Abstract:
Prostheses, methods and related instrumentation for treating disorders of the knee by displacing a connective tissue acting on the patella to alter the location, angle or magnitude of forces exerted by the tissue on the patella so as to achieve a therapeutic effect in patellofemoral compartment of the knee.

Description:
RELATED APPLICATION DATA 
     This application claims the benefit of priority of U.S. Provisional Patent Application Ser. No. 61/951,469, filed Mar. 11, 2014, and U.S. Provisional Patent Application Ser. No. 61/951,470, filed Mar. 11, 2014, both of which are entitled “Apparatus and Methods for Treatment of Patellofemoral Conditions”; this application is also a continuation-in-part of U.S. Nonprovisional patent application Ser. No. 13/002,829, filed Aug. 27, 2010, and titled “Method and Apparatus for Force Redistribution in Articular Joints”; which application is a 371 of International Patent Application No. PCT/US10/46996, filed Aug. 27, 2010, and titled “Method and Apparatus for Force Redistribution in Articular Joints”, which claims the benefit of priority of U.S. Provisional Patent Application Ser. No. 61/237,518, filed Aug. 27, 2009, and U.S. Provisional Patent Application Ser. No. 61/288,692, filed Dec. 21, 2009, each entitled “Method and Apparatus for Force Redistribution in Articular Joints”; this application is also a continuation-in-part of U.S. Nonprovisional patent application Ser. No. 13/843,128, filed Mar. 15, 2013, and titled “Method and Apparatus for Altering Biomechanics of Articular Joints”; which application claims priority to U.S. Provisional Patent Application Ser. No. 61/620,756 filed on Apr. 5, 2012 and U.S. Provisional Patent Application Ser. No. 61/695,406 filed on Aug. 31, 2012; and which application is also a continuation-in-part of U.S. Nonprovisional patent application Ser. No. 12/870,462, filed on Aug. 27, 2010 (now U.S. Pat. No. 8,597,362 issued Dec. 3, 2013), which claims priority to U.S. Provisional Patent Application Ser. No. 61/237,518, filed Aug. 27, 2009, and U.S. Provisional Patent Application Ser. No. 61/288,692, filed Dec. 21, 2009. Each of the foregoing applications is incorporated by reference herein in its entirety. 
    
    
     The present invention generally relates to the field of orthopedic prostheses and procedures. In particular, the present invention is directed to apparatus and methods for treatment of patellofemoral conditions. 
     BACKGROUND 
     As the present invention is directed to apparatus and methods for treatment of patellofemoral conditions, a basic discussion of the anatomy of the knee with a focus on the patellofemoral structures may assist in describing the various embodiments of the invention.  FIG. 1  is a schematic portrayal in side view of a human knee. The bones of the knee joint comprise the femur (F), tibia (T), and patella (P). The fibula (fib) is another bone in the lower leg that attaches to the tibia. As primarily relevant to embodiments of the present invention, connective tissues of the knee joint include the patellar tendon (PT) and the quadriceps tendon (QT). The patellar tendon is also referred to as the patellar ligament as its primary function is to create a connection between bones, i.e., the tibia and patella. The patellar tendon extends from the caudal (lower) extent of the patella to an attachment point on the tibial tuberosity (TT) of the tibia. The tibial tuberosity comprises a raised area on the anterior (forward) aspect of the tibia, caudally positioned (toward the foot or lower) with respect to the cranial (upper) end of the tibia. The quadriceps tendon extends from the cranial extent of the patella and joins with the quadriceps-femoris muscle. 
     The knee is a synovial joint, meaning that the bones are not directly joined but are surrounded by dense connective tissues forming an articular capsule (C) lined by a synovial membrane. The capsule defines a synovial cavity or intracapsular space (IC) that contains the articular cartilage of the joint (not shown) and synovial fluid that acts to reduce friction between the articular cartilages. The approximate extent of the capsule is indicated in  FIG. 1  by dashed lines (C′). The quadriceps and patellar tendons lie outside the capsule. Also outside the capsule is the infrapatellar fat pad (FP). The fat pad is situated posteriorly and caudally with respect to the patella and joins with the patellar tendon on its posterior side over much of its length. The knee joint also includes a number of bursae to protect and facilitate movement between various bony and soft tissues. One of these bursae is the deep infrapatellar bursa (B), which allows for movement of the patellar tendon over the tibia. This bursa is positioned between the upper part of the tibia and the patellar tendon and lies in a pocket outside of the infrapatellar fat pad. 
     Because of the importance of the capsule in protecting and lubricating the articular cartilage, it is usually preferable, whenever possible in a knee intervention, to avoid penetrating the capsule. Because of the role of the infrapatellar fat pad in protecting the knee, it is also usually preferable to avoid dissecting the fat pad during knee interventions. Previously, removal of all or part of the fat pad was common in arthroscopic procedures in order to permit better visibility for the surgeon. However, it has been discovered that damage to the fat pad can lead to scarring, which can be painful and even crippling in some patients. 
     Treatments for various patellofemoral pathologies such as patellofemoral pain and patellofemoral osteoarthritis (PFOA) have been increasingly investigated. One early treatment, which involves anteriorization of the patellar tendon by a relatively invasive surgical procedure, was devised by Dr. Paul Maquet in the early 1960s. See, P. Maquet, 30 Revue Du Rhumatisme, No. 12, December 1963, pp. 779-783, “Biomechanical Treatment of Patellofemoral Osteoarthritis, Advancement of the Patellar Tendon” (translated title). In this procedure, an iliac bone autograft is implanted under the patellar tendon to relieve pressure in the patellofemoral space. Later Dr. Maquet evolved his technique to cut the tibial tuberosity away from the tibia and reposition it. This became known as the Maquet Osteotomy, which has been performed on tens of thousands of patients over the years with positive results. See, e.g, Maquet, Biomechanics of the Knee, pp. 134-143 (pub. Springer-Verlag 1976). However, the Maquet Osteotomy is a highly invasive procedure, which carries with it all of the risks and costs associated with highly invasive orthopedic surgeries. 
       FIG. 2  is a schematic illustration of a relatively square bone implant  2  of the type proposed by Dr. Maquet implanted on the tibia (T) under the patellar tendon (PT). The patellar tendon is attached cranially to a caudal portion of the patella, and caudally to the tibia at the Tibial Tuberosity (TT). The natural line of action of the patella tendon would be generally along a line (L) extending between the two attachment points. Placing the bone implant  2  under the patellar tendon moves the patellar tendon anteriorly between its two attachment points and thus alters the line of action with respect to the patella. The new line of action (L 1 ), oriented more away from the patellofemoral space can reduce the pressure on that space. Said another way, anteriorizing the patellar tendon renders the angle between the patellar tendon and the quadriceps tendon more obtuse, reducing the resultant force pressing the patella against the femur. 
     However, the success of such a procedure may depend heavily on the configuration of the implant used. If the anterior, tissue-engaging surface of the implant is roughly perpendicular to the caudal face of the implant and/or parallel to the underlying surface of the tibia, it displaces the patellar tendon relatively directly anteriorly (perpendicular to the tibial surface) and creates an abrupt step at the caudal edge of the implant, which can produce a number of complications. First is the creation of an unsightly bump on the knee. This is not merely a cosmetic problem, as the bump may catch on clothing or other, harder objects that could cause bruising or injury in the course of daily activity. Second, such an implant could be extremely uncomfortable in certain common positions. For example, if a patient with such an implant were to kneel on that knee, all of the load would be placed on that implant, which could be painful and also damaging to the patellar tendon. 
     A third possible complication arises from the fact that an implant shaped such as implant  2  also pulls the patella caudally, creating an undesirable misalignment. This condition is referred to as “Patella Infera” or “Patella Baja”. The symptoms of this misalignment can include pain on quadriceps contraction, inadequate quadriceps contraction, swelling, edema, joint stiffness, limited joint motion and limited patellar mobility. 
     Further, it may be desirable to maximize the area of the posterior surface of the implant that lies against the bone in order to spread the forces on the implant over as wide an area as possible. And, in order to minimize Patella Baja, it may be desirable to engage the patellar tendon as far cranially as possible without interfering with the patella or other tissues during knee movement. Yet, the space in which the implant is to be located, between the tibial tuberosity and the fat pad, bursa, and/or capsular tissues, is extremely limited. If the posterior surface of the implant extends too far cranially along the bone surface it may interfere with the fat pad, bursa, or joint capsule, causing pain or other complications. What is needed, therefore, are devices and methods for relieving patella-femoral pain due to osteoarthritis or other conditions that overcome the foregoing challenges. 
     SUMMARY OF THE DISCLOSURE 
     In one implementation, the present disclosure is directed to a prosthesis for treating disorders of the knee in the patellofemoral compartment of the knee. The prosthesis includes a fixation portion configured to be mounted to the tibia proximate the upper tibial extremity and medially or laterally of the tibial tuberosity, a spanning section configured and dimensioned to extend cranially and laterally or medially from the fixation portion in a direction towards the tibial mid-line, and a displacement portion configured and dimensioned to (i) extend from the spanning section further laterally or medially under patellar tendon and in engagement therewith, and (ii) displace the patellar tendon anteriorly sufficiently to alter the location, angle or magnitude of forces exerted thereby on the patella so as to achieve a therapeutic effect in patellofemoral compartment of the knee. 
     In another implementation, the present disclosure is directed to a prosthesis for treating disorders of the knee in the patellofemoral compartment of the knee. The prosthesis includes a fixation portion configured to be mounted to the tibia proximate the upper tibial extremity and medially or laterally of the tibial tuberosity, a spanning section configured and dimensioned to extend cranially and laterally or medially from the fixation portion in a direction towards the tibial mid-line, a displacement portion configured and dimensioned to extend from the spanning section further laterally or medially under the patellar tendon, defining a space between at least a part of the displacement portion and tibial surface, to displace the patellar tendon at least anteriorly from a normal, anatomical path; and a supplemental support element with a bone engaging surface disposed at an end of the displacement portion opposite the spanning section, the displacement portion being further configured and dimensioned to dispose the bone engaging surface against the tibial surface when the fixation portion is mounted to the tibia. 
     In yet another implementation, the present disclosure is directed to a prosthesis for repositioning a target tissue, the target tissue comprising a connective tissue or muscle relative to a bone on which the target tissue acts. The prosthesis includes a fixation portion having one or more fixation features configured to receive fixation elements for securing the implant to the bone, and a displacement portion having a first end connected to the fixation portion and a free end opposite the first end, the displacement portion having a bearing surface configured to atraumatically engage and reposition the target tissue relative to the bone wherein the displacement portion has a base portion configured to engage the bone and a cantilevered portion extending from the base portion to the free end, the cantilevered portion being configured to be spaced apart from bone when the base portion is engaging the bone. 
     In still another implementation, the present disclosure is directed to a femorally mountable prosthesis for treating patellofemoral osteoarthritis or patellar maltracking. The prosthesis includes a fixation portion including one or more fixation holes, the fixation portion being generally straight and elongated, and configured for fixation to the femur at least approximately aligned with the femoral shaft on a lateral, medial or anterior-medial/lateral side of the femur, and cranially with respect to the patella; a displacement portion configured and dimensioned to (i) be positioned under the quadriceps tendon cranially with respect the attachment point of the quadriceps tendon to the patella, and (ii) to atraumatically engage and displace the quadriceps tendon anteriorly relative to the femur to increase space in the patellofemoral area; and a spanning section interconnecting the fixation portion and the displacement portion, the spanning section configured and dimensioned to position the displacement portion to engage and displace the quadriceps tendon. 
     In still yet another implementation, the present disclosure is directed to an instrument for reshaping a bone surface. The instrument includes a shaft having proximal and distal ends arranged along a longitudinal axis and an elongated file element coupled to the distal end of the shaft and extending in a direction transverse to the shaft and longitudinal axis, the file element having a curvature about a second axis transverse to the longitudinal axis and having anterior and posterior surfaces lying in respective planes intersected by the second axis, at least one of the anterior and posterior surfaces having features configured to reshape bone when the file is moved in engagement therewith. 
     In another implementation, the present disclosure is directed to a method for treating patellofemoral osteoarthritis or patellar maltracking. The method includes mounting a prosthesis on the tibia at a fixation site outside of the knee joint capsule proximate the upper tibial extremity and medially or laterally of the tibial tuberosity without rupturing the capsule and with a portion of the prosthesis extending under the patellar tendon to displace the patellar tendon from a normal, anatomical path in at least an anterior direction and without disrupting the attachment of the infrapatellar fat pad to the tibia and without dissecting the infrapatellar fat pad. 
     In yet another implementation, the present disclosure is directed to a method of implanting a device on the tibia. The method includes inserting a file element through an incision on a medial or lateral side of the tibia such that the file element extends in a medial-lateral direction across an anterior surface of the tibia cranially of the tibial tuberosity and a handle coupled to the file element extends in a cranial-caudal direction along the tibia outside the incision; moving the handle such that file element reshapes the anterior surface of the tibia to a first shape; placing an implant through the incision and positioning a base portion of the implant in engagement with the anterior surface of the tibia with a displacement portion of the implant under the patellar tendon, the base portion having a posterior surface with a shape complementary to the first shape; and securing the implant to the tibia. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For the purpose of illustrating the invention, the drawings show aspects of one or more embodiments of the invention. However, it should be understood that the present invention is not limited to the precise arrangements and instrumentalities shown in the drawings, wherein: 
         FIG. 1  is a schematic diagram illustrating anatomical features of a human knee joint in a side view; 
         FIG. 2  is a schematic representation of an implant in accordance with early work of Dr. Paul Maquet; 
         FIG. 3  is a schematic illustration of an implant according to one disclosed embodiment; 
         FIG. 3A  is a combined view with the embodiment shown in  FIG. 3  superimposed over the implant shown in  FIG. 2 ; 
         FIGS. 4A and 4B  are diagrams illustrating profiles for tissue bearing surfaces in accordance with disclosed embodiments; 
         FIGS. 5A, 5B and 5C  are a series of schematic illustrations showing disclosed embodiments implanted on the tibia at the knee, with the knee at flexion angles of about 0°, 45° and 90°, respectively; 
         FIGS. 6A and 6B  illustrate another embodiment disclosed by the inventors in an incorporated application; 
         FIGS. 7A, 7B, 7C, 7D, 7E, 7F and 7G  illustrate features of another embodiment disclosed by the present inventors in another incorporated application; 
         FIGS. 8A, 8B and 8C  show alternative embodiments, wherein  FIG. 8A  is a schematic illustration of an implant on the tibia,  FIG. 8B  is a perspective view of the implant, and  FIG. 8C  illustrates a further alternative, as would be seen along line C-C in  FIG. 8B ; 
         FIGS. 9A and 9B  show further alternative embodiments, wherein  FIG. 9A  is a schematic illustration of the implant on the tibia and  FIG. 9B  is a cross-sectional view through section A-A of  FIG. 9A ; 
         FIG. 9C  illustrates another alternative embodiment, again in a view through section A-A of  FIG. 9A ; 
         FIG. 9D  is a cross-sectional view through section D-D of  FIG. 9B ; 
         FIG. 10  is a schematic diagram of a human knee in side view illustrating positioning of an implant according to another disclosed embodiment; 
         FIGS. 11A, 11B and 11C  are perspective views of further alternative embodiments employing supplemental fixation/support means; 
         FIG. 11D  is a schematic illustration of an embodiment such as shown in  FIG. 11C  in place on a portion of the tibia; 
         FIGS. 12A, 12B, 12C, 12D and 12E  are various views of yet another alternative embodiment employing a separate fixation base member; 
         FIG. 12F  is a schematic illustration of a further alternative embodiment, also employing a separate fixation base in place on a portion of the tibia; 
         FIGS. 13A, 13B, 13C, 13D and 13E  are views of other alternative embodiments having differently shaped fixation portions; 
         FIGS. 14A and 14B  are schematic illustrations of further alternative embodiments adapted for femoral fixation; 
         FIGS. 15A and 15B  are views of an embodiment of disclosed instrumentation useful in procedures for placement of implants disclosed herein; 
         FIG. 16  is a drawing of another alternative embodiment viewed from the anterior aspect of an implant with a configuration generally as described herein; 
         FIG. 17  is a drawing of another alternative embodiment viewed from the anterior aspect of an alternative implant including features similar to the embodiment shown in  FIGS. 9A and 9B ; 
         FIG. 18  is a drawing of another alternative embodiment viewed from a cranial aspect of another alternative implant with a displacement portion having an increasing thickness; 
         FIG. 19  is a drawing of another alternative embodiment viewed from an anterior aspect of a further alternative implant including features similar to the embodiments shown in  FIGS. 11B and 11C ; and 
         FIG. 20  is a drawing of another alternative embodiment viewed from a caudal aspect of yet another alternative implant with features to facilitate positioning prior to fixation; 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention employ an improved implant geometry with an appropriately curved cross-section to address drawbacks of some prior devices and procedures, such as an unsightly and uncomfortable bump, concerns about tissue damage, and the caudal movement of the patella as previously discussed. Other embodiments of the present invention employ supplemental support/fixation means and specially shaped fixation portions to facilitate implantation, increase fixation security and resist torqueing forces. Further embodiments of the present invention encompass less invasive methods for treatment of patellofemoral conditions, including employing implant embodiments disclosed herein. 
     The present inventors have disclosed implants for treating PFOA in United States Patent Publication US 2011/0213466, entitled “METHOD AND APPARATUS FOR FORCE REDISTRIBUTION IN ARTICULAR JOINTS,” and, more recently in United States Patent Publication US 2013/0211521, entitled “METHOD AND APPARATUS FOR ALTERING BIOMECHANICS OF ARTICULAR JOINTS,” each of which is incorporated herein by reference in their entirety. In certain embodiments therein disclosed, an implant portion is inserted underneath the patellar tendon, just cranial to the attachment of the patellar tendon to the tibial tuberosity. This implant portion displaces the patellar tendon anteriorly, flattening the angle between the patellar tendon and the quadriceps tendon. This change in angle reduces the resultant pressure of the patella against the femur, reducing patellar pain and patellofemoral cartilage wear. The implant may also improve patellar tracking, or shift the location, angle or loading of the patella against the femur. 
       FIG. 3  schematically illustrates features of the embodiments in this disclosure. The schematic side view of  FIG. 3  helps to illustrate the positioning and curved shape of the bearing surface  11  and displacement portion  12  of embodiments disclosed herein. The bearing surface  11  is the surface of the displacement portion  12  in contact with the patellar tendon. Line L 2  shows the approximate line of action of the patellar tendon after repositioning over displacement portion  12 . To simplify  FIG. 3  for discussion purposes, fixation means and other implant structures such as the fixation portion and spanning section discussed in more detail below are not called out. The various fixation and support structures discussed below facilitate the cantilevering of displacement portion to better accommodate soft tissue structures while properly positioning the bearing surface as discussed in more detail below. 
     In general, implants according to embodiments of the invention will be configured and dimensioned to displace the tissue targeted for treatment by between about 5 mm to about 30 mm from the natural, anatomical tissue path. In some embodiments, the displacement will be greater than about 10 mm. Overall displacement amounts can be set through a combination of shape and size of the fixation portion, spanning section and displacement portion of the implant as previously described. Working within those parameters, it has been discovered that by shaping the bearing surface at least approximately as a quarter-circle in cross-section with a minimum radius of about 8 mm, caudal biasing of the patella can be reduced. Further flattening the curvature of the bearing surface, by increasing radius, or making the surface more oval, elliptical, hyperbolic, or of another complex shape, can further reduce caudal biasing, but space limitation arising from the anatomy and need for a minimum displacement to achieve therapeutic effects may limit the amount of such flattening that may be applied to the implant. In addition, the displacement portion and/or bearing surface may be shaped and dimensioned to provide different magnitudes of displacement at different points along the surface such that the tissue is displaced different amounts at different joint positions, e.g. at different points in the gait cycle. As used herein, “therapeutic effect” means an effect on a treated joint that reduces forces acting on the articular surfaces, reduces wear, lessens pain or provides another positive outcome for the patient whether across the joint as a whole or in particular compartments of the knee. “Therapeutic effect,” however, does not imply, and should not be understood as requiring, any specific, quantified outcome other than as stated above. 
     As shown in  FIG. 4A , for a bearing surface (B) with a depth and length of 1 mm in each dimension, the patella is pulled caudally by only about (Π/2−1) or 0.57 millimeters. In an alternative embodiment, the bearing surface curve may be flattened even further with a length of about 2 mm while maintaining the 1 mm depth, as shown in  FIG. 4B . In this embodiment, the caudal displacement of the patella would be less than half of the anterior displacement. The generally elliptical shape of the bearing surface causes such an implant to extend generally twice as far cranially as it does anteriorly and thus would pull the patella caudally by an amount approximately equal to 0.42 times the anterior displacement. Caudal displacement with an elliptically shaped bearing surface may be estimated based on a corresponding elliptical circumference. For example, using an “ellipse calculator” (e.g. as available online at http://www.cleavebooks.co.uk/scol/callipse.htm) and selecting a major axis of 4 and a minor axis of 2, a circumference of 9.69 can be determined. Dividing the circumference by 4 gives 2.422 (approximately one quarter of the elliptical circumference corresponding to the overall length of the bearing surface). With this information, it may be estimated that a curved bearing surface with a cranial-caudal length of 2 cm and an anteriorization of 1 cm will pull the patellar tendon caudally approximately 0.42 cm. 
     A geometry as described in the preceding paragraphs should dramatically reduce the complications caused by patella baja from square or steeply profiled implants. For example, a prior art implant with a square cross-section, such as shown in  FIG. 2 , would pull the patella caudally by approximately one millimeter for each millimeter of anteriorization; about twice the amount of caudal displacement created by embodiments disclosed herein for the same amount of anteriorization. 
     In certain embodiments, the bearing surface  11  will be positioned with its outer most point (apogee) at a perpendicular distance from the surface of the tibia below it of about 0.3-3 cm, or more typically about 0.5-1.5 cm for an implant configured to treat an average adult knee. The width of the bearing surface in the generally cranial-caudal direction will be about 0.5-3.0 cm, or more typically about 1.0-2.5 cm. While distance from the tibia to the apogee of the bearing surface can equal the bearing surface width, in some embodiments the width will be greater than that distance, about 1.1-3.0 times greater, or more typically about 1.5-2.0 times greater. Further alternative embodiments may employ bearing surfaces with compound curvatures comprising elements of  FIGS. 4A and 4B  as previously discussed. 
     A further physiologic benefit to an implant with the curved geometry as described is that the forces pressing the patella against the femur are highest when the knee is bent, such as when a person is climbing stairs. As shown in  FIG. 3A , where an embodiment of the present invention is superimposed over a prior implant, the shape of the present invention is more effective in flattening the angle between the patellar tendon and the quadriceps tendon when the knee is bent. It also reduces the focal stress on the sharply angled portion of the patellar tendon caused by such prior implants, especially when the leg is straight. The extreme caudal positioning of the prior implant  2  is indicated at (A) in  FIG. 3A . 
     The beneficial effect of embodiments of the present invention as related to knee flexion are further illustrated in  FIGS. 5A-C . From these figures, it can be seen that the angle α of anteriorization is increased from the natural line of the patella tendon from α 1  with the knee fully extended, to α 2  at partial flexion, up to α 3  at 90° flexion in  FIG. 5C , where α 1 &lt;α 2 &lt;α 3 . The unloading provided by the implant thus increases with knee flexion, providing the greatest relief when the patella is maximally loaded. 
     Further embodiments are shown in  FIGS. 6A and 6B  (corresponding to  FIGS. 24 and 25 , respectively, of the present inventors&#39; first incorporated publication above), and  FIGS. 7A-F  (corresponding to  FIGS. 8 and 9A -E, respectively, of the present inventors&#39; second incorporated publication above). In the embodiments of  FIGS. 6A and 6B , implant  210  includes a support member  212  and bearing member  214 . The support and bearing members are functionally divided into displacement portion  216 , spanning section  218  and fixation portion  220 . The displacement portion, with the bearing member is partly cantilevered over the tibia so that a portion of the fat pad may be received thereunder. 
       FIGS. 7A-G  depict an exemplary prototype of implant  300  for treating patellofemoral osteoarthritis and/or patellar maltracking for the right knee. Implant  300  has a fixation portion  312  having one or more holes  315  for receiving screws for anchoring the implant to bone. Fixation portion  312  is generally straight and elongated, being configured for positioning in general alignment with the tibial shaft on the medial or anterior-medial side of the tibia. Bone engaging surface  313  is provided on the bone facing side of the fixation portion. Holes  315  are preferably positioned in approximate alignment with a longitudinal centerline of fixation portion  312 . 
     Displacement portion  314 , is configured and dimensioned to be positioned under the patellar tendon caudally separated from the insertion point of the tendon in the tibia. The displacement portion  314  is configured to atraumatically engage the tendon and displace it anteriorly relative to the tibia. The displacement portion  314  has a length in the lateral-medial direction generally selected to accommodate the full width of the tendon so that the tendon remains engaged along its entire width as it slides on the displacement portion. Displacement portion  314  preferably has a convex curvature on its outer tissue-engaging surface (bearing surface  309 ), preferably being curved at least around an axis generally parallel to the tibial shaft, usually being curved also around an axis perpendicular to the tibial shaft, and more preferably being spherical or partially spherical. Displacement portion  314  has a width in the caudal-cranial direction is selected so that it does not interfere with the patella or engage the insertion point of the tendon, typically being less than its length. A spanning section  316  interconnects fixation portion  312  and displacement portion  314 . Spanning section  316 , in the embodiment illustrated, extends cranially and laterally from fixation portion  312  to displacement portion  314 , forming a curve of about 90° about a dorsal-ventral axis. Where fixation portion  312  is configured for attachment to a more medial aspect of the tibia, spanning section  316  will extend ventrally as well as cranially and laterally from fixation portion  312 , preferably being curved about an axis generally parallel to the tibial shaft. Displacement portion  314  appropriately displaces the patellar tendon in cooperation with the fixation portion  312  and spanning section  316 . 
     Displacement of the target tissue can be altered by changing the length, curvature and angle of the spanning section among other features. For example, the angle α between the displacement portion  314  and the fixation portion  312  (as measured at the intersection of the center line axes of the two portions in the top view of the implant in  FIG. 7B ) may range from about 80 degrees to 135 degrees, more specifically from about 85 degrees to 120 degrees, and in some embodiments about 90 degrees to 110 degrees. 
     The width W 1  of the fixation portion  312  ( FIG. 7C ) typically will be large enough to span a substantial portion of the width of the tibia and to accommodate one or more screw holes of sufficient size, ranging from about 10 mm to 25 mm. In some embodiments, width W 1  may be about 12 mm to 20 mm, and in other embodiments about 14 mm to 18 mm. The length L 1  of the fixation portion  312  will be selected to accommodate a sufficient number of screw holes in the cranial-caudal direction along the tibia, usually at least two and in some embodiments up to five or more, and may range from about 20 mm to 50 mm, more specifically about 25 mm to 45 mm, and in some embodiments about 30 mm to 40 mm. 
     The width W 2  (generally cranial-caudal direction) of the displacement portion  314  ( FIG. 7B ) is generally selected to provide a broad area of contact with the tendon to spread the force and reduce wear, while not interfering with the patella or the tendon insertion point throughout the full range of joint motion. Width W 2  may thus range from about 10 mm to 25 mm, more specifically about 12 mm to 20 mm, and in some embodiments about 14 mm to 18 mm. The length L 2  (generally medial-lateral direction) of the displacement portion  314  is selected so that the displacement portion extends under the full width of the tendon so that the entire width of the tendon remains in engagement and displaced the desired amount throughout the range of joint motion. Length L 2  may thus range from about 20 mm to 50 mm, more specifically about 25 mm to 45 mm, and in certain embodiments about 30 mm to 40 mm. 
     As best seen in  FIGS. 7E-G , implant  300  also includes a supporting section  320  extending along the caudal extent of displacement portion  314 , into spanning section  316  and merging into the bone engaging surface  313  of fixation portion  312 . As will be appreciated by persons of ordinary skill, contour line  322  illustrates the approximate extent of supporting section  320  from the displacement portion, through the spanning section and into the fixation portion. 
     Supporting section  320  rests on the surface of tibia between the tendon insertion point and the fat pad and/or capsular tissue. The cranial-caudal length of bone engaging surface  313 , i.e., the approximate distance from cranial most location of contour line  322  delineating the cranial extent of the supporting section, to the caudal end of fixation portion  312  is preferably greater than the distance from same point on contour line  322  to the cranial edge of displacement portion  314 . The appropriate distance ratios between these two regions increases the moment arm resisting torqueing force applied by the patellar tendon through the cantilevered displacement portion  314  to help fix the implant in place and resist loosening over time due to the cyclic torqueing forces applied by knee flexion and extension. 
     Displacement portion Height (H), shown in  FIG. 7G , is the perpendicular distance from the apogee of bearing surface  309  to the bone engaging surface  313 . Height (H) directly effects the amount of displacement of the tendon achievable with the implant. In general terms, the displacement distance will approximately equal Height (H) minus the normal anatomical distance between the patellar tendon and the tibial surface below it at the location of the displacement portion when implanted. 
     Implant depth D, along with the radius of curvature R 1  of the outer surface of displacement portion  314 , shown in  FIG. 7F , are selected to balance tendon displacement throughout the range of joint motion with the appropriate fixation location. Radius of curvature R 1  is usually 20-35 mm, more preferably 22-33 mm, and most preferably 25-30 mm. For average patient anatomy, an overall implant depth (D), shown in  FIGS. 7C and 7F , as measured from the outermost surface of displacement portion  314  to the centerline of the screw holes in fixation portion  312 , would be in the range of 10-45 mm in order to provide target tissue displacements in the ranges cited hereinabove to achieve a therapeutic effect. 
     The inferior edge  304  of the spanning section  316  can also be curved to minimize or eliminate any contact with the medial edge of the patellar tendon. The superior surface edge  305  of the displacement portion  314  can be curved to allow for easy motion of the patellar tendon during flexion as well as to vary the displacement of the patellar tendon during flexion by varying the region of the implant surface in contact with the tendon at higher flexion angles. In one exemplary embodiment, implant  300  is placed on the medial side of the distal tibia such that fixation portion  312  is substantially aligned with the tibial shaft, the spanning section  316  is positioned to minimize contact with the medial edge of the patellar tendon, and the displacement portion  314 , extending laterally from the spanning section, is substantially parallel to the tibial plateau. 
     A supporting section as generally described above may be incorporated into other embodiments disclosed herein to facilitate locating the fixation portion (and in particular bone screw site) at a distance from the area where displacement portion acts to allow for easier placement of the device, without a need to place fixation elements such as nails or screws under or close to the patellar tendon, the joint capsule or the infrapatellar fat pad. It also means that the displacement element can be appropriately rounded and smooth, without any surface roughness or disturbances due to fixation elements. And although the fixation portion is at a distance from the displacement portion, much of the force from the patellar tendon is transmitted through the supporting section directly onto the tibia behind it. Further, by extending the fixation portion  312  caudally down the tibia relative to the supporting section (and contour line  322 ), the leverage applied by the fixation screws is increased so as to counter any tendency of the displacement portion to be tilted toward the tibia under the forces exerted by the patellar tendon. 
     Another alternative embodiment of the present invention is shown in  FIGS. 8A and 8B . While typically it would be preferable to position the fixation portion more caudally to locate the fixation means such as screws more distant from the joint capsule and other sensitive structures of the joint, in some patients and in some clinical situations this may not be possible, or fixation adjacent to the displacement portion may have other advantages. In this embodiment, implant  20  may have a displacement portion  22  with a bearing surface  24 . The bearing surface may be curved as described above in connection with  FIGS. 4A and 4B . The bearing surface also may be optionally provided with a concave groove or channel  25  extending in the cranial-caudal direction to assist in guiding the displaced tissue as it passes thereover. However, the same embodiment also may be provided without the channel. Implant  20  also may have a fixation portion  26  with a bottom surface having a slight concavity  27  in the cranial-caudal direction configured to be seated on the tibia just cranially of the tibial tuberosity. Fixation means  28  such as screw holes, spikes or bone ingrown facilitating elements may be included in fixation portion  26 . Persons of ordinary skill in the art will appreciate that the features of the concave groove or channel  25  and/or bottom surface concavity  27  may be employed with other embodiments as described herein and are not restricted to use with implant  20 . 
     As shown in  FIG. 8A , and as employed by other embodiments of the invention disclosed herein, displacement portion  22  includes a cantilevered portion  22 A extending in the cranial direction, forming an undercut region  22 B on the inferior (bone facing) side of cantilevered portion  22 A, cranially of fixation portion  26 . Cantilevered region  22 A is configured to extend cranially over the anterior surface of the tibia and the overlying fat pad such that the cranial edge of bearing surface  24  extends a distance X of about 5-30 mm, more preferably about 10-25 mm, and most preferably about 15-22 mm, from the cranial edge of fixation portion  26 . This facilitates engagement of bearing surface  24  with the patellar tendon as far in the cranial direction as possible without interfering with the patella or femur, while undercut region  22 B provides a space in which the fat pad, capsular ligaments or other soft tissues may reside. By engaging the patellar tendon further in the cranial direction, the displacement force applied to the patella has less of a caudal component, reducing “Patella Baja”. 
     In another alternative, a cover member  21  may be provided as shown in  FIG. 8C  to protect and retain the tendon when it is received in channel  25 . Cover  21  is securable over bearing surface  24 , such as with screws  23 . 
     A further alternative embodiment of the present invention is shown in  FIGS. 9A and 9B . In this embodiment, implant  30  may have a displacement portion  32  with a bearing surface  34 . Bearing surface  34  also may be curved as described above in connection with  FIGS. 4A and 4B . Displacement portion may be supported and positioned by spanning section  36 , which is in turn supported by fixation portion  38 . Spanning section  36  and displacement portion  32  may be configured and dimensioned to provide varying amounts of cantilever for the bearing surface  34 . Such a cantilever can provide clearance for the fat pad and/or other critical tissues behind the implant. Fixation means  39  such as screw holes, spikes or bone ingrown facilitating elements may be included in fixation portion  38 . 
     Referring to  FIGS. 9B and 9D , it can be seen that displacement portion  32  can be provided with a concavity on the bone-facing posterior side of displacement portion  32  that spaces the displacement portion away from the tibial, forming a space (S) therebetween with a height indicated by the double arrow. Space S preferably has a maximum height between an underside  32 U of displacement portion  32  and the surface of the tibia in a range of about 5-25 mm, or more typically about 10-20 mm, in order to accommodate the fat pad and other tissues beneath the displacement portion  32 . Displacement portion  32  has a supporting section  32 B, as described above, that sits in engagement with the tibia in the space between the tibial tuberosity TT and the caudal edge of the fat pad, thereby supporting displacement portion  32 . Height (H), the perpendicular distance between the bone engaging surface  326  of supporting section  32 B and the apogee of bearing surface  34 , as also described above, is shown in  FIG. 9D . 
     To fit displacement portion  32  within the available space, with reference again to  FIG. 9A , supporting section  32 B preferably extends in the cranial-caudal direction a distance (SS) of no more than about 20 mm, usually being about 5-15 mm, more preferably about 8-12 mm. The cranial end  32 C of displacement portion  32  preferably extends a distance (CE) of about 5-30 mm, typically about 10-25 mm, or more typically about 15-22 mm in the cranial direction from the upper (cranial) extent of supporting section  32 B, which lies against the tibia approximately at dimension line (DL) in  FIG. 9A  The concave shape of the posterior side of displacement portion  32  causes its lateral margin  33  to extend around the lateral side of the tibia (if the fixation portion of the implant is mounted on the medial side). In some embodiments, for example as shown in  FIG. 9B , lateral margin  33  may be configured to contact the tibia to provide additional support for the displacement portion and bearing surface in resisting forces applied by the patellar tendon, particularly at high flexion angles. An additional embodiment may have an additional fixation portion on the lateral end of the implant as shown in  FIG. 9C , to provide additional support and stabilization for the implant. 
       FIG. 10  illustrates an embodiment of the present invention, such as implant  30  described above, after implantation on the tibia. In this embodiment, implant  30  also employs an extended fixation portion  38 A for enhanced torque resistance as described in more detail below. Fixation portion  38 A may have an extension portion that wraps around either the anterior or posterior side of the tibia, or both, to further stabilize the implant. Multiple screw holes  39  and bone screws  31  are used as dictated by patient anatomy and clinical factors such as condition of the bone. 
     Placement and fixation of an implant according to embodiments of the present invention can often be accomplished through a single surgical incision adjacent the patient&#39;s knee. The implant is then placed through the incision with the displacement portion inserted under the patellar tendon cranially with respect to its attachment point to the tibia at the tibial tuberosity. A therapeutic location that is a target area for placement of the displacement portion includes the caudal pocket below the infrapatellar fat pad containing the infrapatellar bursa (B). Reference letter (B) is provided in  FIG. 10  to identify the target area, but the bursa itself is not shown because in some situations it may be necessary to remove part or all of the bursa to accommodate the implant. However, unlike the articular capsule or the infrapatellar fat pad, there are not significant potential negative indications associated with removal or dissection of the infrapatellar bursa. 
     Placement of the implant as shown for example in  FIG. 10 , allows the implant to be placed and fixed through a single incision without penetrating the capsule (C) or dissecting the infrapatellar fat pad (FP) or separating it from its attachment along the posterior of the patellar tendon (PT). Of course, depending on the shape and size of the implant as clinically determined by the surgeon, it may be necessary to push on and somewhat reposition the fat pad as indicated in  FIG. 10  as compared to  FIG. 1 . Also, as previously described, the smooth, curved shape of the bearing surface  34  and displacement portion  32  moves the patella anteriorly and away from the femur while limiting the amount of movement caudally, thus reducing or avoiding a Baja effect. The shape of implant  30  also effectively avoids and preserves the natural attachment point of the patellar tendon (PT) to the tibia at the tibial tuberosity (TT). 
       FIG. 10  also further illustrates how the shape of the spanning section  36  and displacement portion  32  provides a cantilevered bearing surface  34  to define space (S) under the implant to accommodate the fat pad and its attachment to the side of the tibia. The cantilevered portion of the implant may be configured to deflect under high loading conditions to reduce strain on the tendon. Such deflection may be engineered into the implant by selection of shape, thickness and material so as to allow the implant to flex, or more active means such as springs or hydraulic cylinders may be used. In further alternatives, bearing surfaces of implants according to embodiments of the invention may include resilient elements such as fluid filled pillows and/or pressure control volumes utilizing check or relief valve systems. 
       FIGS. 11A and 11B  illustrate further alternative embodiments employing supplemental support and fixation elements  50  and  54 . Implants  40 A and  40 B each include fixation portion  42  with fixation means such as bone screw holes  43 , spanning section  44  and displacement portion  46  with bearing surface  48 , all as previously described. Positioned at the end of displacement portion  46  on implant  40 A is supplemental support and fixation tab member  50  with at least one bone screw hole  52 . In some embodiments, implant  40 A may be generally shaped in a manner similar to implant  30  of  FIG. 9B , with tab member  50  disposed at the lateral margin  33  of the implant having a bone engaging surface in contact with the tibia. In other embodiments, the shape may be generally reversed such that tab member  50  would be disposed at a medial margin of the implant displacement portion. In a further embodiment shown in  FIG. 11C , tab member  50  has no bone screw hole, but simply provides additional surface area resting against the tibial surface to stabilize the device and to more widely distribute the pressure of the device due to the force of the patellar tendon against the device. 
     The position of tab member  50  with respect to fixation portion  42  may necessitate a second surgical incision site when placing implant  40 A. In order to provide supplemental fixation and support means without necessitating a second incision site, means such as shown in  FIG. 11B  for implant  40 B may be alternatively employed. In this embodiment, displacement portion extension  56  extends the displacement portion in a caudal direction around the lateral side of the tibia (if the fixation portion  42  is mounted to the medial side of the tibia). Supplemental support and fixation tab  54  is disposed at the caudal and/or lateral margin of the extended displacement portion and provided with at least one fixation hole  57 . To allow for placement and fixation from a single incision site, fixation hole  57  is configured to accommodate fixation rod  58  and is aligned with a corresponding fixation hole  57  in fixation portion  42 . Fixation rod  58  may comprise a threaded rod or elongated bone screw, and fixation hole  57  may be threaded so as to receive the threaded tip of the fixation rod. In an alternative embodiment, fixation rod  58  may be threaded over its entire length with a pointed distal end and a bone screw head at the proximal end adapted to receive a torqueing tool such as a hex driver. 
     Placement of an embodiment such as implant  40 B is achieved by positioning the fixation portion on one side of the tibial tuberosity with the extended displacement portion  56  extending around the attachment of the patellar tendon to the tibia and back down caudally on the opposite side of the tibial tuberosity. With fixation holes  57  thus aligned on opposite sides of the tibia, fixation rod  58  may be inserted through the same surgical incision and through a portion of the tibia to fix both holes  57  in a single operation. Additional fixation screws may be placed in other holes  43 , again through the same surgical incision. A specialized drill guide might be employed to ensure accurate alignment while drilling the hole. 
     Depending on patient anatomy and other clinically determined parameters, placement of an implant according to embodiments of the present invention may present a challenge because of the torqueing forces exerted on the displacement portion after insertion under the patellar tendon, even before fixation means, such as bone screws, are secured. Such torqueing forces would tend to lift the fixation portion away from the bone surface to which it was to be affixed. In this situation, a separate fixation base may be employed as shown, for example, in  FIGS. 12A-E . In this alternative embodiment, implant  60  has a fixation portion that comprises a body member fixation portion  62 A and a base member fixation portion  62 B.  FIG. 12A  shows the part unassembled before placement of the body member fixation portion  62 A, and  FIG. 12B  shows the assembled parts as they may appear after placement. 
     As shown in  FIGS. 12A and 12B , implant  60  also includes spanning section  66  and displacement portion  68  with bearing surface  69  generally as previously described. In one embodiment, screw holes  63 , configured to receive bone screws  64 , are provided only in base member fixation portion  62 B. In a further alternative embodiment, illustrated only in  FIG. 12A , additional fixation screw holes  63 A may be provided in both the base member and body member fixation portions and positioned so that the holes align when the body member is received in the base member. In another alternative embodiment, bone screw access holes  63 B may be provided as discussed further below. Access holes  63 B are shown in  FIG. 12A  in dashed lines as optional features. 
     Body member fixation portion  62 A and base member fixation portion  62 B are provided with complementary, mating shapes to permit them to be securely fitted together. Persons of ordinary skill in the art may select from various complementary shapes, one example of which is shown in  FIGS. 12C and 12D , which are end views at lines C-C and D-D, respectively, in  FIG. 12A . In this exemplary embodiment, base member fixation portion  62 B defines a channel  70  with retaining edge  72  that extends therearound. The complementary shape of body member fixation portion  62 A is provided by guide channel  74 , which in this exemplary embodiment extends around the caudal end and onto both sides of the body member fixation portion  62 A. In other embodiments, separate mating features may be provided only on the sides, not extending around the caudal end of the implant. 
     The two-piece design of an embodiment such as implant  60  permits the base member fixation portion  62 B to be first secured at a selected location without an eccentric or torqueing forces applied by the target tissue through the displacement portion  68 . Fixation means such as holes  63  and bone screws  64  may be used to secure the base member fixation portion  62 B. Once proper placement is confirmed, displacement portion  68  may be inserted under the target tissue, such as the patellar tendon, and then base member fixation portion  62 A inserted into base member fixation portion  62 B with a relatively straightforward sliding action as indicated by the arrow in  FIG. 12A  to provide a combined implant generally as shown in  FIG. 12B . Screw holes  63  and screws  64  are shown in phantom lines in  FIG. 12B  because they are covered by body member fixation portion  62 A. For this reason screws  64  preferably are low profile screws with flat heads to avoid interference with the body member when inserted into the base member. Channel  74  is also shown in phantom lines because it is received behind edge  72 . 
     Various locking means for securing the body member to the base member are possible. One such locking means embodiment is schematically illustrated in  FIG. 12E . In this illustrative embodiment, inter-engaging teeth  76 A and  76 B are provided on the facing surfaces, respectively, of the body member channel  74  and the base member retaining edge  72 . When body member is inserted into the base member as indicated by the arrow in  FIG. 12E , the inter-engaging teeth act in a ratchet-like manner, permitting insertion but preventing removal. In some embodiments, the teeth may themselves be formed with resiliency to permit insertion. In other embodiments, the teeth may be relatively short with less resiliency to permit insertion provided by elastic deformation of base member fixation portion  62 B and retaining edge  72 . For even greater fixation security, after the body member is fully received in the base member, additional bone screws may be inserted through optional, additional fixation screw holes  63 A, which become aligned as described above. 
     It will also be appreciated by persons of skill in the art, that inter-engaging teeth or other ratchet-type locking means may be difficult to disengage if it becomes necessary to remove or reposition the base member during the initial implant procedure or a later intervention. Disengagement may be achieved, for example, by deformation of base member fixation portion  62 B and retaining edge  72 . In one alternative, bone screw access holes  63 B may be provided in body member fixation portion  62 A as shown in  FIG. 12A . Access holes  63 B are positioned to align with bone screw holes  63  in base member fixation portion  62 B when the body member is received in the base member after implantation. Using access holes  63 B, bone screws  64  may be removed without separating the body member from the base member. 
     In another alternative embodiment, inter-engaging or ratchet-type locking means is not provided. Instead, locking means may be provided by one or more additional fixation screw holes  63 A. In such an embodiment, body member fixation portion  62 A may be freely inserted and removed from base member fixation portion  62 B once the base member is installed. The complementary shape of the mating parts as described initially carries the torqueing force of the target tissue acting on displacement portion  68  and then the two members are locked together using bone screws through one or more additional fixation screw holes  63 A. Bone screw access holes  63 B also may be included as desired to provide further removal options. 
       FIG. 12F  shows an additional alternative body member-base member geometry, with a curved interface between the parts which allows adjustment for any variation in the angle of the tibial surface against the fixation portion. This allows the body member  69  to be positioned so that the posterior edge of the displacement section rests firmly against the tibia as the fixation screws are tightened. 
     As mentioned above, torqueing and other complex forces applied to the fixation portion through the target tissue acting on the displacement portion may be significant. In order to better resist such forces various supplemental fixation and support embodiments may be provided. Two such exemplary embodiments have been described above in connection with  FIGS. 11A and 11B . Additional exemplary embodiments are shown in  FIGS. 13A-E . Implants  80 A-E of  FIGS. 13A-E  each include fixation portion  82  with screw holes  83 , spanning section  84  and displacement portion  86  with bearing surface  88  generally as previously described. In addition to these basic structures, implant  80 A may include extension portions extending from fixation portion  82  in a direction generally transverse to that of fixation portion  82 . For example, as shown in  FIG. 13A , an extension portion  90 A may be configured to extend generally in the same direction as the displacement portion, e.g., if the fixation portion  82  is mounted on the lateral side of the tibia, in a medial direction (or angled medially and caudally), across the anterior surface of the tibia transverse to the longitudinal axis of fixation portion  82 , but caudally of the tibial tuberosity. In another alternative, as shown in  FIG. 13B , implant  80 B includes extended fixation portion  90 B, which extends in a direction opposite from the displacement portion  86 , e.g. laterally, or laterally and caudally, relative to fixation portion  82  so as to extend around the lateral side of the tibia. In another example, if the fixation portion  82  is mounted to the medial side, extended fixation portion  90 B may be configured to extend posteriorly further around the medial and/or posterior side of the tibia (if the fixation portion  82  is mounted to the medial side). Extended fixation portions  90 A and  90 B create a wider base for fixation portion  82  and thus provide greater resistance to torqueing forces as described. Extended fixation portions  90 A and  90 B generally will be configured and dimensioned to match the shape of the tibia in the area of contact. 
     An extended fixation portion configured and positioned as extended fixation portion  90 A will help to resist torqueing applied to the displacement portion  86  by creating a greater surface bearing against the bone at a greater distance from the center of rotation of the torqueing force, which will lie approximately along a centerline of fixation portion  82 . An extended fixation portion configured and positioned as extended fixation portion  90 B will help to resist torqueing force applied to the displacement portion  86  by creating a greater lever arm through which a bone screw in fixation holes  83  may act to resist the torqueing force. In some situations it may be desirable to utilize both extended fixation portions  90 A and  90 B to achieve the benefits of both approaches.  FIG. 13C  shows exemplary implant  80 C employing both laterally- and medially-extended fixation portions  90 A,  90 B on one device. 
     Depending on patient anatomy, it may be desirable to provide a fixation portion that wraps farther around the tibia. In such situations, a split or bifurcated fixation portion  92  may be employed such as shown with implant  80 D in the exemplary embodiment of  FIG. 13D . In this embodiment, split fixation portion  92  forms two fixation arms  92 A and  92 B at its caudal end, one extending laterally and posteriorly, and a second extending medially and anteriorly, that can be configured to wrap around the tibia in opposing directions to the extent appropriate for the patient anatomy and clinical situation presented. These fixation arms  92 A,  92 B can be oriented transverse to longitudinal axis of fixation portion  92  to extend primarily in the medial-lateral direction, or angled caudally as wells as medially or laterally. Alternatively, a single arm  92 A or  92 B may be employed in a manner similar to the exemplary embodiments of  FIGS. 13A and 13B  employing single extended fixation portions  90 A or  90 B. In addition, the caudal ends of fixation arms  92 A,  92 B may have holes configured to receive a rod or screw extending through one fixation arm  92 A and through the tibia to the other fixation arm  92 B. 
     Again, depending on patient anatomy, it also may be desirable to provide differently shaped bone facing fixation surfaces for fixation portion  82 . Alternatives include fixed protrusions  94  and adjustable protrusions  96  as illustrated in  FIG. 13E . Such protrusions allow the fixation portion  82  to contact the bone at discreet locations to accommodate variability in the contour of the bone surface. Fixed protrusions may be ground to a desired height and shape according to the anatomy of each patient to provide further patient specific adaptability. Adjustable protrusions may be provided with an adjustment mechanism  98 , such as a threaded member accessible at the outer surface of fixation portion  82  and rotatable to move adjustable protrusions  96  between an inner position and an outer position (shown in dashed lines in  FIG. 13E ) to adjust the distance the protrusion extends from the fixation portion  32 . Alternatively or additionally, fixed or adjustable protrusions may be provided on the bone-facing side of the displacement portion such that it contacts the bone at discrete locations, accommodating variations in shape of the bone surface on the cranial side of the tibial tuberosity where the displacement portion extends under the patellar tendon. 
     The present disclosure contains multiple alternative embodiments and multiple alternative features within each disclosed embodiments. As will be apparent to persons of ordinary skill in the art based on the teachings herein contained, different features may be employed with embodiments other than those on which they are shown in the drawings for purposes of illustration. Given the number of possible combinations, it is not possible within a concise disclosure to separately illustrate each combination of features as would be understood by those skilled in the art. As non-limiting examples, each of the different supplemental fixation or support means shown in  FIG. 11A or 11B , the fixation base member shown in  FIGS. 12A-E , and/or the different fixation portion shapes shown in  FIGS. 13A-13E  may be used together in different combinations or individually with each different implant herein. 
       FIGS. 14A and 14B  depict further exemplary embodiments of implants  400 A and  400 B, respectively, also for treating patellofemoral osteoarthritis and/or patellar maltracking, but with femorally mounted implants as shown. As with other embodiments disclosed herein, implants  400 A and  400 B each have a fixation portion  412  including one or more holes  415  for receiving screws, or other fixation means for anchoring the implant to bone. Fixation portion  412  is generally straight and elongated, being configured for positioning in general alignment with the femoral shaft on the lateral, medial or anterior-medial/lateral side of the femur, cranially with respect to the patella. Holes  415  may be positioned in approximate alignment with a longitudinal centerline of fixation portion  412  as shown, however additional holes may be included for added fixation security, similar to embodiments shown, inter alia,  FIG. 10 or 13A -D. Preferably fixation portion  412  is configured to be mounted to the femur outside the joint capsule, cranially with respect to the tendons, ligaments and other tissues that form the capsule. 
     Displacement portion  414 , is configured and dimensioned to be positioned under the quadriceps tendon caudally separated from the insertion point of the tendon in the quadriceps muscle and cranially with respect to its attachment point to the patella, with the entire displacement portion  414  preferably being disposed entirely outside the joint capsule. Thus, for a medially placed implant displacement portion will also extend laterally across an anterior portion of the femur. Likewise, a laterally placed device will have a displacement portion  414  that also extends medially across an anterior portion of the femur. The displacement portion  414  is configured to atraumatically engage the quadriceps tendon and displace it anteriorly relative to the femur, thus increasing space in the patellofemoral area. The displacement portion  414  has a width in the lateral-medial direction selected to accommodate the full width of the quadriceps tendon so that the tendon remains engaged along its entire width as it slides on the displacement portion. Displacement portion  414  has a length in the caudal-cranial direction selected so that it does not interfere with the patella. Displacement portion  414  preferably has a convex curvature on bearing surface  418 , which engages the tendon. In general, the displacement portion and, in particular the bearing surface of the displacement portion, will be free of holes or other fixation means, with configuration similar to the bearing surfaces in other described embodiments. 
     As with tibial mounted embodiments, the displacement portion  414  and/or bearing surface may have a curvature which provides a constant displacement of the tissue throughout the range of motion of the joint, or configured to vary the displacement at different points throughout the range of motion. The curvature may be entirely or partially spherical, elliptical, parabolic, logarithmic spiral, or other curvature or combination thereof. In preferred embodiments the displacement portion  414  is configured such that a cranial aspect of bearing surface  418  slopes or curves gradually away from the femur as it extends in the caudal direction to provide gradually increasing displacement of the tendon. 
     A spanning section  416  interconnects fixation portion  412  and displacement portion  414 . Spanning section  416 , is previously described to appropriately position the displacement portion with respect to the fixation portion and soft and bony tissues in the area of treatment. As with other embodiments, displacement of the target tissue can be altered by changing the length, curvature and angle of the spanning section among other features. Implant  400 A ( FIG. 14A ) may also include a cantilevered portion  420  of displacement portion  414  which has an undercut surface that is spaced apart from the underlying femoral surface. As with other described embodiments, cantilevered portion  420  creates a space (S) between the implant and the bone to accommodate soft tissue structures as needed. Cantilevered portion  420  also extends the displacement portion  414  in the caudal direction toward the patella to optimize displacement while minimizing interference with the joint capsule and other soft tissues. Alternatively, implant  400 B, shown in  FIG. 14B , may be configured without a cantilevered portion such that substantially all of the bearing surface  418  overlies and is supported by portions of the displacement portion  414  that engage the femur. 
     Implants  400 A,  400 B may optionally include various other features described above in connection with tibial-mounted embodiments. For example, implants  400 A and  400 B may include a supporting section extending from a cranial part of the underside of the displacement portion into and merging with the fixation portion as described above in connection with tibially mounted embodiments. The configuration of a supporting section in these embodiments will, however, be generally inverted to accommodate fixation on the femur cranially with respect to the patella, as opposed to, on the tibia, caudally with respect to the patella. Implants  400 A,  400 B may alternatively have a tab or extension portion extending cranially, medially, and/or posteriorly from the displacement portion  414  on the opposite side from the fixation portion  412  which can engage the femur to provide additional support for the displacement portion  414 . As with the embodiments shown in  FIGS. 11A and 11C , such tab or extension may include a hole through which a bone screw may be inserted into the bone, or a hole for receiving a rod or screw extending from fixation portion  412  through the femur. 
     In general, implants according to embodiments of the present invention may be positioned and fixed using well-known instrumentation that is used for other orthopedic implant procedures. However, because of the unique position and seating of embodiments of the present invention, a specially shaped curved file as shown in  FIGS. 15A and 15B  may be useful for preparing the bone surface at the implant location, particularly for tibially mounted implants. Curved file  500  comprises a shaft  502  with a handle  504  at a proximal end and a curved file element  506  at the opposite, distal end. File element  506 , preferably made of a metal suitable for filing bone, extends in a transverse direction from shaft  502  and has a lower surface  506 A and an upper surface  506 B, one or both of which have grooves, knurling, points, bumps, or other features configured to file the bone surface to give it a suitable shape for receiving the implants of the invention. File element  506  is preferably curved about a second axis extending in a direction transverse to shaft  502  longitudinal axis in an anterior-posterior direction, giving file element  506  a convex cranial side  507  and a concave caudal side  509 . Upper and lower surfaces  506 A,  506 B are disposed at least partially in respective planes which are intersected by the second axis. In addition, lower and/or upper surfaces  506 A,  506 B may have a curvature about a third axis parallel to the longitudinal axis of shaft  502 . This latter curvature may have various shapes, e.g. generally matching the curvature of the anterior surface of the tibia in the region just cranial to the tuberosity where the implant will be fixed, or a different curvature as is suitable to provide a stable base for the implant. Further, either or both the convex cranial side  507  or concave caudal side  509  may have grooves or other features to facilitate filing the bone with these surfaces. Moreover, the lower and/or upper surfaces  506 A,  506 B may have either a convex or concave curvature about an axis transverse to shaft  502  as may be suitable to create the particular shape desired for the bone surface. The file element  506  may be inserted through an incision on the lateral or medial side of the tibia just cranial to the tibial tuberosity such that the file element extends across the anterior tibial surface. File  500  may be drawn back and forth in a cranial-caudal direction (parallel to the tibial shaft), or in a medial-lateral direction, to file down the tibial surface cranially of the tuberosity to the desired shape. It should be noted that in addition to their use in implanting the implants of the invention, the files disclosed herein may be used for other treatments, including bone re-shaping for the treatment of Osgood-Schlatter disease or other diseases. 
       FIGS. 16-20  illustrate further exemplary implants made according to the teachings of the present disclosure. For example, implant  610  in  FIG. 16  is made according to the general teachings of the disclosure and includes, inter alia, fixation portion  612 , displacement portion  614  and spanning section  616 , as well as other features described herein for the various embodiments. Implant  620 , shown in  FIG. 17 , includes a fixation portion  622 , displacement portion  624  and spanning section  626 , wherein displacement portion  624  is extended laterally as compared to implant  610  so that the lateral edge  628  of displacement portion  624  may be supported on the tibia, for example on Gerdy&#39;s tubercle or adjacent thereto, to provide a supplemental support element. In this regard, implant  620  includes features similar to embodiments shown in  FIGS. 9A-D . Implant  630 , shown in  FIG. 18 , also includes a fixation portion  632 , displacement portion  634  and spanning section  636 . Displacement portion  634  has a gradually increased thickness area  638  towards the lateral end in order to increase the tendon displacement at the lateral side. Implant  640 , shown in  FIG. 19 , includes fixation portion  642 , displacement portion  644  and spanning section  646 . In this embodiment, displacement portion  644  includes lateral pad  648  at an outer end to provide supplemental support and fixation adjacent the tibial tubercle. In this regard, implant  640  may include features similar to embodiments shown in  FIGS. 11B and 11C . Additionally, lateral pad  648  may be designed to hook the tibial tubercle. Implant  650 , shown in  FIG. 20 , includes fixation portion  652 , displacement portion  654  and spanning section  656 . Fixation portion  652  has a bone engaging surface  658  formed with a convex profile to facilitate positioning along the tibial tuberosity prior to fixation. The convex profile may be in the range of about 10 degrees of convexity. 
     Exemplary embodiments have been disclosed above and illustrated in the accompanying drawings. It will be understood by those skilled in the art that various changes, omissions and additions may be made to that which is specifically disclosed herein without departing from the spirit and scope of the present invention.