detail below, this permits a secure, releasable connection to an instrument used for insertion or manipulation of the anchor  200 . 
     The breakaway structure  220  is configured in terms of its shape, dimensions, and material properties such that it will retain its structural integrity and interconnected the body portion  208  and the extension portion  218  when subjected to tensile loads up to a first magnitude sufficient to complete a swaging process of the anchor  200  as described below. This is referred to herein as a “first predetermined tensile load”. The breakaway structure  220  is further configured in terms of its shape, dimensions, and material properties such that it will fail and permit separation of the body portion  208  and the extension portion  218  when subjected to tensile loads equal to or greater than a second magnitude, referred to herein as a “second predetermined tensile load”. The second tensile load is greater than the first tensile load. The second tensile load may be selected to be sufficiently greater than the first predetermined tensile load such that failure of the breakaway structure  220  is unlikely to occur during the swaging process. Stated another way, the second predetermined tensile load may have a safety margin over the first predetermined tensile load. In one example, the second predetermined tensile load may be selected to be at least 50% to 100% greater than the first predetermined tensile load. 
     In general, the breakaway structure  220  may include one or more stress-concentrating columns which present a known cross-sectional area, thus permitting reliable computation of the tensile stresses in the breakaway structure  220  for a given applied load. 
     In the illustrated example, best seen in  FIG.  45   , the breakaway structure  220  includes a plurality of stress-concentrating columns  228  arrayed around the periphery of the flange  216 , which have a circular cross-sectional shape adjacent to and/or or at the flange  216 . The stress-concentrating columns  228  are separated by openings  230 . It will be understood that other column cross-sectional shapes providing a predictable cross-sectional area may be used, and that the cross-sectional shape may vary over the length of the column. 
     Optionally, the stress-concentrating columns  228  may intersect the flange  216  at the bottom of recesses  232  formed in the flange  216 . In use, this permits the stress-concentrating columns  228  to separate along the fracture plane which is “below” a top surface  234  of the flange  216  or stated another way it is sub-flush to, or recessed from, the top surface  234 . 
     The collet  204  is a hollow member with first and second ends  236 ,  238  and defined by a sidewall  240  having an exterior surface  242 . The collet  204  has a central bore  244  which is sized to receive the tensile member  40  described above. For example, the central bore  244  may be cylindrical, with a minimum inside diameter or characteristic dimension which is initially slightly larger than a diameter D1 of the tensile member  40 . The central bore  244  need not have a circular cross-section; the cross-section may be a polygon shape (e.g. triangular, square) or it may be a lobed shape (e.g., triangular with radiused corners). 
     The collet  204  is configured so as to readily permit it to be swaged, i.e. shaped in such a manner to reduce its cross-section and the size of the central bore  244  so that it firmly engages the tensile member  40  and allows a tensile force to be applied thereto. The act of swaging may involve the collet  204  being deformed, crushed, collapsed, or compressed. The collet  204  is configured, e.g., sized and shaped, such that when subjected to pressure from the sleeve  206 , it will abut the internal flange  214  of the body portion  208 , thus stopping its further axial movement, and permitting the swaging action to take place without axial movement of the collet  204  relative to the tensile member  40  or housing  202 . 
     The exterior surface  242  has a shape adapted to interact with the interior surface of the sleeve  206  described below so as to produce a radially inwardly directed force on the collet  204  in response to the axial movement of the sleeve  206 . Fundamentally, at least one of the exterior surface  242  of the collet  204  and the interior surface of the sleeve  206  incorporates a taper i.e., a diameter or lateral dimension which is larger near one end and smaller near the opposite end of the respective element. In the example shown in  FIG.  45   , the exterior surface  242  has a cylindrical section  246  and a generally frustoconical section  248 . The exterior dimensions and shape of the exterior surface  242  are selected so as to provide a predetermined fit with the sleeve  206  both before and after a compression process.  FIG.  61    shows the collet  204  after swaging. 
     The central bore  244  may include a surface texture which serves to enhance grip on a tensile member  40 . Nonlimiting examples of surface texture structures include teeth, ribs, grooves, dimples, recesses, bumps, pins, ridges, knurling, checkering, and threads. In the example shown in  FIG.  45    this takes the form of longitudinal rows of ramped-shaped teeth. 
     The sleeve  206  is a hollow member with open first and second ends  250 ,  252 . The sleeve  206  is sized is such that the tensile member  40  described above can pass through the first and second ends  250 ,  252 . The sleeve  206  is defined by a peripheral wall having interior and exterior surfaces  254 ,  256 , respectively. In the illustrated example, the sleeve  206  is generally cylindrical in shape. 
     The interior surface  254  has a shape adapted to interact with the exterior surface  242  of the collet  204  described above so as to produce a radially inwardly directed force on the collet  204  in response to the axial movement of the sleeve  206 . As noted above, at least one of the exterior surface  242  of the collet  204  and the interior surface  254  of the sleeve  206  incorporates a taper i.e., a diameter or lateral dimension which is larger near the first end and smaller near the second end of the respective element. In the example shown, the interior surface  254  is tapered, defining a shape like a frustum of a cone, with a larger diameter at the first end  250 . 
     The anchor  200  and its components may be made from any material which is biocompatible and which will engage the other elements so as to transfer tensile force thereto. As used herein, the term “biocompatible” refers to a material which is not harmful to living tissue. Nonlimiting examples of suitable materials include polymers and metal alloys. Nonlimiting example of suitable metal alloys include stainless steel alloys and titanium alloys. The anchor  200  or its components may be fabricated by a technique such as machining, forging, casting, sintering, or additive manufacturing (e.g., “3D printing”). The anchor  200  or its components may be treated with known coating such as titanium nitride, chrome plating, carbon thin films, and/or diamond-like carbon coatings. 
       FIGS.  48 - 54    illustrate an exemplary embodiment of an anchor  300  which incorporates a pivoting function. The anchor  300  is similar in overall construction to the anchor  300  described above. Elements of the anchor  300  not specifically described may be taken to be identical to corresponding elements of the anchor  200 . The anchor  300  includes a washer  316 , a housing  302 , a collet  304  received in the housing  302  and configured to be swaged around and against a tensile member, and a sleeve  306  received in the housing  302  which is capable of moving axially within the housing  302  so as to swage the collet  304 , thus retaining the tensile member  40 . 
     The housing  302  has a body portion  308  extending along a central axis “A” between first and second ends  310 ,  312 . The body portion  308  is defined by a peripheral wall having opposed interior and exterior surfaces, and defining a hollow interior. In the illustrated example, the body portion  308  is generally cylindrical in shape. The first end  310  has an internal flange  314  which is sized to define a stop against axial motion of the collet  304 . 
     A pair of trunnions  315  extend laterally outwards from the body portion  308  at or near the second end  312 . The trunnions  315  may include surfaces which are wholly or partially arcuate or cylindrical. 
     The housing  302  includes an extension portion  318  extending away from the second end  312  of the body portion  308 . The extension portion  318  is coupled to the body portion  308  by a breakaway structure  320 . In the illustrated example, the breakaway structure  320  includes stress-concentrating columns  328  which join to the trunnions. 
     The extension portion  318  extends between a first end  322  and a second end  324 . The second end  324  is interconnected to the breakaway structure  320 . The first end  322  may be provided with a mechanical connector  326  for being connected to an insertion instrument, such as screw threads. 
     The washer  316  is shaped like a disk and has an upper side  358 , an opposed lower side  360 , and a peripheral surface  362  interconnecting the upper and lower sides  358 ,  360 . The washer  316  may be considered a “flange element”. An aperture  364  shaped and sized to receive the body portion  308  passes through the washer  316 . The washer  316  includes a pair of spaced-apart pivot recesses  366  which are complementary to the trunnions  315 . When the body portion  308  is assembled into the washer  316 , it can pivot about the trunnions  315 . Depending on how the body portion  308  is pivoted, a reference plane “P” passing through the washer  316  may be oriented perpendicular to the central axis A, or at an angle θ oblique to the central axis A. This pivoting action accommodates a range of hole angles relative the bone cortical surface. As best seen in  FIG.  52   , the upper and lower sides  358 ,  360  may incorporate reliefs  367 ,  368  to permit pivoting of the body portion  308  to angles at the extreme ends of its range. 
       FIGS.  55 - 59    illustrate an exemplary embodiment of an anchor  400  which incorporates a pivoting function. The anchor  400  is similar in overall construction to the anchor  400  described above. Elements of the anchor  400  not specifically described may be taken to be identical to corresponding elements of the anchor  200 . The anchor  400  includes a housing  402 , a collet  404  received in the housing  402  and configured to be swaged around and against a tensile member, and a sleeve  406  received in the housing  402  which is capable of moving axially within the housing  402  so as to swage the collet  404 , thus retaining the tensile member  40 . 
     The housing  402  has a body portion  408  extending along a central axis “A” between first and second ends  410 ,  412 . The body portion  408  is defined by a peripheral wall having opposed interior and exterior surfaces, and defining a hollow interior. In the illustrated example, the body portion  408  is generally cylindrical in shape. The first end  410  has an internal flange  414  which is sized to define a stop against axial motion of the collet  404 . 
     An annular ball surface  415  extends laterally outwards from the body portion  408  at or near the second end  412 . 
     The housing  402  includes an extension portion  418  extending away from the second end  412  of the body portion  408 . The extension portion  418  is coupled to the body portion  408  by a breakaway structure  420 . In the illustrated example, the breakaway structure  420  includes stress-concentrating columns  428  which join to the trunnions. 
     The extension portion  418  extends between a first end  422  and a second end  424 . The second end  424  is interconnected to the breakaway structure  420 . The first end  422  may be provided with a mechanical connector  426  for being connected to an insertion instrument, such as screw threads. 
     The housing  402  is received in a hollow cup  470  having first and second ends  472 ,  474 . The first end  472  of the cup  470  defines a concave seat  476  which is complementary to the ball surface  415  of the body portion  408 . Thus assembled, the housing  402  can pivot to various orientations relative to the cup, as seen in  FIGS.  56  and  57   . A generally annular flange  416  is located at or near the second end  474  of the cup  470 . The cup  470  may be considered a “flange element”. 
       FIGS.  60 - 62    illustrate some possible alternative anchors for the tensile member  40 .  FIGS.  60  and  61    illustrate an anchor  480  with a ball pivot structure similar to anchor  400 , but lacking the deep sub-cortical cup.  FIG.  62    illustrates an anchor  484  with an externally-threaded housing. 
       FIG.  63    illustrates an exemplary insertion instrument  500  which may be used to insert, tension, and activate any of the swage-type anchors described above. 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  40  passes through the hollow interior of tensioner  514  and is secured to the spool  522 . The tension applied to the tensile member  40  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  40  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 kg ( 222  N) to about (50 lb.) 
       FIG.  64    illustrates protective sleeves  600  which may be implanted at the exit holes on the inboard sides of the passages through the femur F and/or tibia T. The sleeves  600  are wear-resistant semi-flexible members and may be constructed from a material such as a biocompatible or bio-resorbable polymer. Tensile members  40 ,  40 ′ are routed through the sleeves. In use, they extend “around the bend” of the exit hole to prevent degenerative action of the tensile member  40  against the exit hole in the bone. 
     The apparatus and method described herein have numerous advantages over prior art apparatus and techniques. They provide a desirable knee outcome, give the ability to have a reinforced PCL with uni-condylar constructs, and have a potential for long term cost reduction, compared to prior art cruciate-sacrificing methods. Additionally, a PCL-preserving or reinforcing knee arthroplasty technique may contribute to better patient outcomes, higher patient satisfaction, and a reduction in the cost of overall patient health. 
     The foregoing has described apparatus and methods for reinforced 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.